US20030066998A1 - Quantum dots of Group IV semiconductor materials - Google Patents

Quantum dots of Group IV semiconductor materials Download PDF

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Publication number
US20030066998A1
US20030066998A1 US10/212,001 US21200102A US2003066998A1 US 20030066998 A1 US20030066998 A1 US 20030066998A1 US 21200102 A US21200102 A US 21200102A US 2003066998 A1 US2003066998 A1 US 2003066998A1
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quantum dot
quantum dots
core
quantum
material
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Howard Lee
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OmniPV Inc
OEpic Inc
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UltraDots Inc
OEpic Inc
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Priority to US30990501P priority
Priority to US10/212,001 priority patent/US20030066998A1/en
Application filed by UltraDots Inc, OEpic Inc filed Critical UltraDots Inc
Assigned to OEPIC, INC. reassignment OEPIC, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: PAO, YI-CHING, TZUANG, CHING-KUNG, RIAZIAT, MAJID LEONARD
Assigned to OEPIC, INC. reassignment OEPIC, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: TZUANG, CHING-KUNG, PAO, YI-CHING, RIAZIAT, MAJID LEONARD
Assigned to ULTRAPHOTONICS, INC. reassignment ULTRAPHOTONICS, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: LEE, HOWARD WING HOON
Publication of US20030066998A1 publication Critical patent/US20030066998A1/en
Priority claimed from PCT/US2003/024245 external-priority patent/WO2005017951A2/en
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Assigned to OmniPV, Inc. reassignment OmniPV, Inc. CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: ULTRADOTS, INC.
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    • BPERFORMING OPERATIONS; TRANSPORTING
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    • G02F1/015Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating, or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on semiconductor elements with at least one potential jump barrier, e.g. PN, PIN junction
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L29/00Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof; Multistep manufacturing processes therefor
    • H01L29/02Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/16Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic System
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L29/00Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof; Multistep manufacturing processes therefor
    • H01L29/02Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/16Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic System
    • H01L29/1602Diamond
    • HELECTRICITY
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    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L33/00Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/20Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular shape, e.g. curved or truncated substrate
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L33/00Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/26Materials of the light emitting region
    • H01L33/34Materials of the light emitting region containing only elements of group IV of the periodic system
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10STECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S977/00Nanotechnology
    • Y10S977/70Nanostructure
    • Y10S977/813Of specified inorganic semiconductor composition, e.g. periodic table group IV-VI compositions
    • Y10S977/814Group IV based elements and compounds, e.g. CxSiyGez, porous silicon

Abstract

The invention relates to a quantum dot. The quantum dot comprises a core including a semiconductor material Y selected from the group consisting of Si and Ge. The quantum dot also comprises a shell surrounding the core. The quantum dot is substantially defect free such that the quantum dot exhibits photoluminescence with a quantum efficiency that is greater than 10 percent.

Description

    CROSS-REFERENCE TO RELATED APPLICATIONS
  • This application claims the benefit of U.S. Provisional Application Serial No. 60/309,898, filed on Aug. 2, 2001, U.S. Provisional Application Serial No. 60/309,905, filed on Aug. 2, 2001, U.S. Provisional Application Serial No. 60/309,979, filed on Aug. 2, 2001, U.S. Provisional Application Serial No. 60/310,090, filed on Aug. 2, 2001, and U.S. Provisional Application Serial No. 60/310,095, filed on Aug. 2, 2001, the disclosures of which are incorporated herein by reference in their entirety.[0001]
  • FIELD OF THE INVENTION
  • This invention relates generally to quantum dots. More particularly, this invention relates to quantum dots of Group IV semiconductor materials. [0002]
  • BACKGROUND OF THE INVENTION
  • Over the past several years, there has been an increasing interest in exploiting the extraordinary properties associated with quantum dots. As a result of quantum confinement effects, properties of quantum dots can differ from corresponding bulk values. These quantum confinement effects arise from confinement of electrons and holes along three dimensions. For instance, quantum confinement effects can lead to an increase in energy gap as the size of the quantum dots is decreased. Consequently, as the size of the quantum dots is decreased, light emitted by the quantum dots is shifted towards higher energies or shorter wavelengths. By controlling the size of the quantum dots as well as the material forming the quantum dots, properties of the quantum dots can be tuned for a specific application. [0003]
  • Previous attempts at forming quantum dots have largely focused on quantum dots of direct band gap semiconductor materials, such as Group II-VI semiconductor materials. In contrast to such direct band gap semiconductor materials, Group IV semiconductor materials such as Si and Ge have energy gaps, chemical properties, and other properties that render them more desirable for a variety of applications. However, previous attempts at forming quantum dots of Si or Ge have generally suffered from a number of shortcomings. In particular, formation of quantum dots of Si or Ge sometimes involved extreme conditions of temperature and pressure while suffering from low yields and lack of reproducibility. And, quantum dots that were produced were generally incapable of exhibiting adequate levels of photoluminescence that can be tuned over a broad spectral range. Also, previous attempts have generally been unsuccessful in producing quantum dots of Si or Ge that are sufficiently stable under ambient conditions or that can be made sufficiently soluble in a variety of matrix materials. [0004]
  • It is against this background that a need arose to develop the quantum dots and methods for forming quantum dots described herein. [0005]
  • SUMMARY OF THE INVENTION
  • In one innovative aspect, the present invention relates to a quantum dot. In one embodiment, the quantum dot comprises a core including a semiconductor material Y selected from the group consisting of Si and Ge. The quantum dot also comprises a shell surrounding the core. The quantum dot is substantially defect free such that the quantum dot exhibits photoluminescence with a quantum efficiency that is greater than 10 percent. [0006]
  • In another embodiment, the quantum dot comprises a core including a semiconductor material Y selected from the group consisting of Si and Ge. The quantum dot also comprises a ligand layer surrounding the core. The ligand layer includes a plurality of surface ligands. The quantum dot exhibits photoluminescence with a quantum efficiency that is greater than 10 percent.[0007]
  • BRIEF DESCRIPTION OF THE DRAWINGS
  • For a better understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: [0008]
  • FIGS. [0009] 1(a), 1(b), 1(c), and 1(d) illustrate quantum dots according to some embodiments of the invention.
  • FIG. 2 illustrates the energy gap of quantum dots fabricated from silicon plotted as a function of the size of the quantum dots, according to an embodiment of the invention. [0010]
  • FIG. 3 illustrates photoluminescence (PL) spectra from six samples with different sizes of silicon quantum dots, according to an embodiment of the invention. [0011]
  • FIG. 4([0012] a) illustrates the energy gap of quantum dots fabricated from germanium plotted as a function of the size of the quantum dots, according to an embodiment of the invention.
  • FIG. 4([0013] b) illustrates size-selective photoluminescence (PL) spectra for different sizes of germanium quantum dots, according to an embodiment of the invention.
  • FIG. 5([0014] a) illustrates concentration dependence of the linear index of refraction of engineered nonlinear nanocomposite materials doped with silicon and germanium quantum dots, according to an embodiment of the invention.
  • FIG. 5([0015] b) illustrates concentration dependence of the optical nonlinearity of engineered nonlinear nanocomposite materials doped with silicon and germanium quantum dots, according to an embodiment of the invention.
  • FIGS. [0016] 6(a), 6(b), 6(c), 6(d), 6(e), and 6(f) illustrate nonlinear directional couplers comprising engineered nonlinear nanocomposite materials, according to some embodiments of the invention.
  • FIGS. [0017] 7(a), 7(b), 7(c), 7(d), 7(e), and 7(f) illustrate an embodiment of a nonlinear Mach-Zehnder (MZ) interferometer comprising an engineered nonlinear nanocomposite material.
  • FIGS. [0018] 8(a), 8(b), 8(c), and 8(d) illustrate an alternative embodiment of a nonlinear MZ interferometer comprising an engineered nonlinear nanocomposite material.
  • FIG. 9 illustrates a figure-of-merit (FOM) for all-optical switching with an engineered nonlinear nanocomposite material as a function of quantum dot size, according to an embodiment of the invention. [0019]
  • FIGS. [0020] 10(a) and 10(b) illustrate photoluminescence spectra of silicon quantum dots made in accordance with an embodiment of the invention.
  • FIGS. [0021] 11(a) and 11(b) illustrate photoluminescence spectra of germanium quantum dots made in accordance with an embodiment of the invention.
