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Abstract
We designed and fabricated a linear polarization-separation metalens (PSM) made of single-crystal silicon (sc-Si) for long-wavelength infrared (LWIR) imaging. The PSM comprises sc-Si dielectric waveguide pillar meta-atoms with rectangular cross-sections, providing a full 2π phase delay range for two orthogonal linear polarization components with high transmittances (>70%). Electron beam lithography and deep reactive ion etching were used to fabricate the PSM. Polarization-separation imaging of elevated and ambient temperature objects was demonstrated with high extinction ratios of 21.8 dB and 12.8 dB for the x- and y-polarizations, respectively. Additionally, polarization-sensitive imaging was demonstrated by distinguishing the surfaces of a hand and toy windows. Our work enables the visualization of invisible information in the LWIR region and has widespread applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The long-wavelength infrared (LWIR) spectrum, ranging from 7 to 14 micrometers, corresponds to thermal radiation emitted by objects near room temperature as governed by Wien’s Displacement Law. This property enables thermal imaging without the need for illumination, which has led to a broad range of applications in fields such as medicine [1], automotive engineering [2], and security [3]. The development of uncooled bolometer image sensors utilizing microelectromechanical systems (MEMS) technology [4] has greatly facilitated the advancement of thermal imaging. The LWIR region offers great potential for discovering novel applications by visualizing previously unobservable phenomena.
Polarization imaging, also known as polarimetric imaging, relies on the differences between images taken with multiple polarization components, which have been used to visualize invisible information. For example, the difference in reflectance and transmittance between TE and TM polarization has been employed to detect vehicles and measure photoelastic stress distribution, respectively [5]. These measurements are primarily performed in the visible to near-infrared region, and single-shot measurements are carried out with compact optics using polarization image sensors with tiling micro polarizers at each pixel [6]. However, attempts to visualize more complex information have been made using not only linearly polarized light but also elliptically polarized light [7,8].
The application of polarized imaging to the LWIR band is challenging due to the weak intensity of the radiated light. Various efforts have been made to improve the sensitivity of uncooled microbolometers and quantum well-infrared photodetectors (QWIPs) in the LWIR region, for example, attempts have been made to enhance sensitivity by using plasmon resonators [9]. However, the use of absorptive polarizers for polarimetric imaging leads to a decrease in the signal-to-intensity ratio. This results in increased integration time, making live imaging difficult.
We propose an alternative method of applying polarization separation imaging with metasurface lenses (metalenses) to the LWIR band. Metasurfaces are flat sheets of metamaterials composed of tailored arrangements of subwavelength-scale optical elements (meta-atoms) [10–13]. Phase lattices are one of the key implementations of metasurfaces and have been used for lenses [14–24], beam deflectors [25,26], polarization converters [27–32], holographies [33–38], and other applications. Metasurfaces can control the polarization state of the emitted light by utilizing the structural birefringence of meta-atoms. Therefore, metalenses can focus and form images of two orthogonal polarization states at different locations [39–42]. Unlike conventional polarizers, this method separates orthogonal polarization components rather than absorbing them, thus increasing light utilization efficiency. Furthermore, high-performance image sensors manufactured using advanced fabrication processes can be used without modification by replacing existing lenses with metalenses. However, despite previous research on polarization-dependent focal length selection [43], polarization-separation imaging with LWIR metalenses has not been reported.
In this paper, we present the design, fabrication, and demonstration of a linear polarization-separation metalens (PSM) at the LWIR band made of single-crystal silicon (sc-Si). A full 2$\pi$ phase coverage was achieved for both two orthogonal linear polarization components at the design wavelength of 10.6 µm. We successfully achieved polarization separation with a high extinction ratio of 21.8 dB.
2. Principle
Figure 1(a) shows the schematic drawing of the imaging with a PSM. The paths of the object light transmitted through the PSM are separated into $x$- and $y$-polarization components, which form images at positions symmetrical to the $y$-axis.
Fig. 1. (a) Schematic drawing of imaging with a linear polarization-separation metalens (PSM). (b) Focal points of the PSM with a plane-wave incidence.
Figure 1(b) shows the ray diagram of the PSM with plane wave incidence. The phase profile of the PSM for $x$- and $y$-polarizations, $\phi _{x,y}(x,y)$, are given by:
3. Design and fabrication of the polarization-separating metalens
3.1 Electromagnetic field analysis for meta-atoms
We conducted an electromagnetic field analysis using a commercial finite element method software (COMSOL Multiphysics 5.6, COMSOL Inc., USA) to obtain the design parameters of meta-atoms. Figure 2(a) shows the parameters and implementation for the meta-atom simulation: $p$ for the period, $h$ for the height, $w_x$ and $w_y$ for the widths at the $x$- and $y$-directions, respectively.
