Effects of boundary reflection on photoacoustic spectrum of porous silicon

Photoacoustic (PA) amplitude and phase spectra are studied on porous silicon (PS) samples. For the sample with a thinner PS layer and a rough interface observed by field-emission scanning electron microscope (FE-SEM), PA amplitude decays rapidly at short wavelengths but stays at a higher level above 650 nm compared with a sample with a thicker PS layer and a smooth interface. In this latter long-wavelength region, phase delay for the former sample is smaller. A model calculation for the two-layer model taking account of scattering of light in the porous media and interface reflection of light gives at least a qualitative explanation of these differences. Received: 30 June 1999 / Accepted: 11 October 1999 / Published online: 23 February 2000     

Real-time nondestructive imaging with THz waves

We present a real-time imaging measurement in the terahertz (THz) frequency region. The dynamic subtraction technique is used to reduce long-term optical background drift. The reflective images of two targets, a Nikon camera’s lens cap and a plastic toy gun, are obtained. For the lens cap, the image data were processed to be false-color images. For the toy gun, we show that even under an optically opaque canvas bag, a clear terahertz image is obtained. It is shown that terahertz real-time imaging can be used to nondestructively detect concealed objects.     

Probe-beam incidence angle-effects on thermal diffusivity measurement by photothermal deflection technique

Received: 27 February 1998     

Distributed imaging using an array of compressive cameras

We describe a distributed computational imaging system that employs an array of feature specific sensors, also known as compressive imagers, to directly measure the linear projections of an object. Two different schemes for implementing these non-imaging sensors are discussed. We consider the task of object reconstruction and quantify the fidelity of reconstruction using the root mean squared error (RMSE) metric. We also study the lifetime of such a distributed sensor network. The sources of energy consumption in a distributed feature specific imaging (DFSI) system are discussed and compared with those in a distributed conventional imaging (DCI) system. A DFSI system consisting of 20 imagers collecting DCT, Hadamard, or PCA features has a lifetime of 4.8× that of the DCI system when the noise level is 20% and the reconstruction RMSE requirement is 6%. To validate the simulation results we emulate a distributed computational imaging system using an experimental setup consisting of an array of conventional cameras.     

Photoacoustic measurements of the thermal properties of AlyGa1-yAs alloys in the region 0<y<0.5

y Ga_1-yAs alloys grown by liquid phase epitaxy on GaAs substrates, by means of the open photoacoustic cell detection technique and the temperature-rise method under continuous light illumination. The values of the thermal conductivity, diffusivity and specific heat were obtained in the 0<y<0.5 region, where the Al_yGa_1-yAs band gap is mainly direct. The technique presented here is based upon an effective sample model which is shown to be suitable for the determination of the thermal properties of two layer semiconductor specimens. Received: 15 November 1996/Accepted: 5 March 1997     

Statistical modeling of imaging process in active night vision systems with gate-light detection

Characteristics of object images observed through scattering media with an active vision system operating in the mode of spatial selection are estimated using the Monte Carlo method. The effect of optical and geometrical conditions of observation on the contrast in images of reflecting objects is considered. Interpretation is given to the obtained dependences. Received: 11 February 2002 / Revised version: 31 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +7-38/2225-9086, E-mail: belov@iao.ru     

A method for single-pass distortion correction using holography in rhodium-doped barium titanate

We present a holographic method of one-way image transmission with ‘real-time’ correction of distortion through an optical system. The technique is demonstrated using a crystal of photorefractive rhodium-doped barium titanate as the holographic medium and corrects for phase distortions imposed upon a US Air Force test chart. This technique is suitable for applications in both cw and pulsed regimes, and for either fixed or changing distortions. Received: 26 October 1998 / Revised version: 19 December 1998 / Published online: 12 April 1999     

Imaging and post-processing of laser-induced fluorescence from NO in a diesel engine

