Benzimidazole fungicide residue in food products poses a risk to consumer health. Due to its localized electric-field enhancement and high-quality factor value, the metamaterial sensor is appropriate for applications regarding food safety detection. However, the previous detection method based on the metamaterial sensor only considered the resonance dip shift. It neglected other information contained in the spectrum. In this study, we proposed a method for highly sensitive detection of benzimidazole fungicide using a combination of a metamaterial sensor and mean shift machine learning method. The unit cell of the metamaterial sensor contained a cut wire and two split-ring resonances. Mean shift, an unsupervised machine learning method, was employed to analyze the THz spectrum. The experiment results show that our proposed method could detect carbendazim concentrations as low as 0.5 mg/L. The detection sensitivity was enhanced 200 times compared to that achieved using the metamaterial sensor only. Our present work demonstrates a potential application of combining a metamaterial sensor and mean shift in benzimidazole fungicide residue detection.

A high-speed and high-sensitivity avalanche photodetector (APD) is a critical component of a high-data-rate and low-power optical-communication link. In this paper, we study a high-speed and high-efficiency Ge/Si heterostructure APD. First, we numerically study the speed performance of the APD by analyzing frequency response. An optimized epitaxial structure of the high-speed APD is designed. In the absence of RC time effects, the APD exhibits a fast pulse response (full-width at half-maximum) of 10 ps and a high 3 dB bandwidth of 33 GHz at a high-gain value of 10. Taking device size and the corresponding RC time effects into account, the APD still achieves a high 3 dB bandwidth of 29 GHz at a gain value of 10. Moreover, a novel subwavelength periodic hole array is designed on the normal-incidence APD for enhancing light absorption without sacrificing speed performance. Near-perfect absorption is almost achieved by an infinite-period hole array due to the coupling of dual-resonance modes. A high-absorption efficiency of 64% is obtained by a limited-sized hole array in the high-speed APD. This work provides a promising method to design high-speed and high-efficiency normal-incidence Ge/Si heterostructure APDs for optical interconnect systems.