Determination of thermal conductivity of top side illuminated n-type and p-type porous silicon (PS) samples have been performed by means of photo-acoustic and optical pump-probe techniques. Concerning the n-type samples, the values found can be explained in the framework of Looyenga effective medium theory, leading to a thermal conductivity of the solid phase of 13 W/mK. This result suggests that n-type thick PS samples formed by top side illumination has a complex multilayer morphology in which mesophases, nanophases and macrophases coexist. The investigation on p-type nanoporous silicon shows that the thermal conductivity does not scale with porosity p as (1-p)3, suggesting that the percolation of the solid phase is more pronounced respect to the case of a regular “Looyoenga-like” mesophase. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, the thermal diffusivity of free-standing films of nanoporous silicon has been studied. The films were obtained by electrochemical etching of p-type silicon. Measures were performed under vacuum and in the temperature range 300–600 K, so that the internal sample surface was not contaminated by ambient pollutants and hydrogen desorption did not affect significantly the surface chemistry due to high temperature effects. An investigation technique, based on thermal lensing, was adopted since it seemed to be more suitable than other techniques generally used for the determination of thermal properties of porous layers on crystalline substrates. The comparison between theoretical previsions and experimental results obtained for thin layers of known properties shows that the developed technique is reliable and of easy application for solid samples of low thermal conductivity. The diffusivity of the investigated nanoporous silicon samples is reduced with respect to the one of crystalline silicon due to both the porosity of the material and to the reduction in diffusivity of each individual nanocrystal, resulting from the enhanced boundary scattering of lattice waves.

Non-destructive thermal and optical characterization of materials can be successfully performed by the photo-acoustic technique. In this work, this technique has been applied to the measure of thermal conductivity in porous silicon by considering the photo-acoustic response at fixed frequency of samples having the same porosity but different thicknesses. Experimental data are interpreted in terms of a model which takes into account both scattering effects and the contribution to the photo-acoustic signal of the interstitial gas expansion. The measured thermal conductivity is found to be lower than the one reported for crystalline silicon by two orders of magnitude. A discussion of the photo-acoustic signal dependence on the morphology of the porous medium is also presented.

Measurements of the thermal conductivity for free-standing porous silicon layers were performed by means of an optical pump-probe experimental set-up. By transient heating due to laser pumping, a refractive index modulation was induced in the sample and, solving the heat propagation equation for inside the solid film, it is shown that the time decay of the nonlinear transmittance can be related to the thermal conductivity. The optical technique demonstrated here is contactless, quite simple and does not require much effort in data analysis, and is therefore very useful for thin-film characterization. The thermal conductivities of the porous silicon samples, whose porosities lay in the range 60-70%, were taken into account, and we found good agreement with results obtained with different techniques.

The photoacoustic technique is used largely for the thermal and optical non-destructive characterization of materials. In this paper we present a new and alternative method for the measure of thermal conductivity in porous silicon by considering the photoacoustic response at fixed frequency of samples with the same porosity but different thickness. The analysed samples are fabricated by etching n-type, 1 Omega cm, 550 mu m thick crystalline silicon; they have a porosity ranging from 40-70% and thickness from 45-250 mu m. The measured thermal conductivity is lower than the one reported for crystalline silicon by two orders of magnitude.

The photoacoustic technique is applied to study the thermal properties of n-type porous silicon samples having different porosity. The photoacoustic response of the investigated samples is referenced to samples having the same porosity but larger thickness and analysed as a function of the modulation frequency of the incident absorbed light. The response points out interference effects due to multiple reflections of the excited thermal wave inside the thin layer of the porous material. Taking into account also the contribution to the signal of the internal gas expansion, an estimation of the thermal conductivity of the porous samples is derived. The measured thermal conductivity is found to be lower than that reported for crystalline silicon by two orders of magnitude.

CW transillumination of tissue phantoms at 1064 and 820 nn has been performed, by measuring the profile of transmitted light transverse to the direction of the incident beam for various values of the scattering and absorption coefficients and for different viewing angles of the photodetector and sample thicknesses. The set of measurements, compared with the results of Monte Carlo simulations, allows to derive a CW optical characterisation of tissue-like samples and to assess the possible resolution improvement of collimated detection with narrow viewing angles.

