Frequency-chirped Laser Pulses Research Articles

Laser pulse compression by a density gradient plasma for exawatt to zettawatt lasers

We propose a new method of compressing laser pulses to ultrahigh powers based on spatially varying dispersion of an inhomogeneous plasma. Here, compression is achieved when a long, negatively frequency-chirped laser pulse reflects off the density ramp of an over-dense plasma slab. As the density increases longitudinally, high-frequency photons at the leading part of the laser pulse penetrate more deeply into the plasma region than lower-frequency photons, resulting in pulse compression in a similar way to that by a chirped mirror. Proof-of-principle simulations performed using particle-in-cell simulation codes predict compression of a 2.35 ps laser pulse to 10.3 fs—a ratio of 225. As plasma is robust and resistant to damage at high intensities—unlike solid-state gratings commonly used in chirped-pulse amplification—the method could be used as a compressor to reach exawatt or zettawatt peak powers.

Microscopic dynamics and an effective Landau-Zener transition in the quasiadiabatic preparation of spatially ordered states of Rydberg excitations

We examine the adiabatic preparation of spatially-ordered Rydberg excitations of atoms in finite one-dimensional lattices by frequency-chirped laser pulses, as realized in a number of recent experiments simulating quantum Ising model. Our aims are to unravel the microscopic mechanism of the phase transition from the unexcited state of atoms to the antiferromagnetic-like state of Rydberg excitations by traversing an extended gapless phase, and to estimate the preparation fidelity of the target state in a moderately sized system amenable to detailed numerical analysis. We find that, in the basis of the bare atomic states, the system climbs the ladder of Rydberg excitations predominantly along the strongest-amplitude paths towards the final ordered state. We show that, despite its complexity, the interacting many-body system can be described as an effective two-level system involving a pair of lowest-energy instantaneous collective eigenstates of the time-dependent Hamiltonian. The final preparation fidelity of the target state can then be well approximated by the Landau-Zener formula, while the nonadiabatic population leakage during the passage can be estimated using a perturbative approach applied to the instantaneous collective eigenstates.

Proton acceleration in vacuum using frequency-chirped laser pulses

Recent investigations demonstrate the achievement of protons with energies of several hundred MeV via vacuum laser acceleration mechanisms. However, the symmetric electric field from an unchirped laser pulse is not efficient in generating a high energy proton beam. It is known that a proton can gain energy from a chirped laser pulse through the optimal phase synchronization between the proton and the laser field. In this study, we investigate the proton acceleration by different frequency-chirped laser pulses in vacuum both analytically and numerically. We consider four different situations constituting the unchirped, linear, quadratic, and sinusoidal chirped laser pulses. For the unchirped laser case, the pulse symmetry results in no net energy gain for the proton from the laser field. Conversely, using frequency-chirped laser pulses renders high energy to the proton compared with the unchirped laser pulse. We compared three different chirped cases, and found that a sinusoidal chirped laser pulse renders maximum acceleration to the protons compared to the linear and quadratic chirped situations. There exists an optimal chirp parameter for each type of chirped laser pulse.

Coherent manipulation of trapped Rb atoms by overlapping frequency-chirped laser pulses: theory and experiment

We present results of experimental and theoretical studies of coherent momentum transfer to rubidium atoms in a magneto-optical trap by pairs of counter-propagating frequency-modulated (chirped) laser pulses. The counter-propagating pulse pairs partially overlap each other leading to multiphoton interaction processes. We show experimentally that the mechanical momentum transferred to atoms in this scheme of interaction is larger than in the case of non-overlapping pulse pairs acting separately on the atoms. Results of numerical simulations that take into account all relevant hyperfine energy states of Rb along with the influence of relaxation and re-pumping processes are in good agreement with obtained experimental results.Graphical abstract

Terahertz radiation generation through nonlinear interaction of frequency chirped laser pulses with hot inhomogeneous plasmas

Terahertz (THz) radiation generation is investigated by nonlinear interaction of frequency chirped laser pulses with hot inhomogeneous plasmas. The plasma has a density ripple on its surface and linear inhomogeneity of the electron temperature. A high power laser obliquely incidents on a plasma and exerts a quasistatic nonlinear ponderomotive force on plasma electrons. This force oscillates electrons having a drift velocity with finite transverse component. The drift velocity beats with inhomogeneous density and temperature and consequently a nonlinear current density is produced which leads to the THz wave. The linear and nonlinear current densities are coupled with the wave equation and this equation is solved in the electrostatic and electromagnetic cases. In another case, the wave equation is solved numerically by considering both electrostatic and electromagnetic cases. The effects of frequency chirped parameter, incident laser intensity, and the inhomogeneities of electron density and electron temperature are studied on the amplitude of the THz wave. Results show that by increasing the THz frequency, the amplitude of the THz wave decreases. Moreover, by increasing frequency chirped parameter, incident laser intensity, and the inhomogeneities of electron density and electron temperature, the amplitude of the THz wave increases.

