photo-EMF Detectors Research Articles

Speckle Measurement for Small In-Plane Vibration Using GaAs.

In this study, the measurement characteristics of speckles based on the photoinduced electromotive force (photo-emf) effect for high-frequency, small-amplitude, and in-plane vibration were theoretically and experimentally studied. The relevant theoretical models were utilized. A GaAs crystal was used as the photo-emf detector for experimental research, as well as to study the influence of the amplitude and frequency of the vibration, the imaging magnification of the measuring system, and the average speckle size of the measuring light on the first harmonic of the induced photocurrent in the experiments. The correctness of the supplemented theoretical model was verified, and a theoretical and experimental basis was provided for the feasibility of using GaAs to measure in-plane vibrations with nanoscale amplitudes.

Characterization of Laser Generated Lamb Wave Modes after Interaction with a Thickness Reduction Discontinuity Using Ray Tracing Theory

The effect that a circular discontinuity (due to thickness reduction) in an Aluminum plate has on the direction of Lamb wave propagation was experimentally and theoretically studied. Broadband Lamb waves were generated by a pulsed Nd:YAG laser and optically detected with a photo-EMF detector to increase spatial resolution. The experimental results show that thickness reduction modifies the time of flight (TOF) for S0 and A0 vibration modes and generates a change in direction of the ultrasonic Lamb wave. The change in the TOF as a function of distance and thickness reduction was numerically determined using ray theory and then compared to experimental results. It is shown that the change in the direction of propagation depends on the vibrational mode and frequency of the Lamb waves and this can affect the detection and characterization of a hidden discontinuity.

Laser ultrasound detection by CdTe:V monopolar photo-EMF detector with responsivity close to theoretical limit

We report on the experimental demonstration of semiconductor photo-electromotive force (photo-EMF) adaptive detector for laser ultrasonic applications with responsivity close to the theoretical limit for monopolar device of this type. The photo-EMF detector fabricated of a monopolar CdTe:V crystal was characterized in the spectral region 827–852 nm which is very close to the fundamental optical absorption edge of CdTe ( ≈ 825 nm). At λ = 833 nm the experimentally observed photo-EMF signal amplitude reached its maximal value, which corresponds to the theoretical limit for a monopolar photoconductor with quantum efficiency of photoconductivity 0.6–0.75.

Wavelength dependence of the photo-electromotive-force effect in CdTe:V crystal with bipolar photoconductivity

We report on detailed investigation of the photo-electromotive-force (photo-EMF) effect in semiconductor with bipolar photoconductivity of impurity type in a spectral region close to the fundamental absorption edge, where both the photoconductivity and the photo-EMF response reach their maxima. The experiments performed with CdTe:V crystal (Δ E ≈ 1.51 eV) in the spectral range of 826–853 nm show that dynamics of the photo-EMF signal formation in this crystal is governed by relaxation of slow majority photocarriers–holes with the life-time which goes down from τ ≈ 9 μs at 850 nm to ≈6.5 μs at 826 nm. The minority photocarriers (electrons), which contribute approximately 10% of the total photoconductivity, also demonstrate significant wavelength dependence of their diffusion length with L D ≈ 8.5 μm at 850 nm and ≈6.3 μm at 830 nm. The reported results show significant improvement in responsivity of the bipolar CdTe:V photo-EMF detector when it is operated at λ = 840 nm (by factor ≈2.5 as compared with that at 852 nm).

ULTRASONIC DETECTION BY PHOTO-EMF SENSOR AND BY WIDEBAND AIR-COUPLED TRANSDUCER

Two non-contact remote ultrasonic detection systems are compared. The first consists of a continuous wave (CW) Nd:YVO4 laser operating at 532 nm used in conjunction with a photo-induced electromotive-force (EMF) sensor. The second system employs an air-coupled capacitance transducer that has a bandwidth from 100 kHz up to 2 MHz. Both systems exhibit higher sensitivity to out-of-plane than in-plane measurements of stress wave signals. Ultrasound generation is carried out by an Nd:YAG pulsed laser at 532 nm in the thermoelastic regime in a 1.6 mm aluminum plate. A0 Lamb mode is generated at frequency-thickness product with strong out-of-plane displacements. Ultrasonic waveforms acquired are reported and the performance advantages of the two remote non-contact detectors are compared with each other. For lower frequency tests (up to 2 MHz), better results have been achieved with the air-coupled transducer that also has superior overall performance and benefits. The tests enabled direct comparison of air-coupled transducer and photo-EMF detector.

