Chirped Laser Pulses Research Articles

Wakefield evolution and electron acceleration in interaction of frequency-chirped laser pulse with inhomogeneous plasma

The nonlinear interaction of an ultra-short intense frequency-chirped laser pulse with an underdense plasma is studied. The effects of plasma inhomogeneity and laser parameters such as chirp, pulse duration, and intensity on plasma density and wakefield evolutions, and electron acceleration are examined. It is found that a properly chirped laser pulse could induce a stronger laser wakefield in an inhomogeneous plasma and result in higher electron acceleration energy. It is also shown that the wakefield amplitude is enhanced by increasing the slope of density in the inhomogeneous plasma.

Production of excited hydrogen molecule in a two-frequency chirped laser field

We have studied the population transfer in a 1+1 ladder system from v = 0, j = 0 of X 1 Σ g + state to J 1 Δ g +(v = 2, j = 2) state using two frequency-chirped laser pulses and thus to create excited hydrogen molecule. The first chirped frequency connects the ground level with two nonadiabatically coupled (i.e. dressed) intermediate levels, B 1 Σ u +(v = 14, j = 1) and C 1 Π u (v = 3, j = 1), while the second chirped pulse excites the intermediate levels to the final target level. Here both the chirped fields produce adiabatic crossings due to the chirping actions and facilitate the population transfer to the higher levels. We reported complete population transfer to the highly excited bound electronic level J 1 Δ g +(v = 2, j = 2) using appropriate laser parameters of the two chirped laser pulses for all combinations of negatively and positively chirped fields. We explained the population transfer by drawing adiabatic dressed states including nonadiabatic interaction.

External injection and acceleration of electron bunch in front of the plasma wakefield produced by a periodic chirped laser pulse

Herein, we present the analytical results on the behavior of the electron bunch injected in front of the plasma wakefield produced by a chirped laser pulse. In particular, a periodic chirped pulse may produce an ultra-relativistic electron bunch with a relatively small energy spread. The electrons are trapped near the region of the first accelerating maximum of the wakefield and are compressed in both the longitudinal and transverse directions (betatron oscillation). Our results are in good agreement with the one-dimensional results recently published.

Optical design and studies of a tiled single grating pulse compressor for enhanced parametric space and compensation of tiling errors

A new optical design of tiled single grating pulse compressor has been proposed, set-up and studied. The parametric space, i.e. the laser beam diameters that can be accommodated in the pulse compressor for the given range of compression lengths, has been calculated and shown to have up to two fold enhancement in comparison to our earlier proposed optical designs. The new optical design of the tiled single grating pulse compressor has an additional advantage of self compensation of various tiling errors like longitudinal and lateral piston, tip and groove density mismatch, compared to the earlier designs. Experiments have been carried out for temporal compression of 650ps positively chirped laser pulses, at central wavelength 1054nm, down to ~235 fs in the tiled grating pulse compressor set up with the proposed design. Further, far field studies have been performed to show the desired compensation of the tiling errors takes place in the new compressor.

Pulse chirping effect on controlling the transverse cavity oscillations in nonlinear bubble regime

The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λp, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier–envelope phase (CEP) breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional (2D) particle-in-cell (PIC) simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.

Spatially and spectrally resolved quantum path interference with chirped driving pulses

We measure spectrally and spatially resolved high-order harmonics generated in argon using chirped multi-cycle laser pulses. Using a stable, high-repetition rate laser we observe detailed interference structures in the far-field. The structures are of two kinds; off-axis interference from the long trajectory only and on-axis interference including the short and long trajectories. The former is readily visible in the far-field spectrum, modulating both the spectral and spatial profile. To access the latter, we vary the chirp of the fundamental, imparting different phases on the different trajectories, thereby changing their relative phase. Using this method together with an analytical model, we are able to explain the on-axis behaviour and access the dipole phase parameters for the short () and long () trajectories. The extracted results compare very well with phase parameters calculated by solving the time-dependent Schrödinger equation. Going beyond the analytical model, we are also able to successfully reproduce the off-axis interference structure.

The effect of a negatively chirped laser pulse on the evolution of bubble structure in nonlinear bubble regime

In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The Carrier-Envelope Phase (CEP) breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional particle-in-cell simulations show that the utilization of a negatively chirped laser pulse is more effective in controlling the pulse depletion rate, and consequently, the effect of the CEP in the bubble regime. The results indicate that the pulse depletion rate diminishes during the propagation of the pulse in plasma that leads to postponing the effect of Carrier-Envelope Phase (CEP) in plasma response, and therefore, maintaining the stability of the bubble shape for a longer time than the un-chirped laser pulse. As a result, a localized electron bunch with higher maximum energy is prod...

Analysis of sinusoidally modulated chirped laser pulses by temporally encoded spectral shifting.

