G. L. Liakhou

University of Rome Tor Vergata, Roma, Latium, Italy

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Publications (56)51.47 Total impact

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    ABSTRACT: Using the photo-deflection method in quasi c.w. regime refractive index variations of CdSx Se1 − x doped glasses have been observed consisting of two independent contributions Δn = Δnt + Δne. The photothermal term Δnt = (dn/dT) ΔT (ΔT is the temperature increase) is positive and slow in time, the electronic term Δne is negative and fast. The electronic term appears at first due to a reversible contribution (during a time scale of 1/ω, where ω is the chopper frequency) and after a time of a few tenths of seconds disappears irreversibly. It is attributed to a redistribution in space and energy levels of electrons in the semiconductor microcrystals. This redistribution is large enough when the electron relaxation is not too fast. The consequent refractive index change decreases in time and disappears under prolonged laser irradiation. A model is presented in which the presence of traps is taken into account.
    Journal of Modern Optics 03/2007; 39(9):1965-1976. · 1.16 Impact Factor
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    ABSTRACT: Photothermal depth profiling is usually applied to inhomogeneous materials, and allows to localize the optical inhomogeneity or retrieve the thermal effusivity depth profile by simple monitoring the surface temperature or any related photothermal signal after the pump beam pulse absorption. In this article we want to discuss two different kind of inverse problems; the heat source depth profiling (1$^{\rm st}$ type), and the effusivity depth profiling (2$^{\rm nd}$ type). Finally we want to show the equivalence of the two problems, and provide some mathematical tools to get the solutions by the Thermal Wave Backscattering (TWBS).
    Journal de Physique IV (Proceedings) 01/2005; · 0.29 Impact Factor
  • G. Liakhou, S. Paoloni, M. Bertolotti
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    ABSTRACT: The effects produced by absorption of an intensity modulated C O <sub>2</sub> laser in C O <sub>2</sub>- N <sub>2</sub> mixture are investigated here by means of the photothermal deflection technique. The deflection signal arises from C O <sub>2</sub> molecule deactivation and energy transfer from C O <sub>2</sub> to N <sub>2</sub> molecules, which are responsible for heating and cooling the mixture, respectively. The dominant mechanism is determined by the pressure of the gas mixture, which, in turn, affects the C O <sub>2</sub> molecule lifetime. A theoretical model based on rate equations for the C O <sub>2</sub>- N <sub>2</sub> system is presented, in which the deflection signal is given as a function of the excited molecule populations. The theoretical predictions are shown to be in reasonable agreement with the experimental results.
    Journal of Applied Physics 11/2004; · 2.21 Impact Factor
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    ABSTRACT: Polyimide polymers can be converted from an insulator to a conductor by suitably pulsed excimer laser irradiation. The ultraviolet light is strongly absorbed by the polymer and pyrolyzes the material into carbon clusters. The formation of those clusters is accompanied by an increase of both electric and thermal conductivity. We observed photothermally the steep rise of thermal diffusivity when the density of the electrically conducting clusters exceeded the percolation threshold. This finding illustrates the charge carriers contribution to the kinetic energy transport in disordered materials. © 1999 American Institute of Physics.
    Journal of Applied Physics 05/1999; 85(11):7540-7543. · 2.21 Impact Factor
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    ABSTRACT: This article describes and proves the basic phenomena which take place when thermal waves approach an interface between two media: the reflection and the refraction. In synthesis the Snell law for plane thermal waves is proved, both theoretically and experimentally, by means of the mirage technique. © 1999 American Institute of Physics.
    Journal of Applied Physics 05/1999; · 2.21 Impact Factor
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    ABSTRACT: In this paper we want to discuss several new aspects in the theory of depth profiling, introducing a new theoretical model for the heat diffusion in media with variable thermal parameters. The case of thermal conductivity and diffusivity, both are functions of depth is widely analyzed. The thermal depth profiles, reconstructed by numerical simulations according to this new model, are in excellent agreement with the original ones. Experimental results for steel samples hardened at the surface are also presented. © 1999 American Institute of Physics.
