WO2013062795A1

WO2013062795A1 - Electrically-tunable optical filter based on fano resonance

Info

Publication number: WO2013062795A1
Application number: PCT/US2012/060035
Authority: WO
Inventors: Martin A. Kykta; Robert J. Petcavich; Daniel K. Van Ostrand
Original assignee: Unipixel Displays, Inc.
Priority date: 2011-10-24
Filing date: 2012-10-22
Publication date: 2013-05-02
Also published as: TW201317648A

Abstract

An optical filter system includes a plurality of metallic nanoparticles embedded in an electro-optical polymer. The system also includes a plurality of conductive electrodes adjacent the polymer. The electrodes are configured to provide an electric field across the plurality of conductive electrodes to thereby alter the refractive index of the electro-optical polymer.

Description

ELECTRICALLY-TUNABLE OPTICAL FILTER BASED ON

FANO RESONANCE

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims priority to U.S. Provisional Patent Application No. 61/550,521 , filed on October 24, 20 1 titled "Electrically Tunable Optical Filter Based on Fano Resonance," and which is incorporated herein by reference.

BACKGROUND

[0002] There are a number of optical devices presently available for constructing wavelength-selective wavelength-division multiplexing (WDM) and wavelength-division demultiplexing (WDD) systems. Some of these devices include, for example, thin film filters that reflects a very narrow band of wavelengths. Such filters are often constructed from several hundred layers of stacked narrow band filters, and are designed to reflect a single narrow band of wavelengths. Arrayed waveguide gratings are also available. A limitation of many of these devices is that they are not wavelength tunable. That is, the operative wavelength cannot be dynamically changed during operation in order to select a different optical data channel during use. This can have negative implications for many wavelength-selective WDM, WDD and routing applications.

[0003] To overcome these limitations, a number of devices have been developed to provide some level of wavelength tunability. Many of these devices, however, require some form of physical motion or movement to achieve the desired tunability. For example, one such device includes a substrate with a diffraction grating. The diffraction grating is provided in the path of an incoming light beam. To provide wavelength tunability, the diffraction grating is rotated, which causes the incoming light beam to strike the diffraction grating at a different incident angle. The change in incident angle alters the selected wavelength of the grating.

[0004] In another example, a Fabry-Perot cavity is provided with two mirrors separated by an intervening space. The mirrors are moved either toward or away from each other to vary the size of the intervening space, which changes the selected wavelength of the Fabry-Perot cavity. A limitation of many of these devices is that the required physical movement tends to limit the resolution that can be achieved, and may reduce the reliability and/or stability of such devices. SUMMARY

[0005] Various embodiments are described herein of an electrically-tunable optical filter comprising one or more clusters of seven-member nanoparticles (disks) with adjacent electrodes. A voltage applied to the electrodes causes an electric field to be generated across the nanoparticles thereby altering the refractive index of the filter. Different voltages result in the filter being tuned to different wavelengths of light.

[0006] in one example, an optical filter system includes a plurality of metallic nanoparticles embedded in an electro-optical polymer. The system also includes a plurality of conductive electrodes adjacent the polymer. The electrodes are configured to provide an electric field across the plurality of conductive electrodes to thereby alter the refractive index of the electro-optical polymer.

[0007] In another example, an optical system comprises an array of seven-member heptamer clusters embedded in a transparent electro-optical polymer. The system also comprises a plurality of conductive electrodes on opposing sides of each cluster.

[0008] In a method embodiment, the method includes electrically tuning a heptamer cluster of nanoparticles to a first optical wavelength by applying a first voltage to a pair of electrodes on opposing sides of the heptamer cluster. If desired, the filter can be tuned to a different frequency by the application of a second (different) voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] For a detailed description of exemplary embodiments of the invention, reference will now be made to the accompanying drawings in which:

[0010] Figure 1 a top view of a seven-member heptamer cluster in accordance with an embodiment of the invention;

[0011] Figure 2 shows a graph of the size dependence of the scattering spectrum of a seven-member heptamer cluster in accordance with an embodiment of the invention;

[0012] Figure 3 shows a top view of the electrically tunable heptamer optical filter in accordance with an embodiment of the invention;

[0013] Figure 4 illustrates an array of heptamer clusters in accordance with an embodiment of the invention;

[0014] Figure 5 illustrates a system in which control logic controls a voltage signal generator to cause a certain electric field to be applied to a heptamer filter in accordance with an embodiment of the invention; P T/US2012/060035

