Theses
< 2019>
2019-01-18
ION HANCU
2019-01-29
MARIA MAFFEI
2019-02-22
FRANCESCO RICCI
2019-03-06
CLARA GREGORI
2019-04-16
PETER SCHMIDT
2019-05-02
LUCIANA VIDAS
2019-05-03
HANYU YE
2019-05-17
FLAVIO BACCARI
2019-07-02
OZLEM YAVAS
2019-07-03
ALESSANDRO SERI
2019-07-11
RENWEN YU
2019-09-06
ALEXANDER BLOCK
2019-10-04
MARCO PAGLIAZZI
2019-10-07
RINU MANIYARA

Optomechanical resonators based on transition metal dichalcogenide monolayers


Nicolas Morell
December 17th, 2018 NICOLÁS MORELL Quantum NanoMechanics
ICFO-The Institute of Photonic Sciences


Suspended monolayer transition metal dichalcogenides (TMD) aremembranes that combine ultralow mass and exceptional optical prop-erties, making them intriguing materials for opto-mechanical applica-tions. However, the low measured quality factor of TMD resonatorshas been a roadblock so far. In this thesis, we first show an ultra-sensitive optical readout of monolayer TMD resonators that allows usto reveal their mechanical properties at cryogenic temperatures. Wefind that the quality factor of monolayer WSe2resonators greatly in-creases below room temperature, reaching values as high as 16000 at liquid nitrogen temperature and 47000 at liquid helium temper-ature. This surpasses the quality factor of monolayer graphene res-onators with similar surface areas. Upon cooling the resonator, the res-onant frequency increases significantly due to the thermal contractionof the WSe2lattice. These measurements allow us to experimentallystudy the thermal expansion coefficient of WSe2 monolayers for thefirst time. High Q-factors are also found in resonators based on MoS2 and MoSe2 monolayers. The high quality-factor found in this workopens new possibilities for coupling mechanical vibrational states totwo-dimensional excitons, valley pseudospins, and single quantumemitters and for quantum opto-mechanical experiments based on theCasimir interaction.The sensing capabilities offered by these high Q-factor nanome-chanical oscillators are also of interest for studying thermodynamicproperties in condensed matter regimes that are difficult to access. Inthe second part of the thesis, we use optomechanical systems basedon a MoSe2 monolayer to probe the thermal properties of phononsin two-dimensional lattices. We measure the thermal conductivityand the specific heat capacity down to cryogenic temperature. Thephonon transport crossovers from the diffusive to the ballistic regimewhen lowering the temperature below~100 K. The temperature de-pendence of the specific heat capacity approaches a quadratic depen-dence, the signature of two-dimensional lattices. Both the thermalconductivity and the specific heat capacity measurements are consis-tent with predictions based on first-principles. Our result establishes anew strategy to investigate thermal transport in two-dimensional ma-terials, and allows for exploring the phonon hydrodynamic regime,the anomalous heat conduction, and the phase transitions of electronicmany-body collective phenomena in monolayers.


Monday December 17, 12:00 h. ICFO Auditorium
Thesis Advisor: Prof Dr Adrian Bachtold