We study the excitation of the two-wave mixing and non-steady-state photoelectromotive force signals using uniformly accelerated motion of the recording light pattern. Such illumination is created by the linear frequency modulation of the interfering light beams. The pulse response is predicted theoretically and observed experimentally in Bi12TiO20 and GaAs crystals at λ=633 nm. We analyse both the diffusion and space-charge wave regimes of signal excitation. The evolution of the pulse shape versus the chirp rate is demonstrated and explained in the frames of the developed theory. The application of the effects in laser Doppler velocimeters and accelerometers is discussed as well.
We report the results of utilization of wide-gap photorefractive sillenite crystals as adaptive photodetectors (AP) for
optical inspection of micro-electromechanical systems. The operation principle of AP is based on two-wave mixing and
non-steady-state photoelectromotive force effects in photorefractive crystals. The results of measurements of small
vibration amplitudes and resonant frequencies of the diffusely scattering objects and micro-electromechanical systems
are given. The presented adaptive interferometric systems are suitable for industrial applications.
We report the results of utilization of wide-gap photorefractive sillenite crystals as adaptive photodetectors (AP) for
vibration measurements. The operation of the adaptive system of non-destructive testing was studied for the real operation
conditions (diffusely scattering objects). The operation principle of AP is based on the effect of the non-steady-state
photoelectromotive force (photo-EMF). The mechanism responsible for the effect can be described as follows.
Illumination of wide-gap semiconductor by an interference pattern produces a non-uniform excitation of free carriers
(photoconductivity grating). Diffusion of the photo-excited carriers towards the dark regions leads to charge redistribution
between deep traps in the photoconductor. A space charge field grating arises. Small vibrations of the light pattern excite
an alternating current through the crystal. The results of measurements of small vibration amplitudes and resonant
frequencies of the diffusely scattering objects are given. The presented adaptive interferometric system is suitable for
industrial applications.
We report the excitation of the non-steady-state photoelectromotive force (photo-EMF) in high-resistance wide-gap
materials: layered boron nitride (BN), GaN nanoparticles in porous glasses, polypyrrole nanostructures in chrysotile
asbestos matrix. The non-steady-state photo-EMF appears in semiconductor material illuminated by an oscillating
interference pattern. Non-uniform illumination gives rise to the photoconductivity and space charge gratings which
periodically shift with respect to each other resulting in an alternating photocurrent. Since the conductivity and space
charge take part in current generation a number of photoelectric parameters can be measured: Maxwell relaxation time,
lifetime, diffusion length and mobility of carriers, concentration of impurity centers.
We report on simple high-sensitivity interferometric technique of detecting vibrations and present characteristics of
laser vibrometer using GaAs, CdTe and CdZnTe 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
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 results of measurements of semiconductor material's
parameters of CdTe and CdZnTe are presented. The experiments are carried out for diffusion regime of signal excitation at
light wavelength &lgr;=1.15 &mgr;m. The sign, conductivity and diffusion length are estimated from the dependencies of the signal
on the temporal and spatial frequencies. Preliminary studies at 1.06 &mgr;m showed that it is possible to detect ultrasonic
vibrations with the amplitude of 0.2 Å with a signal power of 20 mW and a bandwidth of 15.5 MHz. This optical
phase-to-electrical signal converter is not sensible to ambient vibrations, thermal drift, amplitude laser noise and is
therefore appropriated for industrial applications.
We report the excitation of the non-steady-state photoelectromotive force (photo-EMF) in the layered boron nitride crystal. Boron nitride crystals are characterized by an unique combination of physicochemical properties that advance its wide application in various areas of science and technology such as vacuum technology,
production of the microelectronic devices, x-ray lithography. The main goal of our work is to present possibility to expand the non-steady-state photo-EMF technique for the new class of diamond related materials with extremely wide band gap (Eg = 5.67 eV for BN). The investigated pyrolytic BN crystal was grown using the chemical vapor deposition method. It consists of the monocrystal layers of the rhombohedral modification with thickness of 2000 A separated by thin (~100 A) layers of the hexagonal modification. The photo- EMF signal is generated by two coherent laser beams (λ = 532 nm) one of which is phase modulated. Both the diffusion regime of signal excitation and the excitation in an external sinusoidal electric field are investigated. The measurements performed using standard diffusion regime of photo-EMF generation reveals rather low hole photoconductivity σ0 = (0.9-2.4)x10- 10Ω-1 cm-1 at light intensities I0 = 0.22-0.86 W/cm2 and small diffusion length LD = 35 nm of photoholes. We also measure the μτ- product and diffusion length using novel technique of signal generation in an external ac field: μτ= 1.0 x 10-9 cm2/V, LD = 50 nm. The influence of the layered structure of the investigated BN crystal on the effect of the non- steady-state photo-EMF is discussed.
