Second Harmonic Generation In Nonlinear Crystals Research Articles

Quasi-phase-matched second-harmonic generation in nonlinear crystals with harmonic modulation of nonlinearity

We propose a new type of nonlinear photonic crystal with the lattice representing a harmonic function of the propagation coordinate. This type of spatial modulation of the quadratic nonlinearity is implemented to enhance the second-harmonic generation (SHG) efficiency. The process resembles that realized in periodic nonlinear photonic crystals with rectangular nonlinearity modulation under quasi-phase-matching. Residual periodic oscillations of the second-harmonic intensity were found. Comparison of the unipolar and bipolar modulations showed that they are equivalent with regard to the oscillations. The SHG efficiency in the harmonic lattices is 1.6 times lower compared with the efficiency corresponding to the quasi-phase-matching in periodic nonlinear photonic crystals with rectangular lattices.

On-axis intensity modulation-free, segmented, zero-order Bessel beams with tunable ranges

We propose and experimentally demonstrate a novel experimental scheme to generate high on-axis peak intensity, segmented, smooth, zero-order quasi-Bessel beams with tunable ranges. Illuminating the axicon with hollow Gaussian beams (HGBs) of different orders, we have generated Bessel beams of varying ranges at different positions away from the axicon. The presence of a dark core at the center of the HGBs removes the effect of imperfection in the axicon tip. As a result, the entire power of the input beam is transformed into a zero-order Bessel beam without any on-axis intensity modulation. We observe the decrease in range and increase in on-axis peak intensity of the zero-order Bessel beam with the order of HGBs. Controlling the superposition of the HGBs of different orders to the axicon, we have demonstrated the increase in the range of the Bessel beam. The current technique can also produce Bessel beams of different intensity distributions, including single-peak or multiple-peak Bessel beams. Using single-pass second-harmonic generation in nonlinear crystals of different lengths, we have further verified the increase of on-axis peak intensity of the Bessel beam with the order of the HGBs and the increase in the range of the Bessel beam due to the superposed HGBs.

Thermally poled fused silica as a second-order autocorrelation crystal

Autocorrelation is one of the most commonly used methods for ultrashort pulses (<1 ps) characterization. Many measurement devices have been made with various nonlinear materials: second-harmonic generation in nonlinear crystals and Langmuir–Blodgett films, fluorescence in fused silica, absorption in semiconductors. We propose the use of a poled fused-silica glass as an autocorrelation crystal. This cheap and robust material can be an interesting alternative for ultrashort pulses autocorrelation.

Effect of thermal blooming in second-harmonic generation in nonlinear crystals with a regular domain structure

Analytical expressions are obtained for the power of the second harmonic in nonlinear crystals having a regular domain structure with allowance for thermal blooming for a Gaussian beam of basic radiation in the quasi-stationary plane-wave approximation. Calculations were carried out in both the approximation of an assigned field of basic radiation and a substantially nonlinear regime of exhaustion of basic radiation.

Elastooptic distortions in the second-harmonic generation in nonlinear crystals in the approximation of an elastic isotropic medium

The question of applicability of the approximation of an elastic isotropic medium to the description of elastooptic distortions in the second-harmonic generation in nonlinear crystals is analyzed. A method for a change to this approximation is proposed and the numerical calculation of elastooptic distortions in the secondharmonic generation is made for some crystals.

Thermo-optical distortions in second-harmonic generation in nonlinear crystals

An analysis is made of the elasto-optical contribution to changes in the refractive index in the course of second-harmonic generation in crystalline samples which are in the form of circular cylinders and have a steady-state parabolic temperature field. The associated changes in the wave mismatch and in the intrinsic polarisations of the interacting waves are also considered.

Influence of a constant temperature gradient on the spectral-bandwidth of second-harmonic generation in nonlinear crystals

Analysis of Type I second-harmonic generation in the quasi-continuous-wave undepleted-fundamental regime indicates that a constant axial temperature gradient significantly increases the tuning spectral-bandwidth for fixed crystal orientation. The conversion efficiency spectral distribution is analogous to the light intensity spatial distribution in single-slit diffraction.

Efficiency of second-harmonic generation in nonlinear crystals with a longitudinally nonuniform refractive index

Efficiency of second-harmonic generation in nonlinear crystals with a longitudinally nonuniform refractive index

Theory of thermal self-action in second-harmonic generation in nonlinear crystals

An analysis is made of the influence of radial thermal gradients, which appear in a nonlinear crystal because of the absorption of incident pump radiation, on the process of second-harmonic generation. A complete solution of the steady-state heat conduction equation is obtained for an infinitely long but radially bounded cylinder illuminated with a Gaussian fundamental-frequency beam. The distortions of the temporal and spatial distributions are discussed and the dependences of the conversion coefficient on the length of a crystal, average power, and mismatch along the beam axis are considered. It is shown that, in particular, in some cases the harmonic emerging from a nonlinear crystal can have an annular structure. It is shown that radial thermal gradients in lithium metaniobate crystals can be partly compensated by the induced birefringence.