Reaching the Nonlinear Regime of Raman Amplification of Ultrashort Laser Pulses

Abstract

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...

Demonstration of detuning and wavebreaking effects on Raman amplification efficiency in plasma

“…2). In this setup, both the amplified pulse power and the efficiency of amplification were increased by more than a factor of 4 compared to previous results [8]. This new data suggests that the improvement in efficiency is caused by the plasma density variation along the path of the pulses interaction.…”

“…This scaling remains even if strong detuning is present [6,31] and was observed in current experiments [8,13].…”

“…It was previously suggested that the nonlinear stage of amplification, accompanied by noticeable pump depletion, was reached [8]. An increase of the amplified pulse bandwidth, suggesting pulse compression, and linear growth of the pulse energy with the pump energy was noted.…”

Amplification of an ultrashort pulse laser by stimulated Raman scattering of a 1ns pulse in a low density plasma

“…That is, the seed beam wavelength can be adjusted to seed SRS in a plasma which has low enough density that inverse bremsstrahlung absorption of the beam is sufficiently small for the desired plasma size. Recent experiments testing the concept for the compression of <~100 ps pump pulses to < 1ps durations have shown large amplification of short pulses can be achieved in cold laser produced plasmas [7][8][9][10] where the product of plasma wave number and Debye length (k λ D ) remains small for the brief duration of the experiment.…”

Backward Raman compression of x-rays in metals and warm dense matters

“…It is shown that the backward Raman scattering (BRS) [5,6], where two light pulses and a Langmuir wave interact one another to exchange the energy, is one promising approach to create an ultra short light pulse (down to a few attoseconds [7,8]); a pulse gets compressed and intensified via this laser-plasma interaction. The BRS is successfully used to create intense pulses of the visible light frequencies [9,10]. It is natural to consider the same technique for the x-ray compression [7,8].…”