R. I. Tugushev

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.

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.

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.

Nonlinear optical parameters and optical limitation in cobalt-doped polyvinylpyrrolidone solutions

Nonlinear optical parameters (nonlinear refractive indices and nonlinear absorption coefficients) of solutions of polyvinylpyrrolidone doped by cobalt to various concentrations are measured at the lasing and second-harmonic wavelengths of a picosecond Nd:YAG laser (λ=1064 and 532 nm, respectively, and τ=35 ps). Data on optical limitation in these solutions are presented. The absence of nonlinear absorption in the IR spectral range and its significant effect in the visible range are demonstrated. Optical limitation at a wavelength of 1064 nm is related to defocusing, whereas at 532 nm, this effect is caused by two-photon absorption and partially by inverse saturated absorption and defocusing. Nonlinear optical parameters of metal-polymer complexes are reported.

Harmonic generation in organic dye vapors

Abstract Investigations of nonlinear optical frequency conversion of Nd:YAG laser radiation in naphthalene vapors are presented. The third harmonic generation (THG) caused by difference frequency generation in six-photon process has been studied. Phase-matching temperature for naphthalene vapor was found to be 170°C, at which synchronous conversion of pump radiation to the third harmonic was carried out. The THG conversion efficiency varied within 10 −10 –10 −8 depending on pump intensity and cell temperature. Third- and fifth-order nonlinear optical susceptibilities were calculated and compared with the experimentally measured fifth-order susceptibility of naphthalene vapors.

Investigation of nonlinear optical properties of various organic materials by the Z-scan method

We have studied the nonlinear optical properties of various organic materials (vegetable oil, juice, wine, cognac, Coca-Cola and Fanta drinks, Nescafé coffee, tea, gasoline, clock oil, glycerol, and polyphenyl ether) that are used in everyday life. Their nonlinearities have been studied by the Z-scan method in the near-IR and visible spectral ranges. We have shown that the majority of samples possess a nonlinear absorption; however, some of the studied materials show a strong saturated absorption and nonlinear refraction. Red wine and glycerol proved to be the most interesting materials. For these samples, we have observed a change in the sign of the nonlinear absorption with increasing laser intensity, which was attributed to the competition between two-photon absorption and saturated absorption.

Effect of higher order nonlinear optical processes on optical absorption in the photorefractive BSO and BGO crystals

The processes of nonlinear refraction and nonlinear absorption are studied in the photorefractive Bi12SiO20 (BSO) and Bi12GeO20 (BGO) crystals at a wavelength of a picosecond Nd:YAG laser of 1064 nm. The nonlinear refraction in the crystals is shown to be related to the Kerr effect, and the nonlinear absorption at this wavelength, to three-photon absorption. The three-photon absorption coefficients of the BSO and BGO crystals are equal, respectively, to (2.5±0.8) ×10−20 and (4.4±1.3) ×10−20 cm3W−2.

Laser third harmonic generation in organic dye vapors

The phenomenon of nonlinear optical conversion of a YAG:Nd laser radiation frequency from IR to UV spectral range in naphthalene vapors was studied. The optimum temperature of naphthalene vapors (170°C) was found at which the pumping radiation is synchronously converted into the third harmonic emission. A mechanism of the third harmonic generation is proposed based on the difference frequency generation in a six-photon process. The efficiency of conversion to the third harmonic generation varied within 10−10–10−8, depending on the pumping intensity and cell temperature. Nonlinear optical characteristics of naphthalene vapors were calculated for the generation using nonlinearities of the third and fifth order.

Nonlinear optical characteristics of polymethine dyes

Nonlinear refraction, nonlinear absorption, and saturated absorption of a number of polymethine dyes are studied and their nonlinear optical parameters are measured by the Z-scan method (λ=1064 nm). Simultaneous occurrence of several nonlinear optical processes in the case of excitation of dye solutions by picosecond pulses is analyzed. Saturated absorption of dye solutions is considered in terms of different models.