Anti-stokes emission in an additively colored KCl caused by excitation with a Nd: YAG laser

Abstract

We have observed the quadratic laser power dependence of two anti-Stokes emission bands at 15100cm^?1 and 14000cm^?1 in an additively colored KCl when excited with a Nd:YAG laser. The cross section of two photon absorption to cause the 14000cm^?1 band is estimated to be 4×10^?40cm⁴sec. We have observed that emission intensities are dependent on temperature in the range between 4. 2K and 130K. They are enhanced by means of optical bleaching and also with the increase of the F concentration. From a qualitative analysis of these observations, we propose that these emissions are correlated to the loose aggregation of the F centers.     

A study of M center formation in additively colored KCl

Additively colored crystals of KCl were cooled rapidly from a high temperature and then exposed to light absorbed by the F band. The rate of growth of the M band was initially low and increased markedly during the first part of the irradiation giving rise to what is called a “delay period” in the M center formation. During this delay period it was observed that 1. the quantum efficiency of F center fluorescence decreases by a factor of 3 or 4, and 2. the ability to convert F centers to F′ centers is reduced by about a factor of 3. These effects are interpreted as arising because of an appreciable decrease in the average nearest neighbor F center separation during this period. Results are also presented concerning the kinetics of M center formation, oscillator strengths of the M center transitions, the degree of polarization of the M center fluorescence, and the transfer of energy from an excited F center to an M center.     

Photoinduced dichroism in additively coloured KCl: Na at room temperature

Spectral and kinetic investigations of the reversible photoinduced dichroism in KCl : Na containing F_A and F_A + F_B centers are done. It is suggested that in presence of F_B centers the observed 〈100〉 and 〈110〉 dichroism is connected with the F_B(II) centers.     

Optimization of the recording conditions for holograms recorded in additively colored KCl crystals

The dependences of the characteristics of volume holograms recorded in additively colored crystals of potassium chloride on the spatial frequency and the recording temperature are studied. It is found that these dependences are nonmonotonic, which indicates the presence of nonlocal mechanisms of holographic recording in the crystals. These mechanisms involve diffusion and drift processes of redistribution of color centers between the nodes and antinodes of a recorded interference pattern. The results obtained are used to determine the recording conditions providing the maximum possible diffraction efficiency of the holograms.     

Effect of UV radiation on the diffraction efficiency of holograms when recording in additively colored KCl crystals

We have shown that the increase in diffraction efficiency of holograms recorded in additively colored crystals as a result of preliminary UV irradiation may be due not only to an increase in the concentration of F centers but also to radiolysis of impurity hydroxyl ions, which are always present in nominally pure crystals. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 85–87, January–February, 2008.     

Interband excitation induced absorption by deep impurities in GaP

Transient optical absorption at energies below the band gap in GaP crystals following pulsed interband excitation was observed. The additional absorption attained its maximum about 25 μsec after the excitation pulse and lasted about 100 μsec. The spectral dependence of the rise time and decay time of the absorption and that of the induced optical cross section were determined. The results suggest that the induced absorption is caused by several deep impurity levels, which are most probably due to Cu, being populated by the recombination of excited electrons.     

Diffusion of water in additively colored potassium iodide crystals

The diffusion of water into additively colored potassium iodide has been studied in the range 15–45°C. Penetration depths, measured by decrease in the F-band absorption, increase with $\text{t}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. The diffusion coefficient, D = 0·58 exp (?6496/T) cm² sec^?1 agrees very well with that determined by other workers. The Henry's law constant, $\text{K =}\text{C}{}_0\text{p}{}_{\mathrm{w}}\text{= 1·3 × 10}{}^9\text{exp}\text{(+4882/T)}\text{cm}{}^{\mathrm{?3}}\text{torr}{}^{\mathrm{?1}}$ implies a water concentration of C₀ ? 10¹⁷ molecules per cm³ in the surface of KI crystals in equilibrium with an environment at 25°C and 35 per cent relative humidity. The large C₀ makes penetration very rapid. Diffusion occurs by interstitial migration of water molecules with an entropy of activation of 9.4 cal/mol deg and an enthalpy of activation of 12·9 kcal/mol.     

Impurity induced far infrared absorption in doped NaCl and KCl

Far infrared absorption measurements with NaCl single crystals doped with AgCl and CuCl respectively show resonant band mode lines at 4.2 ?K at 52.5 cm^?1 and 23.6 cm^?1 respectively. Their measured halfwidths are 10 cm^?1 and 0.7 cm^?1 respectively. The halfwidth of the NaCl: Cu⁺ line increases strongly with increasing temperature. In NaCl:Mg⁺⁺, NaCl:Ca⁺⁺ and NaCl:Ag⁺ a temperature independent broad absorption is observed above 50 cm^?1, which behaves roughly as the density of phonon states in the unperturbed crystal. In KCl: Cu⁺ and KCl: Br^? a temperature independent absorption with a maximum at 108 cm^?1 is obtained corresponding to an expected region of high density.     

