Open Access
Add to BibSonomy
Abstract
With current trends to progressively miniaturize optical systems, it is now essential to look for alternative methods to control light at extremely small dimensions. Metalenses are composed of subwavelength nanostructures and have an excellent ability to manipulate the polarization, phase, and amplitude of incident light. Although great progress of metalenses has been made, the compact metalens-integrated devices have not been researched adequately. In the study, we present compact imaging devices for near-infrared microscopy, in which a metalens is exploited. The indicators including resolution, magnification, and image quality are investigated via imaging several specimens of intestinal cells to verify the overall performance of the imaging system. The further compact devices, where the metalens is integrated directly on the CMOS imaging sensor, are also researched to detect biomedical issues. This study provides an approach to constructing compact imaging devices based on metalenses for near-infrared microscopy, micro-telecopy, etc., which can promote the miniaturization tending of futural optical systems.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the development of endoscopes, micro-robots, smartphones, and micro-satellites, etc., compact imaging devices have become an important research field [1]. Although the conventional imaging equipment with mature techniques has high resolution and high imaging quality, the bulky optical systems are heavy, and expensive, due to the dependence on the discrete optical lens. Hence, the flat lenses, which can make imaging devices smaller, lighter, and more convenient to fabricate at low cost, have become an important issue in futural optical systems. Fresnel lenses can compress bulky lenses into a flat configuration and produce incident field imaging through diffraction, thus greatly reduce the weight and size of optical systems. However, the theoretical limit of the maximum diffraction efficiency of a Fresnel lens (the energy ratio of the primary focus to the incident light) is very low (only 41%) [2,3]. Meanwhile, computational imaging, including lensless imaging [4,5] and scattering medium imaging [6–8], is an effective way to replace traditional lenses to form miniature imaging devices. Nevertheless, the post-imaging reconstruction mainly relies on complex algorithms, such as coded apertures [9,10], convolutional neural networks [11], deep neural networks [12,13], and point spread functions [14,15]. Therefore, computational imaging requires a lot of computing resources and consumes a long time.
In recent years, metasurfaces have attracted the increasing interest of researchers as new optical elements, due to their excellent ability to manipulate light [16,17]. The properties such as amplitude, phase, and polarization can be arbitrarily tuned by changing the shape, size, and arrangement of nanopillars at the subwavelength scale [18–20]. As one branch of metasurfaces, metalenses are one approach to realize compact optical imaging systems. In the last few years, the imaging performance, including efficiency [21,22], field of view [23,24], achromatism [25,26], polarization manipulation [27,28], and operating wavelength [29,30], of metalenses has been greatly improved, which indicates that they are close to practical application. However, most metalenses are only researched on the optical features [31,32], and only a few of these works applied metalenses in compact imaging systems [33]. Thus, the compact metalens-integrated devices have not been researched adequately.
In this study, we present a compact imaging device for near-infrared microscopy, in which a metalens is exploited. The metalens was composed of polysilicon (p-Si) square nanocolumns on a SiO2 substrate, of which the theoretical focusing efficiency reached 96.3%. Via imaging several specimens of rabbit intestinal cells, we investigated resolution, magnification, and image quality of the metalens-integrated microscopic imaging device and show the ability to diagnose the biological tissues. Further, we directly locate the metalens on the imaging sensor and carry out the imaging of biological tissues, showing the potential of miniaturizing imaging optical systems into millimeter scales. The imaging results of the rabbit’s jejunum and colon show that it can be used for imaging identification of biological lesions. To our knowledge, this investigation has not been reported in the previous literature. This study provides an approach to constructing compact imaging devices and to miniaturizing optical systems.
2. Metalenses
The schematic diagram and electron microscopy (SEM) image of the metalens used in the study are shown in Fig. 1. It is constructed by nanopillars with a rectangular shape on a SiO2 substrate. The height of the rectangular silicon nanopillars is 140 nm, with a fixed edge-to-edge gap of 100 nm to the adjacent nanopillars. The phase distribution of the metalens is expressed as [34]:
where r represents the radius of the metalens, f represents the focal length, and λ=830 nm represents the operating wavelength.
Fig. 1. Structure of the metalens: (a) Schematic diagram of a NIR metalens operating in the transmission mode. Here, the rectangular silicon nanopillars were arranged in a square lattice with the operating wavelength λ=830 nm, the isometric distance P=100 nm, and the pillar height H=140 nm. (b) SEM image (Top view) of the metalens with a diameter of 120 µm and a focusing distance of 60 µm.
