Simulation Investigation Of Tunable Hybrid Waveguides

Terahertz(THz) waves lie between the microwave and infrared regions in the electromagnetic spectrum, showing potential applications in the basic research and practical application, such as in the aspects of THz imaging, wireless communication and detection of biological specimen. Design novel tunable waveguide devices is very important to the further development of THz technology. As an important kind of hybrid waveguides, dielectric loaded surface plasmons(DLSPs) waveguide(i.e. a dielectric strip deposited on metal substrate), demonstrates the merits of low bending loss, simple structure and good confinement.Graphene can serve as a good platform for the investigation of tunable waveguides devices because of its excited electronic transport property, e.g. high tunability and extreme confinement. Based on the DLSPs structure, we have theoretically investigated the tunable propagation of the hybrid waveguide structure by using the finite element methods. The main results are shown in the following.(1) By thermal control the carrier concentration of semiconductor substrates, the tunable propagation properties have been studied in the THz regime, the results show that the modulation depth(MD) can reach more than 80% if the temperature changes in the range of 300 K to 600 K.(2) By inserting multiple periods of graphene-Al2O3 unit cell structure between the dielectric loaded stripe and metal substrate, the graphene supported tunable DLSPs waveguides have been proposed in the near-infrared spectral region, including the influences of the graphene Fermi level, periodic number of unit cell structure and operation frequency. The results manifest that the MD of propagation length can reach about 70% if the Fermi level changes in the range of 0.1-0.45 e V.(3) In the THz regime, the intra-band contribution dominates because of the small energy of THz waves, resulting into the propagation properties can be modulated in a wide range. The results manifest that as frequency increases, the effective index and propagation length of hybrid mode increase. As the period number of unit cell increases, the modulation properties improve significantly. For example, if the Fermi level changes in the range of 0.1-1.0 e V, the modulation depth of propagation length can reach more than 94% on condition that the period number is eight. In addition, by adjusting the structural parameters, the mode confinement and propagation length improves simultaneously, e.g. for the elliptical stripe structure, as the minor semi-axis increases, the effective index from 1.016 to 1.208.