We propose and numerically simulate a kind of graphene-silicon-based perfect absorber which possesses high-quality optical bistability in the near-IR band. In the structure, monolayer graphene is sandwiched between two silicon layers and is coupled with a 1D dielectric grating. With the help of the absorption rate of the monolayer graphene and the leakage rate of the grating structure, the critical coupling condition is satisfied and the incident light can be almost completely absorbed by the graphene at the resonant wavelength, causing prominent enhancement of electric field and light-matter interaction. Considering the third-order susceptibility of silicon together with the electric field enhancement, the perfect absorber shows significant bistable characteristics with ultralow thresholds and ultrahigh extinction ratio. The demonstrated switching thresholds can be lower than 90 kW/cm2, which can be further cut down by changing the structural parameters. The extinction ratio of the bistable states reaches an ultrahigh level of over 30 dB, since the structure is critically coupled with no reflected light coming out at the off state. The proposed bistable structure with compact size, low thresholds and high extinction ratio can be of great applications in the optical communication field.
Vanadium dioxide (VO2), a thermochromic material with a low phase transition temperature at 68℃ and the magical hysteresis property near the phase transition temperature, has been widely investigated because of its potential application as infrared detector, optical switch, memristor, and ‘smart window’. Graphene, due to its special electrical, optical and thermal properties, has been extensively studied in recent years with layered hybrid structures of other materials. In this paper, to realize flexible manipulation of VO2 phase transition, we co-designed a graphene heater to locally modulate temperature of VO2. Typical thermal induced phase transition by temperature control stage has been achieved through electrical measurements, which shows the resistance of VO2 has gone through a dramatical change over 3 order of magnitude around the critical temperature (68℃). As for the in situ heating manipulation, we applied current to the graphene to generate joule heating, which would result in a hot-spot to locally modulate the temperature of VO2 to reach the phase transition point. Meanwhile, the electrical current of graphene that results in the structural phase transition of VO2 could be smaller by inserting Al2O3 capping layer between graphene and VO2. Compared with the traditional macroscale VO2 device, our nanosized co-designed structure shows both low power consumption and fast response, which would benefit a lot in exploring VO2 based on-chip electronic applications. In addition, we believe that the designed composite structure has a wide range of research significance in optoelectronic devices.
We present a general review on using computer visualization technology to assist teaching electrodynamics for undergraduates in several Chinese universities. Based on our own teaching activities during the past decade in National University of Defense and Technology, China, we propose and discuss the necessity of computer visualization in electrodynamics course teaching for undergraduates major in optics for the first time. Then we will show how to help students to comprehend fundamental concepts and to understand the effect of parameters on core physical quantities through some teaching designs. At last, specified content inside the course that are suitable to be assisted by computer visualization are demonstrated.
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