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We report on the mid-infrared nonlinear photothermal spectrum of the neatliquid crystal 4-Octyl-4'-Cyanobiphenyl (8CB) using a tunable Quantum CascadeLaser (QCL). The nonequilibrium steady state characterized by the nonlinearphotothermal infrared response undergoes a supercritical pitchfork bifurcation.The bifurcation, observed in heterodyne two-color pump-probe detection, leadsto ultrasharp nonlinear infrared spectra similar to those reported in thevisible region. A systematic study of the peak splitting as function ofabsorbed infrared power shows the bifurcation has a critical exponent of 0.5.The surprising observation of an apparently universal critical exponent in anonequilibrium state is explained using a simple model reminiscent of meanfield 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 newfield of mid-infrared microspectroscopy using QCLs.

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