  • DETAILED DESCRIPTION OF THE INVENTION
  • Definitions [0022]
  • The following definitions may apply to some of the elements described with regard to some embodiments of the invention. These definitions may likewise be expanded upon herein. [0023]
  • As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the content clearly dictates otherwise. Thus, for example, reference to “a quantum dot” includes a mixture of two or more such quantum dots and may include a population of such quantum dots. [0024]
  • “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur and that the description includes instances where the event or circumstance occurs and instances in which it does not. For example, the phrase “optionally surrounded with a shell” means that the shell may or may not be present and that the description includes both the presence and absence of such a shell. [0025]
  • Embodiments of the invention relate to a class of novel materials comprising quantum dots. As used herein, the terms “quantum dot”, “dot”, and “nanocrystal” are synonymous and refer to any particle with size dependent properties (e.g., chemical, optical, and electrical properties) along three orthogonal dimensions. A quantum dot can be differentiated from a quantum wire and a quantum well, which have size-dependent properties along at most one dimension and two dimensions, respectively. [0026]
  • It will be appreciated by one of ordinary skill in the art that quantum dots can exist in a variety of shapes, including but not limited to spheroids, rods, disks, pyramids, cubes, and a plurality of other geometric and non-geometric shapes. While these shapes can affect the physical, optical, and electronic characteristics of quantum dots, the specific shape does not bear on the qualification of a particle as a quantum dot. [0027]
  • For convenience, the size of quantum dots can be described in terms of a “diameter”. In the case of spherically shaped quantum dots, diameter is used as is commonly understood. For non-spherical quantum dots, the term diameter, unless otherwise defined, refers to a radius of revolution (e.g., a smallest radius of revolution) in which the entire non-spherical quantum dot would fit. [0028]
  • A quantum dot will typically comprise a “core” of one or more first materials and can optionally be surrounded by a “shell” of a second material. A quantum dot core surrounded by a shell is referred to as a “core-shell” quantum dot. [0029]
  • The term “core” refers to the inner portion of the quantum dot. A core can substantially include a single homogeneous monoatomic or polyatomic material. A core can be crystalline, polycrystalline, or amorphous. A core may be “defect” free or contain a range of defect densities. In this case, “defect” can refer to any crystal stacking error, vacancy, insertion, or impurity entity (e.g., a dopant) placed within the material forming the core. Impurities can be atomic or molecular. [0030]
  • While a core may herein be sometimes referred to as “crystalline”, it will be understood by one of ordinary skill in the art that the surface of the core may be polycrystalline or amorphous and that this non-crystalline surface may extend a measurable depth within the core. The potentially non-crystalline nature of the “core-surface region” does not change what is described herein as a substantially crystalline core. The core-surface region optionally contains defects. The core-surface region will preferably range in depth between one and five atomic-layers and may be substantially homogeneous, substantially inhomogeneous, or continuously varying as a function of position within the core-surface region. [0031]
  • Quantum dots may optionally comprise a “shell” of a second material that surrounds the core. A shell can include a layer of material, either organic or inorganic, that covers the surface of the core of a quantum dot. A shell may be crystalline, polycrystalline, or amorphous and optionally comprises dopants or defects. The shell material is preferably an inorganic semiconductor with a bandgap that is larger than that of the core material. In addition, preferred shell materials have good conduction and valence band offsets with respect to the core such that the conduction band is desirably higher and the valence band is desirably lower than those of the core. Alternatively, the shell material may have a bandgap that is smaller than that of the core material, and/or the band offsets of the valence or conduction bands may be lower or higher, respectively, than those of the core. The shell material may be optionally selected to have an atomic spacing close to that of the core material. [0032]
  • Shells may be “complete”, indicating that the shell substantially completely surrounds the outer surface of the core (e.g., substantially all surface atoms of the core are covered with shell material). Alternatively, the shell may be “incomplete” such that the shell partially surrounds the outer surface of the core (e.g., partial coverage of the surface core atoms is achieved). In addition, it is possible to create shells of a variety of thicknesses, which can be defined in terms of the number of “monolayers” of shell material that are bound to each core. A “monolayer” is a term known in the art referring to a single complete coating of a shell material (with no additional material added beyond complete coverage). For certain applications, shells will preferably be of a thickness between approximately 0 and 10 monolayers, where it is understood that this range includes non-integer numbers of monolayers. Non-integer numbers of monolayers can correspond to the state in which incomplete monolayers exist. Incomplete monolayers may be either homogeneous or inhomogeneous, forming islands or clumps of shell material on the surface of the quantum dot. Shells may be either uniform or nonuniform in thickness. In the case of a shell having nonuniform thickness, it is possible to have an “incomplete shell” that contains more than one monolayer of shell material. For certain applications, shell thickness will preferably range between approximately 1 Åand 100 ÅA. [0033]
  • It will be understood by one of ordinary skill in the art that there is typically a region between the core and shell referred to herein as an “interface region”. The interface region may comprise an atomically discrete transition between the material of the core and the material of the shell or may comprise an alloy of the materials of the core and shell. The interface region may be lattice-matched or unmatched and may be crystalline or noncrystalline. The interface region may contain one or more defects or be defect-free. The interface region may be homogeneous or inhomogeneous and may comprise chemical characteristics that are graded between the core and shell materials such that a gradual or continuous, transition is made between the core and the shell. Alternatively, the transition can be discontinuous. The width of the interface region can range from an atomically discrete transition to a continuous graded alloy of core and shell materials that are purely core material in the center of the quantum dot and purely shell material at the outer surface. Preferably, the interface region will be between one and five atomic layers thick. [0034]
  • A shell may optionally comprise multiple layers of a plurality of materials in an onion-like structure, such that each material acts as a shell for the next-most inner layer. Between each layer there is optionally an interface region. The term “shell” is used herein to describe shells formed from substantially one material as well as a plurality of materials that can, for example, be arranged as multi-layer shells. [0035]
  • A quantum dot may optionally comprise a “ligand layer” comprising one or more surface ligands (e.g., organic molecules) surrounding a core of the quantum dot. A quantum dot comprising a ligand layer may or may not also comprise a shell. As such, the surface ligands of the ligand layer may bind, either covalently or non-covalently, to either the core or the shell material or both (in the case of an incomplete shell). The ligand layer may comprise a single type of surface ligand (e.g., a single molecular species) or a mixture of two or more types of surface ligands (e.g., two or more different molecular species). A surface ligand can have an affinity for, or bind selectively to, the quantum dot core, shell, or both at least at one point on the surface ligand. The surface ligand may optionally bind at multiple points along the surface ligand. The surface ligand may optionally contain one or more additional active groups that do not interact specifically with the surface of the quantum dot. The surface ligand may be substantially hydrophilic, substantially hydrophobic, or substantially amphiphilic. Examples of the surface ligand include but are not limited to an isolated organic molecule, a polymer (or a monomer for a polymerization reaction), an inorganic complex, and an extended crystalline structure. [0036]
  • It will be understood by one of ordinary skill in the art that when referring to a population of quantum dots as being of a particular “size”, what is meant is that the population is made up of a distribution of sizes around the stated “size”. Unless otherwise stated, the “size” used to describe a particular population of quantum dots will be the mode of the size distribution (i.e., the peak size). [0037]
  • As used herein, the “size” of a quantum dot will refer to the diameter of a core of the quantum dot. If appropriate, a separate value will be used to describe the thickness of a shell surrounding the core. For instance, a 3 nm silicon quantum dot with a 1.5 nm SiO[0038] 2 shell is a quantum dot comprising a 3 nm diameter core of silicon surrounded by a 1.5 nm thick layer of SiO2, for a total diameter of 6 nm.
  • For certain applications, the thickness of the ligand layer is a single monolayer or less and can sometimes be substantially less than a single monolayer. [0039]
  • As used herein, the term “photoluminescence” refers to the emission of light of a first wavelength (or range of wavelengths) by a substance (e.g., a quantum dot) that has been irradiated with light of a second wavelength (or range of wavelengths). The first wavelength (or range of wavelengths) and the second wavelength (or range of wavelengths) can be the same or different. [0040]
  • As used herein, the term “quantum efficiency” refers to the ratio of the number of photons emitted by a substance (e.g., a quantum dot) to the number of photons absorbed by the substance. [0041]
  • As used herein, the term “monodisperse” refers to a population of quantum dots wherein at least about 60% of the population, preferably 75% to 90% of the population, or any integer or noninteger therebetween, falls within a specified particle size range. A population of monodispersed particles deviates less than 20% root-mean-square (rms) in diameter, more preferably less than 10% rms, and most preferably less than 5% rms. [0042]
  • “Optically pure” refers to a condition in which light passing through or past a material is substantially unchanged in mode quality as a result of inhomogeneities in the material or modulations at the interface between materials. This does not include mode disruption resulting from changes in index of refraction of waveguides. For instance, a material with large aggregates of quantum dots capable of scattering light would not be optically pure. The same material with aggregates of a size that do not significantly scatter light, however, would be optically pure. It will be apparent to one of ordinary skill in the art that what is meant above by “substantially unchanged” will depend on the optical requirements of a particular application. To this end, “optically pure” refers to the level of optical purity required for the application in which the material is to be used. [0043]
  • “Optically homogeneous” is defined as being homogeneous across a length scale that is significant for optical waves, preferably greater than 250 nm, more preferably greater than 4 μm, and most preferably greater than ˜1000 μm. [0044]
  • A “waveguide structure” is a term of art and refers to an optical device capable of transmitting light from one location to another. A waveguide structure can transmit light through the use of guiding by localized effective index differences. One example of this involves total internal reflection within a “waveguide core”, with an index of refraction n[0045] 1, surrounded by a “cladding”, with an index of refraction n2, wherein n1>n2. Another example of a waveguide structure involves appropriately micro or nanostructured materials such as photonic bandgap materials where the guiding results from the periodic micro- or nano-structure of the materials.
  • “Cladding” is any material that surrounds the waveguide core in a waveguide structure such that n[0046] 1>n2. In a typical waveguide structure, light propagates as a traveling wave within and along the length of the “waveguide core” and evanescently decays within the cladding with a decay constant related to the ratio of n1 to n2. Light trapped within, and traveling along, the length of a waveguide core is referred to as being “guided”.
  • The shape of a waveguide core or a cladding can typically be described in terms of its “cross-section”. The cross-section is the shape created by cutting the waveguide core or the cladding along the axes perpendicular to the longitudinal axis of the waveguide structure. The longitudinal axis is the axis in which guided light travels. [0047]
  • “Optical fibers” and “planar waveguides” are two common forms of waveguide structures known in the art. “Optical fiber”, as the term is commonly used, typically refers to a structure comprising a substantially cylindrical waveguide core surrounded by a substantially cylindrical cladding and optionally comprising a flexible, protective outer-coating. Alternatively, or in conjunction, an optical fiber can comprise a non-cylindrical waveguide core with a cross-section shaped as a trapezoid, a circle, an oval, a triangle, or another geometric and nongeometric shape. [0048]
  • “Planar waveguides” are waveguide structures fabricated on a substrate by a variety of methods. “Planar waveguides” typically comprise a substantially rectangular waveguide core. Alternatively, or in conjunction, planar waveguides can comprise non-rectangular waveguide cores with cross-sections of trapezoids, circles, ovals, triangles, or a plurality of other geometric and nongeometric shapes. While the term “planar” suggests a flat structure, the term “planar waveguide”, as used herein, also refers to structures comprising multiple flat layers. Optionally, one or more layers in a planar waveguide are not flat. One of skill in the art will appreciate that the key aspect of a “planar waveguide” is that it is a waveguide structure fabricated on a “substrate”. Unless otherwise stated, the term “waveguide structure” will be used herein to describe a planar waveguide. [0049]
  • “Waveguide substrate” or “substrate” is used herein to describe the material on which a planar waveguide is located. It is common that a planar waveguide is fabricated directly on the surface of the substrate. The substrate typically comprises a solid support such as, for example, a silicon wafer and optionally comprises an additional “buffer layer” that separates the, waveguide structure from the solid support. The buffer layer optionally comprises a plurality of layers comprising one or more materials or combination of materials. The buffer layer may optionally act, in part, as a cladding. Alternatively, the waveguide substrate may be a flexible substrate serving the same purpose. [0050]
