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Abstract
The performance of a charge-coupled device is important in detection accuracy for terminal sensitivity of a short-wave near-infrared spectrometer. The sizes of pixel pitch and pixel itself are reducing with the development of CCD technologies. However, the fill factor of CCD pixels has not been significantly improved due to the limits of the shift registers, which makes a lower utilization rate of incident light of CCD. In recent years, the advance of metasurface optics provides a new solution for solving this problem. In this paper, a polarization-insensitive metalens is experimentally demonstrated to increase the fill factor of short-wave near-infrared CCD pixels by 4 times, and the simulated results show that the designed metalens has an excellent optical crosstalk (≤0.8%). It proves that the fill factor of CCD pixels can be further improved by the proposed approach which would pave the way for the overall integration of metalens array and photodetectors, as well as the development of CCD miniaturization and lightweight.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The short-wave near-infrared (SW-NIR) (700-1100 nm) detection technique can fast, non-destructively, and quantitatively determine various components in drinks [1,2], biological and biomedical materials [3–5]. In addition, it has been proven that the SW-NIR spectroscopy is capable of measuring glucose concentrations in blood which has great potential in clinical use and determination of glycaemic index in foods [6]. As the terminal detection equipment for a SW-NIR spectrometer, the charge-coupled device (CCD) plays a vital role in the detection process. Since its invention, it has been developing in the directions of miniaturization, high resolution, and high sensitivity. The pixels forming a whole CCD are metal-oxide-semiconductor (MOS) capacitors converting optical signals into electrical signals and then transferring to a shift register before the terminal output. Due to the existence of the shift register, there is a large gap between adjacent pixels, so that the pixels cannot obtain most of the incident light, resulting in low efficiency of the incident light illumination and insufficient detection accuracy for a CCD. Increasing the fill factor (FF) of each pixel can improve the selectivity and the detectivity of the CCD. Furthermore, if the energy of the incident light remains constant, the increase in FF can increase the F-number of the imaging optics, which means that it is possible to realize a simple and lightweight optical system [7]. The normal method is to focus the incident light to the pixels through the integrated microlens array, thereby increasing the FF of the pixels [8]. Due to the first-order diffraction existing in the adjacent pixels, this increase of FF using a microlens array inevitably leads to the optical crosstalk between adjacent pixels. Besides, the complicated manufacturing process of microlenses and their thicknesses that are larger than the wavelength of the incident light further restrict the wide application of microlenses in CCDs. In addition, broadband achromatic diffractive microlens arrays (MDLs) with low F-number and extended depth of focus (DOF) based on 3D spatial point-spread function (PSF) engineering seem to be good choices to reduce the size of the CCDs [9,10], however, the spatially varying topography of diffractive lenses seems unfavorable for integration with planar devices such as CCDs, Additionally, MDLs generally have large periodicity.
In recent years, the rapid development of metasurface optics has provided a new way to solve the problem of increasing the FF of CCD pixels. The previous studies have shown that metasurfaces have great potential to replace traditional bulky optical components [11–15]. By properly arranging the nanostructures on the substrate, the metasurface can almost arbitrarily control the amplitude, phase, and/or polarization of the incident light in the sub-wavelength scale, resulting in a large number of planar device applications, such as holograms [16–22], wave plates [23–25], vortex beam generator [26–29], Bessel beam generator [30], etc. Compared with the traditional optical microlenses, metalenses [31–40] are thinner, lighter, and lower in cost [31]. The previous research about infrared metalens mainly focused on the mid-wave infrared range, and several metallic metalenses [41], dielectric metalenses [42], and solid-immersion metalenses [43] integrated with mid-wave infrared focal plane arrays (FPAs) have been demonstrated. Among them, the focusing efficiency of the metallic metalens is 11%, which is too low to be applied in practical applications because of the intrinsic ohmic loss of metallic materials. The dielectric metalens array can obtain high focusing efficiency and excellent optical crosstalk ($\leq$3%) at the design wavelength. However, optical crosstalk due to chromatic effects [40] and slow narrow-band response limit its use in a broad spectrum. The solid-immersion metalens is expected to achieve monolithic integration with infrared FPA, while the depth of focus is significantly larger than the wavelength of the incident light. So far, to the best of our knowledge, there has been no research on a metalens integrated with SW-NIR CCD.
In this paper, we demonstrate a polarization-insensitive metalens that operates in SW-NIR and can focus the incident light in an area in the same size as the CCD pixel to improve the FF of CCD pixels. Experimental results show that the average efficiency of the designed metalens within the wavelength range of 780-1060 nm is 51% with a peak value of 57%, which means that this novel metalens increases FF of SW-NIR CCD pixels by 4 times, from 10% to 51%. An optimized manufacturing process was used to ensure the sharply vertical nanoposts could be produced as the standard unit cells of the metasurface, which could excellently confine the incident light inside the nanoposts with a maximum aspect ratio of 1/3. Within the design bandwidth, the average optical crosstalk indicates that the designed metalens is beneficial to the miniaturization of CCD pixels. Our work proves that, to a certain extent, the metalens has great potential in improving the performance of photoelectric detection equipment.
