Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
  • G02B6/107—Subwavelength-diameter waveguides, e.g. nanowires
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
  • H01S5/041—Optical pumping
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/30—Structure or shape of the active region; Materials used for the active region
  • H01S5/32—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures
  • H01S5/323—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
  • H01S5/32308—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser emitting light at a wavelength less than 900 nm
  • H01S5/32341—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser emitting light at a wavelength less than 900 nm blue laser based on GaN or GaP
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/30—Structure or shape of the active region; Materials used for the active region
  • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
  • H01S5/3401—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers having no PN junction, e.g. unipolar lasers, intersubband lasers, quantum cascade lasers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/30—Structure or shape of the active region; Materials used for the active region
  • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
  • H01S5/341—Structures having reduced dimensionality, e.g. quantum wires
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/30—Structure or shape of the active region; Materials used for the active region
  • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
  • H01S5/347—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIBVI compounds, e.g. ZnCdSe- laser
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/1042—Optical microcavities, e.g. cavity dimensions comparable to the wavelength

Definitions

• This invention pertains generally to optical waveguides, and more particularly to nanoribbons and nanowires employed as subwavelength optical waveguides as well as optical probes, sensors, routers and other devices based on nanoribbon/wire optical waveguides.
• nanowires represent a unique class of building blocks for the construction of nanoscale electronic and optoelectronic devices. Since nanowire synthesis and device assembly are typically separate processes, nanowires permit more flexibility in the heterogeneous integration of different materials than standard silicon technology allows, although the assembly itself remains a major challenge.
• the toolbox of nanowire device elements is growing and currently includes various types of transistors, light emitting diodes, lasers, and photodetectors. While the electrical integration of simple nanowire circuits using lithography has been demonstrated, optical integration, which promises higher speeds and greater device versatility, remains unexplored.
• Photonics, the optical analogue of electronics shares the logic of miniaturization that drives research in semiconductor and communications technology.
• the ability to manipulate pulses of light within sub-micron spaces is vital for highly integrated light-based devices, such as optical computers, to be realized.
• Recent advances in using photonic bandgap and plasmonic phenomena to control the flow of light are impressive in this regard.
• both of these approaches typically rely on difficult and costly lithographic processes for device fabrication and are in early stages of understanding and development.
• BRIEF SUMMARY OF THE INVENTION A potentially simpler and equally versatile concept is to assemble photonic circuits from a collection of nanoribbon/nanowire elements that assume different functions, such as light creation, routing and detection.
• the present invention generally comprises a subwavelength optical waveguide formed from a nanoribbon or nanowire having a diameter that is less than the wavelength of light to be guided.
• Such a subwavelength waveguide can serve a fundamental element of photonic circuits of various types.
• Chemically synthesized nanoribbons and nanowires have several features that make them good building blocks, including inherent one- dimensionality, a variety of optical and electrical properties, good size control, low surface roughness and, in principle, the ability to operate both above and below the diffraction limit.
• An important step toward integrated nanoribbon/wire photonics is to develop a nanoribbon/wire waveguide that can couple pairs of nanoribbon/wire elements and provide the flexibility in interconnection patterns that is needed to carry out complex tasks, such as logic operations.
• one aspect of the invention is the assembly of photonic circuit elements from nanostructures such as SnO 2 nanoribbon and ZnO nanowire waveguides.
• high aspect ratio e.g., >1000
• nanoribbons/nanowires with diameters below the wavelength of light typically 100 nm to 400 nm
• PL internally generated photoluminescence
• nonresonant UV/visible light emitted from adjacent, evanescently coupled, nanoribbons/wires or external laser diodes
• the length, flexibility and strength of these single-crystalline structures enable them to be manipulated and positioned on surfaces to create various single-ribbon shapes and multi- ribbon optical networks, including ring-shaped directional couplers and nanoribbon/wire emitter-waveguide-detector junctions.
• Another aspect of the invention is that the ability to manipulate the shape of active and passive nanoribbon/wire cavities provides a new tool for investigating the cavity dynamics of subwavelength structures. Moreover, future advances in assembling the diverse set of existing nanowire building blocks could lead to a novel and versatile photonic circuitry.
• Another aspect of the invention is that nanoribbons/nanowires push subwavelength optical fibers beyond silica.
• the scores of materials that can be made in nanoribbon/wire form include active, passive, nonlinear and semiconducting inorganic crystals, as well as a wide variety of polymers. Simultaneous photon, charge carrier and spin manipulation is possible within and between nanowires of different compositions. Also, many of these materials have higher refractive indices than silica-based glasses, permitting light of a given wavelength to be confined within thinner structures for denser integration.
• Another aspect of the invention is waveguiding in liquids using subwavelength nanoribbon/wire optical waveguides.
• nanoribbons/wires are freestanding, mechanically flexible elements that can be manipulated on surfaces or used as mobile probes in fluids. As such, they offer a type of versatility difficult to achieve with lithographically-defined structures that are permanently affixed to their substrates.
• Another aspect of the invention is a nanoribbon/wire optical waveguide having a high aspect ratio and a diameter less than the wavelength of light to be guided.
• the aspect ratio is greater than approximately 1000.
• the diameter is in the range of approximately 100 nm to approximately 400 nm.
• nanoribbon/wire comprises SnO 2 .
• the nanoribbon/wire comprises ZnO.
• the nanoribbon/wire comprises GaN.
• Another aspect of the invention is to provide a nanoribbon/wire laser and a nanoribbon/wire photodetector coupled by a nanoribbon/wire optical channel.
• Another aspect of the invention is an optical waveguide comprising a nanoribbon/wire dispersed on an SiO 2 or mica substrate.
• a further aspect of the invention is an apparatus for guiding light through liquid media, comprising a nanoribbon or nanowire waveguide.
• the nanoribbon waveguide comprises a SnO 2 nanoribbon waveguide.
• the nanowire waveguide comprises a ZnO nanowire waveguide.
