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.

Molecular-level understanding of the rovibrational spectra of N2O in gaseous, supercritical, and liquid SF6 and Xe.

The transition between the gas-, supercritical-, and liquid-phase behavior is a fascinating topic, which still lacks molecular-level understanding. Recent ultrafast two-dimensional infrared spectroscopy experiments suggested that the vibrational spectroscopy of N2O embedded in xenon and SF6 as solvents provides an avenue to characterize the transitions between different phases as the concentration (or density) of the solvent increases. The present work demonstrates that classical molecular dynamics (MD) simulations together with accurate interaction potentials allows us to (semi-)quantitatively describe the transition in rotational vibrational infrared spectra from the P-/R-branch line shape for the stretch vibrations of N2O at low solvent densities to the Q-branch-like line shapes at high densities. The results are interpreted within the classical theory of rigid-body rotation in more/less constraining environments at high/low solvent densities or based on phenomenological models for the orientational relaxation of rotational motion. It is concluded that classical MD simulations provide a powerful approach to characterize and interpret the ultrafast motion of solutes in low to high density solvents at a molecular level.

Ultrafast 2DIR comparison of rotational energy transfer, isolated binary collision breakdown, and near critical fluctuations in Xe and SF6 solutions.

The density dependence of rotational and vibrational energy relaxation (RER and VER) of the N2O ν3 asymmetric stretch in dense gas and supercritical Xe and SF6 solutions for near critical isotherms is measured by ultrafast 2DIR and infrared pump-probe spectroscopy. 2DIR analysis provides precise measurements of RER at all gas and supercritical solvent densities. An isolated binary collision (IBC) model is sufficient to describe RER for solvent densities ≤ ∼4M where rotational equilibrium is re-established in ∼1.5-2.5 collisions. N2O RER is ∼30% more efficient in SF6 than in Xe due to additional relaxation pathways in SF6 and electronic factor differences. 2DIR analysis revealed that N2O RER exhibits a critical slowing effect in SF6 at near critical density (ρ* ∼ 0.8) where the IBC model breaks down. This is attributable to the coupling of critical long-range density fluctuations to the local N2O free rotor environment. No such RER critical slowing is observed in Xe because IBC break down occurs much further from the Xe critical point. Many body interactions effectively shield N2O from these near critical Xe density fluctuations. The N2O ν3 VER density dependence in SF6 is different than that seen for RER, indicating a different coupling to the near critical environment than RER. N2O ν3 VER is only about ∼7 times slower than RER in SF6. In contrast, almost no VER decay is observed in Xe over 200 ps. This VER solvent difference is due to a vibrationally resonant energy transfer pathway in SF6 that is not possible for Xe.

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.

Erratum: Label-free imaging of fibroblast membrane interfaces and protein signatures with vibrational infrared photothermal and phase signals: publisher's note.

[This corrects the article on p. 303 in vol. 12, PMID: 33520386.].

Plasmonic nano-antimicrobials: properties, mechanisms and applications in microbe inactivation and sensing.

Bacterial, viral and fungal infections pose serious threats to human health and well-being. The continuous emergence of acute infectious diseases caused by pathogenic microbes and the rapid development of resistances against conventional antimicrobial drugs necessitates the development of new and effective strategies for the safe elimination of microbes in water, food or on surfaces, as well as for the inactivation of pathogenic microbes in human hosts. The need for new antimicrobials has triggered the development of plasmonic nano-antimicrobials that facilitate both light-dependent and -independent microbe inactivation mechanisms. This review introduces the relevant photophysical mechanisms underlying these plasmonic nano-antimicrobials, and provides an overview of how the photoresponses and materials properties of plasmonic nanostructures can be applied in microbial pathogen inactivation and sensing applications. Through a systematic analysis of the inactivation efficacies of different plasmonic nanostructures, this review outlines the current state-of-the-art in plasmonic nano-antimicrobials and defines the application space for different microbial inactivation strategies. The advantageous optical properties of plasmonic nano-antimicrobials also enhance microbial detection and sensing modalities and thus help to avoid exposure to microbial pathogens. Sensitive and fast plasmonic microbial sensing modalities and their theranostic and targeted therapeutic applications are discussed.

