Flat optics presents a new path to control the phase, amplitude, and polarization state of light with ultracompact devices. Here we demonstrate chip-integrated metasurface devices for polarization detection of mid-infrared light with arbitrary polarization states. Six high-performance microscale linear and circular polarization filters based on vertically stacked plasmonic metasurfaces (with total thickness ) are integrated on the same chip to obtain all four Stokes parameters of light with high accuracy. The device designs can be tailored to operate at any wavelength in the mid-infrared spectral region and are feasible for on-chip integration with mid-infrared (mid-IR) photodetectors and imager arrays. Our work will enable on-chip mid-IR polarimeters and polarimetric imaging systems, which are highly desirable for many applications, such as clinical diagnosis, target detection, and space exploration.
© 2019 Chinese Laser Press
Polarization state is one of the most important properties of light and essential for applications such as communication [1,2], remote sensing , astronomy , polarization imaging , polarization navigation [6–8], chemical analysis , and biomedical diagnosis [10–12]. Polarization detection in the mid-infrared (mid-IR) spectral range (3–12 μm) is especially attractive due to its broad applications in molecular spectroscopy [13,14], biomedical diagnosis [15,16], target detection [17,18], and face recognition [18,19]. Conventional mid-IR polarization detection methods require rotating optical components, which are bulky and difficult for device integration and miniaturization [19–21]. Moreover, these systems are also limited in measurement speed and accuracy of polarization state; therefore, it is highly desirable, though challenging, to realize mid-IR polarimetric detection using a compact and economic system. Monolithically integrated polarimetric imaging systems in visible and IR wavelength ranges have been demonstrated [22–25] by stacking micropolarization filters on top of silicon photodetectors. In these devices, the incoming light is filtered by the spatially distributed microscale polarizers before being collected by the imaging detectors. Such a spatial division measurement scheme avoids the requirement of moving parts, which makes on-chip integration much easier and reliable. The spatial division approach requires four to six different types of polarization filters, including both linear polarization (LP) and circular polarization (CP) filters, to measure the polarization state of light at each pixel. Various types of polarization filters have been used, such as birefringent materials , thin-film polarizers [23–26], and metallic nanowires [27–29]. Yet the applications of these polarimetric imaging systems are hindered by various limitations. Methods based on organic materials, such as liquid crystal polymer [30,31] and chiral organic molecules , are incompatible with scalable manufacturing technology, structurally and chemically unstable, and highly absorptive in mid-IR spectral regions. On the other hand, polarimetric imaging sensors utilizing metallic nanowires [21,27–29] as LP filters have been proven feasible from visible to mid-IR wavelength ranges; however, these devices are not suitable for complete polarization state (full-Stokes) measurement due to the lack of CP filters.
Recently, flat optics based on metamaterials and metasurfaces opens a new path for on-chip polarization detection as a result of the ultracompact form factors, great design flexibility, and broad wavelength coverage [33–41]. Among these designs, artificial three-dimensional (3D) chiral plasmonic metamaterials in the forms of helical and spiral shapes [33,34] can differentiate the handedness of circularly polarized light (CPL) with a circular polarization extinction ratio (CPER) up to 15 but face serious challenges in scalable manufacturing due to structural complexity. In comparison, CP filters made of planar metasurfaces, such as gammadions , Z-shaped antennas , nanoslits , and twist-stacked multiple-layer structures (crosses , nanorods [40,41], etc.), are more suitable for scalable fabrication, yet with relatively low CPER (). Moreover, planar metasurface structures, such as phase-gradient nanoantenna arrays [42,43], plasmonic aperture antennas , and dielectric metasurfaces [44–46], have also been designed to realize full-Stokes polarization detection in visible and near IR wavelength regions. Among these designs, dielectric metasurfaces provide the best efficiencies, yet scaling up the operation wavelength to the mid-IR wavelength range would require growth and etching of thick dielectric layers, which can be challenging for device fabrication and integration. Phase-gradient nanoantenna arrays are most promising for extending into the mid-IR wavelength range because they have demonstrated the best polarization measurement accuracy (measurement errors of Stokes parameters ) [42,43] and broad operation bandwidth (200–300 nm); however, these devices work in reflection/diffraction mode and thus are challenging for direct integration onto photodetectors and imaging sensors. More recently, polarization detection based on electrically tunable graphene-integrated metasurfaces has been demonstrated at mid-IR (6.8 μm) ; yet the devices still work in reflection mode and thus are not feasible for monolithic integration on photodetectors and imaging sensors. To date, the realization of on-chip mid-IR polarimetric detection and imaging arrays remains elusive.
