Six-photon resonant scattering of collinear laser pulses in plasma.

To achieve the highest possible laser intensities with the least laser energy, shorter-wavelengths lasers are advantaged if they can be focused to spots of a few laser wavelengths and durations of several laser periods. However, the top laser pulse energies available nowadays are megajoules at near-optical wavelengths and millijoules at shorter wavelengths. Thus, to produce the highest laser intensities, what is required is an efficient spectral transfer of the huge near-optical energies to shorter wavelengths. It is proposed here that the desired spectral transfer could occur via resonant photon interactions associated with nonlinearity of mildly relativistic motions of plasma electrons in intense laser fields, specifically via the six-photon resonant scattering of collinear laser pulses in plasma. The six-photon interaction can, in fact, be the dominant resonant photon interaction to achieve collinear frequency up-conversion.

Super-resonant four-photon collinear laser frequency multiplication in plasma.

Resonant four-photon scattering could nearly double frequencies of intense laser pulses in plasma. However, transverse slippage between pulses presents a technological challenge, while collinear four-photon scattering is forbidden for classical light dispersion in plasma. Nonlinear renormalization of intense laser pulses can enable collinear four-photon resonance. However, such a very intensity-sensitive resonance is difficult to maintain for evolving pulses. Remarkably, there is a lower-dimensionality submanifold of the resonant four-photon manifold where the evolving pulses stay in resonance. This could enable an all-optical frequency doubling of mildly relativistic-intense laser pulses in collinear geometry, advantageously free of the transverse slippage challenges.

Towards megajoule x-ray lasers via relativistic four-photon cascade in plasma.

A theoretically highly efficient mechanism, operating at high laser intensities and powers, is identified for spectral transferring huge laser energies to shorter ultraviolet and x-ray wavelengths. With megajoule laser energies currently available at near-optical wavelengths, this transfer would, in theory, enable megajoule x-ray lasers, a huge advance over the millijoules x-ray pulses produced now. In fact, enabling even kilojoule x-ray lasers would still be a fantastic advance, and a more likely achievable one, considering practical experimental inefficiencies.

Resonant four-photon scattering of collinear laser pulses in plasma.

Exact four-photon resonance of collinear planar laser pulses is known to be prohibited by the classical dispersion law of electromagnetic waves in plasma. We show here that the renormalization produced by an arbitrarily small relativistic electron nonlinearity removes this prohibition. The laser frequency shifts in collinear resonant four-photon scattering increase with laser intensities. For laser pulses of frequencies much greater than the electron plasma frequency, the shifts can also be much greater than the plasma frequency and even nearly double the input laser frequency at still small relativistic electron nonlinearities. This may enable broad range tunable lasers of very high frequencies and powers. Since the four-photon scattering does not rely on the Langmuir wave, which is very sensitive to plasma homogeneity, such lasers would also be able to operate at much larger plasma inhomogeneities than lasers based on stimulated Raman scattering in plasma.

Transition between inverse and direct energy cascades in multiscale optical turbulence.

Multiscale turbulence naturally develops and plays an important role in many fluid, gas, and plasma phenomena. Statistical models of multiscale turbulence usually employ Kolmogorov hypotheses of spectral locality of interactions (meaning that interactions primarily occur between pulsations of comparable scales) and scale-invariance of turbulent pulsations. However, optical turbulence described by the nonlinear Schrodinger equation exhibits breaking of both the Kolmogorov locality and scale-invariance. A weaker form of spectral locality that holds for multi-scale optical turbulence enables a derivation of simplified evolution equations that reduce the problem to a single scale modeling. We present the derivation of these equations for Kerr media with random inhomogeneities. Then, we find the analytical solution that exhibits a transition between inverse and direct energy cascades in optical turbulence.

Extended Propagation of Powerful Laser Pulses in Focusing Kerr Media.

Powerful incoherent laser pulses can propagate in focusing Kerr media much longer distances than can coherent pulses, due to the fast phase mixing that prevents transverse filamentation. This distance is limited by 4-wave scattering, which accumulates waves at small transverse wave numbers, where phase mixing is too slow to retain the incoherence and thus prevent the filamentation. However, we identify how this theoretical limit can be overcome by countering this accumulation through transverse heating of the pulse by random fluctuations of the refractive index. Thus, the laser pulse propagation distances are significantly extended, making feasible, in particular, the generation of unprecedentedly intense and powerful short laser pulses in a plasma by means of backward Raman amplification in new random laser regimes.

