Infrared spectrum detection is to detect and analyze the light wavelength information of matter through detectors, like Sun Wukong's piercing eyes, which can see the essence through appearance and accurately identify the material information that cannot be judged by humans eyes. The short-wave infrared (SWIR) band is rich in material spectral information, and has the ability to penetrate clouds and fog. It has a wide range of applications in food, agriculture, remote sensing imaging and other fields.
Conventional detectors do not have the capability of spectral identification and need to use gratings, interferometers or other spectroscopic devices to detect the spectral information of targets. For traditional multispectral detection systems, their detectors and spectroscopic devices are separated, requiring mechanical assembly and scanning, resulting in large volumes and complex systems, which are not suitable for portable and on-site detection scenarios.
In recent years, photonic crystals, metamaterials and quantum dots, which have received extensive attention and research, can reduce the spectral detection system to chip level, which has good application prospects. In the SWIR band, the photonic crystals and metamaterials need a high-precise fabrication process with high cost because of their sub-wavelength structures. The quantum dot filters array is restricted by its low repeatable synthetic preparation process and insufficient long-term stability. In contrast, pixel-level spectroscopic arrays based on Fabry-Perot (FP) microcavity can be fabricated by UV lithography require easier fabrication process. Its transmission spectrum is easy to be precisely tuned by cavity thickness, and its structure is stable and easy to be integrated with detectors.
Professor Wang Shaowei's team from Shanghai Institute of Technical Physics, Chinese Academy of Sciences, has reported an on-chip SWIR multispectral detector based on monolithically integrated Fabry–Perot microcavities array. By monolithically integrating the pixel-level FP microcavity structures on InGaAs focal plane array (FPA), a multispectral detection chip is realized in SWIR band. This research has been published in Chinese Optics Letters, Vol. 20, Issue 6, 2022 (Zhiyi Xuan, et al. On-chip short-wave infrared multispectral detector based on integrated Fabry–Perot microcavities array) and selected as the Cover paper.
The researchers monolithically integrated pixel-level FP microcavity arrays with different transmission spectra onto an InGaAs FPA by a combinatorial etching process, each FP microcavity structure corresponds to a multispectral detection channel, to achieve chip-level SWIR Multispectral detection.
In this work, a pixel-level FP microcavities array with 16 different spectral channels is monolithically integrated on a 64×64 pixel InGaAs FPA through 4 times combinatorial etching processes. For the first time, the multi-spectral detection chip in the 1400 ~ 1700 nm SWIR band has been demonstrated, which makes the chip itself have the ability of spectral identification, and can identify the SWIR spectral information of 16 different wavelengths, realizes the integration of spectroscopy and detection, and greatly simplifies the structure of SWIR multispectral detection system, minimized its size.
As shown in Fig. 1(a), when the incident light is irradiated on the chip, it first resonates in the FP microcavity and performs wavelength selection, and the light of the transmitted wavelength is received by the detector. By adjusting the thickness of the FP microcavity cavity layer at different positions, 16-channel on-chip multispectral detection is realized. Figure 1(b) shows the SWIR multispectral detector chip, which is only 2 mm2 in size, the same as the original InGaAs FPA. Figure 1(c) shows the response spectrum of the chip's 16 channels. The number of spectral channels of the multi-spectral detection chip can also be greatly increased according to needs. Only 7 combined etchings can achieve a wider range of spectral detection with 128 channels (the number of spectral channels increases exponentially by 2N). The multi-spectral detection chip turns the original detector that can only respond to light intensity information into a spectral detection chip, so that it has the ability to identify material components. Figure 1(d) shows the gas detection channel of the chip, which can detect CO2, C2H2, H2S, CH4 gases at the same time.
Figure 1 (a) Schematic diagram of the on-chip integrated FP microcavities array SWIR multispectral detector; (b) SWIR multispectral detector chip; (c) 16-channel response spectrum of the SWIR multispectral detector; (d) The gas detection response spectral channel of the SWIR multispectral detector chip.
The bandwidth and center wavelength of the multi-channel response spectrum of the chip can be flexibly adjusted by the integrated FP microcavity structure, and the spectral resolution can be designed to be within 1 nm. In addition, each detection channel of the multispectral detector chip has a flexible arrangement, which can be set as one-dimensional array, two-dimensional array or super-pixel arrangement according to requirements. The unit size can be reduced to the size of the detector pixel. The SWIR multispectral detection chip has no moving parts, is especially suitable for portable micro-miniature spectroscopy instruments, and has broad application prospects in the fields of gas detection, food safety, agricultural production, biomedicine.