Pitchfork Bifurcation and Zharov Splitting in Nonlinear Mid-infrared Photothermal Spectroscopy in a liquid crystal using a Quantum Cascade Laser

TL;DR

This paper investigates nonlinear photothermal spectroscopy of a liquid crystal using a Quantum Cascade Laser, revealing a supercritical pitchfork bifurcation and ultrasharp spectra, with potential applications in mid-infrared microspectroscopy.

Contribution

It demonstrates the occurrence of a pitchfork bifurcation in nonlinear photothermal response and introduces a universal critical exponent in a nonequilibrium state, advancing understanding of mid-infrared spectroscopy.

Findings

Observation of supercritical pitchfork bifurcation in photothermal response

Identification of a universal critical exponent of 0.5

Potential for enhanced spectral contrast in mid-infrared microspectroscopy

Abstract

We report on the mid-infrared nonlinear photothermal spectrum of the neat liquid crystal 4-Octyl-4'-Cyanobiphenyl (8CB) using a tunable Quantum Cascade Laser (QCL). The nonequilibrium steady state characterized by the nonlinear photothermal infrared response undergoes a supercritical pitchfork bifurcation. The bifurcation, observed in heterodyne two-color pump-probe detection, leads to ultrasharp nonlinear infrared spectra similar to those reported in the visible region. A systematic study of the peak splitting as function of absorbed infrared power shows the bifurcation has a critical exponent of 0.5. The surprising observation of an apparently universal critical exponent in a nonequilibrium state is explained using a simple model reminiscent of mean field theory. Apart from the intrinsic interest for nonequilibrium studies, nonlinear photothermal methods lead to a dramatic narrowing of spectral lines, giving rise to a potential new contrast mechanism for the rapidly emerging new field of mid-infrared microspectroscopy using QCLs.