We report an experimental study of highly transient stimulated Raman scattering (SRS) in a BaWO4 crystal pumped by a subpicosecond ytterbium fiber laser at a wavelength of 1030 nm, with adjustable pump-pulse chirp and pulse durations ranging from 0.25 to 6 ps (positive and negative external chirp). It is shown that, in agreement with theoretical predictions, SRS is observed when the chirped pump pulse duration exceeds 0.73 ps-allowing the system to overcome the suppressive effect of Kerr nonlinearities; otherwise, up to 62% of the pump energy is converted into supercontinuum. The optimal pulse-duration ranges for chirped pump pulses are found to be 2-5 ps and 1-4 ps for positive and negative external chirp, respectively. In these ranges, efficient multiwavelength SRS generation occurs, involving both a high-frequency shift associated with the strong totally symmetric vibrational mode (925 cm-1) and a low-frequency shift associated with the weak bending vibrational mode (332 cm-1), with a total SRS conversion efficiency of 31.5% for the first Stokes components (wavelengths of approximately 1138 and 1066 nm). The shift of this optimal range toward lower pulse durations with negative external pump-pulse chirp is attributed to a reduction in the resulting chirp caused by the presence of positive internal chirp induced by nonlinear self- and cross-phase modulation effects. Adjustable pump-pulse chirp also enables the controlled tuning of the wavelength of the first Stokes component of high-frequency-shifted SRS radiation within the 1136-1141 nm range.
- We report the experimental measurement of the optical damage threshold of the BaGa2GeS6 crystal surface irradiated with laser pulses with a wavelength of 1.03 mu m and pulse durations ranging from 0.3 to 10 ps. Under single-pulse irradiation, the threshold energy density for optical damage was approximately 0.6 J/cm2 (intensity similar to 1.8 TW/cm2) for 0.3 ps pulses and increased gradually to 2.2 J/cm2 (intensity similar to 0.22 TW/cm2) for 10 ps pulses. The optical damage threshold of the BaGa2GeS6 crystal surface is found to exhibit a weak dependence on the pulse repetition rate: as the frequency increases from 1 Hz to 100 kHz, the optical damage threshold decreases from 0.6 to 0.45 J/cm2. Analysis of literature data shows that the optical damage threshold for BaGa2GeS6 irradiated by femto- and picosecond laser pulses with a wavelength of 1 mu m is an order of magnitude higher than that of traditional nonlinear crystals such as AgGaS2 and LiGaS2.
We propose an algorithm for determining the ionization threshold, above which tracks may appear during the etching of heavy-ion tracks.
As is known, coherent imaging (CI), which emerged in the second half of the last century, offers an alternative to traditional optical imaging systems. The essence of the method is the mathematical processing of diffraction patterns produced by an object placed in a coherent beam. In this case, focusing optical elements and systems are not used. The set of necessary components can be reduced to a source of coherent radiation, a matrix receiver, and a computer that performs numerical calculations. This eliminates the need for expensive optical instruments. Moreover, CI characterizes the object not only by the modulus of the field of the transmitted wave, but also by its phase. These obvious advantages stimulate research and engineering efforts aimed at improving the method and facilitating its practical application. It is shown that the paraxial approach allows the CI method to be applied not only to plane waves incident on a sample, but also to coherent probing beams of arbitrary transverse structure. This simplifies the requirements for radiation sources and expands the possibilities for practical implementation. The conditions for the applicability of this approach are formulated.
We investigate the possibility of controlling the energy of an electron beam in applied linear accelerators operating at energies of up to 10 MeV and generating bremsstrahlung, when direct access of the beam to the measurement path is impossible. A method for estimating the beam energy based on the angular distribution of bremsstrahlung generated at the accelerator target is proposed. The approach is based on approximating the measured angular distribution of the bremsstrahlung dose by an expression obtained from Monte Carlo simulations (GEANT4) for the actual target geometry. To approximate the simulation results in the energy range of 3–9 MeV, an analytical formula is proposed that takes into account the energy dependence of the angular distribution parameter. An analysis of the influence of the shape of the electron energy spectrum on the accuracy of the method was carried out. The effectiveness of the method was confirmed by comparison with experimental data obtained using a 6 MeV radiographic accelerator.
