Growth and spectral-luminescent study of SrMoO<Subscript>4</Subscript> crystals doped with Tm<Superscript>3+</Superscript> ions

SrMoO₄ crystals doped with Tm³⁺ ions have been produced from a melt using the Czochralski method; their spectral-luminescent characteristics have been studied, and laser radiation has been generated at the wavelength of 1.94 μm using laser-diode excitation. The high absorption section at the wavelength of 795 nm, the fairly high luminescence section, the long lifetime at the upper laser level ³F4 of 1.5 ms, and a wide luminescence band allow one to hope for developing efficient tunable Tm³⁺: SrMoO₄ crystal lasers with diode pumping in the range of 1.7–2.0 μm, which are capable of implementing SRS self-transformation of radiation into the middle IR band.     

Interference method of measuring the extinction coefficient of birefringent optical fibers

The extinction coefficient of a birefringent optical fiber (the ratio between the radiation power output of the polarization mode and the radiation power transferred from this mode to another one) characterizes the capability of a birefringent fiber to retain the polarization state of the radiation. In relatively short birefringent fibers (1–100 m), the extinction coefficient may reach 10⁴–10⁶. Such high values of the extinction coefficient are difficult to measure by standard techniques (excitation of one polarization mode by an incoherent source with subsequent recording of the light intensity at the output of the analyzer). An interference method of measuring the extinction coefficient of birefringent fibers is suggested. It is based on using a coherent source and measuring interference oscillations caused by an additional phase modulation at the input of the fiber. This method does not require precise polarization matching between the laser source and fiber and considerably loosens requirements for the polarizer-analyzer extinction and resolution of the photodetector. As a result, using simple standard components (semiconductor laser, film polarizer, and photodetector), one can measure extinction coefficient as high as 10⁶. The suggested interference method of measuring the extinction coefficient of birefringent fibers receives a theoretical analysis, and experimental data obtained for 2- to 1000-m-long fibers are presented.     

Highly selective multistep ionization of atoms by laser radiation

The possibility of dramatic selectivity increase during the stepwise ionization of atoms due to selective excitation at every step has been examined. Evaluations have been made to estimate the selectivity of excitation and ionization by laser radiation of some atoms, their isotopic shifts of absorption lines being observed at several excitation steps, for instance those of U, Yb, Gd. During three-step ionization of Yb atoms in atomic beam one may obtain an excitation selectivity equal to 4.4 × 10¹⁶ cm².     

Upconverted VUV luminescence of Nd and Er doped into LiYF4 crystals under XeF-laser excitation

The upconverted VUV (185 nm) and UV (230 and 260 nm) luminescence due to 5d-4f radiative transitions in Nd³⁺ ions doped into a LiYF₄ crystal has been obtained under excitation by 351/353 nm radiation from a XeF excimer laser. The maximum upconversion efficiency, defined as the ratio of intensity for 5d-4f luminescence to overall intensity for 5d-4f and 4f-4f luminescence from the ⁴D_3/2 Nd³⁺ level, has been estimated to be about 70% under optimal focusing conditions for XeF laser radiation. A redistribution of intensity between three main components of 5d-4f Nd³⁺ luminescence is observed under changing the excitation power density, which favors the most long-wavelength band (260 nm) at higher excitation density level. The effect is interpreted as being due to excited state absorption of radiation emitted. The upconverted VUV and UV luminescence from the high-lying ²F(2)_7/2 4f level of Er³⁺ doped into a LiYF₄ crystal has also been obtained under XeF-laser excitation the most intense line being at 280 nm from the spin-allowed transition to the ²H(2)_11/2 4f level of Er³⁺, but the efficiency of upconversion for Er³⁺ emission is low, less than 5%.     

Weak antilocalization in a three-dimensional topological insulator based on a high-mobility HgTe film

The up-conversion luminescence of Er³⁺ from the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 levels in nanocrystals of Y_0.95(1?x)Yb_0.95xEr_0.05PO₄ (x = 0, 0.3, 0.5, 0.7, 1) orthophosphates activated with Er³⁺ ions has been studied under the excitation of Yb³⁺ ions to the ²F_5/2 level by 972-nm cw laser radiation. Broadband radiation in the wavelength range of 370–900 nm has been observed at certain power densities of exciting laser radiation; this broadband radiation is absent in the case of excitation of the powders under study by pulsed laser radiation with a wavelength of 972 nm at a pulse repetition frequency of 10 Hz and a duration of a pulse of 15 ns. Experimental data indicating that this radiation is thermal in nature have been presented.     