  • “Single mode” waveguide structures are those waveguide structures (either planar or fiber optic) that typically support a single optical mode (e.g., TEM00). Such waveguide structures are preferred according to some embodiments of the invention. “Multi-mode” waveguide structures are those waveguide structures that typically support multiple optical modes simultaneously. [0051]
  • “Waveguide diameter” is herein used to describe the diameter of a substantially cylindrical waveguide core of an optical fiber. Waveguide diameter is also used to describe the diameter of a substantially cylindrical core on a planar waveguide. [0052]
  • “Waveguide width” or “width” is used herein to describe the cross-sectional dimension of a substantially rectangular waveguide core that is oriented parallel to the substrate surface. This is also referred to as the “horizontal dimension” of the waveguide core. “Waveguide height” or “height” is used herein to describe the cross-sectional dimension of a substantially rectangular waveguide core that is oriented perpendicular to the substrate surface. This is also referred to as the “vertical dimension” of the waveguide core. Based on the definitions of “width” and “height” described here, one of ordinary skill in the art will understand the translation of these terms to other geometrically or nongeometrically shaped waveguide cores. Unless otherwise stated, the standard definitions of width and height used in geometry will be used to describe geometric cross-sectional shapes. [0053]
  • “Core taper” refers to a region of the waveguide core in which the geometry of the waveguide core is changed. This may comprise changing the size and/or shape of the waveguide core in one or two dimensions. A core taper, for example, may comprise a transition of a waveguide core with a square cross-section of 15 μm×15 μm to a waveguide core with a square cross-section of 7 μm×7 μm. A core taper may also, for example, comprise a transition from a waveguide core with a square cross-section of 15 μm×15 μm to a waveguide core with a circular cross-section of 10 μm in diameter. Many other forms of core-tapers are possible and will be understood from the above definition. [0054]
  • A “core taper” is typically engineered to gradually change the characteristics of the waveguide structure over a defined distance, referred to as the “taper length”. Ideally, the taper length will be long enough so that the transition preserves the mode structure of an optical signal through the taper. In particular, it is preferred, but not required, that a single optical mode entering a taper remains a single mode after exiting the taper. This retention of the mode-structure is referred to as an “adiabatic transition”. While the term “adiabatic transition” is commonly used, those of ordinary skill in the art will recognize that it is typically not possible to have a perfectly adiabatic transition, and that this term can be used to describe a transition in which the mode structure is substantially undisrupted. [0055]
  • A “cladding taper” is a novel embodiment disclosed herein that is similar to a core taper; however, it refers to a change in width of the cladding around the waveguide core. Similar to a core taper, a cladding taper can be used to change the size and/or shape of the cladding and can be defined to have a taper length. The taper length can be such as to produce an adiabatic or nonadiabatic transition. [0056]
  • Both core and cladding tapers may optionally refer to the case in which the index of refraction of the materials in the core or cladding are gradually changed, or “graded” over the taper length. As used herein, the term “gradually” refers to changes that occur continuously or in small steps over a given nonzero distance. Core and cladding tapers may optionally comprise changes to the index, size, and/or shape of the core or cladding, respectively. [0057]
  • A “bend” is used herein to describe a portion of a planar waveguide in which the planar waveguide displays a degree of curvature in at least one dimension. Typically, the cross-section of the waveguide is substantially unchanged within the bend. Typically, bends will be smooth and continuous and can be described in terms of a radius of curvature at any given point within the bend. While bends can curve the planar waveguide both parallel and perpendicular to the substrate (e.g., horizontal or vertical bends, respectively), unless otherwise stated, the term “bend” will herein refer to horizontal bends. Optionally, bends can also comprise tapers. [0058]
  • A “multimode interference device” or multimode interferometer (MMI) refers to an optical device in which the cross-section of the waveguide core is substantially changed (typically increased) within a short propagation length, leading to a region of waveguide core in which more than one mode (but typically fewer than 10 modes) may propagate. The interaction of these propagating multiple modes defines the function performed by the MMI. MMI devices include fixed ratio splitters/combiners and wavelength multiplexers/demultiplexers. [0059]
  • As used herein, a “waveguide coupler”, “optical coupler”, and “directional coupler” are synonymous and refer to a waveguide structure in which light is evanescently coupled between two or more waveguide cores within a coupling region such that the intensity of the light within each of the individual cores oscillates periodically as a function of the length of the coupling region. A more detailed description of a waveguide coupler is disclosed below. [0060]
  • A “nonlinear waveguide coupler” is a waveguide coupler in which the region between and/or around two or more coupled waveguide cores is filled with a material (e.g., an “active material”) with an index of refraction that can be changed. By changing the index of refraction of the active material, the coupling characteristics of the nonlinear waveguide coupler can be modified. Alternatively, the active material may be contained within one or more of the coupled waveguide cores (e.g., as a section of one of the waveguide cores). [0061]
  • A “Mach-Zehnder interferometer” or “MZ interferometer” (MZI) is a waveguide structure in which light from a waveguide core (e.g., an “input waveguide core”) is split into two or more separate waveguide cores (e.g., “waveguide arms” or “arms”). Light travels a defined distance within the arms and is then recombined into a waveguide core (e.g., an “output waveguide core”). In a MZ interferometer, the history of the optical signals in each arm affects the resulting signal in the output waveguide core. A more detailed description of a MZ interferometer is disclosed below. [0062]
  • A “nonlinear MZ interferometer” is a MZ interferometer in which one or more of the waveguide arms comprise an active material. The active material may be in the core and/or cladding of the waveguide arm. Modifying the index of refraction of the active material modulates the signal in the output waveguide core by changing the degree of constructive and/or destructive interference from the waveguide arms. [0063]
  • “Active material” refers to any material with nonlinear optical properties that can be used to manipulate light in accordance with some embodiments of the invention. While the term active material will typically be used to refer to an engineered nonlinear nanocomposite material as described herein, the term may also be used to describe other nonlinear materials known in the art. [0064]
  • “Active region” refers to the region of an optical device in which the index of refraction of the active material is modulated in order to manipulate light. In the case of an electro-optic modulator, the active region is that area of the device where a voltage is applied. In a [0065] χ (3) based device, the active region is that area to which a trigger-signal is applied. Note that while the active region can be the only region of the device in which an intentional change in optical properties occurs, it does not restrict, the location of the active material, which may extend beyond the active region. Regions containing active materials outside the active region are typically not modulated during normal operation of the device. “Active length” describes the length of the active region along the longitudinal axis of the device.
  • In the case of optical devices employing evanescent coupling of light between two waveguide cores (e.g., a waveguide coupler), the “interaction region” or “coupling region” is the region of the optical device in which the coupling occurs. As is typically understood in the art, all waveguides can couple at some theoretically non-zero level. The interaction region, however, is typically considered to be that region of the optical device in which evanescent fields of the waveguides overlap to a significant extent. Here again, the interaction region does not restrict the extent of either the active region or the active material, which may be greater or lesser in extent than the interaction region. [0066]
  • “Interaction length” describes the length of the interaction region. “Interaction width” is the spacing between two coupled waveguides within the interaction region. Unless otherwise stated, the interaction width is assumed to be substantially constant across at least a portion of the interaction length. [0067]
  • “Trigger pulse”, “trigger signal”, “control pulse”, “control signal”, “control beam”, and “activation light” are synonymous and refer to light that is used to create a transient change in the index of refraction in the materials of some embodiments of the present invention. A trigger pulse can either be pulsed or CW. [0068]
  • “Data pulse”, data signal”, and “data beam” are synonymous and refer to light used to transmit information through an optical device. A data pulse can optionally be a trigger pulse. A Data pulse can either be pulsed or CW. [0069]
  • “CW light” and “CW signal” are synonymous and refer to light that is not pulsed. [0070]
  • “Wavelength range-of-interest” refers to any range of wavelengths that will be used with a particular optical device. Typically, this will include both the trigger and data signals, where the ranges for the trigger and data signals can be the same or different. For instance, if a device is fabricated for use in the 1550 nm telecom range, the data wavelength range-of-interest may be defined as 1.5 μm to 1.6 μm, and the trigger wavelength range-of-interest may be defined as 1.5 μm to 1.6 μm (or a different range). For devices in the 1300 nm range, the data wavelength range-of-interest may be defined as 1.25 μm-1.35 μm. While these are preferred wavelength range-of-interests, it will be understood that the specific wavelength range-of-interest can be different depending on the specific application. The ability to tune the materials of embodiments of the current invention implies that any wavelength range-of-interest may be used. In general, 300 nm to 4000 nm is a preferred wavelength range-of-interest, more preferably 300 nm to 2000 nm, more preferably 750 nm to 2000 nm, more preferably 1260 nm to 1625 nm, most preferably 1310±50 nm and 1580±50 nm. [0071]
  • Quantum Dots [0072]
  • Embodiments of the current invention, in part, exploit the extraordinary properties of quantum dots. Quantum dots have optical and electronic properties that can be dependent (sometimes strongly dependent) on both the size and the material forming the quantum dots. [0073]
  • In nature, it is the size range on the order of a few nanometers in which the quantum mechanical characteristics of atoms and molecules often begin to impact and even dominate the classical mechanics of everyday life. In this size range, a material's electronic and optical properties can change and become dependent on size. In addition, as the size of a material gets smaller, and therefore more atomic-like, many characteristics change or are enhanced due to a redistribution of oscillator strength and density of states. These effects are referred to as “quantum confinement” effects. For example, quantum confinement effects can cause the energy gap of the quantum dot or the energy of the light emitted from the quantum dot to increase as the size of the quantum dot decreases. These quantum confinement effects result in the ability to finely tune many properties of quantum dots (e.g., optical and electronic properties) by carefully controlling their size. This control provides one critical aspect of some embodiments of the current invention. [0074]
  • A quantum dot will typically be in a size range between about 1 nm and about 1000 nm in diameter or any integer or fraction of an integer therebetween. Preferably, the size will be between about 1 nm and about 100 nm, more preferably between about 1 nm and about 50 nm or between about 1 nm to about 20 nm (such as about 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 nm or any fraction of an integer therebetween), and more preferably between about 1 nm and 10 nm. [0075]
  • FIGS. [0076] 1(a), 1(b), 1(c), and 1(d) illustrates quantum dots according to some embodiments of the invention. In particular, FIG. 1(a) illustrates a quantum dot 100 comprising a core 102, according to an embodiment of the invention. A core (e.g., the core 102) of a quantum dot may comprise inorganic crystals of Group IV semiconductor materials including but not limited to Si, Ge, and C; Group II-VI semiconductor materials including but not limited to ZnS, ZnSe, ZnTe, ZnO, CdS, CdSe, CdTe, CdO, HgS, HgSe, HgTe, HgO, MgS, MgSe, MgTe, MgO, CaS, CaSe, CaTe, CaO, SrS, SrSe, SrTe, SrO, BaS, BaSe, BaTe, and BaO; Group III-V semiconductor materials including but not limited to AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb; Group IV-VI semiconductor materials including but not limited to PbS, PbSe, PbTe, and PbO; mixtures thereof; and tertiary or alloyed compounds of any combination between or within these groups. Alternatively, or in conjunction, a core can comprise a crystalline organic material (e.g., a crystalline organic semiconductor material) or an inorganic and/or organic material in either polycrystalline or amorphous form.
  • A core may optionally be surrounded by a shell of a second organic or inorganic material. FIG. 1([0077] b) illustrates a quantum dot 104 according to another embodiment of the invention. Here, the quantum dot 104 comprises a core 106 that is surrounded by a shell 108. A shell (e.g., the shell 108) may comprise inorganic crystals of Group IV semiconductor materials including but not limited to Si, Ge, and C; Group II-VI semiconductor materials including but not limited to ZnS, ZnSe, ZnTe, ZnO, CdS, CdSe, CdTe, CdO, HgS, HgSe, HgTe, HgO, MgS, MgSe, MgTe, MgO, CaS, CaSe, CaTe, CaO, SrS, SrSe, SrTe, SrO, BaS, BaSe, BaTe, and BaO; Group III-V semiconductor materials including but not limited to AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb; mixtures thereof; and tertiary or alloyed compounds of any combination between or within these groups. Alternatively, or in conjunction, a shell can comprise a crystalline organic material (e.g., a crystalline organic semiconductor material) or: an inorganic and/or organic material in either polycrystalline or amorphous form. A shell may be doped or undoped, and in the case of doped shells, the dopants may be either atomic or molecular. A shell may optionally comprise multiple materials, in which different materials are stacked on top of each other to form a multi-layered shell structure.
  • As illustrated in FIGS. [0078] 1(c) and 1(d), a quantum dot may optionally comprise a ligand layer comprising one or more surface ligands (e.g., organic molecules) surrounding a core, according to some embodiments of the invention. In FIG. 1(c), a quantum dot 110 comprises a core 112 and a ligand layer 114 surrounding the core 112. In FIG. 1(d), a quantum dot 116 comprises a core 118 and a ligand layer 122 surrounding the core 118. Here, the quantum dot 116 also comprises a shell 120 surrounding the core 118, where the shell 120 is positioned between the core 118 and the ligand layer 122.
