mid-IR Pump Research Articles

Ferroelectric polarization reversal versus pump spot shape

It has recently been shown that the polarization of a ferroelectric can be strongly affected by light pumping or even transiently reversed by mid-IR pumping within a record short subpicosecond timescale. It was then suggested that the reversal of polarization is most likely prevented by the depolarizing field that appears during the process. To minimize the effect of the depolarizing field, I propose to create an elongated domain by choosing a corresponding pump spot shape. I calculate the polarization dynamics for a particular experimental setup with mid-IR pumping and show that the polarization reversal is possible in this case.

Antiresonant hollow core fiber-assisted photothermal spectroscopy of nitric oxide at 5.26 μm with parts-per-billion sensitivity

Laser-based gas sensors are undergoing constant and rapid development due to their usability in numerous applications focused on e.g. monitoring of greenhouse gases concentration, safety in harsh environments or medicine. Amongst many types of such sensors, gas detectors employing the photoacoustic or photothermal effect are characterized by an exceptional capability of delivering high selectivity combined with superb signal-to-noise ratio (SNR). In this work, we demonstrate a novel, compact and sensitive nitric oxide (NO) sensor targeting its strong transition near 5.26 μm by combining the photothermal interferometry (PTI) technique and a gas absorption cell based on a self-fabricated antiresonant hollow-core fiber (ARHCF). Having only a 25 cm long interaction length, the sensor reached a minimum detection limit (MDL) of 11 ppbv (parts per billion by volume) for an integration time of 144 s. Proper design of the ARHCF utilized as the gas cell enabled combining a mid-infrared (mid-IR) pump laser to induce the refractive index modulation and a Fabry-Perot interferometer-based readout of the photothermal effect with a near-infrared probe laser. The presented results document the first experimental attempt to detect gas molecules having transitions beyond 4.5 μm in an ARHCF-aided photothermal detection scheme.

Transient CO desorption from thin Pt films induced by mid-IR pumping.

Resonant and off-resonant mid-infrared pump-probe spectroscopy is used to measure the vibrational dynamics of CO adsorbed to thin (0.2 nm, 2 nm, and 10 nm) heterogeneous Pt layers in an aqueous solution. The transient signals observed with resonant pumping are dominated by vibrational relaxation of the CO internal stretch vibration with a lifetime of T1 ∼ 3 ps in all cases. Off-resonant pumping suppresses that contribution to the signal and singles out a signal, which is attributed to heating of the metal layer as well as transient desorption of the CO molecules. Due to the small photon energy (0.2 eV) used as pump pulses, the mechanism of desorption must be thermal, in which case the desorption yield depends exclusively on the fluence of absorbed light and not its wavelength. The thin Pt layers facilitate CO desorption, despite a relatively low pump pulse fluence, as they concentrate the absorbed energy in a small volume.

Standoff pump-probe photothermal detection of hazardous chemicals

A novel pump-probe Photothermal methodology using Quartz Tuning Fork (QTF) detector has been demonstrated for the first time. A tunable mid-IR Quantum Cascade Laser (QCL) and a CW fixed wavelength visible laser have been used as the pump and probe beam respectively. The developed Photothermal (PT) technique is based on Quartz Tuning Fork (QTF) detector for the detection of hazardous/explosive molecules adsorbed on plastic surface and also in aerosols form. PT spectra of various trace molecules in the fingerprinting mid- infrared spectral band 7–9 µm from distance of 25 m have been recorded. The PT spectra of explosives RDX, TNT and Acetone have been recorded at very low quantities. Acetone is the precursor of explosive Tri-Acetone Tri-Phosphate (TATP). The experimentations using pump and probe lasers, exhibit detection sensitivity of less than 5 μg/cm2 for RDX, TNT powders and of ~ 200 nl quantity for Nitrobenzene (NB) and Acetone (in liquid form) adsorbed on surfaces, from a distance of ~ 25 m. The sensitivity of the same order achieved from a distance of 15 m by using only a mid-IR tunable pump laser coupled to QTF detector. Thus the pump-probe PT technique is more sensitive in comparison to single tunable QCL pump beam technique and it is better suited for standoff detection of hazardous chemicals for homeland security as well as for forensic applications.

