Optimization of stimulated rotational Raman scattering over vibrational scattering in a hydrogen-filled fiber.

We present the first, to the best of our knowledge, investigation of the gain competition between rotational and vibrational stimulated Raman scattering (SRS) in the transient regime for a hydrogen (H2)-filled antiresonant fiber (ARF) with the aim of generating multispectral emission composed of only rotational SRS. We show numerically and experimentally that purely rotational emission requires optimization of ARF length and spectral transmission, pump power and polarization, and H2 pressure. In this work, the H2-filled ARF is pumped by 40 kW, 7 ns pulses at λ = 1.06 µm to produce six discrete rotational lines from 1.1 to 1.7 µm with unique temporal profiles and pulse energies up to tens of microjoules.

Wavelength conversion through stimulated Raman scattering in an oxygen-filled fiber for multi-band LiDAR.

Wavelength conversion afforded by stimulated Raman scattering within a hollow core fiber is potentially useful for multispectral light detection and ranging (LiDAR). Herein, we make use of the ideal 1550 cm-1 vibrational Raman shift of an antiresonant fiber filled with gaseous oxygen so that the first and second Raman orders as well as the transmitted pump are all located in separate atmospheric transmission windows. To the best of our knowledge, this is the first report of stimulated Raman scattering in an oxygen-filled fiber. The host of closely spaced rotational stimulated Raman scattering (SRS) lines (12 cm-1) accompanying the transmitted pump and vibrational Raman orders form continuum bands allowing for much greater spectral coverage of the atmospheric transmission windows. The temporal profiles of the Raman orders can be separated without the use of a grating to potentially achieve a multi-band LiDAR.

Charting a course to efficient difference frequency generation in molecular-engineered liquid-core fiber.

Although χ(2) nonlinear optical processes, such as difference frequency generation (DFG), are often used in conjunction with fiber lasers for wavelength conversion and photon-pair generation, the monolithic fiber architecture is broken by the use of bulk crystals to access χ(2). We propose a novel solution by employing quasi-phase matching (QPM) in molecular-engineered hydrogen-free, polar-liquid core fiber (LCF). Hydrogen-free molecules offer attractive transmission in certain NIR-MIR regions and polar molecules tend to align with an externally applied electrostatic field creating a macroscopic χ e f f(2). To further increase χ e f f(2) we investigate charge transfer (CT) molecules in solution. Using numerical modeling we investigate two bromotrichloromethane based mixtures and show that the LCF has reasonably high NIR-MIR transmission and large QPM DFG electrode period. The inclusion of CT molecules has the potential to yield χ e f f(2) at least as large as has been measured in silica fiber core. Numerical modeling for the degenerate DFG case indicates that signal amplification and generation through QPM DFG can achieve nearly 90% efficiency.

Spectrally pure photons generated in a quasi-phase matched xenon-filled hollow-core photonic crystal fiber.

Spectrally pure photons heralded from unentangled photon pair sources are crucial for any quantum optical system reliant on the multiplexing of heralded photons from independent sources. Generation of unentangled photon pairs in gas-filled hollow-core photonic crystal fibers specifically remains an attractive architecture for integration into quantum-optical fiber networks. The dispersion design offered by selection of fiber microstructures and gas pressure allows considerable control over the group-velocity profile which dictates the wavelengths of photon pairs that can be generated without spectral entanglement. Here, we expand on this design flexibility, which has previously been implemented for four-wave mixing, by modeling the use of a static, periodically poled electric field to achieve an effective quasi-phase-matched three-wave mixing nonlinearity that creates spontaneous parametric downconversion. Electric-field-induced quasi-phase-matched spontaneous parametric downconversion enables control of phase matching conditions that is independent of the group velocity, allowing phase matching at arbitrary wavelengths without affecting the entanglement of photons at those wavelengths. This decoupling of entanglement engineering and phase matching facilitates spectrally pure photon pair generation with efficiency and wavelength-tunability that is otherwise unprecedented.

Modeling quasi-phase-matched electric-field-induced optical parametric amplification in hollow-core photonic crystal fibers.

Laser sources in the short- and mid-wave infrared spectral regions are desirable for many applications. The favorable spectral guidance and power handling properties of an inhibited coupling hollow-core photonic crystal fiber (HC-PCF) enable nonlinear optical routes to these wavelengths. We introduce a quasi-phase-matched, electric-field-induced, pressurized xenon-filled HC-PCF-based optical parametric amplifier. A spatially varying electrostatic field can be applied to the fiber via patterned electrodes with modulated voltages. We incorporate numerically modeled electrostatic field amplitudes and fringing, modeled fiber dispersion and transmission, and calculated voltage thresholds to determine fiber lengths of tens of meters for efficient signal conversion for several xenon pressures and electrode configurations.