The phase of every silicon nanopillar is designed based on the Huygens’ principle, which is widely used in designing metasurfaces [35–37]. The metalens is designed to work at a wavelength of 830 nm with a diameter of 120 µm and a focusing distance of 60 µm. To avoid high-order diffractions, the equidistant nanopillars and the aperiodic arrays are exploited with an isometric distance of 100 nm [38]. The simulation results show that the focusing efficiency is as high as 96.3%, and the multistage diffraction is eliminated, which can be used as an efficient planar lens for micro imaging systems. The metalens is fabricated using electron-beam lithography and dry etching. The SEM image of the metalens is shown in Fig. 1(b).
3. Microscopic imaging
The schematic of the NIR microscopic imaging device is shown in Fig. 2(a), in which the metalens replaces one lens of a conventional microscopic system. The whole setup is composed of a light source, an attenuator (Thorlabs, NDC-50C-4M), a linear polarizer, an object, an achromatic lens (Thorlabs, AC080-010-B), the metalens, a tube lens, and a CMOS image sensor (Sempric, s-200-M-R-U, pixel size: 6.5 µm × 6.5 µm). The light source emits light with a wavelength of 830 nm as the illumination. Since the metalens is sensitive to the polarization direction, the linear polarizer is used to remove the undesirable polarization. Then, an attenuator is used to control the intensity of the incident light. Because the focusing distance of the metalens (as objective) is very short, i.e. 60 µm, it could be easily scratched by objects when objects are very close. Therefore, an achromatic lens is added to transfer the image of objects to the area before the metalens as a virtual object, which is 7 times smaller than the real object, as shown in the insert image of Fig. 2(a). The metalens as objective and the tube lens as ocular lens compose the microscopic imaging system. Compared to the conventional microscopic system, the metalens-based configuration can compress the system volume and cost while keeping the same magnifying power and resolution.
Fig. 2. Device architecture and imaging performance: (a) Schematic image of the NIR microscopic imaging device. (b) Image of NBS 1952 resolution test chart taken from the microscopic device with the resolution of 2.1 µm (c) Image of the logo “3.4” on the test chart with an amplification factor 12×
To characterize the imaging performance of the microscopic device, the NBS 1952 resolution test chart (Thorlabs, R1L3S10P) is used as the imaging object. Figure 2(b) shows the microscopic imaging of the test chart and the resolution is 2.1 µm, which is equivalent to 238 line pairs per millimeter (lp/mm). This experimental result confirms that the metalens-based microscopic system has a high resolution. The microscope also images a logo “3.4” on the test chart with the actual size of 0.1×0.16 mm2, as shown in Fig. 2(c). In the measurement, the final image on the CMOS sensor is 1.2×1.92 mm2, expanding to 12 times the real size of the object. The experimental results show that the NIR microscopic system has a small size (about 8 cm), high resolution(2.1 µm), and large magnification (12×). If the microscopic system with the same magnification and resolution is built with conventional lenses, the size will be 48 cm, 6 times the setup.
As an example of applications, we used the microscopic system to image several biospecimens of the jejunum and colon of rabbits (as shown in Fig. 3, images are denoised). Figure 3(a)-(c) show the images of the health issues and Fig. 3(d)-(f) show the diseased ones. Figure 3(c) & (f) are captured by a commercial microscope (Nikon, 4×objective, NA=0.13) with the illumination of wavelength 830 nm while the others are obtained from our metalens-based imaging device. The results show that the images of the semi-transparent biological specimens captured by our device have good consistency with those obtained by the commercial one. Figure 3(a) & (b) show the complete structure of the colon and jejunum with neatly arranged cells, respectively, in which there are no obvious histopathological lesions. Figure 3(d) indicates local necrosis and shedding of epithelial cells in the mucosal layer of colon tissue, making the surface of colon mucosa smooth. Figure 3(e) shows that the mucosal epithelial structure of the jejunum tissue is destroyed, and a large number of cells are necrotic and fall off into the intestinal lumen, indicating that morphological damage has occurred. Therefore, The results indicate that the metalens-based microscopic device can be used for high-throughput biological tissue imaging and diagnosis.
Fig. 3. Imaging of biological specimens of rabbit jejunum and colon: The images of (a) Normal colon, (b) Healthy jejunum, (d) Diseased colon, and (e) Diseased jejunum are captured by the metalens-based microscopic device. Images of the (c) Healthy colon and (f) Diseased colon captured by a commercial microscope (Nikon, 4×objective, NA=0.13) illuminated by the light source 830 nm. Red arrows indicate pathological features.