Here we demonstrate experimentally full-Stokes polarization detection in the mid-IR wavelength range based on chip-integrated plasmonic metasurfaces operating in transmission mode. We have adopted the spatial division scheme and implemented six micrometer-size metasurface polarization filters, including both LP and CP filters, on the same chip to measure the full-Stokes parameters of incident light with arbitrary polarization. Our design is featured with an ultracompact form factor and high measurement accuracy and capability of monolithic integration onto a variety of mid-IR photodetectors and imaging arrays.
2. DEVICE DESIGN
The polarization detector design proposed here [Fig. 1(a)] is based on a spatial division measurement scheme. It is composed of six spatially distributed microscale polarization filters ( to ) and one unstructured cell () without any filter. All seven cells have the same area. The unstructured cell is used to transmit the total intensity of incident light . The LP filters (, , , and ) are based on metallic nanogratings oriented in four different directions to pass LP components oriented 0°, 90°, , and 45° with respect to -axis, respectively. Two CP filters ( and ) transmit right-handed circular polarization (RCP) and left-handed circular polarization (LCP), respectively.
By detecting the transmitted light intensity through each cell separately, one can retrieve all the polarization components in the input light. The Stokes parameters (, , , and ) of the input light can be calculated as 
Such a device design can enable detecting an arbitrary polarization state of the incident light, including partially polarized light . The Stokes parameters satisfy the relation for fully polarized light and for partially polarized light. To perform complete measurement of the arbitrary polarization states with high accuracy, one needs to realize four LP and two CP polarization filters with high extinction ratios. In the following, we will present the design and experimental demonstration of individual polarization filters, followed by the overall performance characterization of Stokes parameter detection of light with an arbitrary polarization state.
While the design and fabrication of LP filters such as metallic nanogratings are straightforward, chip-integratable CP filters in mid-IR wavelength range with high CP extinction ratio (CPER) are much more challenging to realize. Here, inspired by the extraordinary CP light vision of Stomatopods (or mantis shrimps) [49,50], we design mid-IR CP filters with high CPER based on ultrathin double-layer plasmonic metasurfaces (). The CP light vision of Stomatopods originates from the unique structures of their compound eyes [50,51]. In each compound eye, a top retinular cell and seven bottom retinular cells form a natural CP filter. The top retinular cell acts as a quarter-wave plate (QWP) to convert CPL to linearly polarized light (LPL) while the microvilli of bottom retinular cells function as wire-grid polarizers, which are oriented 45° with respect to the long axis of the QWP. Inspired by such a simple configuration, we designed bio-inspired right-handed circular polarization and left-handed circular polarization filters based on vertically integrated metasurfaces, as shown in Figs. 1(b) and 1(d). Both the RCP and LCP filters have two key elements, i.e., a plasmonic metasurface QWP composed of cross-shaped antennas to mimic the top retinular cell and a nanograting LP filter to mimic the bottom retinular cells. By vertically integrating the two layers of achiral planar plasmonic structures, we create a chiral plasmonic structure, which is not superimposable on its mirror image. For RCP filter design [Fig. 1(c)], the input RCP (LCP) light is first converted to LPL oriented at an angle of 45° () with respect to the -axis via the metasurface QWP. Then the nanograting LP filter selectively transmits the LPL oriented at 45° while blocking the LPL oriented at , thus resulting in a much higher transmission for RCP than LCP light. For the LCP filter, the plasmonic metasurface QWP remains the same but the nanograting polarizer is instead oriented at a 45° angle with respect to the -axis, leading to a much higher transmission for LCP than RCP light.
The plasmonic metasurface QWPs are designed based on the fact that an abrupt phase shift is introduced between the incident light and scattered light when light interacts with optical antennas. The phase shift is wavelength-dependent and can be controlled by engineering the antenna geometry . Here the metasurface QWPs are composed of cross-shaped antennas with different arm lengths [Fig. 2(a)]. For incident electric fields and , the resonance “dips” in the transmission spectra [Fig. 2(b)] correspond to the first-order resonance modes for the linear antennas oriented along - and -axes, respectively. Due to the different resonance wavelengths, the scattered fields along the two axes experience different phase shifts, especially in the wavelength region between the two antenna resonances.