  • Optical Properties [0079]
  • Linear Optical Properties: [0080]
  • One of the most dramatic examples of “quantum confinement” effects is that, for a semiconductor material, the energy gap shifts as a function of size. This can be seen in FIG. 2, where the energy gap of quantum dots fabricated from silicon, referred to herein as “silicon quantum dots”, is plotted as a function of the size (e.g., diameter) of the quantum dots, according to an embodiment of the invention. The silicon quantum dots were made as described herein. The vertical axis represents the energy gap of the silicon quantum dots, and the horizontal axis represents the size of the silicon quantum dots. The observed values for the energy gap (dots with error bars) are compared against pseudopotential and tight-binding models (solid line) and against the simple effective mass theory (dashed line). [0081]
  • The same effect can be seen for the emission wavelength as a function of the size of quantum dots. FIG. 3 illustrates photoluminescence (PL) spectra from six samples with different sizes of silicon quantum dots, according to an embodiment of the invention. The silicon quantum dots were made as described herein and include shells formed of an oxide. The vertical axis represents a normalized PL signal, and the horizontal axis represents the emission wavelength. The PL spectra illustrated in FIG. 3 is obtained by optically exciting the silicon quantum dots with ultraviolet light. The wavelength of the optical excitation is shorter than the wavelength at the absorption edge of the silicon quantum dots. FIG. 3 demonstrates the range of sizes that can be made with the methods described herein. The quantum dots shown at the top of FIG. 3 are not drawn to scale and are meant to illustrate the relative size of the quantum dots responsible for the PL spectra. FIGS. 2 and 3 demonstrate the unprecedented control that can be obtained over absorption and emission characteristics of the silicon quantum dots. [0082]
  • Through a series of relations called the Kramers-Kroenig equations, the properties of refractive index and dielectric constant can be related to absorption. As such, size-dependent control of absorption allows control of refractive index. [0083]
  • In addition to the size of a quantum dot, the optical and electronic properties are also strongly influenced by the material from which it is fabricated. Quantum confinement effects represent a modulation of the bulk properties of the material. As such, any changes resulting from a reduction in size are made relative to the bulk properties of the material. By selecting (e.g., independently selecting) the appropriate combination of quantum dot size and material, an even greater control of the optical and electronic properties of a quantum dot is provided. As an example, FIGS. [0084] 4(a) and (b) show the size dependent absorption and emission of germanium quantum dots, which differ from those of silicon quantum dots, according to an embodiment of the invention. The germanium quantum dots were made as described herein. In FIG. 4(a), the vertical axis represents the energy gap of the germanium quantum dots, and the horizontal axis represents the size of the germanium quantum dots. The observed values for the energy gap (open dots with error bars) are compared against theoretical predictions (solid dots and solid line). In FIG. 4(b), a size-selective PL spectrum is shown, where the vertical axis represents a normalized PL signal, and the horizontal axis represents the emission wavelength. The far right curve is offset vertically for clarity. The PL spectra shown in FIG. 4(b) are collected using different excitation wavelengths, such that only quantum dots with energy gaps less than or equal to the photon energy of the excitation light (i.e., greater than a certain quantum dot size) are excited.
  • Relation of Size and Material to Dielectric Constant and Index of Refraction [0085]
  • For most materials, the index of refraction far from resonance decreases as the energy gap of the material increases (a consequence of the Kramers-Kroegnig equations). This explains, for example, why the index of refraction of transparent materials (e.g., silica, metal halides, and organics) is less than that for inorganic semiconductors with smaller relative absorption energies. This effect also typically applies to quantum dots. In this case, as the size of the quantum dot decreases, the energy gap increases, decreasing the index of refraction. Thus, for quantum dots, the off-resonant index of refraction (at a fixed wavelength) typically correlates with size, affording another method to control the optical properties of the quantum dots. [0086]
  • Relation of Concentration of Quantum Dots to Dielectric Constant and Index of Refraction [0087]
  • Embodiments of the invention involve altering the index of refraction of a material by varying the concentration of quantum dots in the material. An example of this is shown in FIG. 5([0088] a), which illustrates concentration dependence of the linear index of refraction of engineered nanocomposite materials doped with silicon and germanium quantum dots, according to an embodiment of the invention. The silicon and germanium quantum dots were made in accordance with the methods described herein. The index of refraction is plotted as a function of the quantum dot concentration expressed in weight percent. In this figure, the index of refraction is measured in the visible range (sodium D line).
  • This concentration dependence provides yet another method of controlling the overall refractive index of a material by utilizing the properties of quantum dots. The ability to embed quantum dots into a variety of host materials will be discussed in a later section. [0089]
  • Nonlinear Optical Properties [0090]
  • In general, a wide variety of nonlinear optical phenomena can arise when materials are exposed to high-intensity light. Some of these nonlinear phenomena are used in certain aspects of telecommunications (e.g., Raman amplifiers) and many are being considered for future use (e.g., four-wave mixing, cross-phase modulation, and solitons). Although nonlinear phenomena are typically associated with high-intensities, these phenomena are also observed at lower intensities due to phase matching, resonant enhancement, and/or long interaction lengths. [0091]
  • Light incident on a material can induce a polarization (P), which can be expressed as (in SI units) [0092]
  • P=ε E=ε 0χ (1) E+ χ (2) E×E+ χ (3) E×E×E+. . . ┘,
  • where E is the electric field strength, ε[0093] 0 is the electric permittivity, χ is the overall optical susceptibility, and χ (n) is the nth order optical susceptibility. Since χ (2) phenomena are typically only present in materials that lack inversion symmetry (e.g., non-centrosymmetry), certain embodiments of the invention primarily exploit χ (3) phenomena, which can be exhibited by all materials. It should be recognized that tensor elements of χ (3) are in general complex quantities. The induced refractive index change Δn and the nonlinear index of refraction γ are related to the real part of appropriate tensor elements of χ (3) e.g., Re[χ (3) 1111], while the two-photon absorption coefficient β is related to the imaginary part of appropriate tensor elements of χ (3) , e.g., Im[χ (3) 1111]). In particular, certain embodiments of the invention exploit phenomena that change the index of refraction of a material by creating an effective optical susceptibility
  • χ eff =χ (1)+χ (3) E×E= χ (1)+χ (3) I,
  • where I is the intensity of the particular light beam creating the effective optical susceptibility (and where the higher order terms are assumed to be small and are therefore neglected here, although they can be utilized as well), which can affect the same light beam or another light beam at the same or different frequency. This leads to an effective or overall index of refraction given by [0094]
  • n(ω′)=n 0+γ(ω′,ω)I(ω),
  • and an operational definition for a nonlinear index of refraction γ given by [0095]
  • γ(ω′,ω)=n(ω′)−n 0 /I(ω)
  • where n(ω′) is the effective index of refraction at ω′, no is the low-intensity refractive index (e.g., the linear index of refraction), and I(ω) is the intensity of light with optical frequency ω that creates the effective optical susceptibility or index change. The nonlinear index of refraction γ(ω′,ω) is related to [0096] χ (3) ijkl(−ω′, ω′, ω,−ω), e.g., χ (3) 1111(−ω′, ω′, ω,−ω. If only one light beam is involved, then ω′ can be set equal to ω. If two light beams are involved, then ω′ and ω can be the same or different. Situations where ω′ and ω are the same can correspond to degenerate conditions (which is further discussed herein), in which case the nonlinear index of refraction γ can be referred to as a degenerate nonlinear index of refraction (or γdeg). Situations where ω′ and ω are different can correspond to non-degenerate conditions (which is further discussed herein), in which case the nonlinear index of refraction γ can be referred to as a non-degenerate nonlinear index of refraction (or γnondeg). As one of ordinary skill in the art will understand, an optical frequency of a light beam (e.g., ω or ω′) is inversely related to a wavelength of the light beam (e.g., λ or λ′).
  • This intensity dependent refractive index n(ω′) can be exploited for all-optical switching and optical signal processing. For certain applications, nonlinear absorption processes are of particular importance, in which case, optimization of Im[[0097] χ (3) ijkl] is preferred.
  • Nonlinear Optical Properties of Quantum Dots [0098]
  • In general, three mechanisms are principally responsible for [0099] χ (3) nonlinearities in quantum dots. These effects fall into the broad categories of resonant, nonresonant, and near-resonant effects. These categories can be further subdivided into degenerate (e.g., all light beams have the same wavelength) and non-degenerate (e.g., one or more light beams have different wavelengths) cases.
  • 1) Resonant Effects: [0100]
  • Resonant processes typically result from a change in electronic properties upon resonant excitation (e.g., the linear absorption of light). This leads to a corresponding change in refractive index, following the Kramers-Kroenig relations. The magnitude of an absorption change, and hence the optical nonlinearity, is directly related to the ground state absorption cross-section modified by any excited state absorption. In the case of a material with discrete states, such as molecules or quantum dots, the optical nonlinearity results from state-filling and is related to (σ[0101] g−σe), where σg and σe are the absorption cross sections of the material in the ground and excited states respectively, with a reduction in refractive index occurring for a reduction in absorption. For quantum dots, further enhancement of χ (3) results from unique physical phenomena such as quantum confinement, local electric field effects, and quantum interference effects.
  • As indicated above, the optical nonlinearity is related to (σ[0102] g−σe), so that increasing the oscillator strength of optical transitions from the ground state generally increases the optical nonlinearity. In the case of quantum dots, a decrease in size increases the spatial overlap of the electron and hole wave functions, which in turn increases the oscillator strength. Resonant nonlinearity therefore tends to increase with decreasing size. This enhancement, however, can be limited by any size dispersion.
