The intensity of a subpicosecond laser pulse was amplified by a factor of up to 1000 using the Raman backscatter interaction in a 2 mm long gas jet plasma. The process of Raman amplification reached the nonlinear regime, with the intensity of the amplified pulse exceeding that of the pump pulse by more than an order of magnitude. Features unique to the nonlinear regime such as gain saturation, bandwidth broadening, and pulse shortening were observed. Simulation and theory are in qualitative agreement with the measurements. The invention of chirped pulse amplification (CPA) [1,2] led to a tremendous increase of ultrashort laser pulse intensities to above 10 20 W=cm 2 [3,4]. However, such an ultrahigh intensity laser system was achieved using very large (on the order of 1 m 2 ) and expensive compressor gratings [4]. A further increase of the pulse intensity using the CPA technique would require even larger gratings in order not to exceed the material damage threshold. Such a system would be very difficult to implement in universityscale laboratories.In order to overcome the CPA material limit at ultrahigh intensities, different backscattering coupling techniques were proposed in plasma, including Compton scattering [5], resonant Raman backscattering [6], and Raman backscattering at an ionization front [7]. The experiments reported here utilize the resonant Raman mechanism [6], where a short seed laser pulse is amplified by a counterpropagating long pump pulse, with their frequencies satisfying the resonance relation, ! pump ! seed ! pe where ! pump , ! seed , and ! pe are frequencies of pump, seed, and plasma, respectively; ! pe 4 e 2 n e =m e p , n e is the plasma electron density, and m e and e are mass and charge of an electron. The energy transfer from pump to seed is in proportion to their frequencies, so for ! pump 10! pe , the efficiency can be as high as 90%. What makes the resonant Raman backscatter regime attractive is that it is a simple resonant interaction, with the seed amplification strong enough to outrun other deleterious competing instabilities (such as modulational instability that can lead to the filamentation of the laser beam) [6] or to avoid superluminous precursor solutions [8], and with realizable highly compressed ultrashort pulse solutions [9].The Raman backscattering (RBS) amplification can be divided into linear and nonlinear regimes. In the linear regime the pump depletion is negligible and the gain is independent of the seed intensity. The seed pulse is amplified and increased in duration due to the narrow bandwidth of the linear amplification. The nonlinear regime, the socalled -pulse regime, is characterized by pump depletion and the simultaneous temporal compression of the amplified pulse. In this regime, the Raman amplification and compression of ultrashort pulses in a plasma allow intensities to reach 10 20 -10 21 W=cm 2 in a compact universityscale device, and unprecedented high intensity on the order of 10 25 W=cm 2 in a larger system [9]. Such intensities open new frontiers i...