Extension of time window into nanoseconds in single-shot ultrafast burst imaging by spectrally sweeping pulses.

We achieved single-shot 2D-burst imaging with a ∼22ps temporal resolution in a nanosecond time window using sequentially timed all-optical mapping photography with a spectral filtering (SF-STAMP) scheme, where a single snapshot of spectral images measured with a linear frequency chirped laser pulse forms time-resolved snapshots. We combined a pulse-stretching scheme of a free-space angular-chirp-enhanced delay (FACED) composed of a pair of tilted mirrors and a 4f-system. With a 4f-FACED system, we generated collinearly propagating burst laser pulses with a different center wavelength and a tunable time interval and demonstrated single-shot burst imaging with a 303 ps interval in a 1.5 ns time window by an SF-STAMP with spectrally sweeping probe pulses.

Acceleration of electron bunches in plasma wakefields generated by frequency-chirped laser pulse

A comparative study of different injection configurations in the wakefield generated by a chirped laser pulse is done. The preferable electron injection region is considered for different injection schemes. For injection in the chirped laser pulse region, the energy spread has approximately the same final value independently of its injection position while energy spread changes strongly for injection in the wakefield region. It is demonstrated that an externally injected electron bunch with an initial energy of 100 MeV can achieve an energy gain of 8 GeV by using periodical chirped laser pulses. In particular, strong bunch decoherence is observed for off-axis injection in a plasma-channel-guided-laser wakefield. This strong bunch decoherence leads to a considerable increase in the bunch emittance growth. The emittance growth in off-axis injection is at least ten orders of magnitude higher than in on-axis injection.

Control of the populations and phases of two-level quantum systems by a single frequency-chirped laser pulse

We present an analytical electric field expression to simultaneously control the population and phase dynamics of two-level quantum systems. The presented electric field is obtained by a reverse engineering technique after the desired population and phase evolution pathways of the system have been specified. Upon application of this laser field, the system is driven from an arbitrary quantum state superposition to the desired population distribution; meanwhile the phase of one of the states proceeds according to a predefined path. The robustness of the engineered electric field is demonstrated by numerical simulations on the example of the 3s 3p transition of atomic sodium. Furthermore, the limitations of the presented technique, which arise due to the application of the rotating wave approximation, are thoroughly analyzed and discussed.

Effect of frequency-chirped laser pulses on terahertz radiation generation in magnetized plasma

The effect of frequency chirp on the generated terahertz (THz) wave by the interaction between intense laser pulses and under dense plasma in the presence of transverse magnetic field is investigated theoretically in the present model. An expression for the electron Lorentz factor coupling cyclotron motion nonlinearity is derived for static magnetic field perpendicular to the laser propagation axis. The beating lasers produce a nonlinear ponderomotive force due to their oscillatory motion. This force drives a nonlinear current, at beat frequency, which produces the THz radiation. The influence of the external magnetic field on the optimization process of THz field amplitude is investigated numerically. A linear frequency chirp increases the duration of nonlinear interaction of laser pulse with plasma electrons and hence, enforces the interaction for longer duration. The presence of magnetic field further provides the additional momentum to the THz photon to obtain a significant gain in output yield. Our numerical simulations reveal that there is a significant enhancement in the THz field strength for optimized value of the chirp parameter and magnetic field.

Tailored laser pulse chirp to maintain optimum radiation pressure acceleration of ions

Ion beams generated with ultra-intense laser-plasma accelerators hold promises to provide compact and affordable beams of relativistic ions. One of the most efficient acceleration setups was demonstrated to be direct acceleration by the laser's radiation pressure. Due to plasma instabilities developing in the ultra-thin foils required for radiation pressure acceleration, however, it is challenging to maintain stable acceleration over long distances. Recent studies demonstrated, on the other hand, that specially tailored laser pulses can shorten the required acceleration distance suppressing the onset of plasma instabilities. Here, we extend the concept of specific laser pulse shapes to the experimentally accessible parameter of a frequency chirp. We present a novel analysis of how a laser pulse chirp may be used to drive a foil target constantly maintaining optimal radiation pressure acceleration conditions for in dependence on the target's areal density and the laser's local field strength. Our results indicate that an appropriately frequency chirped laser pulse yields a significantly enhanced acceleration to higher energies and over longer distances suppressing the onset of plasma instabilities.

