Photothermal spectroscopy of GaAs-Based Multiple Quantum Wells

with Raj Gupta

GaAs-based multiple quantum wells (MQW) are of enormous technological importance. At the same time they are pedagogically very interesting because they represent a very simple quantum system. We are using photothermal deflection spectroscopy (PTDS) to determine optoelectronic and thermal transport properties of MQW samples. PTDS is a particularly valuable technique because it provides information on both the optoelectronic and thermal transport properties simultaneously. The REU student will work along side a graduate student on this project. This is a particularly suitable project for a REU student because the physics of both PTDS and MQW can be understood at the undergraduate level. Moreover, the student will be exposed to a topic of enormous current interest (MQW) and to important instrumentation such as Ar+ laser, ring dye laser, digital processing lock-in amplifier, etc.

The principle of PTDS is very simple and can be understood as follows: A laser beam is normally incident on a MQW sample. the sample could be free standing or deposited on a substrate. The laser beam (called pump beam) is tuned to a wavelength where the sample has a significant optical absorption. The absorbed optical energy appears as heat, and the temperature of the sample rises. The heat diffuses backward as well as laterally in the sample. At the same time some of the heat is conducted to the fluid in which the sample is immersed. The fluid gets slightly heated and its refractive index is modified. The change in refractive index is monitored by a second, weaker laser beam (called the probe beam). The fluid is transparent to both the pump beam and the probe beam. The probe beam gets deflected due to the uniform refractive index created by the heading of the fluid, much like the bending of light rays in a mirage, and can be detected by a position-sensitive detector. The size of the deflection signal is proportional to the heat produced, and thus it is a sensitive measure of the optical absorption coefficient of the sample. Measurement of the optical absorption coefficient as a function of the wavelength of the pump laser and temperature of the sample provides a wealth of information on the optoelectronic properties of the sample. As one might imagine, the PTDS signal depends, in addition to the optical absorption coefficient, on the thermal transport properties of the sample. Thus, the PTDS technique also offers a convenient way of measuring thermal transport properties. Such measurements are valuable in themselves because of the problem of heat dissipation in modern semiconductor devices.

We use radiation from an Ar+ laser-pumped ring dye laser as the pump beam and a HeNe laser provides the probe beam. The pump beam is modulated and a lock-in detection of the signal is performed. Theoretical models for the analysis of the data have already been developed and are currently in use in our laboratory. The project should proceed smoothly and the REU student should be able to get significant results within the 10-week period.


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