2. Design of metalens
The metalens composed of a large number of nanoposts on the substrate focuses the collimated incident light to a spot as is shown in Fig. 1(a). The circular cross-section and symmetric distribution of the nanoposts determine that the metalens is polarization-insensitive. The phase delay between the nanoposts at the position $(x, y)$ and center of the metalens is written as
Fig. 1. (a) Schematic of the transmission metalens. (b) The side view and top view of the building blocks of the designed metalens. (c) and (d) The simulated phase and transmission of the nanopost as a function of the unit cell size and the post diameter. (e) Top and side view of the normalized magnetic energy density in a periodic array for c-Si nanopost with a diameter of 160 nm. The white dashed circles and rectangles show the boundaries of the c-Si nanopost. Scale bar, 200 nm. (f) For the transmission metalens, the simulated transmission and phase of the nanopost as a function of nanopost diameters at the design wavelength of 790 nm. (g) Calculated transmission as a function of the wavelength and the post diameter for the transmission metalens.
To construct the metalens, we discretize the required phase profile $\varphi (x,y)$ and select an appropriate diameter for the nanopost at each position $(x, y)$. A single unit cell can be considered as a truncated waveguide, the propagation length is equal to the height H of the nanopost, then the accumulated propagation phase of the unit cell can be expressed as
where $n_{eff}$ is the effective refractive index of the unit cell. The effective refractive index of the unit cell changes with the diameter of the nanoposts when the wavelength of the incident light and the height of the nanoposts are fixed. Therefore, the accumulated propagation phase can be modulated by changing the diameter of the nanoposts to obtain the desired phase distribution.As shown in Fig. 1(b), the building block (BB) of the designed metalens is composed of a circular crystalline silicon (c-Si) nanopost at the center of the square sapphire substrate. Sapphire has a low absorption within the design bandwidth, and compared with amorphous silicon (a-Si), c-Si has a lower loss in SW-NIR range and a high enough refractive index to ensure $2\pi$ phase coverage while well-confining incident light in the nanoposts. At the design wavelength of 790 nm, the refractive indices of c-Si and sapphire are 3.68 and 1.76, respectively. Rigorous coupled-wave analysis (RCWA) is used to optimize the geometric parameters of the nanopost [44]. The distance (U) between adjacent nanoposts should be small enough to meet the Nyquist sampling standard (U<$\lambda$/2NA). At the design wavelength of 790 nm, the parameter sweep results of phase and transmission are demonstrated in Fig. 1(c) and 1(d) for the nanoposts with the height of H = 500 nm, diameter D from 100 to 280 nm, and unit cell size U range from 300 to 420 nm. It is shown that the nanoposts with the unit cell size of 330 nm can obtain a phase coverage that is greater than 2$\pi$ within the diameter range from 100 to 280 nm (the black horizontal dashed lines in Fig. 1(c)), while maintaining high transmission (the black horizontal dashed lines in Fig. 1(d)). Meanwhile, the unit cell size is smaller than the equivalent wavelength range (780–1060 nm) in the sapphire substrate ($780 nm/n_{sapphire} = 443 nm$) but greater than the diffraction condition ($1060 nm/2n_{sapphire} = 302 nm$), which ensures that only zero-order diffraction exists at normal incidence. Figure 1(e) shows the magnetic energy density of the periodic arrays for the nanopost with a diameter of 160 nm at the wavelength of 790 nm. It can be seen that the scattering of the nanopost is a local effect, and the optical coupling effect between the neighboring nanopost is very weak. The unit cell with a Sub-wavelength scale ensures that only zeroth-order diffraction exists for normal incidence. For the designed metalens, the ideal dimensions of the nanopost are U = 330 nm, D from 150 to 276 nm, and H = 500 nm, and the transmission and propagation phase at the design wavelength are plotted in Fig. 1(f), showing that the phase varies from 0 to 2$\pi$ by changing the nanopost diameters and transmission over 90% can be obtained. The abrupt change in transmittance and phase (the grey rectangles in Fig. 1(f)) caused by electric dipole resonance should be excluded from the design. Figure 1(g) illustrates the simulated transmission of the nanoposts in the operation band of 780-1100 nm, which manifests that the proposed nanoposts support relatively high transmission in the most operating band besides the reduction of the transmission caused by the electric dipole resonance.
