Collinear Geometry Research Articles

Terahertz wave excitation from preexisting air plasma

Terahertz (THz) wave generation process in air plasma was studied using femtosecond laser pulse pairs. The effect of preexisting air plasma was investigated using two optical geometries: collinear and orthogonal pumping geometry. When the air plasma is excited in collinear geometry, the power of the generated THz wave shows asymmetric dependence on the relative delay of these pulses. The asymmetry depends on the relative intensities of these pump pulses, and it disappears when the pulses have equal intensities. In the orthogonal pumping geometry, the power of the THZ wave generated by the main pump pulse decreases in the presence of the preexisting plasma generated by the prepulse, and it recovers as the relative delay between the prepulse and main pulses becomes longer. Possible mechanisms of the results are discussed in terms of the dynamics of free electrons in the air plasma.

Two-Color Two-Dimensional Electronic Spectroscopy Using Dual Acousto-Optic Pulse Shapers for Complete Amplitude, Phase, and Polarization Control of Femtosecond Laser Pulses

We demonstrate a dual pulse-shaper setup capable of independent polarization, phase, and amplitude control over each pulse. By using active phase stabilization, we achieve a phase stability of ~λ/314 between the two pulse shapers, making the dual-shaper setup suitable for both two-quantum and one-quantum measurements. The setup is compact and easily switchable between pump-probe and collinear geometries. We further illustrate the functionality of the dual-shaper setup by performing two-color 2D visible spectroscopy on colloidal CdSe quantum dots in pump-probe geometry.

Emission signal enhancement and plasma diagnostics using collinear double pulse for laser-induced breakdown spectroscopy of aluminum alloys

A time and spatially resolved technique was used for the investigation of emission signal enhancement in collinear double-pulse LIBS. Two Q-switched Nd:YAG lasers at 1064 nm wavelength have been employed to produce laser-induced plasmas on aluminum-based alloys by single- (SP) and doublepulse (DP) LIBS. Time and spatial evolution of the plasma temperature and electron number density was investigated in these two experimental schemes. The enhancement of the emission line intensities was investigated, and a relation between the increases in intensity and excitation energy level was established. The results shows that the production of the second plasma by second laser pulse was important in DP experiment in collinear geometry.

Accurate Study on the Quantum Dynamics of the He + HeH+ (X1Σ+) Reaction on A New ab Initio Potential Energy Surface for the Lowest 11A′ Electronic Singlet State

A time-dependent quantum wave packet method is used to investigate the dynamics of the He + HeH(+)(X(1)Σ(+)) reaction based on a new potential energy surface [Liang et al., J. Chem. Phys.2012, 136, 094307]. The coupled channel (CC) and centrifugal-sudden (CS) reaction probabilities as well as the total integral cross sections are calculated. A comparison of the results with and without Coriolis coupling revealed that the number of K states N(K) (K is the projection of the total angular momentum J on the body-fixed z axis) significantly influences the reaction threshold. The effective potential energy profiles of each N(K) for the He + HeH(+) reaction in a collinear geometry indicate that the barrier height gradually decreased with increased N(K). The calculated time evolution of CC and CS probability density distribution over the collision energy of 0.27-0.36 eV at total angular momentum J = 50 clearly suggests a lower reaction threshold of CC probabilities. The CC cross sections are larger than the CS results within the entire energy range, demonstrating that the Coriolis coupling effect can effectively promote the He + HeH(+) reaction.

Ultrafast two-dimensional terahertz spectroscopy of elementary excitations in solids

Recent experimental progress has allowed for the implementation of nonlinear two-dimensional (2D) terahertz (THz) spectroscopy in the ultrafast time domain. We discuss the principles of this technique based on multiple phase-locked electric field transients interacting in a collinear geometry with a solid and the phase-resolved detection of the THz fields after interaction with the sample. To illustrate the potential of this new method, 2D correlation spectra of coupled intersubband-longitudinal optical phonon excitations in a double quantum well system and a study of ultrafast carrier dynamics in graphene are presented.

Two-dimensional spectroscopy using dual acousto-optic pulse shapers for complete polarization, phase and amplitude control

We demonstrate a pulse-shaper capable of independent polarization, phase and amplitude control over each pulse. The set-up is compact and easily switchable between pump-probe and collinear geometries.

