Photodetectors Based on MASnI3/MoS2 Hybrid-Dimensional Heterojunction Transistors: Breaking the Responsivity-Speed Trade-Off.

Hybrid-dimensional heterojunction transistor (HDHT) photodetectors (PDs) have achieved high responsivities but unfortunately are still with unacceptably slow response speeds. Here, we propose a MASnI3/MoS2 HDHT PD, which exhibits the possibility to obtain high responsivity and fast response simultaneously. By exploring the detailed photoelectric responses utilizing a precise optoelectronic coupling simulation, the electrical performance of the device is optimally manipulated and the underlying physical mechanisms are carefully clarified. Particularly, the influence and modulation characteristics of the trap effects on the carrier dynamics of the PDs are investigated. We find that the localized trap effect in perovskite, especially at its top surface, is primarily responsible for the high responsivity and long response time; moreover, it is normally hard to break such a responsivity-speed trade-off due to the inherent limitation of the trap effect. By synergistically coupling the photogating effect, trap effect, and gate regulation, we indicate that it is possible to achieve an enhancement of the responsivity-bandwidth product by about 3 orders of magnitude. This study facilitates a fine modulation of the responsivity-speed relationship of hybrid-dimensional PDs, enabling breaking the traditional responsivity-speed trade-off of many PDs.

Unbiased and Signal-Weakening Photoelectrochemical Hexavalent Chromium Sensing via a CuO Film Photocathode.

Photoelectrochemical (PEC) sensors show great potential for the detection of heavy metal ions because of their low background noise, high sensitivity, and ease of integration. However, the detection limit is relatively high for hexavalent chromium (Cr(VI)) monitoring in addition to the requirement of an external bias. Herein, a CuO film is readily synthesized as the photoactive material via reactive sputtering and thermal annealing in the construction of a PEC sensing photocathode for Cr(VI) monitoring. A different mechanism (i.e., Signal-Weakening PEC sensing) is confirmed by examining the electrochemical impedance and photocurrent response of different CuO film photoelectrodes prepared with the same conditions in contact with various solutions containing concentration-varying Cr(VI) for different durations. The detection of Cr(VI) is successfully achieved with the Signal-Weakening PEC response; a drop of photocathode signal with an increasing Cr(VI) concentration from the steric hindrance effect of the in situ formed Cr(OH)3 precipitates. The photocurrent of the optimized CuO film photocathode linearly declines as the concentration of Cr(VI) increases from 0.08 to 20 µM, with a detection limit down to 2.8 nM (Signal/Noise = 3) and a fitted sensitivity of 4.22 µA·µM-1. Moreover, this proposed sensing route shows operation simplicity, satisfactory selectivity, and reproducibility.

Ultra-narrowband absorption filter based on a multilayer waveguide structure.

We propose a six-layer waveguide structure embedded in a single-layer grating based on guided-mode resonance (GMR), which can realize ultra-narrowband filtering with a tunable resonance wavelength. The filtering characteristics were analyzed and calculated by rigorous coupled-wave analysis (RCWA) and COMSOL Multiphysics. The narrowband resonance wavelength and absorption are tuned by changing the geometry and physical parameters of the structure such as the grating period and width, layer thickness, and materials. We designed and calculated the full width at half maximum (FWHM) and resonance absorption spectra in detail under different polarization states of electromagnetic waves. We obtained an absorption FWHM of 8.51e-5 nm for the transverse electric (TE) mode and 0.023 nm for the transverse magnetic (TM) mode, with the absorption coefficients having a value over 99.2%. The GMR filtering structure shows a good sensitivity and figure of merit (FOM) for refractive index sensing. For instance, a very high FOM of 17782.6/RIU for TM incidence is observed. These structures can have possible applications in optical information devices and sensors.

Simultaneously performing optical and electrical responses from a plasmonic sensor based on gold/silicon Schottky junction.