3

[0015] Figure 6 shows a cross-sectional side view of Figure 3 in accordance with an embodiment of the invention;

[0016] Figure 7 shows a top view of an embodiment of the electrically tunable heptamer optical filter in accordance with an alternative embodiment of the invention;

[0017] Figure 8 shows a cross-sectional side view of the filter of Figure 5 in accordance with an embodiment of the invention;

[0018] Figure 9 shows a representative example in accordance with an embodiment of the invention; and

[0019] Figure 10 shows a representative example in accordance with an embodiment of the invention; and

[0020] Figure 1 1 illustrates an application of an electrically-tunable optical filter for detecting drift in a laser.

DETAILED DESCRIPTION

[0021] The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment.

[0022] The term "approximately" means plus or minus 10%. The term "system" refers to a collection of one or more components. A system may refer to a single cluster of nanoparticles, a cluster of nanoparticles with electrodes connected to a voltage signal generator, etc.

[0023] The devices noted above may provide only a limited amount of optical wavelength selectability and generally require physical movement of a part of the device (e.g., rotation of a diffraction grating, relative movement of the mirrors of a Fabry-Perot cavity). Such devices are of limited use and may be insufficient to meet the increasingly complex requirements of optical systems.

[0024] Various embodiments are described herein of heptamer optical filters, which is an optical filter having a heptamer arrangement of disks.

[0025] Figure 1 shows a top view of a cluster 100 comprising a number of disks 102. In the particular embodiment of Fig. 1 , the cluster 100 comprises seven disks 102 and thus the cluster is referred to as a heptamer cluster. In other embodiments, the number of disks can be other than seven. An example of a heptamer filter is discussed herein, but the filter can be other than a seven disk filter in other embodiments as noted above. Each disk 102 can be formed from a variety of materials. Gold is one suitable material. In Fig. 1, disks 102 may comprise metallic nanoparticles that may be 30 nm thick and have a diameter size in the range of approximately 85 nm to 170 nm with a constant gap size 104 between the particles of approximately 15 nm. In Fig. 1 , disks 102 are embedded in a dielectric media 06 (i.e., a material with negligible electrical or thermal conductivity). Clusters of plasmonic nanoparticles and nanostructures support Fano resonances. The clusters have a spectral feature, produced by the interference between bright and dark modes of the nanoparticle cluster and are strongly dependent upon both geometry and local dielectric environment. The feature allows a highly sensitive tunability of the Fano dip in both wavelength and transmission amplitude by varying clusters dimensions, geometry, and relative size of the individual nanoclusters components.

[0026] Preferably, the distances between disks 102 are kept constant at approximately 15 nm. In some embodiments, the disks 102 may comprise metallic nanoparticles fabricated by, for example, electron beam lithography and may be composed of 30 nm thick Au disks on a 1 nm Ti adhesion layer, evaporated onto a silicon substrate coated with a 100 nm thick silicon dioxide layer. The dielectric permittivity of the substrate therefore may be similar to a glass substrate.

[0027] Figure 2 shows scattering spectra 200 of three different size heptamer disks. In Fig. 2, a seven-member heptamer cluster 00 is represented by three different sizes wherein size 202 corresponds to 85nm diameter particles with transmission wavelength shift 208 at 625 nm, size 204 corresponds to 128 nm diameter particles with transmission wavelength shift 210 at 750 nm, and size 206 corresponds to 170nm diameter particles with transmission wavelength shift 212 at 900 nm. Figure 2 also shows how the transmission wavelength shift moves towards longer wavelengths as the size of the disks increases. One suitable range of wavelengths is between 800 nm and 1600 nm to match the infrared (IR) wavelengths used in the telecom industry.

[0028] Figure 3 shows a top view of an illustrative electrically tunable heptamer optical filter 300. The example of Fig. 3 includes a seven-member heptamer cluster 100 formed of 7 metallic nanoparticles embedded in a conformal layer 302 of an Electro-Optic Polymer (EOP). EOP a polymer whose refractive index is a function of a characteristic of an applied electric field (e.g., magnitude). The EOP layer 302 may have an index of refraction n in a range of approximately 1.58 to 1.68. Figure 3 also includes a thin layer of conductive material (e.g., gold or other material) 306 on opposing sides of the seven- member heptamer cluster 100. The conductive material 306 acts as electrodes, and thus is also referred to as electrodes 306. A voltage signal generator 308 can be connected to the electrodes 306 to create an electric field across the heptamer cluster 100 that changes the index of refraction of the EOP layer 302, as described by the Pockels Effect. The electrodes 306 preferably are placed in a range of about 750 mm to 1 micron from the edges of conformal layer 302. For this particular configuration, the electric field is set transverse to EOP layer 302 Voltage signal generator 308 may generate a 5 volt signal to create the electric field that emanates from the positive electrode (see + sign in Fig. 3), through the seven-member heptamer cluster 100, to the negative terminal (-), which changes the index of refraction of EOP layer 302 to a value smaller or greater than the original index (i.e. , the index of refraction without an applied voltage/electric field).

