  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.

Optics