Emission of organic dyes in the case of nonlinear absorption upon excitation by the XeCl* laser

Spectral luminescent properties of eight organic dyes are studied upon excitation by focused radiation from the XeCl* laser at the pump power up to 200 MW/cm². The transmission of pump radiation by dyes is investigated as a function of the pump power. It is shown that variations in the energy, spectral, and temporal parameters of radiation of organic dyes upon high-power excitation are explained by the development of superluminescence (amplified spontaneous emission).     

Saturation of the Mn2+ ionic subsystem in Cd0.5Mn0.5Te and Zn0.91Mn0.09Se bulk samples and in Cd0.6Mn0.4Te/Cd0.5Mg0.5Te superlattices

It is experimentally shown that the Mn²⁺ ionic subsystem in the three-and two-dimensional structures of diluted magnetic semiconductors of the II–VI groups is saturated upon pulsed excitation by high-power laser radiation with a wavelength of 532.1 nm at temperatures of 77 and 4 K. The direct excitation under these conditions leads to saturation of the inhomogeneously broadened ⁴ T ₁ level of a part of Mn²⁺ ions, which is confirmed by the estimation of the fraction of excited ions. Processing of the integral kinetic curves of the intracenter luminescence band in the region of 2 eV shows that an increase in the excitation intensity gives rise to a fast component with a decreasing lifetime. Based on the data obtained, it is assumed that, along with the previously studied cooperative effect, redistribution occurs of excitation from localized to delocalized states as a result of more effective saturation of the former.     

On the possibility of obtaining incoherent femtosecond X-ray pulses from a laser plasma

It is proposed to use a high rate of collisional ionization in a superdense laser plasma to generate incoherent femtosecond X-ray pulses. The calculations indicate that the use of picosecond laser pulses with a contrast of about 10¹⁰ will allow the generation of an X-ray pulse with a duration of about 10 fs. The adequacy of the proposed model of the excitation of linear X-ray radiation from the plasma has been tested in the experiments with a picosecond laser of a moderately high contrast.     

Selective excitation of the 62 D 3/2 state of the atomic thallium beam

The absorption of the tunable narrow-band laser radiation with λ = 276.9 nm at the 6² P _1/2 → 62 D _3/2 transition in the atomic thallium beam is experimentally studied. The isotopic selectivity and efficiency of the excitation of the 6² D _3/2 state is investigated. The splittings between the ²⁰³Tl and ²⁰⁵Tl isotopes were found to be Δv = 0.98 and 1.25 GHz for the transitions with the nuclear momenta F = 1 → F = 1, 2 and F = 0 → F = 1, respectively, which is sufficient for a relatively high isotopic selectivity of the 6² D _3/2 state excitation with the subsequent ionization for the laser separation of the thallium isotopes. The effects that lead to the broadening of the resonance and a decrease in the selectivity are analyzed. The main contribution is related to the field broadening by the laser radiation, which limits the mean laser power density.     

Laser-induced photoprocesses in solutions and films of the CdSe/ZnS nanoparticles

The photophysical properties of solutions and films with relatively high concentrations of CdSe/ZnS nanoparticles are studied in the presence of the visible laser irradiation in a wide range of power densities. The short-wavelength wing detected in the photoluminescence spectra of the solutions of quantum dots is due to the selective laser excitation of small-size nanoparticles. A comprehensive analysis of the anti-Stokes photoluminescence of the nanoparticles in solutions and films indicates the thermal mechanism of this phenomenon. The dimensional quantization effect, narrow spectra, and a relatively high luminescence yield are retained in the films with a high nanoparticle concentration. The luminescence spectra of the films remain unchanged when the laser flux density increases to 1 × 10⁶ W/cm². The effect of the laser radiation on the nanoparticle films is studied at the flux densities exceeding the damage threshold (5 × 10⁶–1 × 10⁹ W/cm²).     

