Mertiri, AlketAltug, HaticeHong, Mi K.Mehta, PankajMertz, JeromeZiegler, Lawrence D.Erramilli, Shyamsunder2016-08-162016-08-162016-08-16201410.1021/ph500114hhttps://infoscience.epfl.ch/handle/20.500.14299/128607We 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 super-critical 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 a function of absorbed infrared power shows the bifurcation has a critical exponent of 0.5. The observation of an apparently universal critical exponent in a nonequilibrium state is explained using an analytical model analogous 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.infrared spectroscopyliquid crystalnonlinear spectroscopyphotothermal spectroscopypitchfork bifurcationquantum cascade lasersZharov splittingtext::journal::journal article::research article