A dynamic phase-conjugate mirror based on CdF<Subscript>2</Subscript> crystals with bistable centers

A four-wave phase-conjugate mirror on reflection gratings recorded in a CdF₂ crystal with bistable centers has been studied experimentally. The mirror reflectivity and speed have been measured, and the coefficient of third-order nonlinearity of this medium has been estimated. The quality of the wave reflected from the phase-conjugate mirror has been investigated in model experiments.     

New class of holographic materials based on semiconductor CdF2 crystals with bistable centers: III. Mechanisms of recording and decay of holographic gratings

The mechanisms of recording and decay of holographic gratings in CdF₂ crystals with bistable gallium or indium centers are considered. The analysis of the decay kinetics allows one to determine potentialities and conditions for using these crystals both for static recording of holograms and for holography in real time. This time scale is fairly broad and covers the time range from 1 s to 10 μs or shorter. Energy characteristics of the bistable centers, namely, the binding energies of the deep and shallow levels and heights of the barriers between them, are refined.     

A new class of holographic materials based on semiconductor CdF2 crystals with bistable centers. Part II. Growth of optically perfect crystals

The crystal-chemical processes occurring during the growth of cadmium fluoride crystals doped with bistable gallium and indium impurities and during conversion of the crystals to the semiconductor state are considered. It is found that doping with indium does not strongly affect the optical performance of the crystals, while gallium, due to its low solubility in cadmium fluoride, strongly impairs this performance. It is shown that co-doping of the crystals with highly soluble impurities (yttrium, scandium, and gadolinium fluorides) makes it possible to obtain optically perfect gallium-doped crystals.     

Dynamics of the transformation of bistable centers and lattice in CdF<Subscript>2</Subscript> crystals under femtosecond pulsed photoexcitation

The kinetics of photoinduced absorption spectra of CdF₂ crystals with bistable indium and gallium centers under femtosecond pulsed excitation has been experimentally investigated. Based on the example of indium ions, it is shown that the transmission band in the absorption spectrum of deep centers is formed for 0.8–1 ps, which significantly exceeds the photon absorption time. This process is interpreted as a result of displacement of indium ion from the initial interstitial position to a site of neighboring unit cell; the displacement velocity is estimated to be 200–250 m/s, a value close to the thermal velocity of this ion at room temperature. The times characteristic of the formation of free polarons as a result of the displacement of neighboring lattice ions have been experimentally estimated for the first time at a level of 0.8–1.2 ps. The capture times of free polarons by trivalent gallium and indium ions are estimated (5 and 10 ps, respectively), as well as the corresponding cross sections (2 × 10⁻¹⁶ and 8 × 10⁻¹⁶ cm²).     

Microwave measurements of electrical conductivity of CdF<Subscript>2</Subscript> semiconductor crystals

The electrical conductivity of CdF₂ semiconductor crystals is measured using the microwave intracavity technique at a frequency of ～35 GHz. The crystals are activated with yttrium donor impurities and indium and gallium ions forming bistable one-electron donor impurity and two-electron DX centers. The conclusion is drawn that the concentration of electrons in the conduction band of CdF₂: Ga crystals has an anomalously high value. This confirms the results obtained in earlier NMR investigations of CdF₂ semiconductor crystals at room temperature.     

A high-stability medium based on CaF<Subscript>2</Subscript>:Na crystals with colloidal color centers: I. Photothermochemical conversion of colloidal color centers in CaF<Subscript>2</Subscript>:Na crystals

Photothermochemical conversion of simple color centers (which include from one to four anionic vacancies) and highly aggregated ones in additively colored crystals of calcium fluoride doped by sodium is studied. The annealing of crystals with a low sodium content in a reducing atmosphere (additive coloration) leads to the predominant formation of simple color centers, which convert into highly aggregated centers under the joint action of heating and irradiation in absorption bands of simple centers. The irradiation of highly aggregated centers into their absorption bands and simultaneous heating causes these centers to convert into simple centers. The additive coloration of crystals with a relatively high sodium content leads to the predominant formation of highly aggregated centers. The heating of these crystals along with the irradiation in absorption bands of highly aggregated centers causes these centers to convert into simple centers. The formation of different color centers in the course of additive coloration of crystals with different impurity content and different results of photothermochemical conversion of centers in these crystals are connected with the dual action of impurities. Anion vacancies, which compensate the charge of the impurity alkali metal, facilitate the aggregation of color centers. At the same time, the alkali impurity stabilizes simple color centers.     