Mid-infrared laser-induced thermal grating spectroscopy of hot water lines for flame thermometry

In this work we report the impact of using mid-infrared laser-induced thermal grating spectroscopy (IR-LITGS) for temperature measurements in flames by probing hot water lines. The measurements have been performed in the product zone of laminar atmospheric CH4/H2/air flat flames with equivalence ratio ranging between 0.6 and 1.05. LITGS is a technique based on thermalization through collisions of the excited molecules for generation of a laser-induced grating, which then decays through thermal diffusion. As such, it tends to have limited application in atmospheric flames compared to flame measurements at elevated pressures due to the faster decay at low gas densities. However, by using mid-IR pump laser beams, it enables the generation of laser-induced gratings with large grating spacing, resulting in strong signal intensities and long signal durations. Single-shot IR-LITGS signals were recorded in the different CH4/H2/air flames that covered a temperature range between 1500 and 1800 K. To test the accuracy, the IR-LITGS flame temperature measurements were compared with laser Rayleigh scattering measurements and the result were in good agreement with each other. The IR-LITGS flame temperature measurements show a repetitive single-shot temperature precision better than 1% and an accuracy of 2.5% of the flame temperature. An IR-LITGS excitation scan of water in the flame shows that some ro-vibrational transitions exhibit no IR-LITGS signal, probably due to less efficient collisional energy transfer mechanism. This is important when deciding the wavelength to use for IR-LITGS flame temperature measurements using water absorption.

Visible pump-mid infrared pump-broadband probe: Development and characterization of a three-pulse setup for single-shot ultrafast spectroscopy at 50 kHz.

We report here an experimental setup to perform three-pulse pump-probe measurements over a wide wavelength and temperature range. By combining two pump pulses in the visible (650 nm-900 nm) and mid-IR (5 μm-20 μm) range, with a broadband supercontinuum white-light probe, our apparatus enables both the combined selective excitation of different material degrees of freedom and a full time-dependent reconstruction of the non-equilibrium dielectric function of the sample. We describe here the optical setup, the cryogenic sample environment, and the custom-made acquisition electronics capable of referenced single-pulse detection of broadband spectra at the maximum repetition rate of 50 kHz, achieving a sensitivity of the order of 10-4 over an integration time of 1 s. We demonstrate the performance of the setup by reporting data on a mid-IR pump, optical push, and broadband probe in a single crystal of Bi2Sr2Y0.08Ca0.92Cu2O8+δ across the superconducting and pseudogap phases.

Noise and spectral stability of deep-UV gas-filled fiber-based supercontinuum sources driven by ultrafast mid-IR pulses

Deep-UV (DUV) supercontinuum (SC) sources based on gas-filled hollow-core fibers constitute perhaps the most viable solution towards ultrafast, compact, and tunable lasers in the UV spectral region, which can even also extend into the mid-infrared (IR). Noise and spectral stability of such broadband sources are key parameters that define their true potential and suitability towards real-world applications. In order to investigate the spectral stability and noise levels in these fiber-based DUV sources, we generate an SC spectrum that extends from 180 nm (through phase-matched dispersive waves - DWs) to 4 μm by pumping an argon-filled hollow-core anti-resonant fiber at a mid-IR wavelength of 2.45 μm. We characterize the long-term stability of the source over several days and the pulse-to-pulse relative intensity noise (RIN) of the DW at 275 nm. The results indicate no sign of spectral degradation over 110 hours, but the RIN of the DW pulses at 275 nm is found to be as high as 33.3%. Numerical simulations were carried out to investigate the spectral distribution of the RIN and the results confirm the experimental measurements and that the poor noise performance is due to the high RIN of the mid-IR pump laser, which was hitherto not considered in numerical modelling of these sources. The results presented herein provide an important step towards an understanding of the noise mechanism underlying such complex light-gas nonlinear interactions and demonstrate the need for pump laser stabilization.

Multifunctional integrated photonics in the mid-infrared with suspended AlGaAs on silicon

The microscale integration of mid- and long-wave-infrared photonics could enable the development of fieldable, robust chemical sensors, as well as highly efficient infrared frequency converters. However, such technology would be defined by the choice of material platform, which immediately determines the strength and types of optical nonlinearities available, the optical transparency window, modal confinement, and physical robustness. In this work, we demonstrate a new platform, suspended AlGaAs waveguides integrated on silicon, providing excellent performance in all of these metrics. We demonstrate low propagation losses within a span of nearly two octaves (1.26–4.6 μm) with exemplary performance of 0.45 dB/cm at λ=2.4 μm. We exploit the high nonlinearity of this platform to demonstrate 1560 nm-pumped second-harmonic generation and octave-spanning supercontinuum reaching out to 2.3 μm with 3.4 pJ pump pulse energy. With mid-IR pumping, we generate supercontinuum spanning from 2.3 to 6.5 μm. Finally, we demonstrate the versatility of the platform with mid-infrared passive devices such as low-loss 10 μm-radius bends, compact power splitters with 96±1% efficiency, and edge couplers with 3.0±0.1 dB loss. This platform has strong potential for multifunctional integrated photonic systems in the mid-infrared.

