期刊
ACS PHOTONICS
卷 1, 期 8, 页码 696-702出版社
AMER CHEMICAL SOC
DOI: 10.1021/ph500114h
关键词
photothermal spectroscopy; infrared spectroscopy; quantum cascade lasers; Zharov splitting; pitchfork bifurcation; nonlinear spectroscopy; liquid crystal
类别
资金
- NIH [1 R21 EB013381-01]
- NSF I/UCRC [NSF IIP-1068070]
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 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.
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