The present disclosure is related to polarimetric imaging, and more particularly to devices including metasurface masks for full-stokes division of focal plane polarization of cameras.
Polarization is a degree of freedom of light carrying important information about the light source, the surfaces that the light has been reflected off, or the materials that the light has passed through. Such information is usually absent in intensity and spectral content. The state of polarization is typically described by the four Stokes parameters.
Imaging polarimetry is the process of determining the polarization state of light, either partially or fully, over an extended scene. It has found several applications in various fields of science from remote sensing to biology. Among different devices for imaging polarimetry, division of focal plane polarization cameras (DoFP-PCs) are more compact, less complicated, and less expensive. In recent years, there have been significant improvements in the performance of DoFP-PCs. However, an unresolved limitation is that they can only partially measure the state of polarization, as the degree of circular polarization and helicity are two important properties of polarization that DoFP-PCs conventionally miss. This means that circularly polarized and unpolarized light appear the same for the current DoFP-PCs.
Generally, polarimetric imaging techniques can be categorized in division of amplitude, division of aperture, and division of focal plane. All of these methods are based on measuring the intensity in different polarization bases and using them to estimate the full Stokes vector (i.e. a vector containing information about all four Stokes parameters) or part of it. Among various systems, DoFP-PCs are less expensive, more compact, and require less complicated optics. In addition, they require much less effort for registering images of different polarizations as the registration is automatically achieved in the fabrication of the polar-ization sensitive image sensor. The advances in micro/nano-fabrication have increased the quality of DoFP-PCs and reduced their fabrication costs, making them commercially available. DoFP-PCs either use a birefringent crystal to split polarizations, or thin-film or wire-grid polarization filters.
The main problem with all the above-mentioned methods is that they only work for linear polarization bases, and therefore, as already noted above, cannot measure the degree of circular polarization and helicity. Although form-birefringent quarter waveplates can be integrated with linear polarizers to make circular polarization filters in the mid-IR, their performance as DoFP-PCs with full-Stokes characterization capability has not been disclosed. A secondary issue with current DoFP-PCs is that they all have a theoretical efficiency limit of 50% due to using polarization filters, or spatially blocking half of the aperture.
The disclosed methods and devices address the described challenges and provide practical solutions to the above-mentioned problems.
According to a first aspect of the disclosure, a metasurface-based electromagnetic wave splitting device is provided, comprising: a substrate, and an array of nano-posts on the substrate, the nano-posts having C2-symmetric shapes; wherein: the nano-posts are configured to split an incident electromagnetic wave into a plurality of polarization bases and to focus the split incident electromagnetic wave onto a plurality of target areas according to the plurality of polarization bases.
According to a second aspect of the disclosure, an imaging method is disclosed, comprising: providing an array of nano-posts resting on a substrate; providing an image sensor including a superpixel, and applying light to the array of nano-posts, wherein dimensions of the nano-posts, orientations of the nano-posts, and distances between adjacent nano-posts are configured to: scatter the light off the array of nano-posts; split the light into a plurality of polarization bases, and focus the light onto pixels of the superpixel according to the plurality of polarization bases.
Further aspects of the disclosure are provided in the description, drawings and claims of the present application.
Throughout the present disclosure, the term “superpixel” is used to refer to a combination of several pixels of an image sensor. For example, three pairs of adjacent pixels represent a “superpixel”.
Throughout the present disclosure, the term “C2 symmetry axis” of an object is used to refer to an axis around which a rotation by 180° results in an object indistinguishable from the original. This object is referred to as an object having a C2-symmetric shape.
Throughout the present disclosure the terms “nano-post” or “nano-scatterers” are used to refer to a miniaturized scatterer object having dimensions that are substantially comparable with the operational wavelength of the device implementing such a scatterer.
Optical dielectric metasurfaces are a category of micro-fabricated diffractive optical elements comprised of dielectric nano-scatterers on a surface, judiciously designed to control the wavefront. They have enabled high-efficiency phase and polarization control with large gradients. In addition, their compatibility with conventional microfabrication techniques allows for their integration into optical metasystems.
Metasurfaces have previously been used for polarimetry, but not polarimetric imaging. As disclosed in the above-mentioned U.S. Pat. No. 9,739,918 B2, a notable capability of high contrast dielectric metasurfaces is the simultaneous control of polarization and phase. Such concept is adopted by the teachings of the present disclosure to build metasurface masks for DoFP-PCs with the ability to fully measure the Stokes parameters, including the degree of circular polarization and helicity. Instead of polarization filtering, the disclosed methods and devices are based on splitting and focusing light in, for example, three different polarization bases. Such an approach makes the full-Stokes characterization of the state of polarization possible while overcoming the 50% theoretical efficiency limit of the polarization-filter-based DoFP-PCs as described previously.
There are several representations for polarization of light. Among them, the Stokes vector formalism has some conceptual and experimental advantages since it can be used to represent light with various degrees of polarization, and can be directly determined by measuring the power in certain polarization bases. Therefore, most imaging polarimetry systems determine the Stokes vector, which is usually defined as S=(1/I)[I, (Ix−Iy), (I45−I-45), (IR−IL)], where I is the total intensity, Ix, Iy, I45, and I-45 are the intensity of light in linear polarization bases along the x, y, +45-degree, and −45-degree directions, respectively. IR and IL denote the intensities of the right-hand and left-hand circularly polarized light. To fully characterize the state of polarization, all these intensities should be determined.