An extended series of transillumination experiments has been performed in vitro on animal samples (bovine muscle, up to 30- mm-thick; chicken wing and quail femur, 12-mm-thick) and in vivo on the human hand (thickness, about 20 mm), using a pulsed light source (7 ns, about 10-4 J/pulse, 10 Hz rep rate) from a collimated (1.2 m) Nd:YAG laser beam (1064 nm). A PIN photodiode connected to a digital oscilloscope was used to measure the maximum intensity of the beam pulse transmitted through the sample (i.e., no temporal discrimination of the output signal was attempted) while it was scanned across the source/detector assembly. One dimensional scans were performed on bovine muscle samples in which thin metallic test objects were embedded, in order to study the spatial resolution of the technique (for bovine muscle at 1064 nm, absorption and reduced scattering coefficients are reported to be about 1 cm-1 and 3 cm-1, respectively). The measured spatial resolution was as good as 3.6 mm in 30 mm of tissue thickness. In the two-dimensional scans of the chicken and quail sample, fat and bone tissues can be easily seen with good resolution, whereas imaging of the middle finger of a human hand shows cartilaginoid and bone tissue with 1 - 2 mm resolution. Hence, this simple collimated quasi-cw technique gives significantly better results for tissue imaging than pure cw transillumination. Use of (pulsed) light above 1000 nm and a high energy content per pulse are supposed to explain the positive experimental findings.

We have analysed the propagation of two continuous-wave Ar+-laser beams travelling in opposite directions in a two-component nonlinear heterogeneous medium (the polymeric blend PMMA-EVA), in a mirror retroreflection geometry. Bistable reflection associated with the laser-induced suppression of scattering is observed. This phenomenon has a thermal origin. Based on light- and heat-transport theory inside the material, a simplified explanation of the experimental evidence is presented, showing the conditions for the onset of optical bistability. Qualitative agreement with our experimental results is found.

A novel polymeric material, a poly(methylmethacrylate) (PMMA)/ethylene-co-vinylacerate (EVA) blend, is presented. The blend shows outstanding impact behaviour, at the same level as that of commercially-available ultra-tough PMMA. Moreover, transparency is retained with a transparent-to-opaque transition at about 50°C. The authors show the behaviour of the optical transmission as a function of the temperature, and measure some physical properties of this material. An attempt is made to correlate the thermo-optical behaviour to the variation, across the transition, of fundamental parameters, such as the melting of sub-included rubber particles, the refractive indices of PMMA and EVA, and the particular morphology of the blend. The latter is very interesting, as the rubber particles, by etching techniques, have been shown to be composed of sub-included PMMA domains, leading to a so-called multiple core-shell structure.

Scanning PhotoAcoustic Microscopy (SPAM) was used for the imaging of surface and sub-surface layers in electronic circuits and ad hoc specimens. Depth-profiling SPAM was used, as it allows the imaging of opaque samples and sub-surface features; the signal contrast comes from variations in the optical, thermal or mechanical properties of the samples. In SPAM an intensity-modulated, highly-focused light beam (ion argon laser, 488 nm, 20 mW) scans the surface of the sample in the photoacoustic cell, with micrometric resolution. By varying the modulation frequency of the light beam, the thermal diffusion length of the heat waves inside the sample is changed, thus probing different layers beneath the sample surface. From the comparison of the SPAM images of multi-layer microcircuits obtained at different modulation frequencies, features of the first layers, circuit tracks and conductive connections between the layers can be detected. The SPAM images also correspond dimensionally with the actual layout of the circuits and may reveal the presence of defective parts. The preliminary results presented here suggest the introduction of the SPAM technique in the production process for NDE of layered microcircuits and, in general, for the quality control of sample parts.

The well known self-transparency effect, originated from the nonlinear optical behavior of a scattering sample, has been observed for a solid polymeric blend (PMMA-EVA) at a low intensity threshold (25 W cm-2), using a Ar+ laser cw radiation. An interpretation of the experimental results in terms of a nonlinearity of thermal origin is given, using the theory of nonlinear scattering in heteroheneous media. Use of this polymeric blend as a nonlinear material is suggested.

