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Abstract
The long-wavelength infrared (LWIR) micro-lens arrays, as one of the important components in wafer level thermal optics, have been applied for wavefront sensing, beam shaping, integral imaging, and other thermal optical applications. Recently, electromagnetic metasurfaces provide a promising platform for designing high-performance, lightweight and ultracompact optical elements. Here, we experimentally demonstrate a 60 × 60 transmissive type, polarization-independent LWIR micro-lens array based on all-silicon metasurfaces with a fill factor approaching 100%. Each single micro-metalens with a pitch of 100 μm and a focal length of 100 μm operating at λ = 10.6 μm, can focus the light to a spot with a full-width at half-maximum (FWHM) of 12.7 μm (~1.2λ) at the focal plane. Considering the fact of single-step photolithography and standard integrated circuit (IC) compatible fabrication processes, these metasurface-based micro-lens arrays may have great potentials in compact thermal imaging systems.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the development of wafer level packaging, a lot of long-wavelength infrared (LWIR) optical systems with wafer level thermal optics (WLTOs) [1] are moving towards the trend of miniaturization, high integration and low cost. In particular, the LWIR micro-lens array, as one of the most important components in the WLTOs, has been applied to beam shaping [2], wavefront sensing [3], integral imaging [4,5] and other thermal optical systems [6–8]. For example, a Shack–Hartmann wavefront sensor (SHWFS) combined with LWIR micro-lens array can be used to characterize a thermal imaging system in adaptive optics [3]. In addition, an ultra-thin system that uses an LWIR micro-lens array can receive multiple images to reconstruct a higher-resolution image, which is called thin observation module by bound optics (TOMBO) [9]. Besides providing these specific optical functionalities, LWIR micro-lens arrays are always placed before the focal plane arrays (FPA) to improve their performance of detecting photons and operating temperature [10]. Typically, LWIR micro-lens has cured surface formed by heating the constitution materials [11], which makes it difficult to keep the curvature of each lenslet precise and consistent. Although laser direct writing [12], focused ion-beam machining [13] and gray scale mask technique [14] have been applied to guarantee the precision of manufacture, the thickness of these LWIR refractive micro-lens arrays are usually much larger than the wavelength of light in order to precisely control the wavefronts by phase accumulation of light propagating in refractive materials. Furthermore, the majority of diffractive LWIR micro-lens arrays are fabricated by binary optical lithography [15]. However, the process requiring multiple photolithography and etching is more complicated and it is difficult to align precisely. Therefore, bulky size and complex manufacture processes limit the extensive applications of LWIR micro-lens arrays in compact thermal optical systems.
Recently, metasurfaces composed of subwavelength optical scatters have attracted much attention due to their fascinating functionalities [16–20]. By adjusting geometric parameters of these optical scatters with the subwavelength spatial distribution, metasurfaces can fully control the properties of light, including phase, polarization and amplitude. Different from traditional optical devices, metasurfaces with ultrathin thickness introduce the abrupt changes of optical phase and thus provide an efficient platform for flat and ultrathin optical components. Up to now, a serial of metasurface-based optical devices have been reported, such as metalens [21–25], holograms [26–29], waveplates [30], beam deflectors and splitters [31,32], orbital angular momentum (OAM) generators [33] and vortex beam generator [34]. The metasurface-based micro-lens arrays have also been experimentally implemented in visible [35], mid-wavelength infrared [36] and THz region [37].
In this paper, we propose and experimentally demonstrate a 60 × 60 transmissive and polarization-independent LWIR micro-lens array based on all-silicon (Si) metasurface with a fill factor [38] approaching 100%. At the incident wavelength λ = 10.6 μm, each single metalens with a pitch of 100 μm can focus the light to a small spot with a full-width at half-maximum (FWHM) of 12.7 μm (~1.2λ) at the focal plane. The whole micro-lens array consisting of circular Si pillars is directly fabricated on the Si wafer using single step photolithography and standard etching process, which is easy to realize mass-production. The metasurface-based micro-lens arrays are an alternative method to the convectional convex micro-lens arrays because of their ultrathin thickness and light weight. We envision this type of all-Si metasurface micro-lens array may have great potentials in compact thermal imaging systems.