• the waveguides comprise high dielectric waveguides.
• the nanowire waveguide comprises a GaN nanowire waveguide.
• a further aspect of the invention is an optical router comprising at least two coupled nanoribbon waveguides.
• the nanoribbon waveguides comprise SnO 2 nanoribbon waveguides.
• Another aspect of the invention is an optical router comprising at least two coupled nanowire waveguides.
• the nanowire waveguides comprise ZnO nanowire waveguides.
• Still another aspect of the invention is an optical router comprising a network of nanoribbon waveguides configured to separate white light and route individual colors based on a short-pass filtering effect.
• the nanoribbon waveguides comprise SnO 2 nanoribbon waveguides.
• Another aspect of the invention is an optical crossbar grid comprising two pairs of orthogonal nanoribbon waveguides configured to conduct light through abrupt 90° angles.
• the nanoribbon waveguides comprise Sn ⁇ 2 nanoribbon waveguides.
• FIG. 1A-C illustrate optical waveguiding in a 715 ⁇ m long Sn ⁇ 2 nanoribbon.
• FIG. 2A-F illustrate panchromatic waveguiding in a 425 ⁇ m long nanoribbon.
• FIG. 3A-G illustrate shape manipulation of nanoribbon waveguides.
• FIG. 4A-H illustrate an ⁇ 600 ⁇ m long nanoribbon slightly suspended above a substrate that undergoes physical manipulation by an etched tungsten probe.
• FIG. 5A-F illustrate dark-field images taken before and after manipulating a nanoribbon's cavity shape.
• FIG. 6A-C illustrate nanoribbon coupling, optical components and devices. [0036] FIG.
• FIG. 7A-C show optical coupling between a ZnO nanowire and a SnO 2 nanoribbon waveguide.
• FIG. 8A-B show a hetero-junction created between a single ZnO nanowire and a SnO 2 nanoribbon.
• FIG. 9A-C show a Sn ⁇ 2/SnO 2 junction created by coupling two nanoribbon waveguides at their end facets.
• FIG. 10A-B illustrates nanoribbon short-pass filters.
• FIG. 11 A-C illustrate waveguiding in water.
• FIG. 12A-B show dark field images of waveguiding in water.
• FIG. 13A-D shows fluorescence and absorbance detection of R6G with a nanoribbon cavity.
• FIG. 14A-C illustrate the concept of SERS sensing with subwavelength waveguides.
• FIG. 15 shows PL/dark-field image of two nanoribbons (NR1 and NR2) evanescently coupled at arrow 1.
• FIG. 16A-C illustrate the integration of waveguides into a fluidic device.
• FIG. 17A-F illustrate the routing of GaN PL and lasing emission.
• FIG. 18A-B illustrate multi-laser waveguiding.
• FIG. 19A-B illustrate GaN nanowire lasing.
• FIG. 20A-E show color filtering in a nanoribbon network.
• FIG. 21 is a typical PL spectrum of a SnO 2 nanoribbon, showing its two defect bands.
• FIG. 22A-B illustrate optical routing in a rectangular nanoribbon grid.
• DETAILED DESCRIPTION OF THE INVENTION [0052] Nanoscale ribbon-shaped crystals of binary oxides exhibit a range of interesting properties including extreme mechanical flexibility, surface- mediated electrical conductivity, and lasing. However, as part of a recent study of the photoluminescence (PL) of SnO 2 nanoribbons in our laboratory, we discovered that nanoribbons with high aspect ratios (>1000) act as excellent waveguides of their visible PL emission.
• PL photoluminescence
• SnO 2 is a wide-bandgap (3.6 eV) semiconductor characterized by PL bands at 2.5 eV (green) and 2.1 eV (orange), and finds application in gas sensors and transparent electrodes.
• the structures synthesized possessed fairly uniform (+/- 10%) rectangular cross-sections with dimensions as large as 2 ⁇ m x 1 ⁇ m and as small as 15 nm x 5 nm.
• Many of the nanoribbons we synthesized were 100 nm to 400 nm wide and thick, which we found to be an optimal size range for efficient steering of visible and ultraviolet light in a subwavelength cavity.
• photonic circuit elements can be assembled from, for example, SnO 2 nanoribbon and ZnO nanowire waveguides.
• High aspect ratio nanoribbons/wires with diameters below the wavelength of light typically 100 nm to 400 nm
• PL internally generated photoluminescence
• nonresonant UV/visible light emitted from adjacent, evanescently coupled, nanowires or external laser diodes typically 100 nm to 400 nm
• the length, flexibility and strength of these single-crystalline structures enable them to be manipulated and positioned on surfaces to create various single-ribbon shapes and multi-ribbon optical networks, including ring-shaped directional couplers and nanowire emitter-waveguide- detector junctions.
• nanoribbons/wires as optical waveguides is based on the nanoribbons/wires having diameters which are smaller than the wavelength of light.
• nanoribbons/wires may not have circular cross-sections.
• ZnO nanowires typically have a hexagonal cross-section and Sn ⁇ 2 nanoribbons typically have a rectangular cross- section.
• the term “diameter” is intended generally to refer to the effective diameter, as defined by the average of the major and minor axis of the cross-section of the structure.
• the term “diameter” is not limited to the foregoing definition and is also intended to encompass dimensions of a nanoribbon/wire which allow for the nanoribbon/wire to function as a subwavelength waveguide.
• FIG. 1 and FIG. 2 illustrate representative data collected from single nanoribbons with lengths of 715 and 425 ⁇ m, respectively.
• FIG. 1 illustrates optical waveguiding in a 715 ⁇ m long SnO 2 nanoribbon that we synthesized.
• FIG. 1 A is a dark-field image of a (350 nm wide by 245 nm thick) meandering nanoribbon 10 and its surroundings. The scale bar shown is 50 ⁇ m.
• FIG. 1 B is the PL image of the nanoribbon under laser excitation.
• FIG. 1 C shows the spectra of the emission from the bottom terminus of the waveguide, collected at room temperature and at 5 K.
