Bismuth Silicon Oxide Crystal Research Articles

In-vivo measurement of radio frequency electric fields in mice brain

With the development of novel technologies, radio frequency (RF) energy exposure is expanding at various wavelengths and power levels. These developments necessitate updated approaches of RF measurements in complex environments, particularly in live biological tissue. Accurate dosimetry of the absorbed RF electric fields (E-Fields) by the live tissue is the keystone of environmental health considerations for this type of ever-growing non-ionizing radiation energy. In this study, we introduce a technique for direct in-vivo measurement of electric fields in living tissue. Proof of principle in-vivo electric field measurements were conducted in rodent brains using Bismuth Silicon Oxide (BSO) crystals exposed to varying levels of RF energy. Electric field measurements were calibrated and verified using in-vivo temperature measurements using optical temperature fibers alongside electromagnetic field simulations of a transverse electromagnetic (TEM) cell.

Generation and decay of surface charge in a background charge environment during surface dielectric barrier discharges

Previously, we reported on the decay mechanism of the positive charge filaments generated by the first microdischarge during surface dielectric barrier discharges using a high-speed camera combined with the Pockels effect of a bismuth silicon oxide crystal. With the existing work, the next milestone is achieved by using this powerful approach to the generation and decay of surface charge in a background charge environment. It is found that the generation locations of new positive charge filaments are influenced by the surface charge memory effect. The background charge environment is an important factor in the decay of surface charge.

Spatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges in helium with nitrogen admixtures

This work reports on the spatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges driven by sine-wave feeding voltage at a frequency of in helium with small nitrogen admixtures. The discharge gap of is bounded by glass plates on both plane electrodes. Priority is given to the lateral discharge inhomogeneities, underlying volume- and surface-memory effects, and the breakdown mechanism. Therefore, relevant processes in the discharge volume and on the dielectric surfaces were investigated by ICCD camera imaging and optical emission spectroscopy in combination with electrical measurements and surface charge diagnostics using the electro-optic Pockels effect of a bismuth silicon oxide crystal. The number of current pulses per half-cycle of the sine-wave voltage rises with increasing nitrogen admixture to helium due to the predominant role of the Penning ionization. Here, the transition from the first glow-like breakdown to the last Townsend-like breakdown is favored by residual species from the former breakdowns which enhance the secondary electron emission during the pre-phase of the later breakdowns. Moreover, the surface charge measurements reveal that the consecutive breakdowns occur alternately at central and peripheral regions on the electrode surface. These spatial inhomogeneities are conserved by the surface charge memory effect as pointed out by the recalculated spatio-temporal development of the gap voltage.

Surface charge measurements on different dielectrics in diffuse and filamentary barrier discharges

Previously, we reported on the measurement of surface charges during the operation of barrier discharges (BDs) using the electro-optic Pockels effect of a bismuth silicon oxide (BSO) crystal. With the present work, the next milestone is achieved by making this powerful method accessible to various dielectrics which are typically used in BD configurations. The dynamics and spatial distribution of positive and negative surface charges were determined on optically transparent borosilicate glass, mono-crystalline alumina and magnesia, respectively, covering the BSO crystal. By variation of the nitrogen admixture to helium and the pressure between and , both the diffuse glow-like BD and the self-stabilized discharge filaments were operated inside of a gas gap of . The characteristics of the discharge and, especially, the influence of the different dielectrics on its development were studied by surface charge diagnostics, electrical measurements and ICCD camera imaging. Regarding the glow-like BD, the breakdown voltage changes significantly by variation of the cathodic dielectric, due to the different effective secondary electron emission (SEE) coefficients. These material-specific SEE yields were estimated using Townsend’s criterion in combination with analytical calculations of the effective ionization coefficient in helium with air impurities. Moreover, the importance of the surface charge memory effect for the self-stabilization of discharge filaments was quantified by the recalculated spatio-temporal behavior of the gap voltage.

Study of material properties important for an optical property modulation-based radiation detection method for positron emission tomography.