Effect of Osmolytes on the Conformational Behavior of a Macromolecule in a Cytoplasm-like Crowded Environment: A Femtosecond Mid-IR Pump-Probe Spectroscopy Study.

Osmolytes found endogenously in almost all living beings play an important role in regulating cell volume under harsh environment. Here, to address the longstanding questions about the underlying mechanism of osmolyte effects, we use femtosecond mid-IR pump-probe spectroscopy with two different IR probes that are the OD stretching mode of HDO and the azido stretching mode of azido-derivatized poly(ethylene glycol) dimethyl ether (PEGDME). Our experimental results show that protecting osmolytes bind strongly with water molecules and dehydrate polymer surface, which results in promoting intramolecular interactions of the polymer. By contrast, urea behaves like water molecules without significantly disrupting water H-bonding network and favors extended and random-coil segments of the polymer chain by directly participating in solvation of the polymer. Our findings highlight the importance of direct interaction between urea and macromolecule, while protecting osmolytes indirectly affect the macromolecule through enhancing the water-osmolyte interaction in a crowded environment, which is the case that is often encountered in real biological systems.

Water Dynamics in Cytoplasm-Like Crowded Environment Correlates with the Conformational Transition of the Macromolecular Crowder.

Polyethylene glycol (PEG) is a unique polymer material with enormous applicability in many industrial and scientific fields. Here, its use as macromolecular crowder to mimic the cellular environment in vitro is the focus of the present study. We show that femtosecond mid-IR pump-probe spectroscopy using three different IR probes, HDO, HN3, and azido-derivatized crowder, provides complete and stereoscopic information on water structure and dynamics in the cytoplasm-like macromolecular crowding environment. Our experimental results suggest two distinct subpopulations of water molecules: those that interact with other water molecules and those that are part of a hydration shell of crowder on its surface. Interestingly, water dynamics even in highly crowded environment remains bulk-like in spite of significant perturbation to the tetrahedral H-bonding network of water molecules. That is possible because of the formation of water aggregates (pools) even in water-deficient PEGDME-water solutions. In such a crowded environment, the conformationally accessible phase space of the macromolecular crowder is reduced, similar to biopolymers in highly crowded cytoplasm. Nonetheless, the hydration water on the surface of crowders slows down considerably with increased crowding. Most importantly, we do not observe any coalescing of surface hydration water (of the crowder) with bulk-like water to generate collective hydration dynamics at any crowder concentration, contrary to recent reports. We anticipate that the present triple-IR-probe approach is of exceptional use in studying how conformational states of crowders correlate with structural and dynamical changes of water, which is critical in understanding their key roles in biological and industrial applications.

Invited Article: Multiple-octave spanning high-energy mid-IR supercontinuum generation in bulk quadratic nonlinear crystals

Bright and broadband coherent mid-IR radiation is important for exciting and probing molecular vibrations. Using cascaded nonlinearities in conventional quadratic nonlinear crystals like lithium niobate, self-defocusing near-IR solitons have been demonstrated that led to very broadband supercontinuum generation in the visible, near-IR, and short-wavelength mid-IR. Here we conduct an experiment where a mid-IR crystal is pumped in the mid-IR. The crystal is cut for noncritical interaction, so the three-wave mixing of a single mid-IR femtosecond pump source leads to highly phase-mismatched second-harmonic generation. This self-acting cascaded process leads to the formation of a self-defocusing soliton at the mid-IR pump wavelength and after the self-compression point multiple octave-spanning supercontinua are observed. The results were recorded in a commercially available crystal LiInS2 pumped in the 3-4 μm range with 85 fs 50 μJ pulse energy, with the broadest supercontinuum covering 1.6-7.0 μm. We measured up 30 μJ energy in the supercontinuum, and the energy promises to scale favorably with an increased pump energy. Other mid-IR crystals can readily be used as well to cover other pump wavelengths and target other supercontinuum wavelength ranges.

Generation of Transient Photocurrents in the Topological Surface State of Sb_{2}Te_{3} by Direct Optical Excitation with Midinfrared Pulses.

