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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. 

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. 

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. 

Vibrational fingerprints and combination bands are a direct measure of couplings that control molecular properties. However, most combination bands possess small transition dipoles. Here we use multiple, ultrafast coherent infrared pulses to resolve vibrational coupling between CH3CN fingerprint modes at 918 and 1039 cm(-1) and combination bands in the 2750-6100 cm(-1 )region via doubly vibrationally enhanced (DOVE) coherent multidimensional spectroscopy (CMDS). This approach provides a direct probe of vibrational coupling between fingerprint modes and near-infrared combination bands of large and small transition dipoles in a molecular system over a large frequency range. 

The Paternò–Büchi reaction is the [2+2] photocycloaddition of a carbonyl with an alkene to afford oxetane products. Enantioselective catalysis of this classical photoreaction, however, has proven to be a long-standing challenge. Many of the best-developed strategies for asymmetric photochemistry are not suitable to address this problem because the interaction of carbonyls with Brønsted or Lewis acidic catalysts can alter the electronic structure of their excited state and divert their reactivity towards alternate photoproducts. We show herein that an alternative triplet rebound strategy enables the stereocontrolled reaction of an excited-state carbonyl compound in its native, unbound state. These studies have resulted in the development of the first highly enantioselective catalytic Paternò–Büchi reaction, cata-lyzed by a novel hydrogen-bonding chiral Ir photocatalyst. 

The Paternò−Büchi reaction is the [2 + 2] photocycloaddition of a carbonyl with an alkene to afford an oxetane. Enantioselective catalysis of this classical photoreaction, however, has proven to be a long-standing challenge. Many of the best-developed strategies for asymmetric photochemistry are not suitable to address this problem because the interaction of carbonyls with Brønsted or Lewis acidic catalysts can alter the electronic structure of their excited state and divert their reactivity toward alternate hotoproducts. We show herein that a triplet rebound strategy enables the stereocontrolled reaction of an excited-state carbonyl compound in its native, unbound state. These studies have resulted in the development of the first highly enantioselective catalytic Paternò−Büchi reaction, catalyzed by a novel hydrogen-bonding chiral Ir photocatalyst.