  • Another important effect arises from the presence of one or more defects in a quantum dot. Defects can be present as trap states within the quantum dot. Due to the enormous surface to volume ratio in the size range of quantum dots, most relevant traps exist on the surface. If not passivated correctly, resonant excitation of a quantum dot creates electron-hole pairs that quickly relax into these surface-states. Holes, with their relatively large effective mass, tend to trap more easily, while the electrons, with their smaller effective mass, remain largely delocalized. The result is a spatial separation of the electron and hole wavefunctions and a decrease in oscillator strength, reducing the magnitude of the resulting nonlinearity. Furthermore, by tailoring the rate of relaxation between the delocalized quantum dot states and the localized surface states, it is possible to control the response time of the resonant optical nonlinearity. [0103]
  • Resonant nonlinearities can be utilized in both the degenerate and non-degenerate cases with respect to the wavelength of control and data beams. In the degenerate case, the wavelength range-of-interest lies near the absorption edge. For a single beam, the absorption can be saturated, leading to an intensity dependent absorption, commonly known as saturable absorption. For degenerate control and data beams, the control beam can modulate the transmission of the data beam, leading to an optical modulator. The refractive index change caused by the absorption change can also be utilized. Due to the broad electronic absorption in semiconductors in general and quantum dots in particular, resonant nonlinearities can be observed for the case where the control beam and the data beam are non-degenerate. In this case, the control beam can be of higher photon energy, such that carriers are generated which relax (primarily via phonon emission) towards the band edge, where the absorption bleaching and/or excited state absorption can affect the data beam of lower photon energy (but still resonant). [0104]
  • 2) Nonresonant Effects: [0105]
  • In contrast to resonant nonlinearities, where linear absorption of light is typically required, non-resonant nonlinearities typically do not require single-photon absorption of light. As a result, nonresonant nonlinearities are intrinsically fast since excited state relaxation is not required. However, nonresonant nonlinearities are generally smaller than resonant nonlinearities, due to the lack of strong single-photon resonance enhancement (although multi-photon resonance can be utilized to enhance the nonresonant nonlinearity). [0106]
  • There are three primary enhancement factors that can be utilized for nonresonant nonlinearities in quantum dots: quantum confinement, multi-photon resonance enhancement, and local-field effects. Quantum confinement provides an increase in oscillator strength due to enhanced wavefunction overlap (as described above), which enhances [0107] χ (3). Multi-photon resonances can be utilized in the absence of single-photon resonances to enhance the nonresonant nonlinearity. However, multi-photon resonances can introduce unwanted nonlinear absorptive losses. For certain applications, the ideal situation is one where the relevant light beams are just below the threshold of a multi-photon resonance, thereby allowing some resonant enhancement without significant nonlinear absorption loss. Finally, local field effects can be utilized to enhance the nonresonant χ (3). In particular, for a nanocomposite material in which quantum dots with dielectric constant ε1 are imbedded in a matrix material with dielectric constant ε2, an externally applied electric field (such as that originating from an electromagnetic light source) can be locally enhanced at the quantum dots if ε12, with the magnitude of the enhancement related to Δε=ε1−ε2. Such a situation can arise by embedding the quantum dots in a lower index matrix material. When illuminated by light, the electric field at the quantum dots is enhanced compared to the incident external field, in turn leading to an increase in the overall nonlinear response. This enhancement increases with size of the quantum dots as the quantum dot bandgap energy decreases, resulting in an increase in dielectric constant (ε1).
  • Nonresonant nonlinearities can be utilized in the non-degenerate case as well. In this case, the control beam can have either higher or lower photon energy than the data beam. One advantage of the non-degenerate case is that enhancement of cross-phase modulation (the control beam inducing an index change seen by the data beam) can occur without enhancement of self-phase modulation (the data beam affecting itself by the self-induced index change), which can cause some deleterious effects for telecommunications data streams. [0108]
  • 3) Near-Resonant Effects: [0109]
  • Near-resonant nonlinearities can be classified into two categories: degenerate (typically close to resonance) or non-degenerate (typically with one beam resonant and the other beam nonresonant). In the former case, the beams are typically very close to the resonance edge, i.e., just above, just below, or exactly at the edge of resonance, so that either no direct excitation of the material occurs through linear absorption or very little direct absorption occurs. The non-degenerate case is perhaps the more useful situation, as the refractive index change induced by resonant excitation via a control beam causes a phase change for the data beam that is below resonance (so as to minimize losses due to single- or multi-photon absorption). For example, the refractive index change due to the absorption saturation that extends to photon energies well below the absorption edge can be utilized, where carriers can be directly generated using the control beam instead of generating carriers via two-photon absorption using a high-intensity data beam. In addition, the excitation of free carriers in quantum dots due to absorption of control beam photons can lead to a refractive index change caused by other free carrier effects. For example, due to their small size, quantum dots typically intrinsically have high free carrier densities for even single photon absorption (e.g., ˜10[0110] 18 carriers/cm3 for one photon absorption in a single quantum dot). This leads to effects such as quantized Auger recombination and enhanced reflectivity (due to a large plasma frequency) at high enough carrier densities (e.g., ˜1020 carriers/cm3).
  • Size Dependence [0111]
  • From the discussion above, the size dependence (for a given quantum dot material) of both resonant and nonresonant nonlinear processes can be derived. Typically, for resonant optical nonlinearity, the magnitude of the nonlinearity increases as the quantum dot size decreases, decreases as the number of quantum dots with traps that localize electrons or holes increases, and decreases as the size dispersion increases. [0112]
  • Typically, for nonresonant processes, the optical nonlinearity increases with increasing quantum dot size, increases with increasing index of refraction of the quantum dot, increases with decreasing index of refraction of the surrounding matrix material. There is the caveat that these trends may not continue indefinitely to all sizes of quantum dots but can be useful as aids in practical design considerations. By carefully tailoring the specific size of the quantum dot, resonant effects, nonresonant effects, or both, can be used to optimize the resulting nonlinear response. [0113]
  • Quantum Dot Material Dependence [0114]
  • One important consideration for a material forming a quantum dot is that, for bound electrons, the optical nonresonant nonlinearity typically depends on the energy gap of the material as 1/E[0115] g n, where n typically ranges from about 4 to about 6. The nonresonant nonlinearity therefore can increase significantly as the energy gap decreases. This trend favors a combination of large quantum dot sizes and materials with intrinsically small bandgap energies. At the same time, however, the photon energy in the wavelength range-of-interest can affect the choice of material and quantum dot size in order to avoid significant linear and nonlinear absorption. Specifically, the material in the bulk form desirably should have an energy gap roughly equal to or greater than the photon energy in the wavelength range-of-interest for a data beam in order to exploit quantum confinement effects that shift the energy gap to higher energies. At the same time, to avoid significant multi-photon absorption effects, the energy gap of the material desirably should be sufficiently large that the energy gap of the resulting quantum dot is greater than two times the photon energy of the data beam photons.
  • For the case of nonresonant optical nonlinearities, these two concerns specify opposing trends that bracket the energy gap of the material of choice for quantum dots according to some embodiments of the present invention. The material in the bulk form desirably should have an energy gap less than this bracketed energy in order to exploit quantum confinement effects that shift the energy gap to higher energies. As an example, to avoid two-photon losses in degenerate all-optical switching components operating near 1550 nm (corresponding to a photon energy of 0.8 eV) and to also take advantage of the 1/E[0116] g n behavior of the nonlinear response, the quantum dot energy gap should be less than but close to 775 nm (or greater than but close, to 1.6 eV).
  • Enhanced Optical Properties [0117]
  • In addition to size-dependent spectral characteristics, quantum confinement can also result in an enhancement in the magnitude of various optical and electronic properties due to a redistribution of the density of states. Properties such as absorption cross-section and excited-state polarizability have been found to be enhanced by several orders of magnitude over bulk materials. [0118] χ (3) can also be enhanced by quantum confinement, as described previously.
  • Additional Effects [0119]
  • There are many practical definitions of a figure-of-merit (FOM) that take into account the many parameters that can be important and relevant for all-optical switching. One example of such a FOM is defined as [0120] Δ n α · τ ,
    Figure US20030066998A1-20030410-M00001
  • where Δn is the induced refractive index change, a is the linear and nonlinear absorption coefficient, and τ is the response time of the material. For this FOM, which is particularly relevant for resonant optical nonlinearities where light absorption is used, the larger the FOM, the better will be the performance of the all-optical switching. A definition of a FOM useful for nonresonant optical nonlinearities, where ideally no or little light absorption occurs, is 2γ/βλ, where γ is the nonlinear index of refraction, β is the two-photon absorption coefficient, and λ is the wavelength of operation. In this case, useful all-optical switching typically occurs when FOM>1. According to some embodiments of the invention, the following effects can be important for the formation of nanocomposite materials with a FOM in a usable range for practical optical switching. [0121]
  • The Effect of Defects on FOM: [0122]
  • Defects within quantum dot materials can have a substantial negative impact on their performance as nonlinear optical materials. Defects in the core and/or surface of the quantum dot can yield direct absorption of below-bandgap photons, increasing optical losses, and decreasing the overall FOM. As a result, while [0123] χ (3) may be high, the material can still be inappropriate for optical switching. The effect of defects on optical switching using quantum dots has not been previously considered as discussed herein.
  • One important aspect of some embodiments of the invention is that, for quantum dots to be used as a nonlinear optical material, they desirably should comprise a substantially defect-free core. In this case, the term “defect” typically refers to defects with energy below the energy gap of the quantum dot core or within the energy range of the wavelength range-of-interest. Additionally, the surface of quantum dots should be well passivated, such that there are substantially no defect states. Passivation can be accomplished, for example, through the inclusion of appropriate surface ligands in the ligand layer to bind to defect sites and remove them from the energy gap. Alternatively, or in conjunction, passivation can be achieved by applying a shell to the quantum dot core to fill or eliminate the defect sites. In this case, the shell material is preferably a material with an energy gap that is higher than that corresponding to the wavelength range-of-interest, and more preferably higher than the energy gap of the quantum dot core. Additionally, the shell desirably should be substantially defect-free or should have defects that can be eliminated through the inclusion of appropriate surface ligands. [0124]
  • Concentration Effects: [0125]
  • One important aspect of some embodiments of the invention is that the nonlinear properties of a material including quantum dots can be substantially affected by correlated interactions between two or more quantum dots. In particular, while [0126] χ (3) can be proportional to concentration of quantum dots at low concentrations, as the concentration increases, the individual quantum dots can get close enough to interact with each other, producing collective phenomena that can further enhance nonlinearity. This effect is seen in FIG. 5(b), which illustrates concentration dependence of the optical nonlinearity of engineered nonlinear nanocomposite materials doped with silicon and germanium quantum dots, according to an embodiment of the invention. The silicon and germanium quantum dots were made in accordance with the, methods described herein. The vertical axis represents the nonlinear index of refraction γ, and the horizontal axis represents the relative concentration of quantum dots in a matrix material. As shown in FIG. 5(b), γ can increase superlinearly with concentration at sufficiently high concentrations. The effect of concentration (and particularly the superlinear concentration dependence) on optical switching using quantum dots has not been previously considered as discussed herein.
  • For FIG. 5([0127] b), γ arises as a result of nonresonant degenerate nonlinearities. The values attained for γ are particularly large. As shown in FIG. 5(b), the nanocomposite material doped with silicon quantum dots has γ as high as about 8×10−5 cm2/W, which is 9 orders of magnitude larger than the bulk material from which the silicon quantum dots are fabricated (bulk silicon has a nonresonant degenerate γ of about 8×10−14 cm2/W). Additional nonlinear enhancement can be induced through the appropriate selection of molecular species in the ligand layer (see discussion below on Molecular Tethers).
  • Summary of Nonlinear Optical Properties of Quantum Dots [0128]
  • Enhancement and tunability of the optical nonlinearity in individual quantum dots and multi-quantum dot nanocomposites, combined with substantially defect free and/or well passivated quantum dot cores, provide the engineered nonlinear nanocomposite materials according to some embodiments of the current invention. Such nanocomposite materials can satisfy various characteristics for an ideal [0129] χ (3) based optical material that include (but are not limited to): large Re[χ (3) ijkl] in the wavelengths range-of-interest; a multi-photon transition that can be tuned to maximize near-resonance enhancement while minimizing optical loss due to absorption; the use of non-degenerate control and data beams where the control beam is resonant and induces a large index change at the data beam wavelength while introducing low optical loss at that wavelength; the use of degenerate control and data beams to allow cascading of devices; and low optical loss due to absorption by defects.
