Collinear Geometry Research Articles

Effect of pre-plasma on terahertz radiation from two-color laser plasma filaments in a collinear geometry.

Terahertz (THz) radiation from air plasma in the presence of pre-plasma in a collinear geometry is investigated experimentally, where the pre-plasma is formed by a pre-pulse with a Gaussian beam profile and the measured THz radiation is driven by a main laser pulse. The pre-plasma has a de-focusing effect for the main pulse passing through it, which reduces the effective length of the plasma filament formed by the main laser pulse for THz radiation. It is found that only the part not overlapped by the pre-plasma can actually produce THz radiation. Thus, the amplitude of the THz pulse driven by the main pulse can be modified by changing the spatial separation between two plasma filaments. The experimental observations are qualitatively in agreement with our numerical simulation results. It is also found that the change of the time delay between the pre-pulse and the main pulse does not change the THz radiation amplitude for a given spatial separation. This study suggests a practical way for the manipulation of THz waves through an interaction between laser plasma filaments.

Efficient strong-field THz generation from DSTMS crystal pumped by 1030 nm Yb-laser

We report collinear strong-field terahertz (THz) generation from the organic crystal 4-N, N-dimethlamino-4-N-methyl-stilbazolium 2,4,6-trimethyl benzenesulfonate (DSTMS), driven by an industrial-grade ytterbium (Yb) laser operating at 1030 nm. The laser pulse duration is compressed from 800 to 90 fs by using a multi-pass-cell compressor. The output THz energy reaches 0.4 μJ, with a conversion efficiency of 0.22%. The spectrum covers over 0.1–6 THz with a peak electric field of 236 kV/cm. The phase matching condition and material absorption are illustrated. We demonstrate that a 1030 nm Yb-laser is used to pump organic crystal DSTMS for efficient THz generation in a simple collinear geometry, which provides a promising THz source based on an industrial-grade ultrafast laser.

Extracting double-quantum coherence in two-dimensional electronic spectroscopy under pump-probe geometry.

Two-dimensional electronic spectroscopy (2DES) can be implemented with different geometries, e.g., BOXCARS, collinear, and pump-probe geometries. The pump-probe geometry has the advantage of overlapping only two beams and reducing phase cycling steps. However, its applications are typically limited to observing the dynamics with single-quantum coherence and population, leaving the challenge to measure the dynamics of the double-quantum (2Q) coherence, which reflects the many-body interactions. We demonstrate an experimental technique in 2DES under pump-probe geometry with a designed pulse sequence and the signal processing method to extract 2Q coherence. In the designed pulse sequence, with the probe pulse arriving earlier than the pump pulses, our measured signal includes the 2Q signal as well as the zero-quantum signal. With phase cycling and data processing using causality enforcement, we extract the 2Q signal. The proposal is demonstrated with rubidium atoms. We observe the collective resonances of two-body dipole-dipole interactions in both the D1 and D2 lines.

Half-broadband two-dimensional electronic spectroscopy with active noise reduction

Two-dimensional electronic spectroscopy (2DES) provides detailed insight into coherent ultrafast molecular dynamics in the condensed phase. Here we report a referenced broadband pump-compressed continuum probe half-broadband (HB) 2DES spectrometer in a partially collinear geometry. To optimize signal-to-noise ratio (SNR) we implement active noise reduction referencing, which has not previously been applied in 2DES. The method is calibrated against the well characterized 2DES response of the oxazine dye cresyl violet and demonstrated at visible wavelengths on the photochromic photoswitch 1,2-Bis(2-methyl-5-phenyl-3-thienyl) perfluorocyclopentene (DAE). The SNR is improved by a factor of ∼2 through active referencing. This is illustrated in an application to resolve a low frequency mode in the excited electronic state of DAE, yielding new data on the reaction coordinate. We show that the active noise reduction referencing, coupled with the rapid data collection, allows the extraction of weak vibronic features, most notably a low frequency mode in the excited electronic state of DAE.

