Dramatic enhancement of optical-field-ionization collisional-excitation x-ray lasing by an optically preformed plasma waveguide

Dramatic enhancement of optical-field-ionization collisional-excitation x-ray lasing is achieved by using an optically preformed plasma waveguide. With a 9-mm-long pure krypton plasma waveguide prepared by using the axicon-ignitor-heater scheme, lasing at 32.8 nm is enhanced by 400 folds relative to the case without the plasma waveguide. An output level of 8 x 10(10) photon/shot is reached at an energy conversion efficiency of 2 x 10(-6). The same method is used to achieve x-ray lasing in a gas jet for the high-threshold low-gain transition at 46.9 nm in neonlike argon.     

High-brightness optical-field-ionization collisional-excitation extreme-ultraviolet lasing pumped by a 100-TW laser system in an optically preformed plasma waveguide

Recent study on optical-field-ionization collisional-excitation extreme-ultraviolet lasing of Ni-like krypton at 32.8 nm pumped by a 100-TW laser system with an optically preformed plasma waveguide is reported. By using a 9-mm-long pure krypton plasma waveguide fabricated with the axicon-ignitor-heater scheme, the 32.8-nm extreme-ultraviolet laser provided an average output of 10¹² photons/pulse at pump energy of 1 J, more than one order of magnitude enhancement relative to the previous results with the same scheme at pump energy of 235 mJ. It is also found the far-field pattern of laser beams varies from a single peak profile at low pump energy to an annular profile at high pump energy due to over-ionization of krypton ions at the center of the plasma channel.     

Guiding of relativistic laser pulses by preformed plasma channels

Guiding of relativistically intense (>10(18) W/cm2) laser pulses over more than 10 diffraction lengths has been demonstrated using plasma channels formed by hydrodynamic shock. Pulses up to twice the self-guiding threshold power were guided without aberration by tuning the guide profile. Transmitted spectra and mode images showed the pulse remained in the channel over the entire length. Experiments varying guided mode power and simulations show a large plasma wave was driven. Operating just below the trapping threshold produces a dark current free structure suitable for controlled injection.     

Enhancement of harmonic generation by Fresnel-lensing effects

We present experimental results of enhanced second- and third-harmonic generation efficiency from Fresnel-lensing effects. We obtained enhancement by a factor of 6 in the third-harmonic yield by simply limiting the laser beam with an iris. Higher enhancement factors (up to 16) were obtained with a Fresnel zone plate. The experimental findings are in good agreement with the results of a simple theoretical analysis.     

Observation of phase-matched relativistic harmonic generation

Phase-matched relativistic harmonic generation in plasmas is observed for the first time. Third-harmonic light is detected and discriminated spectrally and angularly from the harmonics generated from competing processes. Its angular pattern is a narrow forward-directed cone, which is consistent with phase matching of a high-order transverse mode in a plasma. The signal level is found to be on the same order of magnitude for a circularly polarized pump pulse as for a linearly polarized pump pulse.     

Enhanced high harmonic generation from an optically prepared excited medium

We investigate high harmonics generated from rubidium atoms irradiated simultaneously by an intense 3.5 microm fundamental field and a weak cw diode laser. When 5p, 5d, and 4d excited states are populated through cascade excitation or deexcitation, orders-of-magnitude increases in harmonic yield as compared with the ground state are observed. It appears that, quite unexpectedly, the population accumulated in the 4d state alone is responsible for the observed enhancement.     

Nonlinear analysis of relativistic harmonic generation by intenselasers in plasmas

A linearly polarized, ultraintense laser field induces transverse plasma currents which are highly relativistic and nonlinear, resulting in the generation of coherent harmonic radiation in the forward direction (i.e., copropagating with the incident laser field). A nonlinear cold fluid model, valid for ultrahigh intensities, is formulated and used to analyze relativistic harmonic generation. The plasma density response is included self-consistently and is shown to significantly reduce the current driving the harmonic radiation. Phase detuning severely limits the growth of the harmonic radiation. The effects of diffraction are considered in the mildly relativistic limit. No third-harmonic signal emerges from a uniform plasma of near-infinite extent. A finite third-harmonic signal requires the use of a semi-infinite or finite slab plasma. For an initially uniform plasma, no second-harmonic radiation is generated. Generation of even harmonics requires transverse gradients in the initial plasma density profile     

Enhanced third harmonic generation in periodic structures

We consider third harmonic generation in a periodic layered medium with alternate nonlinear media. We show enhanced third harmonic generation when the fundamental frequency matches one of the mode frequencies of the distributed feedback structure. The observed feature is explained in terms of large local field enhancement for the fundamental wave.     

Surface acceleration of fast electrons with relativistic self-focusing in preformed plasma

We report an observation of surface acceleration of fast electrons in intense laser-plasma interactions. When a preformed plasma is presented in front of a solid target with a higher laser intensity, the emission direction of fast electrons is changed to the target surface direction from the laser and specular directions. This feature could be caused by the formation of a strong static magnetic field along the target surface which traps and holds fast electrons on the surface. In our experiment, the increase in the laser intensity due to relativistic self-focusing in plasma plays an important role for the formation. The strength of the magnetic field is calculated from the bent angle of the electrons, resulting in tens of percent of laser magnetic field, which agrees well with a two-dimensional particle-in-cell calculation. The strong surface current explains the high conversion efficiency on the cone-guided fast ignitor experiments.     