• a higher resolution emission profile shows fine structure in three of the central peaks. This fine structure was found to be present in every peak.
• FIG. 2 illustrates panchromatic waveguiding in a 425 ⁇ m long nanoribbon.
• FIG. 2A is a dark-field image of the nanoribbon 12, which has cross-sectional dimensions of 520 nm x 275 nm.
• the scale bar is 50 ⁇ m.
• FIG. 2B is a PL image with the UV excitation spot centered near the middle of the nanoribbon, showing waveguided emission from both ends.
• FIG. 2C is a magnified dark-field PL view of the right end of the nanoribbon, with the laser focused on the left end. A wide ( ⁇ 1 ⁇ m) nanoribbon 14 lies across the nanoribbon of interest.
• the inset in FIG. 2C is a scanning electron micrograph of the right terminus of the nanoribbon, showing its rectangular cross-section. The scale bar is 500 nm.
• 2F are digital images of the guided emission 16a, 16b, 16c, respectively, at the output end of the nanoribbon during nonresonant excitation of the input end of the nanoribbon with monochromatic light of wavelengths 652 nm (red), 532 nm (green) and 442 nm (blue) light, respectively.
• the leftmost emission spots 18a, 18b, 18c in FIG. 2D, FIG. 2E and FIG. 2F, respectively, were caused by scattering at the nanoribbon-nanoribbon junction and were quenched by selectively removing the wide nanoribbon 14 with a micromanipulator.
• red waveguiding was rare, green waveguiding was common, and blue waveguiding was ubiquitous.
• critical diameter below which all higher order optical modes are cut off and waveguiding becomes increasingly difficult to sustain. More specifically, by treating a nanoribbon waveguide as a cylinder of SnO 2 embedded in air, we found cutoff diameters for higher order transverse modes of about 270 nm, 220 nm and 180 nm for the 652 nm, 532 nm and 442 nm light used in our experiment, respectively.
• nanoribbons we also found the nanoribbons to be of sufficient length and strength to be pushed, bent and shaped using a commercial micromanipulator under an optical microscope.
• the large aspect ratio and elastic flexibility of Sn ⁇ 2 nanoribbons allowed us to manipulate the location and shape of individual nanoribbons under the optical microscope using a commercial micromanipulator tipped with sharp tungsten probes.
• Waveguiding nanoribbons with one end dangling in air could be elastically bent to large angles (e.g., up to about 180°) without kinking or fracturing, which is remarkable for an oxide that is brittle in its bulk form.
• FIG. 3 through FIG. 5 illustrate experimental results of our shape manipulation of nanoribbon waveguides.
• these crystalline nanoribbon waveguides are to be useful as interconnects in optical circuits, they need to be capable of coupling light from one nano-object to another and to be facilely transportable from one location to another. To realize the latter, we attempted to bend and move the nanoribbons using the micromanipulator.
• FIG. 3A is an SEM image of a simple shape 20, demonstrating the high level of positional control afforded by the micromanipulator. This shape was created from a single straight nanoribbon of dimensions 400 nm x 115 nm that was cut into two pieces and then assembled.
• FIG. 3B and FIG. 3C are optical images of the emission end of a long nanoribbon (aspect ratio ⁇ 5200), showing the minimal effect of curvature on waveguiding.
• FIG. 3B is a black and white rendering of a true color photograph taken after crafting a single bend.
• FIG. 3C is a black-and-white dark-field/PL image captured after an S- turn was completed. We found that blue light could be guided around 1 ⁇ m radii curves with low loss.
• FIG. 3D through FIG. 3F are a series of dark-field images and FIG. 3G is the corresponding guided PL spectra for a single nanoribbon 22 bent into different shapes. Collection was at the right end of the nanoribbon in each case. An unguided PL spectrum of the nanoribbon is included for reference. Spectra are normalized and offset for clarity.
• FIG. 4 shows an approximately 600 ⁇ m long nanoribbon 24 slightly suspended above the substrate, which undergoes physical manipulation by an etched tungsten probe.
• FIG. 4A, FIG. 4C, FIG. 4D and FIG. 4F are dark-field images during the bending process, from no bend (FIG. 4A) to a > 90° angle (FIG. 4F), illustrating the extreme flexibility of the nanoribbons.
• FIG. 4B, FIG. 4E and FIG. 4G are PL images taken at different bend angles.
• FIG. 4H illustrates spectra taken at the bottom terminus as a function of arbitrary bend angle.
• the curves identified as Bend 1 , 2, and 3 in FIG. 4H correspond to the images in FIG. 4C, FIG. 4D and FIG. 4F, respectively.
• the mode structure was found to be significantly dependent on the size and shape of the cavity.
• the dark field images (FIG. 4A, FIG. 4C, FIG. 4D and FIG. 4F) were taken during the process of bending a nanoribbon that was slightly suspended above the substrate. This was the first direct indication of the degree of flexibility of these oxide nanostructures.
• FIG. 4B, FIG. 4E, and FIG. 4G provide additional information on the waveguiding behavior of the cavity as the nanoribbon is bent to angles > 90°.
• spectra were taken from the waveguided terminus of the nanoribbon.
• FIG. 4H shows the resulting emission profiles as a function of arbitrary bend angle. It is apparent that the mode structure emerges as the semi-linear nanoribbon begins to take physical shape, and leads to the possibility of using these nanoribbons as high quality (Q) factor cavities. To further pursue and explore the limitations of physically perturbing these nanoribbons, we focused on thinner nanoribbons that still exhibited outstanding waveguiding properties.
• FIG. 5 clearly demonstrates the potential of these structures in nano- photonic circuits.
• FIG. 5A and FIG. 5C are dark-field images taken before (FIG. 5A) and after (FIG. 5C) manipulating the cavity shape of a nanoribbon 26. The flexibility of the nanoribbon allows it to maintain its shape integrity even after the tungsten probe is removed.