Diagnosing the waveform of an ultrashort optical pulse by collinear interferometry

During the high-harmonic generation (HHG) process, information about field interaction with the medium is imprinted in the extreme ultraviolet (XUV) radiation. Here, we present a method for using HHG to diagnose the electric field of an optical pulse under a collinear geometry. When mixing a weak signal pulse with a strong driving pulse collinearly, the far field divergence of the XUV HHG is sensitive to the relative delay between the two pulses, which can be used as an ultrafast subcycle gate to reconstruct the electric field of the signal pulse. This collinear configuration is efficient, easy to operate and avoids artificial high order frequency components in reconstruction as compared to non-collinear schemes.

Methods for Calculating the Shape of the Spectral Transmission Window of an Acousto-Optic Filter

The problem of calculation of acousto-optic filter characteristics with control of the number of volumetric diffraction gratings and their structure is taken into consideration. Special-case calculation methods are described including those in collinear geometry. It is demonstrated that there is an approach allowing to perform detailed calculation of spectral characteristics in wide-aperture diffraction geometry used for problems of spectral analysis of object images. It allows to divide the entire luminous flux into separate beams with the partial transmission function of each of them being described by a simple path integral. This method allows for determining the impact of acoustic wave modulation on the filter spectral transmission function.

Collinear opto-optical loss modulation for carrier-envelope offset stabilization of a fiber frequency comb.

Opto-optical loss modulation (OOM) for stabilization of the carrier-envelope offset (CEO) frequency of a femtosecond all-fiber laser is performed using a collinear geometry. Amplitude-modulated 1064 nm light is fiber coupled into an end-pumped semiconductor saturable absorber mirror (SESAM)-mode-locked all-polarization-maintaining erbium fiber femtosecond laser, where it optically modulates the loss of the SESAM resulting in modulation of the CEO frequency. A noise rejection bandwidth of 150 kHz is achieved when OOM and optical gain modulation are combined in a hybrid analog/digital loop. Collinear OOM provides a simple, all-fiber, high-bandwidth method for improving the CEO frequency stability of SESAM mode-locked fiber lasers.

Two bridge-particle-mediated RET between chiral molecules.

The problem of resonance energy transfer between a pair of chiral molecules mediated by two electrically polarizable bridging particles is solved using molecular quantum electrodynamics theory. In this framework, a single virtual photon propagates between any two-coupled entities and is responsible for the conveyance of excitation energy from the emitter to the absorber. Electric dipole and quadrupole, and magnetic dipole couplings linear in the Maxwell fields are employed for donor and acceptor, while each mediator scatters two virtual photons and responds quadratically to the electric displacement field via its electric dipole polarizability. This enables fourth-order diagrammatic perturbation theory to be used to compute the probability amplitude for the process. Individual multipole moment contributions to the Fermi golden rule rate are then extracted for oriented and isotropic systems. Discriminatory transfer rates arise when either the donor or the acceptor are electric-magnetic dipole and the other has a pure multipole moment, or when both are chiral, with mixed electric dipole-quadrupole contributions vanishing in the fluid phase. The bridge-mediated transfer rate is found to be a maximum for a collinear geometry. Moreover, a multi-level model of the mediator is necessary for energy migration. Asymptotically limiting rates for arbitrary and collinear geometries are also obtained for one center purely electric dipolar and the other purely quadrupolar, or both donor and absorber purely quadrupolar. Understanding is gained of radiationless and radiative transfer mechanisms between chiral moieties in a dielectric medium.

Identifiability Analysis for Configuration Calibration in Distributed Sensor Networks

In this work, the parameter identifiability of sensor position perturbations in a distributed network is analyzed through establishing the link between rank of the Jacobian matrix and parameter identifiability under Gaussian noise. Here, the calibration is classified as either external or internal, dependent on whether auxiliary sources are exploited. It states that, in the case of internal calibration, sensor position perturbations can be precisely calibrated when the position of a sensor and orientation to a second sensor along with the coordinate of a third sensor along some axis, are known. In the case of external calibration where auxiliary sources are introduced to support the process, the identifiability condition for configuration calibration is to have at least three noncollinear auxiliary sources with the distributed sensor network avoiding the collinear and coplanar geometries. As the assumption of small perturbations is considered, the parameter identifiability is capable of being measured by virtue of the Bayesian Cramer–Rao lower bound (BCRLB), after asymptotical tightness of the BCRLB is verified. Simulations corroborate well with the theoretical development.

