Thermoreflectance techniques have become popular to measure the thermal properties of thin films such as thermal conductivity and thermal boundary conductance (TBC). Varying the focused spot sizes of the beams increases the sensitivity to in-plane heat transport, enabling the characterization of thermally anisotropic materials. However, this requires realignment of the optics after each spot size adjustment. Offsetting the probe beam with respect to the pump beam and modulating over a wide range of frequencies (5 kHz to 50 MHz) yield better sensitivity to the thermophysical properties of anisotropic materials without varying the spot sizes. We demonstrate how beam-offset frequency domain thermoreflectance can be used to measure the in-and out-of-plane thermal conductivity as well as the TBC simultaneously from a single data set by working at reduced spot sizes. Lowering the laser spot size allows us to detect signals over a wide range of frequencies and use larger beam offsets, thanks to the increase in the thermoreflectance signal. We measure the anisotropic thermal properties of a range of materials, including single layer Graphene on SiO 2 , which is of interest for novel electronic devices.
The rapidly increasing number of 2-dimensional (2D) materials that have been isolated or synthesized provides an enormous opportunity to realize new device functionalities. Whereas their optical and electrical characterization have been more readily reported, quantitative thermal characterization is more challenging due to the difficulties with localizing heat flow. Optical pump-probe techniques that are well-established for the study of bulk materials or thinfilms have limited sensitivity to in-plane heat transport, and the characterization of the thermal anisotropy that is common in 2D materials is therefore challenging. Here we present a new approach to quantify the thermal properties based on the magneto-optical Kerr effect that yields quantitative insight into cross-plane and in-plane heat transport. The use of a very thin magnetic material as heater/thermometer increases in-plane thermal gradients without complicating the data analysis in spite of the layer being optically semi-transparent. The approach has the added benefit that it does not require the sample to be suspended, providing insight of thermal transport in supported, device-like environments. We apply this approach to measure the thermal properties of a range of 2D materials which are of interest for device applications, including single layer graphene, few-layer h-BN, single and few layer MoS2, and bulk MoSe2 crystal. The measured thermal properties will have important implications for thermal management in device applications.
The authors report on the impact of wetting layer thickness and quantum dot size on the electronic and optical properties of dome-shaped InAs/GaAs quantum dots (QDs) with strained potential. Two wetting layer thicknesses of 0.5 and 2.0 nm were compared. A strong size dependence of P-to-S transition energy, transition dipole moment, oscillator strength, and linear and third-order nonlinear susceptibilities were concluded. The P-to-S transition dipole moment was shown to be purely in-plane polarization. The linear and nonlinear absorption and dispersion showed a red shift when the wetting layer thickness was increased. Our results revealed that the nonlinear susceptibility is much more sensitive to QD size compared to the linear susceptibility. An interpretation of the results was presented based on the probability density of finding the electron inside the dot and wetting layer. The results are in good agreement with previously reported experimental data.
In this work, we present the implementation of a new method to perform high-frequency thermoreflectance measurements on thin films. The so-called differential broad-band frequency domain thermoreflectance method follows broad-band frequency domain thermoreflectance developed previously [Regner , Rev. Sci. Instrum.(6), 064901 (2013)], without the use of expensive electro-optic modulators. Two techniques are introduced to recover the thermal phase of interest and to separate it from the unwanted instrumental contributions to the recorded phase. Measuring a differential thermal phase by either varying the spot size or offsetting the pump and probe beams, the thermophysical properties of materials can be extracted. This approach enables the study of nanoscale heat transport where non-equilibrium phenomena are dominating.
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