Photothermal characterization of thermochromic materials for tunable thermal devices

The optical and infrared properties in thermochromic materials have been deeply investigated in the last years due to the large variety of photonic devices that can be realised thanks to the metal/insulator phase transition: “smart” windows which autonomously control sun rays into a room, thermally controllable localized-plasmon, hysteresis enhancement of a phase transition by nanoparticulation, optical memory by the means of phase transition, and ultrafast switching of the photonic stop band in photonic crystals. Among the thermocromic materials (niobium dioxide, vanadium sesquioxide etc..) vanadium dioxide (VO2) represents the most widely used for many applications [1]. Its crystalline lattice exhibits an abrupt semiconductor-to-metal phase transition at a temperature of about TC≈68°C, characterized by a change of the crystalline cell from monoclinic to tetragonal, and consequently by an ultrafast change in the optical and IR properties (i.e. reflectivity and emissivity). In this paper we show how photothermal techniques may represent a useful non-destructive and non contact tool to study the optical and thermal properties changes of vanadium dioxide thin films (both single layer and multilayers deposited onto a silicon substrate [2,3]) during the phase transition. We have performed the standard optical and infrared measurements of reflectance, transmittance, and emittance by changing the temperature of the vanadium dioxide film from 20°C (below Tc) to 90°C (above Tc) so to determine the entity of the thermal hysteresis loops and compare the refractive index in both metallic and semiconductor states with respect to the reference values. We have applied photothermal radiometry (PTR) by using a diode laser modulated at a frequency in the range 1Hz -100 kHz, and a MCZT infrared detector, in order to measure the effective thermal parameters of the structure, as well as the induced emissivity changes from the vanadium dioxide film. Experimental results show how PTR represent an good tool for a quantitative measurement of the thermal properties of vanadium dioxide thin films. Acknowledgments This work has been performed in the framework of a collaboration between Sapienza University of Rome and the Defence R&D Canada Valcartier research center. Part of the work has been granted by Italian Ministry of Defence.