Super-resonant four-photon collinear laser frequency multiplication in plasma.

Resonant four-photon scattering could nearly double frequencies of intense laser pulses in plasma. However, transverse slippage between pulses presents a technological challenge, while collinear four-photon scattering is forbidden for classical light dispersion in plasma. Nonlinear renormalization of intense laser pulses can enable collinear four-photon resonance. However, such a very intensity-sensitive resonance is difficult to maintain for evolving pulses. Remarkably, there is a lower-dimensionality submanifold of the resonant four-photon manifold where the evolving pulses stay in resonance. This could enable an all-optical frequency doubling of mildly relativistic-intense laser pulses in collinear geometry, advantageously free of the transverse slippage challenges.

Examining measuring the degree of linear photon-beam polarization in the energy range up to 100 MeV with the use of He4(γ⃗,p)H3 and He4(γ⃗,n)He3 reactions

The paper gives physical justification of a new method for measuring the degree of linear photon-beam polarization, which relies on the experimental data for the total cross section of spin $S$ = 1 transitions in the $^{4}\mathrm{He}(\ensuremath{\gamma},p)^{3}\mathrm{H}$ and $^{4}\mathrm{He}(\ensuremath{\gamma},n)^{3}\mathrm{He}$ reactions. The experimental information on the cross section was obtained from both the mentioned reactions and the reactions of radiative capture of protons by tritium nuclei. The total cross section for transitions with spin $S$ = 1 is $\ensuremath{\approx}1%$ of the total cross section for the reaction. The ratio of the differential cross section in the collinear geometry, specified by the $S$ = 1 transitions, to the differential reaction cross section at the nucleon emission angle ${\ensuremath{\theta}}_{N}={90}^{\ensuremath{\circ}}$ in the energy range 20 $\ensuremath{\le}{E}_{\ensuremath{\gamma}}\ensuremath{\le}$ 100 MeV is independent of the photon energy within the experimental error. The total cross section for the $S$ = 1 transitions can be also determined from the experimental data on the cross-section asymmetry of the reaction with linearly polarized photons $\mathrm{\ensuremath{\Sigma}}({\ensuremath{\theta}}_{N})$. However, the total cross section for transitions with spin $S$ = 1, calculated from the asymmetry of the cross section $\mathrm{\ensuremath{\Sigma}}({\ensuremath{\theta}}_{N})$, is several times greater than that calculated from the differential cross section. This inconsistency of the experimental data may be due, in particular, to the overestimate of the degree of linear photon-beam polarization. The above-mentioned reactions seem to be more convenient for measuring the degree of linear photon-beam polarization than the deuteron photodisintegration reactions.

Millijoule 265 fs Tm:YAP regenerative amplifier for driving ultrabroad band collinear mid-infrared optical parametric amplifiers.

Generation of 265-fs millijoule pulses at 1940 nm from a solid-state regenerative amplifier has been demonstrated. The amplification chain consists of a thulium-doped fluoride (Tm:ZBLAN) fiber oscillator, a two stage Tm:ZBLAN fiber preamplifier, and a regenerative amplifier with a thermoelectrically cooled thulium-doped yttrium aluminium perovskite crystal. The newly developed light source is used for pumping an ultra broadband mid-infrared optical parametric amplifier based on a gallium selenide crystal. The 2.5-4 µm range of a multioctave supercontinuum, generated in a polarization-maintaining ZBALN fiber, is used as the MIR seed. The amplified signal in combination with the corresponding idler pulses spread from 2.5 to 10µm in a collinear geometry.

