The use of hot electrons arising from the nonradiative decay of surface plasmons (SPs) is increasingly attracting interests in photodetection, photovoltaics, photocatalysis, and surface imaging. Nevertheless, the quantum efficiency of the hot-electron devices has to be improved to promote the practical applications. We propose an architecture of conformal TCO/semiconductor/metal nanowire (NW) array for hot-electron photodetection with a tunable optical response across the visible and near-infrared bands. The wavelength, strength, and bandwidth of the plasmonic resonance are tailored by controlling the lattice periodicity and topology. Finite-element simulation demonstrates that the near-perfect, polarization-insensitive, and ultranarrow-band optical absorption can be achieved in the conformal NW system. By the excitation of localized SPs, a strong field concentrates at the top corner of the NWs with a high hot-electrons generation rate. The analytical probability-based electrical calculation further shows that the SPs-enhanced photoresponsivity can be more than five times larger than that of the flat reference.
We propose a stop-band filter in infrared region using a periodically chirped subwavelength structure. The structure is made of a stack of metal and dielectric pattern made on a thick metal layer that is deposited a PMMA substrate. It is found that an appropriately designed microstructure of metal-insulator-metal patches can generate a wideband infrared absorption, resulting in an infrared stop-band filter. Different width of metal-insulator-metal resonator arranged in one unit cell generate trough in the reflection spectrum at different wavelengths. The full width at half magnitude (FWHM) of the stop-band filter can thus be adjusted by tuning the width of the resonators. The larger the range of the resonator width, the wider the bandwidth will be. Under the condition of subwavelength dimension of the structure compared with the working wavelength, it is found that a FWHM of 4μm at central wavelength of ~9μm and a high absorption efficiency of up to 80% can be achieved. The proposed structure provides a novel method in the design of wideband efficient plasmonic absorbers in infrared or THz spectral regions with simultaneously wide bandwidth and high efficiency of absorption.
The static and dynamic characteristics of two-mode vertical-cavity surface-emitting lasers (VCSELs) under mode-selective feedback are investigated numerically. The model includes the spatial dependences of the optical modes, the carriers and the injection current. The research is respectively done for two different cases: weak coupling and strong coupling cases. For weak coupling case, the disc injection current is introduced and the LP<sub>01</sub> and LP<sub>11</sub> modes are included; while for strong coupling case, LP<sub>11</sub> and LP<sub>21</sub> modes are excited simultaneously by using a ring injection current. Besides, LP<sub>01</sub> and LP<sub>11</sub> modes are selectively reflected back by the external mirror for the weak and strong coupling cases, respectively. For short external cavities, the laser keeps its continuous wave (CW) operation. The modal power varies periodically with respect to the external cavity length and increases (or decreases) with the increase of the external mirror reflectivity. The variation trend of modal power is determined by the external cavity length, which decides the relative phase of the reflected light and the in-cavity one. Moreover, we find that the two modes under consideration exhibit anti-phase behaviors with the variation of the feedback conditions. Especially, for the strong coupling case, the enhancement of one mode can result in the thorough suppression of the other mode. For long external cavities, however, the relaxation oscillation process of laser becomes undamped, which induces rich nonlinear dynamics for both the reflected mode and the one without feedback. Through computer simulation, many typical dynamics, such as the CW, period-one, period-two, period-four, quasi-period, and chaotic states, are observed for both modes. It is also shown that fixing the feedback level, the dynamics shown by the laser exhibit periodic evolution with respect to the external cavity length; however, for a given external cavity length, a distinct period-doubling route to chaos is found with increasing external mirror reflectivity. Therefore, although just one mode is directly affected by the external feedback, the other mode still exhibits the similar nonlinear behaviors due to the carriers dynamics induced by the mode with feedback.
The authors propose a new compound cavity model with optical feedback for study the nonlinear dynamic behavior of VCSEL. The results show that system can be controlled to its fixed point, periodic orbits by using modulation parameters. In addition, the stable range be obtained and indicate the introduction of external reflectivity enhances chaotic characteristics. As increase of the external mirror reflectivity the CW state become unstable and the output power exhibit single period oscillation, higher order bifurcation, eventually leading to chaos.