Counter-propagating Pump Pulse Research Articles

Generation of subcycle isolated attosecond pulses by pumping ionizing gating

We present an interesting approach named as pumping ionizing gating (PIG) for the generation of isolated attosecond pulses (IAPs). In this regime, a short laser is used to ionize a preexisting gas grating, creating a fast-extending plasma grating (FEPG) having an ionization front propagating with the velocity of light. A low-intensity long counterpropagating pump pulse is then reflected by a very narrow region of the ionization front, only where the Bragg conditions for resonant reflection is satisfied. Consequently, the pump reflection is confined within a subcycle region called PIG, and forms a wide-band coherent IAP in combination with the frequency up-conversion effect due to the plasma gradient. This approach results in a new scheme to generate IAPs from long picosecond pump pulses. Three-dimensional (3D) simulations show that a 1.6 ps, 1 µm pump pulse can be used to generate a 330 as laser pulse with a peak intensity approximately 33 times that of the pump and a conversion efficiency of around 0.1%. These results highlight the potential of the PIG method for generating IAPs with high conversion efficiency and peak intensity. Published by the American Physical Society 2024

The role of transient plasma photonic structures in plasma-based amplifiers

High power lasers have become useful scientific tools, but their large size is determined by their low damage-threshold optical media. A more robust and compact medium for amplifying and manipulating intense laser pulses is plasma. Here we demonstrate, experimentally and through simulations, that few-millijoule, ultra-short seed pulses interacting with 3.5-J counter-propagating pump pulses in plasma, stimulate back-scattering of nearly 100 mJ pump energy with high intrinsic efficiency, when detuned from Raman resonance. This is due to scattering off a plasma Bragg grating formed by ballistically evolving ions. Electrons are bunched by the ponderomotive force of the beat-wave, which produces space-charge fields that impart phase correlated momenta to ions. They inertially evolve into a volume Bragg grating that backscatters a segment of the pump pulse. This, ultra-compact, two-step, inertial bunching mechanism can be used to manipulate and compress intense laser pulses. We also observe stimulated Compton (kinetic) and Raman backscattering.

Geometry-dependent spectra and coherent-transient measurement of nearly degenerate four-wave mixing using two-photon resonance

We study nearly degenerate four-wave mixing using a two-photon-allowed vibrational transition of parahydrogen. A signal photon is generated by a trigger photon and coherence among parahydrogen, which is prepared by two counterpropagating pump pulses, and we investigate the dependence of the signal pulse energy on the trigger frequency. The measured spectra depend on the geometry, shifting depending on the direction of the signal pulse and on the small angle formed by the counterpropagating pump pulses. Furthermore, we investigate the dependence of signal pulse energy on the incident time of the trigger pulse. The measured signal pulse energy is high if the trigger pulse is delayed slightly with respect to the pump pulses. We demonstrate that these geometry-dependent spectra and coherent-transient response can be explained by using simple models.

Compression of pulses during their amplification in the field of a focused counterpropagating pump pulse of the same frequency and width in media with electrostriction nonlinearity

Efficient compression of focused ∼0.9-ns pulses of a miniature Nd:YAG laser to less than 60 ps is experimentally obtained at their interaction with counterpropagating pulses of the same carrier frequency and width in CCl4. In this case, electrostriction interaction (amplification) begins not from the level of spontaneous-scattering noise; therefore, the counterpropagating pulses can be compressed at pump pulse energies below the stimulated Brillouin scattering (SBS) threshold energies. When counterpropagating seed pulses are used, the energy and temporal stability of compressed pulses are several times higher, and their time jitter is smaller than that for SBS compression from the level of spontaneous-scattering noise.

Moving electron density gratings induced in the beat-wave field of two counterpropagating laser pulses

The production and physical characteristics of moving electron density gratings (MEDGs) are investigated by analytical model and particle-in-cell (PIC) simulations. When the frequency difference of two counterpropagating pump pulses is equal to the electron plasma frequency, large amplitude electron plasma wave is resonantly excited in the plasma. As a result, electrons are excited to form a moving, gratinglike structure in the interfering region of pump pulses, while the ions keep at rest. The spatial period and velocity of the MEDG are obtained by solving the electron density modulation equation, which agrees well with one-dimensional PIC simulation results. When a short signal laser pulse is scattered on the MEDG, the frequency of the backscattering wave shifts due to the stimulated Raman scattering.

Relaxed damage threshold intensity conditions and nonlinear increase in the conversion efficiency of an optical parametric oscillator using a bi-directional pump geometry

A novel bi-directional pump geometry that nonlinearly increases the nonlinear optical conversion efficiency of a synchronously pumped optical parametric oscillator (OPO) is reported. This bi-directional pumping method synchronizes the circulating signal pulse with two counter-propagating pump pulses within a linear OPO resonator. Through this pump scheme, an increase in nonlinear optical conversion efficiency of 22% was achieved at the signal wavelength, corresponding to a 95% overall increase in average power. Given an almost unchanged measured pulse duration of 260 fs under optimal performance conditions, this related to a signal wavelength peak power output of 18.8 kW, compared with 10 kW using the traditional single-pass geometry. In this study, a total effective peak intensity pump-field of 7.11 GW/cm(2) (corresponding to 3.55 GW/cm(2) from each pump beam) was applied to a 3 mm long periodically poled lithium niobate crystal, which had a damage threshold intensity of 4 GW/cm(2), without impairing crystal integrity. We therefore prove the application of this novel pump geometry provides opportunities for power-scaling of synchronously pumped OPO systems together with enhanced nonlinear conversion efficiency through relaxed damage threshold intensity conditions.

