Thermoreflectance of optical phonon resonances

Ultrashort laser-based techniques such as time-domain and frequency-domain thermoreflectance have been repeatedly identified as effective tools for observing thermal transport processes at the nanoscale. However, there are many limiting factors inhibiting the indoctrination of these metrologies into industrial processes due to the need for a metal transducer. This thin metal film severs a dual purpose: A known absorbance converting the optical energy into thermal energy, and large temperature dependent reflectivity changes for high signal. In order to bring thermoreflectance based techniques in-situ optimizing these values in the absence of a thin metal film is key.
The highest thermoreflectance coefficients currently used in thermorefectance measurements are near the energies associated with inter-band transitions where the electrons of metals transition from a dense sub band to a more free band at the Fermi surface. However, the root cause of theses high coefficients are the shape and intensity of Lorentzian resonances within the dielectric function of a material. Thus, any strong resonance in the dielectric character of a material should give rise to strong thermoreflectivity. In this work, we analytically quantify and experimentally measure the thermoreflectance coefficient due to lattice resonances in dielectric films through temperature dependent infrared variable-angle spectroscopic ellipsometry (IR-VASE) measurements. We demonstrate thermoreflectivities in the mid-infrared spectral region rivaling the preeminent values reported in Gold and Aluminum in the visible. The intense thermoreflectance response outlined in this work from optical phonon modes provides an opportunity for transducer-less in-situ characterization of thermal and interface properties. We demonstrate the ability to directly measure the thermal relaxation of optical phonon modes we measure with IR ellipsometry by performing pump probe measurements on amorphous silicon dioxide without a transducer. For this purpose, we employ an ultrafast pump-probe spectroscopy technique with a mid-infrared probe, which provides sub-picosecond resolution of optical phonon resonances. The capability of directly monitoring the ultrafast relaxation of these modes demonstrates the potential of infrared pump-probe measurements of optical phonons a solution for nanoscale in-situ characterization of semiconductor devices.