Ab Initio studies of the interaction potential for the Xe–NO(X 2Π) van der Waals complex: Bound states and fully quantum and quasi-classical scattering

Adiabatic potential energy surfaces for the ground electronic state of the Xe⋅⋅⋅NO(X(2)Π) van der Waals complex have been calculated using the spin-restricted coupled cluster method with single, double, and non-iterative triple excitations (RCCSD(T)). The scalar relativistic effects present in the Xe atom were included by an effective core potential and we extended the basis with bond functions to improve the description of the dispersion interaction. It has been found that the global minimum on the A(') adiabatic surface occurs at a T-shaped geometry with γ(e) = 94° and R(e) = 7.46 a(0), and with well depth of D(e) = 148.68 cm(-1). There is also an additional local minimum for the collinear geometry Xe-NO with a well depth of 104.5 cm(-1). The adiabat of A('') symmetry exhibits a single minimum at a distance R(e) = 7.68 a(0) and has a skewed geometry with γ(e) = 64° and a well depth of 148.23 cm(-1). Several C(nl) van der Waals dispersion coefficients are also estimated, of which C(6, 0) and C(6, 2) are in a reasonable agreement with previous theoretical results obtained by Nielson et al. [J. Chem. Phys. 64, 2055 (1976)]. The new potential energy surfaces were used to calculate bound states of the complex for total angular momentum quantum numbers up to J = 7/2. The ground state energy of Xe⋅⋅⋅NO(X(2)Π) is D(0) = 117 cm(-1), which matches the experimental value very accurately (within 3.3%). Scattering calculations of integral and differential cross sections have also been performed using fully quantum close coupling calculations and quasi-classical trajectory method at a collision energy of 63 meV. These calculations reveal the important role played by L-type rainbows in the scattering dynamics of the heavier Rg-NO(X) systems.

Depolarization of rotational angular momentum in CN(A2Π, v = 4) + Ar collisions

Angular momentum depolarization and population transfer in CN(A(2)Π, v = 4, j, F(1)e) + Ar collisions have been investigated both experimentally and theoretically. Ground-state CN(X(2)Σ(+)) molecules were generated by pulsed 266-nm laser photolysis of ICN in a thermal (nominally 298 K) bath of the Ar collision partner at a range of pressures. The translationally thermalized CN(X) radicals were optically pumped to selected unique CN(A(2)Π, v = 4, j = 2.5, 3.5, 6.5, 11.5, 13.5, and 18.5, F(1)e) levels on the A-X (4,0) band by a pulsed tunable dye laser. The prepared level was monitored in a collinear geometry by cw frequency-modulated (FM) spectroscopy in stimulated emission on the CN(A-X) (4,2) band. The FM lineshapes for co- and counter-rotating circular pump and probe polarizations were analyzed to extract the time dependence of the population and (to a good approximation) orientation (tensor rank K = 1 polarization). The corresponding parallel and perpendicular linear polarizations yielded population and alignment (K = 2). The combined population and polarization measurements at each Ar pressure were fitted to a 3-level kinetic model, the minimum complexity necessary to reproduce the qualitative features of the data. Rate constants were extracted for the total loss of population and of elastic depolarization of ranks K = 1 and 2. Elastic depolarization is concluded to be a relatively minor process in this system. Complementary full quantum scattering (QS) calculations were carried out on the best previous and a new set of ab initio potential energy surfaces for CN(A)-Ar. Collision-energy-dependent elastic tensor and depolarization cross sections for ranks K = 1 and 2 were computed for CN(A(2)Π, v = 4, j = 1.5-10.5, F(1)e) rotational/fine-structure levels. In addition, integral cross sections for rotationally inelastic transitions out of these levels were computed and summed to yield total population transfer cross sections. These quantities were integrated over a thermal collision-energy distribution to yield the corresponding rate constants. A complete master-equation simulation using the QS results for the selected initial level j = 6.5 gave close, but not perfect, agreement with the near-exponential experimental population decays, and successfully reproduced the observed multimodal character of the polarization decays. On average, the QS population removal rate constants were consistently 10%-15% higher than those derived from the 3-level fit to the experimental data. The QS and experimental depolarization rate constants agree within the experimental uncertainties at low j, but the QS predictions decline more rapidly with j than the observations. In addition to providing a sensitive test of the achievable level of agreement between state-of-the art experiment and theory, these results highlight the importance of multiple collisions in contributing to phenomenological depolarization using any method sensitive to both polarized and unpolarized molecules in the observed level.

Consistent calculation of diabatic energies and diabatic coupling terms in the van der Waals region: Application to the [formula omitted] system

In this work we carry out calculations of the long-range interactions between a ground state O atom and a hydrogen molecule at Cs geometries in order to describe the diabatic and adiabatic surfaces that connect the 3Π and 3Σ states at collinear geometries with the 3B1, 3B2 and 3A2 states at C2v configurations. We show that the usual procedure to correct the basis set superposition error gives inconsistent results for the diabatic potentials and their coupling terms. Consistent results can only be obtained using high quality basis sets for which this error can be negligible.