Self-trapped excitons perturbed by Na in KCl crystal

The optical absorption and emission induced by electron irradiation of Na⁺-doped KCl was measured and new absorption and emission bands which decay at the same time constant were found and ascribed to the self-trapped exciton perturbed by Na⁺ impurity.     

Paramagnetic susceptibility of additively colored CdF2:In photochromic crystals

The paramagnetic susceptibility K _para of CdF₂:In crystals with metastable indium centers has been measured in darkness after photobleaching the crystals in visible light in the temperature interval 4–300 K. For crystals cooled in darkness to liquid-helium temperature K _para is wholly determined by the accompanying impurity Mn²⁺ with magnetic moment J=5/2. Illumination of the crystals leads to the appearance of an induced signal δ K _para due to the formation of centers with J=1/2. The results of the experiments indicate the absence of paramagnetism in the deep state of the indium center and its existence in the shallow (donor) state, i.e., they confirm the two-electron (negative-U) nature of the deep indium level in CdF₂. Fiz. Tverd. Tela (St. Petersburg) 39, 1205–1209 (July 1997)     

Electron-trapping centers in additively colored alkali halide crystals doped with divalent impurities

A study has been made of the temperature dependences of the dielectric loss and structure-sensitive conductivity of additively colored KCl single crystals having a Sr²⁺ impurity. The optical absorption spectra were recorded before and after heat treatment of the colored crystals. A model for Z-centers is proposed on the basis of the results.Translated from Izvestiya VUZ. Fizika, No. 10, pp. 26–29, October, 1969.In conclusion the author thank Professor E. K. Zavadovskaya for suggesting thistopic and for discussion of the results.     

Stochastic oscillations induced by colored noise

We investigate the effect of additive colored noise on Hopf-bifurcating systems in the limit of small correlation times. It is shown that it results in an advancement of the oscillating regime. Several examples are studied.     

Transient absorption spectra of tetraazaporphine and its reduced derivatives upon picosecond excitation

Transient absorption spectra of tetraazaporphine and substituted derivatives of tetraazachlorin and tetraazabacteriochlorin were obtained upon picosecond excitation with various probe pulse delay times in order to resolve the open question about the reasons for fluorescence quenching in tetraazaporphine hydrogenated derivatives. The quantum yield of triplet state T ₁ formation has been estimated. It has been shown that in all investigated cases radiationless de-excitation of the fluorescent level S ₁ occurs by two channels: S ₁ ⤳ T ₁ and S ₁ ⤳ S ₀, the latter being predominant. As the S ₁ level becomes lower, the transition rate for this channel increases. For tetraazaporphine and its derivatives, a quasi-photochemical mechanism is proposed that accounts for the anomalous efficiency of the S ₁ ⤳ S ₀ channel in the dissipation of the electronic excitation energy.     

Magnetic circular dichroism in the absorption of theF A centre in KCl doped with Li and Na

Summary The spin-orbit structure ofF _A centres in KCl:Li and KCl:Na have been studied by means of the magnetic circular dichroism. Due to theirC _4V symmetry theF _A centres have two different spin-orbit parameters, Δ^‖ and Δ^⊥, which only in the KCl:Li case follow the relation: Δ^‖ < Δ_F < Δ^⊥ as expected from the theory. For a close comparison we have also reported our measurement of the spin-orbit coupling ofF centre in KCl. The spin-orbit parameters of theF andF _A centres have been determined using the method of moment.     

Photothermal bleaching of Ca colloids in additively colored CaF2

Cation colloidal particles in additively colored CaF₂ crystals have been bleached with light in the colloidal band. At 360 K the colloid band decreases and a new band in the near i.r. region is created which is probably due to highly coagulated F-centers. At 430 K the colloidal band is destroyed at the same time that a band appears at 383 nm. This band has been tentatively associated to a kind of F-center. Photochromic conversions between colloids and i.r. bands at 360 K and between colloids and 383 nm band at 430 K have been obtained. A mechanism for the bleaching of colloids has been proposed.     

Transformation of color centers during hologram recording in an additively colored CaF2 crystal

When holograms are recorded on color centers in calcium fluoride crystals, these centers undergo spatial redistribution in the crystal bulk, which is accompanied by their transformation. The nature of this transformation has been investigated by optical spectroscopy and confocal scanning microscopy. It is shown that, under the recording conditions we used, the degree of center aggregation increases in both minima and maxima of the interference field in which the recording performed. The enhanced aggregation in field minima is caused by the increase in the concentration color centers, while the additional aggregation in maxima is determined by the specific conditions of hologram recording: the wavelength and power density of recording radiation and the crystal temperature.