Toroidal Dipole-Induced Photocurrent Enhancement in Silicon Nanodisk Hexagonal Array beyond 1400 nm in Wavelength

A laser with the emission wavelength longer than 1400 nm is “eye safe”, because light in the wavelength range is strongly absorbed in the cornea and lens and cannot reach more sensitive retina. The eye safe lasers have been used as a light source for LiDARs in atmospheric physics, autonomous driving, etc. For the development of low-cost LiDARs for autonomous driving, a silicon-based CMOS compatible photodetector operating beyond 1400 nm is crucial. Several methods have been proposed to extend the detection range of a silicon-based photodetector beyond the band gap wavelength. The most widely studied one is using internal photoemission at a Schottky junction between silicon and metal nanoantennas. Another approach is enhancing defect-mediated weak absorption of silicon by optical resonances.<br/> In this work, we show that a hexagonal array of low-aspect-ratio silicon nanodisks formed on a silicon thin film has the sub-radiant high-Q toroidal dipole (TD) resonances that strongly enhance light absorption in the low material loss wavelength range, i.e., sub-band gap range. We also demonstrate that the absorption enhancement appears as a photocurrent enhancement at the resonance wavelength, and thus the metasurface can be used as a narrow-band photodetector operating in the silicon sub-band gap wavelength range [1-2]. To further boost the performance of that device, we design a structure having perfect absorption by introducing the concept of the bound states in the continuum (BICs) in the TD resonances of a silicon nanodisk hexagonal array. The idea is introducing a mirror below the metasurface via a silicon dioxide spacer. In proper spacer thicknesses, interference between a TD resonance in a nanodisk array and the image dipole in the mirror cancels the radiation perfectly and BICs emerge. Slight detuning of the BIC condition by controlling the spacer thickness makes the radiation loss finite and to be equal to the material loss. This results in perfect absorption at the TD resonance wavelengths. Since the absorption is predominantly in the silicon region, the photocurrent is expected to be strongly enhanced. In this structure, the absorption spectrum is relatively insensitive to the incident angle and is independent of the polarization direction. Furthermore, since the TD resonances are very narrow, the device can detect photons at specific wavelengths without a bandpass filter. The device is composed of silicon, silica and aluminum mirror and electrodes, and does not require narrow band gap semiconductors and noble metal nanostructures for the detection of long wavelength photons. Therefore, the device can be easily integrated in silicon-based electronic devices.<br/><br/>[1] H. Hasebe, H. Sugimoto, T. Hinamoto, and M. Fujii, Advanced Optical Materials, 22, 8 ,2001148 (2020).<br/>[2] H. Hasebe, K. Moriasa, K. Yamashita, H. Sugimoto, and M. Fujii, ACS Photonics, 9, 10, 3302-3309, (2022).