Label-free imaging of fibroblast membrane interfaces and protein signatures with vibrational infrared photothermal and phase signals.

Label-free vibrational imaging of biological samples has attracted significant interest due to its integration of structural and chemical information. Vibrational infrared photothermal amplitude and phase signal (VIPPS) imaging provide label-free chemical identification by targeting the characteristic resonances of biological compounds that are present in the mid-infrared fingerprint region (3 µm - 12 µm). High contrast imaging of subcellular features and chemical identification of protein secondary structures in unlabeled and labeled fibroblast cells embedded in a collagen-rich extracellular matrix is demonstrated by combining contrast from absorption signatures (amplitude signals) with sensitive detection of different heat properties (lock-in phase signals). We present that the detectability of nano-sized cell membranes is enhanced to well below the optical diffraction limit since the membranes are found to act as thermal barriers. VIPPS offers a novel combination of chemical imaging and thermal diffusion characterization that paves the way towards label-free imaging of cell models and tissues as well as the study of intracellular heat dynamics.

Two-dimensional infrared spectroscopy from the gas to liquid phase: density dependent J-scrambling, vibrational relaxation, and the onset of liquid character.

Ultrafast 2DIR spectra and pump-probe responses of the N2O ν3 asymmetric stretch in SF6 as a function of density from the gas to supercritical phase and liquid are reported. 2DIR spectra unequivocally reveal free rotor character at all densities studied in the gas and supercritical region. Analysis of the 2DIR spectra determines that J-scrambling or rotational relaxation in N2O is highly efficient, occurring in ∼1.5 to ∼2 collisions with SF6 at all non-liquid densities. In contrast, N2O ν3 vibrational energy relaxation requires ∼15 collisions, and complete vibrational equilibrium occurs on the ∼ns scale at all densities. An independent binary collision model is sufficient to describe these supercritical state point dynamics. The N2O ν3 in liquid SF6 2DIR spectrum shows no evidence of free rotor character or spectral diffusion. Using these 2DIR results, hindered rotor or liquid-like character is found in gas and all supercritical solutions for SF6 densities ≥ρ* = 0.3, and increases with SF6 density. 2DIR spectral analysis offers direct time domain evidence of critical slowing for SF6 solutions closest to the critical point density. Applications of 2DIR to other high density and supercritical solution dynamics and descriptions are discussed.

Ultrafast Two-Dimensional Infrared Spectroscopy of a Quasifree Rotor: J Scrambling and Perfectly Anticorrelated Cross Peaks.

Ultrafast two-dimensional infrared (2DIR) spectra of the N_{2}O ν_{3} mode in moderately dense SF_{6} gas exhibit complex line shapes with diagonal and antidiagonal features in contrast to condensed phase vibrational 2DIR spectroscopy. Observed spectra for this quasifree rotor system are well captured by a model that includes all 36 possible rovibrational pathways and treats P (ΔJ=-1) and R (ΔJ=+1) branch resonances as distinct Kubo line shape features. Transition frequency correlation decay is due to J scrambling within one to two gas collisions at each density. Studies of supercritical solvation and relaxation at high pressure and temperature are enabled by this methodology.

Micromechanical resonator with dielectric nonlinearity.

Nonlinear response of dielectric polarization to electric field in certain media is the foundation of nonlinear optics. Optically, such nonlinearities are observed at high light intensities, achievable by laser, where atomic-scale field strengths exceeding 106-108 V/m can be realized. Nonlinear optics includes a host of fascinating phenomena such as higher harmonic frequency generation, sum and difference frequency generation, four-wave mixing, self-focusing, optical phase conjugation, and optical rectification. Even though nonlinear optics has been studied for more than five decades, such studies in analogous acoustic or microwave frequency ranges are yet to be realized. Here, we demonstrate a nonlinear dielectric resonator composed of a silicon micromechanical resonator with an aluminum nitride piezoelectric layer, a material known to have a nonlinear optical susceptibility. Using a novel multiport approach, we demonstrate second and third-harmonic generation, sum and difference frequency generation, and four-wave mixing. Our demonstration of a nonlinear dielectric resonator opens up unprecedented possibilities for exploring nonlinear dielectric effects in engineered structures with an equally broad range of effects such as those observed in nonlinear optics. Furthermore, integration of a nonlinear dielectric layer on a chip-scale silicon micromechanical resonator offers tantalizing prospects for novel applications, such as ultra high harmonic generation, frequency multipliers, microwave frequency-comb generators, and nonlinear microwave signal processing.

