Abstract:
An optical system that includes a reconfigurable phase-change material (PCM) layer that includes a plurality of individually controllable pixel areas. Each individually controllable pixel area is variable between a first refractive index and a second refractive index. The PCM layer is configured to pass radiation incident on the PCM layer in accordance with a first mask pattern through the PCM layer in a downstream direction. A PCM controller is configured to control the plurality of individually controllable pixel areas to have respective refractive indices in accordance with the first mask pattern.

Description:
PRIORITY 
       [0001]    This application is a continuation of co-pending U.S. patent application Ser. No. 13/585,577, filed on Aug. 14, 2012, entitled “RECONFIGURABLE PHASE CHANGE MATERIAL MASKS FOR ELECTRO-OPTICAL COMPRESSIVE SENSING,” which claims the benefit of U.S. Provisional Patent Application No. 61/523,672, filed on Aug. 15, 2011, entitled “RECONFIGURABLE PHASE CHANGE MATERIAL MASKS FOR ELECTRO-OPTICAL COMPRESSIVE SENSING,” the disclosures of which are hereby incorporated herein by reference in their entireties. 
     
    
     TECHNICAL BACKGROUND 
       [0002]    A key challenge to high resolution imaging sensors used in observing terrestrial activities over a very wide field-of-view (WFOV) (e.g., 50 km 2 ) is to achieve the resolution needed to observe and make inferences regarding events and objects of interest while maintaining the area coverage, and minimizing the cost, size, weight, and power of the sensor system. One particularly promising approach to the data deluge problem is compressive sensing, which involves collecting a small amount of information-rich measurements rather than the traditional image collection from a traditional pixel-based imager. 
         [0003]    There is no current solution for compressive sensing architectures, especially in the infrared. An eyelid technology, liquid crystal (LC), and microelectromechanical system (MEMS) digital mirror arrays (DMA) have been postulated as potential solutions in a lab environment, but there is no current hardware available. The closest technology to production scale is a visible/short wave infrared compressive sensing camera that uses the DMA array, but this is a reflective design. 
         [0004]    A DMA solution is limited in resolution by the number of pixels and also to a±degree tilt in the reflective element(s). Also, the solution is complex and failure-prone due to the complex optics, and the sampling modulation is limited. 
       SUMMARY 
       [0005]    Aspects of the techniques and solutions disclosed herein are directed at coded masks that include phase change material (PCM). Such masks may be suitable for use with various types of photo-detectors, such as photo-detectors of the type disclosed in U.S. Pat. No. 7,687,871, issued to Shimon Maimon on Mar. 30, 2010, the entire contents of which are hereby incorporated by reference. Other detector types, such as p-n junction detectors, photodiodes, charge-coupled device (CCD) photodetectors, active-pixel sensors/CMOS sensors, and other detector types. Wavelengths detected by the photodetector and/or filtered or otherwise affected by a coded PCM mask applied to the detector may include long-wave, mid-wave, and/or short-wave infra-red, millimeter-wave, visible spectrum, and/or ultra-violet radiation. 
         [0006]    Aspects of the techniques and solutions discussed herein may pertain to a pixel-level mask for a photo-detector, the mask comprising: a layer of reconfigurable phase-change material (PCM) configured to vary between a first refractive index and a second refractive index; said PCM layer being divided into individual pixel areas such that each individual pixel area may be set to have the first refractive index or the second refractive index; said PCM layer being disposed on a photo-detector such that incident radiation detected by the photo-detector must pass through the PCM layer in order to be detected; and a PCM controller that controls the refractive index of an individual pixel area. 
         [0007]    In some variations, each pixel area may have a refractive index within a range of values between the first refractive index and the second refractive index, inclusive. In some variations, the PCM includes Ge2Sb2Te5 (GST); the first refractive index is associated with a crystallized state of GST; and the second refractive index is associated with an amorphous state of GST. 