Making the chemical or biological sensors simpler and more compatible with other measurements is a key enabling technology for commercial application. In this work, we propose a plasmonic refractive-index sensor only based on gold/silicon Schottky junction to simultaneously perform optical and electrical read-out responses. Via exciting surface plasmon resonance (SPR), the designed device shows a few characteristic reflection valleys and greatly enhances the narrowband light absorption. Calculated results indicate that the SPR resonance wavelength can be tuned in the wavelength range of 1100-2000 nm by manipulating the period, width of the Si nanochannel and the incident angle of light. When the analyte is changed, the SPR resonance wavelength generated at the bottom surface of the Au layer barely shifts, while the one at the top surface shows a significant linear shift. The optimally designed system shows an optical response with a refractive index sensitivity of over 1000 nm per refractive index unit (nm/RIU). Moreover, the electrodynamic calculation of the hot electrons predicts the electrical response can be up to 14.5 mA/(W·RIU) for an example of detecting trichloromethane, where the employed light can be a monochromatic light and the sensing operation can be much simpler relative to the conventional spectral work mode.

Planar, narrowband, and tunable photodetection in the near-infrared with Au/TiO2 nanodiodes based on Tamm plasmons.

There is increasing interest in hot-electron photodetection due to the extended photoresponse well below the semiconductor band edge. However, the photoresponsivity is extremely low and the metallic nanostructures used to excite surface plasmons (SPs) for improved quantum yield are too complex for practical applications. Here, we show that by exciting Tamm plasmons (TPs), a planar device consisting of a thin metal film of 30 nm on a distributed Bragg reflector (DBR) can absorb ∼93% of the incident light, resulting in a high hot-electron generation that is over 34-fold enhanced compared to that of the reference without the DBR. Besides, the electric field increases with the light penetration depth in the metal, leading to hot-electron generation that is strongly concentrated near the Schottky interface. As a result, the photoresponsivity can be over 30 (6) times larger than that of the reference (conventional grating system). Moreover, the planar device exhibits an easily tunable working wavelength from the visible to the near-infrared, sustained performance under oblique incidences, and a multiband photodetection functionality. The proposed strategy avoids the complicated fabrication of the metallic nanostructures, facilitating the compact, large-area, and low-cost photodetection, biosensing, and photocatalysis applications.

Polarization-insensitive hot-electron infrared photodetection by double Schottky junction and multilayer grating.

Infrared photodetection based on hot electrons is drawing increasing interest due to the capabilities of below-bandgap detection, high tunability of working wavelength, compact size, and room-temperature operation. However, conventional hot-electron photodetectors are mostly based on surface plasmons with a strong polarization preference. In this Letter, we propose a multilayer grating double-junction hot-electron photodetector by introducing an ultrathin Au layer sandwiched between two Au-Si-Au cavities. The multilayer grating system allows the excitation of the guided-mode resonance that shows a weak reliance on the incident polarization and, therefore, realizes the polarization-insensitive optical absorption up to 98%. The special multilayer design facilitates hot-electron generation in the ultrathin Au layers with high carrier transport efficiency, as well as enabling the formation of a double Schottky junction, which doubles the carrier emission probability. The optical and electrical benefits ensure a polarization-independent photoresponsivity ∼1 mA/W at the wavelength of 1470 nm.

Tunable light absorbance by exciting the plasmonic gap mode for refractive index sensing.

The tunable and narrowband optical response from the surface plasmon resonances usually requires periodic metal nanostructures; however, it is usually expensive and challenging to construct such macroscale and defect-free devices. Herein, we make it possible to obtain a characteristic and sharp absorbance via exciting the plasmonic gap mode, which can be obtained in a large-area sample prepared with relatively low cost. The resonant wavelength can be tuned via changing the bottom-facet area of the top structured metal or the spacer thickness. Furthermore, we design the hexagonal arrangement gold microholes atop the gold continuous film with a spacer, which possesses a sharp reflectance dip from the intense plasmonic gap mode. Numerical calculations show that the resonant wavelength is linearly changed with the varying environmental refractive index (RI). The sensitivity is up to ∼1287 nm per RI unit, and the figure of merit for an RI sensor is over 300.

Refractive index sensor based on graphene-coated photonic surface-wave resonance.

