Harmonic Generation Of Laser Radiation Research Articles

A self-consistent model of second harmonic generation of laser radiation in the periodically poled nonlinear-optical crystal conditioned by its nonuniform heating

We introduce a self-consistent model that describes the process of the second harmonic generation (SHG) of the laser radiation in a nonlinear-optical crystal in conditions of its nonuniform heating induced by optical absorption. A periodically poled crystal pumped by the pulse laser radiation focused into its center was considered. The iteration procedure of solving the SHG problem relies on the interrelated solution of two equation systems describing the temperature dependent SHG and the heat conduction processes. The heat sources are represented by the absorbed power of both pump and generated radiations. Along with the absorption of pump and generated waves the diffraction effect was also taken into account. Eventually the resulting distribution of the crystal temperature should correspond to the radiation intensity distributions of pump and second harmonic.

Higher-order harmonic generation of laser radiation due to relativistic plasma resonance at nonrelativistic laser intensity

The well-known plasma resonance (transformation into plasma waves) at a critical density in an inhomogeneous plasma easily becomes relativistic nonlinear even at nonrelativistic laser intensities because the generated electrostatic field is significantly increased. Therefore, the widely used theory of harmonic generation by plasma resonance in a laser-produced plasma should be re-examined. We formulate the corresponding analytical theory of higher-order harmonic generation by nonrelativistic intense laser radiation propagating in a spatially inhomogeneous plasma. We find the spectral and angular characteristics of the harmonic radiation field and demonstrate the role of relativistic nonlinearity at plasma resonance in forming the harmonic spectra. The applicability range of the developed theory is determined by the plasma wave-breaking condition in the vicinity of the critical plasma density, which we analyze explicitly. The proposed theory is compared with the standard perturbation approach. Because the latter corresponds to low laser intensities, this comparison clearly shows the failure of the theory of harmonic generation via linear plasma resonance. The presented relativistic theory, which is applicable up to the laser intensity corresponding to plasma wave breaking, demonstrates the formation of a smooth power-law energy spectrum of higher-order laser harmonics in contrast to the standard perturbation theory. A spectral modulation of harmonics is also shown, which is a unique feature of relativistic nonlinearity.

Advanced properties of extended laser-produced plasmas for efficient generation of the high-order harmonics of ultrashort laser pulses

The review of the studies showing the advanced properties of extended plasma plumes (5 mm) for efficient high-order harmonic generation of laser radiation compared with the short lengths of plasmas (~0.3–0.5 mm) used in previous studies is presented. The harmonic conversion efficiency quadratically increased with the growth of plasma length at the optimal conditions of plasma formation. The studies of this process using the long plasma jets produced on various metal surfaces, particularly including the resonance-enhanced harmonic generation, two-color pump, and extended nanoparticle plasma, are discussed.

Crystal equivalent temperature model in process of nonlinear conversion of laser radiation

Theoretical modeling was performed for estimation of influence of thermal effects in nonlinear-optical crystals on process of second harmonic generation of laser radiation. Due to absorption of both pump and generated radiation the temperature distribution inside crystal is substantially nonuniform as in cross-section as along its length. Three dimensional heat conduction problem was solved considering two types of heat sources, conditioned by exponential decay and parabolic growth of laser radiation intensity along crystal length. Longitudinal temperature gradient inside crystal is detrimental for process of laser radiation conversion.

Advanced properties of extended plasmas for efficient high-order harmonic generation

We demonstrate the advanced properties of extended plasma plumes (5 mm) for efficient harmonic generation of laser radiation compared with the short lengths of plasmas (∼0.3–0.5 mm) used in previous studies. The harmonic conversion efficiency quadratically increased with the growth of plasma length. The studies of this process along the whole extreme ultraviolet range using the long plasma jets produced on various metal surfaces, particularly including the resonance-enhanced laser frequency conversion and two-color pump, are presented. Such plasmas could be used for the quasi-phase matching experiments by proper modulation of the spatial characteristics of extended ablating area and formation of separated plasma jets.

Quasi-phase-matching of high-order harmonics in multiple plasma jets

Harmonic generation of laser radiation from ionized particles in the extreme ultraviolet range at the conditions of the phase matching between the femtosecond driving field and harmonics is an advanced concept for the development of efficient coherent sources in this spectral region. Here we present experimental evidence of quasi-phase-matched high-order harmonic generation in multiple plasma jets produced during laser ablation of the silver target. We observed both the enhancement of some groups of harmonics along the plateau range and the variation of maximally enhanced harmonic order; additionally we analyzed the influence of the number of plasma jets on the harmonic yield. The enhancement factor of 13 was achieved for the 33th harmonic of Ti:sapphire laser using the five-jet plasma configuration.