We compare the performance of two detector materials, cadmium telluride (CdTe) and bismuth silicon oxide (BSO), for optical property modulation-based radiation detection method for positron emission tomography (PET), which is a potential new direction to dramatically improve the annihilation photon pair coincidence time resolution. We have shown that the induced current flow in the detector crystal resulting from ionizing radiation determines the strength of optical modulation signal. A larger resistivity is favorable for reducing the dark current (noise) in the detector crystal, and thus the higher resistivity BSO crystal has a lower (50% lower on average) noise level than CdTe. The CdTe and BSO crystals can achieve the same sensitivity under laser diode illumination at the same crystal bias voltage condition while the BSO crystal is not as sensitive to 511-keV photons as the CdTe crystal under the same crystal bias voltage. The amplitude of the modulation signal induced by 511-keV photons in BSO crystal is around 30% of that induced in CdTe crystal under the same bias condition. In addition, we have found that the optical modulation strength increases linearly with crystal bias voltage before saturation. The modulation signal with CdTe tends to saturate at bias voltages higher than 1500V due to its lower resistivity (thus larger dark current) while the modulation signal strength with BSO still increases after 3500V. Further increasing the bias voltage for BSO could potentially further enhance the modulation strength and thus, the sensitivity.

The effect of discharge area variation on stochastic characters of PD magnitude

Surface charges generated by previous discharges at the solid-gas interface in a void can affect the characteristics of subsequent ones. In this paper, the effect of discharge area variation on the stochastic characters of partial discharge (PD) magnitude was investigated by changing the void size. It is found that the uniformity of PD magnitude increased when void diameter was reduced. In order to clarify this, the corresponding discharge area variation was obtained by Pockels effect of Bismuth Silicon Oxide (BSO) crystal. As void diameter became smaller, due to the limitation of cavity walls, the fluctuation of discharge area became weaker, and hence the uniformity of PD magnitude increased. Besides, the result that the fluctuation of PD magnitude decreased when PD void height became larger was obtained. Considering the safety of BSO crystal, surface charge distribution was not measured by experiment for this case. Instead, a simulation model was constructed to obtain it. As a result, discharge area became larger when void height increased. According to this, it is inferred that the expanding of discharge area may be affected by cavity walls in the experiment. Therefore, the changing tendency of the uniformity of PD magnitude with the void height can be explained when the void area remained unchanged. These results suggest that the variation of discharge area can affect the stochastic character of PD magnitude and further investigation is needed to clarify the PD mechanism.

Light-induced Effects in Sillenite Crystals with Shallow and Deep Traps

This paper presents the light-induced effects in bismuth silicon and bismuth titanium oxide crystals associated both with the electron transitions into the conduction band and with the filling of shallow and deep traps, which determine the optical and electroconductive properties of these crystals. The dynamics of photoconductivity and light-induced absorption is analyzed under conditions of pulsed laser illumination at the wavelength of 532nm. The possibility to describe the relaxation processes of a population for trapping levels with the use of two-exponential function is demonstrated. The photoconductivity dynamics is characterized by two relaxation times on the order of 100ns and 10μs, whereas for light-induced absorption the lifetimes about 10μs and several days for short- and long-lived traps, respectively, have been obtained. Because of this, the relaxation transitions may be occurred both to the shallow trap centers with energy located close to the conduction band and to the deep-lying traps, which should be included into a diversified theoretical model adequately describing the light-induced phenomena in photorefractive sillenite-family crystals.

Interferometer with bismuth silicon oxide crystal for vibration measurement

We present a small-amplitude, high-frequency vibration measurement system. This system is based on the reflective holographic grating in a crystal of bismuth silicon oxide without applying an external electric field. A quarter-wave plate is applied in the reference beam path, with a polarizer after the crystal, to fulfill the quadrature condition when no electric field is applied to the crystal. A reflection configuration is used to obtain a good overlapping of the interference beams, which increases the beam coupling. The factors that affect the diffraction efficiency, including the signal-to-reference-beam intensity ratio and the recording angle, has been investigated. The experimental results coincide with the theoretical results, and the optimal conditions are obtained. The results of comparisons of our system with the vibrometer TEMPO show that the nanometer vibrations of a piezoelectric transducer can be reliably detected.

Systematic investigation of the barrier discharge operation in helium, nitrogen, and mixtures: discharge development, formation and decay of surface charges

As a logical extension to previous investigations of the barrier discharge (BD) in helium and nitrogen, the present work reports on the operation in any mixtures of both pure gases. Using a well-established plane-parallel discharge cell configuration allows to study the influence of the He/N2 mixing ratio on the formation of different discharge modes. Their characterization was made by measuring the discharge emission development together with the formation and decay of surface charges on a bismuth silicon oxide (Bi12SiO20, BSO) crystal. This was realized by the simultaneous application of the spatio-temporally resolved optical emission spectroscopy, and the electro-optic Pockels effect in combination with a CCD high speed camera. The existence diagram for diffuse and filamentary BDs was determined by varying the amplitude and shape of the applied voltage. Over the entire range of the He/N2 ratio, the diffuse mode can be operated at moderate voltage amplitudes whereas filamentation occurs at significant overvoltage and is favoured by a high voltage slew rate. Irrespective of the discharge mode, the overall charge transfer during a discharge breakdown is found to be in excellent agreement with the amount of accumulated surface charges. An exponential decay of the surface charge deposited on the BSO crystal is induced by LED illumination beyond a typical discharge cycle. During the decay process, a broadening of the radial profiles of positive as well as negative surface charge spots originating from previous microdischarges is observed. The investigations contribute to a better understanding of the charge accumulation at a dielectric.

