Abstract:We report terahertz surface plasmon resonance (SPR) sensing based on prism-coupling to the spoof surface plasmon polariton (SSPP) mode existing on periodically grooved metal films. It was demonstrated that, except for the fundamental mode of the SSPP, there was also a higher mode SSPP wave when the depth of groove was larger. Both fundamental and high-order modes of SSPP could be used for terahertz sensing. We compared the performance of different modes of SSPP on the sensing sensitivity using both reflection amplitude and phase-jump information. The results indicated that the gap distance between the prism base and the metal film had a significant influence on the reflectivity of SPR sensing by affecting the coupling efficiency of an evanescent wave to an SSPP wave; also, high-order mode SSPP-based sensing had a high sensitivity of up to 2.27 THz/RIU, which nearly doubled the sensitivity of the fundamental mode. The application of high-mode SSPP has enormous potential for ultra-sensitive SPR sensing in the terahertz regime.

We report on the fabrication and transmission properties of free-standing single-layer and double-layer THz bandpass filters. These filters are fabricated on aluminum foils using femtosecond laser micro-machining. The aluminum foils are periodically patterned with cross apertures with a total area of 1.75×1.75 cm, also known as frequency-selective surfaces. Their terahertz transmission properties were simulated using the FDTD method and measured using a time-domain terahertz spectroscopy system. The simulation results agree with the measurements results very well. The performance of single-layer bandpass filters is as good as the commercial equivalents on the market. The double-layer filters show extraordinary transmission peaks with changing spacing between the two layers. We show the contour map of the electric field distribution across the apertures, and ascribe the new transmission peaks to the interference and coupling of surface plasmon polaritons between the two layers.

Herein, we have theoretically investigated the sensing performance—including enormous increase in the sensitivity and figure of merit (FOM)—of a magneto-optical surface plasmon resonance (MOSPR) sensor, which is based on the transverse magneto-optical Kerr effect (T-MOKE) in a ferromagnet coupled with a noble-metal grating. Specifically, we propose to use a CoFeB magnetic slab covered by a subwavelength, periodic gold grating configured as a magnetoplasmonic heterostructure. In such a device, sharp, Fano-like T-MOKE signals of high amplitude can be achieved due to the surface plasmon resonances (SPRs) excited in the presence of the gold grating, especially after optimizing the grating period. Tiny changes in the refractive index of an analyte surrounding the MOSPR sensor can be measured by analyzing the shift in the angle of incidence of the resonance positions of the T-MOKE signals. By calculating these resonance positions, we have demonstrated that it is possible to achieve a considerable sensitivity of 105° RIU−1 and a FOM as high as ∼102. Such a MOSPR sensing system can be exploited in biosensors with high detection limits.

We investigated the spoof surface plasmon polaritons (SSPPs) on 1D grooved metal surface for terahertz sensing of refractive index of the filling analyte through a prism-coupling attenuated total reflection setup. From the dispersion relation analysis and the finite element method-based simulation, we revealed that the dispersion curve of SSPP got suppressed as the filling refractive index increased, which cause the coupling resonance frequency redshifting in the reflection spectrum. The simulated results for testing various refractive indexes demonstrated that the incident angle of terahertz radiation has a great effect on the performance of sensing. Smaller incident angle will result in a higher sensitive sensing with a narrower detection range. In the meanwhile, the higher order mode SSPP-based sensing has a higher sensitivity with a narrower detection range. The maximum sensitivity is 2.57 THz/RIU for the second-order mode sensing at 45° internal incident angle. The proposed SSPP-based method has great potential for high sensitive terahertz sensing

Ultrafast all-optical switches have attracted considerable attention for breaking through the speed limitation of electric devices. However, ultrafast and high-efficiency all-optical switches based on two-dimensional (2D) materials can be achieved due to their strong nonlinear optical response and ultrafast carrier dynamic. For this reason, we propose the pump-probe method to achieve an ultrafast optical switcher with a response time of 192 fs and a switching energy of 800 nJ by using transition metal carbide (Nb2C). The response time and switching energy are far smaller than that of the all-optical device based on the saturable absorption effect of 2D materials. It is believed that the Nb2C-based all-optical switch provides a novel idea to achieve a high-performance all-optical device and has the potential for application in high-speed photonics processing.

A terahertz metamaterial refractive index/thickness sensor with flexible substrate, exhibiting low-frequency Fano resonance and high-frequency electromagnetically induced transparent (EIT) resonance, is proposed. The physical formation mechanisms of Fano and EIT resonances are investigated by calculating the electromagnetic field. Simulated results demonstrate that the refractive index sensing sensitivity based these two resonances are up to 60 and 100 GHz/RIU, and the thickness sensing sensitivity are up to 1 and 1.7 GHz/µm, respectively. Furthermore, the proposed sensor was fabricated using femtosecond laser etching technology, and its sensing performance was verified by the experimental results that it can distinguish different liquids and detect the polyimide film with different thicknesses less than 180 µm. The remarkable performances make the proposed metamaterial sensor has feasible capability for biological and chemical sensing in terahertz range.

Single nanowires (NWs) are of great importance for optoelectronic applications, especially solar cells serving as powering nanoscale devices. However, weak off-resonant absorption can limit its light-harvesting capability. Here, we propose a single NW coated with the graded-index dual shells (DSNW). We demonstrate that, with appropriate thickness and refractive index of the inner shell, the DSNW exhibits significantly enhanced light trapping compared with the bare NW (BNW) and the NW only coated with the outer shell (OSNW) and the inner shell (ISNW), which can be attributed to the optimal off-resonant absorption mode profiles due to the improved coupling between the reemitted light of the transition modes of the leak mode resonances of the Si core and the nanofocusing light from the dual shells with the graded refractive index. We found that the light absorption can be engineered via tuning the thickness and the refractive index of the inner shell, the photocurrent density is significantly enhanced by 134% (56%, 12%) in comparison with that of the BNW (OSNW, ISNW). This work advances our understanding of how to improve off-resonant absorption by applying graded dual-shell design and provides a new choice for designing high-efficiency single NW photovoltaic devices.

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