We study the effect of simultaneous excitation of the space charge and conductivity running gratings in photorefractive crystals. For the realization of such regime we illuminate the crystal by the interference pattern oscillating with frequency ω. The combination of external dc and ac (with frequency Ω) voltages is applied to the crystal as well. If the frequencies of the phase modulation and ac field are equal to the eigenfrequency of the conductivity grating oscillation (high-frequency branch of the space charge oscillation) ω,Ω=ωpc, and their difference is equal to the eigenfrequency of the space charge grating oscillation (low-frequency branch) |ω-Ω|=ωsc, then the former grating oscillation effectively interacts with the ac component of applied field giving rise to the latter one. The experiments are performed in the photorefractive sillenite crystals (Bi12SiO20, Bi12TiO20) using both the diffraction and non-steady-state photo-EMF techniques. The dependencies of the detected signal amplitudes on the difference frequency ω-Ω (for fixed Ω) and dependencies of the detected signal amplitudes on the frequency of phase modulation ω (for fixed ω-Ω) are measured. They demonstrate the excitation of both the low and high-frequency eigenmodes of the space charge oscillation and provide estimations of material parameters, namely, the mobility of photoelectrons: μ=(1.1-1.4)×10-2 cm2/Vs (Bi12SiO20, λ=532 nm, T=296-298 K), μ=1.5×10-2 cm2/Vs (Bi12SiO20, λ=442 nm, T=293 K), and μ=3.1×10-3 cm2/Vs (Bi12TiO20, λ=532 nm, T=293 K). The application of the developed techniques for the characterization of widegap semiconductors and for the detection of optical phase-modulated signals is discussed.
We report high-sensitive experimental investigations of the SiC powders transmission spectra in the lattice oscillations region. For the SiC powders with the grain size of the order of the wave length the transmission minimum is shifted to the low-frequency range with respect to the transverse oscillation frequency of a single crystal ωT orresponding to phonon absorption with greater wave vectors. The decrease of the grain size leads to the shift and narrowing of the lattice absorption band to the high-frequency region. The observed change in the form and position of an absorption band corresponds to the narrowing of an optical phonon band due to decrease of the crystalline grain size, and approaching the lattice oscillation frequency to the frequency of molecular oscillations of Si-C in carbosilane owing to the surface energy contribution increase.
We report the nonresonant excitation of the non-steady-state photocurrents in an external sinusoidal electric field. We demonstrate that the application of an AC field sufficiently increases the photocurrent amplitude. At the same time the frequency transfer function of the effect maintains the form typical for the diffusion mechanism of photocurrent excitation. The dependencies of the current amplitude and characteristic cut-off frequency versus applied voltage are utilized for the determination of the photocarrier's μτ-product. The advantages of the proposed technique are discussed. The experiments are carried out in the photorefractive n-type Bi12SiO20 crystal. The signal enhancement of 50 dB has been achieved and μτ-product is found to be μτ = (0.8 - 1.2) 10-10 m2/V.
A novel interferometric modulation technique for optical thin film testing using GaAs:Cr adaptive photodetectors based on the effect of the non-steady-state photoelectromotive force is presented. The technique needs no special vibroinsulation and automatically adjusts and keeps the operation point of the interferometer. Two different interferometric setups with GaAs:Cr adaptive photodetectors are reported. A modified Mach-Zehnder interferometer with adaptive photodetector is used to study piezoelectric coefficient, d$_33) of the thin film and the influence of the bending motion of the substrate. A two beams polarization interferometer with adaptive photodetector is developed for effective differential Pockels coefficient, rc equals r33-(n0/ne)$=3)r13, measurement. It is shown that the proposed two beams polarization technique allows measurement of the Pockels coefficient of thin films with a strong Fabry-Perot effect usually present in ferroelectric thin film. Strong hysteresis effect with a slightly asymmetric form of the hysteresis loop was observed at the dependence of the d33 and re coefficients of the PZT thin film from the DC electric field. The values of d33 and Re are in agreement with known data.
We report on simple high-sensitivity interferometric technique of detecting vibrations and present characteristics of laser vibrometer using GaAs 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 wave reflected from the vibrating object into an output electrical signal with concomitant setting of optical 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 micrometers showed that it is possible to detect ultrasonic vibrations with the amplitude of 0.2 angstrom with a signal power of 20 mW and a bandwidth of 15.5 MHz. The sensitivity of GaAs adaptive photodetectors at 0.63 micrometers was found to be 0.1 angstrom for excitation frequency 1 kHz, registration bandwidth 1 Hz and average laser power on the photodetector 200 (mu) W. This optical phase-to- electrical signal converter is not sensible to ambient vibrations, thermal drift, amplitude laser noise and is therefore appropriated for industrial applications.
We present results of detailed experimental study of threshold sensitivity of GaAs adaptive photodetector at 632.8 nm. The minimal experimentally detected sinusoidal phase modulation was approximately equals 1.6 X 10-6 rad(root)mW/Hz that approached the theoretical limit for these devices (approximately equals 2 X 10-7 rad(root)mW/Hz) and allowed detection of approximately equals 1.6 X 10-7 micrometers vibration amplitudes with 1 mW laser power in 1 Hz bandwidth.
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