As described in the above-mentioned U.S. Pat. No. 9,739,918 B2, an optical metasurface with the ability to fully control phase and polarization of light can perform the same task over a substantially smaller volume and without changing any optical components. The metasurface can split any two orthogonal states of polarization and simultaneously focus them to different points with high efficiency and on a micro-scale. This is schematically shown in
According to an embodiment of the present disclosure, the metasurface of
With reference to
With reference to
The operation of a nano-post can be modeled by a Jones matrix related the input and output electric fields (i.e. Eout=TEin). For the rotated nano-post (401) shown in
where R(θ) denotes the rotation matrix by the angle θ as shown in
where * is used to show complex conjugation. Based on equation (2), the Jones matrix to transform any input field with a given phase and polarization to any desired output field with a different phase and polarization can be calculated, and therefore a complete and independent phase and polarization is made possible through such equation.
In the case where the determinant of the matrix on the left hand side of equation (2) is zero, the following can be obtained:
wherein φ1 and φ2 represent the phase relation between the input and output polarizations. Equation (3) essentially indicates that given any two orthogonal input polarizations (E1in,E2in), their phases can be independently controlled using the Jones matrix given by equation (3).
When the Jones matrix is calculated from equation (3) (or equation (2) depending on the functions), the two phases (φx′,φy′) and the rotation angle (θ) can be calculated from equation (1) by finding the eigenvalues and eigenvectors of the Jones matrix. This can be repeated independently for each nano-post, meaning that the polarization basis can be changed from one nano-post to the next.
In order to fabricate metasurface (300) of
According to the teachings of the present disclosure, the same design principle and concept described above can also be applied to electromagnetic waves of any frequency range given the use of appropriate material systems and scaling the designs accordingly. For example, the same principles can be used to design division of focal plane polarization cameras in the visible range using silicon nitride, titanium oxide, or crystalline silicon nano-posts, in the near and mid IR ranges using amorphous or crystalline silicon, and in various ranges of far IR using different dielectric or metallic materials
A metasurface was fabricated based on the concepts detailed above. 650-nm-thick layer of α-Si was deposited on a 500-μm-thick fused silica wafer in a plasma enhanced chemical vapor deposition process. The metasurface pattern was defined using electron-beam lithography, and transferred to the α-Si layer through a lift-off process (to make a hard etch-mask) followed by dry etching.
To characterize the metasurface mask, it was illuminated with light from an 850-nm LED (filtered by a 10-nm bandpass filter) with different states of polarization, and the plane corresponding to the image sensor location was imaged using a custom-built microscope.
Using the DoFP metasurface mask described above, one could perform polarimetric imaging. To do this, a metasurface polarization mask (using the polarization-phase control method described above was fabricated. Such mask is configured to convert x-polarized input light to an output polarization state characterized by the Stokes parameters shown in the left column of
A single image was captured of the sensor-location plane in front of the DoFP mask, and the Stokes parameters were extracted from that single image. The results are shown in
The existing errors in estimating these coordinates (resulting from small tilts in the setup, aberrations of the custom-built microscope, etc.) cause a degraded performance over some superpixels. In a polarization camera made using the metasurface DoFP mask, both of these issues will be solved as the resolution can be much higher, and the mask and the image sensor are lithographically aligned. To extract the polarization information of the image, the intensity was integrated inside the area of two adjacent image sensor pixels, and the corresponding Stokes parameter were simply calculated by dividing their difference by their sum. While straightforward, this is not the optimal method to perform this task as there is non-negligible cross-talk between different polarization intensities measured by the pixels (
To address the above-mentioned issue, a better polarization data extraction method is to form a matrix that relates the actual intensity of different input polarizations to the corresponding measured values for a specific DoFP metasurface mask design. This allows one to reduce the effect of the cross-talk and measure the polarization state more precisely. The designed small distance between the metasurface and the image sensor (e.g., 9.6 μm for the 4.8-μm pixel) results in a diffraction-limited bandwidth of about 40%. Therefore, the actual bandwidth of the device is limited by the focusing and polarization control efficiencies that drop with detuning from the design wavelength. In addition, it is expected that the same level of performance achieved from the 2.4-μm pixel in this work, can be achieved from a ˜1.7-μm pixel if the material between the mask and the image sensor has a refractive index of 1.5, which is the case when the DoFP mask is separated from the image sensor by an oxide or polymer layer, as in a realistic device.
The present application claims priority to U.S. Provisional Patent Application No. 62/802,143 filed on Feb. 6, 2019 and may be related to U.S. Pat. No. 9,739,918 B2 issued on Aug. 22, 2017, titled “Simultaneous Polarization and Wavefront Control Using a Planar Device”, both disclosures of which are incorporated herein by reference in their entirety.
This invention was made with government support under Grant No. HR0011-17-2-0035 awarded by DARPA. The government has certain rights in the invention.
Number | Date | Country | |
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62802143 | Feb 2019 | US |