Department of Mechanical Engineering
Kenneth E. Goodson
We use high speed pump-probe optics to characterize a broad variety of materials and interfaces. The short duration of the heating pulse and the high temporal precision of thermometry allows for detailed study of the impact of properties below the surface on the temperature respose at the surface. Our setup falls in the general domain of Time Domain Thermoreflectance (TDTR) methods and plays a central role in our fundamental studies of the thermal and optical properties of thin films, interface, and nanostructured materials. We use periodic pump pulses from a mode-locked Nd:YVO4 laser (9.2 ps pulse width, 82 MHz repetition rate, ∼10 nJ/pulse energy, 1064 nm wavelength) deposit heat in the metal film, establishing a transient temperature field within the sample. The surface temperature of the metal is then measured by an optically delayed probe beam, derived from the pump, which is reflected off of the sample and collected by a fast photodetector. The pump beam is externally modulated at 8 MHz for lockin detection and converted to 532 nm with a second harmonic generator to enable the rejection of pump leakage at the detector. The coaligned pump and probe beams are focused on the sample surface with Gaussian waist diameters of 10.0 and 5.0 μm and powers of 20 and 5 mW, respectively. The temperature excursion (< 10 K) due to the laser heating is significantly less than the variations in temperature we are studying due to annealing or other processes.
Sample design and preparation is an essential component of any TDTR measurement, and the focus in this laboratory has been on developing unique geometries consisting of nanoscale multilayers, transparent substrates (for backside access), and other new features to enable first-of-kind data and improved experimental uncertainty. Most of the measurements feature a thin metallization film, typically at the top of the sample (closest to the laser source), which serves to absorb the heating pulse and precisely define the region at which thermometry is being performed. The nearly linear variation of the reflectivity of this film with temperature yields an optical response that is representative of the transient surface temperature over a few nanoseconds with temporal precision in the 10 ps range.