Plasmonic Enhancement of Selective Photonic Virus Inactivation.

Femtosecond (fs) pulsed laser irradiation techniques have attracted interest as a photonic approach for the selective inactivation of virus contaminations in biological samples. Conventional pulsed laser approaches require, however, relatively long irradiation times to achieve a significant inactivation of virus. In this study, we investigate the enhancement of the photonic inactivation of Murine Leukemia Virus (MLV) via 805 nm femtosecond pulses through gold nanorods whose localized surface plasmon resonance overlaps with the excitation laser. We report a plasmonically enhanced virus inactivation, with greater than 3.7-log reduction measured by virus infectivity assays. Reliable virus inactivation was obtained for 10 s laser exposure with incident laser powers ≥0.3 W. Importantly, the fs-pulse induced inactivation was selective to the virus and did not induce any measurable damage to co-incubated antibodies. The loss in viral infection was associated with reduced viral fusion, linking the loss in infectivity with a perturbation of the viral envelope. Based on the observations that physical contact between nanorods and virus particles was not required for viral inactivation and that reactive oxygen species (ROS) did not participate in the detected viral inactivation, a model of virus inactivation based on plasmon enhanced shockwave generation is proposed.

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.

Wireless actuation of micromechanical resonators.

The wireless transfer of power is of fundamental and technical interest, with applications ranging from the remote operation of consumer electronics and implanted biomedical devices and sensors to the actuation of devices for which hard-wired power sources are neither desirable nor practical. In particular, biomedical devices that are implanted in the body or brain require small-footprint power receiving elements for wireless charging, which can be accomplished by micromechanical resonators. Moreover, for fundamental experiments, the ultralow-power wireless operation of micromechanical resonators in the microwave range can enable the performance of low-temperature studies of mechanical systems in the quantum regime, where the heat carried by the electrical wires in standard actuation techniques is detrimental to maintaining the resonator in a quantum state. Here we demonstrate the successful actuation of micron-sized silicon-based piezoelectric resonators with resonance frequencies ranging from 36 to 120 MHz at power levels of nanowatts and distances of ~3 feet, including comprehensive polarization, distance and power dependence measurements. Our unprecedented demonstration of the wireless actuation of micromechanical resonators via electric-field coupling down to nanowatt levels may enable a multitude of applications that require the wireless control of sensors and actuators based on micromechanical resonators, which was inaccessible until now.

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.

Three-dimensional printed optical phantoms with customized absorption and scattering properties.

Three-dimensional (3D) printing offers the promise of fabricating optical phantoms with arbitrary geometry, but commercially available thermoplastics provide only a small range of physiologically relevant absorption (µa) and reduced scattering (µs`) values. Here we demonstrate customizable acrylonitrile butadiene styrene (ABS) filaments for dual extrusion 3D printing of tissue mimicking optical phantoms. µa and µs` values were adjusted by incorporating nigrosin and titanium dioxide (TiO2) in the filament extrusion process. A wide range of physiologically relevant optical properties was demonstrated with an average repeatability within 11.5% for µa and 7.71% for µs`. Additionally, a mouse-simulating phantom, which mimicked both the geometry and optical properties of a hairless mouse with an implanted xenograft tumor, was printed using dual extrusion methods. 3D printed tumor optical properties matched the live tumor with less than 3% error at a wavelength of 659 nm. 3D printing with user defined optical properties may provide a viable method for durable optically diffusive phantoms for instrument characterization and calibration.

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.

Engineered absorption enhancement and induced transparency in coupled molecular and plasmonic resonator systems.