Spectral Phase Measurements of Heterodyne Detection in Interfacial Broadband Electronic Spectroscopy

A spectral phase is vital to retrieve both the real and imaginary parts of both second-order vibrational and electronic sum frequency generation spectra. Despite recent efforts, phase measurements in electronic spectra at interfaces are lacking. In particular, references are not well established for frequency-dependent spectral phases in interfacial electronic spectroscopy. In this work, we present broadband heterodyne detection (HD) of interfacial electronic spectroscopy for spectral phases of references. Such a broadband HD method was based upon a broadband short-wave IR laser source from 1100 to 2200 nm. A collinear geometry method was adopted for passive phase stability. We further compared often-used reference samples, including gold (Au) thin films, silver (Ag) thin films, p-type GaP (100), p-type GaAs (100), as well as left-handed and right-handed z-cut α-quartz crystals. The phases of the second-order susceptibilities for Ag thin films, the GaP crystal, the GaAs crystal, and the quartz crystals are independent of frequency from 630 to 730 nm. More importantly, it was found that the often-used Au thin film in the literature is not a good phase reference for frequency-dependent phases.

Controlling the transverse correlation in quasi-phase matching parametric down-conversion

In this work we study the transverse spatial correlation of the pair of photons generated via the process of spontaneous parametric frequency down-conversion, in periodically poled non-linear crystals illuminated by a pulsed laser beam. It is well known that the two-photon state generated in quasi-phase matching (QPM) configurations depends explicitly on the characteristics of the pump beam, on the crystal modulated non-linearity, and on the detection geometry. This has allowed the development of several techniques for controlling the biphoton spectral and spatial properties. Here we discuss another technique for implementing the spatial entanglement modification in QPM gratings. We show, theoretically and experimentally, that in nearly collinear geometries, the spatial shape modulation of the pump beam allows for the control of the biphoton transverse spatial correlation.

Quantum Dynamics of Nonadiabatic Renner-Teller Effects in Atom + Diatom Collisions.

We review the quantum nonadiabatic dynamics of atom + diatom collisions due to the Renner-Teller (RT) effect, i.e., to the Hamiltonian operators that contain the total spinless electronic angular momentum L̂. As is well-known, this rovibronic effect is large near collinear geometries when at least one of the interacting states is doubly degenerate. In general, this occurs in insertion reactions and at short-range, where the potential wells exhibit deep minima and support metastable complexes. Initial-state-resolved reaction probabilities, integral cross sections, and thermal rate constants are calculated via the real wavepacket method, solving the equation of motion with an approximated or with an exact spinless RT Hamiltonian. We present the dynamics of 10 single-channel or multichannel reactions showing how RT effects depend on the product channels and comparing with the Born-Oppenheimer (BO) approximation or coexisting conical-intersection (CI) interactions. RT effects not only can significantly modify the adiabatic dynamics or correct purely CI results, but also they can be very important in opening collision channels which are closed at the BO or CI level, as in electronic-quenching reactions. In the OH(A2Σ+) + Kr electronic quenching, where both nonadiabatic effects (CI and RT) coexist, they are in competition because CI dominates the reactivity but RT couplings reduce the large CI cross section and open a CI-forbidden evolution toward products, so that CI + RT quantum results are in good agreement with experimental or semiclassical findings. The different roles of these couplings are due to the unlike nuclear geometries where they are large: rather far from or near to linearity for CI or RT, respectively. The OH(A2Σ+) + Kr electronic quenching was investigated with the exact RT Hamiltonian, validating the approximated one, which was employed for all other collisions.

Discriminatory Resonance Energy Transfer Mediated by a Third Molecule.

Relay of resonant excitation energy between two chiral molecules by an inert third particle is studied using molecular quantum electrodynamics theory. A single virtual photon propagates between each interacting pair. Fourth-order diagrammatic time-dependent perturbation theory is employed to compute the matrix element. Rate terms dependent upon the chirality of the donor and acceptor species are extracted using the Fermi golden rule. Interestingly, the mediated rate is discriminatory. For freely tumbling particles it exhibits an inverse-square dependence on each interparticle separation distance, indicating a purely radiative exchange mechanism. Furthermore, the isotropic rate is found to be a maximum for a collinear geometry and vanishes when the angle between the donor, mediator, and acceptor is 90°. The indirect rate is compared with direct transfer between two chiral molecules. Insight is gained into discriminatory migration of energy in a dielectric medium.