Short wavelength X-ray emission generated by highly excited cluster beams

X-ray radiation is studied for large clusters consisting of 10⁷–10¹⁰ atoms and irradiated by an intense laser pulse with an intensity ranged from (10¹⁴ up to 10¹⁸ W/cm²). The model is developed for such a laser plasma that includes the radiative transitions and the processes of excitation and quenching of multicharged ions of this plasma by electron impact. Due to interaction of a radiating multicharged ion with a surrounding plasma, spectral lines of emission are broaden and neighboring spectral lines are overlapped. As a result, the spectrum of radiation of multicharged ions is transformed into a continuous spectral band. The model under consideration includes important plasma processes including dielectronic recombination, spontaneous radiation, excitation, quenching and ionization of multicharged ions by electron impact. On the basis of the model developed the X-ray spectrum and spectral power are evaluated. In the range of laser intensities under consideration a laser plasma formed contains multicharged ions with charges Z = 26?36 that corresponds to the 3d-electron shell in the xenon case.     

Mutual quenching of Er photoluminescence under two laser excitation in GaN:Er

Erbium photoluminescence in GaN:Er was studied with above-band-gap excitation, provided by a He–Cd laser and below-band-gap excitation by a tunable Ti–Sa laser. The spectra obtained with these two lasers exhibit different spectral shapes. When both lasers are used at the same time, we observe that the Er³⁺ photoluminescence induced by each of the lasers is partly quenched by the illumination of the other laser. In this experiment, one of the lasers is modulated and a lock-in amplifier is used to filter the corresponding photoluminescence signal. The spectra recorded this way are found to be linear combinations of spectra obtained with each of the lasers used separately. This effect is explained by the presence of defects mediating the excitation towards the Er³⁺ ions. These defects act as electron traps, which can be populated by one specific laser excitation and are photo-ionized by the other laser leading to a large quenching of Er³⁺ emission.     

Phototransformations of substituted <Emphasis Type="Italic">p</Emphasis>-terphenyl upon excitation by a XeCl laser

Phototransformations of 1,8-dicarboxyisobutyl-substituted p-terphenyl in ethanol are studied upon excitation by a XeCl laser at a power density of up to 180 MW/cm² for different types of radiation and different excitation geometries. The absorption and emission spectra are studied before and after irradiation. The resources of the lasing medium, quantum yields of phototransformations, and relative quantum yields of photoproducts are determined. The transmission of the solutions at the excitation wavelength and the shape of radiation pulses are measured as functions of the pump power density.     

Laser atomic photoionization spectral analysis of element traces

A new method for determining trace amounts of elements in a substance has been developed. It is based on thermal evaporation and atomization of a substance in vacuum and detection of the released trace atoms by the laser stepwise photoionization technique. A rather effective way of detecting atoms is their multistep excitation to Rydberg states by the radiation of tunable dye lasers with their subsequent ionization by electric field pulses. The analytical laser spectrometer consisting of several tunable, pulsed dye lasers, a vacuum chamber with an atomizer and a recording system is described. The microimpurities of elements have been detected in pure materials: the contents of Na impurities in CdS crystals (2×10^?6%) and Al impurities in germanium (2×10^?7%). The detection limit of about 10^?9% was determined. A technique for direct determination of the content of elements in natural water and human blood is described. A high sensitivity, universality and other merits of the method enable it to be used in determining traces of many important elements at a level of 10^?8 to 10^?11% almost in any matrix.     

Prospects for improving energy parameters of radiation of a Cu laser excited by a transverse discharge

The production of copper vapors by an electric explosion and their excitation by a transverse discharge are considered. The results of the study are used for determining the optimal explosion regime for a copper conductor. Maximum specific parameters of a Cu laser are obtained. In an active volume of 0.5 cm³, a radiation energy of 0.92 mJ corresponding to an efficiencyη=0.25 % is obtained. For an active volume of 0.25 cm³, the specific radiation energy corresponded to 2.4 mJ/cm³. The specific power was equal to 120 kW/cm³ forη=0.16 %. An analysis of studies devoted to the given excitation method is performed that shows the possibility of further improvement of the efficiency of the laser under study.     