Raman spectroscopy of bismuth silicon oxide single crystals grown by the Czochralski technique

In this work, single crystals of bismuth silicon oxide (BSO; Bi12SiO20) have been grown by the Czochralski method. The growth conditions were studied. The critical diameter and the critical rate of rotation were calculated. Suitable polishing and etching solutions were determined. The structure of the Bi12SiO20 has been investigated by x-ray diffraction (XRD), and Raman and Fourier transform infrared spectroscopy (FTIR) spectroscopy. The results obtained are discussed and compared with the published data. The pale yellow Bi12SiO20 single crystals prepared were without cores. Using spectroscopic measurements 19 Raman and 5 IR modes were observed.

Opto-electric particle manipulation on a bismuth silicon oxide crystal

High-throughput manipulation of microparticles can be efficiently accomplished using electrokinetic effects. In this contribution, we demonstrate the two-dimensional investigation of internal space-charge fields inside a bismuth silicon oxide (BSO) crystal and their use for optically mediated particle trapping. The magnitude of the internal fields as well as the time constant for its build-up are measured by Zernike phase contrast and digital holography. The fast response time of a BSO crystal at very low light powers enables real-time generation of high electric field gradients. We demonstrate that this photoconductive material facilitates both electrophoretic and dielectrophoretic trapping of particles on an accessible surface.

Analysis of output beam polarization in higher-order self diffraction via two-wave mixing in BSO crystal

Generation of two higher-order diffraction beams in two-wave mixing in transmission geometry have been analyzed theoretically in photorefractive cubic crystal of 23 symmetry, BSO (Bismuth Silicon Oxide) and considering its optical activity as one of the important parameter. Results are shown graphically by solving coupled wave equations using fourth-order Runge–Kutta method. The effect of polarization state of pump beam, optical activity, off-Bragg parameter, coupling coefficients, absorption and thickness of crystal has been analyzed on the beam intensities and their respective polarizations. It has been observed that as the value of the off-Bragg parameter reduces the transfer of energy to the higher-order beams increases. It has been also observed that by selecting proper input beam polarization and crystal thickness, diffracted beam intensity and its polarization could be controlled considerably. It has also been seen that by controlling input pump beam polarization, particular higher-order beam can be selectively amplified.

Studies on crystal growth, optical and electrical characterization of pure and Dy-doped bismuth silicate single crystals

(0.5 mol%) Dysprosium (Dy) doped bismuth silicon oxide (BSO) single crystals were grown by the Czochralski technique under air atmosphere. Detailed analysis of Dy-doped BSO with pure BSO has been studied through optical analysis. The absorption edges of pure and Dy-doped BSO crystals are found to be 405 nm and 415 nm, respectively. The shift in the absorption edge is contributed to the defect centers created in the crystal with Dy-doping. The shifts observed in the Raman spectra on doping Dy are found to be lower, when compared with the pure BSO crystal. This effect can be correlated to the lattice distortion induced by the Dy doping. The oxide formation and intrinsic defects in the BSO crystal have been identified by photoluminescence analysis. Dielectric measurements reveal that higher permeability value in the BSO sample is due to the presence of charged defects, which can be related to the space charge polarization. There is a slight decrease in dielectric constant on doping with Dy. The piezoelectric value explains the defects formed in the crystal. On poling, d 33 value of BSO and Dy-doped BSO are 32 pC/N and 40 pC/N, respectively.

Approach to wavelet multiresolution analysis using Coiflets and a two-wave mixing arrangement

An approach to the wavelet multiresolution analysis is realized using Coiflet wavelets and a two-wave mixing arrangement via transmission gratings in a bismuth silicon oxide (BSO) crystal. A Coiflet filter acts not only as a band-pass filter but as a high-pass or low-pass filter. 2-D wavelet bases are generated by a tensorial product of the 1-D Coiflet bases and are separable in x and y. Therefore, the 1-D band-pass property produces direction ability in the 2-D tensorial products. In this way, unlike the Mexican hat-based filters, the Coiflet-based filters allow selecting horizontal, vertical or diagonals details of an original image. The decomposition of a signal in two levels of multiresolution is experimentally demonstrated in this scheme. The original image is discomposed in an average image and several detail images depending on the level of multiresolution. Coif12 wavelets are used to achieve the multiresolution analysis. The photorefractive implementation of the wavelet analysis is numerically simulated and experimentally performed. An agreement between numerical and experimental results is obtained.