Fig. 2. (a) Setup for the electromagnetic field analysis of meta-atoms. (b) Calculated phase delay. Solid and dashed contours show phase delays for $x$- and $y$-polarizations, respectively. The contour lines are shown in ten colors for each 2$\pi$ phase change, indicating a $0.2\pi$ step. Orange-red contour lines indicate phase delays of 2$\pi$, 4$\pi$, and 6$\pi$, respectively. Four thick purple contours indicate phase delays of $\phi _x, \phi _y$ are $2.2\pi$ and $4.2\pi$, respectively. (c) Transmittance for the linear $x-$polarized illumination $T_x$.
As the design wavelength, we selected 10.6 µm, which is the typical emission peak of a CO$_2$ laser, also corresponding to thermal radiation at 0$^\circ$C. sc-Si, which has a low extinction coefficient at the design wavelength, was selected as both the dielectric waveguide meta-atom and substrate material. We applied Floquet periodic boundary conditions to the $\pm x$ and $\pm y$ boundaries to express a square lattice arrangement. The input and output ports are implemented at the $-z$ and $+z$ boundaries, respectively. We used the Electromagnetic Wave, Frequency Domain (ewfd) interface of COMSOL to calculate the transmittance and phase delay of the meta-atoms.
Figure 2(b) and (c) show the results of parameter mapping for phase delays $\phi _x$, $\phi _y$, and $x$-polarization transmittance $T_x$ with varying $w_x$ and $w_y$. The detailed analysis procedure is shown in Supplement 1. In these calculations, the height $h$ and period $p$ were fixed at 20 µm and 3 µm, respectively. In Fig. 2(b), solid and dashed contours show phase delays for $x$- and $y$-polarizations, respectively. The contour lines are shown in ten colors for each 2$\pi$ phase change, indicating a 0.2$\pi$ step. Orange-red contour lines indicate phase delays of 2$\pi$, 4$\pi$, and 6$\pi$, respectively. Four thick purple contours indicate phase delays of $\phi _x$ and $\phi _y$ of 2.2$\pi$ and 4.2$\pi$, respectively. Thus, by adjusting the meta-atom dimensions within the region bounded by these four thick lines, a full phase coverage of 2$\pi$ is obtained for both $\phi _x$ and $\phi _y$. We obtained 64 different meta-atom designs by quantizing this region into $8\times 8$ points. Figure 2(c) shows the transmittance for $x$-polarized illumination, where transmittance higher than 70% was achieved for the entire region.
Based on the obtained meta-atom design library, we performed focusing simulations using the Electromagnetic Wave, Beam Envelope (ewbe) interface of COMSOL. Simulations were performed on a reduced model with the lens diameter $D$ of 250 µm ($\approx 23.6 \lambda _0$), which consists of approximately 5000 meta-atoms due to computational cost issues. The focal length $f$ and the separation $f_x$ were set to be $2D$ and $D/4$, respectively, and the numerical aperture (NA) was calculated to be 0.24. A GDSII layout file was prepared using the Python library gdstk [44].
Figure 3 shows the results of the beam envelope calculation. Figures 3(a-c) show the intensity distribution in the $x-z$ plane with incident linearly-polarized light in the (a) $x$, (b) $45^\circ$, and (c) $y$ directions, respectively. Figure 3(d) shows the intensity distribution along the $x$-axis on the focal plane. The vertical grid lines correspond to $x =\pm f_x=\pm 62.5$ µm. Successful polarization separation was obtained for $x$- and $y$-polarized light, and even intensity distribution was obtained at the two foci for the $45^\circ$ incident light. The position of $y$-focus is in good agreement with the design value and a smooth intensity distribution is obtained, while the $x$-focus shows a slight deviation and intensity disorder near the focal point. This may be due to the fineness of the computational mesh. The extinction ratios for $x$- and $y$-polarized incidences were 29.6 and 26.9 dB, respectively. The full-widths half-maximum (FWHMs) for $x$- and $y$-focal points were 18.4 and 21.5 µm, respectively. Diffraction efficiencies (Total power within 3$\times$FWHM/Total power after the lens) for $x$-, $45^\circ$, and $y$-polarized light were 67.7, 68.2, and 66.8%, respectively.