         [0008]    In some variations, the PCM controller includes a voltage source; and the PCM controller is operably connected to an individual pixel area such that a first voltage level provided by the controller sets the individual pixel area to have the first refractive index and a second voltage level provided by the controller sets the individual pixel area to have the second refractive index. 
         [0009]    In some variations, the mask includes a voltage source operably connected to the PCM controller; the PCM controller includes a multiplexer PCM controller; and the PCM controller controls the voltage source such that the voltage source provides a first voltage level that sets an individual pixel area to have the first refractive index and such that the voltage source provides a second voltage level that sets the individual pixel area to have the second refractive index. In some variations, the first voltage level is six volts. 
         [0010]    In some variations, the individual pixel areas are aggregated into superpixels. In some variations, the superpixels are controlled by the PCM controller such that each superpixel may be set to have a particular imaging mask pattern by changing the refractive indices of the pixel areas within each superpixel. In some variations, each superpixel in the mask is the same size and shape. In some variations, a superpixel corresponds to a pixel of the underlying photo-detector. 
         [0011]    In some variations, the mask includes a laser source; the PCM controller is operably connected to the laser source; the laser source provides a first laser irradiation to the individual pixel area to set the individual pixel area to the first refractive index and a second laser irradiation to the individual pixel area to set the individual pixel area to the second refractive index. In some variations, the first laser irradiation is continuous wave (CW) irradiation and the second laser irradiation is pulsed irradiation. 
         [0012]    In some variations, the photo-detector is an infra-red detector; the superpixels the PCM layer correspond to pixel areas of the infra-red detector; the pixel areas having the first refractive index are opaque to infra-red radiation; and the pixel areas having the second refractive index are transparent to infra-red radiation. In some variations, the pixel areas having the first refractive index and the pixel areas having the second refractive index are arranged to form an imaging mask for compressive imaging. 
         [0013]    In some variations, the mask includes an index variation layer of ZnS-SiO2 disposed beneath the PCM layer; a layer of Aluminum disposed beneath the response variation layer; and a layer of glass disposed beneath the layer of Aluminum; where the photo-detector is disposed beneath the layer of glass such that incoming radiation to be detected by the photo- detector must pass through the PCM layer, the index variation layer, the Aluminum, and the glass before being detected by the photo-detector. 
         [0014]    In some variations, the mask includes a layer of doped silicon and/or alumina disposed beneath the PCM layer, where switching properties of the mask are determined based on a thickness of the doped silicon and/or alumina layers. 
         [0015]    In some variations, the PCM controller controls a pattern of the imaging mask by selectively changing refractive indexes of individual pixel areas. 
         [0016]    In some variations, PCM controller includes a laser source; the laser source providing a first laser irradiation to the individual pixel area to set the individual pixel area to the first refractive index; and the laser source providing a second laser irradiation to the individual pixel area to set the individual pixel area to the second refractive index. 
         [0017]    Aspects of the techniques and solutions discussed herein may pertain to method of controlling the absoption of individual pixel areas in a reconfigurable phase-change material (PCM) mask for a photo-detector, the method comprising: first illuminating at least one pixel area with a continuous wave (CW) laser illumination to increase the absorption of said at least one pixel area from a first value up to a second value; and second illuminating said at least one pixel area with a pulsed laser illumination to set the absorption of said at least one pixel area to the first value; where said first illuminating and said second illuminating are performed selectively on individual pixel areas in the PCM mask to set a mask pattern. 
         [0018]    Aspects of the techniques and solutions discussed herein may pertain to a method of controlling the absoption of individual pixel areas in a reconfigurable phase-change material (PCM) mask for a photo-detector, the method comprising: first providing at least one pixel area with a SET voltage level to increase the absorption of said at least one pixel area from a first value up to a second value; and second providing said at least one pixel area with a RESET voltage level to set the absorption of said at least one pixel area to the first value; where said first providing and said second providing are performed selectively on individual pixel areas in the PCM mask to set a mask pattern. 
