Khwanchai Tantiwanichapan

Fiber-based Bessel beams with controllable diffraction-resistant distance.

We experimentally generate n=0 Bessel beams via higher-order cladding mode excitation with a long period fiber grating. Our method allows >99% conversion efficiency, wide or narrow conversion bandwidth, and accurate control of the number of rings in the beam. This latter property is equivalent to tuning the beam cone angle and allows for control of width and propagation distance of the center spot. We generate Bessel-like beams from LP(0,5) to LP(0,15) cladding modes and measure their propagation-invariant characteristics as a function of mode order, which match numerical simulations and a simple geometric model. This yields a versatile tool for tuning depth of focus out of fiber tips, with potential uses in endoscopic microscopy.

Uniaxial Strain Redistribution in Corrugated Graphene: Clamping, Sliding, Friction, and 2D Band Splitting

Graphene is a promising material for strain engineering based on its excellent flexibility and elastic properties, coupled with very high electrical mobility. In order to implement strain devices, it is important to understand and control the clamping of graphene to its support. Here, we investigate the limits of the strong van der Waals interaction on friction clamping. We find that the friction of graphene on a SiO2 substrate can support a maximum local strain gradient and that higher strain gradients result in sliding and strain redistribution. Furthermore, the friction decreases with increasing strain. The system used is graphene placed over a nanoscale SiO2 grating, causing strain and local strain variations. We use a combination of atomic force microscopy and Raman scattering to determine the friction coefficient, after accounting for compression and accidental charge doping, and model the local strain variation within the laser spot size. By using uniaxial strain aligned to a high crystal symmetry direction, we also determine the 2D Raman Grüneisen parameter and deformation potential in the zigzag direction.

Graphene on nanoscale gratings for the generation of terahertz Smith-Purcell radiation

Generation of THz radiation based on the Smith-Purcell effect in graphene is investigated numerically. The specific device geometry considered involves an electrically biased single-layer sheet of graphene deposited on a periodic array of holes in a solid substrate. Rigorous electrodynamic simulations combined with a basic model of charge transport are presented, showing that technologically significant output power levels can be obtained at geometrically tunable THz frequencies. These results suggest that graphene is a uniquely suited materials platform for the demonstration of THz electron-beam radiation mechanisms in compact solid-state systems.

Graphene on nanoscale gratings: a novel materials platform for THz electron-beam radiation

THz light emission based on electron-beam radiation mechanisms in graphene (i.e., cyclotron-like emission and the Smith-Purcell effect) is investigated numerically. Technologically significant power levels at tunable THz frequencies are computed.

Diffraction control of Bessel beams generated in fiber

LP0,m fiber cladding modes propagate as Bessel beams in free space. We show that selecting the mode order with a long period grating enables tuning of the propagation distance and width of the center spot.