Aperiodic Multilayer Graphene Based Tunable and Switchable Thermal Emitter at Mid-Infrared Frequencies

Abstract

Graphene has attracted an explosion of interest for photonic applications as it provides a degree of freedom to manipulate electromagnetic waves. In this work, we propose a new one-dimensional system composed of multiple graphene and hexagonal boron nitride (hBN) with varying aperiodic thicknesses in between the graphene layers. A hybrid optimization method, consisting of a micro-genetic global optimization algorithm coupled to a local optimization algorithm, indicates that absorption efficiency close to 100% can be achieved at very narrow frequency range under normal incidence. This efficiency can be significantly decreased by tuning the chemical potential and the corresponding carrier density in graphene. It thus enables a promising way to design electrically controllable thermal emitter. Fig. 1(a) shows the proposed device structure composed of 23 graphene layers and 22 alternating layers of hexagonal boron nitride (hBN) insulator, which sandwiches between two thick silicon carbide (SiC) layers. The hBN has two-dimensional atomic structure similar to graphene and has been proposed as its promising complementary insulator layer. The ambient temperature of 873 K is assumed, in which the semi-infinite tungsten substrate creates the thermal emission of black body radiation with maximum emission at infrared range. Since the tungsten substrate is taken to be semi-infinite, the transmittance is identically zero (Absorption = 1-Reflection) and the calculated absorption can be equated to emittance because of Kirchhoff’s second law and conservation of energy. The Kubo formula is used to extract the optical conductivity and consequently the refractive index of graphene while the experimental data is used for the wavelength-dependent indices of refraction of other materials. Fig. 1(b) shows the normalized power radiated per unit area and unit wavelength by the structure. In order to find the optimized structure with narrowband thermal emittance, the chemical potential initially set at the Dirac point of graphene layers (µg=0eV), thus introducing minimum insertion loss or signal attenuation. It can be seen that the proposed device can exhibit narrowband infrared emittance, leading to most prominent emission at λ=3340 nm with narrower wavelength range than black body curve. This wavelength corresponds with the λ/(4nSiC) of 538nm SiC layer, matched with the maximum thermal emission of black body radiation at T = 873 K. This indicates promising application of the proposed structure in design of thermophotovoltaic, in which the frequency of thermal radiation must be matched with the band gap of semiconductor. After obtaining the optimized structure, the dimensions are fixed and the carrier density in graphene is increased by tuning the chemical potential to larger values. It can be seen in Fig. 1(b) that the field effect tuning of carrier density in graphene reduces the normalized thermal emission from 100% at µg = 0eV to 64%, 43%, and 16% corresponding to the chemical potentials of µg = 0.4eV, 0.6eV, and 1.0eV, respectively. The intensity, width and the position of wavelength range is widely tunable, leading to electronic control of thermal emission. As the carrier density is increased, there is a decrease in intensity and a spectral shift toward smaller wavelength. When a bias is applied, two-dimensional electron gases are induced in the graphene layers, resulting in higher contribution of intraband transitions especially at high temperature of black body emitter. This manipulates the optical conductivity of graphene and alters the constructive and destructive interference of thermal incidences and reflections from the graphene/hBN layers, leading to different intensity, width, and position of thermal emittance. In this work, we present a 1D system composed of graphene/hBN heterostructure as a tunable and switchable thermal emitter. While the black-body thermal emission is a broadband and isotropic radiation, the proposed structure may contribute toward the realization of narrowband-wavelength selective thermal emitters with switchable intensity for sensing applications. Figure 1