High-order harmonic generation in laser plasma: Recent achievements

Recent studies of high-order harmonic generation of laser radiation in laser-produced plasma show new attractive developments in this field. Those include generation of extended harmonics in plasma plumes, new approaches in application of two-color pump, generation of extremely broadened harmonics, further developments in harmonic generation in clusters (fullerenes, carbon nanotubes, in-situ produced nanoparticles), destructive interference of harmonics from different emitters, resonance-induced enhancement of harmonics, applications of high pulse repetition rate lasers for the enhancement of average power of generating harmonics, observation of quantum path signatures, etc. We review some of these recent developments.

Use of carbon-containing materials for efficient high-order harmonic generation of laser radiation

A systematic study of the high-order harmonic generation (HHG) of laser radiation in various carbon-containing plasma plumes (CCPPs) is presented. The materials studied are: graphite, boron carbide, C60, polytetrafluoroethylene, polyethylene, carbon nanotubes, and soot. These studies show that CCPPs present the best plasma media for efficient lower order (from 9th to 19th) harmonic generation, while the harmonic cutoff restricted to the 29th order. The advantages of CCPPs for the harmonic generation are confirmed by comparison of the HHG conversion efficiency with that in Ag and In plasma plumes, where the highest conversion efficiency was reported. Use of two-color pump scheme allows further enhancement of harmonic yield from the CCPPs.

Low-order harmonic generation in metal ablation plasmas in nanosecond and picosecond laser regimes

Low-order harmonics, third and fifth, of IR (1064 nm) laser emission have been produced in laser ablation plasmas of the metals manganese, copper and silver. The harmonics were generated in a process triggered by laser ablation followed by frequency up-conversion of a fundamental laser beam that propagates parallel to the target surface. These studies were carried out in two temporal regimes by creating the ablation plasma using either nanosecond or picosecond pulses and then probing the plasma plume with pulses of the same duration. The spatiotemporal behavior of the generated harmonics was characterized and reveals the distinct composition and dynamics of the plasma species that act as nonlinear media, allowing the comparison of different processes that control the generation efficiency. These results serve to guide the choice of laser ablation plasmas to be used for efficient high harmonic generation of laser radiation.

Time-resolved spectroscopy of plasma plumes: A versatile approach for optimization of high-order harmonic generation in laser plasma

The time-resolved studies of laser-produced Ag, In, Pt, V, Mn, and Ga plasmas are presented from the point of view of plasma application as the nonlinear optical medium for high-order harmonic generation of laser radiation. We show that optimization of plasma formation using this technique allows the enhancement of harmonic generation efficiency and extension of maximal harmonic order.

Second harmonic generation of laser radiation in uniaxial crystals. Alternate problem solutions in the assumed field approximation

General questions of the theory of the second harmonic generation of laser radiation in uniaxial crystals are discussed. The derivation of the integral equations determining the form of the interacting fields on an observation plane situated at an arbitrary distance behind the crystal is presented. It is suggested to solve these equations with boundary conditions specified on the same observation plane. It is shown that such a nontraditional choice of the plane of the specification of the boundary conditions allows for a significant simplification of the sought-after second harmonic field representation in the assumed field approximation.

Application of nanoparticle-containing laser plasmas for optical harmonic generation

Nanoparticle-containing media can be used for efficient high-order harmonic generation of laser radiation. We present the results of studies of the harmonics generated in the laser-produced plasmas containing Cr2O3, Ag, Cu, and Au nanoparticles. These results are compared with the harmonics generated in the plasma produced on the surface of bulk targets at different delays between the subnanosecond heating prepulse and femtosecond pulse and different intensities of prepulse on the target surface. We discuss the harmonic enhancement, which was observed in the case of nanoparticle-containing plumes with regard to the monoparticle-containing plasmas. Morphological studies of plasma debris confirmed the presence and integrity of nanoparticles in the plumes.

Synthesis and structure of CsLi0.5Al0.5PO4

A new phosphate CsLi0.5Al0.5PO4 with a structure related to the β-tridymite structure has been synthesized by a precipitation method. X-ray powder diffraction, electron probe microanalysis, second harmonic generation of laser radiation, and IR spectroscopy have been used for studying the compound. Differential scanning calorimetry shows that the synthesized phosphate undergoes phase transitions at Ttr = 68 and 130°C. The crystal structure of CsLi0.5Al0.5PO4 was refined on the basis of X-ray powder diffraction data by the Rietveld method in space group P21/a (Z = 8) with the unit cell parameters a = 17.8323(3) A, b = 5.4250(1) A, c = 9.3666(2) A, β = 89.984(8)°, V = 906.10(9) A3.