We present an analytical formalism elucidating how information is stored in chirped optical probes by describing the effects of sinusoidal temporal modulations on the electric field. We show that the modulations produce spectral sidebands which can be interpreted as temporal sidebands due to the time-wavelength mapping, an effect we call temporally encoded spectral shifting (TESS). A derivation is presented for the case of chirped-pulse spectral interferometry showing how to recover both the amplitude and the periodicity of the modulation from a Fourier transform of the interferogram. The TESS effect, which provides an intuitive picture for interpreting pump-probe experiments with chirped pulses, is illustrated for probing wakefields from a laser-plasma accelerator.

70 GHz arbitrary modulation of chirped laser pulses by means of acousto-optics.

We report the arbitrary modulation of chirped ultrashort laser pulses with a 70 GHz bandwidth. In the proof-of-principle experiment, modulation of Ti:sapphire laser emission was performed by the acousto-optic dispersive delay line, and the temporal pulse shape was measured with a picosecond streak camera.

Resonant second harmonic generation by a chirped laser pulse in a semiconductor

The propagation of a high power laser beam, with frequency ω1 and wave vector k1zˆ, through a semiconductor under chirped pulse effect has been investigated which results in the harmonic generation of second order. When a highly intense beam of laser propagates through a semiconductor under the impact of wiggler magnetic field, the transverse current density starts oscillating with a frequency which is twice of the laser field. Enhancement in the second harmonic generation of chirped pulse laser in semiconductor under the influence of wiggler magnetic field is seen. We have also examined the effect of chirped pulse on power conversion efficiency of the second harmonic generation which increases with the increase of the chirp parameter.

High-order harmonic generation driven by chirped laser pulses induced by linear and non linear phenomena

We present a theoretical study of high-order harmonic generation (HHG) driven by ultrashort optical pulses with different kind of chirps. The goal of the present work is perform a detailed study to clarify the relevant parameters in the chirped pulses to achieve a noticeable cut-off extensions in HHG. These chirped pulses are generated using both linear and nonlinear dispersive media.The description of the origin of the physical mechanisms responsible of this extension is, however, not usually reported with enough detail in the literature. The study of the behaviour of the harmonic cut-off with these kind of pulses is carried out in the classical context, by the integration of the Newton-Lorentz equation complemented with the quantum approach, based on the integration of the time dependent Schr\"odinger equation in full dimensions (TDSE-3D), we are able to understand the underlying physics.

Resonant second harmonic generation of chirped pulse laser in plasma

Resonant second harmonic generation of a relativistic self-focusing chirped pulse laser in plasma has been investigated. The second harmonic undergoes periodic focusing in the plasma channel created by the fundamental wave and the strength of focusing increases with the propagation distance. The normalized second-harmonic amplitude varies periodically with distance and attains maximum value in the focal region. A significant enhancement in the intensity of second harmonic on account of relativistic self-focusing of the chirped pulse laser is seen. We have also examined the effect of chirped pulse on the normalized amplitude of the second harmonic generation. A sharp increase in the normalized second harmonic amplitude is seen for positive chirp.

The role of quadratic frequency chirps in the photodissociation dynamics of [formula omitted

The present study deals with the photodissociation dynamics of the D2+ molecular ion induced by frequency modulated laser pulses. Contrary to our recent works in this field (Csehi et al., 2015, 2016), not only linearly, but also quadratically varying frequency chirped laser pulse effects are investigated in terms of the total dissociation probabilities and kinetic energy release (KER) of the photofragments.The numerical studies are carried out in the recently introduced light-induced conical intersection (LICI) framework. Obtained results undoubtedly demonstrate the movement of the LICI in the configuration space emerging from the frequency modulation of the laser pulses. We present here a detailed comparison between the linear and quadratic frequency chirps for several different values of the chirp parameters and the delay times (with respect to the ionization of D2) which are the control knobs during the search of the maximal dissociation probability.

Degradation of picosecond temporal contrast of Ti:sapphire lasers with coherent pedestals.

Recompressed pulses from Ti:sapphire chirped-pulse lasers are accompanied by a slowly decaying post-pulse pedestal that is coherent with the main pulse. The pedestal typically consists of numerous pulses with temporal separation in the picosecond range. The source of this artifact lies in the Ti:sapphire active medium itself, both in the Kerr-lens mode-locked oscillator and in subsequent amplifiers. In the presence of substantial self-phase modulation, after recompression the post-pedestal generates a mirror-symmetric pre-pulse pedestal. This pedestal severely degrades the leading edge of the output pulse. This degradation is far more limiting than the original post-pedestal and severely lowers the achievable temporal contrast.

Chirped-Standing-Wave Acceleration of Ions with Intense Lasers.

We propose a novel mechanism for ion acceleration based on the guided motion of electrons from a thin layer. The electron motion is locked to the moving nodes of a standing wave formed by a chirped laser pulse reflected from a mirror behind the layer. This provides a stable longitudinal field of charge separation, thus giving rise to chirped-standing-wave acceleration of the residual ions of the layer. We demonstrate, both analytically and numerically, that stable proton beams, with energy spectra peaked around 100MeV, are feasible for pulse energies at the level of 10J. Moreover, a scaling law for higher laser intensities and layer densities is presented, indicating stable GeV-level energy gains of dense ion bunches, for soon-to-be-available laser intensities.