  • Colloidal Quantum Dots [0130]
  • Structures comprising quantum dots can be fabricated using vapor deposition, ion-implantation, photolithography, spatially modulated electric fields, semiconductor doped glasses, strain-induced potential variations in quantum wells, atomic width fluctuations in quantum wells, and a variety of other techniques. Preferably, quantum dots are formed or used in a form that can be easily incorporated into flexible or engineered optical materials or devices. In addition, it is desirable to separate the optical properties of the quantum dots from those of a matrix material to achieve a sufficiently large FOM with reduced absorption and/or scattering by the matrix material. [0131]
  • In a preferred embodiment, the current invention comprises colloidal quantum dots. Colloidal quantum dots are freestanding nanostructures that can be dispersed in a solvent and/or a matrix material. Such colloidal quantum dots are a particularly preferred material for some embodiments of the current invention because they can be more easily purified, manipulated, and incorporated into a matrix material. [0132]
  • It will be apparent to one of ordinary skill in the art that the defining characteristic for a “colloidal” quantum dot is that it is a freestanding nanostructure. The method of fabrication, size, and shape of the particular colloidal quantum dot do not bear on its classification. [0133]
  • Chemical Properties [0134]
  • Chemically Controllable Surface [0135]
  • According to some embodiments of the invention, a unique physical characteristic of quantum dots is that, while the core can comprise a crystalline semiconductor material, the surface can be coated with a variety of different organic and/or inorganic materials. These surface coatings (e.g., shells or ligand layers) can impart stability and chemical activity, as well as passivation of electrically and optically active defect sites on the quantum dot surface. These surface coatings are optionally substantially different in chemical nature than the inorganic core. As a result, while quantum dots can comprise primarily a highly nonlinear semiconductor material, they substantially appear to the surrounding material as surface ligands. As such, the processability and chemical, stability of this highly nonlinear and tunable optical material can primarily be a function of the surface layer and not a function of the material that provides the majority of the optical characteristics. [0136]
  • Surface ligands are preferably bi-functional. By bi-functional, it is meant that there are at least two portions of the surface ligand such that one portion interacts primarily with the quantum dot surface, while the second portion interacts primarily with the surrounding environment (e.g., solvent and/or matrix material). These at least two portions of the surface ligand may be the same or different, contiguous or noncontiguous, and are optionally contained within two or more different molecular species that interact with each other to form the ligand layer. The at least two portions can be selected from a group consisting of hydrophilic groups, hydrophobic groups, or amphiphilic groups. The interaction of each of the at least two portions and the quantum dot or surrounding environment can be covalent or noncovalent, strongly interacting or weakly interacting, and can be labile or non-labile. The at least two portions can be selected independently or together. [0137]
  • In some embodiments of the current invention, the surface ligands are selected such that the portion that interacts with the quantum dot passivates defects on the surface such that the surface is made substantially defect-free. At the same time, the portion that interacts with the environment is selected specifically to impart stability and compatibility (e.g., chemical compatibility or affinity) of the quantum dot within a matrix material that is selected for a specific application. Simultaneously satisfying both of these requirements is an important aspect of certain embodiments of the current invention relating to the development of an engineered nonlinear nanocomposite material. Alternative methods of achieving these requirements include (but are not limited to): 1) Passivating the surface of the quantum dot independent of the ligand layer (e.g., using a shell or creating an intrinsically defect free surface), while the environmental compatibility is imparted by the surface ligands, or 2) imparting both passivation and environmental compatibility independent of the ligand layer. Achieving passivation of the surface of quantum dots is one advantage of using colloidal quantum dots over alternate approaches. [0138]
  • Through the appropriate selection of surface ligands, quantum dots can be incorporated into a variety of matrix materials such as, for example, liquids, glasses, polymers, crystalline solids, and even close-packed ordered or disordered quantum dot arrays. The resulting nanocomposite materials can be formed into homogeneous, high-quality optical films of quantum dots. Alternatively, the chemistry can be selected to allow dispersion of the quantum dots into a matrix material with a controllable degree of aggregation, forming micron or sub-micron sized clusters. The result is an increased local fill-factor and an enhanced local field effect that may further increase the nonlinear response of the nanocomposite materials of embodiments of the present invention. [0139]
  • An important aspect of some embodiments of this invention relates to effectively separating the optical properties of the quantum dots from the optical, chemical, mechanical, and other properties of the matrix material. In this aspect, it is possible to combine the large nonlinearities of quantum dots with the ease of handling and processability of a matrix material such as a standard polymer. Thus, this aspect provides two additional features of an ideal [0140] χ (3) based optical material: physical and chemical compatibility with specific device architectures and the ability to be easily processed for incorporation.
  • Molecular Tethers: [0141]
  • In addition to conveying stability and chemical compatibility with the surrounding environment, the ligand layer can optionally be used to tailor the physical, optical, chemical, and other properties of the quantum dots themselves. In this case, it is not just the chemical nature of the surface ligand but also the interaction of the surface ligand with the quantum dot that imparts an additional level of control over the physical, optical, chemical, and other properties of the resulting nanocomposite material. We refer herein to any molecule, molecular group, or functional group coupled (e.g., chemically attached) to the surface of,a quantum dot that imparts additional functionality to the quantum dot as a “molecular tether”. In some cases, the molecular tether can be electrically active, optically active, physically active, chemically active, or a combination thereof. The inclusion of molecular tethers into a quantum dot structure is an important aspect of some embodiments of the present invention. [0142]
  • Active species are used to precisely control the electrical, optical, transport, chemical, and physical interactions between quantum dots and the surrounding matrix material and/or the properties of individual quantum dots. For instance, a conjugated bond covalently bound to the surface of one or more quantum dots may facilitate charge transfer out of one quantum dot and into another. Similarly, a physically rigid active group bound in a geometry substantially normal to the surface of a quantum dot can act as a physical spacer, precisely controlling minimum interparticle spacing within an engineered nonlinear nanocomposite material. [0143]
  • As described above, collective phenomena (e.g., at high concentrations) are an important aspect of some embodiments of the current invention. This aspect can be further enhanced by allowing individual quantum dots to interact with one another using molecular tethers that foster interactions between quantum dots. At sufficiently high number densities, the molecular tethers begin to make contact with molecular tethers from -other quantum, dots or with other quantum dots directly. This can serve to augment nonlinearity by controlling the interaction between quantum dots and thus increasing the degree of collective phenomena compared to single particle phenomena. Molecular tethers may include, but are not limited, to conducting polymers, charge transfer species, conjugated polymers, aromatic compounds, or molecules with donor-acceptor pairs. These molecular tethers can foster electron delocalization or transport and thus can increase the interaction between quantum dots. Additionally, the molecular tethers can be selected to facilitate high quantum dot number densities without the detrimental aggregation that often plagues high concentration systems. [0144]
  • Molecular tethers can also be selected to impart stability of quantum dots under a variety of environmental conditions including ambient conditions. Molecular tethers can optionally contain chemically active groups to allow quantum dots to be attached to polymer backbones, along with other active molecules. This provides a method for controlling the density of quantum dots within close proximity of molecules that influence a variety of functions such as carrier transport or delocalization. [0145]
  • An additional aspect of the present invention is the use of molecular tethers to physically connect two or more quantum dots in a 1 dimensional, 2 dimensional, or 3 dimensional structure or array. Such quantum dot superstructures can be created to initiate multiple dot quantum interference interactions or collective phenomena yielding new and useful properties such as enhanced, nonsaturating optical nonlinearities. The length and properties of these molecular tethers can be tailored to enhance or generate specific quantum phenomena. These nanostructures can have the properties of single quantum dots or an ensemble of quantum dots depending on the nature of the molecular tethers. For certain applications, more than one type of molecular tether can be used to connect quantum dots. [0146]
  • The quantum dots according to some embodiments of the invention exemplify microscopic conditions that enhance the nonresonant optical nonlinearity arising from local electric field effects described above. Whether the quantum dot surface is terminated with oxide or ligand layer (e.g., molecular tethers), the result is a particle (e.g., a core of the quantum dot) with dielectric constant ε[0147] 1 surrounded by an environment (e.g., the surface oxide layer or molecular tethers) with dielectric constant ε2 where ε12. Therefore, the enhancement of the nonresonant optical nonlinearity can be engineered by the judicious choice of oxide or molecular tether without resorting to a surrounding bulk matrix material. In other words, a single quantum dot as described in this patent should exhibit an enhanced nonresonant optical nonlinearity since the surface layer functions as a surrounding matrix material with a lower dielectric constant. Optionally, molecular tethers can be used to connect quantum dots together without a separate matrix material. In this case, an extrinsic matrix material is not required since the individual interconnected quantum dots exhibit an enhanced local electric field effect.