• FIG. 5B and FIG. 5D are PL images of the shapes in FIG. 5A and FIG. 5C, respectively. Even with two sharp bends, the nanoribbon successfully guided the defect emission from the left coupling end to the right terminus with minimal loss occurring at the bend apexes.
• FIG. 5E and FIG. 5F are dark-field/PL (FIG. 5E) and PL (FIG.
• Tin oxide however, can achieve a higher internal confinement due to its higher index of refraction, nearly double that of silica (2.3 to 1.4), and its unequivocal property of minimizing loss at like-refractive index interfaces.
• nanoribbon waveguides can be coupled together to create optical networks that may form the basis of miniaturized photonic circuitry.
• the approximate size of a nanoribbon can be inferred from the color of its guided PL; namely, large nanoribbons are white, while small nanoribbons are blue.
• a nanoribbon of average size is pumped nearer to one end, it shines blue at the far end and green at the near end, demonstrating the higher radiation losses for longer wavelengths.
• this effect makes nanoribbons excellent short-pass filters with tunable cutoffs based on path length.
• nanoribbon filters spanning the 465 nm to 580 nm region that feature steep cutoff edges and virtually zero transmission of blocked wavelengths.
• nanoribbons Since light diffracts in all directions when it emerges from a subwavelength aperture, nanoribbons must be in close proximity, and preferably in direct physical contact, to enable the efficient transfer of light between them.
• Staggered nanoribbons separated by a thin air gap can communicate via tunneling of evanescent waves. It is also possible to bond two nanoribbons together by van der Waals forces, often simply by draping one over another, to create a robust optical junction.
• FIG. 6 is illustrative of nanoribbon coupling, optical components and devices.
• FIG. 6A is a black-and-white dark-field/PL image of two coupled nanoribbons 28, 30 (both nanoribbons are 750 nm x 250 nm, 630 ⁇ m total length). Light is incident on the right terminus of the right nanoribbon 30 and collected at the left terminus of the left nanoribbon 28. The arrow denotes the location of the junction. The SEM image in the inset of FIG. 6A resolves the junction layout.
• FIG. 6B illustrates raw emission spectra of the left nanoribbon 28 before (upper curve) and after (lower curve) forming the junction.
• FIG. 6C is a black and white rendering of a true color PL image of a three-ribbon ring structure that functions as a directional coupler.
• the ring nanoribbon 32 (135 ⁇ m x 540 nm x 175 nm) is flanked by two linear nanoribbons 34, 36 (34 at left, 120 ⁇ m x 540 nm x 250 nm; 36 at right, 275 ⁇ m x 420 nm x 235 nm).
• Light input at branch 1 exits preferentially at branch 3 (as shown), while light input at branch 2 exits branch 4.
• FIG. 6A and FIG. 6B illustrate an example of two-ribbon coupling.
• more functional geometries such as Y-junctions, branch networks, Mach-Zehnder interferometers and ring oscillators can also be constructed.
• the three-ribbon ring structure illustrated in FIG. 6C operates by circulating light that is injected from one branch around a central cavity, which can be tapped by one or more output channels to act as an optical hub.
• optical modulators based on nanoribbon assemblies that utilize the electro-optic effect for phase shifting.
• Single-crystalline nanoribbons are intriguing structures with which to manipulate light, both for fundamental studies and photonics applications.
• FIG. 7 illustrates successful optical coupling between a ZnO nanowire 38 and a Sn ⁇ 2 nanoribbon waveguide 40.
• FIG. 7A is a black and white rendering of a true color dark-field/PL image of the nanowire 38 (56 ⁇ m long, at top, pumped at 3.8 eV) channeling light into the nanoribbon 40 (265 ⁇ m long, at bottom). The arrow denotes the location of the junction.
• FIG. 7B is an SEM image of the nanowire/nanoribbon junction.
• FIG. 7C illustrates spectra of the coupled structures taken at different excitation and collection locations.
• FIG. 8 illustrates another example of a hetero-junction created between a single ZnO nanowire and a SnO 2 nanoribbon.
• FIG. 8A is a dark-field image of the junction after pushing a ZnO nanowire up to the end facet of the Sn ⁇ 2 nanoribbon. The inset in FIG.
• FIG. 8A is a magnification of the active coupling region showing the short ( ⁇ 6-7 ⁇ m) ZnO nanowire and the upper terminus of the Sn ⁇ 2 nanoribbon. The total length of the nanoribbon was - 600 ⁇ m.
• FIG. 8B shows spectra collected at the passive end (bottom terminus) while pumping either the ZnO nanowire (On ZnO) or the Sn ⁇ 2 nanoribbon directly (On NR). A profile of the band gap emission collected over the ZnO nanowire (ZnO Only) is included for reference.
• the Modulation in the "On ZnO" spectrum is a direct result of the broad emission from the ZnO propagating through a high Q-factor SnO 2 cavity.
• FIG. 8A The 50x dark-field image and 100x dark-field inset of FIG. 8A pictorially demonstrate the basic components of an active/passive nanophotonic device. However, to ensure that we had devised a complete junction between the two nanosystems, we optically pumped the ZnO nanowire active end and collected at the passive Sn ⁇ 2 nanoribbon end. As seen in FIG. 8B, ZnO band gap emission created from the pump source was directed across the intervening air space by the ZnO cavity and into the neighboring Sn ⁇ 2 waveguide. The light output from the ZnO nanowire emerged at the distant end of the nanoribbon and clearly showed a modulated emission profile similar to the PL line shape seen in FIG. 4.
• FIG. 9 illustrates a Sn ⁇ 2 /SnO 2 junction created by coupling two nanoribbon waveguides 42, 44 at their end facets.
• FIG. 9A and FIG. 9B are dark-field images before (FIG. 9A) and after (FIG.
• FIG. 9B is a PL image of the same nanoribbon junction and end terminus shown in FIG. 9B demonstrating that multi-junction networks between SnO 2 nanoribbon waveguides can be realized.