We propose a graphene-coated photonic system with the excitation of Bloch surface waves (BSWs) for refractive index sensing. Through manipulation of the BSW resonance in the truncated photonic crystal under a Kretschmann configuration, the absorption in a graphene monolayer is significantly enhanced, assisted by the strong electromagnetic confinement of BSWs. The sharp and low reflectivity dip and the strong wave-environment interaction enable highly sensitive optical sensing. First-order perturbation theory and transfer-matrix calculation indicate that the system sensitivity is strongly related to the ratio of the electric field energy in the detection area, operation wavelength, and incident angle. Study shows that the wavelength sensitivity and figure of merit of the optimized system can reach 7023 nm/RIU and 196.44, respectively. More generalized BSW system configurations, e.g., aperiodic BSW design, are proposed for refractive index sensing application.

Silicon-gold core-shell nanowire array for an optically and electrically characterized refractive index sensor based on plasmonic resonance and Schottky junction.

This work reports the plasmonically enhanced refractive index sensor consisting of silicon nanowire array (Si-NWA) coated by a conformal gold (Au) nanoshell. Compared to the pure Si or Au NWA system, the Si-Au core-shell setup leads to substantially enhanced optical in-coupling to excite strong surface plasmon resonance (SPR) for highly sensitive sensors. Results indicate that the SPR wavelength can be subtly tuned by manipulating the nanowire radius, and it shows a strong shift with very small variation of the refractive index of the analyte. Furthermore, we configure the system into the Schottky junction, which can separate the photogenerated hot electrons so that the electrical outputs under various incident wavelengths can be measured. The capabilities of optical and electrical measurements ensure a high flexibility of the sensing system. Through our optoelectronic evaluation, the optimally designed system shows a sensitivity up to 1008 nm per refractive index unit and a full width at half-maximum of 9.89 nm; moreover, the high sensing performance can be sustained in a relatively large range of the incident angle.

Broadband and wide-angle light harvesting by ultra-thin silicon solar cells with partially embedded dielectric spheres.

We propose a design of crystalline silicon thin-film solar cells (c-Si TFSCs, 2 µm-thick) configured with partially embedded dielectric spheres on the light-injecting side. The intrinsic light trapping and photoconversion are simulated by the complete optoelectronic simulation. It shows that the embedding depth of the spheres provides an effective way to modulate and significantly enhance the optical absorption. Compared to the conventional planar and front sphere systems, the optimized partially embedded sphere design enables a broadband, wide-angle, and strong optical absorption and efficient carrier transportation. Optoelectronic simulation predicts that a 2 µm-thick c-Si TFSC with half-embedded spheres shows an increment of more than 10 mA/cm2 in short-circuit current density and an enhancement ratio of more than 56% in light-conversion efficiency, compared to the conventional planar counterparts.

Comprehensively characterizing ophthalmic freeform lenses with contour plots and astigmatic axis.

An accurate evaluation method is proposed for comprehensively characterizing ophthalmic freeform lenses (OFLs) by computing the curvatures of the freeform surface at an arbitrary direction in the transformed Cartesian coordinates based on space analytic geometry. The proposed method could especially produce the astigmatic axis, which is an indispensable characteristic of OFLs. An OFL designed with an analytical formula is characterized by our comprehensive method, not only with power and astigmatism contour plots that are consistent with the results from the conventional method, but also with the astigmatic axis, which can be analytically confirmed by combining a conventional quadratic equation with directional derivatives. Benefiting from the astigmatic axis, the comprehensive method could produce the combined astigmatism of both surfaces for double OFLs, which is not possible with the conventional method. The evaluation results of a double OFL are also presented and can be experimentally verified by fabrication and measurement. This might provide a new idea and a more comprehensive methodology for evaluatiing OFLs with high precision.

Simulation method for evaluating progressive addition lenses.

Since progressive addition lenses (PALs) are currently state-of-the-art in multifocal correction for presbyopia, it is important to study the methods for evaluating PALs. A nonoptical simulation method used to accurately characterize PALs during the design and optimization process is proposed in this paper. It involves the direct calculation of each surface of the lens according to the lens heights of front and rear surfaces. The validity of this simulation method for the evaluation of PALs is verified by the good agreement with Rotlex method. In particular, the simulation with a "correction action" included into the design process is potentially a useful method with advantages of time-saving, convenience, and accuracy. Based on the eye-plus-lens model, which is established through an accurate ray tracing calculation along the gaze direction, the method can find an excellent application in actually evaluating the wearer performance for optimal design of more comfortable, satisfactory, and personalized PALs.