Received: 11 July 1996/Revised version: 12 November 1996     

Photoelastic effect and mirage deflection in anisotropic materials

The collinear mirage technique is widely used to measure the thermal diffusivity of semi-transparent materials. However, in a recent paper [A. Salazar, M. Gateshki and A. Sánchez-Lavega: Appl. Phys. Lett. 76, 2665 (2000)], it was shown that for isotropic materials, because of the influence of photoelastic effect, the method was sensitive to the polarization state of the probe beam. The present paper extends the previous work to include anisotropic materials. In particular, we focus on the experimental conditions under which the thermal diffusivity of each crystal system can be measured using the phase method. Our theoretical model indicates that while the thermal diffusivity of isotropic materials can be measured using an unpolarized probe beam, for anisotropic materials, even the use of an unpolarized probe beam does not guarantee the validity of the method in all crystal systems. Experimental measurements performed on cubic, hexagonal and monoclinic crystals confirm the validity of the model. Received: 17 March 2001 / Accepted: 17 March 2001 / Published online: 25 July 2001     

Quantitative measurements of OH concentration fields by two-dimensional laser-induced fluorescence

Received: 7 May 1996/Revised version: 21 October 1996     

Temperature measurements in gases using laser-induced electrostrictive gratings

Received: 1 December 1997 / Revised version: 27 January 1998     

Measurement of the thermal properties of liquid mixtures using a thermal wave interferometer

In this paper we discuss the use of an alternative photothermal technique for measurements of thermal properties of liquid mixtures. The proposed technique is based upon the concept of a thermal wave interferometer. The liquid sample is confined between two thin pyroelectric detectors. One of these detectors acts as a modulated light absorber while the other is used for sensing the temperature fluctuations transmitted through the liquid layer. It is demonstrated that the proposed experimental configuration allows us to fully characterize the thermal properties of the constituent liquids. Received: 19 February 2001 / Published online: 18 July 2001     

Dynamic and static contributions to the zero-field splitting term of divalent nickel in fluosilicate crystals

Previously published electron paramagnetic resonance (EPR) data for Ni²⁺(3d⁷) in hydrated and deuterated crystals of zinc and nickel fluosilicate have been re-analyzed to separate the static and dynamic contributions to the fine-structure (or axial zero-field-splitting) term D in the spin-Hamiltonian. While the Debye parameters of ZnSiF₆·6H₂O and ZnSiF₆·6D₂O were determined in an earlier low-temperature study¹, those of the undiluted nickel crystals, for which good data between 4 and 77 K is lacking, have been estimated here by comparison of the data above 60 K with those for the corresponding zinc compounds. The numerical results indicate that the measured values of D are composed of a negative dynamical contribution produced by the rotational motions of the water ligands, together with both negative and positive static contributions produced by the water ligands, and the more distant [SiF₆]²⁻ and [Ni.6H₂O]²⁺ complexes, respectively. A point-charge calculation shows the contribution of the latter to be roughly +2.5 cm⁻¹ in NiSiF₆·6H₂O. The exceptionally large temperature variations of D in these crystals is attributed to the near-cancellation of the positive and negative static contributions, leaving the effect of the dynamical contribution dominant.     

Speciation of actinide ions in aqueous solution by laser-induced pulsed photoacoustic spectroscopy

The oxidation-state specific detection of actinide ions, U(VI), Pu(IV), Pu(VI), and Am(III), was investigated by pulsed laser photoacoustic spectroscopy. With a single-beam differential technique, the solvent effect is compensated and thus the detection sensitivity substantially increased. The observed detection sensitivities are8×10^–7M/L for U(VI)7×10^–8M/L for Pu(IV)3×10^–8M/L for PU(VI)2×10^–8M/L for Am(III)The present method facilitates the study of complexation and polymerization of actinide ions in the submicromole concentration range.     