Second harmonic generation by periodic laminar structure of nonlinear optical crystal

The phase-matched second harmonic generation by periodic structures composed of laminae of nonlinear optical crystals has been studied experimentally. It is shown that the intensity of the second harmonic is approximately proportional to the square of the number of laminae under phase-matched condition, which is satisfied over a broad frequency range by the variation of the angle of incidence when a laminar structure with a period much larger than the coherence length is used.     

Harmonic balance approach to the periodic solutions of the (an)harmonic relativistic oscillator

The first-order harmonic balance method via the first Fourier coefficient is used to construct two approximate frequency-amplitude relations for the relativistic oscillator for which the nonlinearity (anharmonicity) is a relativistic effect due to the time line dilation along the world line. Making a change of variable, a new nonlinear differential equation is obtained and two procedures are used to approximately solve this differential equation. In the first the differential equation is rewritten in a form that does not contain a square-root expression, while in the second the differential equation is solved directly. The approximate frequency obtained using the second procedure is more accurate than the frequency obtained with the first due to the fact that, in the second procedure, application of the harmonic balance method produces an infinite set of harmonics, while in the first procedure only two harmonics are produced. Both approximate frequencies are valid for the complete range of oscillation amplitudes, and excellent agreement of the approximate frequencies with the exact one are demonstrated and discussed. The discrepancy between the first-order approximate frequency obtained by means of the second procedure and the exact frequency never exceeds 1.6%. We also obtained the approximate frequency by applying the second-order harmonic balance method and in this case the relative error is as low 0.31% for all the range of values of amplitude of oscillation A.     

Enhancement of second harmonic generation by double-pass configuration in barium borate

Double-pass type-I second harmonic generation (SHG) external to a laser cavity is reported for the first time in a BBO crystal resulting in an enhancement of the generated harmonic intensity by a factor of 3.4 from that obtained in the single-pass generation.     

Nonlinear harmonic interaction in a waveguide with periodic layered walls

We study the harmonic interaction in the nonlinear guiding layer located between two periodic layered structures. Analytical relations for the first-and second-harmonic amplitudes are obtained. It is shown that the nonlinear interaction coefficients become maximum at points where the Bragg resonance conditions are fulfilled for all structure layers of the studied waveguide including the guiding layer. The appreciable enhancement of the nonlinear interaction within the forbidden band described in experimental paper [1] is explained. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 5, pp. 416–424, May 2006.     

Sub-millimeter third harmonic generation in n-InSb waveguide

Resonant third harmonic generation of a sub-millimeter wave in n-InSb waveguide embedded with a density ripple is investigated. The non-linearity arises through the modulation of free electron mass while the ripple accounts for the phase mismatch. The efficiency of the third harmonic generation is large. However, as the plasma frequency increases the attenuation rate of the third harmonic increases and the third harmonic efficiency decreases as (ω_pb/c) is raised.     

Odd harmonic generation of ultra-intense laser pulses reflectedfrom an overdense plasma

A method of generating odd harmonics from an intense laser incident upon a sharp vacuum-overdense plasma interface is presented. One- and two-dimensional simulations are used to investigate the interaction of ultra-intense laser pulses with a sharp vacuum-plasma interface. With an intensity greater than 10¹⁸ W/cm² , these pulses have a pressure greater than 10³ Mbar creating large density oscillations and relativistic electron velocities at the surface. This results in efficient odd harmonic generation. The author present is a physical model for this harmonic generation mechanism, along with some scaling of the power in the third harmonic. This is compared with relativistic particle-in-cell simulations that include mobile and immobile ions, as well as a variety of upper shelf densities. A discussion on how ion profile modifications influence the harmonics is also presented     

High-order harmonic generation from plasma mirrors

We discuss the two mechanisms involved in high-order harmonic generation from plasma mirrors, and show that they can be clearly identified experimentally. The very different phase properties of the corresponding harmonics lead to light beams with different divergences. This can be exploited to select a particular type of harmonic by spatial filtering in the far-field.     

Transition to the relativistic regime in high order harmonic generation

Experiments and computer simulations on the generation of high order harmonics from steep plasma gradients using intense femtosecond laser pulses are presented. Qualitative changes in the harmonic emission take place when the intensities are increased above 10(19) W/cm2 and/or the plasma scale length is varied. Good agreement between experimental and calculated spectra makes it possible to clearly distinguish between nonrelativistic and relativistic mechanisms of harmonic generation.     

Locally periodically poled LNOI ridge waveguide for second harmonic generation

Periodically poled lithium niobate on insulator[LNOI]ridge waveguides are desirable for high-efficiency nonlinear frequency conversions,and the fabrication proc...