Effective broadband detection of nanometer laser-induced ultrasonic surface displacements by CdTe:V adaptive photoelectromotive force detector

Results of experiments studying the utilization of adaptive CdTe:V photoelectromotive force (photo-EMF) detector for high-sensitivity broadband detection of laser-generated ultrasound are presented. Unlike widely used GaAs photo-EMF detectors, the devices used here demonstrate no significant electron-hole competition. This ensures effective detection of ≅1 nm surface displacements with ≅0.1 mW of the signal beam power in 10 MHz detection frequency band, even in simple transverse detector configuration. For the wavelength of λ=852 nm used in the experiments, the dielectric cutoff frequency of a typical device was approaching 1 MHz. This, in principle, enables monitoring of as-processed objects moving with in-plane velocities up to 10 m/s.

Optimizing transverse photo-electromotive force detectors for laser ultrasonic applications

The sensitivity of the adaptive photoelectromotive-force (photo-EMF) detectors arranged in the conventional transverse configuration and optimized for laser ultrasonic applications is analyzed. It is shown that for the devices based on bipolar photoconductors operating under typical conditions, i.e. with approximately pixels in the detected speckle-like signal wave, the amplifier input noise current and the probe beam wavelength of the optimal matching with the preamplifier can be achieved with a detected signal beam power P S greater than ≈0.1 mW. The limit on the sensitivity of these optimized photo-EMF detectors (with the inter-electrode spacing equal to 5 optimal fringe periods) found to be determined by the noise of the detector itself and is approximately a factor of 14 below the fundamental limit set by the shot noise of the detected signal power. In particular, for and the detection frequency range of the minimal detectable phase modulation amplitude is rad, corresponding to the detection of ≈0.13 nm ultrasound induced surface displacement amplitude. For lower signal laser power the sensitivity of the detection configuration proves to be limited by the noise of the preamplifier and deteriorates linearly with the detected signal power. Significant growth of the cut-off frequency makes difficult to deploy the typically more effective monopolar photoconductors in such optimal small-size devices.

Interferometric applications of GaAs adaptive photo-EMF detectors

Interferometric applications of GaAs adaptive photo-EMF detectors

Effective laser ultrasonic detection with biased GaAs photoelectromotive force detectors

Improvement of the sensitivity of laser-induced ultrasonic detection by GaAs adaptive photoelectromotive force (photo-EMF) devices under external dc bias is reported. The gain in responsivity of the detector about 7× in ≈10 MHz frequency band was observed under application of an external dc field ≈50 V/mm at λ=633 nm. Detailed experiments show that this growth is paid by proportional shift of maximum of the photo-EMF response to lower spatial frequencies and by appearance of a relatively broad resonance pick located at modulation frequencies lower than the cutoff frequency of the unbiased GaAs photo-EMF detector. These results are consistently interpreted using a theoretical model of the photo-EMF effect in bipolar photoconductor with large lifetime of one type of the photocarriers.

Estimation of wall thinning in mild steel using laser ultrasound Lamb waves and a non-steady-state photo-emf detector

In this work, a non-steady-state photo-emf receiver has been used to detect the lower frequency fundamental a 0 Lamb waves in mild steel. Experimentally, the Lamb waves are laser-generated in the thermoelastic regime using a Q-switched, 20 ns, pulsed Nd:YAG laser and a line source. Typical Lamb waves had centre frequencies of 250 kHz but with frequency components that extended beyond 1 MHz. In mild steel, higher order Lamb wave modes were not considered to be significant below a frequency thickness product of 1.6 MHz mm. Below this level, associated velocity dispersion curve offered phase velocity changes that were sensitive to thickness change. Samples up to 5 mm in thickness were examined without significant interference from higher order modes. A non-steady-state photo-emf detector used as the ultrasonic detector had the advantage of a lower frequency cut-off at 100 kHz compared to a confocal Fabry–Pérot interferometer (CFPI) of about 2 MHz. Both schemes offered greater stand-off distance (>20 cm) than is possible with EMATs, which have a stand-off distance less than 1.0 mm. Progress made in detecting wall thinning in steel plate with thickness up to 5 mm is reported.

