Paratellurite Crystal Research Articles

Study of the defect structure of paratellurite crystal using multiwave diffraction and normal X-ray diffractometry methods

Multiwave X-ray diffraction has been used to study the structure of crystalline samples. A cycle of local measurements of rocking curves (RCs) of the (220) and (371) reflections under conditions of multiwave diffraction (MD) has been carried out in a paratellurite crystal. The data obtained are used to compare the sensitivity to structure defects of two-beam diffraction with that of MD, which makes it possible to study the X-ray wave phase. The comparison has revealed a higher ability of the phase-sensitive method to detect defects.

Optical parameters of paratellurite crystals

Refractive indices and transmittances of paratellurite single crystals have been measured. The specific optical rotation is determined and the Verdet constants are calculated for a radiation wavelength of 355 nm.

Propagation of acoustic beams in a paratellurite crystal

A general solution to the problem of acoustic beam diffraction in an anisotropic medium has been obtained. The effect of acoustic anisotropy of paratellurite crystal on the structure of beams propagating in the XY crystallographic plane has been analyzed. Ray spectra are calculated for different propagation directions of ultrasound. The effects of beam focusing, defocusing, and autocollimation caused by acoustic anisotropy have been analyzed.

Acousto-optic study of the unusual cases of reflection of bulk elastic waves in a paratellurite crystal

The propagation and reflection of bulk ultrasonic waves in a paratellurite crystal have been investigated by the acousto-optic method. Simultaneous excitation of two acoustic waves by one piezoelectric transducer has been observed in a cell of complex, specially chosen configuration. Maximally efficient conversion of the energy of a quasi-longitudinal wave into a single reflected quasi-shear wave with a walkoff angle 72° is implemented for the reflection from a free crystal boundary. The wave beam compression by a factor of 7 is observed for this reflection. A reflection (close to backward) of waves with an angle of spatial separation of the incident and reflected beams equal to 8° is also implemented. The acousto-optic figure of merit values of the medium are calculated for arbitrary propagation and polarization directions of the interacting light and ultrasonic waves. Possible applications of the effects studied in acousto-optics and acoustoelectronics are indicated.

Bragg diffraction of multicolor radiation in a paratellurite crystal

Two variants of acousto-optic interaction of multicolor radiation with one acoustic wave propagating in a paratellurite crystal both at an angle to the optical axis of the crystal and orthogonally to the axis are analyzed. It is shown that, in the band of wavelengths 0.475–0.525 μm, both variants have practically identical characteristics. However, the preference is given to the second variant, because it is devoid of the drift of the acoustic wave, which makes it possible to produce acousto-optic cells with smaller dimensions.

Acousto-optic cell based on paratellurite crystal with surface excitation of acoustic waves

An acousto-optic cell based on a paratellurite (TeO2) crystal, in which bulk acoustic waves are excited directly from the surface due to an intrinsic piezoelectric effect in the material, has been studied. The bulk shear acoustic waves with a frequency of 50 MHz propagate along the [001] and [110] axes with a polarization along the \(\left[ {\bar 110} \right]\) axis. The ultrasound has been excited by a simple system of two electrodes formed on one face of the crystal. Characteristics of the acousto-optic cell have been determined and the parameters of acoustic waves have been measured at 633 nm by optical beam diffraction on the acoustic diffraction grating.

Acousto-optic interaction in crystals with large acoustic anisotropy

The Bragg diffraction of light from a sinusoidal phase grating induced in an anisotropic medium by an acoustic wave has been studied theoretically for the case of a large acoustic energy walk-off. Based on the modified coupled-wave equations, the frequency and angular characteristics of an anisotropic diffraction in a paratellurite crystal have been calculated. It is shown that the acoustic beam walk-off significantly changes the angular and frequency ranges of acousto-optic interactions.