Fig. 3. Intensity distributions in the $x-z$ plane with incident linear polarizations to (a) $x$-, (b) $45^\circ$-, and (c) $y$-direction. (d) Intensity profile along $x$-axis on the focal plane for $x$-, $45^\circ$, and $y$-polarized incident light. The vertical grid lines correspond to $x =\pm f_x$.
3.2 Fabrication and implementation
The PSM was designed to replace an existing refractive lens ($f=3.7$ mm, $F_\#=1.1$) on a commercial LWIR camera (LW10F42-ET, Tamron, Japan) to simplify mounting. The lens dimensions were decided to be $D=4.25$ mm, $f=3.96$ mm, $f_x=1.00$ mm (NA $=0.47$) to have separated image circles on a MEMS bolometer image sensor (80$\times$80 pixels, 2.72-mm-square). A double-polished, 525-µm-thick non-doped silicon wafer ($> 20,000$ Ωcm) was diced into a 2-cm square chip and used as the starting material. The transmittance of the wafer at the design wavelength of 10.6 µm was measured to be 55% by a spectrometer (FT/IR-6600AC, JASCO, Japan.). Figure 4(a-c) shows the schematic of the fabrication process flow.
- (a) The lens pattern was directly drawn on the negative resist (OEBR-CAN040AE, TOKYO OHKA KOGYO, CO., LTD. Japan) by an electron beam lithography apparatus (F5112+VD01, ADVANTEST, Japan) with the exposure dose of 24 µC/cm2.
- (b) Si pillars were formed by a deep reactive ion etching (deep-RIE) apparatus (MUC-21 ASE Pegasus, SPP Technologies, Japan) with an etching depth of 20 µm. The parameter ramping technique was applied to etch deep trenches [45].
- (c) The resist was removed by an O$_2$ asher (FA-1, Samco, Japan), and the substrate was diced into 5.2-mm-square chips.
Figure 4(d) shows a photograph of the fabricated PSM. Two concentric patterns reflecting the phase profile in Eq. (1) are clearly visible. Figure 4(e, f) shows the SEM images of the edge (e) and the cross-section (f) of the PSM. We successfully etched vertically to the bottom of the deep trenches. The lens chip was mounted on a jig shown in Fig. 4(g) and assembled with the LWIR camera (LW10F42-ET, Tamron, Japan).
Fig. 4. (a)-(c) Schematic drawing of the fabrication process flow. (a) EB lithography on the Si substrate. (b) Deep-RIE. (c) Resist removal by O$_2$ asher. (d) Photograph of the fabricated PSM. (e),(f) SEM images of the edges (e) and cross-section (f) of the PSM. (g) Photograph of the camera-mount jig and the PSM diced into a 5.2-mm-square chip.
4. Results and discussion
Imaging tests were performed by the PSM-mounted LWIR camera as shown in Fig. 5. The camera has an 80 $\times$ 80 MEMS bolometer array image sensor with a pixel pitch of 34 µm $\times$ 34 µm. Figure 5(a)-(e) shows captured pictures. Note that all the pictures were captured from the live movie with 8 frames per second (fps), which is the maximum speed of the bolometer array. Figure 5(a) shows an elevated-temperature object (a soldering iron). Fine structures such as tips and screw heads can be observed. Figure 5(b) shows an ambient-temperature object (a right hand with V-sign). Although the background noise is larger than that of the soldering iron, the V shape can be observed.
Fig. 5. (a)-(e) Captured pictures with the fabricated PSM. (a) Soldering iron. (b) Right hand with V-sign. (c)-(e) 1-mm-diameter aperture illuminated by a 250 $^\circ$C hot plate with (c),(d) and without (e) linear polarizer. Polarization directions are $x$ (c) and $y$ (d), respectively. (f) Normalized intensity distributions of (c)-(e).