Characterization of electrons and x-rays produced using chirped laser pulses in a laser wakefield accelerator

The electron injection process into a plasma-based laser wakefield accelerator can be influenced by modifying the parameters of the driver pulse. We present an experimental study on the combined effect of the laser pulse duration, pulse shape, and frequency chirp on the electron injection and acceleration process and the associated radiation emission for two different gas types—a 97.5% He and 2.5% N2 mixture and pure He. In general, the shortest pulse duration with minimal frequency chirp produced the highest energy electrons and the most charge. Pulses on the positive chirp side sustained electron injection and produced higher charge, but lower peak energy electrons, compared with negatively chirped pulses. A similar trend was observed for the radiant energy. The relationship between the radiant energy and the electron charge remained linear over a threefold change in the electron density and was independent of the drive pulse characteristics. X-ray spectra showed that ionization injection of electrons into the wakefield generally produced more photons than self-injection for all pulse durations/frequency chirp and had less of a spread in the number of photons around the peak x-ray energy.

Single-shot electro-optic experiments for electron bunch diagnostics at Tsinghua Accelerator Laboratory

The electro-optic (EO) technique detects the Coulomb electric field distribution of relativistic electron bunches to obtain the associated longitudinal profile. This diagnostic method allows the direct time-resolved single-shot measurement and thus the real-time monitoring of the bunch profile and beam arrival time in a non-destructive way with sub-picosecond temporal resolution. In this paper, we report the measurement of the longitudinal profile of an electron bunch through electro-optic spectral decoding detection, in which the bunch profile is encoded into the spectra of the linearly chirped laser pulse. The experimental setup and measurement results of a 40MeV electron bunch are presented, with a temporal profile length of 527fs rms (~1.24ps FWHM) and a beam arrival time jitter of 471fs rms. Temporal resolution and future experimental improvement are also discussed.

Erratum: Measures of electronic-vibrational entanglement and quantum coherence in a molecular system [Phys. Rev. A92, 042323 (2015)

We characterize both entanglement and quantum coherence in a molecular system by connecting the linear entropy of electronic-nuclear entanglement with Wigner-Yanase skew information measuring vibronic coherence and local quantum uncertainty on electronic energy. Linear entropy of entanglement and quantifiers of quantum coherence are derived for a molecular system described in a bipartite Hilbert space H=Hel x Hvib of finite dimension Nel x Nv, and relations between them are established. For the specific case of the electronic-vibrational entanglement, we find the linear entropy of entanglement as having a more complex informational content than the von Neumann entropy. By keeping the information carried by the vibronic coherences in a molecule, linear entropy seizes vibrational motion in the electronic potentials as entanglement dynamics. We analyze entanglement oscillations in an isolated molecule, and show examples for the control of entanglement dynamics in a molecule through the creation of coherent vibrational wave packets in several electronic potentials by using chirped laser pulses.

Controlling nanoscale acoustic strains in silicon using chirped femtosecond laser pulses

The influence of femtosecond laser pulse chirp on laser-generated longitudinal acoustic strains in Si (100) monocrystal substrates is studied. Degenerate femtosecond pump-probe transient reflectivity measurements are performed using a layered structure of thin Ti transducer film on an Si substrate. Experimental results show that acoustic strains, manifested as strong Brillouin oscillations, are more effectively induced when negatively chirped femtosecond laser pulses pump the transducer. These results are theoretically supported by a modified thermo-mechanical model based on the combination of a revised two-temperature model and elasticity theory that takes into account the instantaneous frequency of the chirped femtosecond laser pump pulses.

On some theoretical problems of laser wake-field accelerators

Enhancement of the quality of laser wake-field accelerated (LWFA) electron beams implies the improvement and controllability of the properties of the wake waves generated by ultra-short pulse lasers in underdense plasmas. In this work we present a compendium of useful formulas giving relations between the laser and plasma target parameters allowing one to obtain basic dependences, e.g. the energy scaling of the electrons accelerated by the wake field excited in inhomogeneous media including multi-stage LWFA accelerators. Consideration of the effects of using the chirped laser pulse driver allows us to find the regimes where the chirp enhances the wake field amplitude. We present an analysis of the three-dimensional effects on the electron beam loading and on the unlimited LWFA acceleration in inhomogeneous plasmas. Using the conditions of electron trapping to the wake-field acceleration phase we analyse the multi-equal stage and multiuneven stage LWFA configurations. In the first configuration the energy of fast electrons is a linear function of the number of stages, and in the second case, the accelerated electron energy grows exponentially with the number of stages. The results of the two-dimensional particle-in-cell simulations presented here show the high quality electron acceleration in the triple stage injection–acceleration configuration.