Ultraslow Gaussian pulse propagation induced by a dispersive phase coupling in photorefractive bismuth silicon oxide crystals at room temperature

By use of the highly dispersive phase coupling effect in a photorefractive wave mixing process, we have observed ultraslow propagation of a single Gaussian light pulse with a group velocity ∼0.5m/s in a photorefractive Bi12SiO20 crystal at room temperature. The ultraslow Gaussian pulse is amplified due to an intensity coupling effect but keeping its Gaussian profile with high fidelity. The group velocity of the Gaussian pulse can be controlled to a large extent. This technique is useful for controllable optical delay lines.

Running gratings in photoconductive materials

Starting from the three-dimensional version of a standard photorefractive model (STPM), we obtain a reduced compact set of equations for an electric field based on the assumption of a quasi-steady-state fast recombination. The equations are suitable for evaluation of a current induced by running gratings at small-contrast approximation and also are applicable for the description of space-charge wave domains. We discuss spatial domain and subharmonic beam formation in bismuth silicon oxide (BSO) crystals in the framework of the small-contrast approximation of STPM. The experimental results confirming holographic current existence in BSO crystal are reported.

Raman study of gaseous bubble inclusions in bismuth germanate and bismuth germanium silicon oxide single crystals

Gaseous bubble inclusions in bismuth germanate (BGO) and bismuth germanium silicon oxide (BGSO) crystals were studied by means of Raman spectroscopy at room temperature. Their Raman spectra in the range from 60 to 70 cm−1 showed three peaks for the rotational Raman modes of O2 and N2. Vibrational Raman modes of O2 and N2 were also recorded for BGO and BGSO crystals. It was found that all the rotational and vibrational modes were blue shifted from those of free molecules due to the hydrostatic pressure in the bubbles. Internal pressure in the bubbles was estimated from the rotational and vibrational Raman mode frequencies. O2 gas pressure in the bubble was estimated as 140 GPa, and N2 gas pressure, as 31 GPa. The pressure coefficient of the vibrational mode frequency of O2 (0.368 cm−1/GPa for O2 vibrational mode of 1580 cm−1) and N2 (0.322 cm−1/GPa for N2 vibrational mode of 2331 cm−1) was also obtained from the blue shift and the calculated bubble pressure.

Optical characterization of sillenite crystals using polarization self-modulation effect

Abstract We propose to use the polarization self-modulation effect for the optical characterization of sillenite crystals. The proposed method allows the characterization of crystals in a broader spectral range compared to conventional wave mixing techniques. The nominally pure Bi 12 SiO 20 bismuth silicon oxide (BSO) crystals are characterized at wavelengths λ =633, 810, and 980 nm demonstrating the unusually fast operation speed in the infrared part of the spectrum. We show that the intensity dependence of the material's response is sublinear and can be approximated as ∝ I −0.7 . The mobility-lifetime product μτ is determined for BSO by fitting the experimentally measured data with the above function.

Defect analysis in Czochralski grown Bi 12SiO 20 crystals

Bismuth silicon oxide (BSO—Bi 12SiO 20) crystals were grown by Czochralski technique employing both resistive and inductive heating systems. The effect of rotation rate, pulling rate and melt level in crucible is discussed. The growth conditions were optimized to achieve core-free crystals with minimum incorporation of bubbles into the crystals. The morphology of the crystals was analysed with respect to the growth parameters employed. Doped crystals were grown with 0.1 mol% of group III elements (B, Al and Ga). Chemical etching studies showed that the etch pit densities vary with different dopants. The optical transparency indicated varying absorption edges for different dopants. The results obtained from dielectric studies are presented. Hardness of these crystals vary with different dopants. Based on these results, the defect nature in the crystal is analyzed.

Edge-enhanced correlation with four-wave mixing in bismuth silicon oxide crystal by use of moving gratings

The nonlinearity of phase-conjugate beam reflection with four-wave mixing in bismuth silicon oxide crystal by use of moving gratings at large fringe modulation formed by the incident-beam ratio is investigated. On the basis of this investigation, the edge enhancement of an object and the edge-enhanced optical correlation with improved discrimination capability are achieved by use of moving gratings at an appropriate fringe velocity. Experimental results are presented.