• the dark-field images in FIG. 9A and FIG. 9B capture the junction before and after successfully adjoining the two nanoribbons.
• the PL image in FIG. 9C verifies that light traveling down the small nanoribbon can be directly coupled into a secondary like-cavity.
• the oxide waveguides serve as important interconnects between active light sources, such as LEDs and lasers, and optical detectors based on photoconducting nanowires.
• the optical loss of several nanoribbon waveguides was measured by systematically varying the distance between UV excitation (50 ⁇ m spot size) and PL collection in the near-field. We estimate a loss of about 2 dB mm "1 at a wavelength of 550 nm for a nanoribbon with a 400 x 150 nm 2 cross-section, which is significantly greater than losses reported recently for subwavelength silica waveguides.
• nanoribbon waveguides are excellent materials with which to study the interplay between mechanics, microstructure and optical confinement in nanoscale cavities. They can be manipulated and assembled to serve as photonic interconnects between single nano-objects, such as nanowire lasers, in optical circuits and devices.
• nanoribbon waveguides can be used as filter devices.
• FIG. 10 illustrates the use of nanoribbons as short-pass filters.
• FIG. 10A shows room temperature PL spectra of five different nanoribbons, each 200 ⁇ m to 400 ⁇ m long, with 50% intensity cut-off wavelengths ranging from 465 nm to 580 nm.
• Cross-sectional dimensions of the 465 nm, 492 nm, 514 nm, 527 nm and 580 nm filters were 310 nm x 100 nm (0.031 ⁇ m 2 ), 280 nm x 120 nm (0.037 ⁇ m 2 ), 350 nm x 115 nm (0.04 ⁇ m 2 ), 250 nm x 225 nm (0.056 ⁇ m ), and 375 nm x 140 nm (0.053 ⁇ m ), respectively.
• the spectra were normalized and offset for clarity.
• Example 1 SnO 2 nanoribbon waveguides were synthesized by the chemical vapor transport of SnO powder in a quartz tube reactor operating at 1100 °C and 350 Torr of flowing argon (50 seem). Milligram quantities of nanoribbons were collected on an alumina boat near the center of the reactor and deposited onto clean substrates by dry transfer.
• Optical measurements were carried out using a dark-field microscope outfitted with a cryostat (Janis X-100).
• the PL excitation source was a HeCd laser operating at 325 nm.
• Laser pointers (532 and 652 nm) and the HeCd laser (442 nm) provided nonresonant illumination.
• the size of the laser spot was -50 ⁇ m for all measurements.
• Spectra were collected with a fiber- coupled spectrometer (SpectraPro 300i, Roper Scientific) and liquid N2 cooled CCD detector. Images were captured using both a microscope-mounted camera (C00ISNAP, Roper Scientific) and a handheld digital camera (PRD- T20, Toshiba).
• photonic circuit elements can be assembled from SnO 2 nanoribbon and ZnO nanowire waveguides.
• High aspect ratio nanoribbons/wires with diameters below the wavelength of light typically 100 nm to 400 nm were shown to act as excellent waveguides of both their own internally generated photoluminescence (PL) and nonresonant UV/visible light emitted from adjacent, evanescently coupled, nanowires or external laser diodes.
• PL photoluminescence
• UV/visible light emitted from adjacent, evanescently coupled, nanowires or external laser diodes.
• Waveguiding in Liquids [0095] Waveguiding in Liquids [0096] Quite surprisingly, we have also found that these one-dimensional (1 D) nanostructures can guide light through liquid media. The fact that light can be delivered through these cavities in solution offers a unique application for high dielectric (n > 2) waveguides in fluidic sensing and probing. Waveguiding in liquids is especially important for integrated on-chip chemical analysis and biological spectroscopy in which small excitation and detection volumes are required. Subwavelength nanostructures can be assembled to probe molecules in a fluorescence or absorption scheme, both of which utilize the decaying light field outside of the cavity to induce photon absorption. The waveguide is strongly coupled to emitted photons near the cavity, allowing the generated fluorescence to be directed back to the point of injection.
• nanoscale dimensions of the waveguides afford small liquid volumes ( ⁇ picoliters) to be sensed and presage the way for miniaturized optical spectrometers.
• pulsed light must be transmissible if nanowire photonic devices are to be useful in communications or computing.
• Simple networks of SnO 2 nanoribbons are then used to separate white light and route individual colors based on a short-pass filtering effect.
• the PL is generated with a CW HeCd laser (325 nm).
• FIG. 11 also shows how the guided PL spectrum of this thin nanoribbon changes when it is immersed in water.
• FIG. 11 A is a combined PL/dark-field image of the nanoribbon 46 on a dry oxide surface. The inset shows a magnified view of the blue end emission.
• FIG. 11 A is a combined PL/dark-field image of the nanoribbon 46 on a dry oxide surface. The inset shows a magnified view of the blue end emission.
• FIG. 11 B shows the same nanoribbon in a water environment, under a quartz coverslip. The inset shows resultant green emission.
• FIG. 11C shows the spectra of the two situations.
• the large red shift of the empirical cutoff wavelength (from 483 nm in air to ⁇ 570 nm in water) is caused by the decrease in refractive index profile between the substrate and the cap medium.
• the more homogeneous cladding index improves wave confinement in the nanoribbon core. The effect was reversible by evaporating the water.
• n W aveguide n SU btrate > n CO ver when n W aveguide n SU btrate > n CO ver), as it does here, raising the index of the cover reduces its asymmetry with the substrate and improves confinement.
• FIG. 12A shows a dark-field image of various sized droplets of 1 ,5-pentanediol on a silicon substrate (with a 1 ⁇ m thermal oxide). The radii and corresponding volumes are displayed by each droplet.
• FIG. 12B is a magnified dark-field image of smaller droplets ( ⁇ 1 fL).
• Ribbon waveguides can also sense molecules, proteins or larger biological entities in solution by means of either an emission or absorption mechanism as mentioned above. In the former, a nanoribbon provides local excitation for fluorophores passing through the cone of scattered light at its output end, and the emission is collected by a fiber or microscope. [00104] Referring to FIG.