Coupled plasmonic resonators have become the subject of significant research interest in recent years as they provide a route to dramatically enhanced light-matter interactions. Often, the design of these coupled mode systems draws intuition and inspiration from analogies to atomic and molecular physics systems. In particular, they have been shown to mimic quantum interference effects, such as electromagnetically induced transparency (EIT) and Fano resonances. This analogy also been used to describe the surface-enhanced absorption effect where a plasmonic resonance is coupled to a weak molecular resonance. These important phenomena are typically described using simple driven harmonic (or linear) oscillators (i.e., mass-on-a-spring) coupled to each other. In this work, we demonstrate the importance of an essential interdependence between the rate at which the system can be driven by an external field and its damping rate through radiative loss. This link is required in systems exhibiting time-reversal symmetry and energy conservation. Not only does it ensure an accurate and physically consistent description of resonant systems but leads directly to interesting new effects. Significantly, we demonstrate this dependence to predict a transition between EIT and electromagnetically induced absorption that is solely a function of the ratio of the radiative to intrinsic loss rates in coupled resonator systems. Leveraging the temporal coupled mode theory, we introduce a unique and intuitive picture that accurately describes these effects in coupled plasmonic/molecular and fully plasmonic systems. We demonstrate our approach's key features and advantages analytically as well as experimentally through surface-enhanced absorption spectroscopy and plasmonic metamaterial applications.

Mid-infrared photothermal heterodyne spectroscopy in a liquid crystal using a quantum cascade laser.

We report a technique to measure the mid-infrared photothermal response induced by a tunable quantum cascade laser in the neat liquid crystal 4-octyl-4'-cyanobiphenyl (8CB), without any intercalated dye. Heterodyne detection using a Ti:sapphire laser of the response in the solid, smectic, nematic and isotropic liquid crystal phases allows direct detection of a weak mid-infrared normal mode absorption using an inexpensive photodetector. At high pump power in the nematic phase, we observe an interesting peak splitting in the photothermal response. Tunable lasers that can access still stronger modes will facilitate photothermal heterodyne mid-infrared vibrational spectroscopy.

Predicting the melting temperature of ice-Ih with only electronic structure information as input.

The melting temperature of ice-Ih was calculated with only electronic structure information as input by creating a problem-specific force field. The force field, Water model by AFM for Ice and Liquid (WAIL), was developed with the adaptive force matching (AFM) method by fitting to post-Hartree-Fock quality forces obtained in quantum mechanics∕molecular mechanics calculations. WAIL predicts the ice-Ih melting temperature to be 270 K. The model also predicts the densities of ice and water, the temperature of maximum density of water, the heat of vaporizations, and the radial distribution functions for both ice and water in good agreement with experimental measurements. The non-dissociative WAIL model is very similar to a flexible version of the popular TIP4P potential and has comparable computational cost. By customizing to problem-specific configurations with the AFM approach, the resulting model is remarkably more accurate than any variants of TIP4P for simulating ice-Ih and water in the temperature range from 253 K and 293 K under ambient pressure.

Rational design and optimization of plasmonic nanoarrays for surface enhanced infrared spectroscopy.

We present an approach for rational design and optimization of plasmonic arrays for ultrasensitive surface enhanced infrared absorption (SEIRA) spectroscopy of specific protein analytes. Motivated by our previous work that demonstrated sub-attomole detection of surface-bound silk fibroin [Proc. Natl. Acad. Sci. U.S.A. 106, 19227 (2009)], we introduce here a general framework that allows for the numerical optimization of metamaterial sensor designs in order to maximize the absorbance signal. A critical feature of our method is the explicit compensation for the perturbative effects of the analyte's refractive index which alters the resonance frequency and line-shape of the metamaterial response, thereby leading to spectral distortion in SEIRA signatures. As an example, we leverage our method to optimize the geometry of periodic arrays of plasmonic nanoparticles on both Si and CaF2 substrates. The optimal geometries result in a three-order of magnitude absorbance enhancement compared to an unstructured Au layer, with the CaF2 substrate offering an additional factor of three enhancement in absorbance over a traditional Si substrate. The latter improvement arises from increase of near-field intensity over the Au nanobar surface for the lower index substrate. Finally, we perform sensitivity analysis for our optimized arrays to predict the effects of fabrication imperfections. We find that <20% deviation from the optimized absorbance response is readily achievable over large areas with modern nanofabrication techniques.