Generating isolated terawatt-attosecond x-ray pulses via a chirped-laser-enhanced high-gain free-electron laser

A feasible method is proposed to generate isolated attosecond terawatt x-ray radiation pulses in high-gain free-electron lasers. In the proposed scheme, a frequency chirped laser pulse is employed to generate a gradually-varied spacing current enhancement of the electron beam and a series of spatiotemporal shifters are applied between the undulator sections to amplify a chosen ultra-short radiation pulse from self-amplified spontaneous emission. Three-dimensional start-to-end simulations have been carried out and the calculation results demonstrated that 0.15 nm x-ray pulses with peak power over 1TW and duration of several tens of attoseconds could be achieved by using the proposed technique.

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.

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.

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.

Multi-GeV electron acceleration by a periodic frequency chirped radially polarized laser pulse in vacuum

Linear and periodic effects of frequency chirp on electron acceleration by radially polarized (RP) laser pulse in vacuum have been investigated. A frequency chirp influences the electron dynamics, betatron resonance, and energy gain by electron during interaction with the RP laser pulse and ensures effective electron acceleration with high energy gain (~GeV). The electron energy gain with a periodic frequency chirped laser pulse is about twice as high as with a linear chirp. Our observations reveal electron energy gain of about 10.5 GeV with a periodic chirped RP petawatt laser pulse in vacuum.

Tracking the photodissociation probability of D(2)(+) induced by linearly chirped laser pulses.

In the presence of linearly varying frequency chirped laser pulses, the photodissociation dynamics of D2(+) is studied theoretically after ionization of D2. As a completion of our recent work [A. Csehi et al., J. Chem. Phys. 143, 014305 (2015)], a comprehensive dependence on the pulse duration and delay time is presented in terms of total dissociation probabilities. Our numerical analysis carried out in the recently introduced light-induced conical intersection (LICI) framework clearly shows the effects of the changing position of the LICI which is induced by the frequency modulation of the chirped laser pulses. This impact is presented for positively, negatively, and zero chirped short pulses.

Enhancement of proton acceleration by frequency-chirped laser pulse in radiation pressure mechanism

The transition from hole-boring to light-sail regime of radiation pressure acceleration by frequency-chirped laser pulses is studied using particle-in-cell simulation. The penetration depth of laser into the plasma with ramped density profile increases when a negatively chirped laser pulse is applied. Because of this induced transparency, the laser reflection layer moves deeper into the target and the hole-boring stage would smoothly transit into the light-sail stage. An optimum chirp parameter which satisfies the laser transparency condition, a0≈πnel/ncλ, is obtained for each ramp scale length. Moreover, the efficiency of conversion of laser energy into the kinetic energy of particles is maximized at the obtained optimum condition. A relatively narrow proton energy spectrum with peak enhancement by a factor of 2 is achieved using a negatively chirped pulse compared with the un-chirped pulse.

Ion Acceleration by Short Chirped Laser Pulses

Direct laser acceleration of ions by short frequency chirped laser pulses is investigated theoretically. We demonstrate that intense beams of ions with a kinetic energy broadening of about 1% can be generated. The chirping of the laser pulse allows the particles to gain kinetic energies of hundreds of MeVs, which is required for hadron cancer therapy, from pulses of energies in the order of 100 J. It is shown that few-cycle chirped pulses can accelerate ions more efficiently than long ones, i.e., higher ion kinetic energies are reached with the same amount of total electromagnetic pulse energy.

Electron heating enhancement by frequency-chirped laser pulses

Propagation of a chirped laser pulse with a circular polarization through an uprising plasma density profile is studied by using 1D-3V particle-in-cell simulation. The laser penetration depth is increased in an overdense plasma compared to an unchirped pulse. The induced transparency due to the laser frequency chirp results in an enhanced heating of hot electrons as well as increased maximum longitudinal electrostatic field at the back side of the solid target, which is very essential in target normal sheath acceleration regime of proton acceleration. For an applied chirp parameter between 0.008 and 0.01, the maximum amount of the electrostatic field is improved by a factor of 2. Furthermore, it is noticed that for a chirped laser pulse with a0 = 5, because of increasing the plasma transparency length, the laser pulse can penetrate up to about ne ≈ 6nc, where nc is plasma critical density. It shows 63% increase in the effective critical density compared to the relativistic induced transparency regime for an unchirped condition.