For controllable generation of an isolated attosecond relativistic electron bunch [relativistic electron mirror (REM)] with nearly solid-state density, we propose using a solid nanofilm illuminated normally by an ultraintense femtosecond laser pulse having a sharp rising edge. With two-dimensional (2D) particlein-cell (PIC) simulations, we show that, in spite of Coulomb forces, all of the electrons in the laser spot can be accelerated synchronously, and the REM keeps its surface charge density during evolution. We also developed a self-consistent 1D theory, which takes into account Coulomb forces, radiation of the electrons, and laser amplitude depletion. This theory allows us to predict the REM parameters and shows a good agreement with the 2D PIC simulations. Generation of attosecond relativistic electron beams is of great importance for modern physics. These beams can be used in modern laser-wakefield accelerators and freeelectron lasers for injection, in attosecond electron diffraction and microscopy, in generation of ultrashort coherent xray radiation via Thomson scattering, and in many other applications, providing time-resolved studies in physics, biology, chemistry, etc., with the attosecond time-scale resolution. The main requirement for attosecond electron bunches is the controllability of their parameters, including length, charge, and energy.In high-density (overcritical) plasmas, two mechanisms for generation of ultrashort electron beams -the v B heating and the vacuum heating -were investigated recently [1][2][3][4]. The length of the electron beam is about the laser pulse length here with a wide energy spread for electrons; in addition, the beam parameters are difficult to control. In low-density (underdense) plasmas, a single electron bunch can be generated by laser-wakefield acceleration mechanism [5][6][7] but the length of the bunch is not shorter than 1-5 m (several femtoseconds). In a vacuum, a single ultrashort electron beam can be generated through laser compression of a longer electron beam [8,9]; however, the charge of the bunch here is considerably smaller than 1 pC. The same compression can be applied for thin (1 m and less) plasma layers of low (gas) density [10 -12], but the practical realization of such layers is under question now.In this Letter, we propose using a nanofilm (film with a thickness of 10 nm or less) as a solid-state density target for generation of an attosecond relativistic electron bunch. It is shown that, when this target is irradiated normally by a superhigh intensity laser pulse with a sharp rising edge (nonadiabatic laser pulse), all electrons of the plasma layer can achieve relativistic longitudinal velocities synchronously when the dimensionless field amplitude becomes large enough, a 0 [a 0 jejE 0 = mc! ,, where e and m are the charge and the mass of an electron, c is the speed of light, E 0 , !, and are the amplitude, the frequency, and the wavelength of the laser field in a vacuum, ! p 4 n 0 e 2 =m p is the characteristic plasma frequency, n 0 and l are th...

We augment the usual three-wave cold-fluid equations governing Raman backscatter (RBS) with a new kinetic thermal correction, proportional to an average of particle kinetic energy weighted by the ponderomotive phase. From closed-form analysis within a homogeneous kinetic three-wave model and ponderomotively averaged kinetic simulations in a more realistic pulsed case, the magnitude of these new contributions is shown to be a measure of the dynamical detuning between the pump laser, seed laser, and Langmuir wave. Saturation of RBS is analyzed, and the role of trapped particles illuminated. Simple estimates show that a small fraction of trapped particles (6%) can significantly suppress backscatter. We discuss the best operating regime of the Raman plasma amplifier to reduce these deleterious kinetic effects. Electron kinetic effects on stimulated Raman scattering in plasmas have been explored intensively in various contexts, especially in connection with the role of Raman backscatter (RBS) in the ignition phase of inertial confinement fusion [1,2]. These investigations were motivated by discrepancies between the results observed in fluid-based simulations and those based on kinetic models. In fully kinetic simulations [1], Vu et al. observed the saturation of Raman reflectivity followed by quasiperiodic bursting. These behaviors have been attributed to a nonlinear phase shift between the three waves associated with trapped particles [1], or to a breakup of the plasma wave by the trapped-particle instability [2].However, the particle trapping effect described in [1] is more or less phenomenological, in the sense that certain physical terms responsible for the secular phase shift are omitted, and its ultimate dynamical origin remains somewhat unclear. In this Letter, we derive a new kinetic thermal correction from averaging the dynamical equations used in free-electron-laser and averaged-particle-incell (aPIC) models [3]. Its inclusion in the three-wave model provides a clear mathematical encapsulation of electron trapping and other nonlinear effects on RBS saturation and bursting. Analysis indicates that in certain regimes RBS may be saturated predominantly by small levels of electron trapping, rather than by the previously proposed mechanism invoking the breakup of the plasma wave [2]. Our kinetic three-wave model is also useful for the analysis of the Raman backward laser amplifier [3][4][5][6][7][8], which provided the basis for our study.We begin with the coupling between pump and seed lasers and the plasma electrons, corresponding to the wave equations implemented in the aPIC computer model [3], used for the kinetic simulations discussed below. Dynamics are derived under a number of simplifying assumptions: viz., field and particle data vary appreciably only in the longitudinal (z) direction; the plasma is underdense; electron motion remains nonrelativistic; ions remain immobile on the relevant time scales; the seed and pump lasers can be represented as eikonal fields with slowly varying envelopes modulating t...