         [0019]    Further scope of applicability of the techniques, devices, and solutions described herein will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the techniques, devices, and solutions described herein, are given by way of illustration only, since various changes and modifications within the spirit and scope of the disclosure will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]    The techniques, devices, and solutions described herein will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present disclosure, and wherein 
           [0021]      FIG. 1   a  depicts a variation of a PCM coded mask as described herein; 
           [0022]      FIG. 1   b  depicts a variation of a PCM coded mask as described herein; 
           [0023]      FIG. 1   c  depicts a variation of a PCM coded mask as described herein; 
           [0024]      FIG. 1   d  depicts a variation of a PCM coded mask as described herein; 
           [0025]      FIG. 2   a  depicts a variation of a PCM coded mask as described herein; 
           [0026]      FIG. 2   b  depicts a variation of a PCM coded mask as described herein; 
           [0027]      FIG. 3   a  depicts a variation of refraction index changes in a PCM material; 
           [0028]      FIG. 3   b  depicts variations of reflectivity changes in variations of a PCM material. 
           [0029]    The drawings will be described in detail in the course of the detailed description. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the techniques, devices, and solutions described herein. Instead, the scope of the techniques, devices, and solutions described herein is defined by the appended claims and equivalents thereof. 
         [0031]    In a solution to the above-noted problem, phase change materials (PCM) may be used as the active components to create coded apertures (i.e. sub-pixel/sub-wavelength patterns), which in, combination with image read-out and processing algorithms, optimize the “best” possible compressible image that fits the observed measurements for perfect image reconstruction. 
         [0032]    In some variations, a PCM coded mask may be disposed onto a focal plane array (FPA) such as an infra-red (IR) detector. Other variations may use different types of detectors, such as detectors that operate in some or all of the visible, millimeter-wave, and infra-red spectra. A variation of a PCM coded mask disposed over a pixel array is shown in  FIG. 1   a.    
         [0033]    In the variation shown, a pixel array  1001  such as an FPA may include several individual pixels  1020 . A PCM coded mask  1010  may be disposed over the FPA  1001 . In some variations, the PCM coded mask may include several mask PCM elements  1030 . In some variations, many mask PCM elements  1030  may cover one FPA pixel  1020 . In other variations, a PCM coded mask may be a continuous surface configured for sub-pixel variations in mask structure. 
         [0034]    In some variations, a PCM coded mask can be used in the Fourier planes as well as in the image plane. In such variations, the PCM coded mask will result in the detection of the image convolved with the PCM mask. For applications in compressive sensing, this would be particularly useful for image sparse images (i.e. imaging objects of interest against a bland background). 
         [0035]    In some variations, a PCM coded mask may be used to generate one or more masks for compressive sensing applications. Such a variation of a mask is shown in  FIG. 1   b . In the variation shown, a PCM film  1110  may be divided into discrete regions  1100 ,  1120  which may correspond to pixels on an underlying photodetector (not shown). The film regions may then each be set or otherwise configured to have particular transmission properties. Some film regions may be set to transmit or pass  1120  a certain wavelength or wavelength range. Other film regions  1100  may be set to suppress that wavelength or wavelength range. In further variations, the suppressive regions  1100  may suppress or otherwise reject all incoming radiation that could/would otherwise be detected by an underlying photodetector. In such variations, the PCM film  1110  may be arranged into a compressive imaging mask such that only the transmissive regions  1120  of the mask allow electro-optical radiation to pass through for detection by an underlying photodetector (such as, for instance, an infra-red detector). 
         [0036]    In some variations, a PCM film may be used in combination to other non-active materials such as alumina on a film stack, to generate masks for a compressive sensing application. In variations where the PCM coded mask elements correspond to one or more particular pixels on an underlying FPA, each individual element or array of elements in the mask (e.g. row or column) can be individually addressed by an external laser or voltage stimuli allowing the PCM material to change its transmission properties. In some variations, this is accomplished by causing the PCM material to change between crystalline and amorphous states; which in turns produces a change in the optical properties of the material (e.g. refractive index, absorption, etc). In some variations, these changes can occur in the nanosecond response time. 