• FIG. 13 shows fluorescence and absorbance detection of R6G with a nanoribbon cavity.
• FIG. 13A is a fluorescence image of a droplet of 1 mM R6G in 1 ,5- pentanediol excited by blue light from a nanoribbon waveguide 48 (240 nm by 260 nm by 540 ⁇ m). The nanoribbon crosses the frame from upper left to lower right. A notch filter was used to block the excitation light.
• FIG. 13A The left inset of FIG. 13A is a dark-field image showing the droplet and the bottom half of the nanoribbon.
• the right inset of FIG. 13A is a magnified view of the droplet emission, showing the light cone and evanescent pumping of the dye along the nanoribbon length.
• FIG 13B shows the spectra taken of the droplet region (direct) and the fluorescence coupled back into the nanoribbon (guided). The red shift of the guided emission is a microcavity effect.
• FIG. 13C is a dark-field image of the nanoribbon with a droplet deposited near its middle (absorbance geometry). The nanoribbon was UV pumped on one side of the droplet and probed on the other side, as indicated.
• FIG. 13D shows the spectra of the guided PL without liquid present and with droplets of pure 1 ,5- pentanediol and 1 mM R6G.
• the arrow indicates the absorption maximum of R6G.
• blue light (442 nm) launched into the far end of the nanoribbon resulted in strong fluorescence from within the droplet, where the R6G emission mapped out the spatial intensity distribution of the waveguide output as a cone of light (FIG. 13A and Inset). A fraction of this fluorescence was captured by the nanoribbon cavity and guided back to its far end, demonstrating that these waveguides are capable of routing signals both from and to liquids. Spectra acquired from both ends of the nanoribbon are shown in FIG.
• FIG. 13B shows strong fluorescence originating from the segment of the nanoribbon wet by the droplet through capillary action.
• dye molecules in proximity to the nanoribbon surface are excited in a subwavelength version of total internal reflection fluorescence (TIRF).
• TIRF total internal reflection fluorescence
• excitation of a macroscopic waveguide (such as a microscope coverslip) generates an evanescent field of light that decays exponentially with distance from the waveguide surface, limiting the depth of excitation to a distance of ⁇ 100 nm and enabling the local probing of structures such as cell membranes.
• subwavelength fibers can carry a larger fraction of their modal power outside of the core, they enhance the intensity of this evanescent field and increase its penetration depth into the surroundings, making proportionally more power available to excite nearby molecules. Calculations indicate that roughly thirteen to fifteen per cent of the electric field intensity exists outside of the nanoribbon for the wavelength of light used in this experiment.
• the radial field intensity decays to ten per cent of its maximum value at the center of the waveguide by about 135 nm into the liquid solution. Since TIRF detection sensitivity scales with the fractional power present in the waveguide cladding, one-dimensional nanostructures are promising waveguides for local fluorescence sensing using this approach. [00107] Another way that 1 D nanostructures may be used for optical detection in solution relies on producing an absorption spectrum of molecules located on and near the nanoribbon surface. Absorbance detection, while inherently less sensitive than fluorescence methods, is applicable to a wider range of molecules and avoids the complications of fluorescent tagging.
• subwavelength 1 D nanostructures into microfluidic devices and to apply them as flexible probes in the study of live cells.
• a third way that subwavelength nanoribbons/wires can be used for chemical/biological sensing relies on the surface enhanced Raman spectroscopic (SERS) effect.
• SERS surface enhanced Raman spectroscopic
• the nanoribbons/wires described here were fashioned into subwavelength SERS fibers by decorating their surfaces with a high density of silver nanoparticles. By exposing the nanoparticles-coated nanoribbon/wire to an analyte solution while injecting monochromatic light down the nanoribbon/wire, it is possible to detect the SERS signal of the analyte molecule.
• FIG. 14A shows a schematic picture of this concept
• FIG. 14B and FIG. 14C show an image of a nanoribbon (NR) coated with 40 nm silver nanoparticles attached by exposing the nanoribbon to a flowing nanoparticle solution.
• the particles are seen to scatter the waveguided light very effectively.
• an analyte solution of interest it is possible to generate a SERS signal.
• the device is reusable by simply dissolving the Ag nanoparticles in an acidic solution (e.g., HNO3) and then reintroducing fresh Ag particles.
• an acidic solution e.g., HNO3
• FIG. 15 shows a PL/dark-field image of two nanoribbons (NR1 and NR2) evanescently coupled at arrow 1.
• the top inset is a magnified dark-field image of the coupled nanoribbons with a glycol droplet designating where the analyte would sit in this configuration.
• the bottom inset is a dark-field image of NR1 with NR2 removed showing a coupled ring structure (junction - denoted be arrow 2) that would serve as a multi-pass beam path in a subwavelength optical spectrometer.
• FIG. 15 shows a PL/dark-field image of two nanoribbons (NR1 and NR2) evanescently coupled at arrow 1.
• the top inset is a magnified dark-field image of the coupled nanoribbons with a glycol droplet designating where the analyte would sit in this configuration.
• the bottom inset is a dark-field image of NR1 with NR2 removed showing a coupled
• FIG. 15 illustrates that ring shapes can be easily fashioned using our manipulation capabilities to create a subwavelength cavity shape that would sample an analyte repetitively.
• the glycol droplet (top inset) serves to identify where the analyte would sit in this particular configuration.
• the PL/dark-field image shows a two nanoribbon device evanescently coupled (arrow 1 denotes the junction), illustrating the first step to design a multi-pass spectrometer based on free-standing 1 D nanostructures.
• the bottom inset was taken after manipulating the end of NR1 into a ring structure (arrow 2 denotes the junction) showing the second step for creating a multi-cycle instrument.
• FIG. 16 shows the microfluidic channels (MFC) of a PDMS stamp bridged by multiple nanoribbons (NR). This is shown schematically in FIG. 16A.