Interaction of a high-power laser pulse having a sharp front with a thin plasma layer is considered. General one-dimensional numerical-analytical model is elaborated, in which the plasma layer is represented as a large collection of electron sheets, and a radiation reaction force is derived analytically. Using this model, trajectories of the electrons of the plasma layer are calculated numerically and compared with the electron trajectories obtained in particle-in-cell simulations, and a good agreement is found. Two simplified analytical models are considered, in which only one electron sheet is used, and it moves transversely and longitudinally in the fields of an ion sheet and a laser pulse (longitudinal displacements along the laser beam axis can be considerably larger than the laser wavelength). In the model I, a radiation reaction is included self-consistently, while in the model II a radiation reaction force is omitted. For the two models, analytical solutions for the dynamical parameters of the electron sheet in a linearly polarized laser pulse are derived and compared with the numerical solutions for the central electron sheet (positioned initially in the center) of the real plasma layer, which are calculated from the general numerical-analytical model. This comparison shows that the model II gives better description for the trajectory of the central electron sheet of the real plasma layer, while the model I gives more adequate description for a transverse momentum. Both models show that if the intensity of the laser pulse is high enough, even in the field with a constant amplitude, the electrons undergo not only the transverse oscillations with the period of the laser field, but also large (in comparison with the laser wavelength) longitudinal oscillations with the period, defined by the system parameters and initial conditions of particular oscillation.

Emission of radiation from electrons undergoing plasma oscillations (POs) at the plasma frequency has attracted interest because of the existence of intriguing and non-trivial coupling mechanism between the electrostatic PO and the emitted electromagnetic wave. While broadband emission from plasma waves in inhomogeneous plasma is well known, the underlying physics of narrowband emission at the plasma frequency observed in experiments and in solar radio-bursts is obscure. Here we show that a spatially-localized plasma dipole oscillation (PDO) can be generated when electrons are trapped in a moving train of potential wells produced by the ponderomotive force of two slightly detuned laser pulses that collide in plasma and give rise to a burst of quasi-monochromatic radiation. The energy radiated in the terahertz spectral region can reach an unprecedented several millijoules, which makes it suitable for applications requiring short pulses of high-intensity, narrowband terahertz radiation.

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Stimulated Raman backscattering in plasma is potentially an efficient method of amplifying laser pulses to reach exawatt powers because plasma is fully broken down and withstands extremely high electric fields. Plasma also has unique nonlinear optical properties that allow simultaneous compression of optical pulses to ultra-short durations. However, current measured efficiencies are limited to several percent. Here we investigate Raman amplification of short duration seed pulses with different chirp rates using a chirped pump pulse in a preformed plasma waveguide. We identify electron trapping and wavebreaking as the main saturation mechanisms, which lead to spectral broadening and gain saturation when the seed reaches several millijoules for durations of 10’s – 100’s fs for 250 ps, 800 nm chirped pump pulses. We show that this prevents access to the nonlinear regime and limits the efficiency, and interpret the experimental results using slowly-varying-amplitude, current-averaged particle-in-cell simulations. We also propose methods for achieving higher efficiencies.

Raman amplification arising from the excitation of a density echelon in plasma could lead to amplifiers that significantly exceed current power limits of conventional laser media. Here we show that 1–100 J pump pulses can amplify picojoule seed pulses to nearly joule level. The extremely high gain also leads to significant amplification of backscattered radiation from “noise”, arising from stochastic plasma fluctuations that competes with externally injected seed pulses, which are amplified to similar levels at the highest pump energies. The pump energy is scattered into the seed at an oblique angle with 14 J sr−1, and net gains of more than eight orders of magnitude. The maximum gain coefficient, of 180 cm−1, exceeds high-power solid-state amplifying media by orders of magnitude. The observation of a minimum of 640 J sr−1 directly backscattered from noise, corresponding to ≈10% of the pump energy in the observation solid angle, implies potential overall efficiencies greater than 10%.