[0029] Note that in Fig. 3, the positive and negative terminals of voltage signal generator 308 are connected to the electrodes 306 on opposing sides of the filter 100. Electrode 306a is on the left-hand side and electrode 306b is on the right-hand side of the filter. In some embodiments, multiple instances of the seven-member heptamer cluster 100 are provided. Preferably repeating patterns of the heptamer optical filter 300 are provided in an array 150 as shown in Figure 4. The array 150 includes multiple rows and columns of seven-member heptamer filters 300. The left-hand side electrode 306a of each of the filters 300 in the array 10 may be connected together. The right-hand side electrode 306b of each filter also may be connected together as well, but not to the left- hand side electrode 306a. As such, the positive terminal of the voltage signal generator 308 may be connected to all of the left-hand side electrodes 306a in parallel, and similarly the negative terminal of the voltage signal generator 308 may be connected to all of the right-hand side electrodes 306b in parallel

[0030] In various embodiments, the voltage can be varied from, for example, 0 volts to 5 volts (or other voltage limits). Figure 5 illustrates an embodiment in which control logic 155 is coupled to the voltage signal generator 308 which in turn is connected to the heptamer filter array 150. The control logic 155 may be implemented as a discrete circuit, a computing device (e.g., a computer, a smart phone, etc.). The control logic 155 preferably asserts a signal to the voltage signal generator 308 to cause a particular voltage level to be provided to the heptamer filter array 150. The control logic 155 may have various inputs 158 to dictate which voltage is selected for the array. An example of an input is a user control (e g., a button, a knob, etc.) that a user activates to specify a particular voltage or wavelength of light. Another example is a digital signal (e.g., a command, a message, etc.) from another device such as another computer. [0031] Figure 6 shows a cross sectional side view of Fig. 3. In Fig. 6, EOP layer 302 is sandwiched between two transparent dielectric layers 402. A bottom layer comprising a transparent substrate 404 also is provided adjacent one of the dielectric layer 402 as shown. Transparent substrate 404 may be made from, for example, silicon, germanium, or other suitable material. Fig. 6 also shows a seven-member heptamer cluster 100 on top of the bottom transparent dielectric Iayer402. Transparent dielectric layer 402 may be made from silicon dioxide (S1O2). As explained previously, the electric field across the

EOP layer 302 is created by a voltage signal generator 308 which is connected to thin layer of conductive material 306 (left and right electrodes). Note that in Fig. 6 the positive terminal of voltage signal generator 308 is connected to the left electrode and the negative terminal is connected to the right electrode. As in Fig. 3, voltage signal generator 308 generates variable (or fixed) voltage (e.g., 5 volts) to create the electric field emanating from the positive terminal connected to electrode 306a, through seven- member heptamer cluster 100, to the negative electrode 306b and to the negative terminal of the voltage signal generator 308. As a result, the index of refraction of EOP layer 302 changes (smaller or greater) relative to the original index. The change of the index depends on the original poling done to the EOP layer 302 and the direction of the applied electric field. Poling refers to the process of applying an electric field above the glass transition temperature of an EOP to polarize it. To maintain the polarization, the temperature is decreased below the glass transition temperature while maintaining the electric field.

[0032] A plane 505 is shown in Fig. 6 that passes through the nanoparticles which form the heptamer cluster. The electrodes 306a and 306b are provided on opposing sides of the cluster in such a way that the plane 505 also passes through the electrodes. The electrodes 306a and 306b are illustrated in Fig. 3 to have a height H1 that exceeds the height H2 of the nanoparticles themselves, although in other embodiments, the electrodes' height H1 may be the same as or less than the height H2 of the nanoparticles.