Crystal structure and nonlinear optical properties of the K2UO2(SO4)2 · 2H2O compound at 293 K

Single crystals of potassium uranyl sulfate are grown, and their atomic structure is determined using X-ray diffraction analysis. The compound crystallizes in the orthorhombic crystal system with space group Pna2{sub 1} [a = 13.773(4) A, b = 7.288(2) A, c = 11.556(4) A, R{sub 1} = 0.033, wR{sub 2} = 0.0892 for 2630 reflections with I > 2{sigma}(I)]. The crystal structure of the K{sub 2}UO{sub 2}(SO{sub 4}){sub 2} . 2H{sub 2}O compound is built up of two-dimensional infinite, negatively charged layers of the composition [UO{sub 2}(SO{sub 4}){sub 2}.H{sub 2}O]{sub 2{infinity}}{sup {delta}}{sup -}], which are linked together through the K{sup +} ions. The specific features of the atomic arrangement in the structure of this compound are analyzed, and the second harmonic generation of laser radiation is investigated.

Study of nonlinear optical characteristics of various media by the methods of z-scan and third harmonic generation of laser radiation

The nonlinear optical parameters of colloidal solutions of metals, semiconductor chalcogenide films, vapours and solutions of organic dyes, as well as of fullerene-doped polyimide films are studied by the methods of z-scan and third harmonic generation (THG). The nonlinear refractive indices, nonlinear susceptibilities, and nonlinear absorption coefficients of these materials are measured at the fundamental 1064-nm emission wavelength of a 35-ps pulsed Nd:YAG laser and its second harmonic at 532 nm. The study of the optical limitation of picosecond and nanosecond light pulses in these media showed that the appearance of the long-wavelength absorption band at the final stage of aggregation of colloidal metals resulted in nonlinear absorption of picosecond pulses. The nonlinear susceptibilities χ(3)(—3ω,ω,ω,ω) of colloidal metals, fullerenes, and dye vapours are measured by the picosecond THG method, and the effect of self-action on THG in these media is analysed.

Third harmonic generation of laser radiation in Fe- and Zn-doped polyvinylpyrrolidone

The results of third-order nonlinear susceptibilities studies of Fe- and Zn-doped polyvinylpyrrolidone (PVP) aqueous solution in processes of third harmonic generation of Nd:YAG laser radiation are presented. Nonlinear susceptibilities of PVP:Fe and PVP:Zn were calculated to be 5×10-13 esu and 3×10-13 esu respectively. Third harmonic conversion efficiencies in these metallocomplexes were measured to be 8×10-7 and 5×10-7 respectively. The Z-scan method was applied to determine Kerr effect influence on frequency conversion process. The value of nonlinear refractive index of PVP:Fe at the wavelength of λ = 1064 nm was measured to be n2 = - 6.7×10-13 esu.

Second harmonic generation in periodically poled crystals for two types of interaction

Second harmonic generation of laser radiation in periodically poled crystals is considered for two types of interaction simultaneously. The possibility of highly efficient frequency conversion of depolarised laser radiation is demonstrated.

Investigation of Nonlinear Optical Characteristics of Colloidal Metals, Semiconductors, Fullerenes and Organic Dyes by Z-Scan Method and Third Harmonic Generation of Laser Radiation

The nonlinear optical parameters of colloidal metal solutions, semiconductor chalcogenide films, dye vapors and solutions, and fullerene-doped polyimide films and solutions are investigated by Z-scan method and third harmonic generation (THG). We measure nonlinear refractive indices, nonlinear susceptibilities and nonlinear absorption coefficients of such materials at the wavelengths of Nd:YAG laser radiation and its second harmonic ( ; = 1064 nm and 532 nm, = 35 ps). We study the optical limiting of picosecond and nanosecond radiation in these media. We show that the appearance of long-wave wing of absorption at the final stages of colloidal metals aggregation leads to the nonlinear absorption of picosecond pulses. THG studies of picosecond Nd:YAG laser radiation in colloidal metals, fullerenes and dye vapors are presented. We measure their nonlinear optical susceptibilities ; (3) ( ; 3 , , , ) and discuss the influence of self-action processes on THG in these media.

Peculiarities of self-action in second harmonic generation of laser radiation in nonlinear crystals

By using the generalized method of strong interaction of nonlinear waves and computer simulation, an analysis of the influence of self-action effects on second harmonic generation of an intense diverging picosecond pulse is performed. Using the approximation of strong wave interaction, an analytic solution is obtained for frequency conversion of the laser radiation taking into account the pump intensity depletion, the influence of the angular dispersion effect (ADE), and the higher nonlinearities. It is shown that the self-action effects, the ADE, and the linear phase mismatch can compensate for each other, increasing the efficiency of second harmonic generation. Optimum conditions for second harmonic generation in converging and diverging laser beams were found. An asymmetry of the angular dependence of the second harmonic intensity was experimentally observed and theoretically explained. The asymmetry is caused by the reverse energy conversion of the second harmonic into the pump by the influence of the ADE and Kerr nonlinearities.

Phase-matched third harmonic generation of laser radiation in a plasma channel: nonlocal effects

Abstract A laser beam propagating through a plasma channel produces density oscillations at the second harmonic. The density oscillations in conjunction with the oscillatory velocity at the fundamental produce third harmonic current density, driving the third harmonic wave. The presence of a background density perturbation accounts for phase matching.