Suggested design of gold-nanoobjects-based terahertz radiation source for biomedical research

TL;DR

This paper proposes a novel design for a terahertz radiation source using gold nanoparticles that emit THz photons via two-phonon processes, with potential biomedical applications.

Contribution

It introduces a specific mechanism involving two-phonon processes in gold nanoparticles for generating tunable THz radiation, including numerical estimates and a potential implementation method.

Findings

Potential to generate soft (~0.54 THz) and hard (~8.7 THz) THz radiation.

Numerical estimates for 5-7 nm rhombicuboctahedral GNPs.

Proposed deposition of GNPs on Teflon substrate for practical realization.

Abstract

Gold nanoparticles (GNPs) may serve as "devices" to emit electromagnetic radiation in the terahertz (THz) range, whereby the energy is delivered by radio frequency or microwave photons which won't by themselves induce transitions between sparse confinement-shaped electron levels of a GNP, but may borrow the energy from longitudinal acoustic phonons to overcome the confinement gap. Upon excitation, the Fermi electron cannot relax otherwise than via emitting a THz photon, the other relaxation channels being blocked by force of shape and size considerations. Within this general scope that has been already outlined earlier, the present work specifically discusses two-phonon processes, namely (i) a combined absorption-emission of two phonons from the top of the longitudinal acoustic branch, and (ii) an absorption of two such phonons with nearly identical wavevectors. The case (i) may serve as a source of "soft" THz radiation (at ~0.54 THz), the case (ii) the "hard" THz radiation at 8.7 THz. Numerical estimates are done for crystalline particles in the shape of rhombicuboctahedra, of 5 - 7 nm "diameter". A technical realisation of this idea is briefly discussed, assuming the deposition of GNPs onto / within the substrate of Teflon, the material sustaining high temperatures and transparent in the THz range.