         [0037]    A variation of such a PCM mask is shown in  FIG. 1   c . In the variation shown, each element  1210 ,  1220  or the PCM mask  1230  corresponds to one pixel of an underlying photodetector. In the variation shown, the percentage of light transmitted through each pixel  1210 ,  1220  of the PCM mask  1230  can be adjusted by controlling an input voltage directed to that pixel  1210 ,  1220  by a multiplexer PCM controller  1200 . In some variations, the application of a bias voltage (SET pulse) to a pixel  1210  crystallizes the material and a different bias (RESET), or further increasing the bias voltage, may cause the material to re-amorphize. In other variations, the PCM material may be placed in a crystalline state by a particular voltage pulse and may be triggered to change to an amorphous state by a different voltage pulse. In some variations, a SET pulse may be ˜6 volts and a RESET voltage may be ˜10 volts. 
         [0038]    In some variations, the multiplexer PCM controller  1200  may be a specialized or otherwise distinct component of an overall PCM mask device. In some variations, such a multiplexer PCM controller  1200  may include a separate voltage supply source to provide the SET and RESET voltages. In the variation shown, the PCM controller  1200  is disclosed as a multiplexer that addresses the individual PCM mask elements  1210 ,  1220 . Other variations may address the mask elements by column, by row, or may address the individual elements in non-multiplexed ways. In one variation, each mask element may have a separate signal pathway between it and a PCM controller. 
         [0039]    In another variation, shown in  FIG. 1   d , the percentage of light transmitted through each pixel  1220 ,  1230  of the PCM mask  1200  can be adjusted by controlling  1920  a laser power. In one variation, a CW laser  1900  crystallizes the material while a pulsed laser  1910  re-amorphizes it. In the variation shown, a PCM controller  1920  may be equipped with or connected to two different laser sources  1900 ,  1910 . A first laser source may be a continuous wave (CW) laser  1900  whereas a second laser source  1910  may be a pulsed laser. In other variations, the PCM controller  1920  may be equipped with or connected to a single laser such as a femtosecond laser. 
         [0040]    In some variations, reversible switching in PCM can be accomplished, in some variations, by crystallizing with a laser in CW mode and then re-amorphizing with a 40 ns pulse laser at 16 mW. Other variations may use different types or intensities of lasers to SET and RESET the PCM. 
         [0041]    In some variations using PCM GST (Ge 2 Sb 2 Te 5 ) films, for example, for optical excitation, the energy density required to SET and RESET are: SET ˜24 mJ/cm2 and RESET˜50 mJ/cm2. Molecular dynamics indicate quenching GST at dT/dt=−15K/ps produces crystalline/amorphous phase transitions. 
         [0042]    In some optically switched variations of a coded PCM mask, the mask may be equipped with multiple waveguides or optical fibers feeding each individual pixel to change their properties at adjustable laser power levels. 
         [0043]    In some variations, the elements of a PCM coded mask may be smaller than the individual pixels in an FPA (focal plane array) covered by the mask. In some cases, a “superpixel” made up of smaller individual PCM coded mask elements may be coded onto one pixel of an FPA or other photodetector. In some variations, a single “pixel” of a PCM coded mask may be  10  microns or smaller. In some variations, the size of the “superpixel” may not necessarily match the pixel size of an underlying detector pixel. In some variations, the “superpixels” may be 2-dimensional arrays of PCM elements. The sizes of such arrays may match those of underlying detector pixels or may have larger or smaller sizes depending on an intended use or desired effect(s) of the PCM coded mask. Such a variation is shown in  FIG. 2   a.    