• FIG. 16B is an image showing microfluidic channels in detail and
• FIG. 16C is an image showing several nanoribbons bridging the microfluidic channels shown in FIG. 16B. This microfluidic layout is important for the practical use of these structures for fluorescence, absorbance and SERS sensing.
• the instrument is generally used to detect 1 ⁇ L to 2 ⁇ L nucleic acid aliquots with a sample detection limit of 2 ng/ ⁇ L (dsDNA).
• the path length for the Xe flash lamp (220 nm to 750 nm) is held relatively fixed at 1 mm.
• the major advantages of a subwavelength spectrometer over the commercially available unit is smaller volume size ( ⁇ 10 6 times smaller), shorter path lengths ( ⁇ 10 times shorter), and possibly higher sensitivity with the advanced multi-pass geometries.
• Optical Routing With Nanorihhons And Nanowire Assemblies [00114] The manipulation of optical energy in structures smaller than the wavelength of light is key to the development of integrated photonic devices for computing, communications and sensing.
• SnO 2 nanoribbons were synthesized by the chemical vapor transport of SnO at 1100°C in flowing argon.
• ZnO nanowires were grown as epitaxial arrays on sapphire substrates by the oxidation of metallic zinc at 800°C, using gold as a catalyst.
• GaN nanowires were made by the chemical vapor transport of gallium in a NH 3 /H 2 mixture at 900°C, with nickel as the catalyst.
• the SnO 2 nanoribbons were dry transferred en masse to oxidized silicon substrates (600 nm Si ⁇ 2, Silicon Sense Inc.).
• Example 4 Nanoribbons and nanowires were manipulated with the probe under a dark-field microscope.
• a HeCd laser provided continuous wave (CW) resonant illumination (325 nm), while the fourth-harmonic of a Nd:YAG laser (266 nm, 8 nm, 10 Hz) was used for pulsed pumping.
• CW continuous wave
• Nd:YAG laser 266 nm, 8 nm, 10 Hz
• Laser diodes (652 nm and 532 nm) and the HeCd laser (442 nm) supplied visible light for the filtering and fluorescence demonstrations.
• the lasers were focused to a beam diameter of approximately 50 ⁇ m, giving a CW power density of approximately 175 W/cm 2 and a pulsed energy density of approximately 10 ⁇ J/cm 2 .
• Spectra were acquired with a fiber-coupled spectrometer (gratings at 150 and 1200 grooves/mm, SpectraPro 300i, Roper Scientific) and liquid N 2 -cooled CCD setup. Black-and-white and color images were recorded with two microscope- mounted CCD cameras (CoolSnap fx and CoolSnap cf, Photometries).
• the nanoribbons/wires described herein operated as single- mode fibers for some of the experimental wavelengths, while others were multi-mode.
• FIG. 17 illustrates the routing of GaN PL and lasing emission.
• FIG. 17A is a dark-field optical image of a coupled GaN nanowire 50 and SnO 2 nanoribbon 52.
• FIG. 17B shows direct excitation of the SnO 2 nanoribbon at location B generates white PL that is guided to the ends of the SnO 2 cavity. Some of the light is scattered by a large particle found at C.
• the inset in FIG. 17B is a magnified view of the bottom emission spot.
• FIG. 17C is a magnified view of the junction area.
• the inset in FIG. 17C is a SEM image showing that the two structures are staggered over 9 ⁇ m and touch for approximately 2 ⁇ m.
• FIG. 17D shows direct CW excitation of the GaN nanowire generates UV band-edge emission at 365 nm and a small amount of visible defect emission at 650 nm.
• FIG. 17E is an optical image of the routing of UV laser pulses from nanowire to nanoribbon.
• the GaN cavity was pumped above its lasing threshold by a pulsed 266 nm source (itself invisible to this detector).
• FIG. 17F shows spectra comparing the GaN PL and lasing emission before and after passage through the nanoribbon cavity.
• the broad pseudo-Gaussian spontaneous emission peak (top) is broken into a series of sharp modes during its transit through the nanoribbon (WG PL).
• the lasing emission at moderate pump power which shows multiple modes (GaN lasing) is severely modulated by the mode structure of the SnO 2 cavity (bottom).
• Spectra are normalized and offset for clarity.
• FIG. 17A shows a GaN nanowire (130 nm by 65 ⁇ m) that has been coupled to a SnO 2 nanoribbon (240 nm by 260 nm by 460 ⁇ m) with the micromanipulator.
• the magnified SEM view of the GaN-SnO 2 junction indicates that the two structures are in physical contact over an interaction length of approximately 2 ⁇ m.
• This staggered- bonded configuration provides good optical coupling between the cavities and some degree of inter-wire adhesion (via electrostatic forces), which aids in the construction of multi-wire networks.
• Butt-end coupling is also effective, and it is possible for us to detect the transfer of light between nanowire cavities that are weakly coupled across an air gap of up to several hundred nanometers (not shown). If two nanoribbons are crossed instead of staggered, the coupling losses decrease with shallower intersection angles, which has also been observed recently for crossed CdS nanowires.
• To demonstrate the routing of continuous wave light we excited the GaN nanowire with the focused beam of a HeCd laser operating at 325 nm. Band-edge PL from the GaN cavity was channeled through the Sn ⁇ 2 nanoribbon to emerge primarily at its far end. A fraction of the light was also scattered by imperfections along the length of the nanoribbon (i.e., attached particles or macroscopic step edges).
• FIG. 18A is a dark-field image of a GaN nanowire 54 and a ZnO nanowire 56 coupled to the same nanoribbon 58.
• the scale bar is 10 ⁇ m.
• FIG. 18B shows the spectrum of guided light collected at the far end of the nanoribbon when both nanowires were pumped above their lasing thresholds by the same train of optical pulses.
• the nanoribbon is the same used in FIG. 13 and FIG. 17.
• FIG. 19 illustrates GaN nanowire lasing.