By engineering the dimension of the cross-shaped antenna array, we can introduce a phase difference between and with the same transmission coefficients at the wavelengths roughly in the middle of the two antenna resonances. The bottom panel in Fig. 2(b) shows the phase difference introduced by the wavelengths roughly in the middle of the two antenna resonances. The bottom panel in Fig. 2(b) shows the phase difference introduced by the metasurface QWP calculated via full-wave finite difference time-domain (FDTD) simulation for an operation wavelength of around 3.8 μm. The operation wavelength of the metasurface QWP can be engineered from mid-IR (MIR) to near-IR (NIR) by changing the geometric parameters of the cross-shaped antenna (see Appendix A for more details). The nanograting polarizers are designed to provide a high linear polarization extinction ratio (LPER), i.e., , as indicated in Fig. 2(c). We have designed a gold nanograting LP filter with an at and 90% transmission of selected LPL () for wavelengths longer than 1.5 μm. Even higher LPER can be achieved by engineering the nanograting design parameters, such as the grating duty cycle and thickness.
The double-layer plasmonic CP polarization filters [Fig. 2(a)] are composed of a gold plasmonic metasurface QWP, a gold nanograting LP filter, and a dielectric spacer layer (silicon oxide) between them. The thickness of the spacer layer is chosen to maximize the CPER of the device (see more details in Appendix C). Meanwhile, the separation between the metasurface QWP and the nanograting LP filter ensures that the near-field interaction between the two plasmonic layers does not significantly change their optical response. Moreover, it also helps to smooth the surface of the nanogratings to facilitate subsequent integration of the QWP on top. We also investigated the impact of the lateral displacement between the metasurface QWP and nanograting LP filter. Full simulation results show that our device designs do not rely on accurate alignment between the two to realize high CPER performance (see more details in Appendix D), which reduces the complexity of device fabrication. The total thickness of the whole structure is close to 600 nm, about one-sixth of the operational wavelength. Figure 2(d) shows the transmission spectra of RCP and LCP input light for an RCP filter designed at an optimal operation wavelength of 3.8 μm. The extracted CPER reaches a maximum value of over 300 and provides high CPER () over a bandwidth of 150 nm [Fig. 3(a)]. The operational wavelength of the CP filters can be engineered by changing the geometric parameters of the structure. Figure 3(b) shows the tuning of operational wavelengths from 1.6 to 3.8 μm as a function of the longer arm length () of the cross-shaped antenna (see more details in Appendix B).
3. EXPERIMENTAL RESULTS
We first fabricated the CPL filters on sapphire substrates, which have a mid-IR transmission cutoff wavelength (around 5 μm) longer than the designed operational wavelengths. The fabrication process is illustrated in Fig. 4(a). First, gold nanogratings with 200 nm period, 50% duty cycle, and 120 nm thickness were patterned on the substrate with electron beam lithography (EBL), metal film deposition (Cr/Au 5/120 nm), and metal lift-off. Figure 4(b) shows a scanning electron microscopy (SEM) image of the fabricated nanogratings. After solvent cleaning and brief Ar plasma cleaning, a 350 nm thick silicon oxide spacer layer was deposited on top of the nanogratings via a sputtering process. Atomic force microscopy (AFM) images of the nanogratings covered by the oxide spacer layer [Fig. 4(c)] indicate a dramatically reduced surface roughness (). The greatly reduced surface roughness facilitates the subsequent integration of plasmonic metasurface QWP. The metasurface QWPs were fabricated on top of the nanogratings by EBL, metal deposition (Cr/Au 3/50 nm), and lift-off. Figure 4(d) shows an SEM image of fabricated CPL polarization filters for an operation wavelength around 4 μm.