  • A preferred approach of attaching appropriate molecular tethers to a quantum dot surface can be thought of as essentially treating a quantum dot as a very large molecule (e.g., a macro-molecule) and the molecular tethers as functionalizations of this large molecule. This creates a large three-dimensional structure with enhanced nonlinear optical properties resulting from the combination of quantum effects from the quantum dot and carrier polarization and delocalization effects from the molecular tethers and from the interaction of these two effects. These properties can be tailored by the choice of molecular tethers. In addition, a quantum dot can also represent a large and stable reservoir of polarizable charge that also contributes to a large nonlinear optical response. [0148]
  • Macroscopic Quantum Dot Solids [0149]
  • Macroscopic solids can be fabricated in which quantum dots form a substantially close-packed array (e.g., a cubic closed-packed array) in the absence of an extrinsic matrix material. These “quantum dot solids” can either be crystalline, polycrystalline, or amorphous. While containing a relatively high density of quantum dots, quantum dot solids can still be easily processed since, during formation, the quantum dots can be dispersed in a solvent that is subsequently removed. Uniform solid quantum dot films, for instance, can be formed using standard spin-coating techniques as, for example, described in C. R. Kagan et al., “Long-range resonance transfer of electronic excitations in close-packed CdSe quantum-dot solids,” Phys. Rev. B 54, 8633 (1996), the disclosure of which is incorporated herein by reference in its entirety. In addition, surface ligands can still be selected to impart solvent compatibility and appropriate chemical stability to the final quantum dot solid. In contrast to the interconnected material described above, these macroscopic quantum dot solids are typically not held together by molecular bonds but rather by Van der Waals forces. [0150]
  • High-quality optical materials can be fabricated from quantum dot solids with substantially homogeneous optical properties throughout the material. The density of quantum dots can be tuned by modifying the length and/or structure of the surface ligands. Careful selection of surface ligands can produce continuously tunable densities up to a maximum fill-factor of about 75% by volume of the quantum dot solid, preferably between about 0.005% and 75% by volume (e.g., between about 10% and 75% by volume, between about 30% and 75% by volume, between about 50% and 75% by volume, or between about 60% and 75% by volume). The surface ligands are optionally removed partially or completely by heating or chemical treatment after the quantum dot solid is formed. More specifically, the length of the surface ligands can be used to define the spacing between quantum dots. By combining the ability to create density-controlled quantum dot solids with variable density quantum dots in a matrix material, the concentration of quantum dots, and therefore the nonlinear index of refraction of the materials described herein, can be tuned over many orders of magnitude. [0151]
  • In the case of quantum dot solids, the surface ligands can take the place of an extrinsic matrix material according to some embodiments of the current invention. In the case of close-packed quantum dots in which the surface ligands have typically been removed, the quantum dots themselves are considered to form their own “intrinsic” matrix material. Quantum dot solids according to some embodiments of the invention can be fabricated in a variety of ways, such as, for example, described in C. B. Murray et al., “Self-Organization of CdSe Nanocrystallites into Three-Dimensional Quantum Dot Superlattices,” Science 270, 1335 (1995), C. R. Kagan et al., “Long-range resonance transfer of electronic excitations in close-packed CdSe quantum-dot solids,” Phys. Rev. B 54, 8633 (1996), and U.S. Pat. No. 6,139,626 to Norris et al., entitled “Three-dimensionally patterned materials and methods for manufacturing same using nanocrystals” and issued on Oct. 31, 2000, the disclosures of which are incorporated herein by reference in their entirety. Quantum dot solids according to some embodiments of the invention can be fabricated with a variety of different quantum dot materials, sizes, and size distributions. It is also possible to form mixed quantum dot solids comprising a plurality of quantum dot materials, sizes, and size distributions. [0152]
  • Engineerable Nonlinear Nanocomposite Materials [0153]
  • One embodiment of the present invention comprises an engineered nonlinear nanocomposite material that combines the large nonlinear and size dependent optical properties of quantum dots with the processability and chemical stability of a matrix material and/or a chemically controlled quantum dot surface. By separately selecting the size and material of the quantum dot, the surface ligands, the matrix material, and the density of quantum dots within the matrix material, one can independently tune various significant characteristics in designing an ideal nonlinear optical material. [0154]
  • In particular, this embodiment of the present invention comprises the following characteristics that, taken together, in part or in whole, provides a substantially improved nonlinear optical material over what is known in the art. [0155]
  • The effects of quantum confinement and the specific selection of quantum dot material is used to create extremely large optical nonlinearities, specifically Re[[0156] χ (3) ijkl], in the data beam wavelength range-of-interest, while the energies of single- and multi-photon absorption features are selected to minimize absorptive loss of the data beam and heating and to optimize resonant enhancement effects. This optimization can include the use of appropriately chosen non-degenerate control and data beams. Alternatively, the nonlinear absorption mechanisms can be enhanced, e.g., Im[χ (3) ijkl] can be optimized, depending upon the application.
  • The matrix material is selected, independent of the quantum dot material and size, with the desired chemical and mechanical properties to impart physical and chemical compatibility with the specific device architecture and materials as well as the process of incorporation into devices. [0157]
  • The surface ligands of the quantum dots are selected to facilitate homogeneous incorporation of the quantum dots into the selected matrix material and are optionally selected to facilitate controlled aggregation of quantum dots within the selected matrix material. [0158]
  • The density of quantum dots in the matrix material is selected to precisely tune the linear index of refraction to match the boundary conditions for a given device architecture (in the case of high-index materials, a quantum dot solid can be used). [0159]
  • EXAMPLE 1
  • This example describes a preferred embodiment in which an engineered nonlinear nanocomposite material is incorporated into a nonlinear directional coupler that utilizes nonresonant or near-resonant nonlinearities. In the current example, the waveguide core is fabricated from doped silica with an index of refraction of 1.52 at 1.55 μm. It will be recognized by one of ordinary skill in the art that doped silica can have an index of refraction over a wide range of values. The current example is not meant to limit the scope of the invention, and it will be understood that variations on this example can extend to waveguide cores with an arbitrary index of refraction. [0160]
  • In the case of a nonlinear directional coupler, light is evanescently coupled between two waveguide cores such that a signal entering one waveguide core oscillates between the two as a function of the interaction length. By choosing an appropriate length, the light can be coupled completely into one or the other of the two waveguide cores (i.e., the “off” state can be transmission through one or the other waveguide core by appropriate device design). By changing the index of refraction between the waveguide cores, it is possible to switch the output waveguide core from the “off” state to the other waveguide core (i.e., the “on” state) for a fixed length device. An index change from a [0161] χ (3) based nonlinear material can yield extremely fast optical switching. However, so far no single material has been appropriate for a commercial optical switch based on a nonlinear direction coupler.
  • The active material in this optical device desirably should have a large nonlinear response in the data beam wavelength range-of-interest. It is also desirable (primarily for nonresonant nonlinearities) to maximize resonant enhancement, while simultaneously avoiding significant single-or multi-photon absorption. At the same time, the linear index of refraction of the active material desirably should be less than that of the core material and be close to that of the rest of the cladding to avoid disruption of the optical mode as light is guided into the active region. [0162]
  • In this example, depicted in FIGS. [0163] 6(a) through 6(e), the device comprises doped silica waveguide cores 602 and 604 (n=1.552) fabricated on a doped silica substrate 606 (n=1.515). As shown in FIG. 6(b), the other three sides of the waveguide cores 602 and 604 are initially surrounded by air (n=1), as is the space between the waveguide cores 602 and 604 in the interaction region. The space around the waveguide cores 602 and 604 is then filled with an engineered nonlinear nanocomposite material 608 (n=1.515) to match the waveguide boundary conditions of the substrate 606, as shown in FIG. 6(c). By illuminating the interaction region with trigger-pulses as shown in FIGS. 6(d) and 6(e), the index of refraction between the waveguide cores 602 and 604 is changed, activating the switch.
  • Operation of this switch is slightly different than what is commonly described in the art. It is best understood by presupposing that the directional coupler length is chosen such that in the inactivated state, the two waveguide cores [0164] 602 and 604 exchange energy such that each output will receive substantially half of the power from each input (acting as a 3 dB coupler). If the illumination is such that the index of refraction increases in the nonlinear nanocomposite material 608, the interaction between the two propagating waveguide cores 602 and 604 will decrease, leading to a reduction in the data energy transferred between the cores 602 and 604, forcing the switch closer to a bar state. If the illumination is such that the index of refraction decreases in the nonlinear nanocomposite material 608, the interaction between the two cores 602 and 604 increases, increasing the energy transferred between the cores 602 and 604, forcing the switch closer to a cross state. One skilled in the art will recognize that this transfer function is cyclic and that further reduction of the index of refraction of the nonlinear nanocomposite material 608 will result in oscillations between the cross and bar states. If desired, the length of nonlinear directional coupler may be chosen to include several oscillations in the inactive state, leading to an effective bias in the total oscillations.
  • The engineered nonlinear nanocomposite material [0165] 608 for this example comprises silicon oxide coated silicon quantum dots or organic-terminated silicon or germanium quantum dots dispersed in a poly(methyl methacrylate) polymer matrix material (PMMA; n=1.49). PMMA is chosen here due to its desirable optical properties for use in the 1.55 μm range and its ease of processing in waveguide structures. Examples of these desirable optical properties include high optical transmissivity in the visible wavelength, relatively low absorption near 1550 nm, and low birefringence (as low as 0.0002 at 1550 nm has been observed).
  • In order to optimize degenerate nonresonant switching at 1.55 μm, silicon quantum dots with a diameter of around 4 nm are used, placing the 2-photon absorption peak at higher energy than the spectral energy range-of-interest. This is sufficient to minimize 2-photon absorption that may result in signal loss and heating, while maintaining a significant resonance enhancement at the wavelength of the trigger pulse. This particular combination of quantum dot material and size also yields a maximum in [0166] χ (3) at 1.55 μm. To maximize near-resonant switching, the appropriate choice of control wavelength and quantum dot resonance at the control wavelength desirably should be chosen that minimizes or reduces absorption loss at the data wavelength.
  • To facilitate incorporation of the quantum dots into PMMA, the silicon or germanium quantum dots can be coated with a ligand layer comprising a long-chained hydrocarbon with a methacrylate functional group at the end. Alternatively, any functional group compatible with PMMA can be used. Quantum dots and PMMA are dissolved in an organic solvent, such as toluene, and applied to the device as shown in FIG. 6([0167] c). The concentration of PMMA is determined based on the desired thickness of the final nanocomposite material and the method of application. In the case of spin-coating, a 5% PMMA solution is appropriate. The concentration of quantum dots is selected such that the final nanocomposite material, after deposition, has a linear index of refraction of 1.515. This is determined by calibrating the initial concentration of quantum dots (as measured by the absorption characteristics) to the final index of refraction of a PMMA-quantum dot film deposited in the method to be used. The linear index of the film can be measured using ellipsometry or the like.
  • After spin-coating the polymer-quantum dot solution over the device, the solvent is allowed to evaporate, leaving an engineered nonlinear nanocomposite coated device as shown in FIG. 6([0168] c). The index of refraction around all sides of the waveguide cores 602 and 604 is matched and optimized for the specific device. At the same time, χ (3) and the resonance conditions for 1.55 μm are independently tuned for optimum switching performance. As a final aspect of the current example, based on the known intensity of the trigger-pulse and the resulting nonlinear response of the engineered nonlinear nanocomposite material 608, the active length of the device is selected to provide optimal switching performance. This can be done by limiting the illumination area of the trigger-pulse to define the active area as in FIG. 6(d) or by designing the specific waveguide structure with the appropriate interaction length as in FIG. 6(e). The actual active length can be determined empirically or through simulation.
  • By increasing the index of refraction of waveguide cores, substantially larger concentrations of quantum dots can be incorporated into the active material while retaining functionality of the switch. This can yield substantially higher switching efficiency. For example, as shown in FIG. 6([0169] f), with silicon waveguide cores 610 and 612 having an index of refraction of ˜3.4, an active material 614 desirably should have an index of refraction equal to or less than 3.39 to achieve efficient waveguiding through the active region. This allows densities of quantum dots as high as those of close-packed quantum dot solids (either crystalline or amorphous).
  • EXAMPLE 2
  • To highlight the flexibility of embodiments of the current invention, this example describes a second preferred embodiment in which an engineered nonlinear nanocomposite material may be used in a waveguide nonlinear Mach-Zehnder (MZ) interferometer. In this case, as shown in FIGS. [0170] 7(a) through 7(f), a waveguide core is fabricated from partially oxidized silicon with an index of refraction of 2.4 at 1.55 μm. Once again, it will be apparent to one of ordinary skill in the art that partially oxidized silicon can have a range of indices of refraction, and that 2.4 is not meant to limit the scope of the invention. Variations on this example comprising other possible indices can be used depending on the specific application.
  • In the nonlinear MZI of the present example, a data signal traveling along a waveguide core is split into two separate and uncoupled waveguide arms with a defined phase relation between them. The signals travel along the arms for a predetermined length and are then recombined. Phase differences resulting from the propagation of the light in each arm result in constructive or destructive interference of the signals in the output waveguide core. By modulating the index of refraction of one or both of the arms, the output signal can be switched on or off by creating a relative 0- to π-phase shift between the signals. One of ordinary skill in the art will realize that further changes in index of refraction will result in cyclic exchange between the on and off states. An index change from a [0171] χ (3) based nonlinear material would yield extremely fast optical switching; however, so far no single material has been appropriate for a commercial switch based on this device.
