M.Sc Thesis

M.Sc StudentPanna Dmitry
SubjectUltrafast Dynamics in Hybrid Devices Based on Novel Phases
of Matter
DepartmentDepartment of Electrical and Computer Engineering
Supervisor ASSOCIATE PROF. Alex Hayat
Full Thesis textFull thesis text - English Version


The scope of our research is focused on fundamental dynamic processes of carriers necessary for successful integration of 2D materials into semiconductor microcavities supporting exciton-polariton condensates. This can be subsequently implemented for all-optical ultrafast switches based on the ac Stark effect with no need for a cryogenic environment. Towards these goals, we investigated a wide range of ultrafast phenomena in novel materials such as 2D transition metal dichalcogenide (TMDC) monolayers, exciton-polariton condensates embedded in semiconductor microcavities and topological insulators for potential optical and electronic device implementation required in the quantum electronics and computing. 

The field of quantum technologies developed significantly over the past few decades and became a promising candidate for substitution of standard, classical electronic and optoelectronic technology. Recently, several novel materials exhibiting interesting optical and electronic properties attractive for quantum science have been discovered. However, there is no unique development vector, and several possible implementations of quantum devices exist. One of them based on exciton-polariton condensates observed in semiconductor microcavities at cryogenic temperatures. The novel family of 2D TMDC materials exhibits a variety of exciting properties such as extremely large exciton binding energies, valley dependent electron angular momentum and presence of dark excitonic states. Most of the TMDC monolayers are direct band-gap semiconductors capable of light emission in the visible and NIR spectrum ranges useful for optoelectronic applications.

In our research, we measured dark exciton lifetime using two-photon ultrafast spectroscopy enabling a direct access to the dark states, which are spin-forbidden in one-photon absorption. Subsequently, we monitored the population of the bright excitonic state, emerging due to the scattering of the dark excitons to the bright states via the spin-flip process. Our finding sheds new light on the potential implementation of quantum memory devices utilizing dark states in the TMDC monolayers.

The second part of our research concentrated on semiconductor microcavities exhibiting exciton-polariton condensates. Where in presence of a pump pulse the dynamic ac Stark effect occurs, shifting spectral polariton lines for an ultrafast time period, limited mostly by the pump pulse duration. This effect enables implementation of all-optical switches, where logical states are represented by the polariton spectral position. In our experiments, we managed to obtain extremely large Stark shifts due to a specially designed semiconductor microcavity, the large shift ensures robustness of the logical switch operation. Moreover, we conducted several studies distinguishing between two fundamental switching regimes - diabatic and adiabatic, resulting in drastically different dynamics and as consequence in switching operation. 

Among other novel materials promising for quantum science are topological insulators with spin-polarized electron surface states, where current flows on the surface without carrier scattering and thus dissipating zero thermal energy even at the room temperatures. We performed experimental studies demonstrating selective optical access to the surface states by the circularly polarized laser beam and recorded surface electron dynamics contrasting it with the bulk electrons one, which allowed deducing characteristic lifetimes of the surface and bulk electron spins.