We combine tunable midinfrared (mid-IR) pump pulses with time- and angle-resolved two-photon photoemission to study ultrafast photoexcitation of the topological surface state (TSS) of Sb_{2}Te_{3}. It is revealed that mid-IR pulses permit a direct excitation from the occupied to the unoccupied part of the TSS across the Dirac point. The novel optical coupling induces asymmetric transient populations of the TSS at ±k_{∥}, which reflects a macroscopic photoexcited electric surface current. By observing the decay of the asymmetric population, we directly investigate the dynamics of the long-lived photocurrent in the time domain. Our discovery promises important advantages of photoexcitation by mid-IR pulses for spintronic applications.

Water Hydrogen-Bonding Network Structure and Dynamics at Phospholipid Multibilayer Surface: Femtosecond Mid-IR Pump-Probe Spectroscopy.

The water hydrogen-bonding network at a lipid bilayer surface is crucial to understanding membrane structures and its functional activities. With a phospholipid multibilayer mimicking a biological membrane, we study the temperature dependence of water hydrogen-bonding structure, distribution, and dynamics at a lipid multibilayer surface using femtosecond mid-IR pump-probe spectroscopy. We observe two distinguished vibrational lifetime components. The fast component (0.6 ps) is associated with water interacting with a phosphate part, whereas the slow component (1.9 ps) is with bulk-like choline-associated water. With increasing temperature, the vibrational lifetime of phosphate-associated water remains constant though its relative fraction dramatically increases. The OD stretch vibrational lifetime of choline-bound water slows down in a sigmoidal fashion with respect to temperature, indicating a noticeable change of the water environment upon the phase transition. The water structure and dynamics are thus shown to be in quantitative correlation with the structural change of liquid multibilayer upon the gel-to-liquid crystal phase transition.

Recent Progress in Widely Tunable Single-Mode Room Temperature Terahertz Quantum Cascade Laser Sources

We present the operating principle, design, and performance of external-cavity (EC) and monolithic terahertz (THz) tuners based on intracavity difference-frequency generation (DFG) in midinfrared (mid-IR) quantum cascade lasers (QCLs). A DFG-QCL gain chip employed in a Littrow-type THz EC system was optimized for wide tunability in 1–6 THz range using broad mid-IR gain bandwidth active region with integrated optical nonlinearity and Cherenkov THz phase-matching scheme. The EC system demonstrated ultrabroadband single-mode tuning from 1.2 to 5.9 THz. Beam steering in THz far field due to refractive index dispersion of InP substrate has been successfully suppressed by bonding the QCL chip on a high-resistivity silicon substrate. We also discuss the design of compact widely tunable monolithic THz DFG-QCL sources. Devices use two electrically isolated grating sections for independent electronic tuning of the two mid-IR pump frequencies. THz DFG frequency tuning from 3.44 to 4.02 THz, corresponding to 580 GHz or 15% of center frequency, is obtained.

Two-dimensional vibrational-electronic spectroscopy.

Two-dimensional vibrational-electronic (2D VE) spectroscopy is a femtosecond Fourier transform (FT) third-order nonlinear technique that creates a link between existing 2D FT spectroscopies in the vibrational and electronic regions of the spectrum. 2D VE spectroscopy enables a direct measurement of infrared (IR) and electronic dipole moment cross terms by utilizing mid-IR pump and optical probe fields that are resonant with vibrational and electronic transitions, respectively, in a sample of interest. We detail this newly developed 2D VE spectroscopy experiment and outline the information contained in a 2D VE spectrum. We then use this technique and its single-pump counterpart (1D VE) to probe the vibrational-electronic couplings between high frequency cyanide stretching vibrations (νCN) and either a ligand-to-metal charge transfer transition ([Fe(III)(CN)6](3-) dissolved in formamide) or a metal-to-metal charge transfer (MMCT) transition ([(CN)5Fe(II)CNRu(III)(NH3)5](-) dissolved in formamide). The 2D VE spectra of both molecules reveal peaks resulting from coupled high- and low-frequency vibrational modes to the charge transfer transition. The time-evolving amplitudes and positions of the peaks in the 2D VE spectra report on coherent and incoherent vibrational energy transfer dynamics among the coupled vibrational modes and the charge transfer transition. The selectivity of 2D VE spectroscopy to vibronic processes is evidenced from the selective coupling of specific νCN modes to the MMCT transition in the mixed valence complex. The lineshapes in 2D VE spectra report on the correlation of the frequency fluctuations between the coupled vibrational and electronic frequencies in the mixed valence complex which has a time scale of 1 ps. The details and results of this study confirm the versatility of 2D VE spectroscopy and its applicability to probe how vibrations modulate charge and energy transfer in a wide range of complex molecular, material, and biological systems.