Comprehensive Finite-Difference Time-Dependent Beam Propagation Model of Counterpropagating Picosecond Pulses in a Semiconductor Optical Amplifier

In this paper, we present a numerical model to study counter pulse propagation in semiconductor optical amplifiers. An improved finite-difference beam propagation method for solving the modified nonlinear Schrodinger equation is applied for the first time in the counterpropagation regime. In our model, group velocity dispersion, two-photon absorption, ultrafast nonlinear refraction, and the change in the gain peak wavelength with carrier density are included, which have not been considered simultaneously in previous counterpropagation models. The model is applied to demonstrate how a subpicosecond and picosecond probe pulse shape and spectrum can be modified by a counterpropagating pump pulse. Based on the results obtained by this model, while subpicosecond probe pulses can be compressed by in this scheme, their time-bandwidth product are also improved significantly. Furthermore, the effects of several parameters are analyzed to obtain the proper probe spectral peak shift using counterpropagating probe pulses. The accuracy and computational efficiency of the new scheme are assessed through numerical examples and are shown to be superior to previously published approaches.

Femtosecond pulse shaping using counter-propagating pulses in a semiconductor optical amplifier

An efficient pulse shaping method using counter-propagating pulses in the femtosecond regime is proposed and investigated. The effects of pump pulse power and pulsewidth on probe pulse are studied in counter-propagation scheme. It is shown that, with the proposed method, output probe pulse temporal and spectral peak shift due to femtosecond nonlinearities can be compensated, while the output pulse is amplified sufficiently. Furthermore, in relatively high power regime, the probe pulsewidth and time-bandwidth product are improved using counter-propagating pump pulse.

Light slowdown in the vicinity of cross-over resonances

Pulse propagation is considered in an inhomogeneously broadened medium of three-level atoms in a V-configuration, dressed by a counter-propagating pump pulse. A significant enhancement of the group index and the resulting signal slowdown are demonstrated in three frequency windows of a reduced absorption and a steep normal dispersion, in particular in that window which is due to a cross-over resonance. Particular properties of the group index in the vicinity of such a resonance are demonstrated in the case of closely spaced upper levels.

SRS-amplification of picosecond stokes pulses in the field of counterpropagating pump pulses

with the duration of the amplified pulses. At the same time the use of backward SRS removes this limitation. In the present work results of theoretical as well as experimental study of amplification of picosecond Stokes pulses with backward SRS in methane in the field of counterpropagating nano- and subnanosecond pumping pulses arc given. A modified installation from 131 was used in the experiments, in which the SRS-mirror in pulse laser pumping unit II was also used for compressing the pump pulses when necessary, and in amplification unit III instead of Ti:S-amplifiers SRS-cuvettes of length LI = 120 and L2 = 25 cm were placed. We present here only the main parameters of this laser installation. At the output of the second cascade of the SS-compressor (unit I) the Stokes pulses (,t S = 0.63 F4m) had an energy of Ws < 8 mJ with a pulse length of T S < 20 ps. The pulse laser pumping unit (2 L = 0.532/~m) made it possible to obtain energies W1 <_ 300 mJ in the nanosecond (rl < 4 nsc) pulse mode and W2 < 110 mJ in the subnanosecond (r2 < 0.65 ns) pulse mode. A delay line additionally inserted between units I and III made it possible to change the moment of arrival of the pump pulses by Ato = +--1.3 ns relative to the arrival time of the signal pulse. Furthermore, the meeting point of the pulses in the SRS-cuvette was adjusted by moving it longitudinally. Thus, by changing the focusing, the meeting point of the pulses, and the methane pressure in the cuvette, the conditions of interaction were optimized to achieve the maximum output energy of the first Stokes pulse for specified input energies of the pump signal pulses. To transform the energy of the pump pulses efficiently with backward SRS, as in the case of laser amplifiers 13 I, it is necessary that the energy density of the Stokes pulses be close to the saturation energy density. In the quasistationary case, where the duration of timewise smooth laser and Stokes pulses TL, S is much more than the time of transverse relaxation T2 of the excited molecules, the saturation energy flux density is found by Fsa t = ;tL(nL+ns)/gc S = 2/gc [61, where nL, S are the refractive indices of the SRS-medium at the laser and Stokes frequencies, g is the specific gain (in intensity) of the Stokes component [7 l, which depends on the pressure; c is the velocity of light. At a methane pressure of p -- 30 aim the time of dephasing of the oscillations is T2 = 16 ps, the gain g = 0.86 cm/GW [71, and the saturation energy density is Fsa t = 78 mJ/cm 2. Thus, assuming by analogy with laser amplifiers [3 ] that in the nonstationary mode of amplification of short Stokes pulses the saturation energy Fsa t has the same (relatively low) order of magnitude, one can hope to achieve rather high efficiencies of