Abstract
We report measurements of the temperature dependence of the optical reflectivity, dR/dT of fifteen metallic elements at a wavelength of λ = 1.03 μm by time-domain thermoreflectance (TDTR); and the thermoreflectance of thin-films of Pt, Ta, Al, Au, SrRuO(3), and LaNiO(3) over the wavelength range 0.4 < λ < 1.6 μm using variable angle spectroscopic ellipsometry. At λ = 1.03 μm, Al, Ta, Re, Ru, have high values of thermoreflectance, dR/dT > 6∙10(-5) K(-1), and are good choices as optical transducers for TDTR experiments using a Yb:fiber laser oscillator. If low optical reflectivity and the associated high degree of steady-state heating are not a concern, LaNiO(3) provides an exceptionally sensitive thermometer in the infrared; (1/R)(dR/dT) > 2.5∙10(-4) K(-1) in the wavelength range 0.85 < λ < 1.3 μm. This compilation of data will assist in the design and interpretation of optical pump-probe studies of thermal properties.
Highlights
Time-domain thermoreflectance [1] (TDTR) and frequency-domain thermoreflectance [2, 3] (FDTR) are optical-pump-probe techniques for studying heat flow on ultrafast time scales and nanoscale length scales
In time-domain thermoreflectance (TDTR) and FDTR measurements, the signal strength depends on the product of the optical absorbance of the metal, which determines the temperature excursion from heating by the pump beam, and the temperature dependence of the optical reflectivity, which determines the change in the intensity of the reflected probe beam created by the temperature excursion
Where G is the gain of the preamplifier, V0 is the average dc voltage read by the InGaAs detector, and R is the optical reflectivity of the metal
Summary
Time-domain thermoreflectance [1] (TDTR) and frequency-domain thermoreflectance [2, 3] (FDTR) are optical-pump-probe techniques for studying heat flow on ultrafast time scales and nanoscale length scales. TDTR and FDTR monitor the evolution of the surface temperature of a thin-metal-film in the time and frequency domain The flexibility of these techniques has enabled experimental studies of thermal transport in a diverse set of systems for a wide variety of applications. While use of an opaque transducer on the sample surface prevents these artifacts, it is a severely limiting constraint that would preclude the use of ultrafast thermal analysis as a technique for studying many geometries of interest Two examples of such systems that partially motivate the present study are perovskite oxide interfaces and plasmonic nanomaterials. The wavelengths of pump and probe beams can be chosen so that the artifact is minimized and the response of the transducer is maximized This type of design necessitates quantitative values for dn/dT and dk/dT of the transducer as a function of optical wavelength; values that are not currently available in the literature. The values of dn/dT and dk/dT reported for SrRuO3 and LaNiO3 will assist in using ultrafast thermal analysis as a tool for probing interfacial properties of oxide heterostructures
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