2. Design and simulation of the LWIR micro-lens array
As schematically shown in the left side of Fig. 1(a), the size of single micro-metalens is P × P. The LWIR micro-lens array composed of the square micro-metalenses has a fill factor of 100%. The basic unit structure of a single micro-lens is a circular Si pillar arranged on the center of a square Si substrate, which is illustrated in the right side of Fig. 1(a). The height and diameter of Si pillars are H and D, respectively. The size of Si pillar unit cell is U × U. For modeling and numerically simulating the Si pillar, full-wave simulations are performed using Lumerical finite-difference time-domain (FDTD) solver. The incident light is a plane wave with the wavelength of 10.6 μm. The refractive index of the Si coming from the data of the literature [39] is set to be 3.42. Along x and y axes, periodic boundary conditions are applied and perfectly matched layer (PML) boundary condition is used in z direction. Each Si pillar behaves as a truncated waveguide achieving the phase accumulation with propagation of a subwavelength distance and a weakly coupled low-quality factor resonator which highly confines the field in them. Figure 1(b) shows the magnetic energy density of the periodic arrays for the Si pillar diameter D = 2.5 μm. It can be seen that the scattering of the Si pillar is a local effect and the optical coupling effect between the neighboring pillars is very weak. The phase shift ѱ which the light accumulates propagating through the Si pillar is expressed by , where ne is the effective refractive index modified by the size of the pillar and its surroundings medium. The Si pillar has a fixed height of 6.8 μm which is tall enough to achieving full 2π phase cover. The circular Si pillar can perform polarization-independent operation due to their symmetric cross-sections. Figures 1(c) and 1(d) show the simulated transmission amplitude and phase shift as a function of the lattice constant and the pillar diameter for the square lattice periodic arrays of the pillars. Considering fabrication constraints, a set of circular Si pillars with the lattice constant of 5.5 μm are selected for achieving full 2π phase coverage with relatively high transmission amplitude.
Fig. 1 (a) Left side: schematic of a LWIR micro-metalens. Right side: side-view and top-view of the micro-metalens unit cell (a Si pillar on a Si substrate with a square lattice). Note that two different colors (blue and gray) are only used to distinguish the silicon pillar from the silicon substrate. (b) The normalized magnetic energy density in a periodic array for the Si pillar diameter D = 2.5 μm. The dashed white circles and rectangles show the boundaries of the Si pillars. Scale bar, 2 μm. (c), (d) Transmission amplitude and phase of the transmission coefficient variation as a function of a Si pillar diameter (D) and period (U) at λ = 10.6 μm.
To design a metasurface micro-lens, the phase profile of the metalens for the normal incident light can be calculated as follows:
where λ is the operation wavelength and f is the designed focal length. After discretizing the desired phase profile , silicon pillars with different diameters are selected at each position (x, y) to form the metasurface micro-lens. In this work, we design a 60 × 60 transmissive LWIR micro-lens array with a pitch of 100 μm and focal length of 100 μm for each micro-lens. The phase profile of a single metasurface micro-lens is given in Fig. 2(a). To reduce the size of three-dimensional simulation region, a 2 × 2 micro-lens array is simulated with a plane wave at the wavelength λ = 10.6 μm normally incident on the micro-lens array from the substrate. The distribution of normalized far-field intensity in the x-z plane is shown in Fig. 2(b), where the simulated focal length is 100 μm that agrees well with the design. Figure 2(c) shows the light intensities of the focusing spots in the x-y plane at the focal length. The FWHMs of the four focal spots along x-directions are 11.7 μm (~1.1λ). In addition, there is no obvious cross-talk between neighboring metalens. These results exhibit good focusing properties of the designed LWIR metasurface micro-lens array.Fig. 2 (a) The discrete phase profile of a micro-metalens. (b) Simulated far-field intensity distribution of the 2 × 2 micro-lens array in the x-z plane. (c) Simulated far-field intensity distribution of the 2 × 2 micro-lens array in the x-y plane at the focus. Here, the simulations are performed with either TM or TE polarized light at λ = 10.6μm.