Theoretical Analysis of the Difference-Frequency Generation of Terahertz Radiation in Lithium Niobate with Regular Domain Structure

The difference-frequency generation of terahertz (THz) radiation in single crystalline lithium niobate with periodic domain structure has been studied theoretically. The quadratic nonlinear dielectric with lithium niobate parameters has been considered in collinear geometry of wave interaction and absence of absorption for several types of the regular domain structures. The features of the generated THz and their dependence on the domain structure have been revealed.

Diagnostic study of four-wave-mixing-based electric-field measurements in high-pressure nitrogen plasmas

We present the results of a diagnostic study of the use of coherent four wave mixing for in situ measurement of an electric field in air or in nitrogen-containing plasmas. Static electric fields in air at a nominal pressure of 625 Torr and temperature of 300 K are detected using vibrational CARS of nitrogen. It is shown that the ratio of the infrared signal to the vibrational N(2) CARS signal is equal to approximately 10(-8) at 8.33 kV/cm, a factor of approximately 50 less than that predicted assuming equal third-order nonlinear susceptibilities. It is also shown that the spatial resolution of a typical collinear geometry measurement is approximately 1 cm. Finally, it is shown that achieving sensitivities of the order of 1 kV/cm requires that the coherent Raman pumping be performed in the highly saturated and Stark broadened regime.

Efficiency and temporal response of crystalline Kerr media in collinear optical Kerr gating

Optical Kerr gating is widely used in ultrafast measurements ranging from pulse characterization to spectroscopy and microscopy. We examined the efficiency and the temporal response of three cubic lattice Kerr media, YAG, GGG and BGO, and compared them with the well studied fused silica (fast response, low efficiency) and STO (high efficiency, slow response). YAG and GGG emerged as superior materials for ultrafast spectroscopy and microscopy applications thanks to their fast Kerr response and considerably higher gating efficiency than silica at low gating energies. Importantly, it was found that in collinear geometry all tested materials except STO are capable of reaching nearly 100% transmission.

Methyl groups at dielectric and metal surfaces studied by sum-frequency generation in co- and counter-propagating configurations

The optimum experimental geometry for visible-infrared sum-frequency generation experiments depends rather sensitively on the molecules adsorbed at the surface, their orientation, and the nature of the adjacent bulk media. We consider the commonly encountered case of methyl groups situated at air-water, air-gold, and polymer-water interfaces. We provide expressions that may be used to determine the optimal visible and IR beam incident angles, considering the symmetric and antisymmetric modes separately and then together. The analysis is carried out for co-propagating (collinear and non-collinear geometries) and counter-propagating configurations. We first consider that one or more vibrational modes are of interest, and the goal is to study them quantitatively under a single polarization scheme; our results enable the user to set the beam angles for such an experiment. In the second case, molecular orientation information is desired, and so the calibrated response is required in all accessible polarization schemes for full characterization of the nonlinear susceptibility tensor.

Ion optical design of a collinear laser-negative ion beam apparatus

An apparatus for photodetachment studies on atomic and molecular negative ions of medium up to heavy mass (M ≃ 500) has been designed and constructed. Laser and ion beams are merged in the apparatus in a collinear geometry and atoms, neutral molecules and negative ions are detected in the forward direction. The ion optical design and the components used to optimize the mass resolution and the transmission through the extended field-free interaction region are described. A 90° sector field magnet with 50 cm bending radius in combination with two slits is used for mass dispersion providing a resolution of M∕ΔM≅800 for molecular ions and M∕ΔM≅400 for atomic ions. The difference in mass resolution for atomic and molecular ions is attributed to different energy distributions of the sputtered ions. With 1 mm slits, transmission from the source through the interaction region to the final ion detector was determined to be about 0.14%.

Sum frequency generation in chiral carbon nanotubes

The process of Sum Frequency Generation (SFG) in chiral Carbon Nanotubes (CNTs), with the exciting laser beams propagating along the CNT symmetry axis, is here investigated. First the general form of the first hyperpolarizability third rank tensor β for chiral CNTs is determined with group projecto r technique method applied to the Lnp22 line group describing their symmetry. For this purpose, the group natural factorization has been adopted and the associated irreducible representations, parameterized in terms of helical quantum numbers, then used to construct the group projector operator in the relevant vector space. The method demonstrates that second harmonic generation is symmetry forbidden in the collinear geometry while, conversely, SFG is symmetry allowed. Subsequently, the SFG far-field radiation pattern has been modelled with a calculation scheme derived from antenna's theory and already used for simulation of Rayleigh scattering from a CNT. The method has been overhauled for describing the here considered nonlinear optical interaction. The study of the role played by two induced counter-propagating current density retarded waves in shaping the radiation pattern is addressed and simulations are reported in the limit regimes of strong and weak attenuation.

