(Invited) Detection of Wavelength Information by Filter-Free Wavelength Sensor and Its Applications
Authors: Choi Yong-Joon , Sakae Tsugumi , Nakano Kakeru , Ryoma Mibu , Matsubara Ryuya , Ide Tomoya , Takahashi Kazuhiro , Noda Toshihiko , Ishii Hiromu , Sawada Kazuaki 
Published: February 2024

This paper introduces the wavelength and light intensity detection method using the filter-free wavelength sensor without optical filters. Optical detection methods are widely used in medical, environmental, chemical, biofield, and agricultural analyses. In the biofield, the optical detection method makes it possible to predict, diagnose, and analyze diseases by detecting the wavelength information and light intensity, such as absorption, fluorescence, and spectral shift.

In order to measure a specific wavelength and intensity of light, mainly used a detection method such as a filter cube of a fluorescence microscope or a monochromator of a spectrometer system. Even if this system has high sensitivity and selectivity, it is not easy to apply to POCTs that require portability due to the high price and increased system volume due to the integration of optical filters and other parts. Therefore, various studies have reported using semiconductor processes to detect the wavelength and light intensity with a high-performance and miniaturized system.

An on-chip photodetection device has been reported, such as a fluorescence detection sensor to integrate interference or absorption filters on CMOS image sensors with high selectivity and sensitivity. Since the specific wavelength of light intensity detection is possible, it can apply to biofields such as fluorescence analysis. However, detecting different wavelength changes by fluorescent reagent is difficult because the optical filter is integrated into the CMOS image sensor. A method of multi-wavelength analysis with the single-pixel without an optical filter is reported using a CMOS-buried quad p-n junction photodetector. Since the light wavelength has a different absorption depth depending on the silicon absorption coefficient, it is possible to separate wavelength by measuring the current generated at each p-n junction. Therefore, it will provide a high-space-resolution image in the biofield. However, the possibility detection wavelength band is fixed according to the buried p-n junction numbers and depth.

Previously, we reported a filter-free wavelength sensor with a photogate structure. Sensor can detect light intensity and wavelength without optical filters based on the dependence of the light absorption depth in silicon. The sensor fabricates using the 1-polysilicon, 2-metal process in the LSI facility of the Toyohashi University of Technology in Japan, as shown in Fig1(a). The sensor consists of a three-layer structure where deep n-well and p-well were formed on a p-type silicon substrate, as shown in Fig1(b). Applying a voltage to the photogate and n-well forms a potential peak W as a mountain shape below the sensing area. Therefore, the photocurrent generated by the incident light was divided into the surface side (I
PG), and bottom side (I
n-well) based on the potential peak W and wavelength identification is possible from the current ratio (I
n-well/I
PG). To detect the wavelength information of multi-peak light for applications in the biofield, compare the centroid wavelength and current ratio. We experimentally confirmed that the centroid wavelength was proportional to the current ratio by the light absorption depth of silicon. Since the light absorption depth depends on the light wavelength, the current ratio is constant even when the light intensity changes. Therefore, estimation of the light intensity is possible by the sum of each current.

We proposed a wavelength, spectral shift, and absorbance measure method using a filter-free wavelength sensor to apply various fields. Figure 2(a) shows a schematic for detecting the fluorescence wavelength. Without using optical filters, fluorescence can be quantified by calculating the centroid wavelength of the light even when the excitation light is emitted simultaneously. In addition, it successfully identified fluorescence emitted from Legionella bacteria [1]. Figure 2(b) shows a schematic diagram that can identify the change in the spectrum using a sensor. Integrating an LSPR biosensor into a filter-free wavelength sensor makes it possible to measure the amount of spectrum changed by molecular binding. Therefore, miniaturization of the LSPR detection system becomes possible, and we succeeded in detecting and quantifying the S-protein of SARS-CoV-2 [2]. Figure 2(c) shows a schematic diagram of the absorbance measurements method for samples that absorb a specific wavelength range. By the proposed absorbance measurement method was possible to measure the ratio of chlorophyll content and chlorophyll a/b ratio contributing to plant photosynthesis [3].

The filter-free wavelength sensor fabricated by the semiconductor process has been confirmed for its application as an optical detection system. In the future, we will continue research to develop an optical sensor system that is more compact, sensitive, and selective and can be applied to various fields.

[1] Biosensors, vol.12, no.11, p.1033 (2022)

[2] Applied Physics Express, vol.16, no.1, p.012012 (2023)

[3] Jpn. J. Appl. Phys., vol.61, p.SD1041 (2022)

Figure 1