Thermal transport across membranes and the Kapitza length from photothermal microscopy.

An analytical model is presented for light scattering associated with heat transport near a cell membrane that divides a complex system into two topologically distinct half-spaces. Our analysis is motivated by experiments on vibrational photothermal microscopy which have not only demonstrated remarkably high contrast and resolution, but also are capable of providing label-free local information of heat transport in complex morphologies. In the first Born approximation, the derived Green's function leads to the reconstruction of a full 3D image with photothermal contrast obtained using both amplitude and phase detection of periodic excitations. We show that important fundamental parameters including the Kapitza length and Kapitza resistance can be derived from experiments. Our goal is to spur additional experimental studies with high-frequency modulation and heterodyne detection in order to make contact with recent theoretical molecular dynamics calculations of thermal transport properties in membrane systems.

Flatfield ultrafast imaging with single-shot non-synchronous array photography.

We present a method for acquiring a sequence of time-resolved images in a single shot, called single-shot non-synchronous array photography (SNAP). In SNAP, a pulsed laser beam is split by a diffractive optical element into an array of angled beamlets whose illumination fronts remain perpendicular to the optical axis. Different time delays are imparted to each beamlet by an echelon, enabling them to probe ultrafast dynamics in rapid succession. The beamlets are imaged onto different regions of a camera by a lenslet array. Because the illumination fronts remain flat (head-on) independently of beamlet angle, the exposure time in SNAP is fundamentally limited only by the laser pulse duration, akin to a "global shutter" in conventional imaging. We demonstrate SNAP by capturing the evolution of a laser induced plasma filament over 20 frames at an average rate of 4.2 trillion frames per second (Tfps) and a peak rate of 5.7 Tfps.

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

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.

Vibrational mid-infrared photothermal spectroscopy using a fiber laser probe: asymptotic limit in signal-to-baseline contrast.

We report on a mid-infrared photothermal spectroscopy system with a near-infrared fiber probe laser and a tunable quantum cascade pump laser. Photothermal spectra of a 6 µm-thick 4-octyl-4'-cyanobiphenyl liquid crystal sample are measured with a signal-to-baseline contrast above 103. As both the peak photothermal signal and the corresponding baseline increase linearly with probe power, the signal-to-baseline contrast converges to an asymptotic limit for a given pump power. This limit is independent of the probe power and characterizes the best contrast achievable for the system. This enables sensitive quantitative spectral characterization of linear infrared absorption features directly from photothermal spectroscopy measurements.

Nonlinear Midinfrared Photothermal Spectroscopy Using Zharov Splitting and Quantum Cascade Lasers.

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.

Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays.

Infrared absorption spectroscopy enabling direct access to vibrational fingerprints of the molecular structure is a powerful method for functional studies of bio-molecules. Although the intrinsic absorption cross-sections of IR active modes of proteins are nearly 10 orders of magnitude larger than the corresponding Raman cross-sections, they are still small compared to that of fluorescence-label based methods. Here, we developed a new tool based on collective excitation of plasmonic nanoantenna arrays and demonstrated direct detection of vibrational signatures of single protein monolayers. We first tailored the geometry of individual nanoantennas to form resonant structures that match the molecular vibrational modes. The tailored nanoantennas are then arranged in such a way that their in-phase dipolar coupling leads to a collective excitation of the ensemble with strongly enhanced near fields. The combined collective and individual plasmonic responses of the antenna array play a critical role in attaining signal enhancement factors of 10(4)-10(5). We achieved measurement of the vibrational spectra of proteins at zeptomole levels for the entire array, corresponding to only 145 molecules per antenna. The near-field nature of the plasmonic enhancement of the absorption signals is demonstrated with progressive loading of the nanoantennas with varying protein film thicknesses. Finally, an advanced model based on nonequilibrium Green's function formalism is introduced, which explains the observed Fano-type absorption line-shapes and tuning of the absorption strengths with the antenna resonance.