Intersubband polaritons in a one-dimensional surface plasmon photonic crystal

In this work we demonstrate the achievement of light-matter strong coupling regime between an intersubband transition and the photonic bandlike mode supported by a metallic grating. Polaritonic resonances have been identified in the reflectivity spectra at 10° incidence, with a clear anticrossing appearing when the photonic mode is tuned across the transition line. Experimental results are in good agreement with the simulations performed with a scattering-matrix approach. This cavity design can be further optimized and is likely to open the way to a new class of time-resolved measurements, as it allows pump-and-probe measurements to be performed in collinear geometries.

A Theoretical Investigation of the Photophysical Consequences of Major Plant Light-Harvesting Complex Aggregation within the Photosynthetic Membrane

Spectroscopic measurements of Arabidopsis leaves have shown that the energy-dependent component of non-photochemical quenching (NPQ), known as qE, is associated with an absorption change at 535 nm (ΔA(535)). Identical measurements on the zeaxanthin-deficient mutant npq1 reveal a similar spectroscopic signature at 525 nm (ΔA(525)). We investigated whether these red-shifts may arise from excitonic interactions among homodimers of xanthophylls, zeaxanthin, and violaxanthin, bound at the peripheral V1 binding site on adjacent light-harvesting complex II (LHCII) trimers. Estimates of the relative geometries of these pigment pairs were obtained from the structure of LHCII. The excitonic couplings of zeaxanthin and violaxanthin dimers were probed using the time-dependent density functional theory method (TD-DFT). Calculations indicated that dimers formed between zeaxanthin or violaxanthin molecules using the published LHCII structure resulted in absorption blue shifts, typical of an H-type (parallel) geometry. In contrast, if the volume of the LHCII structure was modified to reflect the change in membrane thickness that occurs upon ΔpH formation, then both zeaxanthin and violaxanthin dimers adopted a J-type (collinear) geometry, and the resulting spectral shift was to the red region. The magnitudes of these predicted red-shifts are in good agreement with the experimental magnitudes. We therefore conclude that the observed xanthophyll red-shift results from the combination of both LHCII aggregation and changes in membrane thickness during qE. ΔA(535) may therefore be considered a "marker of aggregation" between LHCII trimers upon qE formation.

Generation of 25 μJ vacuum ultraviolet pulses with sub-50 fs duration by noncollinear four-wave mixing in argon

Generation of sub-50fs vacuum UV pulses with more than 2.5μJ energy at a 1kHz repetition rate is reported. The pulses at 160nm are produced using noncollinear difference-frequency four-wave mixing between the fundamental and third harmonics of an amplified Ti:sapphire laser in argon. While the pulse duration is maintained by increasing the phase-matching pressure, noncollinear interaction improves the conversion efficiency by 1 order of magnitude in comparison with the previous results in collinear geometry.

The electron affinity of tungsten

The electron affinity of tungsten has been measured using laser photodetachment threshold spectroscopy in a collinear geometry. The electron affinity was determined to 6583.6(6) cm-1 by observing the onset of the process when W- ions in the $5d^56s^2$ 6S5/2 ground state are photodetached producing neutral W atoms in the $5d^46s^2$ 5D0 ground state. The measured value is in agreement with previous measurements and improves the accuracy by almost two orders of magnitude. Further, a photodetachment signal below the ground state photodetachment threshold was found, which indicates the existence of a bound excited state in W-.

Source for ultrafast continuum infrared and terahertz radiation

A compact and stable method for generating high-intensity linearly polarized continuum mid-IR and terahertz light using ultrafast femtosecond (fs) laser pulses is demonstrated. Continuous light generation from <400 cm(-1) (12 THz, 25 microm) to >3300 cm(-1) (100 THz, 3 microm) in a sub-100 fs laser pulse is facilitated by nonlinear mixing of the fundamental, second harmonic, and third harmonic of an ultrafast amplified laser source through filamentation in air. Including the third harmonic in the mixing scheme leads to a tenfold increase in the generated IR power. The compact optical configuration utilizing a delay plate in a collinear geometry serves to simplify alignment and increase stability, making it a practical source for transient IR spectroscopy.