Igor A. Sokolov

Adaptive photodetectors: novel approach for vibration measurements

Abstract We report on a simple high-sensitivity interferometric technique of detecting vibrations and present characteristics of laser vibrometer using GaAs and SnS2 adaptive photodetectors based on the effect of the non-steady-state photoelectromotive force. It enables efficient direct conversion of high-frequency phase modulation of speckle-like optical waves reflected from the vibrating object into an output electrical signal with concomitant setting of optimal operation point of the interferometer and suppression of amplitude laser noise. The results of measurements of small vibration amplitudes of the mirror and diffusely scattering objects are presented. Preliminary studies at 1.06 μm showed that it is possible to detect ultrasonic vibrations with the amplitude of 0.2 A with a signal power of 20 mW and a bandwidth of 15.5 MHz. This optical phase-to-electrical signal converter is not sensitive to ambient vibrations, thermal drift, amplitude laser noise and is therefore appropriate for industrial applications.

Volume and contact photo-emf in GaAs adaptive photodetectors

The generation of non-steady-state photo-emf in an adaptive photodetector made of semi-insulating GaAs is studied. Single-and double-frequency excitation modes are considered. It is found that a contact component of the photocurrent arises in the detector when the spatial frequencies of the interference pattern and phase modulation frequencies are low. The contact signal, being sensitive to a slow drift of the interference pattern, adversely affects the adaptive properties of the device. It is shown theoretically and confirmed experimentally that the contact emf signal at the frequency of basic phase modulation can effectively be suppressed by specially selecting the amplitude of additional phase modulation and the spatial frequency of the interference pattern. The spectral components of the non-steady-state emf signal are calculated for the double-frequency excitation mode.

Non-steady-state photoelectromotive force in the wide-band-gap dielectric MnO

Although manganese oxide is a wide-band-gap dielectric with a band gap Eg ≃ 4 eV and, in the undoped state, is characterized by a high electrical resistivity, it has been found experimentally that there is a non-steady-state photoelectromotive force in dynamic space-charge gratings formed in MnO by light with a photon energy of 1.96 eV (λ = 633 nm). The dependences of the photoelectromotive force signal amplitude on the phase modulation frequency, light intensity, and spatial frequency of the interference pattern have been investigated. The unusual frequency dependence of the signal has been explained in terms of the two-level semiconductor model taking into account the presence of shallow traps in the crystal. The photoelectric parameters of the material have been determined.

Space-and-time current spectroscopy of a β-Ga 2 O 3 crystal

We report the excitation of the non-steady-state photoelectro-motive force in a monoclinic gallium oxide crystal. The crystal grown in an oxygen atmosphere is insulating and highly transparent for a visible light, nevertheless, the formation of dynamic space-charge gratings and observation of the photo-EMF signal is achieved under the laser illumination with wavelength λ = 532 nm. The induced ac current is studied for the cases of zero and non-zero external electric fields, which imply the non-resonant and resonant mechanisms of space-charge recording. The dependencies of the signal amplitude versus the frequency of phase modulation, light intensity, spatial frequency, light polarization and value of the external dc electric field are measured. The material demonstrates the anisotropy along the [100] and [010] directions, namely, there is a weak difference of the transport parameters and a pronounced polarization dependence of the signal. The photoconductivity and diffusion length of electrons are estimated for the chosen light wavelength.

Adaptive interferometers for sensor applications

We report on simple high-sensitivity interferometric techniques of detecting vibrations and present characteristics of laser vibrometer using Bi12SiO20 crystal. Adaptive interferometer enables efficient conversion of high-frequency phase modulation of speckle-like optical wave reflected from the vibrating object into an output signal with concomitant setting of optimal operation point of the interferometer and suppression of amplitude laser noise.

Self-diffraction of frequency-modulated light in the Bi12TiO20 crystal

The self-diffraction of frequency-modulated light in the photorefractive B12TiO20 crystal was studied experimentally. To observe the effect, the crystal was illuminated by two light beams with the relative frequency shift Δf(t). In the experiments, linear frequency modulation was used: Δf(t) = At. As a result of the light self-diffraction on a hologram moving with a constant acceleration, the power of the light beams at the crystal output changed in the form of a chirp pulse. It was found that the pulse appears at the instant of stopping the interference pattern, and its duration is determined by the rate of frequency change A and the hologram recording time τsc.

Nonstationary photovoltage induced in tin disulfide crystals under strong surface excitation

The specific features of nonstationary photovoltage excitation in molecular crystals of tin disulfide are investigated. A substantial change in the dependence of the photocurrent amplitude on the spatial frequency of the interference pattern is revealed. The observed behavior is explained in terms of the model of a photoconductor with different-type charge carriers of the same sign.

Space-and-time current spectroscopy of nanostructured selenium in the chrysotile asbestos matrix

The non-steady-state photoelectromotive force effect was experimentally studied in a semiconductor nanowire array, i.e., in a composite representing selenium in a chrysotile asbestos matrix. The sample was exposed to an oscillating interference pattern, and the material response was measured as an alternating electric current. The experiments were performed for two geometries in which the excited photocurrent was parallel or perpendicular to nanowires. The dependences of the signal amplitude on the phase modulation frequency, spatial frequency, light polarization, and temperature were obtained. The photoelectric parameters of the material were determined for the light wavelength λ = 633 nm. The effect was theoretically analyzed for the semiconductor model with shallow traps, which allowed the explanation of the observed increase in the signal amplitude in the presence of additional phase modulation.

Non-steady-state photoelectromotive force in SiC crystals irradiated by reactor neutrons

The effect of the non-steady-state photoelectromotive force in 6H-SiC crystals irradiated by reactor neutrons has been investigated experimentally. The dependences of the signal amplitude on the phase modulation frequency, spatial frequency, light intensity, and amplitude of an external alternating-current (ac) electric field have been analyzed. The unusual frequency dependence of the signal has been explained in terms of the two-level semiconductor model taking into account shallow traps. The photoelectric parameters of the crystals have been determined for the light wavelength λ = 532 nm.

Adaptive photodetectors for vibration monitoring

Abstract We present characteristics of laser vibrometer using semiconductor GaAs and molecular SnS 2 adaptive photodetectors (AP) based on the effect of the non-steady-state photoelectromotive force. AP enable efficient direct conversion of high-frequency phase modulation of speckle-like optical wave reflected from the vibrating object into an output electrical signal with concomitant setting of optimal operation point of the interferometer and suppression of amplitude laser noise. The sensitivity of the setup is analyzed and further improvements in operation of AP are discussed.