3. Fabrication of metasurface micro-lens array
The substrate for the LWIR micro-lens array is double-side polished crystalline silicon wafer. The pattern of 60 × 60 micro-lens array is written on the chromium (Cr) film deposited on the soda glass substrate using the laser direct writing system (Heidelberg Instruments, MLA150). A positive photoresist (S1813) layer with a thickness of 2 μm is spin-coated on the silicon wafer and baked at 100 °C for 2mins. Then the pattern of the micro-lens array is transferred into the photoresist using the standard ultra-violet lithography. After the resist development, the patterned photoresist is then used as the hard mask in dry etching of the silicon wafer. The dry etching is performed in a mixture of SF6 and C4F8 plasmas using an inductively-coupled plasma (ICP) system. Finally, the photoresist mask is dissolved in piranha solution. Figure 3(a) shows the optical photograph of fabricated 60 × 60 LWIR micro-lens array. Figure 3(b) shows the zoom-in optical microscope picture of a portion of the fabricated micro-lens array. Scanning electron micrograph images of a micro-metalens at different views are shown in Figs. 3(c) and 3(d).
Fig. 3 (a) Top-view optical photograph of the fabricated 60 × 60 LWIR micro-lens array. Scale bar: 1 mm. (b) Top-view optical microscope picture of the 6 × 6 microlens array. Scale bar: 100 μm. (c) Top-view scanning electron micrograph image of a single micro-metalens. Scale bar: 20 μm. The inset is the zoomed-in SEM image. Scale bar: 5 μm. (d) Side-view scanning electron micrograph image of a single micro-metalens. Scale bar: 20 μm. The inset is the zoomed-in SEM image. Scale bar: 5 μm.
4. Characterization of the LWIR micro-lens array
We characterize the performance of fabricated metasurface micro-lens array using the experimental setup illustrated in Fig. 4(a). A CO2 laser emitting 10.6 μm light is used as the LWIR source. The light propagating through the expanded beam lens system and attenuator illuminates the substrate of the micro-lens array. The intensity distribution in the x-y plane is detected by a LWIR focal plane array (Guide-Infrared Inc., UA840, resolution: 800 × 600 pixels) attached with a 15 × objective lens (Thorlabs, LMM-15X-P01-160). The focal plane array and the objective lens are fastened to a translation stage. For a single micro-lens, Fig. 4(b) shows the focal spot profiles at different z positions around the focal plane. The measured (blue circle) and simulated (black solid line) intensity distributions of the focal spot along x-directions on the focal plane are shown in Fig. 4(c). The measured FWHM of the focal spot is about 12.7 μm, which is a little larger than the simulated result of 11.7 μm. We can only measure several micro-lenses in one field of view limited by the objective lens. Actually, we can obtain the focal spot profile of the whole 60 × 60 micro-lens array by moving the whole micro-lens array in the x-y plane. Due to the similar performance of every micro-lens, Fig. 4(d) only shows the measured optical field intensity of a portion of the whole micro-lens array (4 × 4 micro-lens array) at the focal plane. In order to analyze the focusing efficiency quantificationally, we define the efficiency as the ratio of the optical power in the focal spot area (a circular aperture with the diameter of 50 μm spanning the center of the focal spot) to the incident optical power over a micro-metalens (100 μm × 100 μm). The measured focusing efficiency of a micro-lens is approximately 34%, which is smaller than the simulated values of 43%. The difference between the two efficiencies mainly stems from measurement artifacts and dimensional or morphological differences in the structures of the fabricated devices compared with that of the ideal designed structures (such as differences in the diameters, heights or roughness of the silicon pillars). However, the major energy loss of the micro-lens array is due to the reflection from substrate backside interface (air/Si) and the interface between the front of the micro-lens array and the air above it. The focusing efficiency can be improved by depositing an anti-reflection film on the substrate and replacing the silicon substrate with LWIR transparent materials such as halide glass.
