Aleksandr S. Aleksandrovsky

Ultrashort pulses characterization by nonlinear diffraction from virtual beam

Noncollinear frequency doubling in a random nonlinear photonic crystal of strontium tetraborate via nonlinear diffraction from virtual beam is investigated. This effect is shown to be useful for ultrashort pulses characterization in broad spectral range. Nonlinear photonic crystals of strontium tetraborate are a promising medium for ultrafast diagnostics in short-wavelength applications.

Diagnostics of fs pulses by noncollinear random quasi-phase-matched frequency doubling

Noncollinear random quasi-phase-matched frequency doubling in nonlinear photonic crystal (NPC) of strontium tetraborate (SBO) can be used for background-free autocorrelation measurements of ultrashort pulses in wide spectral range without any angular tuning of nonlinear photonic crystal. Nonuniformity of reciprocal superlattice vectors spectrum of random structure does not affect the accuracy of measurement. Nonlinear photonic crystals of strontium tetraborate are attractive medium for cross-correlation measurements in deep and vacuum ultraviolet.

Generation of coherent radiation in the vacuum ultraviolet using randomly quasi-phase-matched strontium tetraborate

Tunable coherent radiation is generated in the vacuum ultraviolet down to 121 nm using random quasi-phase matching in strontium tetraborate, the shortest wavelength ever produced with a second-order nonlinear optical process in a solid-state material. Relevant properties of this radiation, the nonlinear process, and the nonlinear crystal are investigated.

Čerenkov nonlinear diffraction in random nonlinear photonic crystal of strontium tetraborate

Cerenkov-type second harmonic generation of femtosecond pulses in random one-dimensional nonlinear photonic crystal of strontium tetraborate is investigated experimentally and theoretically. Sidebands in angular-spectral dependence of generated second harmonic were detected and found to be in perfect agreement with calculations. Spectral maximum of generated radiation experiences deviation from Cerenkov relation due to influence of nonlinear photonic crystal structure. Pulse duration and local spectrum of Cerenkov second harmonic are measured and found to be in fair agreement with each other.

Random quasi-phase-matched nonlinear optical conversion of supercontinuum to the ultraviolet

Conversion of fs supercontinuum to the ultraviolet (UV) range from 260 to 305 nm in nonlinear photonic crystal of strontium tetraborate is obtained. Spectral shape of generated UV radiation is governed by the shape of supercontinuum spectrum, focusing conditions and phase mismatch in the material of nonlinear photonic crystal. Maximum integral UV power of 2.6 μW was obtained in the case of weaker focusing, and peaks with the spectral width 1–3 nm dominate in the spectrum. Using tight focusing, broadband radiation in the range 265–300 nm was obtained.

Laser-induced wavelength-controlled self-assembly of colloidal quasi-resonant quantum dots.

Self-assembly of colloidal semiconductor quantum dots controlled solely by laser-induced interaction is demonstrated for the first time. Pairs of CdTe nanoparticles are formed under irradiation with nanosecond pulses at wavelengths 555 or 560 nm. Formation of pairs is justified by corresponding changes of absorption spectra. Conditions of the experiment are in excellent agreement with those predicted by the theory of laser-induced dipole-dipole interaction of QDs. The fraction of QDs assembled into pairs is up to 47%.

Spectral fringes in non-phase-matched SHG and refinement of dispersion relations in the VUV

We consider second harmonic generation (SHG) of ultrashort pulses in the case of strong phase- and group-velocity mismatch. Spectral fringes appear in the second harmonic related to two delayed replicas of the fundamental pulse in the time domain. The fringe separation can be used to evaluate the group-velocity and refractive index of nonlinear crystals at extreme wavelengths. Experimental results with femtosecond pulses in SrB(4)O(7) (SBO) are used to refine the Sellmeier equation describing the n(c) refractive index down to 160 nm, essential for the use of this unique nonlinear crystal for random quasi-phase-matching in the VUV.

Random spectrally resolved Maker fringes

The Maker fringes technique is extended to the case of nonlinear media with one-dimensional second-order nonlinear susceptibility modulation. For broadband radiation, second harmonic intensity oscillates in both spectral and angular domains, which can be considered random spectrally resolved Maker fringes. A theoretical approach is developed for modeling the second-harmonic generation in such domain structures, and the calculations are in excellent agreement with experimental results.

Deep UV generation and fs pulses characterization using strontium tetraborate

The properties of NPC structures in strontium tetraborate are analyzed. Different types of NPC structures are revealed that possess different nonlinear properties, and their spectral dependences of frequency conversion efficiency are calculated and compared. Experimental study of these structures is reported for the process of doubling of the second harmonic of fs Ti:S laser. Tuning of generated radiation is obtained in the range 187.5 - 232.5 nm, with extreme insensitivity to the angular orientation of NPC. Behavior of tuning curve along investigated fundamental wave range is similar in all studied samples, but efficiency obtained depends on the type of structure. Conversion efficiency and spectral quality of generated radiation is experimentally shown to grow better when using NPC with improved structure. Prospects of VUV converter on a single NPC are discussed. NPCs of SBO are demonstrated to be useful for autocorrelation diagnostics both in random QPM geometry and in the geometry of nonlinear diffraction from virtual beam.

Structure, Thermal Stability, and Spectroscopic Properties of Triclinic Double Sulfate AgEu(SO4)2 with Isolated SO4 Groups

Silver-europium double sulfate AgEu(SO4)2 was obtained by solid-phase reaction between Ag2SO4 and Eu2(SO4)3. The crystal structure of AgEu(SO4)2 was determined by Monte Carlo method with simulated annealing, and after that, it was refined by the Rietveld method from X-ray powder diffraction data. The compound crystallizes in the triclinic symmetry, space group P1̅ ( a = 0.632929(4), b = 0.690705(4), c = 0.705467(4) nm, α = 98.9614(4), β = 84.5501(4), γ = 88.8201(4)°, V = 0.303069(3) nm3). Two types of sulfate tetrahedra were found in the structure, which significantly affects the spectroscopic properties in the IR-range. In the temperature range of 143-703 K, the average linear thermal expansion coefficients of cell parameters a, b, and c are very similar, (1.11-1.67) × 10-5 K-1 in magnitude, and therefore, AgEu(SO4)2 expands almost isotropically. Upon heating in argon flow, AgEu(SO4)2 is stable up to 1053 K. The luminescence spectra in the region of ultranarrow 5D0-7F0 transition contain a single narrow and symmetric line at 579.5 nm that is evidence of good crystalline quality of AgEu(SO4)2 and uniform local environment of Eu3+ ions in the structure. Distribution of luminescence bands is determined by the environment of Eu3+ ions in the structure. Influence of Ag+ ions on the electron density distribution at Eu sites is detected.