Depth resolution of a laser profilometer based on photo-EMF detector

Abstract A laser profilometer based on adaptive photo-EMF detector that can find application for testing optically rough surfaces is described. Depth resolution of the configuration is shown to depend on not only the coherence length of the wide-spectrum light source used, but also the interelectrode spacing of the detector and crossing angle of interfering beams. Experimental demonstration of the device having an approximately 10 mm resolution and using a GaAs photo-EMF detector and a superluminescent diode was performed in tests of multisurface reflecting object.

Laser-ultrasound detection systems: a comparative study with Rayleighwaves

Using laser-generated Rayleigh waves in the thermoelastic regime asan ultrasonic source, a range of ultrasonic detector systems have beenevaluated experimentally for their sensitivity. Systems examined were thosebased on electromagnetic acoustic transducers (EMATs), a non-steady statephoto-emf detector, a confocal Fabry-Pérot interferometer (CFPI) and adifferential optical beam deflection (OBD) detector. These detector systemshad their own range of frequency responses within the range of approximately100 kHz to 140 MHz. However, experimental conditions in this study limited theupper frequency range to 22 MHz. For comparison, signal-to-noise ratios werenormalized with respect to both the experimental bandwidth, and, in the caseof laser detection, the incident laser intensity. Results showed that forremote monitoring at a distance greater than 0.2 m, optical systems based onthe photo-emf detector and the CFPI offered similar performances from amachine-finished aluminium surface. OBD measurements are not possible on sucha surface. However, for typical stand-off distances of 0.25 mm or less,cost-effective EMATS provided an alternative form of detector.

Asymmetric interdigitated metal-semiconductor-metal contacts for improved adaptive photoinduced-electromotive-force detectors

The use of interdigitated metal-semiconductor-metal contacts on semi-insulating GaAs enhances the responsivity of photoinduced-electromotive-force (photo-EMF) adaptive detectors by reducing the carrier transit time between electrical contacts. The unique polar character of the photo-EMF effect prevents the use of conventional interdigitated contact designs. For photo-EMF enhancement, the photoconductivity must be suppressed in every odd electrode pair. This is achieved through proton implantation to reduce the photocarrier lifetimes. The need to minimize the amount of area occupied by the back-action pairs while maintaining maximum collecting area between the active pairs is the source of the asymmetry. We present theoretical analysis of the scaling of the responsivity with the number of pairs. The theory is verified with experimental asymmetric interdigitated contact devices with 1, 2, 4, 8, 16, and 32 pairs of electrodes. The relative enhancement in responsivity is approximately equal to the number of pairs. The enhanced photo-EMF detectors have potential application to adaptive laser-based ultrasound detection.

Enhanced responsivity of non-steady-state photoinduced electromotive force sensors using asymmetric interdigitated contacts

The responsivity at a constant detection area of non-steady-state photoinduced electromotive force (photo-emf) detectors is improved by a factor equal to the number of contact pairs contained in asymmetric interdigitated surface contacts. The polar nature of photo-emf current generation requires contact asymmetry in which one increases the total signal by blocking the illumination between alternate contact pairs, in distinct contrast to the behavior of conventional interdigitated contacts fabricated upon isotropic photoconductors.

Giant enhancement of low frequency non-steady-state photo-EMF signal in Bi 12SiO 20 crystal under external DC bias

We present experimental results which demonstrate that non-steady-state photo-EMF, induced by vibrating interference fringes in Bi 12SiO 20 photorefractive crystal can be dramatically (by a factor of 10 3 in our experimental conditions) enhanced in a resonance regime under application of an external DC field. We show that for increasing applied electric field the resonance maximum moves from the dielectric relaxation frequency towards the low frequency end. Its magnitude grows as E 0 2 for low values of the field, for high external field significant broadening of the resonance peak and linear growth of the signal resonance amplitude is observed. This effect and non-trivial dependence of the resonance signal amplitude on the spatial frequency of the pattern is explained by saturation of the space charge grating. Finally, the sensitivity of the biased resonance adaptive photo-EMF detector is estimated.