Linear Tikhonov regularization against an edge field: an improved reconstruction algorithm in photothermal depth profilometry

This work introduces the concept of edge-field regularization into photothermal inverse depth profilometry problems. An edge field allows prior information concerning the depth location of material interfaces in a sample to be introduced into a Tikhonov regularization problem by a simple binary encoding. The edge-field regularization allows Nth-order Tikhonov stabilization constraints to be applied independently to multiple zones or segments of a depth profile between defined interface positions. This allows the reconstruction of continuous depth-profile information within known layers, without the globally imposed smoothing and edge oscillations of the classical regularization methods. This method successfully reconstructs both the amplitude of the interface discontinuities and the photothermal depth-contrast variations within the bounding edges, to a resolution limited by the resolving kernel for the underlying Nth-order Tikhonov constraint. The edge-field regularization dramatically reduces the errors associated with profiling photothermal contrast in bounded zones that are depth-displaced in the sample. Received: 19 September 2002 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-514/398-3797, E-mail: joan.power@mcgill.ca     

Classical and photopyroelectric studies of optical and thermophysical properties of starch sheets: dependence on water content and temperature

The optical and thermophysical parameters of starch films (obtained by extrusion) were determined in order to obtain input data for photothermal studies of water migration in such films. The optical spectra, sorption isotherm, and volume change were measured for samples equilibrated at different levels of relative humidities. The photopyroelectric method was used to measure thermal parameters as a function of water content and temperature. The thermal conductivity, thermal effusivity, and volume specific heat all increase with water content while the thermal diffusivity remains almost constant. The temperature behaviour of the thermal parameters in the -15+70 °C temperature range was measured for samples with different water contents. No phase transition was observed at 0 °C, which proves either that water in the starch matrix is not in a free form, or the fact that water dissolved other particles shifting the melting point to a lower temperature. Received: 1 June 1999 / Final version: 13 March 2000 / Published online: 7 June 2000     

Plasmon resonances observed in light scattered by large alkali clusters

Received: 14 October 1996/Revised version: 15 January 1997     

Multiplexing encrypted data by using polarized light

We investigate the feasibility of multiplexing, employing polarized light, a set of security encrypted data. The encryption approach is based on the double random pure-phase enciphering method. Phase conjugation operation is conducted in the reconstruction stage with the aid of a photorefractive crystal which stores the encrypted information. When storing each encrypted image, a polarization change is introduced in the system. This induces decorrelation on the speckle patterns inside the storing medium. We apply this approach for multiple image encryption. We show experimental results that confirm our approach.     

Optical encryption with a reference wave in a joint transform correlator architecture

In this work we introduce a digital holographic configuration in a Joint Transform Correlator (JTC) architecture for encryption purposes. Conceptually, it is a Mach-Zender interferometer, with a JTC in one arm and a reference wave in the other. We describe the practical implementation of this architecture, along with experimental results that support the proposal. We analyze the noise influence caused by intensity saturation during image capture, and this information allows us a filtering process to reduce the information to be handled, with a consequent increase in the speed of the total procedure.     

Structuring of aluminum films by laser-induced local oxidation in water

A simple method for patterning of thin (15–650 nm) aluminum films on glass substrates by direct, low-power, laser-thermal oxidation in water under common laboratory conditions is demonstrated. Local heating of the metal film enhances the formation of aluminum oxide (hydrargillite, Al₂O₃–3H₂O) and provokes breakdown of the passivation layer followed by local corrosion at temperatures close to the boiling point of water. Moving the focus of an Ar-ion laser (λ=488 nm) over the aluminum film with a speed of several μm/s yields grooves flanked by hydrargillite. Upon through oxidation of the metal these structures act as electrically insulating domains. Depending on the film thickness, the minimum width of the line structures measures between 266 nm and 600 nm. The required laser irradiation power ranges from 1.7 mW to 30 mW. It is found that the photo-thermal oxidation process allows for writing of two-dimensional electrode patterns. Received: 16 July 2001 / Accepted: 23 July 2001 / Published online: 2 October 2001