Exceeding the leading spike intensity and fluence limits in backward Raman amplifiers.

The leading amplified spike in backward Raman amplifiers can reach nearly relativistic intensities before the saturation by the relativistic electron nonlinearity. The saturation sets an upper limit to the largest achievable leading spike intensity. It is shown here that this limit can be substantially exceeded by the initially subdominant spikes, which surprisingly outgrow the leading spike after its nonlinear saturation. Furthermore, an initially negligible group velocity dispersion of the amplified pulse in strongly undercritical plasma appears to be capable of delaying the longitudinal filamentation instability in the nonlinear saturation regime. This enables further amplification of the pulse to even larger output fluences.

Seed laser chirping for enhanced backward Raman amplification in plasmas.

Backward Raman compression in plasma enables pulse compression to intensities not available using material gratings. Mediating the compression with higher density plasma generally produces shorter and therefore more intense output pulses. However, very high density plasma, even if sufficiently tenuous to be transparent to the laser, also produces group velocity dispersion of the amplified pulse, deleteriously affecting the interaction. What is shown here is that, by chirping the seed pulse, the group velocity dispersion may in fact be used advantageously, achieving the maximum intensities over the shortest distances while minimizing unwanted effects.

[Luminescence of stable stacking aggregates of adenine and uracil in water].

Luminescence and excitation spectra of the highly luminescent stacking dimers of adenine and uracil in water solutions are studied. By the luminescence excitation spectra method it is shown that the stacking aggregates of adenine and uracil are formed with participation of rare forms of monomer molecules: N7H tautomers of adenine and the uracil molecules in rare forms of hydratation, for example molecules without H-bonds with water. The study of temperature dependence of luminescence intensity of monomers and stacking dimers of uracil has shown that stacking dimers do not dissociate even at 85 degrees C similarly as described earlier for adenine and adenosine. Stable stacking aggregates of nucleic bases are most likely to be the precursors of RNA molecules in chemical evolution. This hypothesis is supported by new data on their stability.

Quasitransient regimes of backward Raman amplification of intense x-ray pulses.

New powerful soft x-ray sources may be able to access intensities needed for backward Raman amplification (BRA) of x-ray pulses in plasmas. However, high plasma densities, needed to provide enough coupling between the pump and seed x-ray pulses, cause strong damping of the Langmuir wave that mediates energy transfer from the pump to the seed pulse. Such damping could reduce the coupling, thus making efficient BRA impossible. This work shows that efficient BRA can survive despite the Langmuir wave damping significantly exceeding the linear BRA growth rate. Moreover, the strong Langmuir wave damping can automatically suppress deleterious instabilities of BRA to the thermal noise. The class of "quasitransient" BRA regimes identified here shows that it may be feasible to observe x-ray BRA within available x-ray facilities.

[Study of spectral and luminescent manifestations of the aggregation of thymine chromophores in aqueous solutions of polythymidylic acid at room temperature in connection with the task of photochemical record of information on thymine].

Luminescence and excitation luminescence spectra of water solutions of polythymidylic acid at room temperature were studied. Three luminescence bands at different excitation wavelengths were observed: at 338 nm, which was known earlier, and two new bands, at 320 and 350 nm. The study of excitation luminescence spectra that have not been studied earlier led us to interpret the band at 320 nm as a band of chromophores that do not interact, the band at 338 nm as a band of photochemically most active densely packed stacking dimers (absorption band exciton splitting approximately 4000 cm(-1)), and the band at 350 nm as a band of photochemically inactive big stacking aggregates (n > or = 10, exciton splitting approximately 8000 cm(-1)). Changes in optical density at 270 nm of poly-T water solutions after consecutive irradiations with UV light at 297+302 and 248 nm were studied. The causes of incomplete reversibility are discussed.

Compression of powerful x-ray pulses to attosecond durations by stimulated Raman backscattering in plasmas.

Backward Raman amplification (BRA) in plasmas holds the potential for longitudinal compression and focusing of powerful x-ray pulses. In principle, this method is capable of producing pulse intensities close to the vacuum breakdown threshold by manipulating the output of planned x-ray sources. The minimum wavelength limit of BRA applicability to compression of laser pulses in plasmas is found.

Relic crystal-lattice effects on Raman compression of powerful x-ray pulses in plasmas.