         [0044]    In the variation shown, a PCM coded mask may be made up of multiple individually controllable PCM elements  2100 ,  220 . Such PCM elements may be aggregated into “superpixels”  2000 . In some variations, such PCM coded mask superpixels  2000  may be equipped with a particular pattern that establishes a transmission pattern within the superpixel  2000 . In some variations, such a pattern may be repeated in some or all of the superpixels  2000 ,  2300 . In some variations, different patterns may be applied to different superpixels  2400  in the PCM coded mask. In some variations, a superpixel in such a PCM coded mask may be controlled by a PCM controller (not shown) by setting a particular predetermine or otherwise preconfigured pattern onto the superpixel. In some such variations, the PCM controller and/or the superpixel(s) of the PCM coded mask may be equipped with one or more preconfigured or otherwise predetermined mask patterns that may be triggered by a particular signal or signal set transmitted from the PCM controller to a superpixel. In other variations, the PCM controller may address each PCM pixel element  2100 ,  2200  individually. In further variations, the PCM controller may optionally address a PCM superpixel  2000  to establish a particular pattern in the superpixel  2000  or address individual PCM coded mask elements  2100 ,  2200  to establish a particular refractive index or transmission state of that element. 
         [0045]    In one particular variation, a coded PCM mask element may be an individually addressable square element measuring 10 microns on a side. In some variations, such individually addressable elements may be aggregated into 16×16 superpixels that each cover one detector pixel of an FPA. In some variations, the coded PCM mask may include a 512×512 array of such 16×16 superpixels. In further variations, each superpixel in such an array may be equipped or otherwise configured with a particular masking pattern. 
         [0046]    In some variations, a superpixel  2000  may correspond to less than one pixel of an FPA. In some such variations, a group of superpixels may correspond to one or more pixels of an FPA. In other variations, a superpixel may correspond to more than one pixel of an FPA and/or have a shape or arrangement that does not overlap directly with an FPA pixel. For example, an FPA having a 30-micron pixel pitch may be covered by a coded PCM mask equipped with superpixels measuring 20 microns by 40 microns. In other variations, the superpixels may not all be the same shape. In some variations, the superpixels may not be square or rectangular. In some variations, the superpixels may have irregular shapes, such as L-shapes. Such variations are depicted in  FIG. 2   b.    
         [0047]    In one variation, irregularly-shaped coded PCM mask superpixels  2900 ,  2920  may be configured to fit together to form a rectangular shape that may be associated with a size of one or more underlying pixels. In some variations, such superpixels  2900 ,  2920  may have different PCM masks. Such different masks may complement each-other or may be individually determined. In other variations, such superpixels  2950 ,  2940  may have the same masks. 
         [0048]    In some variations, the PCM mask superpixels may be asymmetrically shaped  2960 ,  2930  and may or may not be configured to form regular shapes such as squares, rectangles, triangles, or other polyhedrons. In such variations, the mask of each superpixel  2960 ,  2930  may be individually established or separately controlled. 
         [0049]    In further variations, the PCM mask superpixels may be irregular shapes such as “cross” type shapes  2970 ,  2910 , step-sided pyramids, t-shapes, s-shapes, or other shape variations. In some variations, such superpixels  2910 ,  2970  may be shaped differently from each-other. In some variations, such superpixels  2910 ,  2970  may have different mask patterns. 
         [0050]    Although discussed so far with respect to only two states (transmission/absorption, or on/off), some PCM mask variations also allow for graded/scaled masking instead of just switching mask elements/pixels into “on” and “off” states. In some variations large refractive index changes (delta n ˜2.4) can be achieved. In some variations, refractive index can be tailored from n˜3.8 in the amorphous to n˜6.2 in the hexagonal crystalline via meta-stable face centered cubic transition of the material structure.  FIG. 3   a  depicts such a range of refractive indexes for PCM GST in amorphous and crystalline states. 