Effects of conical intersections on hyperfine quenching of hydroxyl OH in collision with an ultracold Sr atom

The effect of conical intersections (CIs) on electronic relaxation, transitions from excited states to ground states, is well studied, but their influence on hyperfine quenching in a reactant molecule is not known. Here, we report on ultracold collision dynamics of the hydroxyl free-radical OH with Sr atoms leading to quenching of OH hyperfine states. Our quantum-mechanical calculations of this process reveal that quenching is efficient due to anomalous molecular dynamics in the vicinity of the conical intersection at collinear geometry. We observe wide scattering resonance features in both elastic and inelastic rate coefficients at collision energies below k_{text {B}}times 10 , hbox {mK}. They are identified as either p- or d-wave shape resonances. We also describe the electronic potentials relevant for these non-reactive collisions, their diabatization procedure, as well as the non-adiabatic coupling between the diabatic potentials near the CIs.

Laser spectroscopy of indium Rydberg atom bunches by electric field ionization

This work reports on the application of a novel electric field-ionization setup for high-resolution laser spectroscopy measurements on bunched fast atomic beams in a collinear geometry. In combination with multi-step resonant excitation to Rydberg states using pulsed lasers, the field ionization technique demonstrates increased sensitivity for isotope separation and measurement of atomic parameters over previous non-resonant laser ionization methods. The setup was tested at the Collinear Resonance Ionization Spectroscopy experiment at ISOLDE-CERN to perform high-resolution measurements of transitions in the indium atom from the text {5s}^2text {5d},^2text {D}_{5/2} and text {5s}^2text {5d},^2text {D}_{3/2} states to text {5s}^2np ^2P and text {5s}^2ntext {f},^2F Rydberg states, up to a principal quantum number of n=72. The extracted Rydberg level energies were used to re-evaluate the ionization potential of the indium atom to be 46,670.107(4),hbox {cm}^{-1}. The nuclear magnetic dipole and nuclear electric quadrupole hyperfine structure constants and level isotope shifts of the text {5s}^2text {5d},^2text {D}_{5/2} and text {5s}^2text {5d},^2text {D}_{3/2} states were determined for ^{113,115}In. The results are compared to calculations using relativistic coupled-cluster theory. A good agreement is found with the ionization potential and isotope shifts, while disagreement of hyperfine structure constants indicates an increased importance of electron correlations in these excited atomic states. With the aim of further increasing the detection sensitivity for measurements on exotic isotopes, a systematic study of the field-ionization arrangement implemented in the work was performed at the same time and an improved design was simulated and is presented. The improved design offers increased background suppression independent of the distance from field ionization to ion detection.

Evaluation of analytical performance of double pulse laser-induced breakdown spectroscopy for the detection of rare earth elements

Double pulse laser-induced breakdown spectroscopy (DP-LIBS) in collinear configuration was demonstrated to enhance the signal intensity of three rare earth elements (REEs), Eu, Gd, Pr, and Y (Yttrium is included because of its similar chemical properties and occurrence in similar deposits as REE). Significant enhancement in spectral signals of all elements was observed with an aid of second laser. Two laser pulses of 1064 nm wavelength were employed in collinear geometry on the Pr2O3, Eu2O3, Gd2O3, and Y2O3 pellets to obtain emission spectra. For selected strong emission lines of Eu and Gd signal enhancement was found to be 3–7-fold and that for Y and Pr lines was observed to be 3–13-fold compared to single-pulse laser-induced breakdown spectroscopy (SP-LIBS). Role of inter-pulse delay between two pulses in DP-LIBS was found to be very crucial. While comparing selected ionic and neutral lines, ionic lines were found to have larger signal enhancement in DP-LIBS at the experimental conditions used in this study.

Estimating the Parameters of Monocrystal-Based Acousto-Optic Devices for Terahertz Radiation Control

The acoustic and optical properties of a number of monocrystals transparent in the terahertz range are systematized. Acousto-optic (AO) figure of merit is calculated for the quasi-orthogonal and collinear geometries of AO interaction. The parameters of AO deflectors and filters of THz radiation are determined.