Light-matter interaction in complex media
Disorder and imperfection are ubiquitous in nature and also have an impact on nanotechnology. Photonic and optomechanical devices are always affected by some degree of disorder that hampers their functionality. At the P2N group, we are exploring novel functionalities induced by disorder particularly related to the photon-phonon interaction. We explore the role of material nonlinearities to modulate the optical modes of these systems. In addition, we do explore the role of disorder in complex optomechanical systems where the mechanical vibrations of matter – phonons - are included as a new degree of freedom in the control of the light-matter interaction.
Dr. Ryan Ng
Quantum 2D materials
In 2D materials strain can be an effective tool to control the electronic Fermi surface topology and electron–phonon coupling. It has been applied to modify the optical properties of semiconducting transition metal dichalcogenides such as MoS2 or WS2. In MoS2 membranes reversible tuning of the optical band gap of suspended monolayer by as much as 500 meV have been reported. At the same time, coupled straintronic-photothermic effect, where coupling between bandgap of the 2D layered semiconductor under localized strains, optical absorption and the photo-thermal effect results in a large chromatic mechanical response.
Van de Waals materials, where 2D layers act as building blocks are expected to exhibit different properties than the constituent materials. These structures can be realized either by mechanical transfer or direct growth by means of chemical vapour deposition (CVD). The weak van der Waals bonding between the layers allows for them to slide, in fact MoS2 makes a perfect solid lubricant commonly used for industrial applications. But the nanoscale study of the interlayer coupling and strain transfer remains to be performed, especially focusing on the twisted angle between the layers.
Nano-scale thermal transport
The PnCs structures with characteristic sizes reported to show coherent effects in the hypersound have been found to exhibit reduced in-plane thermal conductivity, κ, values. Moreover, it has been seen that, for a given membrane thickness, the temperature evolution, κ(T), from room temperature to about 1000 K can be effectively tuned and approaching to a regime where κ is almost insensitive to T by changing the neck distance in between holes. The latter reflects the increasing role of surface scattering on k(T) by limiting the phonon mean free path at expenses of the phonon-phonon scattering. Control and manipulation of heat transport requires devices with analogous functionalities as diodes and transistors in electronics, therefore thermal circuits could be devised but, also used in thermal management and thermoelectric energy generation. This work benefits from European collaborations and membership of the European CRS network on Thermal Nanoscience and Nanoengineering. During this period we will study the tuning of k(T) in “holey” membranes as a mechanism to introduce heat transfer directionality towards efficient and direction controlled heat dissipation and thermal diode and transistor concepts.
The group is developing experimental methods for the characterization of thermal properties in fluids. These techniques will allow us the establishment of new research on the modification of the thermal properties of a base fluid upon the incorporation of nanoparticles.
Dr. E. Chavez-Angel
Phonons for energy
The regulation of temperature is a major energy‐consuming process of humankind. Today, around 15% of the global‐energy consumption is dedicated to refrigeration and this figure is predicted to triple by 2050, thus linking global warming and cooling needs in a worrying negative feedback‐loop. Here, an inexpensive solution is proposed to this challenge based on a single layer of silica microspheres self‐assembled on a soda‐lime glass. This 2D crystal acts as a visibly translucent thermal‐blackbody for above‐ambient radiative cooling and can be used to improve the thermal performance of devices that undergo critical heating during operation. The temperature of a silicon wafer is found to be 14 K lower during daytime when covered with the thermal emitter, reaching an average temperature difference of 19 K when the structure is backed with a silver layer. In comparison, the soda‐lime glass reference used in the measurements lowers the temperature of the silicon by just 5 K. The cooling power of this simple radiative cooler under direct sunlight is found to be 350 W m−2 when applied to hot surfaces with relative temperatures of 50 K above the ambient. This is crucial to radiatively cool down devices, i.e., solar cells, where an increase in temperature has drastic effects on performance.