A multifunctional acousto-optic splitter of dichromatic radiation

The splitting of dichromatic radiation propagating along the optical axis of a crystal resulting from the Bragg acousto-optic diffraction at a single acoustic wave was considered. Several splitting variants, which can be implemented with a single acousto-optic cell, were revealed. Experiments, in which an Ar laser served as a dichromatic radiation source and a paratellurite single crystal was used as an acousto-optic media, confirmed all the splitting variants predicted for dichromatic radiation.

The growth and mechanical properties of paratellurite single crystals at high temperatures

α-TeO2 single crystals have been grown by the Czochralski method on specially developed systems with automatic growth control. The mechanical properties of crystals under compression in the [100], [110], and [221] directions at a constant strain rate of about 10−4 s−1 in the temperature range T = 733−993 K have been investigated. The distribution of plastic shear strain in deformed samples is has been experimentally studied. Manifestations of localized strain in covalent paratellurite crystals at T > 900 K are revealed for the first time. The active slip systems along which localized shear band are oriented are determined. The temperature dependence of the critical stress of transition to localized flow is investigated.

Peculiarities of X-ray multiwave diffraction in paratellurite crystals (TeO2)

Peculiarities of X-ray multiwave diffraction in paratellurite crystals (TeO₂)

Acoustogyration diffraction of optical waves: case of SiO2 and TeO2 crystals

AbstractInteraction of acoustic and light waves with accounting for elastooptic and elastogyration effects is theoretically described. Collinear acoustogyration diffraction in quartz and paratellurite crystals is experimentally investigated and thoroughly analyzed. Piezogyration effect is experimentally studied for TeO2 crystals. The acoustogyration efficiency and the acoustogyration figure of merit calculated for a number of crystals (GaAs, TeO2 and SiO2) are shown to be too small for experimental detection. On the other hand, we demonstrate that the light diffraction at periodical distribution of the imaginary part of dielectric permittivity related to the piezogyration effect should, in principle, be observed for the case of interaction of optical waves with enantiomorphous ferroelastic domain structure and in cholesteric liquid crystals.

Broadband acoustooptic diffraction of two-wavelength light in paratellurite

We suggest acoustooptic deflector operating at two different light wavelengths. Geometry of paratellurite crystal is chosen such that it simultaneously satisfies the Bragg conditions at those wavelengths. This enables beam steering at either of the wavelengths in the same predefined angular range. The wavelengths are switched by means of changing the sound frequency, so that only one of the wavelengths is available at the output of the deflector at each specific moment of time. A technique for calculations of parameters of the device is described. The deflector has been successfully constructed and investigated experimentally.

Cascaded acousto-optical system for modulation of unpolarized light

A cascaded system of two acousto-optical cells is proposed for equalization of multichannel optical signal satisfying coarse wavelength-division multiplexing standard. Two similar acousto-optical modulators for unpolarized light on the basis of 10 degrees-cut paratellurite crystals were used in a free-space gap of a fiber line. The system controlled intensity of several optical carriers in the spectral range from 1200 to 1700 nm. The device was tested in a four-channel regime of operation in the range 1510-1570 nm. Overall optical insertion losses did not exceed -2 dB and less than 1 W electric power per channel was necessary for -20 dB intensity attenuation of the signal in a continuous operation mode. Compensation for birefringence in paratellurite provided diffraction regime that was insensitive to polarization of light. Interchannel crosstalk was less than -10 dB.

Quasi-collinear tunable acousto-optic paratellurite crystal filters for wavelength division multiplexing and optical channel selection

Quasi-collinear acousto-optic interaction is studied in acoustically and optically anisotropic paratellurite crystals. The possible applications of this interaction in acousto-optic tunable filters with a high spectral resolution are discussed. Different modifications of devices are compared and variants of devices intended for processing light beams and selection of light signals in fibreoptic communication systems with wavelength division multiplexing (WDM) at λ ≃ 1550 nm are considered.

