Generation of high-power and high-order terahertz vortex beams using a gyrotron

Abstract

Optical vortex is a type of electromagnetic wave characterized by helically twisted wavefronts around the propagation axis. The optical vortex beams carry Orbital Angular Momentum (OAM), which is transferred to the irradiated target. Terahertz (THz) vortex beams are optical vortex beams in the THz region ranging from 0.1 THz to 10 THz, the intermediate region between radio waves and light. The THz waves possess the transparency of radio waves and the directionality of light. Besides, various elementary excitations exist in the THz region, such as electron spin, molecular rotation spectrum, and superconducting gap. The THz vortex beams are unique electromagnetic waves that combine the characteristics of THz waves and optical vortexes. These intrinsic features make them promising for various applications such as telecommunications, spintronics, and plasma heating.
Gyrotrons are high-power radiation sources in the submillimeter to THz bands capable of producing kilowatts to megawatts class outputs. High-power THz vortex beams can be generated using the gyrotron as a beam source. The Research Center for Development of Far-Infrared Region (FIR-UF) has its own gyrotron development technology and has developed advanced high-frequency gyrotrons.
Recently, we have successfully generated a 1 kW class THz vortex beam at a frequency of 0.265 THz using our advanced gyrotron as a radiation source. Quasi-optical mirrors passively shaped the gyrotron radiation into the Laguerre-Gauss beam, a well-known optical vortex. However, generating high-order vortex beams through mirror shaping is generally challenging.
As a next step, a quasi-optical mode converter is under development. The mode converter consists of a quasi-parabolic mirror and a helical-cut antenna, which converts incident-collimated beams into rotating higher-order transverse electric modes, TE_m,n, where m and n are the azimuthal and radial mode indices, respectively. TE_m,n modes carry OAM corresponding OAM mode number of m–1. We fabricated a prototype of the mode converter and measured the radiation profile using a gyrotron as a beam source.