A. I. Ryskin

CdF2:In: A novel material for optically written storage of information

We demonstrate that semiconducting CdF2 crystals doped with indium is an efficient medium for optical storage of information in static and dynamic regimes. A metastable phototransformation of 1018 cm−3 In centers from a localized deep state to a hydrogenlike shallow state leads to a change of the refractive index Δn of about 10−4 for the probe beam at the wavelength of 500 nm. The diffraction efficiency is temperature dependent due to spontaneous decay of the grating caused by thermal recovery of the In impurity from the metastable hydrogenic state to the localized ground state.

Mechanisms of writing and decay of holographic gratings in semiconducting CdF2:Ga

We consider the mechanisms responsible for the photoinduced change in the optical properties of semiconducting CdF2 crystals with metastable Ga impurities forming DX centers. Unlike the case of compound semiconductors with DX centers (GaAlAs:Si, GaAlAs:Te, CdZnTe:Cl), this change is caused not by free electrons but by a redistribution of electrons between deep and shallow localized states. The resulting modification of the refractive index of the crystal allows writing of persistent holographic gratings at temperatures up to 200 K, high for this class of holographic materials. Holographic characteristics of CdF2:Ga crystals such as refractive index change, sensitivity, and grating decay are described.

Paramagnetic susceptibility of semiconducting CdF2:In crystals: Direct evidence of the negative-U nature of the DX-like center

Paramagnetic susceptibility κpara of CdF2 crystals with bistable In centers is measured in the temperature range T=4–300 K. For crystals cooled in the dark down to liquid helium temperature, κpara is determined by the trace Mn2+ impurity. Illumination of the sample by the ultraviolet-visible light results in the appearance of the photoinduced δκpara signal due to formation of centers with the magnetic moment J=1/2. This gives a clear evidence of an absence of paramagnetism in the deep state of the bistable In center and its presence in the shallow state, i.e., proves the negative-U nature of the deep state.

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.

DX center gratings in real-time holography

Abstract CdF2 crystals doped with Ga or In are the most promising representative of new class of holographic materials, in which a photo-induced change in the density of bistable (DX center) states in a semiconductor crystal produces a local change of its refractive index. Mechanism of holographic gratings decay is considered. Analysis of the decay kinetics shows opportunities of CdF2xa0:xa0Ga, CdF2xa0:xa0In as materials of real-time holography with response times in the range of seconds to 1xa0ms in CdF2xa0:xa0Ga or to 100xa0ns or less in CdF2xa0:xa0In.

Donor impurities and DX centers in the ionic semiconductor CdF2

Group-III impurities in the wide-gap ionic crystal CdF2 are examined. After being heated in a reducing atmosphere, crystals with these impurities acquire semiconductor properties, which are determined by electrons bound in hydrogen-like orbitals near an impurity. Besides these donor states, nontransition impurities form “deep” states accompanied by strong lattice relaxation, i.e. they are strongly shifted along the configuration coordinate. These states are a complete analog of DX centers in covalent and ionic-covalent semiconductors. The difference of the behavior of nontransition impurities from that of transition and rare-earth impurities is analyzed. This difference is attributed to the character of the filling of their valence shells by electrons. A deep, multilevel analogy is drawn between the properties of deep centers in typical semiconductors with an appreciable fraction of a covalent bond component and in predominantly ionic crystal CdF2 with semiconductor properties.

Dynamic grating recording in semiconductor CdF 2 : Ga, Y

Cadmium fluoride codoped with Ga and Y is characterized as a medium for phase grating recording. The dependence of the three-dimensional grating diffraction efficiency on the intensity of the recording light, on the polarization of the recording light, on the fringe contrast, and on the spatial frequency of the fringe was measured together with the dynamic characteristics (grating decay time and grating buildup time). The crystal behaves as a χ(3) medium with its refractive-index change depending on linear intensity and with nearly exponential buildup–decay temporal characteristics for moderate intensities of the green recording light at room temperature.

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.

Dynamic reflection holograms in CdF2 crystals with bistable centers

The diffraction efficiency and the recording and relaxation times of dynamic reflection holograms, recorded in CdF2 crystals with bistable centers are studied experimentally in the temperature range 20–100°C. In the model experiments which measured the quality of the wave reflected from the hologram, the dynamic wavefront distortions are demonstrated to be efficiently compensated using a holographic corrector based on these crystals. CdF2 crystals with bistable centers are likely to be useful in solving problems of correction of laser light wavefront and image correction in observation telescopes with nonideal primary mirrors.

Dielectric response of semiconducting and photochromic CdF2 on microwaves

CdF 2 is the only highly ionic dielectric crystal, which can be converted into n-type semiconductor via doping with column-III elements and annealing in the reduction atmosphere. Among donor impurities, Ga and In dopants form DX centers with the shallow metastable state and ground strongly localized state, making these crystals photochromic. We studied the dielectric permittivity, e = e 1 -ie 2 , of CdF 2 :Ga, CdF 2 : In, and also non-photochromic CdF 2 :Y in the microwave range (λ8mm) at low temperatures (T) down to 1.8K in the darkness and after illumination by the ultraviolet-visible light. Illumination of CdF 2 : Ga and CdF 2 :In results in increase of both the dielectric constant, e 1 , by Δe 1 = (0.5-1.4) and the dielectric loss factor, e 2 , by about an order of magnitude. At T = 1.8 K the low-field dielectric loss factor in these crystals and also in CdF 2 : Y, e 2 = 0.1-0.3, may be decreased by an order of magnitude with increase of the microwave field power; however, e 2 ceases to depend on the field at T >4K. These features are explained on the basis of Tanaka theory of resonant saturated absorption of ionized donor pairs, modified here to cover also far infrared range of spectrum. The study shows, that maximum available concentration of neutral donors in CdF 2 could never exceed 10 19 cm -3 and the compensation degree be less than 0.5 at any doping level because of the existence of impurity clusters. These clusters store an excessive impurity and are inexhaustible sources of interstitial F- ions.