Second-harmonic phonon spectroscopy of α -quartz

Abstract

We demonstrate midinfrared second-harmonic generation as a highly sensitive phonon spectroscopy technique that we exemplify using α-quartz (SiO2) as a model system. A midinfrared free-electron laser provides direct access to optical phonon resonances ranging from 350 cm −1 to 1400 cm −1 . While the extremely wide tunability and high peak fields of a free-electron laser promote nonlinear spectroscopic studies-complemented by simultaneous linear reflectivity measurements-azimuthal scans reveal crystallographic symmetry information of the sample. Additionally, temperature-dependent measurements show how damping rates increase, phonon modes shift spectrally and in certain cases disappear completely when approaching Tc = 846 K where quartz undergoes a structural phase transition from trigonal α-quartz to hexagonal β-quartz, demonstrating the technique's potential for studies of phase transitions.

Controlling the Infrared Dielectric Function through Atomic-Scale Heterostructures

“…Thus, we employed IR SHG, 38−42 collecting spectra from the AlN/GaN SL using the IR emission from a free-electron laser. 43 This technique was recently shown to provide clear identification of phonon modes in multimode systems 44 since it allows the direct measurement of phonon peaks on a nominally flat spectral background.…”

“…Within the SHG spectra, each E 1 (TO) phonon is observed with large contrast. 44 Additionally, A 1 (LO) phonons of the SL can be closely approximated via the Berreman effect, 45 which produces subtle dips in the reflectance near the spectral position where the real part of the permittivity crosses through zero (the so-called epsilon-near-zero condition). The strong confinement of the electromagnetic fields of the Berreman modes 46,47 also results in pronounced peaks in the SHG spectra.…”

“…The second-harmonic phonon spectra 38,44 were obtained using an IR free-electron laser (FEL) 43 as a light source, which provided widely tunable, ∼0.5% bandwidth laser pulses in the mid- to far-IR. Using a noncollinear autocorrelator geometry in reflection, both FEL beams were focused onto the SL wafer in spatial and temporal overlap, incident at angles α 1 i = 28° and α 2 i = 62°, respectively.…”

“…Thus, in order to directly characterize the SL optic phonons, we employed IR second harmonic generation (SHG), [37][38][39][40] collecting spectra from each of the XH structures using an IR free-electron laser 41 . This technique was recently shown to provide clear identification of phonon modes in multimode systems 42 since it allows for the direct measurement of phonon peaks on a nominally flat spectral background.…”

Nonlinear Tunable Vibrational Response in Hexagonal Boron Nitride

“…For sufficiently strong external fields, atomic vibrations in solids and molecules can be pushed beyond the harmonic regime, thus exhibiting nonlinear behavior. Surprisingly, harmonic generation associated with atomic vibrations has only been poorly explored, with just a few works focusing on this phenomenon at terahertz frequencies [65][66][67][68], as well as a parallel effort on parametric amplification of optical phonons in SiC [69]. The strength of vibrational nonlinear effects is a key question that determines the range of applications, but this line of research is still open, in search of robust materials with intense response.…”