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Thermal Lab

Description

The thermal laboratory includes four major experiments:

A confocal Raman Spectrometer:

The Raman spectrometer is home-made. It operates in the backscattering configuration using a 532-nm laser line from an Ar-ion laser or a 632.8-nm laser line from a He-Ne laser. The incident light is focused to a spot size of approximately 1µm using a confocal microscope with a long working-distance 100 or 50 objective protected by a cooling jacket. The laser power on the sample surface can be tuned from 0.05 to 15 mW. The scattered light is analyzed using a triple monochromator equipped with a Peltier-cooled charge-coupled device (CCD) detector. A precise voltage-controlled resistive heater can be used to carry out Raman measurements in the temperature range between room temperature and 1200°C.

A conventional Fourier-Transform Infrared Spectrometer:The Fourier-Transform spectrometer operates in the 40-4000 cm-1 wavenumber range. The sample can be held at different temperatures between room temperature and 1200°C. It uses a nitrogen-cooled deuterated triglycine sulfate (DTGS) detector and potassium bromide (KBr) beam splitter.

A photothermal beam deflection experiment:
The experimental setup is schematized below:
A 10.6 µm wavelength CO2 laser beam is modulated at different frequencies using an acousto-optic modulator and focused on the sample surface using a 3 cm effective focal length lens. The CO2 laser beam is combined with a low-power argon-ion laser beam for visualization and alignment purposes. An IR camera and a single axis positioning stage are used to drive the sample surface to the exact focal point of the heating beam. The IR camera is also used to monitor the variation of the local absorbance of the surface during data acquisition, through the measurement of the back-reflected IR beam. The mechanical and thermal response of the region irradiated by the IR heating beam is monitored by using a He-Ne laser probe beam incident to the sample surface at very small angles. The amplitude and phase of the longitudinal component (parallel to the heating beam) and the transverse component (perpendicular to the heating beam) of the He-Ne probe beam after interacting with the sample and the air layer above the heated region are measured with a four-quadrant photodetector whose outputs are amplified with a two-channel lock-in amplifier. As depicted in the figure inset, the heating beam is moved across the sample surface perpendicularly to the probe beam using a single axis positioning stage. All the measurements are repeated for several different frequencies.

The periodic sample irradiation leads to the generation of thermal waves, which damp rapidly in the air layer in contact with the sample surface. The periodic temperature gradient in the air accompanying the thermal waves gives rise to a periodic gradient of refraction index capable of periodically deflecting the He-Ne laser probe beam passing very close to the sample surface, at the frequency of the modulation of the heating beam. Besides the induced temperature gradient in the air, the thermoelastic deformation of the surface may significantly deflect the longitudinal component of the He-Ne probe beam. It is worth noting here that the important thermoelastic deformation of the surface due to the high absorbance of IR in the surface layers makes the largest contribution to the deflection of the longitudinal component of the probe beam. Thus, the transverse component of the probe beam is deflected due to only the mirage effect, whereas the longitudinal component of the probe beam is deflected primarily by the thermoelastic deformation of the surface. Modeling the deflection of the probe beam due to the mirage effect or the thermoelastic displacement of the surface provides the thermal properties of the measured sample.

Fourier-Transform Infrared Scattering Spectrometer:

This is a homemade experiment. It measures the infrared scattering spectrum by the thermoelastic displacement of the surface induced by local heating caused by a well-focused infrared laser beam. It allows measuring surface electromagnetic modes and obtaining the infrared dielectric properties of ultrathin films which cannot be measured by classical infrared spectroscopy.