Ilya A. Kulagin

Limitation of second-harmonic generation of femtosecond Ti:sapphire laser pulses

The second-harmonic generation of 50-fs Ti:sapphire laser pulses in 1- and 2-mm KDP crystals at 350 mJ of incident energy is measured experimentally and analyzed theoretically. A frequency-conversion efficiency of ∼40% is reached. The limiting influence of self-phase modulation and fundamental phase modulation is demonstrated. The deep modulation of a frequency-doubled spectrum at a high incident intensity is observed. Methods to increase conversion efficiency are proposed.

Frequency conversion of picosecond radiation in fullerene-doped polyimide films and colloidal metals

Investigations of third-harmonic generation (THG) of picosecond Nd:YAG laser radiation in C70-doped polyimide films and colloidal metals (Pt and Cu) are presented. The nonlinear susceptibility χ(3)(-3ω;ω,ω,ω) of the fullerene-doped film was measured to be (9±4.5)×10-13 esu (for 0.5% C70 in polyimide 6B film). The influence of nonlinear refraction of fullerene-doped films on the phase-matching conditions of THG was responsible for deviation from the cubic power dependence of the third-harmonic signal on the fundamental radiation intensity I3ω(Iω) and restriction of THG efficiency in 0.5% C70-doped polyimide film. THG efficiencies were found to be 6×10-6 and 7×10-7 for 0.5% C70-doped polyimide film and colloidal platinum respectively. The maximal predicted efficiency of THG in 0.5% C70-doped polyimide film at weak focusing has been achieved. Nonlinear susceptibilities χ(3)(-3ω;ω,ω,ω) of colloidal platinum and copper were measured to be (1.5±0.75)×10-14 and (1±0.5)×10-14 esu respectively.

Analysis of third-order nonlinear susceptibilities of quadratic nonlinear optical crystals

The relations between the tensor components of the nonlinear susceptibilities responsible for self-action processes are measured in KDP, deuterated KDP, and LiNbO3 crystals at the Nd:YAG laser wavelength (lambda=1064 nm). The measurements are conducted on the basis of the analysis of the polarization dependences of focused radiation transmittance with the z-scan scheme. The obtained polarization dependences in KDP corresponded to the fourth-rank tensor of Kerr nonlinearity. The obtained values of nonlinear refractive indices are compared with the known experimental data.

Efficient selection of single high-order harmonic caused by atomic autoionizing state influence

It is shown that the influence of atomic autoionizing states on the phase matching of high harmonic generation results in efficient selection of the single harmonic generated in the plateau region. The change of relative concentration of the atomic medium components and variation of the fundamental laser frequency can be used for tuning of the selected harmonic frequency. The possibility of achievement of contrast for the selected single harmonic of more than 10(4) at intensity conversion efficiency of approximately 10(-3) has been shown.

The nonlinear refractive indices and nonlinear third-order susceptibilities of quadratic crystals

The third-order nonlinear susceptibilities and nonlinear refractive indices of KDP, BBO, and LiNbO3 crystals at wavelengths of 1064 and 532 nm are measured using the Z-scan technique. The measurements are made for different energies of incident radiation and different focusing conditions and crystal lengths. It is found that, as the angle between the light beam and the principal optic axis of a KDP crystal increases, the nonlinearity drops, with its magnitude at a wavelength of 1064 nm being higher than at 532 nm. For a BBO crystal, the nonlinearity dispersion is normal. The mechanisms of nonlinear losses in KDP, BBO, and LiNbO3 crystals are identified. The values of the nonlinear susceptibilities thus obtained are analyzed and compared with the results of calculations based on an empirical model and with the data of other authors.

Analysis of third-order nonlinear susceptibilities of quadratic nonlinear-optical crystals

The values of tensor components of nonlinear susceptibilities responsible for self-action effects have been measured in quadratic nonlinear-optical crystals at the Nd:YAG laser wavelength. The angular dependences of the nonlinear susceptibilities have been determined on the base of measured relations between tensor components. The variation of the high-order nonlinear susceptibilities at various spectral ranges has been discussed. The measurements have been conducted on the basis of the analysis of polarization dependences of focused radiation transmittance in the Z-scan scheme.

Theory of second-harmonic generation for limited laser beams in nonlinear crystals

The frequency doubling of spatially limited laser beams in nonlinear crystals is investigated theoretically. It is shown that the influence of crystal anisotropy can be adjusted by a spatial shift of the extraordinary polarized fundamental beam relative to the ordinary polarized fundamental one in the perpendicular direction of wave propagation at entry to the crystal in the type-II phase-matching configuration.

Picosecond three-color holographic digital interferometry

The possibilities of multicolor digital holographic interferometry are experimentally studied upon measuring displacements of a surface in radiation of a picosecond Nd:YAG laser with the radiation frequency conversion into harmonics (λ1 = 1.06 µm, λ2 = 0.53 µm, and λ3 = 0.35 µm). It is shown that three-color digital holographic interferometry makes it possible to increase the measurement accuracy of displacements. The peculiarities of multicolor digital holographic interferometry by picosecond pulses are discussed.

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.

Strong-interaction approximation in the theory of second harmonic generation of intense short laser pulses

The method of strong interaction of nonlinear waves has been developed to analyse the second harmonic generation of intense laser radiation in the transient regime. Analytic solution for frequency conversion efficiency – taking into account the pump radiation depletion, influence of phase mismatch, dispersion of group velocities and higher nonlinearities – has been first obtained. A comparison of theoretically derived results and known experimental data has been conducted, and a general agreement of these results has been obtained. The optimal conditions of the second harmonic generation of neodymium laser radiation into femtosecond pulse duration range have been determined.