Fig. 4 (a) Experimental setup for the characterization of the LWIR micro-lens array. The infrared camera attached with the reflective objective lens is fastened to a translation stage. (b) The measured focal spot profiles for a single metalens along the optical axis at different z values. The z value of 100 μm is the position of the focal plane. (c) Measured and simulated intensity distributions of the focal spot along x-directions on the focal plane (d) Measured focal spot profile of a portion of the whole micro-lens array (4 × 4 micro-lens array). Scale bar: 100 μm.
5. Conclusion
In summary, we theoretically and experimentally implement a 60 × 60 LWIR polarization-insensitive micro-lens array based on all-silicon metasurface with the fill factor of 100%. Each square micro-metalens is composed of a set of different diameter Si pillars that can cover 0-2π phase shift with relatively high transmission amplitude. In experiment, each single micro-metalens can focus the incident light to a small spot with FWHM of 1.2λ, close to the simulation result of 1.1λ. The LWIR metasurface micro-lens array with subwavelength thickness is fabricated on the silicon wafer using the processes that are completely compatible with the standard integrated circuit technology. We believe the light-weight, high-integration and low-cost LWIR micro-lens array based on all-silicon metasurfaces will boost the development of WLTOs and have the potentials to be applied for the next generation compact thermal imaging systems.
Funding
National Key R&D Program of China (2016YFA0202103, 2017YFA0303700); National Natural Science Foundation of China (6157509, 11774163).
References
1. A. Symmons and R. Pini, “A practical approach to LWIR wafer-level optics for thermal imaging systems,” Proc. SPIE 8704, 870425 (2013). [CrossRef]
2. A. Bich, J. Rieck, C. Dumouchel, S. Roth, K. J. Weible, M. Eisner, R. Voelkel, M. Zimmermann, M. Rank, M. Schmidt, R. Bitterli, N. Ramanan, P. Ruffieux, T. Scharf, W. Noell, H. P. Herzig, and N. de Rooij, “Multifunctional micro-optical elements for laser beam homogenizing and beam shaping,” Proc. SPIE 6879, 68790Q (2008). [CrossRef]
3. S. Velghe, J. Primot, N. Guerineau, R. Haidar, S. Demoustier, M. Cohen, and B. Wattellier, “Advanced wave-front sensing by quadri-wave lateral shearing interferometry,” Proc. SPIE 6292, 62920E (2006). [CrossRef]
4. P. W. Kruse, Uncooled Thermal Imaging. Arrays, Systems and Applications (SPIE, 2001).
5. M. Shankar, R. Willett, N. Pitsianis, T. Schulz, R. Gibbons, R. Te Kolste, J. Carriere, C. Chen, D. Prather, and D. Brady, “Thin infrared imaging systems through multichannel sampling,” Appl. Opt. 47(10), B1–B10 (2008). [CrossRef] [PubMed]
6. N. T. Gordon, “Design of Hg1-xCdxTe infrared detector arrays using optical immersion with microlenses to achieve a higher operating temperature,” Semicond. Sci. Technol. 6(12C), C106–C109 (1991). [CrossRef]
7. J. Piotrowski, M. Grudzien, Z. Nowak, Z. Orman, J. Pawluczyk, M. Romanis, and W. Gawron, “Uncooled photovoltaic Hg 1-x Cd x Te LWIR detectors,” Proc. SPIE 4130, 175–185 (2000). [CrossRef]
8. A. Rogalski, “Infrared detectors: status and trends,” Prog. Quantum Electron. 27(2–3), 59–210 (2003). [CrossRef]
9. J. Tanida, T. Kumagai, K. Yamada, S. Miyatake, K. Ishida, T. Morimoto, N. Kondou, D. Miyazaki, and Y. Ichioka, “Thin observation module by bound optics (TOMBO): concept and experimental verification,” Appl. Opt. 40(11), 1806–1813 (2001). [CrossRef] [PubMed]
10. J. Piotrowski and A. Rogalski, “Uncooled long wavelength infrared photon detectors,” Infrared Phys. Technol. 46(1-2), 115–131 (2004). [CrossRef]
11. F. T. O’Neill and J. T. Sheridan, “Photoresist reflow method of microlens production Part I: Background and experiments,” Optik (Stuttg.) 113(9), 391–404 (2002). [CrossRef]
12. D. Savastru, S. Miclos, and R. Savastru, “Infrared chalcogenide microlenses,” J. Optoelectron. Adv. Mater. 8(3), 1165–1172 (2006).
