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Absorbers such as photonic crystals, metasurfaces, and nanowires have been demonstrated to have important applications in solar cells, detectors, etc. The application of thin-layer-based absorbers to photodetectors is beneficial to simultaneously improve responsivity and 3 dB bandwidth. In this work, we propose a high-contrast-gratings-like absorber to achieve ultra-high absorption based on a thin Germanium layer. High absorption approximately 95% at wavelength 1310 nm is reached for TE-polarized incidence due to the interference of two guided modes, which is 4 times compared to the planar film with the same thickness. The high absorption of the absorber is also experimentally demonstrated. This work shows a new method for enhanced absorption with thin Germanium layer, indicating a promising prospect for high-efficiency photodetection.

Highly sensitive detection of enhanced terahertz (THz) fingerprint absorption spectrum of trace-amount tiny molecules is essential for biosensing. THz surface plasmon resonance (SPR) sensors based on Otto prism-coupled attenuated total reflection (OPC-ATR) configuration have been recognized as a promising technology in biomedical detection applications. However, THz-SPR sensors based on the traditional OPC-ATR configuration have long been associated with low sensitivity, poor tunability, low refractive index resolution, large sample consumption, and lack of fingerprint analysis. Here, we propose an enhanced tunable high-sensitivity and trace-amount THz-SPR biosensor based on a composite periodic groove structure (CPGS). The elaborate geometric design of the spoof surface plasmon polaritons (SSPPs) metasurface increases the number of electromagnetic hot spots on the surface of the CPGS, improves the near-field enhancement effect of SSPPs, and enhances the interaction between THz wave and the sample. The results show that the sensitivity (S), figure of merit (FOM) and Q-factor (Q) can be increased to 6.55 THz/RIU, 4234.06 1/RIU and 629.28, respectively, when the refractive index range of the sample to measure is between 1 and 1.05 with the resolution 1.54×10−5 RIU. Moreover, by making use of the high structural tunability of CPGS, the best sensitivity (SPR frequency shift) can be obtained when the resonant frequency of the metamaterial approaches the biological molecule oscillation. These advantages make CPGS a strong candidate for the high-sensitivity detection of trace-amount biochemical samples.

Plasmonic photodetection based on the hot-electron generation in nanostructures is a promising strategy for sub-band detection due to the high conversion efficiencies; however, it is plagued with the high dark current. In this paper, we have demonstrated the plasmonic photodetection with dark current suppression to create a Si-based broadband photodetector with enhanced performance in the short-wavelength infrared (SWIR) region. By hybridizing a 3 nm Au layer with the spherical Au nanoparticles (NPs) formed by rapid thermal annealing (RTA) on Si substrate, a well-behaved ITO/Au/Au NPs/n-Si Schottky photodetector with suppressed dark current and enhanced absorption in the SWIR region is obtained. This optimized detector shows a broad detection beyond 1200 nm and a high responsivity of 22.82 mA/W at 1310 nm at −1 V, as well as a low dark current density on the order of 10−5 A/cm2. Such a Si-based plasmon-enhanced detector with desirable performance in dark current will be a promising strategy for realization of the high SNR detector while keeping fabrication costs low.

In the field of modern optical communication systems and photoelectric detection, new components with complex functions and excellent performance are urgently needed. In this paper, a graphene-based parity–time (PT) symmetry structure is proposed, which is achieved by preparing the graphene layer on the top of a PT-symmetry photonic crystal. The transfer matrix method was used to calculate the absorptance of graphene, and a unique amplified absorption effect was found. Meanwhile, the peak value and wavelength position of the absorption can be modulated via the applied electric field. The results show that by adjusting the negative square-wave electric field from −3.5 × 10−5 to −13.5 × 10−5 V/nm (or the positive square-wave electric field from 2 × 10−5 to 11 × 10−5 V/nm), the proposed structure can achieve in-phase (or out-phase) enhanced absorption for the communication wavelength 1550 nm, with the absorption of graphene from 17 to 28 dB (or 30 to 15 dB) corresponding to the square-wave modulation electric field change. The modulable absorption properties of graphene in the structure have potential in optoelectronic devices and optical communication systems.

In this paper, we predict unexpected optical absorption in an optical thick free-standing transmission metallic grating by introducing periodical back grooves into it. The absorption peak could be very high up to 90%. In the structure, the introduced periodical back grooves bring about different orders of transmission minima into the transmission spectrum, which are named as transmission minima introduced by back grooves (TMIBBGs) here. When TMIBBGs approach the Fabry-Pérot (FP)-like resonant transmission peaks supported in the slits, the latter will be strongly suppressed. However, the reflection dips are almost unaffected. As a result, absorption is greatly enhanced. More interesting, the positions of TMIBBGs can be tuned by modulating the structural parameters of back grooves to selectively inhibit different orders of FP-like resonant transmission peaks and realize corresponding enhanced absorption, meanwhile, without influence to other orders of FP-like resonant transmissions. Moreover, due to the asymmetry of the structure, the enhanced absorption is unidirectional. The physical mechanisms are qualitatively and quantitatively analyzed in this paper.