Relaxation processes upon photoexcitation of semiconductor CdF<Subscript>2</Subscript> by laser pulses

The microwave-cavity-based technique is used to study the processes of photoionization of electrons from donor levels to the conduction band in semiconductor CdF₂ crystals doped with Y, In, or Ga. The samples were excited by periodic pulses of Nd-laser (λ = 1.06 μm, pulse width ～10 ns) in the temperature range 6–77 K. The transient processes were detected in the absorption and dispersion modes related to variation of the imaginary and real parts of the complex permittivity ?₁ ? i?₂ induced by the light pulses. The observed signals consisted of short peak at t ～ 0, approximately 40–70 ns in length, and a long tail with a duration of ～100 ms. The short peak is likely to be related to the stay of the photoexcited carriers in the conduction band, while the long tail is associated with the processes of excitation relaxation after the electrons coming back to the donor levels of the impurity band. The weak temperature dependence of the width of the peak at t ～ 0 is explained by the tunneling mechanism of relaxation of electrons through the energy (or, probably, spatial) barrier separating the bound and free states of the carriers in the semiconductor CdF₂.     

Recording dynamic holograms in a semiconductor crystal CdF<Subscript>2</Subscript>:In

The kinetics of decay of a phase hologram in a semiconductor CdF₂ crystal with bistable In centers is studied. Kinetic constants of the hologram decay are found, and the potential relief of the bistable center is plotted. The resolving power of the crystal is evaluated and recording of a transparency is demonstrated.     

A highly stable holographic medium based on CaF<Subscript>2</Subscript>:Na crystals with colloidal color centers: III. Properties of holograms

The properties and characteristics of the holograms recorded on colloidal centers in calcium fluoride crystals have been considered. The amplitude-phase character of the holograms, with a dominance of the amplitude component, is demonstrated. It is shown that the effective thickness of the holograms is somewhat smaller than the sample thickness. The diffraction efficiency and resolution of the holograms have been determined. A characteristic feature of the holograms considered is their high stability.     

Location of the energy levels of the rare-earth ions in BaF<Subscript>2</Subscript> and CdF<Subscript>2</Subscript>

The location of the energy levels of rare-earth (RE) elements in the energy band diagram of BaF₂ and CdF₂ crystals is determined. The role of RE ³⁺ and RE ²⁺ ions in the capture of charge carriers, luminescence, and the formation of radiation defects is evaluated. It is shown that the substantial difference in the luminescence properties of BaF₂: RE and CdF₂: RE is associated with the location of the excited energy levels in the band diagram of the crystals.     

A highly stable holographic medium based on CaF<Subscript>2</Subscript>:Na crystals with colloidal color centers: II. Mechanisms of hologram recording and erasure

The mechanisms of formation and erasure of holographic gratings recorded in calcium fluoride crystals with a low sodium content have been considered. It is found that in the case of recording by UV radiation, an effective holographic grating is formed due to the spatial modulation of the colloidal centers arising under the action of the radiation at temperatures about 200°C. The decisive role of photoinduced diffusion of anion vacancies in this modulation is demonstrated.     

Effective room-temperature broad-band recording of 3D dynamic holograms in CdF<Subscript>2</Subscript>:Ga crystals

Photochromic CdF₂:Ga crystals with bistable impurity centers were effectively used for the dynamic recording of holograms and readout over the visible and near IR spectral regions at spatial frequencies of up to 5000 mm^?1 at room temperature. The diffraction efficiency of the dynamic holograms was as high as 60% at maximum and exceeded 1% when the beams’ intensities were in the ratio 1:100. As one goes from the low temperatures (≤200 K) to 300 K, the peak diffraction efficiency of the dynamic holograms decreases approximately by a factor of 1.5, while the speed of their response and photosensitivity in the long-wavelength spectral region increases by more than an order of magnitude. For the sake of comparison, the dynamic holograms were recorded under the same conditions as the widely used electrooptical SBN crystals. Comparative analysis ascertained a unique combination of the useful features offered by CdF₂:Ga crystals in holography.     

Fabrication of holographic diffraction gratings based on As<Subscript>2</Subscript>S<Subscript>3</Subscript> Layers

The formation of holographic diffraction gratings based on As₂S₃ layers is investigated. The variation of the groove profiles with exposure is studied by atomic force microscopy. The spectral curves of the diffraction efficiency are taken, and a relationship between these curves and grating surface relief is analyzed.     

