Nonlinear Midinfrared Photothermal Spectroscopy Using Zharov Splitting and Quantum Cascade Lasers

Abstract

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 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.

Spaser as a biological probe

“…This leads to the aforementioned blue and red sharp peaks. Recently, these peaks were observed in infrared range by an independent research group26.…”

Quantitative phase imaging with molecular vibrational sensitivity

“…Therefore, the ideal MIR light source would be a kilohertz laser having 10 -100 µJ pulse energy and 10 -100 ns pulse duration which could boost the image acquisition rate by 2 -3 orders of magnitude. Such a high frame rate is, in principle, realizable with MV-QPI due to the wide-field parallelized detection scheme of MIR photothermal signal with a two-dimensional image sensor, and beyond the reach of other state-of-the-art label-free molecular-sensitive microscopes, such as coherent Raman [16,17] and other MIR photothermal [7][8][9][10] microscopes, which employ a point-scanning mechanism for image acquisition. We also note that the label-free, simultaneous and in-situ acquisition of the quantitative morphology and the MV absorption contrast is a unique capability of our MV-QPI which could promote new findings in studies on complex dynamics of an optically-transparent system.…”

“…Likewise, in our MV-sensitive QPI (MV-QPI), we measure the local changes in the sample-specific optical-path-length delay occurring upon absorption of MIR light in the vicinity of the MV-resonant molecules. This effect, called photothermal effect [7][8][9][10], is essentially the change of refractive index of the sample due to increase of temperature as a result of non-radiative decay of the absorbed MIR photon energy. Importantly, with MV-QPI, we can achieve the spatial resolution beyond the MIR diffraction limit due to the visible (VIS) light-based optical-phase-delay detection, maintain high molecular-detection sensitivity based on MIR absorption having ~8 orders of magnitude larger cross-section compared to Raman scattering [8], reduce optical damages to the sample associated with electronic transitions and other nonlinear mechanisms due to wide-field excitation [11], and achieve high-speed imaging ultimately limited by the image sensor's frame rate.…”

Thermoplasmonic Effect of Surface-Enhanced Infrared Absorption in Vertical Nanoantenna Arrays

“…). Nanoscale mid-IR thermoplasmonics with tunable EC-QCLs has been recently exploited for mid-IR absorption nanospectroscopy of few molecules [23][24][25], nonlinear optics in the mid-IR based on phase transitions of liquid crystals [26], and mapping of field-enhancement hotspots in IR metamaterials [27][28][29]. SEIRA has been long sought for in mid-IR plasmonic nanoantenna structures, however it has been elusive up to now due to Fano interference phenomena [30][31][32][33][34][35][36] that can prevent IR absorption enhancement while providing scattering enhancement [37,38].…”