Multiple bifurcations with signal enhancement in nonlinear mid-infrared thermal lens spectroscopy

Abstract

We report a novel nonlinear mid-infrared vibrational spectroscopy regime where multiple bifurcations and signal enhancement are observed in the photothermal spectrum of a 6 μm-thick layer of 4-octyl-4'-cyanobiphenyl (8CB) liquid crystal. For increasing pump power values, the nonlinear evolution of the photothermal spectrum is studied in 8CB samples initially in the crystalline and smectic-A phase and their non-equilibrium transitions are characterized with pump-probe thermal lens spectroscopy. The nonlinear photothermal phenomena can be explained by the nucleation of localized non-equilibrium transitions that leads to the formation of bubbles, which modify the thermal lensing behavior. Analysis of the multiple bifurcations reveals a universal critical exponent for these non-equilibrium dynamics that can be linked to mean field theory. We report for the first time simultaneous measurement of the photothermal signal amplitude and phase behavior in the nonlinear regime. Due to the signal enhancement and spectral narrowing observed, nonlinear photothermal behavior shows promise for improvement in sensitivity and signal contrast in mid-infrared, attractive for sample characterization in the mid-infrared.

Resolution Limit in Infrared Chemical Imaging

“…This study generalizes ideas from optics to chemical imaging microscopes and can be extended to other emerging techniques as well as modalities with different mechanisms of chemical contrast. One exciting new area is the combination of infrared and optical frequencies to record data by using the photothermal effect, − for example. The probing wavelength in these cases is much shorter (typically, visible), which facilitates submicrometer optical resolution but does not yet provide the SNR of all-IR laser scanning microscopy .…”

A case study in bifurcation theory

“…We show numerically that the Lorentzian map is characterized by single bifurcation point, which can be used to simulate certain non linear phenomena reported experimentally in°uid mechanics 12 where the velocity pro¯le of the°uid in rectangular geometry is characterized by two stable states in laminar°ow with respect to the dimensions of the channel. Additionally, similar pattern of double period bifurcation found in Lorentzian map was also observed in optical spectroscopy 13 where liquid crystal characterization using laser showed that the measured spectra with respect to the input power of the laser contains second-order bifurcation.…”

“…The experimental results of recent study, 13 reported the bifurcation of the measured output power of the response of (8CB) liquid crystal, using¯ber laser probe. By changing the temperature, photo thermal spectra gave di®erent phases such as crystalline and disordered phases.…”

“…It was found for¯xed temperature T ¼ 29:3 that changing the input power of the laser and measuring the spectra using wave number units leads to bifurcation of the spectrum which is of second-order such that the bifurcation point corresponds to the value of the input power P ¼ 0:52 mW. 13 The obtained bifurcation process is similar to that given in Fig. 5 using simulation, in this case the input power P corresponds to the growth rate in our simulation and the sequence x nþ1 represents the spectrum F ð À1 Þ.…”

Bond-selective imaging by optically sensing the mid-infrared photothermal effect

“…In a conference proceeding, Furstenberg et al (46) reported chemical imaging of small crystals and polymer coating on microelectromechanical systems using a reflection photothermal microscope. Erramilli and Sander's group reported photothermal IR spectra of liquid crystal and showed the nonlinear Zharov peak splitting with QCL as pump beam (47)(48)(49)(50). Mërtiri et al (51) demonstrated a mid-IR pump, near-IR probe photothermal imaging using a reflective objective lens and showed a photothermal image of a dried bird brain tissue slice.…”