Heat source distribution, vertical structure, and coating influences on the temperature of operating 0.98 mu m laser diodes: Photothermal reflectance measurements

In the present work single-quantum-well laser diodes operating at 0.98 mm are investigated by photothermal reflectance microscopy. Temperature maps were obtained for the output facet of all devices studied. Furthermore, the temperature distribution was determined along the cavity (on the ridge) of lasers soldered with the junction side up. Near the facets, the measured temperature was found to be about seven times the bulk's temperature, indicating the presence of an important surface heat source. The signal phase distribution of the laser facet shows the important role of the vertical structure on the heat confinement. Comparison between experiments and calculations shows that the confinement layers (GaAlAs and GaInP) thermal parameters are the principal responsible for the heat propagation in these structures near the active region. The same calculations show the role of the coating (Al2O3) in the heat propagation, and give a quantitative ratio between surface and bulk heat sources. Measurements made on the facet and on the ridge as a function of injection current were found to present a quite similar behavior, leading to the conclusion that thermal effects are strongly dominant in these measurements, masking any carrier or electroreflectance effects. Finally, measurements made under different light output power conditions and under the same injection current conditions showed that the surface heat source is caused by laser light absorption at the facets. (C) 1998 American Institute of Physics. [S0021-8979(98)05419-X].8473491349

Temperature dependence of the refractive index near the reentrant-isotropic-calamitic-nematic phase transition

The laser-induced nonlinear optical response of a lyotropic liquid crystal system in the reentrant-isotropic and calamitic-nematic phases is investigated by the use of the thermal lens technique. The occurrence of an inversion in the temperature coefficient of the ordinary refractive index, dn(perpendicular to)/dT, near the reentrant-isotropic-calamitic-nematic phase transition, is discussed. This effect is attributed to the behavior of the electronic polarizability due to the change in micelle shape near the isotropic-nematic transition, and correlated with the results obtained near the nematic-isotropic transition, previously reported.641

Sensitivity enhancement in thermoreflectance microscopy of semiconductor devices using suitable probe wavelengths

In this paper we present an experimental and theoretical study of the thermoreflectance response as a function of the probe wavelength for layered microelectronics structures. The investigated sample consists of a polycrystalline silicon conducting track grown on a SiO2-coated Si substrate. Thermoreflectance measurements were carried out in the wavelength range from 450 to 750 nm with the track biased in modulated regime. An oscillating pattern is observed in the spectral region where the upper layer is transparent. Such oscillations are due to the interference resulting from the multiple reflections at the interfaces. Using a thermo-optical model, we show that the optical constants (n and k) of the materials, which are wavelength dependent, as well as their temperature derivatives (dn/dT and dk/dT), strongly influence the thermoreflectance signal. The optical thicknesses of the layers, mainly determined by the real part of the refractive indices, define the period of oscillation. On the other hand, the imaginary part of the refractive indices establishes the cutoff wavelength of the oscillations. Below this cutoff wavelength, the probe light does not penetrate the material and the upper-surface reflectance dominates the signal. (c) 2005 American Institute of Physics.98

Geometrical anisotropy dependence of thermal diffusivity in lyotropic nematics: Mode mismatched thermal lens measurements

In this work the quantitative theoretical treatment for two beam mode mismatched thermal lens spectrometry is applied to investigate the thermal diffusivity anisotropy of two lyotropic mixtures: (1) potassium laurate, decanol and water and (2) potassium laurate, potassium chloride and water in the nematic calamitic phase. The ratio between the thermal diffusivities parallel and perpendicular to the director has been shown to be smaller than those reported for thermotropic liquid crystal. This observation is explained by using a simple model where this ratio is correlated to the micellar shape anisotropy. (C) 1996 American Institute of Physics.68243371337

Magnetocaloric effect in GdGeSi compounds measured by the acoustic detection technique: Influence of composition and sample treatment

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)In this paper we explore the acoustic detection method applied to the investigation of the magnetocaloric effect in Gd and Gd(5)(Ge(1-x)Si(x))(4) compounds, in the temperature range from 230 to 360 K and for magnetic fields up to 20 kOe. Measurements were performed in as-cast materials, both for powder and pellet samples, and in tree samples with compositions around Gd(5)Ge(2)Si(2) that underwent different thermal treatments. Small differences were observed when comparing powder and pellet samples of Gd and Gd(5)(Ge(1-x)Si(x))(4) compounds with 0.500<x <= 1.00. For the alloys with composition around Gd(5)Ge(2)Si(2), which exhibit giant magnetostriction and coexistence of distinct phases, expressive changes were observed when comparing powder and pellet samples. Based on these cases, it is easy to see that the acoustic method can distinguish a second-order phase transition from a first-order magnetic-crystallographic one, and that it presents good sensitivity to detect spurious material phase in small quantities. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3357375]1077Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)FAEPEX-UnicampFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

DEPTH PROFILE OF FERROMAGNETIC LAYERED SAMPLES STUDIED WITH PHOTOTHERMALLY MODULATED MAGNETIC-RESONANCE

Depth profile analysis of a ferromagnetic layered sample composed of gamma-Fe2O3 and CrO2 tapes were performed with photothermally modulated ferromagnetic resonance (PM-FMR). Those analysis were done by using phase-resolved and modulation frequency variation methods. The experiments showed the advantages of this technique over the conventional one. The dependency of the detected signal on the modulation frequency was also studied for single layer samples, thus determining the microwave-absorbing layer thickness of the tapes.4C766767

PHOTOACOUSTIC CHARACTERIZATION OF A 2-LAYER SYSTEM

In this paper the use of the so-called open photoacoustic cell for thermal characterization of two-layer systems of variable thickness is described. It is shown that the thermal diffusivity as well as the thermal conductivity are completely determined, based upon the effective sample model widely used in heat-transfer problems.70117046705