Frequency multiplication in Terahertz band using AlGaN/GaN plasmonic crystals

TL;DR

This paper introduces rotonic plasmons in AlGaN/GaN plasmonic crystals and demonstrates their potential for RF-to-THz frequency multiplication through nonlinear parametric resonances and time-domain excitation.

Contribution

It reports a new class of collective excitations called rotonic plasmons and shows how uniform gate modulation can enable frequency multiplication in plasmonic crystals.

Findings

High-amplitude gate pumping enables frequency multiplication.

Cryogenic temperatures induce parametric instabilities.

Periodic short-pulse excitation achieves RF-to-THz conversion.

Abstract

The plasma oscillations in high-mobility field-effect transistors (HEMTs) have emerged as a key physical mechanism for manipulating electromagnetic radiation in the sub-terahertz (sub-THz) and THz frequency ranges. These collective electron excitations can be excited and tuned electrically offering a compelling route to compact, integrable components for a wide range of next-generation technologies, including sixth-generation (6G) wireless networks, high-resolution biomedical and chemical spectroscopy, industrial process monitoring, and advanced security and defense systems. For these applications, plasmonic crystals -- periodic arrays of many strongly coupled FET channels -- are particularly promising. In this work, we report on a new class of collective excitations in plasmonic crystals termed rotonic plasmons, which arise at plasmonic mode crossings and exhibit a parabolic dispersion law reminiscent of soft-mode and roton-like spectra. We show that uniform gate modulation across plasmonic crystal unit cells induces periodic variations in the sheet carrier concentration and, consequently, in the plasma frequency. This time-periodic modulation drives nonlinear plasmonic parametric resonances enabling RF-to-THz conversion. By solving the generalized Mathieu equation with damping, we demonstrate that high-amplitude gate pumping enables frequency multiplication and, at cryogenic temperatures (77K) leads to parametric instabilities due to enhanced electron mobility. In plasmonic crystals with lower mobility, RF-to-THz conversion can instead be realized via periodic short-pulse excitation, a regime we introduce as Time-Domain Frequency Multiplication (TDFM). Investigation of AlGaN/GaN low-high plasmonic crystals confirm their potential as tunable, compact THz sources.