The thermally induced change of the birefringence of a high-birefringence fused-silica optical fiber, when exposed to a proton beam (2 MeV, 0.5–44 nA, 5-mm spot diameter), was measured with a fiber-optic polarimetric sensor. A short length (5 cm) of 125-μ-diam, single-mode, polarization-maintaining optical fiber was put orthogonally to the beam in vacuo and the linear polarization of a HeNe laser beam launched into the fiber was analyzed by a polarizer and recorded by a photodetector. The response of the sensor both to a continuous and to a modulated proton beam was studied. The photodetector output, which varies with the cosine of the induced relative phase retardance between the fiber eigenmodes, provides a dc or an ac (10 Hz test modulation frequency) measurement of the beam current. This device acts as a proton beam intensity monitor, with minimum beam perturbation and a sensitivity of 0.1 nA. Its possible use also as a beam position monitor or as a wire-scanner beam profiler is suggested.

In a previous paper (SPIE Conf. 1584, 1991, 304-307) we proposed an optical fiber interferometric sensor for X-ray dosimetry. Basically, on absorption of a modulated X-ray beam, a temperature rise is produced in a silica fiber, which is detected with a Mach-Zehnder interferometric scheme. In this paper we report on the test measurements performed with our prototype detector using a X-ray rube. A possible application for X-ray synchrotron radiation monitor is discussed.

A general approach to the light-induced anisotropy (LIA) effects in initially isotropic media subjected to a biharmonic laser field (pump-probe), that is based on a tensor operator formalism, is presented. Using such a theoretical treatment, we propose novel nonlinear spectroscopic techniques. We show new results dealing with advantageous measurement of the 3rd-order nondegenerate nonlinear susceptibility (3NNS), including the 3NNS phase (3NNSP) and the real and imaginary parts of the effective 3NNS. Figure 1 is the schematic illustration of a method for measuring 3NNSP in the framework of optically heterodyned polarization spectroscopy (OHPS) when the components of 3NNS are connected by the Kleinman relationships. In the standard OHPS arrangement with the slightly tilted analyzer and the linearly polarized probe beam at 45 to the major axis of the pump polarization ellipse, under tuning of the pump intensity W) or ellipticity (tans) we can find the minimum OHPS signal (AI which is determined by 3NNSP. Another possibility for measuring 3NNSP occurs when the linearly polarized pump and circularly polarized probe beams are used. In this case, the tangent of double azimuth of the output probe polarization is exactly equal to 3NNSP. The validity of the theoretical treatment presented is supported by the experimental data. Furthermore, we point out that some previous theoretical studies devoted to the similar problems are not quite correct and should be revised. In Fig. 2 the nonlinear polarization spectroscopy signal normalized to the maximum value is plotted as a function of c. The line 1 is fitted with experimental data for malachite green in water.1 The experimental results for crystal violet, methylene-blue and rhodamine-B in aqueous solutions are shown by circles, triangles and squares, respectively. They are in rather good agreement with our experimental curve 2.

A fiber-optic Mach-Zehnder interferometer was used for x-ray detection. A single-mode, polarization-preserving silica fiber is exposed to a high-flux x-ray beam (x-ray tube for diagnostic radiology, 30 kV, 16.8-keV average energy, 17-40-mA anodic current), modulated at f = 9 Hz by a chopper with steel blades. The energy absorbed produces a modulated temperature rise that induces a phase shift in the propagation of a He-Ne laser beam with respect to the unirradiated arm of the interferometer. We measured the linearity of the temperature rise with the energy released to the fiber and the linearity of the peak amplitude at frequency f in the power spectrum of the interferometric signal with the anodic current of the x-ray tube. A possible application of this technique to synchrotron radiation monitoring is discussed.

The transition from the transmission to the reflection regime for an Ar+-laser beam propagating in the new polymeric blend PMMA-EVA at a nonlinear interface has been observed. A comparison between the experimental data and a calculation of the input optical intensity at which this transition should occur (1.45×107 W m−2) is presented using Kaplan's theory. The results suggest the presence of thermally induced optical bistability in PMMA-EVA.

The temperature dependent optical properties of a poly(methyl methacrylate) (PMMA) ethylene-co-vinylacetate (EVA) blend (PMMA-EVA) were investigated in order to study its thermo-optical behaviour in the temperature range 30–70‡C. The thermal coefficients of the refractive index dn/dt of PMMA-EVA, PMMA and EVA were measured as a function of the temperature. The observed trend of the, thermal coefficients is related to a first-order phase transition of the EVA component. A comparison with the results of calorimetric investigations by differential scanning calorimetry (DSC) is also reported. The possible use of this material in optics is described.