Thermal and Optical Properties of SiO2/GaN Opals by Photothermal Deflection Technique

The use of opals in many devices requires the lowest available thermal conductivity. In this case the switching power and switching time are decreased. It is thus evident the importance of the experimental determination of the thermal properties of the opals. Since the typical dimension of these kind of structures is of the order of 5x5x0.5 mm3, the problem is not trivial. The photothermal deflection technique, that is a non-contact method is one possible solution. We applied this technique to different SiO2/GaN opals obtained by assembling SiO2 spheres with a diameter of about 650 nm, and by infiltrating GaN at different percentage. Two different configurations have been used to investigate separately both thermal and optical properties by using the photothermal deflection technique. The first confuguration basically uses two laser beams: one as a pump beam (Ar laser @488nm) and the other as a probe beam (He-Ne laser) [1]. The pump beam modulated in time is weakly absorbed by the opal and generates a timedependent temperature rise field called "thermal-wave". The probe travels in air skimming the sample surface at the minimum possible distance and is deflected by the temperature gradient (mirage effect). The deflection angle is strongly dependent on both thermal and optical parameters. From the plot of the deflection angle versus the distance between the two beams y it is possible to retrieve the sample thermal diffusivity [1]. In fact the amplitude of the deflection signal versus y exhibits an exponential behaviour where the extinction length is pu = D/f where D is the thermal diffusivity and f the modulation frequency. The plot in logarithmic scale should be obviously linear. In fig. 1 the logarithm of the amplitude vs y has been plotted at different modulation frequencies for the opal n. 1 made just of SiO2 spheres. After some distance the slope becomes linear and it is easy to calculate the thermal diffusivity. In the inserted table the thermal diffusivity is reported for all the investigated Si02/GaN opals. It is evident that the thermal diffusivity increases with the content of GaN, but the values are always below the thermal diffusivity value (8.2 1 0-3 cm2/s) for pure SiO2. We applied again photothermal techniques in a second configuration to study the absorption spectra of all the SiO2/GaN opals. The setup consists of a photothermal deflection setup, where the illumination comes from a Xe lamp, and the wavelength of the spectrum is selected by monochromator. The scan is performed in the range from 350nm to 1150nm. From fig. 2 it is evident that the absorbance is not low in the range 350nm - 600nm when the wavelength is less than the dimensions of the opal SiO2 spheres (diameter 650 nm) [2].