EPR of trivalent iron centers in SrF<Subscript>2</Subscript>: Fe crystals

Paramagnetic centers of three types are found in SrF₂: Fe(0.2 at. %) crystals. Two types are observed in the untreated crystals, and the third type appears only in the crystals irradiated by x-rays. The EPR spectra of one type of centers in a nonirradiated crystal and of the centers that appear after irradiation are described by the orthorhombic Hamiltonians with an effective spin S _eff = 5/2. In both cases, the centers are observed at 4.2 and 77 K. The principal axes of the spin Hamiltonians for them are along the 〈001〉, 〈1 \(\overline 1 \) 0〉, and 〈110〉 axes. However, the fine-structure parameters of their EPR spectra differ significantly. An analysis of the superhyperfine structure (SHFS) of the EPR spectra shows that the radiation center forms through substitution of a Fe²⁺ ion for a Sr²⁺ cation. Under x-ray irradiation, the Fe²⁺ ion transforms into the Fe³⁺(⁶ A _1g ) state and is displaced to an off-center position along the C ₂ axis of its coordination cube. The absence of a SHFS in the EPR spectra of the orthorhombic centers in a nonirradiated crystal makes it impossible to determine their molecular structure unambiguously. The most probable model is proposed for this structure. The EPR spectra of centers of the third type were observed only at 4.2 K, and the structure of these centers was not studied.     

Jahn-teller chromium ions in CdF<Subscript>2</Subscript> and CaF<Subscript>2</Subscript> crystals: An EPR spectroscopic study in the frequency range 9.3–300 GHz

The bivalent chromium impurity centers in CdF₂ and CaF₂ crystals are investigated using electron paramagnetic resonance (EPR) in the frequency range 9.3–300 GHz. It is found that Cr²⁺ ions in the lattices of these crystals occupy cation positions and form [CrF₄F₄]^6? clusters whose magnetic properties at low temperatures are characterized by orthorhombic symmetry. The parameters of the electron Zeeman and ligand interactions of the Cr²⁺ ion with four fluorine ions in the nearest environment are determined. The initial splittings in the system of spin energy levels of the cluster are measured.     

Low-temperature Raman spectra of Hg<Subscript>2</Subscript>(Br,I)<Subscript>2</Subscript> mixed crystals

Raman spectra of Hg₂(Br,I)₂ mixed crystals were studied. The spectra revealed multimode behavior of optical vibrations, which were assigned to the existence in these crystals of molecules of three types, namely, Hg₂Br₂, Hg₂I₂, and Hg₂BrI. The spectra exhibit a manifestation of phase transition effects associated with soft modes, the density of states of IR-active vibrational branches, and of nanoclusters, whose nucleation is induced by the Br-Hg-Hg-I dipole molecules.     

The Urbach tails and optical absorption in layered semiconductor TlGaSe<Subscript>2</Subscript> and TlGaS<Subscript>2</Subscript> single crystals

TlGaSe₂ and TlGaS₂ single crystals were grown by the modified Bridgman-Stockbarger method. We report the result of an experimental study of the optical absorption of TlGaSe₂ and TlGaS₂ crystals. The absorption measurements were performed in steps of 10 K. The direct and indirect band gaps for TlGaSe₂ and TlGaS₂ samples were calculated as a function of temperature. The phonon energies in TlGaSe₂ and TlGaS₂ crystals were calculated as (39±4) and (9±4) meV at 240 K, respectively. At 10 K, direct and indirect band gaps were found as 2.294 and 2.148 eV for TlGaSe₂, 2.547 and 2.521 eV for TlGaS₂ crystals, respectively. The abrupt changes were observed in the direct and indirect band gaps in the some temperature ranges. These changes were interpreted as phase transformation temperatures. The steepness parameters and Urbach energy for TlGaSe₂ and TlGaS₂ samples increased with increasing sample temperature in the range (10–320) K.     

Optical properties of CdF<Subscript>2</Subscript> in a wide energy range

For either of the two reflection spectra of cadmium difluoride that are known from experiments, a complete set of the fundamental optical functions is calculated in the energy range 4–45 eV with the Kramers-Kronig relationships. The basic features of the optical spectra are established, and a hypothesis for their origin is suggested based on the known theoretical results for the band structure.     

Local structure of Gd<Superscript>3+</Superscript> and Eu<Superscript>2+</Superscript> impurity centers in a CdF<Subscript>2</Subscript> crystal

The local crystal structure of Gd³⁺ and Eu²⁺ cubic impurity centers in cadmium fluoride is calculated within the shell model in the pair potential approximation. The local compressibility of the cationic and anionic sublattices of the host lattice is determined in the vicinity of the Gd³⁺ (Eu²⁺) impurity ion.     

Electrical instability of LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> crystals

The temperature behavior of I-U curves and the field and temperature dependences of the electrical resistivity and dielectric permittivity of crystals of the LiCu₂O₂ phase have been studied. It was established that the crystals belong to p-type semiconductors and that their static resistivity in the range 80–260 K follows the Mott law ρ=Aexp(T₀/T)^1/4 describing variable-range hopping over localized states. At comparatively low electric fields, the crystals exhibit threshold switching and characteristic S-shaped I-U curves containing a region of negative differential resistivity. In the critical voltage region, jumps in the conductivity and dielectric permittivity are observed. Possible mechanisms of the disorder and electrical instability in these crystals are discussed.