S. V. Averin

New Method for Monitoring the Process of Freeze Drying of Biological Materials

A capacitive sensor was proposed and tested for the monitoring and control of a freeze drying process of a vaccine against the Newcastle disease of birds. The residual moisture of the vaccine was measured by the thermogravimetric method. The vaccine activity was determined by titration in chicken embryos. It was shown that, at the stages of freezing and primary drying, a capacitive sensor measured the fraction of unfrozen liquid phase in a material and allowed one to control the sublimation stage of drying in an optimal way. This prevented the foaming of the material and shortened the total drying time approximately twice. The control range at the sublimation stage of drying expanded up to −70°C. It was found at the final stage of drying that the signal of a capacitive sensor passed through a maximum value. We supposed that this maximum corresponds to the minimum of intramolecular mobility of biological macromolecules and hence to the optimal residual moisture of the material, which ensures long-term preservation of its activity. We also suppose that using the capacitive sensor at the final stage of drying allows one to more precisely detect the time when the residual moisture of dried material reaches the optimal value.

Metal-semiconductor-metal photodiodes based on ZnCdS/GaP wide-gap heterostructures

Good epitaxial ZnCdS layers are grown on GaP semiconductor substrates by metal-organic chemical vapor deposition. The respective photodiode structures are fabricated by the metal-semiconductor-metal method, and their characteristics are studied. The diodes feature low dark currents. The bias dependence of the spectral response of the detector is determined. The long-wavelength edge of the ZnCdS/GaP diodes shifts from 355 to 440 nm as the bias voltage varies from 40 to 80 V. At the maximal photosensitivity wavelength (355 nm), the ampere/watt sensitivity of the detector is 0.1 A/W.

MSM optical detector on the basis of II-type ZnSe/ZnTe superlattice

On the basis of a type-II ZnSe/ZnTe superlattice, a MSM (metal—semiconductor–metal) photodetector is fabricated and investigated. The detector features low dark currents and a high sensitivity. The spectral characteristic of the detector provides the possibility of the selective detection of three separate spectral portions of visible and near-infrared radiation.

A bicolor photodetector of ultraviolet radiation based on a low-dimensional ZnCdS/ZnMgS/GaP heterostructure

A bicolor photodetector of ultraviolet radiation based on a ZnCdS/ZnMgS/GaP heterostructure is designed and analyzed. At low bias voltages, the detector has a narrowband response at a wavelength of 350 nm, which is determined by the composition of the ZnCdS quantum well. As the bias increases to 80–100 V, the maximum sensitivity of the detector is shifted to a wavelength of 450 nm with subsequent rapid decrease. In this case, the spectral sensitivity characteristic of the detector covers the region of the maximum pigmentation effect of solar radiation.

An ultraviolet MSM photodetector with electrically tunable spectral sensitivity

Ultraviolet photodetectors based on interdigital Schottky-barrier contacts to a ZnCdS/GaP heterostructure, which have low dark currents, are fabricated and investigated. It is found that the characteristics of the spectral response of these detectors depend on the bias and the long-wavelength boundary of the response of a ZnCdS/GaP metal-semiconductor-metal (MSM) diode can be shifted from 355 nm to 440 nm as the bias varies from 40 to 80 V. It is found that, at the wavelength of the maximum photosensitivity (355 nm), the ampere-watt sensitivity of the detector is 0.1 A/W.

A method for measuring the shot noise in Schottky diodes with reduced barrier height

A method for measuring the spectral density of shot noise of one-port networks, in particular, semiconductor diodes, is described. The method is applicable at frequencies of up to 200 MHz. Expressions are obtained that relate the spectral density of the diode’s current noise to the measured noise factor and power gain of the diode fixture with the diode mounted in it. With this method, the dependence of the Fano factor of a GaAs Schottky diode with reduced barrier height on the diode current is determined.

Acousto-optic filters based on the superposition of diffraction fields [Invited]

Methods for the calculation of the acousto-optic (AO) transfer functions forming as a result of superposition of two AO fields with similar and orthogonal light polarizations are proposed. Specific calculations are made on the basis of the parameters widely used in practice with uni-axial gyrotropic crystal paratellurite. It is revealed that the resulting field is generally very inhomogeneous but contains domains with two-dimensional behavior properties. These domains can be used for two-dimensional image edge enhancement based on Fourier processing. Theoretical conclusions are verified by means of the optical image Fourier processing with the spatial frequency AO paratellurite filters.

Triple Bragg diffraction in paratellurite crystal

Triple Bragg diffraction in a paratellurite crystal has been considered for the case when the plane of diffraction is oblique to the optical axis of the crystal. It has been shown that effective photoelastic constants for isotropic and anisotropic diffraction depend on the inclination of the plane of diffraction insignificantly. Triple Bragg diffraction of 0.63-μm coherent radiation in paratellurite at a 47.3-MHz slow acoustic wave has been experimentally demonstrated. For an optical power of 0.69 W delivered to a piezoconverter, the relative intensities of diffraction orders equal ~0.4, 0.4, 0.1, and 0.1, respectively.

An acousto-optic modulator of optical radiation at the double acoustic frequency

An acousto-optic (AO) modulator that converts a frequency shift of optical radiation into the amplitude modulation of light at the double acoustic frequency is proposed and described. A ТеО2 singlecrystal modulator mockup that was developed and tested provides the amplitude modulation of linearly polarized optical radiation at a wavelength of 0.63 μm at the double acoustic frequency, which is equal to ~82MHz.

Acousto-optical modulation of light at a doubled sound frequency

A method of acousto-optical (AO) Bragg diffraction is proposed that provides the amplitude modulation of optical radiation at a doubled acoustic frequency. The method is based on the double transmission of the light through the AO modulator made of a gyrotropic crystal and is experimentally tested by the example of the modulation of light with a wavelength of 0.63 μm, controlled by the paratellurite AO cell.