We characterized the metasurface CPL filters with a setup as illustrated in Fig. 5(a). Unpolarized light from a Fourier transform infrared spectrometer (FTIR) first passed through a linear polarizer and a QWP to generate CPL (at wavelengths close to 4 μm). The handedness of CPL was controlled by adjusting the angle between the optical axes of the linear polarizer and the QWP. The measured transmission spectra for LCP and RCP incident light of a fabricated RCP filter are shown in Fig. 5(b). The extracted CPER is over 16 at the best operational wavelength (), as shown in Fig. 5(c). Our device provides high CPER () over a wavelength range of 300 nm (from 3.8 to 4.1 μm). The relatively lower transmission () of the selected polarization state compared to simulated results [Fig. 2(d)] is likely due to higher loss introduced in the plasmonic metasurface QWP and nanogratings as a result of the structural deviations of fabricated devices from the optimal design parameters. There is still much room to further improve the performance of the CP filters by taking into account the difference in material parameters between simulation and experiments as well as more accurate control of structure geometries.
To demonstrate complete polarization state detection, we integrated both CP polarization filters and four LP filters on the same substrate [Fig. 1(a)]. The polarization detection was performed with the setup shown in Fig. 6(a). The output light from FTIR (unpolarized) first passed through a linear polarizer and a QWP. The polarization state of light (at the operation wavelength around 4 μm) was controlled by rotating the linear polarizer and the QWP. A reflective condenser was used to focus light on the device. To avoid the impacts of the condenser on the polarization state of light, the QWP was placed between the condenser and the device under test. The transmitted light through the sample was then collected by an MCT photodetector, with the region of interest selected by an aperture placed at the conjugated image plane. Such a setup allowed us to characterize light transmission through microscale areas down to with sufficient signal to noise ratio (SNR) (see Appendix E). To precisely determine the generated polarization state, we first calibrated all LP and CP filters to obtain their transmission efficiencies and extinction ratios for each filter. Then polarization measurements were carried out for input light with arbitrary polarization states. For each polarization state, the transmitted light of all filters was measured sequentially by controlling the lateral displacement of the motorized stage. As discussed previously [Fig. 1(a)], we obtained all the polarization components of the input light to achieve all four Stokes parameters. To determine the accuracy of the measured Stokes parameters, we replaced our device with a rotating linear polarizer as a polarization analyzer (PA) to obtain the Stokes parameters as a reference (see more details in Appendix F). Since all generated light was purely polarized, we were able to obtain both polar plots [left panels in Figs. 6(b)–6(d)] and polarization ellipse plots for a number of input polarization states [right panels in Figs. 6(b)–6(d), more polarization states provided in the Appendix F]. The measurement results obtained by our device (red lines) agreed well with reference results (black circles) measured by the rotating PA. The blue arrows on the ellipse plots indicate the handedness of the circularly and elliptically polarized light determined with our device, which is not available from the measurement taken by the rotating PA. Based on all nine polarization states we have characterized, the average errors for orientation angle and elliptical angle are 1.2° and 1.9°, respectively. Figure 6(e) shows the measured Stokes parameters (normalized by ) with our device and those obtained by rotating PA for nine polarization states. The average errors of and are 0.035 and 0.025, respectively. The average error of is 0.104, which is limited by the CPERs of the CP filters. From the measurement results of the Stokes parameters, we also extracted the average measurement error for DOLP and DOCP to be 2.3% and 10.3%, respectively.
To the best of our knowledge, our devices exhibit the best polarization measurement accuracy among all structures in the literature so far [42–45]. The measurement accuracy can be further improved via increasing the extinction ratio of all micropolarization filters and reducing the insertion loss. As a proof of concept demonstration, we use a motorized stage to characterize our device. The CP and LP filters demonstrated here operate in transmission mode and can be monolithically integrated onto various mid-IR photodetector arrays and imager arrays to achieve on-chip polarimeter and polarimetric imaging sensors, where the incident light is filtered by the LP and CP filters and then collected by the photodetectors beneath them. Thus, these chip-integrated devices can perform full-Stokes polarization detection and capture polarimetric images without moving parts.
In this paper, we present theoretical modeling and experimental demonstration of complete mid-IR polarization detection with a microscale filter array based on subwavelength-thick plasmonic metasurfaces. Unlike conventional polarization detectors requiring bulky and complex systems, our design can be fabricated monolithically on a single chip with a total device layer thickness of about 600 nm. We experimentally demonstrated full-Stokes parameter detection for arbitrary polarization at mid-IR wavelengths with the best measurement accuracy reported in the literature so far, to the best of our knowledge. Additionally, the design concepts can be applicable over a broad wavelength range by engineering the design parameters and thus are promising for multi-wavelength or broadband polarization detection. Moreover, the metasurface device design is compatible with conventional semiconductor substrates, such as silicon, III-V, and II-VI semiconductors and can be integrated with photodetector arrays on the same chip. Therefore, our technique has great potential to enable mid-IR on-chip polarimetric imaging systems and polarimeters, which are highly desirable for many applications, such as biomedical imaging, target detection, material characterization, and molecular spectroscopy.