[0033] Figure 7 shows a top view of an alternative structure for an electrically tunable heptamer optical filter 320. This alternative structure differs from Fig. 3 in the way the electric field is applied, which is perpendicular to EOP layer 302 instead of transverse to it. Fig. 7 shows an electrically-tunable heptamer optical filter 320 including seven- member heptamer cluster 100, also formed of 7 metallic nanoparticles embedded in a conformal EOP layer 302. Fig. 7 also shows a thin layer of semi-transparent conductive material 502 (e.g., gold) on top of EOP layer 302 (and on the bottom, not shown in Fig. 7). In Fig. 7, semi-transparent conductive material 502 act as electrodes (top and bottom) wherein a voltage signal generator 308 is applied to create an electric field to change the index of refraction of the EOP layer 302. Voltage signal generator 308 generates a voltage (e.g., 5 volts) to create the electric field that emanates from the positive terminal, through one of the semi-transparent conductive material portions 502, through seven- member heptamer cluster 100, to the other semi-transparent conductive material portion 502, and to the negative terminal, which makes the index of refraction of EOP layer 302 different (smaller or larger) than the original index. The change of the index depends on the original poling done to the EOP layer and the direction of the applied electric field. Note that in Fig. 7, the positive terminal (+) of voltage signal generator 308 is connected to the top electrode 502 and the negative terminal (-) is connected to the bottom electrode 502 The conductive electrodes preferably are placed in a range of about 750 mm to 1 micron from the edges of conformal layer EOP 302. An array (similar to that depicted in Fig. 4) of heptamer filters 320 of Fig. 7 can be implemented.

[0034] Figure 8 shows a cross sectional side view of Fig. 7 In Fig. 8, EOP layer 302 is between a semi- transparent gold or transparent conductor 502a layer (top electrode) and transparent dielectric 602. Below transparent dielectric 602 is another semi-transparent conductive material 502b layer (bottom electrode) followed by a bottom layer of transparent substrate 404. Fig. 8 also shows seven-member heptamer cluster 100 embedded in EOP layer 302. Top and bottom electrodes 502a and 502b are on opposing sides of a plane 505 on which the nanoparticles forming the cluster reside. The nanoparticles thus reside in plane 505 sandwiched between the conductive electrodes 502a and 502b . The electric field in EOP layer 302 layer is created by voltage signal generator 308 which is connected as shown to both semi-transparent conductive material 502 layer (top electrode, positive terminal) and semi-transparent conductive material 502 layer (bottom electrode, negative terminal).

[0035] Figure 9 is the cross-sectional side view of Fig. 6. Fig. 9 shows a representative example of how a group of light beams 702 and 704 of different wavelengths, emitted from a collimated light source in, for example, an infrared (IR) wavelength range centered around 1600 nm, are affected when voltage signal generator 308 generates a 5 volt signal to create an electric field. The electric field is transverse to EOP 302, and emanates from the positive terminal, through seven-member heptamer cluster 100, to the negative terminal. The Pockels Effect describes how the refractive index of EOP 302 changes from a smaller or greater value, allowing the wavelengths to be selectively transmitted through the Fano Resonance in the filter. In Fig. 9, when EOP layer 302 changes its refractive index, wavelengths 702 are transmitted and the other wavelengths 704 are reflected. Fig. 9 also includes, again, a bottom layer of transparent substrate 404, seven-member heptamer cluster 100, and EOP layer 302 in between both transparent dielectrics 402. Fig. 9 also shows voltage signal generator 308 positive terminal connected to the left electrode and negative terminal connected to the right electrode.

[0036] Figure 10 is the cross-sectional side view of Fig. 8. Fig. 8 shows a representative example of how a group of light beams 802 and 804 of different wavelengths, emitted from a col!imated light source in an infrared (IR) wavelegth range centered around 1600 nm, are affected when voltage signal generator 308 generates a signal (e.g. , 5 volts) to create an electric field. The electric field is perpendicular to EOP layer 302, and emanates from the positive terminal, through seven-member heptamer cluster 100, to the negative terminal. The Pockels Effect describes how the refractive index of EOP 302 changes from a smaller or greater value, allowing the wavelengths to be selectively transmitted through the Fano Resonance in the filter. In Fig. 8, wavelengths 802 are transmitted and the other wavelengths 804 are scattered. Fig. 8, again, includes a bottom layer of transparent substrate 404, EOP layer 302 in between semi-transparent gold or transparent conductor 502 layer and transparent dielectric 808, and seven-member heptamer cluster 100 resting on top of transparent dielectric 808.