4. Metalens-integrated imaging device
To further miniaturize the imaging devices, the metalens is integrated on the surface of the imaging sensor and the tube lens is moved, which realizes an ultra-compact imaging device. The schematic of the metalens-integrated imaging system is shown in Fig. 4(a), in which the 830 nm light illuminates the object that is transferred by an achromatic lens to the area before the metalens as a virtual object, which is 4.2 times smaller than the real object. The metalens is placed directly on the imaging sensor (AR0144AT, pixel size:3.0 µm × 3.0 µm) and the imaging v distance (distance between the imaging sensor and the metalens) equals 1000 µm. A clear image can be acquired by turning the object distance u (distance between the object and the metalens). Figure 4(b) shows a photograph of the ultra-compact imaging device. Compared to the microscopic imaging device in Fig. 2, the metalens-integrated imaging device can further reduce the size (11.5 mm) and cost of the system.
Fig. 4. Device architecture and imaging performance: (a) Schematic image of the metalens-integrated imaging system. (b) Photograph of the ultra-compact microscopic device. (c) Image of NBS 1952 resolution test chart taken from the metalens-integrated imaging device with the resolution of 22.3 µm. (d) Image of the logo “5.6” on the test chart with an amplification factor of 1.35x.
To characterize the imaging performance of the metalens-integrated imaging device, the NBS 1952 resolution test chart is used to discern the resolution of the imaging system. Figure 4(c) shows the image of the test chart and the resolution is 22.3 µm, which is equivalent to 22.4 line pairs per millimeter (lp/mm). The device also images a logo “5.6” on the test chart, as shown in Fig. 4(d). The results show that the object was magnified by about 1.35 times. Furthermore, to demonstrate the application potentials of the device, we used it to image several biological specimens of rabbit jejunum and colon, as shown in Fig. 5. The images of healthy rabbit jejunum and colon specimens are shown in Fig. 5(a) & (c), respectively. The results show that the intestinal structure of rabbits is complete without pathological features. Then, the images of the diseased colon and jejunum are presented in Fig. 5(b) & (d), respectively. The results show that the diseased colon and jejunum mucosa have necrosis and fall off, forming erosions. Therefore, the metalens-integrated imaging device can see the microstructure of animal tissues and be used for imaging identification of biological samples.
Fig. 5. Imaging of biological specimens of rabbit jejunum and colon: Images of (a) Healthy colon, (b) Diseased colon, (c) Healthy jejunum, and (d) Diseased jejunum. Red arrows indicate pathological features.
5. Conclusion
In the study, we investigate metalens-integrated compact near-infrared microscope devices. Based on the silicon metalens, a metalens-based microscopic device is established, and the experimental results show that the resolution and magnification are 2.1 µm and 12 times, respectively. The images of the rabbit jejunum and colon tissue specimens captured by our device have good consistency with those obtained by the commercial one. To further miniaturize the imaging devices, the metalens is integrated on the surface of the imaging sensor, reducing the imaging optical system to the millimeter scale. The results show that resolution and magnification were 22.3 µm and 1.35 times, respectively. It can clearly image the rabbit jejunum and colon tissues. Summarily, the metalens-based/integrated imaging device has a high resolution and image quality, which can successively identificate the biological samples. However, compared with the traditional equipment, the imaging performance of the current compact device still need further improvement. Fortunately, the development of nanofabrication and CMOS image sensor technology will promote the capabilities of compact imaging devices for higher resolutions. The research provides a way for metalens-based microscopic imaging, which may inspire more compact optical devices.
Funding
National Natural Science Foundation of China (61705204, 61705206); China Postdoctoral Science Foundation (2019M653486); Innovation Team Foundation of China West Normal Universit (CXTD2018-11); Innovation and Development Foundation of China Academy of Engineering Physics (CX20200021).
Disclosures
The authors declare that there are 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.