Generation of narrowband pulses from chirped broadband pulse frequency mixing.

We extend an approach based upon sum-frequency generation of oppositely chirped pulses to narrow the bandwidths of broadband femtosecond pulses. We efficiently generate near-transform-limited pulses with durations of several picoseconds, while reducing the pulse bandwidth by a factor of 120, which is more than twice the reduction reported in previous literature. Such extreme bandwidth narrowing of a broadband pulse enhances the effects of dispersion nonlinearities. Precise chirp control enables us to characterize the efficacy of frequency mixing broadband pulses with nonlinear temporal chirps. We demonstrate the use of these narrowband pulses as probes in coherent anti-Stokes Raman spectroscopy.

Rotational coherence beating in molecular oxygen: Coupling between electronic spin and nuclear angular momenta.

Time-resolved pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (fs/ps RCARS) of oxygen (O2) is performed at pressures from ∼0.04 to 0.4 atm. As the RCARS spectra evolve with probe delay, they exhibit coherence beating between unresolved S-branch triplet transitions (ΔN = 2, ΔJ = 2). The time-domain fitting of the RCARS signal intensity enables the determination of these transition frequency separations, which are as low as 480 MHz (0.016 cm-1). Additionally, we study the underlying pressure-dependent dynamics and the signatures of the time-domain triplet signals compared to the simple decays associated with the O2 self-broadened linewidths. Pressure- and N-dependent O2 linewidths are compared to literature coefficients obtained from experiments and models that have not incorporated the triplet splitting. Our findings are incorporated into a time-domain model for rotational CARS thermometry of O2 and have significant impact for spectral evaluations at probe delays greater than 100 ps for temperature or species concentration determination. The time- and frequency-resolved experiments presented in this work provide insight into the spectroscopic complexities introduced by the electronic ground state of O2 for accurate evaluation of time-resolved coherent Raman spectra.

Implementation of continuous fast scanning detection in femtosecond Fourier-transform two-dimensional vibrational-electronic spectroscopy to decrease data acquisition time.

Femtosecond Fourier transform two-dimensional vibrational-electronic (2D VE) spectroscopy is a recently developed third-order nonlinear spectroscopic technique to measure coupled electronic and vibrational motions in the condensed phase. The viability of femtosecond multidimensional spectroscopy as an analytical tool requires improvements in data collection and processing to enhance the signal-to-noise ratio and increase the amount of data collected in these experiments. Here a continuous fast scanning technique for the efficient collection of 2D VE spectroscopy is described. The resulting 2D VE spectroscopic method gains sensitivity by reducing the effect of laser drift, as well as decreasing the data collection time by a factor of 10 for acquiring spectra with a high signal-to-noise ratio within 3 dB of the more time intensive step scanning methods. This work opens the door to more comprehensive studies where 2D VE spectra can be collected as a function of external parameters such as temperature, pH, and polarization of the input electric fields.

Pure-rotational H2 thermometry by ultrabroadband coherent anti-Stokes Raman spectroscopy.

Coherent anti-Stokes Raman spectroscopy (CARS) is a sensitive technique for probing highly luminous flames in combustion applications to determine temperatures and species concentrations. CARS thermometry has been demonstrated for the vibrational Q-branch and pure-rotational S-branch of several small molecules. Practical advantages of pure-rotational CARS, such as multi-species detection, reduction of coherent line mixing and collisional narrowing even at high pressures, and the potential for more precise thermometry, have motivated experimental and theoretical advances in S-branch CARS of nitrogen (N2), for example, which is a dominant species in air-fed combustion processes. Although hydrogen (H2) is of interest given its prevalence as a reactant and product in many gas-phase reactions, laser bandwidth limitations have precluded the extension of CARS thermometry to the H2 S-branch. We demonstrate H2 thermometry using hybrid femtosecond/picosecond pure-rotational CARS, in which a broadband pump/Stokes pulse enables simultaneous excitation of the set of H2 S-branch transitions populated at flame temperatures over the spectral region of 0-2200 cm-1. We present a pure-rotational H2 CARS spectral model for data fitting and compare extracted temperatures to those from simultaneously collected N2 spectra in two systems of study: a heated flow and a diffusion flame on a Wolfhard-Parker slot burner. From 300 to 650 K in the heated flow, the H2 and N2 CARS extracted temperatures are, on average, within 2% of the set temperature. For flame measurements, the fitted H2 and N2 temperatures are, on average, within 5% of each other from 300 to 1600 K. Our results confirm the viability of pure-rotational H2 CARS thermometry for probing combustion reactions.