[0037] In a method embodiment, a method includes electrically tuning a heptamer cluster of nanoparticles to a first optical wavelength by applying a first voltage to a pair of electrodes on opposing sides of the heptamer cluster. If desired, the filter can be tuned to a different wavelength by the application of a second (different) voltage.

[0038] One application of the embodiments described herein is as a switch within a free space optical communication (FSOC) system that selectively blocks or transmits light. To operate the filters described herein as a switch, an electric field is generated transverse or perpendicular to the EOP layer. When the electric field is on, the index of the EOP layer is changed, enabling the filter to transmit light and when the electric field is off, the filter blocks light. The filter could also be built to do the inverse; block the light when there is an applied field and transmit the light when there is no applied field.

[0039] Another application for the various electrically-tunable heptamer optical filters described herein is to monitor the emission wavelength of a laser. It is known that the emission wavelength of a laser may drift over time, temperature, etc. To monitor the emission wavelength, a tunable filter may be used in a FSOC system. As illustrated in Fig. 11 , the filter (e.g., filter 100) may be positioned between the light beam (in this case a laser 415) and a detector 425. The detector 425preferably is capable of detecting relatively wide range of wavelengths, while the tunable filter 100 only passes a relatively narrow band of wavelengths. With the laser 415 turned on, an electrical field is applied to the tunable filter 100 as described above until the filter passes the current operating wavelength of the laser to the detector 425. When the detector 425 detects the emission, a controller (CTRLR) 430 is notified. By noting the electric field applied to the tunable filter, the controller 430 may determine the current operating wavelength of the laser.

[0040] The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.

Claims

CLAIMS What is claimed is:

1. An optical filter system, comprising:

a plurality of metallic nanoparticles embedded in an electro-optical polymer; and a plurality of conductive electrodes adjacent said polymer, said electrodes configured to provide an electric field across said plurality of conductive electrodes to thereby alter the refractive index of the electro-optical polymer.

2. The optical filter system of claim 1 further comprising a dielectric layer on a transparent substrate, and said polymer rests adjacent the dielectric layer.

3. The optical filter system of claim 1 further comprising a signal generator coupled to said conductive electrodes.

4. The optical filter system of claim 3 wherein the fano resonance of the electro- optical polymer is varied through application of varying voltage from the signal generator.

5. The optical filter system of claim 1 wherein the plurality of metallic nanoparticles comprises seven nanoparticles.

6. The optical filter system of claim 1 wherein the plurality of metallic nanoparticles comprises an array of nanoparticles.

7. The optical filter system of claim 1 wherein the plurality of metallic nanoparticles comprises an array of elements, each element comprising a seven-member heptamer cluster of the nanoparticles.

8. The optical filter system of claim 1 wherein the electrodes are provided on opposing sides of the plurality of nanoparticles, and the nanoparticles reside in a plane that passes through the electrodes.

9. The optical filter system of claim 1 wherein the nanoparticles reside in a plane sandwiched between the conductive electrodes.

10. An optical system, comprising:

an array of seven-member heptamer clusters embedded in a transparent electro- optical polymer; and

a plurality of conductive electrodes on opposing sides of each cluster.

1 1. The optica! device of claim 10 further comprising a voltage signal generator connected to said electrodes, said voltage signal generator varies the fano resonance of each cluster by varying the voltage applied to the electrodes.

12. The optical system of claim 10 wherein the electrodes for a given cluster are on opposite sides of a plane passing through that cluster.

13. The optical system of claim 10 further comprising a voltage signal generator connected to the electrodes and a controller coupled to the voltage signal generator, wherein the controller provides a control signal to the voltage signal generator to cause the voltage signal generator to provide a particular voltage to the electrodes.

14. The optical system of claim 13 wherein the controller varies the control signal to the voltage signal generator to cause the voltage signal generator to vary the voltage to the electrodes thereby varying the wavelengths light that pass through each cluster.

15. The optical system of claim further comprising a voltage signal generator connected to the electrodes and a controller coupled to the voltage signal generator, said controller causing the clusters to function as optical switches in which light is selectively passed through or blocked by each cluster.

16. A method, comprising:

electrically tuning a heptamer cluster of nanoparticles to a first optical wavelength by applying a first voltage to a pair of electrodes on opposing sides of the heptamer cluster.

17. The method of claim 16 further comprising eiectncaliy tuning the heptamer cluster to a second optical wavelength by applying a first voltage to a pair of electrodes on opposing sides of the heptamer cluster.