Hidden harmonic quantum states in proteins: Did Davydov get the sign wrong?

We revisit mid-IR pump–probe experiments at the FELIX picosecond Free Electron Laser which probed the vibrational dynamics of the α-helix rich protein in search of long-lived anharmonic trapped vibrational states (solitons). We analyze and try to understand something puzzling that we observed in the context of unusual ‘hidden’ quantum phenomena in proteins which probably are of no biological consequences, but bears re-examination. We observed in a narrow (0.5 cm−1) spectral range of the amide-I band a very large response in terms of degenerate 4-wave mixing scattering. We propose that this narrow but strong scattering signal is not due to anharmonic trapping but rather is the imaginary (index of refraction) component of a super-linear response of the amide I band to high levels of vibrational excitation.

Conformation of the neurotransmitter γ-aminobutyric acid in liquid water.

We study the conformation and reorientation dynamics of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) under neutral and acidic conditions using a combination of broadband dielectric relaxation spectroscopy and polarization-resolved femtosecond mid-infrared pump-probe spectroscopy. We find that both zwitterionic and cationic GABA adopt nearly linear conformations in aqueous solution, meaning that the two charged functional groups of the GABA zwitterion are hydrated separately.

Polarization sensitive ultrafast mid-IR pump probe micro-spectrometer with diffraction limited spatial resolution

A setup of ultrafast transient infrared IR spectrometer is described in this paper that employed Schwarzschild objectives to focus the probe beam to a diffraction limited spot. Thus measurements were performed with very high spatial resolution in the mid-IR spectral region. Furthermore, modulating the polarization of the probe light enabled detecting transient dichroism of the sample. These capabilities of the setup were applied to study transient absorption of Photosystem II core complex and to image an organized film of methylene blue chloride dye. Moreover, a study of noise sources in a pump probe measurement is presented. The predicted noise level of the current setup was 8.25 μOD in 10(4) acquisitions and compared very well with the experimental observation of 9.6 μOD.

Double resonance 1-D photonic crystal cavities for single-molecule mid-infrared photothermal spectroscopy: theory and design

We propose and theoretically examine a novel mid-infrared (mid-IR) photothermal spectroscopic sensing technique capable of detecting a single small molecule. Our conceptual design attains such high sensitivity by leveraging dramatically amplified photothermal effects in an optical nanocavity doubly resonant at both mid-IR pump and near-IR probe wavelengths. Unlike conventional mid-IR spectroscopy, the technique eliminates the need for cryogenically cooled mid-IR photodetectors, as optical detection is performed solely at the near-IR probe wavelength. A device design based on nested one-dimensional nanobeam photonic crystal cavities is numerically analyzed to demonstrate the technique's potential for single small gas molecule detection.

Role of PufX in Photochemical Charge Separation in the RC-LH1 Complex from Rhodobacter sphaeroides: An Ultrafast Mid-IR Pump–Probe Investigation

Photochemical charge separation in isolated reaction center-light harvesting 1 (RC-LH1) complexes from Rhodobacter sphaeroides was examined using time-resolved mid-infrared pump-probe spectroscopy. Absorption difference spectra were recorded between 1760 and 1610 cm(-1) with subpicosecond time resolution to characterize excited-state and radical pair dynamics in these complexes, via the induced absorption changes in the keto carbonyl modes of the bacteriochlorophylls and bacteriopheophytins. Experiments on RC-LH1 complexes with and without the polypeptide PufX show that its presence is required to achieve generation of the radical pair P(+)Q(A)(-) under mildly reducing conditions. In the presence of PufX, the final radical pair formed over a ~3 ns period was P(+)Q(A)(-), but in its absence the corresponding radical pair was P(+)H(A)(-), implying that Q(A) was either absent in these PufX-deficient complexes or was prereduced. However, P(+)Q(A)(-) could be generated in PufX-deficient complexes following addition of the oxidant DMSO, showing that Q(A) was present in these complexes and allowing the conclusion that under mildly reducing conditions charge separation was blocked after P(+)H(A)(-) due to the presence of an electron on Q(A). The data provide strong support for the hypothesis that one of the functions of PufX is to regulate the stability of Q(B)(-), ensuring the oxidation of Q(A)(-) in the presence of a reduced quinone pool and so preserving efficient photochemical charge separation under anaerobic conditions.