Fourier transform two-dimensional electronic-vibrational spectroscopy using an octave-spanning mid-IR probe.

The development of coherent Fourier transform two-dimensional electronic-vibrational (2D EV) spectroscopy with acousto-optic pulse-shaper-generated near-UV pump pulses and an octave-spanning broadband mid-IR probe pulse is detailed. A 2D EV spectrum of a silicon wafer demonstrates the full experimental capability of this experiment, and a 2D EV spectrum of dissolved hexacyanoferrate establishes the viability of our 2D EV experiment for studying condensed phase molecular ensembles.

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.

Measuring Coherently Coupled Intramolecular Vibrational and Charge-Transfer Dynamics with Two-Dimensional Vibrational-Electronic Spectroscopy.

We demonstrate Fourier transform (FT) 2D vibrational-electronic (2D VE) spectroscopy employing a novel mid-IR and optical pulse sequence. This new femtosecond third-order nonlinear spectroscopy provides the high time and frequency resolutions of existing 2D FT techniques; however, resulting 2D VE spectra contain IR and electronic dipole moment cross terms. We use 2D VE spectroscopy to help understand the vibrational-electronic couplings in the cyanide-bridged transition-metal mixed valence complex [(CN)5Fe(II)CNRu(III)(NH3)5](-) dissolved in formamide. The amplitudes of the cross-peaks in the 2D VE spectra reveal that three of the intramolecular cyanide stretching vibrations lying along the charge-transfer axis are coherently coupled to the metal-to-metal charge-transfer electronic transition with differing strengths. Analysis of the 2D VE line shapes reveals positive and negative correlations of the cyanide stretching modes with the charge-transfer transition depending on the physical orientation of the vibration in the molecule and its interaction with the solvent. The insights found thus far into the vibronic couplings in the mixed valence model system indicate that the 2D VE technique will be a valuable addition to the existing multidimensional spectroscopy toolbox.

Enhanced interferometric detection in two-dimensional spectroscopy with a Sagnac interferometer.

An intrinsically phase-stable Sagnac interferometer is introduced for optimized interferometric detection in partially collinear two-dimensional (2D) spectroscopy. With a pump-pulse pair from an actively stabilized Mach-Zehnder interferometer, the Sagnac scheme is demonstrated in broadband, short-wave IR (1-2 µm), 2D electronic spectroscopy of IR-26 dye.

Absolute measurement of femtosecond pump-probe signal strength.

The absolute femtosecond pump-probe signal strength of deprotonated fluorescein in basic methanol is measured. Calculations of the absolute pump-probe signal based on the steady-state absorption and emission spectrum that use only independently measured experimental parameters are carried out. The calculation of the pump-probe signal strength assumes the pump and probe fields are both weak and includes the following factors: the transverse spatial profile of the laser beams; the pulse spectra; attenuation of the propagating pulses with depth in the sample; the anisotropic transition probability for polarized light; and time-dependent electronic population relaxation. After vibrational and solvent relaxation are complete, the calculation matches the measurement to within 10% error without any adjustable parameters. This demonstrates quantitative measurement of absolute excited state population.

Bulklike hot carrier dynamics in lead sulfide quantum dots.

Hot electronic dynamics in lead sulfide nanocrystals is interrogated by degenerate pump-probe spectroscopy with 20-25 fs pulses over a broad frequency range around three times the nanocrystal band gap. For each nanocrystal diameter, an initial reduction in absorption is seen only at the peak of the quantum confined E1 transition, while increased absorption is seen at all other wavelengths. The signals from the nanocrystals are approximately 300 times weaker than expected for a two-level system with the same absorbance and molar extinction coefficient and are weaker near time zero. These results appear to be inconsistent with quantum confinement of the initially excited high energy states. Arguments based on carrier scattering length, the wave packet size supported by the band structure, and effective mass are advanced to support the hypothesis that, for many direct-gap semiconductor quantum dots, the carrier dynamics at three times the band gap is localized on the 1-2 nm length scale and essentially bulklike except for frequent collisions with the surface.