From Event: SPIE Optical Engineering + Applications, 2016
Mid-infrared photothermal spectroscopy is a pump-probe technique for label-free and non-destructive sample
characterization by targeting intrinsic vibrational modes. In this method, the mid-infrared pump beam excites a
temperature-induced change in the refractive index of the sample. This laser-induced change in the refractive index is
measured by a near-infrared probe laser using lock-in detection. At increased pump powers, emerging nonlinear
phenomena not previously demonstrated in other mid-infrared techniques are observed.
Nonlinear study of a 6 μm-thick 4-Octyl-4’-Cyanobiphenyl (8CB) liquid crystal sample is conducted by targeting the
C=C stretching band at 1606 cm-1. At high pump powers, nonlinear signal enhancement and multiple pitchfork
bifurcations of the spectral features are observed. An explanation of the nonlinear peak splitting is provided by the
formation of bubbles in the sample at high pump powers. The discontinuous refractive index across the bubble interface
results in a decrease in the forward scatter of the probe beam. This effect can be recorded as a bifurcation of the
absorption peak in the photothermal spectrum. These nonlinear effects are not present in direct measurements of the
Evolution of the nonlinear photothermal spectrum of 8CB liquid crystal with increasing pump power shows
enhancement of the absorption peak at 1606 cm-1. Multiple pitchfork bifurcations and spectral narrowing of the
photothermal spectrum are demonstrated. This novel nonlinear regime presents potential for improved spectral
resolution as well as a new regime for sample characterization in mid-infrared photothermal spectroscopy.
Atcha Totachawattana, Shyamsunder Erramilli, and Michelle Y. Sander, "Nonlinear photothermal mid-infrared spectroscopy," Proc. SPIE 9956, Ultrafast Nonlinear Imaging and Spectroscopy IV, 99560Y (Presented at SPIE Optical Engineering + Applications: August 29, 2016; Published: 5 October 2016); https://doi.org/10.1117/12.2238024.
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