3. Fabrication and characterization of the metalens
The metalens was designed to be 22 $\mu$m$\times$22 $\mu$m in size with an FF of 100%, which can collect the incident light to the greatest extent. The metalens was fabricated on a wafer structured with a layer of 500 nm thick c-Si and a 500 $\mu$m thick sapphire substrate. The pattern was generated in high-resolution negative resist (Hydrogen silsesquioxane, HSQ, 200 nm thick film, Dow Corning) by electron beam lithography (EBL, Raith Vistec EBPG-5000plusES). The development was performed in tetramethylammonium hydroxide (TMAH), and the metalens pattern was etched by inductively coupled plasma (ICP, Oxford Instruments PlasmaPro 100ICP180) etching technology with HBr chemistry. Finally, the redundant HSQ was removed by hydrogen fluoride (HF) acid. Scanning electron microscope (SEM, Zeiss Auriga) images of the fabricated metalenses are shown in Figure 2.
Fig. 2. (a) Top-view scanning electron microscope (SEM) image of the designed metalens. Scale bar: 2 $\mu$m. (b) Top-view SEM image of a portion of the metalens at a higher magnification than that in (a), displaying each naopost. Scale bar: 2 $\mu$m. (c) Sideview SEM image of the edge of the metalens, showing the vertical profile of the nanoposts. Scale bar: 200 nm.
We characterize the performance of fabricated metalens using the experimental setup illustrated as shown in Fig. 3. In the experimental setup, the incident beam emitted by a broadband bromine tungsten lamp (Zolix GLORIA-T250A) through a monochromator (Zolix Omni-$\lambda$300i), is collimated by a fiber-coupled collimator (Thorlabs RC04SMA-P01) and is focused to the metalens by an achromatic doublet lens (Thorlabs AC254-100-AB). On the motorized stage, a 100 $\times$ objective lens (Olympus UMPlanFl) paired with a tube lens (Thorlabs AC254-200-AB) is used to form an image of the metalens on a CMOS camera (HIKROBOT MV-CA050-20GN). The square iris (Daheng Optics GCM-5711M) is used to remove the background noise. Then, the stage is moved toward the focal plane of the metalens with a step of 1 $\mu$m. The three-dimensional (3D) far-field profile of the metalens is measured by the two-dimensional (2D) images acquired at each position. For the focusing efficiency and efficiency measurements, the camera was replaced by a power meter (Thorlabs PM100USB with Thorlabs S121C photodiode power sensor). The incident power is measured by focusing the camera on the metalens, adjusting the square iris to match the size of the metalens on the camera, removing the metalens and recording the power. The focusing power is measured by adjusting the iris (Thorlabs SM1D12CZ) to three times of the FWHM of the focus on the image plane, then replacing the camera by the power meter and recording the power. To measure the efficiency, the power within the pixel area is measured by adjusting the square iris to an area of corresponding size on the image plane, and then recording the power.
Figure 4(a) shows the comparision between the experimentally measured and simulated normalized intensity distributions for the metalens at the design wavelength of 790 nm in the x-z plane and x-y plane, respectively. It can be seen that as the transmitted light moves away from the surface of the metalens, the optical density increases, eventually forming a focal point, which is consistent with the theoretical design of the metalens. The symmetry of the focal spots reveals that the designed metalens is polarization-insensitive. The measured intensity profiles for the designed metalens with different z values along the optical axis at 790 nm are shown in Fig. 4(b). The horizontal cuts of the measured and simulated focal spots are shown in Fig. 4(c). The difference between these two full widths at half maximum (FWHMs) is mainly attributed to the dimensional error between the fabricated nanoposts and the design. The intensity profiles within the range of 780-1060 nm with a step of 20 nm are shown in Fig. 6–8 in the Appendix. The measured and simulated focal lengths are plotted in Fig. 4(d), which illustrates that the focal lengths of metalens decrease with the growth of incident wavelength, reflecting the same tendency as the conventional diffraction dispersion of binary optics. The dispersion of the metalens can caused by the resonant phase dispersion of the building blocks and the intrinsic dispersion of the used materials, which can be eliminated by introducing an additional phase [45].
Fig. 4. (a) Simulated (left) and measured (right) normalized intensity profiles in the x-z plane and x-y plane at the design wavelength of 790 nm. (b) Measured intensity profiles for the designed metalens with different z values along the optical axis at the design wavelength. (c) Measured and simulated horizontal cuts of the focus at the design wavelength. (d) Measured and simulated focal length as a function of incident wavelength. (e) Focusing efficiency for the designed metalens as a function of incident wavelength. (f) Measured and simulated efficiency of the SW-NIR metalens at different incident wavelengths.
The focusing efficiency is defined as the ratio of the optical power in the area with a radius which is equal to three times of the FWHM on the focal plane to that for the incident beam. As shown in Fig. 4(e), the measured focusing efficiencies of the metalens can maintain in the range of 42%$\sim$58%, with a peak value of 58.3% at 900 nm. Within the entire operating bandwidth, the measured focusing efficiency is 16.2% lower than the corresponding simulated results. The experimental results prove that the designed metalens is capable of focusing the incident light onto the pixel of the SW-NIR CCD.