Characteristics of Cornu depolarisers made from quartz and paratellurite optically active crystals

Cornu depolarisers made from quartz and paratellurite crystals have been studied. Numerical calculations and experimental data demonstrate that such depolarisers make it possible to significantly alleviate the major drawback to birefringent wedge depolarisers: the strong dependence of the polarisation of the output beam on the input polarisation orientation. We have examined the polarisation extinction ratio in the depolarisers as a function of the rotatory power of the crystals, depolariser length, beam diameter, and power distribution across the beam, and have determined the angle between the circularly polarised output beams in the far field. Data on the optical rotatory power of quartz and paratellurite crystals have been systematized, and the results have been used to derive dispersion formulas for these materials in the visible and near-IR spectral regions.

Taylor vortices formed in the melt during paratellurite crystal growth

The hydrodynamics of tellurium dioxide melt during Czochralski growth of paratellurite crystals with a diameter up to 80 mm was experimentally investigated. The images of the melt surface during crystal growth are obtained. It is shown that a stable system of Taylor vortices in the form of two convection cells is formed at definite Reynolds numbers.

Features of light attenuation in crystals under violation of the Bouguer law

A computer simulation and measurements of the light transmittance of germanium and paratellurite crystals of different thickness were used to show that, at scattering probabilities of photons comparable to their absorption probabilities, the standard methods for calculating light extinction coefficients on the basis of the Bouguer law lead to rough errors in estimation of the optical quality of a material.

Acousto-optic investigation of propagation and reflection of acoustic waves in paratellurite crystal

The paper presents results on acousto-optic investigation of unusual acoustic phenomena taking place in crystals possessing strong anisotropy of elastic properties. Advantages of the applied method of analysis are demonstrated by the example of the commonly used acousto-optic material tellurium dioxide. The major goal of the research consists of experimental verification of theoretical conclusions related to peculiar cases of acoustic propagation and reflection recently observed in the crystalline material. In particular, the case of glancing incidence and the following reflection of elastic energy from a free boundary separating the paratellurite crystal and the vacuum is examined in the paper. It is shown in the acousto-optic experiment that, in the case of glancing incidence, energy flow of a reflected acoustic wave may propagate practically in a reverse direction with respect to an incident wave. It is also proved that strong elastic anisotropy of the crystal is responsible for the unusual propagation and reflection of the acoustic waves. The research confirms the conclusion that the examined acoustic effects may be useful in development of new acousto-optic devices.

Short access time acousto-optic deflector based on two cascaded Paratellurite devices

Acousto-optic deflectors (AODs) are used in many applications requiring optical scanning devices with various specifications regarding access time, resolution and wavelength. High time-bandwidth products can be achieved with AODs made in a tellurium dioxide (TeO2) crystal. As paratellurite crystals are available with relatively large dimensions, the time-bandwidth product can be increased by enlarging the optical aperture. However, the access time is increased which is not suitable for random access scanning mode. A new laser scanning system is then presented based on two wide-band paratellurite acousto-optic deflectors. Anisotropic interactions take place under two different tangential phase matching (TPM) configurations in such a way that the acousto-optic bandwidths add up. The optical arrangement of the two cascaded AODs is detailed. The feasibility of such a cascade deflection system has been demonstrated for the green wavelength λ= 514 nm of an argon laser. The total frequency bandwidth is Δft = 100 MHz, equally distributed between the two acousto-optic deflectors. The total angular scan at the output is Δθt = 4.5 ° leading to more than 120 resolvable spots for a 1 mm truncated Gaussian beam and a short access time (as low as 1.5 μs).

Acoustooptic interaction of two light beams in a paratellurite crystal

Double acoustooptic diffraction of light in a paratellurite crystal is investigated experimentally and theoretically. The anisotropic Bragg interaction with three diffraction maxima is considered in the case when two monochromatic light beams are incident on the crystal simultaneously. For each beam, double diffraction is realized, so that the propagation direction of one of the beams coincides with the propagation direction of the second-order diffracted ray for the other beam. It is shown that the diffraction pattern consists of three diffraction maxima: two of these are formed by the zero- and second-order diffracted rays, while the third maxima is formed by the first-order diffracted rays. The possibility of summation of the radiation intensities at the diffraction maxima is analyzed.