  • As with the example above, the active material in this device desirably should have a high nonlinear response in the wavelength range-of-interest and no significant absorption. In this case, however, the nonlinear material is incorporated directly into the waveguide core. As such, the index of refraction of the engineered nonlinear nanocomposite desirably should be greater than that of the cladding material and be close to that of the core to avoid disruption of the optical mode as light moves into the active region. [0172]
  • In this example, as shown in FIG. 7([0173] a), the device comprises a partially oxidized waveguide core (n=2.4) fabricated on a silica substrate (n=1.45) and surrounded by a silica cladding on three sides. The top of the waveguide core is bounded by air (n=1). A section of one of the waveguide arms is etched away as shown in FIG. 7(b), filled with an engineered nonlinear nanocomposite material (n=2.4) as shown in FIG. 7(c) to match the boundary conditions of the waveguide core, and then polished as shown in FIG. 7(d). By illuminating the active region with trigger-pulses as shown in FIGS. 7(e) and 7(f), the index of refraction in one arm is changed, thus activating the switch. A preferred engineered nonlinear nanocomposite material for this example comprises silicon oxide coated silicon quantum dots formed into a close-packed quantum dot solid with index of refraction tuned to 2.4.
  • In order to optimize switching at 1.55 μm, silicon quantum dots with a diameter of 4 nm are used, placing the 2-photon absorption peak at higher energy than the spectral energy range-of-interest. This is sufficient to eliminate or reduce 2-photon absorption that may result in signal loss and potential heating of the device by the trigger-pulse. This particular combination of material and size also yields a maximum in [0174] χ (3) at 1.55 μm. To maximize near-resonant switching, the appropriate choice of control wavelength and quantum dot resonance at the control wavelength desirably should be chosen that minimizes or reduces any absorption loss at the data wavelength.
  • In order to achieve precise index of refraction control within the waveguide arm, surface ligands desirably should be selected to yield a specific particle-to-particle spacing within the final quantum dot solid. This can be achieved by measuring the index of refraction of many thin-films, formed by the method to be used, with quantum dots comprising different types of surface ligands. By using ellipsometry or the like, the index of refraction resulting from each type of surface ligand and deposition method can be determined and calibrated for determining the optimum conditions for the final device deposition. In the case of the present example, an index of 2.4 corresponds roughly to a packing density of 70% by volume. A short-chained hydrocarbon is preferable in this case, such as a butyl- or other alkyl group. [0175]
  • The quantum dots, in a solvent of hexane or toluene, are spin-coated over the surface of the device, filling the open region of the waveguide arm as shown in FIG. 7([0176] c). A slow spin speed is preferable, since the thickness of the material in the waveguide arm can be controlled by polishing the overflow off the surface (1000 rpm). The concentration of quantum dots in the solution should be high, preferably in the range of 1 nM to 1M, more preferably 10 μM to 1 mM.,
  • After spin-coating, the solvent is allowed to evaporate, creating a close-packed quantum dot solid filling the open region of the waveguide arm as shown in FIG. 7([0177] c). The surface is then polished to provide an optical-quality interface on the topside of the device in the active region as shown in FIG. 7(d). The index of refraction of the engineered nonlinear nanocomposite is matched to that of the waveguide core of the arm and optimized for the specific device. At the same time, χ (3) and the resonance conditions for 1.55 μm are independently tuned for optimum switching performance. As a final aspect of the current embodiment, based on the known intensity of the trigger-pulse and the resulting nonlinear response of the engineered nonlinear nanocomposite material, the active length is selected to provide optimal switching. This can be done by designing the etched length of the waveguide arm to the desired active length as in FIG. 7(e) or by limiting the illumination area of the trigger-pulse as in FIG. 7(f). The specific active length can be determined empirically or through simulation.
  • Alternatively, a nonlinear MZ interferometer can be fabricated without etching a portion of a waveguide core as shown in FIGS. [0178] 8(a) through 8(d). In this case, a engineered nanocomposite material can be simply cast on top of the entire device as shown in FIG. 8(b) with any excess removed as shown in FIG. 8(c), such that the active material is in evanescent contact with the signal passing through each of the arms (as well as elsewhere). By illuminating a portion of one or both arms, the active region can be defined as shown in FIG. 8(d). In this preferred embodiment, the engineered nonlinear nanocomposite desirably should be designed to have an index of refraction that is compatible with waveguiding in the partially oxidized silicon core (e.g., n<2.4). Again, this nanocomposite material is preferably a close-packed quantum dot solid.
  • Had further chemical processing steps been required in either of the above examples, it would also be possible to select the matrix material and/or surface ligands to impart stability of the engineered nonlinear nanocomposite under the required conditions. [0179]
  • The current embodiments not only provide a nonlinear material with a dramatically increased nonlinear response for use in these optical devices, they simultaneously provide materials that have been engineered to have optimum linear index of refraction, 2-photon absorption, near-resonance enhancement, and processability for each application. This level of independent control of optical, chemical, and mechanical properties does not exist in other materials. [0180]
  • Preferred Quantum Dot Materials [0181]
  • Preferred quantum dots according to some, embodiments of the present invention comprise substantially defect free quantum dots with a well-passivated surface. Preferred quantum dots also comprise a bandgap energy that is preferably greater than the photon energy range-of-interest (e.g., for the data beam), and more preferably greater than twice the photon energy range-of-interest (primarily for nonresonant nonlinearities) for its intended applications. While maintaining these requirements, the material and size of the quantum dots can be interchangeable. The specific material and size can be selected as necessary to engineer the optical characteristics for a particular application. The following provides certain preferred characteristics according to some embodiments of the invention: [0182]
  • Core-Shell Quantum Dots: [0183]
  • Core-shell quantum dots are particularly preferred because defects can result in traps for electrons or holes at the surface of a quantum dot core. These traps can degrade the electrical and optical properties of the quantum dot, yielding low-energy states within the bandgap of the material. An insulating layer at the surface of the quantum dot core provides a rise in the chemical potential at the interface, which can eliminate energy states that serve as traps. Surprisingly, these trap states can actually interfere with efficient switching or decrease the FOM of a material by contributing to single or multi-photon absorption. Additionally, shells act to physically protect the core material from chemical interactions such as oxidation, reduction, or dissolution. For instance, one embodiment of the present invention relates to the use of a shell to stabilize intrinsically unstable silicon or germanium quantum dots. Optionally, the shell can provide an appropriate chemical surface for covalent or non-covalent binding of molecules to the quantum dot, wherein the core material may or may not provide an appropriate surface for such binding. [0184]
  • Preferably, a quantum dot will be substantially defect free. By substantially defect free, it is typically meant that within the quantum dot there is fewer than 1 defect per quantum dot, preferably substantially fewer than 1 defect per quantum dot, more preferably less than 1 defect per 1000 quantum dots, more preferably less than 1 defect per 10[0185] 6 quantum dots, more preferably less than 1 defect per 109 quantum dots. Typically, a smaller number of defects within a quantum dot translates into an increased photoluminescence quantum efficiency. For certain embodiments of the invention, a quantum dot that is substantially defect free will typically exhibit photoluminescence with a quantum efficiency that is greater than 6 percent, preferably greater than 10 percent, more preferably at least 20 percent, more preferably at least 30 percent, more preferably at least 40 percent, and more preferably at least 50 percent.
  • Preferably, the core will be substantially crystalline and be substantially defect-free. By substantially defect free, it is typically meant that within the core there is fewer than 1 defect per quantum dot, preferably substantially fewer than 1 defect per quantum dot, more preferably less than 1 defect per 1000 quantum dots, more preferably less than 1 defect per 10[0186] 6 quantum dots, more preferably less than 1 defect per 109 quantum dots.
  • In a similar manner, the shell and/or the interface region preferably will be substantially defect free, where it is typically meant that within the shell and/or the interface region there is fewer than 1 defect per quantum dot, preferably substantially fewer than 1 defect per quantum dot, more preferably less than 1 defect per 1000 quantum dots, more preferably less than 1 defect per 10[0187] 6 quantum dots, more preferably less than 1 defect per 109 quantum dots.
  • Size and Size-Distribution: [0188]
  • Another preferred characteristic of the quantum dots of some embodiments of the present invention is such that a figure-of-merit (FOM) for all-optical switching or processing can be largely insensitive to size dispersion, contrary to results and predictions in the literature. FIG. 9 illustrates a figure-of-merit (FOM) for all-optical switching with an engineered nonlinear nanocomposite material as a function of quantum dot size, according to an embodiment of the invention. Here, the nanocomposite material includes germanium quantum dots made with methods described herein. The FOM in this case is defined as 2γ/βλ, which is applicable for nonresonant nonlinearities. The criteria for effective all-optical switching is FOM>1. FIG. 9 shows how the FOM for all-optical switching depends on the size of the quantum dots. It can be seen that the FOM exceeds 1 for a large size dispersion, e.g., for diameters ranging from 3 nm to 6 nm. Similar results can be obtained with the other quantum dots described herein, e.g., silicon quantum dots. Therefore, some embodiments of the present invention avoid the need for a substantially monodispersed size distribution of quantum dots while substantially improving switching characteristics and efficiency over previous uses of quantum dots as nonlinear materials. The effects of size distribution and specifically how the FOM of switching depends on the quantum dot size has not been previously considered in detail. [0189]
  • Shape and Shape Distribution [0190]
  • Quantum dots can be fabricated in a variety of shapes, including (but not limited to) spheroids, rods, pyramids, cubes, and other geometric and non-geometric shapes. For shapes that are not spherically symmetric, a distribution of orientations can result in an effective broadening of the size distribution as seen by incident light. To avoid the need for orientation of quantum dots within a matrix material, the preferred quantum dot shape is spherical, according to some embodiments of the invention. Spherical quantum dots are also preferred for nanocomposites comprising oriented quantum dots. Alternatively, another preferred embodiment comprises spheroid or substantially spherical quantum dots, with an aspect ratio restricted to between 1±(% size distribution) or with an aspect ratio between approximately 0.8 and 1.2. In this case, orientation plays an insignificant role in the inhomogeneous broadening of the spectral features. For similar reasons, the preferred quantum dot will also be substantially monodisperse in shape. These considerations regarding the importance of shape and/or shape-distribution constitute an improvement in the use of quantum dots as a nonlinear material. [0191]
  • It should be recognized that an arbitrary shape may still be preferred as long, as the relative orientation dependence of the broadening of the linear and nonlinear optical properties is less than the broadening resulting from the size distribution of the quantum dot sample. [0192]
  • Crystal Structure of the Core [0193]
  • For reasons similar to those described above for shape, preferred quantum dots according to some embodiments of the invention will have a core with a crystal structure that is spherically symmetric, more preferably a cubic or diamond crystal structure. Alternatively, the crystal structure may be non-spherically symmetric, preferably cylindrically symmetric, more preferably a wurtzite crystal structure. [0194]
  • It should be recognized that an arbitrary crystal structure may still be preferred as long as the relative orientation dependence of the broadening of the linear and nonlinear optical properties is less than the broadening resulting from the size distribution of the quantum dot sample.