Powerful x-ray pulses might be compressed to even greater powers by means of backward Raman amplification in ultradense plasmas produced by ionizing condensed matter by the same pulses. The pulse durations contemplated are shorter than the time for complete smoothing of the crystal lattice by thermal motion of ions. Although inhomogeneities are generally thought to be deleterious to the Raman amplification, the relic lattice might, in fact, be useful for the Raman amplification. The x-ray frequency band gaps can suppress parasitic Raman scattering of amplified pulses, while enhanced dispersion of the x-ray group velocity near the gaps can delay self-phase-modulation instability, thereby enabling further amplification of the x rays.

[Photochemical revelation of the stacking aggregation of thymine chromophores in water solutions of polythymidylic acid].

The structure heterogeneity of water solutions of polyribothymidylic acid at T(room) was studied from changes caused in their absorption spectra by the photodimerization reaction. Three fractions of thymine chromophores were revealed from the differential absorption spectra: (a) the main fraction consisting of weakly interacting (isolated chromophores) chromophores with the absorption spectrum maximum at approximately 270 nm; (b) pair chromophores of the first type with the absorption spectrum maxima at 260 and 290 nm (exciton splitting 4000 cm(-1)); and (c) pair chromophores of the second type with the absorption spectrum maxima at 250 and 280 nm (exciton splitting 4300 cm(-1)). The revealed aggregates have a relatively high photochemical activity in the photodimerization reaction in comparison with the isolated chromophores. They contribute little to the total absorption spectrum of solutions but make a great contribution to its changes at the initial stages of the UV irradiation of solutions.

Pump side scattering in ultrapowerful backward Raman amplifiers.

Extremely large laser power might be obtained by compressing laser pulses through backward Raman amplification (BRA) in plasmas. Premature Raman backscattering of a laser pump by plasma noise might be suppressed by an appropriate detuning of the Raman resonance, even as the desired amplification of the seed persists with a high efficiency. In this paper we analyze side scattering of laser pumps by plasma noise in backward Raman amplifiers. Though its growth rate is smaller than that of backscattering, the side scattering can nevertheless be dangerous, because of a longer path of side-scattered pulses in plasmas and because of an angular dependence of the Raman resonance detuning. We show that side scattering of laser pumps by plasma noise in BRA might be suppressed to a tolerable level at all angles by an appropriate combination of two detuning mechanisms associated with plasma density gradient and pump chirp.

Finite-duration seeding effects in powerful backward Raman amplifiers.

In the process of backward Raman amplification (BRA), the leading layers of the seed laser pulse can shadow the rear layers, thus weakening the effective seeding power and affecting parameters of output pulses in BRA. We study this effect numerically and also analytically by approximating the pumped pulse by the "pi-pulse" manifold (family) of self-similar solutions. We determine how the pumped pulse projection moves within the pi-pulse manifold, and describe quantitatively the effective seeding power evolution. Our results extend the quantitative theory of BRA to regimes where the effective seeding power varies substantially during the amplification. These results might be of broader interest, since the basic equations are general equations for resonant three-wave interactions.

Collective deceleration of relativistic electrons precisely in the core of an inertial-fusion target.

The energy deposition of a relativistic electron beam in a plasma can be managed through turning on or off fast beam-plasma instabilities in desirable regions. This management may enable new ways of realizing the fast-igniter scenario of inertial fusion. Collisional effects alone can decelerate electrons of at most a few MeV within the core of an inertial-fusion target. Beam-excited Langmuir turbulence, however, can decelerate even ultrarelativistic electrons in the core.

Suppression of superluminous precursors in high-power backward Raman amplifiers.

The very promising scheme for producing ultrapowerful laser pulses through Raman backscattering of pump lasers in plasmas can be jeopardized by superluminous precursors. Growing from the leading part of the Raman pumped seed pulse, these precursors can disturb the plasma and the pump ahead of the pumped pulse. These ruinous effects, however, might be averted by a detuning, large enough to suppress the precursors, yet small enough to allow the desired backscatter effect.

Stimulated raman scattering of rapidly amplified short laser pulses.

The theory of transient forward stimulated Raman scattering (FSRS) of rapidly amplified short laser pulses is put forth to complement the classical theory for FSRS of stationary pulses. Quantitative conditions for FSRS suppression are identified. In particular, it is shown quantitatively how the limitation imposed by pumped pulse FSRS on the output laser intensity in plasma-based ultrapowerful backward Raman amplifiers can be overcome through a selective detuning of the Stokes resonance.