    AIP Conference Proceedings. 03/1999; 463(1):24-26.
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    ABSTRACT: In this paper we want to introduce the theory of depth profiling in time domain. We present a new theoretical model for the heat diffusion in media with variable thermal parameters and show the retrieval procedure to reconstruct the thermal effusivity depth profile from the surface temperature dynamic. The effect of the noise on the quality of the reconstruction is also briefly discussed. © 1999 American Institute of Physics.
    AIP Conference Proceedings. 03/1999; 463(1):37-39.
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    ABSTRACT: The photothermal deflection technique has been used to determine the thermophysical properties of artificially aged paper samples. The deflection amplitude ratio and the phase difference of the deflection angle between front and rear surface illumination have been measured as a function of the pump beam modulation frequency, in order to avoid the dispersion effects mainly introduced by the probe beam vertical offset with respect to the sample surface. A theoretical model for the photothermal signal generation, taking into account the unavoidable light scattering occurring at the paper fibers–air interfaces, has been developed. A photothermal deflection method has been also proposed to measure the transmittance and the reflectance values of a given paper sheet, thus offering a proper constraint to the whole fitting procedure. In this way it has been possible to estimate the thermal diffusivity, the optical absorption and the scattering coefficient values of all the samples examined. A correlation between these values and the aging state of the paper has been found. © 1999 American Institute of Physics.
    Journal of Applied Physics 02/1999; 85(5):2881-2887. · 2.21 Impact Factor
  • 01/1999;
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    ABSTRACT: Photothermal lateral scans respond to both thermal conductivity and heat capacity depth profiles. The depth profiling is studied by two different models introduced to solve the 3D heat diffusion in inhomogeneous media the finite difference method and the thermal wave back scattering technique. Both approaches provide the base for numerical retrieval procedures. Theoretical and experimental inversion results on hardness steel samples are in very good agreement.
    01/1999;
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    ABSTRACT: This article describes and proves the basic phenomena which take place when thermal waves approach an interface between two media: the reflection and the refraction. In synthesis the Snell law for plane thermal waves is proved, both theoretically and experimentally by means of the mirage technique.
    01/1999;
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    ABSTRACT: Although the mechanism of heat transport is a typical diffusion process, in cases in which the heat source is of a sinusoidal type, it may be found convenient to transform the Fourier diffusion equation in a formal Helmolz equation, speaking of thermal waves. In the present paper we will use this formalism for handling data from a photothermal measurement to retrieve the profile of thermal diffusivity under the surface of a solid material, showing that the surface temperature field obtained by the photothermal method can be connected to the thermal properties of the material in a simple and elegant way by using the concept of thermal waves scattering by the small thermal inhomogeneities of the material.
    Proc SPIE 01/1999;
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    ABSTRACT: The thermal response of a semiconductor laser diode is studied through two different methods based on the photothermal deflection technique. The full theoretical model is reviewed; the study of the temperature profile and hence the maximum mirror temperature rise, obtained for different sizes of the device, allow one to expect thermally stable lasers. The way to obtain, from the measurements, the thermal parameters of the entire structure (diffusivity, conductivity) is also considered. The photothermal deflection equipment is described in detail for this application. The experimental results on three different kinds of laser diode are thoroughly discussed: double-heterostructure AlGaAs/GaAs, double-heterostructure InGaAsP/InP, and single-quantum-well (QW) structure AlGaAs/GaAs InGaAs QW.
    Measurement Science and Technology 12/1998; 6(9):1278. · 1.44 Impact Factor
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    ABSTRACT: The photothermal deflection technique has been usually applied, for the thermal diffusivity measurements, in the transverse skimming scheme. To overcome some limitations of the skimming, a surface reflection scheme (i.e., bouncing scheme) has been introduced in which the probe beam is reflected from the sample surface. In this configuration the probe beam deflection is obtained as a result of two different mechanisms: the thermal gradient in the gas near to the heated sample (mirage) and the sample surface deformation due to the thermal expansion (displacement). The superposition of these two effects must be taken into account when deriving the thermal diffusivity. In this article the mirage and the displacement have been studied from a theoretical and experimental point of view, and a new method for the measurement of thermal diffusivity in the bouncing scheme is presented. A special setup is described to obtain separately the mirage and the displacement signals from which the thermal diffusivity and the thermal expansion coefficient can be derived. The experimental values for different samples obtained by applying our method are in agreement with the literature values. © 1998 American Institute of Physics.