APPENDIX A: OPERATION WAVELENGTH ENGINEERING OF THE METASURFACE QWP
The operation wavelength of the metasurface QWP can be engineered by changing the design parameters. One of the critical requirements to design a QWP is introducing a phase difference between two orthogonal electric field components with equal amplitude (i.e., and ). FDTD simulations are performed to optimize different metasurface QWP designs with operational wavelengths from NIR to MIR by scaling the geometric dimensions of the metasurface QWP (Fig. 7). The corresponding design parameters of the metasurface QWP at different wavelengths are shown in Fig. 8.
APPENDIX B: WAVELENGTH ENGINEERING OF METASURFACE CPL FILTERS
FDTD simulations are performed to optimize CPL filter designs at operation wavelengths from 1.6 to 3.8 μm. The extinction ratios of six LCP filters for different wavelengths are shown in Fig. 9.
APPENDIX C: SPACER LAYER THICKNESS AND DEVICE PERFORMANCE
We performed full-wave simulations to determine the spacer layer thickness for the highest CPER (Fig. 10). The dependence of device performance on the spacer layer thickness is mainly due to the multiple reflections at the top metasurface layer (QWP) and the bottom grating layer (linear polarizer).
APPENDIX D: LATERAL DISPLACEMENT AND DEVICE PERFORMANCE
We investigated the influence of the lateral displacement between the top metasurface QWPs and the bottom nanogratings [illustrated in Fig. 11(a)]. According to full-wave simulation results, the CPERs of the device around the operation wavelength remained higher than 200 [Fig. 12(a)], when the displacement between the QWP and the nanogratings linear polarizer was shifted over 200 nm along the -axis.
APPENDIX E: CPL FILTER CHARACTERIZATION AND STOKES PARAMETER DETECTION
For CP filter characterization, a linear polarizer and a mid-IR QWP (Thorlabs, WPLQ05M-4000) were used to generate CP states, which were verified by a rotating linear polarizer. Then the generated CP light was transmitted through the device under test, collected by an objective (15×), and detected by an MCT detector. An aperture (a built-in feature of HYPERION Series FTIR Microscopes, Bruker Inc.) of at the image plane in front of the MCT detector was used to select the measurement area on the device (Fig. 12).
For Stokes parameter detection, the transmission coefficients of the CP and LP filters were first calibrated with CP and LP input light, respectively. Then the input linear polarizer and QWP were rotated to generate arbitrary polarization states of light. The transmitted light through each of the seven detection units [Fig. 1(a)] was sequentially measured utilizing the aperture and the computer-controlled motorized stage. The transmitted light through each filter was divided by the corresponding calibration data to consider different transmission coefficients of the polarization filters. We also characterized the generated polarization states with a rotating linear polarizer to obtain the reference data. Note that with only one linear polarizer, one can only measure the linear components of Stokes parameters and but not , the sign of which indicates the handedness of polarized light. In order to obtain , both the QWP and the linear polarizer are required as rotation components, which is not feasible for us due to the space limitation of our setup.
APPENDIX F: STOKES PARAMETERS DETECTION RESULTS
Besides the experiment results for Stokes parameters detection shown in Figs. 6(b)–6(d), the measurement results for other polarization states are shown [Figs. 13(a)–13(d)]. The polar plots and the ellipse plots correspond to B, F, G, and H in Fig. 6(e).
U.S. Air Force (FA9550-16-1-0183); Directorate for Engineering (1542160, 171141, 1809997); National Science Foundation (NSF) (ECCS-1542160); Arizona State University.
The devices were fabricated in the NanoFab and Eyring Materials Center (EMC) at Arizona State University. P. Amrollahi from Arizona State University participated in the device fabrication for SiO2 sputtering. Y. Yao and C. Wang acknowledge support from Arizona State University.
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