arXiv subjects

J. S. Reparaz

Publications and source records attributed to J. S. Reparaz.

4 recordsLinked to original sources

arXiv2026

Controlling the Sign and Magnitude of the Thermoreflectance Coefficient of Gold-MoS$_2$ Metamaterials

Heating a material changes how much light it reflects. This effect, quantified by the thermoreflectance coefficient $C_\mathrm{TR}$, enables temperature sensing, while it is a nuisance for situations where constant reflection is crucial. Whereas temperature detection requires a large $C_\mathrm{TR}$, the latter situation demands it to be as close to zero as possible. Controlling the $C_\mathrm{TR}$ has so far relied on the selection of materials and wavelengths, rather than engineering the coefficient itself. This work demonstrates that the $C_\mathrm{TR}$ of a 50 nm thin gold film on SiO$_2$/Si can be controlled in sign and magnitude by an ultrathin ($<$8 nm) MoS$_2$ film underneath it. Time-resolved thermoreflectance measurements reveal suppression, sign inversion, and revival of the thermoreflectance signal, controlled by three knobs: ($i$) the buried MoS$_2$ thickness, ($ii$) probe wavelength, and ($iii$) ambient temperature. Transfer-matrix simulations using temperature-dependent dielectric functions of gold and MoS$_2$ reproduce all experimental observations, demonstrating an optothermal metamaterial platform with controllable properties. In particular, these (anti-)thermoreflective metamaterial mirrors bring $C_\mathrm{TR}$ under the same interference design rules previously applied to the optical response itself, opening ways to maximize or minimize the temperature dependence of the optical reflectance, tailored to the specific application.

physics.optics↗

arXiv2021

Anisotropic Thermoreflectance Thermometry: A contactless frequency-domain approach to study anisotropic thermal transport

We developed a novel contactless frequency-domain approach to study thermal transport, which is particularly convenient when thermally anisotropic materials are considered. The method is based on a similar line-shaped heater geometry as used in the 3-omega method, however, keeping all the technical advantages offered by non-contact methodologies. The present method is especially suitable to determine all the elements of the thermal conductivity tensor, which is experimentally achieved by simply rotating the sample with respect to the line-shaped optical heater. We provide the mathematical solution of the heat equation for the cases of anisotropic substrates, multilayers, as well as thin films. This methodology allows an accurate determination of the thermal conductivity, and does not require complex modeling or intensive computational efforts to process the experimental data, i.e., the thermal conductivity is obtained through a simple linear fit ("slope method"), in a similar fashion as in the 3-omega method. We demonstrate the potential of this approach by studying isotropic and anisotropic materials in a wide range of thermal conductivities. In particular, we have studied the following inorganic and organic systems: (i) glass, Si, and Ge substrates (isotropic), (ii) $β$-Ga$_2$O$_3$, and a Kapton substrate (anisotropic) and , (iii) a 285 nm SiO$_2$/Si thin film. The accuracy in the determination of the thermal conductivity is estimated at $\approx$ 5\%, whereas the best temperature resolution is $Δ$T $\approx$ 3 mK.

physics.app-ph↗

arXiv2016

Thermal conductivity of MoS2 polycrystalline nanosheets

We report a technique for transferring large areas of the CVD-grown, few-layer MoS2 from the original substrate to another arbitrary substrate and onto holey substrates, in order to obtain free-standing structures. The method consists of a polymer- and residue-free, surface-tension-assisted wet transfer, in which we take advantage of the hydrophobic properties of the MoS2. The methods yields better quality transferred layers, with fewer cracks and defects, and less contamination than the widely used PMMA-mediated transfer and allows fabrication of few-layer, fee-standing structures with diameters up to 100 micro-m. We report thermal conductivity measurements by means of contactless Raman thermometry on the so-fabricated samples. The measurements revealed a strong reduction in the in-plane thermal conductivity down to 0.5 W/mK. The results are explained using finite elements method simulations for a polycrystalline film.

cond-mat.mtrl-sci↗

arXiv2013

A novel high resolution contactless technique for thermal field mapping and thermal conductivity determination: Two-Laser Raman Thermometry

We present a novel high resolution contactless technique for thermal conductivity determination and thermal field mapping based on creating a thermal distribution of phonons using a heating laser, while a second laser probes the local temperature through the spectral position of a Raman active mode. The spatial resolution can be as small as $300$ nm, whereas its temperature accuracy is $\pm 2$ K. We validate this technique investigating the thermal properties of three free-standing single crystalline Si membranes with thickness of 250, 1000, and 2000 nm. We show that for 2-dimensional materials such as free-standing membranes or thin films, and for small temperature gradients, the thermal field decays as $T(r) \propto ln(r)$ in the diffusive limit. The case of large temperature gradients within the membranes leads to an exponential decay of the thermal field, $T \propto exp[-A \cdot ln(r)]$. The results demonstrate the full potential of this new contactless method for quantitative determination of thermal properties. The range of materials to which this method is applicable reaches far beyond the here demonstrated case of Si, as the only requirement is the presence of a Raman active mode.

cond-mat.mtrl-sci↗