13. X. Y. Zhang, X. J. Yi, M. He, and X. R. Zhao, “128x128-element silicon microlens array fabricated by ion-beam etching for PtSi IRCCD,” Proc. SPIE 3551, 191–198 (1998). [CrossRef]
14. Y. Kumaresan, A. Rammohan, P. K. Dwivedi, and A. Sharma, “Large area IR microlens arrays of chalcogenide glass photoresists by grayscale maskless lithography,” ACS Appl. Mater. Interfaces 5(15), 7094–7100 (2013). [CrossRef] [PubMed]
15. D. C. O’Shea, T. J. Suleski, A. D. Kathman, and D. W. Prather, Diffractive optics: design, fabrication, and test (SPIE, 2004).
16. S. M. Kamali, E. Arbabi, A. Arbabi, and A. Faraon, “A review of dielectric optical metasurfaces for wavefront control,” Nanophotonics 7(6), 1041–1068 (2018). [CrossRef]
17. N. M. Estakhri and A. Alù, “Recent progress in gradient metasurfaces,” J. Opt. Soc. Am. B 33(2), A21–A30 (2016). [CrossRef]
18. X. Luo, “Subwavelength optical engineering with metasurface waves,” Adv. Opt. Mater. 6(7), 1701201 (2018). [CrossRef]
19. X. Luo, “Engineering Optics 2.0: A Revolution in Optical Materials, Devices, and Systems,” ACS Photonics 5(12), 4724–4738 (2018). [CrossRef]
20. X. Luo, D. Tsai, M. Gu, and M. Hong, “Subwavelength interference of light on structured surfaces,” Adv. Opt. Photonics 10(4), 757–842 (2018). [CrossRef]
21. M. Khorasaninejad and F. Capasso, “Metalenses: Versatile multifunctional photonic components,” Science 358(6367), eaam8100 (2017). [CrossRef] [PubMed]
22. Q. Fan, P. Huo, D. Wang, Y. Liang, F. Yan, and T. Xu, “Visible light focusing flat lenses based on hybrid dielectric-metal metasurface reflector-arrays,” Sci. Rep. 7(1), 45044 (2017). [CrossRef] [PubMed]
23. S. Zhang, M. H. Kim, F. Aieta, A. She, T. Mansuripur, I. Gabay, M. Khorasaninejad, D. Rousso, X. Wang, M. Troccoli, N. Yu, and F. Capasso, “High efficiency near diffraction-limited mid-infrared flat lenses based on metasurface reflectarrays,” Opt. Express 24(16), 18024–18034 (2016). [CrossRef] [PubMed]
24. Q. Fan, M. Liu, C. Yang, L. Yu, F. Yan, and T. Xu, “A high numerical aperture, polarization-insensitive metalens for long-wavelength infrared imaging,” Appl. Phys. Lett. 113(20), 201104 (2018). [CrossRef]
25. Q. Fan, Y. Wang, M. Liu, and T. Xu, “High-efficiency, linear-polarization-multiplexing metalens for long-wavelength infrared light,” Opt. Lett. 43(24), 6005–6008 (2018). [CrossRef] [PubMed]
26. F. Zhang, M. Pu, X. Li, P. Gao, X. Ma, J. Luo, H. Yu, and X. Luo, “All-Dielectric Metasurfaces for Simultaneous Giant Circular Asymmetric Transmission and Wavefront Shaping Based on Asymmetric Photonic Spin–Orbit Interactions,” Adv. Funct. Mater. 27(47), 1704295 (2017). [CrossRef]