Two-photon-excited fluorescence in KTiOPO4 polycrystals

The spectra of two-photon-excited fluorescence in KTiOPO₄ were obtained at room temperature. A coppervapor laser was used as a source of excitation light and provided two emission lines (λ=510.6 and 578.2 nm). The laser operated at a high pulse-repetition rate (～ 10⁴ Hz) and featured a peak power of about 10⁴ W, average power of 1 W, and pulse duration of 20 ns. The fluorescence spectra of crystalline KTiOPO₄ are compared with the resonance fluorescence spectra of KTiOPO₄ at 4.2 K. The measured decay time of fluorescence was found to be less than 16 ns. The efficiency of conversion of the laser radiation to fluorescence was about 10^?10 under saturation conditions.     

Tunable TEA CO2 laser for long-range DIAL lidar

The high-power pulse-periodic TEA CO₂ laser intended for use in the structure of mobile ground-based long-range IR DIAL lidar is developed. For achievement of the highest peak power of radiation, the system of laser excitation with a voltage of ±40 kV and effective preionization allowing one to work at high pressure of various gaseous mixtures is created. A study of energetic, temporal and spatial parameters of laser radiation for used gaseous mixtures are executed at a pressure of 0.5–2.4 atm. The pulses of laser emission with an energy more than 10 J and duration on FWHM of ∼30 ns are obtained for the case of heliumless mixtures. The maximal obtained peak power of the laser radiation is 100 MW. The maximal efficiency of the laser including the energy deposited in the glow discharge is 12.6%. At the use of the selective cavity, the laser operation at 85 emission lines of the CO₂ molecule is obtained and at 60 lines, the energy of radiation exceeded 4 J.     

Luminescence and selective heat radiation of Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> upon the resonant and thermal laser excitation

Yb₂O₃ polycrystals with a size of up to 10 mm are synthesized using the sintering and melting of the ultrapure Yb₂O₃ powders by the CO₂-laser radiation with the power P _L ≤ 100 W at the wavelength λ = 10.6 μm at the melting point T _m = 2703 K, forming due to surface tension in melt, and crystallization in air. The analysis of the polycrystal microstructure using the methods of optical and electron microscopy and X- ray diffractometry shows that perfect oxide crystallites are formed in the course of crystallization after melting-through. The transformation of the luminescence and selective heat radiation (SHR) spectra of the Yb₂O₃ polycrystals is studied under the resonant excitation at λ ≈ 975 nm using a laser diode and the laser heating at the wavelength λ = 10.6 μm. When the resonant excitation power of the Yb³⁺ ions increases from 0.15 to 4.5 W, the Stokes luminescence of the Yb₂O₃ polycrystals is sequentially transformed into SHR and the thermal radiation of the crystal lattice. The transformation of the emission spectra of the Yb₂O₃ polycrystals with an increase in the laser heating intensity by about four orders of magnitude can be represented as the low-temperature heat radiation, spectral burst of the thermodynamically nonequilibrium SHR of the Yb³⁺ ions, and the high-temperature radiation of the crystal lattice. The temperature dependence of the luminescence spectra and SHR of the Yb₂O₃ polycrystals on the intensity of the laser and laser-thermal excitation and the concentration quenching of the Yb³⁺ luminescence in oxides indicate the key role of the interaction of the f-electron shell of the Yb³⁺ ions with the natural oscillations of the crystal lattice in the processes of the multiphonon excitation and nonradiative (multiphonon) and radiative (vibronic) relaxation.     

High-order stimulated electronic Raman scattering from lithium vapor

An intense radiation at 395.0 nm has been observed when lithium vapor is optically pumped in a heat pipe with a pulsed dye laser whose output wavelength is tuned near the Li 2s–4s two-photon resonance transition. The radiation is emitted in the direction along the pump laser beam. It is proposed that the 395.0 nm radiation is mainly generated through three-photon excitation and one-Raman-photon scattering followed by two-cascade (spontaneous) emission. The overall reaction mechanism can be described by a parametric six-wave mixing process. The quantum efficiency of the observed process is estimated to be of the order of 2 × 10^–6.This work was partially supported by the USC Faculty Research and Innovation Fund