Figures 5(c)-(e) show polarization-separation imaging of a 1-mm-diameter aperture illuminated by a 250 $^\circ$C hotplate from the backside. In Figs. 5(c), (d), a linear polarizer is placed in front of the camera along with $x$- (c) and $y$-directions (d), while in Fig. 5(e) it is not. These figures show that the orthogonal polarization components are separated and focused. Figure 5(f) shows the normalized intensity distributions of (c)-(e) along with the $x$-axis indicated by an orange single-dash line in (e). As in the simulation, almost evenly split into two foci was observed without the polarizer. On the other hand, when the polarizer is inserted, the focus depends on the direction of the polarizer, and the extinction ratios of 21.8 and 12.8 dB in the $x$- and $y$-directions were observed, respectively. These values can be further improved by making the lens substrate thinner and by depositing an anti-reflection film on the backside of the substrate. Note that these values are better than that of the conventional micropolarizer array for LWIR imaging polarimetry (8–9.8 dB) [46]. The FWHMs for $x$-, non-, and $y$-polarized incident light were 170, 136, and 170 µm, respectively, which corresponds to 5, 4, and 5 pixels of the image sensor. These values are larger than the simulation results, but this is due to the large aperture diameter of 1 mm. Note that in Figs. 5(c)-(f) the focusing directions are flipped between $x$- and $y$-polarizations because of the lens rotation for focusing. In addition, the slight increase around 40–44 pixels is considered to be due to the 0th-order light.
Figure 6 shows the polarization-sensitive imaging result. Figure 6(a) shows the LWIR image with the fabricated PSM, and 6(b) shows an experimental setup. The thermal radiation from the 250 $^\circ$C hotplate irradiates a train toy and a hand. The toy has two windows, the back of which is indicated by the index finger. The reflected rays were imaged by the camera with the PSM. As shown in Fig. 6(a), the two windows are brighter on the image’s left ($y$-polarized) side. On the other hand, there is almost no difference in brightness between the finger and hand. This indicates the selective reflection of $p$- and $s$-polarization on a smooth surface, consistent with an increase in the intensity of the $y$-component corresponding to $s$-polarization.
Fig. 6. Polarization-sensitive imaging. (a) An LWIR image of a train toy using the fabricated PSM. (b) Experimental setup.
Based on these experimental results, we have succeeded in single-shot polarization-separation imaging with a speed of 8 fps. These have advantages in speed over the multi-shot method, which requires rotating analyzers and offers the possibility of live imaging. Furthermore, they offer an advantage over dedicated polarization image sensors as they can be seamlessly integrated with off-the-shelf commercial image sensors, simplifying their use and implementation. These features will open up potential applications of LWIR PSM in fields for example remote sensing, biomedical imaging, and material characterization. Furthermore, the method can be easily extended to the separation of orthogonal circular and elliptical polarizations by adding rotations to the individual columns and obtaining the help of the Pancharatnam-Berry phase (geometric phase). Therefore, it can be applied to full Stokes imaging polarimetry and more complex analyses are possible [39–42]. However, there are challenges in aligning images at the pixel level, such as further optimization of fixtures and adjustment mechanisms. Deposition of an antireflection film on the backside of the lens substrate would be beneficial in reducing stray light and improving light-utilization efficiency. Improvement of chromatic aberration will also be the next major issue.
5. Conclusion
In this study, we have demonstrated the fabrication and characterization of an sc-Si PSM for the LWIR band. From the electromagnetic simulation at the design wavelength of 10.6 µm, the meta-atoms were expected to have full 2$\pi$ phase delay ranges for both $x$- and $y$-directions. Polarization-separation imaging of both elevated and ambient temperature objects has been demonstrated with the high extinction ratios of 21.8 and 12.8 dB for the $x$- and $y$-polarizations, respectively. Furthermore, polarization-sensitive imaging was demonstrated by distinguishing $p$- and $s$-polarized waves reflected from the surfaces of a hand and the toy windows. Our results suggest that the proposed PSM has promising potential for use in various LWIR optical devices, such as thermal imaging and sensing systems, as well as in polarization-related applications, including chemical and biological sensing. The development of such metasurfaces and metalenses could open up new avenues for research and practical applications in the field of infrared optics.
Funding
Japan Science and Technology Agency (JPMJTM20MK); Ministry of Education, Culture, Sports, Science and Technology (JPMXP1222UT1041); Japan Society for the Promotion of Science (21H01781, 22K04894).
Acknowledgments
Part of this work was conducted at Takeda Sentanchi Supercleanroom, The University of Tokyo. The authors thank Prof. Y. Mita, and Mr. M. Fujiwara (The Univ. of Tokyo) for their help and assistance with the sample fabrication and Prof. Lucas H. Gabrielli (The Univ. of Campinas) for the development of the Python library gdstk. Part of the numerical calculations was carried out on the TSUBAME3.0 supercomputer at Tokyo Institute of Technology.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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