27. Y. Deng, X. Wang, Z. Gong, K. Dong, S. Lou, N. Pégard, K. B. Tom, F. Yang, Z. You, L. Waller, and J. Yao, “All-Silicon Broadband Ultraviolet Metasurfaces,” Adv. Mater. 30(38), e1802632 (2018). [CrossRef] [PubMed]
28. G. Zheng, H. Mühlenbernd, M. Kenney, G. Li, T. Zentgraf, and S. Zhang, “Metasurface holograms reaching 80% efficiency,” Nat. Nanotechnol. 10(4), 308–312 (2015). [CrossRef] [PubMed]
29. X. Xie, X. Li, M. Pu, X. Ma, K. Liu, Y. Guo, and X. Luo, “Plasmonic metasurfaces for simultaneous thermal infrared invisibility and holographic illusion,” Adv. Funct. Mater. 28(14), 1706673 (2018). [CrossRef]
30. F. Ding, Z. Wang, S. He, V. M. Shalaev, and A. V. Kildishev, “Broadband high-efficiency half-wave plate: a supercell-based plasmonic metasurface approach,” ACS Nano 9(4), 4111–4119 (2015). [CrossRef] [PubMed]
31. S. Wang, P. C. Wu, V. C. Su, Y. C. Lai, C. Hung Chu, J. W. Chen, S. H. Lu, J. Chen, B. Xu, C. H. Kuan, T. Li, S. Zhu, and D. P. Tsai, “Broadband achromatic optical metasurface devices,” Nat. Commun. 8(1), 187 (2017). [CrossRef] [PubMed]
32. M. Khorasaninejad, W. Zhu, and K. B. Crozier, “Efficient polarization beam splitter pixels based on a dielectric metasurface,” Optica 2(4), 376–382 (2015). [CrossRef]
33. K. Yang, M. Pu, X. Li, X. Ma, J. Luo, H. Gao, and X. Luo, “Wavelength-selective orbital angular momentum generation based on a plasmonic metasurface,” Nanoscale 8(24), 12267–12271 (2016). [CrossRef] [PubMed]
34. M. Q. Mehmood, S. Mei, S. Hussain, K. Huang, S. Y. Siew, L. Zhang, T. Zhang, X. Ling, H. Liu, J. Teng, A. Danner, S. Zhang, and C. W. Qiu, “Visible-Frequency Metasurface for Structuring and Spatially Multiplexing Optical Vortices,” Adv. Mater. 28(13), 2533–2539 (2016). [CrossRef] [PubMed]
35. J. Jin, X. Zhang, P. Gao, J. Luo, Z. Zhang, X. Li, X. Ma, M. Pu, and X. Luo, “Ultrathin Planar Microlens Arrays Based on Geometric Metasurface,” Ann. Phys. 530(2), 1700326 (2018). [CrossRef]
36. S. Zhang, A. Soibel, S. A. Keo, D. Wilson, S. B. Rafol, D. Z. Ting, A. She, S. D. Gunapala, and F. Capasso, “Solid-immersion metalenses for infrared focal plane arrays,” Appl. Phys. Lett. 113(11), 111104 (2018). [CrossRef]
37. Q. Wang, X. Zhang, Y. Xu, Z. Tian, J. Gu, W. Yue, S. Zhang, J. Han, and W. Zhang, “A Broadband Metasurface-Based Terahertz Flat-Lens Array,” Adv. Opt. Mater. 3(6), 779–785 (2015). [CrossRef]
38. H. Yang, C.-K. Chao, M.-K. Wei, and C.-P. Lin, “High fill-factor microlens array mold insert fabrication using a thermal reflow process,” J. Micromech. Microeng. 14(8), 1197–1204 (2004). [CrossRef]
39. E. D. Palik, Handbook of Optical Constants of Solids (Academic Press, New York, 1985).