    Journal of Applied Physics 02/1998; · 2.21 Impact Factor
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    ABSTRACT: The transverse photothermal deflection configuration, which uses the probe beam reflected from the sample surface (bouncing scheme), has been investigated and the theoretical equations governing the probe beam deflection have been derived. In particular the role of the probe beam incidence angle has been studied and found to be analogous to that played by the probe beam vertical height when the photothermal deflection technique is applied in the skimming scheme. A substantial analogy between the two experimental schemes has been found by comparing the deflection signal dependence on the probe incidence angle and on the probe height. To demonstrate the theory, photothermal deflection experiments have been performed on an invar specimen that, owing to its low thermal expansion coefficient, allowed us to neglect the contribution coming from thermally induced surface displacement in a restricted incidence angle range.
    Applied Physics B 01/1998; 67(5):641-646. · 1.78 Impact Factor
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    ABSTRACT: The photothermal deflection technique applied to a gas thermal wave resonator seems to be one of the most powerful techniques to investigate in situ the thermal diffusivity of the gas. After a brief description of what a thermal wave resonator really is, a discussion is presented of the advantages and disadvantages of its use for measuring the gas thermal diffusivity.
    International Journal of Thermophysics 01/1998; 19(2):603-613. · 0.57 Impact Factor
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    ABSTRACT: The photothermal deflection technique is applied in transverse configuration to measure the thermal diffusivity of semiconductor wafers. The large size of these samples inhibits the possibility to make the probe beam skim the sample at a small height which is required for a direct thermal diffusivity measurement. To overcome this problem, three new experimental schemes are proposed, each one based on a different geometry of the heat diffusion (one-, two-, or three-dimensional scheme). In particular for the 3D experimental scheme, a new mirage setup is described which uses two crystalline prisms 6 mm apart from each other to let the probe beam skim 50±3 μm high over the sample surface, with a spot size of 22 μm. The main advantages of this setup, here discussed, are the obtained low probe beam height which is, moreover, independent of the sample dimensions, and the cheap technology to produce the necessary high-quality prisms. The performances of the new schemes have been tested by comparing, for well-known semiconductor wafers (InSb, InAs, InP, GaAs, GaP, Ge, and Si), the experimentally measured thermal diffusivity with the values reported in the literature. © 1997 American Institute of Physics.
    Review of Scientific Instruments 02/1997; 68(3):1521-1526. · 1.60 Impact Factor
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    ABSTRACT: The photodeflection method is able to characterize the propagation losses in optical waveguides. The theory of the transverse configurations in air and in situ is reviewed. The procedure to determine the propagation loss coefficient is widely discussed. The experimental results on two different Ti:LiNbO3 and glass channel waveguides are in good agreement with theory.
    Optical Engineering 01/1997; 36(2):515-520. · 0.88 Impact Factor
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    ABSTRACT: The photothermal deflection technique is applied to measure the surface deformation induced by laser absorption at the surface. The theory of the probe beam reflection due to the surface displacement is reviewed. The first experimental results show that this technique is able to measure displacements of a few Angstroms.
    Proc SPIE 09/1996;
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    ABSTRACT: A theoretical discussion on the resolution of buried air defects by using the photothermal deflection technique is presented. The thermal wave interferometry theory is described for the general case of layered materials. The principle of measurement of thermal parameter is reviewed. The analysis of buried air gap is made considering the defect like a mirror which is able to reflect partially the thermal wave giving rise to an interference effect at the solid surface. The condition of this interference is studied together with the limit to the capability to detect defects. The surprising theoretical result is that an air buried layer of thickness less than 1 percent of the air thermal wavelength is still detectable. The photothermal deflection technique is introduced as a suitable technique to investigate the thermal interference effects.
    Proc SPIE 08/1996;