The FF of the CCD can be defined as:
where $S_p$ and $S$ represent the area of the pixel and the square of the pixel pitch size, respectively. To characterize the performance of the metalens in improving the FF of CCD pixels while distinguishing the focusing efficiency, here, we define the efficiency of the metalens as the ratio of the optical power over the pixels within an area of 7 $\mu$m$\times$7 $\mu$m to the incident power, as shown in Fig. 9 in the Appendix. The measured and simulated efficiencies of the designed metalens are shown in Fig. 4(f). The measured average efficiency within the entire operating bandwidth is 51%, and the corresponding simulated value is 67.5%. The experimental results show that the designed metalens increase the FF of the CCD pixels by 4 times for a CCD with a pixel FF of 10%.The difference between the measured and simulated values is mainly attributed to the reflection of the sapphire substrate and the scattering caused by the roughness of the etched c-Si nanopost surface. In addition, the focal spot size of the metalens can be adjusted by changing the designed numerical aperture (NA) to suit different CCD pixel sizes. For example, further reduction of the beam size to smaller than 7 $\mu$m$\times$7 $\mu$m is achievable by increasing the NA of the metalens. By using single-step photolithography and standard etching process, the designed metalens are scalable and are expected to realize monolithically integration with CCDs in the future.
4. Simulation of the optical crosstalk
Optical crosstalk, as a critical parameter of CCDs, affects the performance of electro-optical used to detect small objects with a low signal-to-noise ratio(SNR). The trade-off between optical crosstalk and pixel pitch size is a major obstacle for the development of next-generation CCD with a higher resolution and a smaller size. To prove that the designed metalens can effectively suppress optical crosstalk, we performed numerical simulations of the optical crosstalk of the designed metalens in the plane of pixels. Here, the optical crosstalk can be defined as the ratio of the PSF distribution of 8 adjacent pixels and that within the central pixel area [42], written as:
As shown in Fig. 5(a), within the designed operating bandwidth, the average of the simulated optical crosstalk of the designed metalens is 0.76%, with a minimum value of 0.53%. Compared with previous studies based on numerical simulation, the designed metalens achieves the best optical crosstalk, even lower than that of metallic metalens($\leq$1%) [41,42], which can be attributed to a good sampling of the required phase of the metalens caused by a relatively small lattice constant. The simulated results show that the optical crosstalk changes relatively smoothly within 780-940 nm, which is different from the previous research conclusions [40]. This is because a small amount of scattered light that is not focused does not fall in adjacent pixels, which also shows from the side that the designed metalens is beneficial to the miniaturization of CCD pixels. To further illustrate the suppression effect of the designed metalens on optical crosstalk, we simulated the normalized intensity distribution of the 2$\times$2 metalens array in the plane of the pixel when the incident wavelengths are 780 nm, 940 nm, and 1060 nm, respectively, as shown in Fig. 5(b)-(d). The white solid line in the figure represents the pixel area. The simulation results show that the converging light spots of a single metalens in the array are uniformly distributed in their respective regions.
Fig. 5. (a) Simulated optical crosstalk of the designed metalens at the different incident wavelengths. (b)-(d) The simulated normalized intensity profiles of the metalens array in the plane of the pixel at incident wavelengths of 780 nm, 940 nm, and 1060 nm, respectively.
5. Conclusion
In summary, we demonstrated that the designed metalenses can focus the incident light to an area of 7 $\mu$m$\times$7 $\mu$m within the bandwidth of 780-1060 nm, and increase the FF of CCD pixels by 4 times. The thickness of the designed metalens is only 500 nm, which is much smaller than that for SW-NIR microlens and beneficial to miniaturization and lightweight of the CCD. Our work is a useful approach for realization of future SW-NIR CCDs with high resolution and small pixel size, and may inspire the researchers working in development of monolithic integration of metalens arrays to optoelectronic devices, such as CCDs, CMOS, and FPAs.
6. Appendix
Fig. 6. (a) Experimental and (b) simulated intensity distribution profiles of the designed metalens at the different incident wavelengths in the x-z plane, respectively.
Fig. 7. (a) Experimental and (b) simulated intensity distribution profiles of the designed metalens at the different incident wavelengths in the x-y plane, respectively.
Fig. 8. Experimental (the red solid line) and simulated (the black dashed line) horizontal cuts corresponding to the focus for different incident wavelengths with a step of 20 nm.
Funding
Chinese Aeronautical Establishment (2019010810063); National Natural Science Foundation of China (61904138); National Natural Science Foundation of China (61774120).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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.
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