References
1. M. L. Tseng, H. H. Hsiao, C. H. Chu, M. K. Chen, G. Sun, A. Q. Liu, and D. P. Tsai, “Metalenses: Advances and Applications,” Adv. Opt. Mater. 6(18), 1800554 (2018). [CrossRef]
2. H.-C. J. Liang-Chen Lin, Tsung-Hsien Lin, and Andy Y.-G. Fuh, “Highly efficient and polarization-independent Fresnel lens based on dye-doped liquid crystal,” Opt. Express 15(6), 2900–2906 (2007). [CrossRef]
3. W. D. L. Zhao and S. F. Yelin, “All-optical Fresnel lens in coherent media: controlling image with image,” Opt. Express 19(2), 981–993 (2011). [CrossRef]
4. A. Ozcan and E. McLeod, “Lensless Imaging and Sensing,” Annu. Rev. Biomed. Eng. 18(1), 77–102 (2016). [CrossRef]
5. J. N. Mait, G. W. Euliss, and R. A. Athale, “Computational imaging,” Adv. Opt. Photonics 10(2), 409–483 (2018). [CrossRef]
6. S. Mukherjee and J. Rosen, “Imaging through scattering medium by adaptive non-linear digital processing,” Sci. Rep. 8(1), 10517 (2018). [CrossRef]
7. W. Harm, C. Roider, A. Jesacher, S. Bernet, and M. Ritsch-Marte, “Lensless imaging through thin diffusive media,” Opt. Express 22(18), 22146–22156 (2014). [CrossRef]
8. E. N. Leith and J. Upatnieks, “Holographic imagery through diffusing media,” J. Opt. Soc. Am. 56(4), 523 (1966). [CrossRef]
9. M. J. Cieślak, K. A. A. Gamage, and R. Glover, “Coded-aperture imaging systems: Past, present and future development – A review,” Radiat. Meas. 92, 59–71 (2016). [CrossRef]
10. R. G. Stephen and E. E. Fenimore, “New family of binary arrays for coded aperture imaging,” Appl. Opt. 28(20), 4344–4352 (1989). [CrossRef]
11. R. Horisaki, Y. Okamoto, and J. Tanida, “Deeply coded aperture for lensless imaging,” Opt. Lett. 45(11), 3131–3134 (2020). [CrossRef]
12. L. Sun, J. Shi, X. Wu, Y. Sun, and G. Zeng, “Photon-limited imaging through scattering medium based on deep learning,” Opt. Express 27(23), 33120–33134 (2019). [CrossRef]
13. A. Sinha, J. Lee, S. Li, and G. Barbastathis, “Lensless computational imaging through deep learning,” Optica 4(9), 1117–1125 (2017). [CrossRef]
14. G. Kim and R. Menon, “Computational imaging enables a “see-through” lens-less camera,” Opt. Express 26(18), 22826–22836 (2018). [CrossRef]
15. V. Boominathan, J. K. Adams, J. T. Robinson, and A. Veeraraghavan, “PhlatCam: Designed Phase-Mask Based Thin Lensless Camera,” IEEE Trans. Pattern Anal. Mach. Intell. 42(7), 1618–1629 (2020). [CrossRef]
16. T. Omatsu, D. P. K. Tsai, Y.-T. Huang, J.-W. Chen, T. L. Chung, R. J. Lin, Y. H. Chen, B. H. Chen, H. Y. Kuo, C. H. Chu, and M. K. Chen, “Metalens for structure light,” presented at the Optical Manipulation Conference2018.
17. C. Jin, J. Zhang, and C. Guo, “Metasurface integrated with double-helix point spread function and metalens for three-dimensional imaging,” Nanophotonics 8(3), 451–458 (2019). [CrossRef]
18. K. Dou, X. Xie, M. Pu, X. Li, X. Ma, C. Wang, and X. Luo, “Off-axis multi-wavelength dispersion controlling metalens for multi-color imaging,” Opto-Electron. Adv. 3(4), 19000501 (2020). [CrossRef]
19. C. Chen, W. Song, J. W. Chen, J. H. Wang, Y. H. Chen, B. Xu, M. K. Chen, H. Li, B. Fang, J. Chen, H. Y. Kuo, S. Wang, D. P. Tsai, S. Zhu, and T. Li, “Spectral tomographic imaging with aplanatic metalens,” Light: Sci. Appl. 8(1), 99 (2019). [CrossRef]
20. W. Zang, Q. Yuan, R. Chen, L. Li, T. Li, X. Zou, G. Zheng, Z. Chen, S. Wang, Z. Wang, and S. Zhu, “Chromatic Dispersion Manipulation Based on Metalenses,” Adv. Mater. 32(27), 1904935 (2019). [CrossRef]
21. H. Chu, J. Qi, and J. Qiu, “An efficiently-designed wideband single-metalens with high-efficiency and wide-angle focusing for passive millimeter-wave focal plane array imaging,” Opt. Express 28(3), 3823–3834 (2020). [CrossRef]
22. B. Li, W. Piyawattanametha, and Z. Qiu, “Metalens-Based Miniaturized Optical Systems,” Micromachines 10(5), 310 (2019). [CrossRef]
23. J. Engelberg, C. Zhou, N. Mazurski, J. Bar-David, A. Kristensen, and U. Levy, “Near-IR wide-field-of-view Huygens metalens for outdoor imaging applications,” Nanophotonics 9(2), 361–370 (2020). [CrossRef]
24. S. Wang, J. Lai, T. Wu, C. Chen, and J. Sun, “Wide-band achromatic flat focusing lens based on all-dielectric subwavelength metasurface,” Opt. Express 25(6), 7121–7130 (2017). [CrossRef]
25. Z. B. Fan, H. Y. Qiu, H. L. Zhang, X. N. Pang, L. D. Zhou, L. Liu, H. Ren, Q. H. Wang, and J. W. Dong, “A broadband achromatic metalens array for integral imaging in the visible,” Light: Sci. Appl. 8(1), 67 (2019). [CrossRef]
26. R. J. Lin, V. C. Su, S. Wang, M. K. Chen, T. L. Chung, Y. H. Chen, H. Y. Kuo, J. W. Chen, J. Chen, Y. T. Huang, J. H. Wang, C. H. Chu, P. C. Wu, T. Li, Z. Wang, S. Zhu, and D. P. Tsai, “Achromatic metalens array for full-colour light-field imaging,” Nat. Nanotechnol. 14(3), 227–231 (2019). [CrossRef]
27. X. Zang, W. Xu, M. Gu, B. Yao, L. Chen, Y. Peng, J. Xie, A. V. Balakin, A. P. Shkurinov, Y. Zhu, and S. Zhuang, “Polarization-Insensitive Metalens with Extended Focal Depth and Longitudinal High-Tolerance Imaging,” Adv. Opt. Mater. 8(2), 1901342 (2020). [CrossRef]
28. M. Decker, W. T. Chen, T. Nobis, A. Y. Zhu, M. Khorasaninejad, Z. Bharwani, F. Capasso, and J. Petschulat, “Imaging Performance of Polarization-Insensitive Metalenses,” ACS Photonics 6(6), 1493–1499 (2019). [CrossRef]
29. 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]
30. T. Hu, Q. Zhong, N. Li, Y. Dong, Z. Xu, Y. H. Fu, D. Li, V. Bliznetsov, Y. Zhou, K. H. Lai, Q. Lin, S. Zhu, and N. Singh, “CMOS-compatible a-Si metalenses on a 12-inch glass wafer for fingerprint imaging,” Nanophotonics 9(4), 823–830 (2020). [CrossRef]
31. S. Uenoyama, Y. Takiguchi, K. Takahashi, K. Hirose, H. Terada, and A. Watanabe, “Contact metalens for high-resolution optical microscope in semiconductor failure analysis,” Opt. Lett. 45(22), 6218–6221 (2020). [CrossRef]
32. A. She, S. Zhang, S. Shian, D. R. Clarke, and F. Capasso, “Large area metalenses: design, characterization, and mass manufacturing,” Opt. Express 26(2), 1573–1585 (2018). [CrossRef]
33. B. Xu, H. Li, S. Gao, X. Hua, C. Yang, C. Chen, F. Yan, S. Zhu, and T. Li, “Metalens-integrated compact imaging devices for wide-field microscopy,” Adv. Photonics 2(6), 066004 (2020). [CrossRef]
34. F. Tang, X. Ye, Q. Li, H. Li, H. Yu, W. Wu, B. Li, and W. Zheng, “Quadratic Meta-Reflectors Made of HfO2 Nanopillars with a Large Field of View at Infrared Wavelengths,” Nanomaterials 10(6), 1148 (2020). [CrossRef]
35. J. Wu, F. Tang, J. Ma, Q. Li, S. Shang, J. Chen, Y. Wu, Y. Wang, X. Ye, W. Zheng, and R. Zhu, “Angle-sensitive dynamic optical modulation based on Huygens metasurfaces,” Results Phys. 18, 103226 (2020). [CrossRef]
36. W. Zhao, H. Jiang, B. Liu, J. Song, Y. Jiang, C. Tang, and J. Li, “Dielectric Huygens’ Metasurface for High-Efficiency Hologram Operating in Transmission Mode,” Sci. Rep. 6(1), 30613 (2016). [CrossRef]
37. M. Kim, A. M. H. Wong, and G. V. Eleftheriades, “Optical Huygens’ Metasurfaces with Independent Control of the Magnitude and Phase of the Local Reflection Coefficients,” Phys. Rev. X 4(4), 041042 (2014). [CrossRef]
38. S. Shang, F. Tang, X. Ye, Q. Li, H. Li, J. Wu, Y. Wu, J. Chen, Z. Zhang, Y. Yang, and W. Zheng, “High-Efficiency Metasurfaces with 2pi Phase Control Based on Aperiodic Dielectric Nanoarrays,” Nanomaterials 10(2), 250 (2020). [CrossRef]
