Electrical control of photonic crystal lasers with graphene electrode

Abstract

A photonic crystal (PhC) is a structure in which two or more materials with different dielectric constants are periodically arranged with a wavelength scale of light. The light passing through the PhC has a unique light dispersion relation that cannot be seen in light passing through a homogeneous medium. One of the unique optical characteristics of PhC is the photonic band-gap and band-edge phenomenon. The photonic band-gap is a frequency region where the mode of the photon cannot exist, and a PhC cavity laser can be demonstrated in such a manner that light is strongly confined within the cavity by using the photonic band-gap. Also, by using a band-edge mode in which the group velocity of light approaches zero, the optical gain can be enhanced by a standing-wave resonance mode in the optical gain material. Thus, a PhC band-edge laser can be demonstrated. PhC lasers have great potential in that they can be fabricated in small areas and have low power consumption. However, in order to actually use PhC lasers in industry, modulation techniques must be preceded. Generally, the modulation speed of the PhC laser has a limitation by the modulation speed of the optical excitation method. Therefore, for high-speed modulation, it is necessary to control the optical absorption electrically. In this study, graphene is used as a light absorber. Graphene is characterized in that the carbon atoms are arranged on a two-dimensional plane and have a large light absorption rate and can control the light absorption rate by controlling the Fermi level. In this thesis, in the first research topic, the effect of the optical absorber on the PhC laser structure was experimentally characterized. First, PhC band-edge and cavity lasers were fabricated using InGaAsP multiple-quantum-well structures, and we observed clear laser emissions around 1.5 μm. Then, graphene monolayer with the partially removed around the cavity structure was integrated on the PhC cavity laser structure. It was confirmed that as the removal region of graphene is widened, the interaction between graphene and PhC cavity mode is reduced and the output intensity of the laser is increased. In addition, it was confirmed that the single-mode laser operation of the PhC cavity laser is possible with the partially removed graphene monolayer. The characteristics of the PhC band-edge laser integrated graphene or metal patterns on the PhC band-edge laser structure were also observed. When the absorber is placed at the antinode of the resonant mode, the resonant mode is attenuated and the laser oscillation is suppressed by the strong interaction between the absorber and the resonant mode. While, when the absorber is placed at the node of the resonance mode, the interaction between the absorber and the resonance mode is not large and the laser oscillation is maintained. In addition, we confirmed that the tendency of the absorber is consistent with the results obtained by FDTD simulation. This study is meaningful in that it has experimentally characterized the effect of light absorber on the PhC resonance mode. Further, applicability to the design of the electrode structure of the electrically driven PhC laser is expected. In the second research topic, electrical modulation of the PhC band-edge laser was demonstrated by controlling the optical absorption of graphene monolayer on the PhC band-edge laser structure. When an electric field is applied to the graphene, the Fermi level of the graphene shifts and the absorption of light can be controlled by inhibiting the band transition. A gate voltage was applied to the graphene to observe a 1.5% of transmittance change. In this experiment, the ion-gel film used as a dielectric material has the advantage that it can be operated at a low gate voltage because of its large capacitance. Subsequently, graphene monolayer was integrated on the PhC band-edge laser structure using the change in transmittance of graphene to demonstrate the electrical modulation. Then, the electrical modulation of the PhC band-edge laser was demonstrated using the change in transmittance of graphene. When the gate voltage was not applied, the laser oscillation was suppressed due to the absorption of the resonance mode by the graphene. However, when a gate voltage of -1.0 V was applied, the laser emission was successful and the characteristics of the laser according to the gate voltage were observed. Observed characteristics are consistent with the results calculated by FDTD simulation, then we have successfully demonstrated the electrical modulation of the PhC band-edge laser. The electrical modulation of PhC band-edge laser is expected to be applied to optical system on chip technology such as optical integrated circuits. In conclusion, in this thesis, we observed the change of the PhC resonance mode when the optical absorber is integrated on the PhC laser structure. Furthermore, the electrical modulation was demonstrated by controlling the optical absorption rate of the graphene monolayer integrated on the PhC band-edge laser structure. The results of this study are expected to enhance understanding of the resonance mode and the PhC laser oscillation, and contribute to the application of the PhC lasers to the system on chip.