         [0051]    The graph in  FIG. 3   a  shows the refractive index (n) and extinction coefficient (k) dispersion of Ge 2 Sb 2 Te 5  at the two extremes (amorphous and hexagonal crystalline phases). The extinction coefficient is related to the absorption. The index is shown by the solid line and the extinction coefficient by the dotted lines. As can be seen in the diagram, the refractive index of the PCM may vary based on a desired wavelength or wavelength range. In the variation shown, an index of refraction is depicted for near infra red (NIR) and mid-wave infra red (MWIR) imaging. As is shown in the diagram, for imaging wavelengths from approximately 1 to 5 microns, the amorphous PCM has an extinction coefficient of ˜zero, making it essentially transparent to IR radiation. By contrast, the crystalline PCM has an extinction coefficient greater than zero, making it essentially opaque (or lossy) to IR radiation. Such variations may be realized using materials such as GST (Ge 2 Sb 2 Te 5 ) or other materials on the Ge-Sb-Te system or other PCM compositions 
         [0052]    Using a PCM coded mask as described herein for compressive sensing enables the optical design to be greatly simplified because there is no mechanical actuation of reflective elements and therefore a much simpler optical design. Furthermore, because the solution discussed herein operates in transmission (as compared to reflective designs using DMA) allowing for a simpler optical configuration; it does not require mechanical actuation (on/off states can be achieved by a phase transition from amorphous to crystalline state and design architecture) and can be adapted to the encoding scheme at the same spatial and/or temporal rate as the desired image/video reconstruction (it can be reconfigured/switched at nanosecond speeds using an external laser or voltage stimuli). This is so because switching times can be controlled by changing/optimizing the film structure in which the PCM layer is deposited. 
         [0053]      FIG. 3   b  shows a chart indicating changes in reflectivity of an example of a PCM film stack using GST based on changes to an underlying layer of ZnS-SiO 2 . As can be seen from the chart, an initially crystallized PCM region  300  may have a reflectivity normalized to 1. When amorphized by either an appropriate voltage or laser stimulus  310 , the reflectivity may drop to approximately 0.8, with thicker layers of ZnS-SiO 2  being associated with a higher reflectivity. Subsequently, when re-crystallized  330 , a PCM layer disposed on a thicker region of ZnS-SiO 2  recovers its reflectivity more quickly. In cases where the ZnS-SiO 2  is less than a certain thickness  320 , a PCM film stack may have some difficulty in recovering an initial reflectivity. In some cases, even after a significant time period (160 nsec or more), reflectivity may not be recovered. 
         [0054]    As can be understood from the diagram in  FIG. 3   b , there is flexibility in the design of a PCM coded mask architecture that can be used to optimize or otherwise configure the device properties. Although the example above depicts a particular film stack configuration using GST over ZnS-SiO 2 , other materials and material combinations may be used. Similarly, although the example shown varies the thickness of the ZnS-SiO 2  to change the film stack properties, the composition of that layer (and/or other layers) and the thicknesses of other layers (such as, for instance, the GST layer) may also be altered to change the properties of the PCM film stack. 
         [0055]    As can be seen in the diagram of  FIG. 3   b , switching time for changing from crystallized to amorphous and back to crystallized states can be realized in as little as ˜15 nanoseconds. In some cases, amorphization may be realized in less than one nanosecond and crystallization may be realized in under 15 nanoseconds. Such fast switching time enables the creation and use of PCM coded masks for that can be reconfigured at fast switching times (few nsec as compared to millisecond for the DMA), providing the ability for better image reconstruction and quality, especially when the target object is moving. 
         [0056]    In the variation shown, the 15 nm, 25 nm, and 50 nm thicknesses of ZnS-SiO 2  require fluences of 52 mJ/cm 2 , 47 mJ/cm 2 , and 31 mJ/cm 2 , respectively, to achieve the transition from crystallized to amorphous states. Such fluence levels may be realized with nanosecond or femtosecond lasers. 
         [0057]    Also, in the variation shown, the third layer of the PCM material stack is Aluminum. In other variations, this layer may be omitted or replaced with materials such as doped silicon or indium tin oxide (ITO). Material composition of the underlying layers of a PCM material stack may be determined based on a desired wavelength or waveband of electro-optical radiation to be detected by an underlying photodetector. Doped silicon and ITO, for example, are transparent to infra-red radiation. 
         [0058]    Only exemplary embodiments of the present invention are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. Such variations are not to be regarded as departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims: