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Four wave mixing (FWM) spectroscopy is used to measure a preresonant hyper-Raman hyperpolarizability. The four wave mixing process known as hyper-difference frequency generation (HDFG), which depends upon the hyper-Raman hyperpolarizability, can be measured simultaneously with other well-studied four-wave mixing processes, which serve as an internal standard. We use the coherent anti-Stokes Raman (CARS) response from deuterobenzene to quantify the acetonitrile ν(CN) hyper-Raman hyperpolarizability. This internal standard method is applicable to any infrared active vibration and greatly diminishes the barriers to quantification of hyper-Raman hyperpolarizabilities. 

Vibrational dephasing, relaxation, and spectra are useful probes of local environments in complex chemical systems. Here, we present the use of an infrared-hyper-Raman fourwave mixing spectroscopy, hyper difference frequency generation (HDFG) spectroscopy, as a method to resolve vibrational dephasing and spectra in the bulk. The coherent vibrations of dichloromethane and cyanocobalamin are resolved in both frequency and time. Tuning the two-photon interaction frequency towards an electronic resonance in cyanocobalamin enhances the vibrational signal, demonstrating electronic resonance enhancement due to vibronic coupling. This work motivates use of mixed-domain HDFG to measure coherent vibrational spectra in both frequency and time and to extract electron-vibration coupling information in molecular systems. 

Abstract This study reports a pulsed laser deposition-assisted synthesis of highly metallic titanium nitride (TiN) and a series of semiconducting titanium oxynitride (TiN_xO_y) compounds in thin film form with tunable plasmonic properties by carefully altering the nitrogen (N)-oxygen (O) ratio. The N/O ratio was controlled from 0.3 (highest oxygen doping of TiN) to ~ 1.0 (no oxygen doping of TiN) by growing the TiN films under nitrogen pressures of 50, 35, and 10 mTorr and high vacuum conditions of 2 × 10⁻⁶ Torr with no external gas introduced. The presence of nitrogen in the deposition chamber during the film growth affects the gas phase oxidation of TiN to TiN_xO_yby increasing the mean free path-dependent N and O inter-collisions per second by two to three orders of magnitudes. The evidence of increased oxidation of TiN to TiN_xO_ywith an increase in nitrogen deposition pressure was obtained using X-ray photoelectron spectroscopy analysis. While the TiN samples deposited in high vacuum conditions had the highest reflectance, TiN_xO_ythin films were also found to possess high reflectance at low frequency with a well-defined edge around 20,000 cm⁻¹. Furthermore, the vacuum-deposited TiN samples showed a large negative dielectric constant of -330 and the largest frequency of zero-crossing at 25,000 cm⁻¹; the TiN_xO_ysamples deposited in the presence of nitrogen ambient also showed promising plasmonic applications at the near-mid infrared range. A comparison of the dielectric constant and loss function data of this research with the literature values for noble metals seems to indicate that TiN and TiN_xO_yhave the potential to replace gold and silver in the visible and near-infrared spectral regions. 

Frequency-domain ultrafast coherent multidimensional spectroscopy has made possible a family of fully coherent spectroscopies that can create and interrogate characteristic superpositions of the quantum-mechanical states of a system under investigation. Typical applications include the resolution of couplings and dynamics among multiple electronic states in atoms, molecules, and materials. These methods require scanning the wavelengths of multiple, ultrafast light sources—often optical parametric amplifiers (OPAs). Spectral calibration of the OPA output (a.k.a. wavelength-tuning) involves optimizing the OPA output intensity by adjusting the angles of its component nonlinear crystals and motorized delay stages. When the spectral range addressed in the experiment is large, optimization and control of the one or more OPAs become complex. This work describes an automated calibration strategy that measures the multidimensional configuration-space of a typical 800-nm OPA over all angular and delay degrees-of-freedom in order to create a global tuning curve that spans its dynamic spectral range with optimal power and smooth interpolation. To accomplish this task, the optimization assesses the wavelength-dependent variations to the temporal and spatial characteristics of the OPA output caused by material dispersion so that compensations may be applied during a wavelength scan. 

Nonlinear, four-wave mixing vibrational spectroscopies are commonly used to probe electron–vibration coupling in isotropic media. Most of these methods rely on infrared and/or Raman transitions, but methods involving hyper-Raman transitions are also possible. Hyper difference frequency generation (HDFG) spectroscopy is an underdeveloped four-wave mixing vibrational spectroscopy based upon both infrared absorption and hyper-Raman scattering transitions. Despite several experimental reports on HDFG, its spectroscopic properties have not been fully explored. To this end, we investigate the selection rules and behavior of HDFG spectroscopy as an upconverted infrared spectroscopy and as a probe of vibronic coupling in molecular systems. We discuss the similarities between HDFG, a four-wave mixing technique, and vibrational sum frequency generation (vSFG) spectroscopy, a three-wave mixing technique. vSFG and HDFG appear to provide similar output intensities, making HDFG feasible for vSFG practitioners. HDFG is shown to be a sensitive probe of vibronic coupling in bulk systems and provides an alternative method to investigate electronic-nuclear coordinate correlations. 

Frequency domain nonlinear spectroscopies are a useful probe of linear and non-linear transitions in a variety of biological, chemical, and materials systems. They require scanning of optical parametric amplifiers (OPAs). Each OPA contains multiple motors that move to prerecorded positions to optimize output at each desired color. OPA optimization and color accuracy are crucial for frequency domain experiments, where OPA color is scanned. Such performance is highly sensitive to environmental fluctuations, so motor positions must be regularly optimized and tuned. Despite the widespread availability of motorized OPAs, this frequent maintenance can make frequency domain spectroscopy a cumbersome and time-consuming process. We have found that fully automated approaches to tuning are invaluable when scanning OPAs. Here, we report four algorithms that accurately and robustly tune a variety of ultrafast laser systems—picosecond and femtosecond, homebuilt and commercial OPAs. Using case studies from previously published work, we illustrate how these four algorithms can be combined to tune all motors of an ultrafast laser system. These algorithms are available through open-source software and can be applied to existing instruments, significantly lowering the threshold for executing frequency domain spectroscopy. 

Floquet state spectroscopy is an optical analogue of multiple quantum coherence nuclear magnetic resonance (MQC-NMR). Tunable ultrafast excitation pulses resonantly excite multiple states in a sample to form the Floquet state. The Floquet state emits multiple coherent beams at frequencies and in directions that conserve energy and momenta. The different output beams differ in the time ordering and coherences created by the excitation beams. They correspond to the different methodologies in the NMR family. Isolating a specific beam and monitoring the output intensity as a function of excitation frequencies creates multidimensional spectra containing cross-peaks between coupled states. The frequency range of the multidimensional spectra is limited by phase matching constraints. This paper presents a new, to the best of our knowledge, active phase matching strategy that increases the versatility of multidimensional Floquet state spectroscopy through both longer sample path lengths and larger spectral ranges.