• FIG. 19A shows a series of emission spectra at different pump fluence for an isolated GaN nanowire with a diameter of 150 nm and length of 45 ⁇ m.
• the inset in FIG. 19A shows the PL spectrum.
• FIG. 19B shows the energy curve for the same nanowire.
• Typical thresholds for GaN NW lasing were 5 ⁇ J to 15 ⁇ J cm "2 .
• the inset in FIG. 19B is an image of lasing emission from a different GaN nanowire, showing its pronounced spatial pattern.
• FIG. 20 shows color filtering in a nanoribbon network 60.
• FIG. 20A is a dark-field image of a four-ribbon assembly as it guides white PL generated at the pump spot (left) and separates it into a different color at the end of each nanoribbon (right).
• the scale bar is 50 ⁇ m.
• FIG. 20B is a magnified view of the emission region.
• FIG. 20B emitted green, aqua and blue light because of their progressively smaller cross-sections (350 nm by 140 nm, 260 nm by 175 nm and 210 nm by 135 nm, respectively). Their 50% cutoff wavelengths were determined by near-field scanning optical microscopy (NSOM) to be 543 nm, 502 nm and 478 nm.
• the stem nanoribbon is 260 nm by 240 nm by 390 ⁇ m.
• FIG. 20C shows that non-resonant blue light is transmitted to the end of all four nanoribbons, while FIG. 20D shows that green light is much more strongly guided by nanoribbon 1 than by nanoribbon 3 and FIG. 20E shows that red light is filtered out by all three branches.
• the scale bar is 20 ⁇ m.
• FIG. 20 we assembled a simple network comprising four nanoribbons of different sizes to show how such a structure may be used to separate colors.
• the large nanoribbon that formed the stem of the network 60 emitted white light composed of two broad SnO 2 PL bands centered at 495 nm and 590 nm, as can be seen from FIG. 21 which is a typical PL spectrum of a SnO 2 nanoribbon showing its two defect bands. Varying amounts of the stem emission then flows into the three shorter and consecutively thinner branch nanoribbons, separating the white light into green, aqua and blue components (ribbons 1-3).
• FIG. 22A is a dark-field image of the four-ribbon structure, with the input channel extending off the frame to the right and the output channels labeled 1-7.
• the nanoribbons vary in size from 300-400 nm on a side.
• FIG. 22B is a PL image as the input channel is pumped at 325 nm.
• the light trajectory is important here since the low reflectivity of their end facets makes nanoribbons poor resonators (with an ideal finesse of -1.3). As such, most photons do not make multiple passes and light flow is highly directional.
• the right-angle intersections present significant obstacles to inter-cavity waveguiding by total internal reflection. At the same time, they act as quasi-isotropic scatterers that feed light between nanoribbons. Nanoribbon-to-ribbon losses, although nearly maximized in this geometry, are still low enough for the activation of channels 2 and 3, which require photons to negotiate two right-angle junctions and transit three separate cavities.
• a ZnO nanowire laser was added to the input channel and used it to launch light into the grid, emission was detected from all channels but 2 and 3; the number of injected photons was simply too small to illuminate the parallel nanoribbon.
• Nanowire grids have already been employed to implement rudimentary electronic logic.
• Integrated optical logic and all-optical switches present exciting prospects, and our results show that grids of nanowires should be capable of routing signals for such tasks.
• n > 2 Due to their high refractive indices (n > 2), the nanoribbons and nanowires discussed here function well as waveguides in water and other liquids. This is a considerable advantage over subwavelength silica waveguides, which cannot efficiently confine visible light in liquids because of a low dielectric contrast (n S iiica * 1.45). Waveguiding in liquids is especially important for integrated on-chip chemical analysis and biological spectroscopy in which small excitation and detection volumes are required.
• nanoribbon and nanowire waveguides have two unique and potentially useful features for subwavelength photonics applications.
• nanowires push subwavelength optical fibers beyond silica.
• the scores of materials that can now be made in nanowire form include active, passive, nonlinear and semiconducting inorganic crystals, as well as a wide variety of polymers.
• Simultaneous photon, charge carrier and spin manipulation is possible within and between nanowires of different compositions.
• many of these materials have higher refractive indices than silica-based glasses, permitting light of a given wavelength to be confined within thinner structures for denser integration.
• nanowires are freestanding, mechanically flexible elements that can be manipulated on surfaces or used as mobile probes in fluids. As such, they offer a type of versatility difficult to achieve with lithographically-defined structures that are permanently affixed to their substrates.
• the disadvantages of nanowire photonics include (i) the paucity of parallel assembly methods for accurately arranging large groups of nanowires into useful structures; (ii) relatively high inter-wire coupling losses compared to monolithic waveguides formed by lithography (coupling losses could be greatly reduced if branched, multi-component nanowires were developed to replace the staggered or crossed nanowire cavities used here); (iii) the lesser geometric perfection of nanowire assemblies relative to the precise shapes and sizes definable with lithography. Geometric imprecision introduces some uncertainty in the resulting light propagation and adds complexity to nanowire experiment/theory comparisons.
• the subwavelength waveguide described herein can be used as a functional element in photonic circuits such as optical networks, optical filters, optical directional couplers, emitter-waveguide- detector junctions, optical probes, optical sensors, optical routers, optical junctions, optical modulators, optical Y-junctions, optical branch networks, Mach-Zehnder interferometers, optical ring oscillators, nanolasers, optical phase shifters, fluidic sensors, fluidic probes, microfluidic devices, optical spectrometers, and optical crossbar grids.
• photonic circuits such as optical networks, optical filters, optical directional couplers, emitter-waveguide- detector junctions, optical probes, optical sensors, optical routers, optical junctions, optical modulators, optical Y-junctions, optical branch networks, Mach-Zehnder interferometers, optical ring oscillators, nanolasers, optical phase shifters, fluidic sensors, fluidic probes, microfluidic devices, optical spectrometers, and optical crossbar grid

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