- Post Doctoral
MIT Unit Affiliation:
- Materials Science and Engineering
Post Doc Sponsor / Advisor:
Date PhD Completed:
Top 3 Areas of Expertise:
I like to pursue a ever challenging path towards solving new problems, which are beneficial to the scientific community at large and exploring new avenues in physics and material science.
Expected End Date of Post Doctoral Position:
1) Studying fundamental properties of Excitons and Trions in semiconducting materials.
2) Characterizing Light emitting Diodes for improvement in performance and mitigating droop.
3) Fabrication of new materials for use in next generation electronics.
Two-dimensional semiconducting systems have become increasingly important
for a variety of applications including photo-detectors, high-power transistors and
optoelectronics. With the discovery of the indirect-to-direct bandgap transition in
atomically thin transition metal dichalcogenide (TMDs’) materials, a plethora of further
applications and advances await. Optical properties in these materials are especially
interesting to measure, due to presence of spin-valley coupling giving rise to valleytronic
applications, and enhanced light emission (and absorption) with applications in
Optical studies in semiconductors near the bandgap primarily relate to the
fundamental optical excitation of semiconductors, an exciton (a Coulomb-bound
electron-hole pair). If the Coulomb interaction is strong enough, excitons may capture an
extra electron or hole, forming charged excitons known as trions. Trions have shown to
carry longer-lived spin information, and can drift under an electric field. The interaction
between excitons and trions, is thus a technologically important issue in optoelectronics.
The purpose of this dissertation is to measure the interactions between excitons
and trions in a variety of two-dimensional systems, primarily in the new class of
semiconducting two-dimensional materials, TMDs’. The interactions are measured for
their character (coherent or incoherent) and dynamics. Utilizing a two-color pump-probe setup we uncover coherent coupling, between excitons and trions in monolayer
molybdenum diselenide, an order of magnitude larger than traditional semiconductors
(like gallium arsenide). Incoherent relaxation pathways towards trions, are measured via
resonant excitation of excitons. A mobility edge within the exciton resonance is
uncovered, with applications in quantifying transport properties of materials under study.
Further, valley sensitive measurements are carried out on monolayer tungsten diselenide,
revealing the long-lived trion spin polarization and ultrafast exciton valley relaxation.
The possible spectroscopy feature of biexcitons is discussed in monolayer tungsten
diselenide. Finally, measurements are extended to high mobility gallium arsenide
quantum well systems, and electron-density dependent spin scattering mechanisms are
uncovered. We further discuss the possibility to suppress spin relaxation, via gate
voltage, in these gallium arsenide quantum wells.
Top 5 Awards and honors (name of award, date received):
5 Recent Papers:
A Singh, G Moody, S Wu, Y Wu, N J Ghimire, J Yan, D Mandrus, X Xu and X Li, (2014) “Coherent Electronic Coupling in Atomically Thin MoSe2”, Physical Review Letters, 112, 216804
A Singh, G Moody, K Tran, M Scott, V Overbeck, G Berghäuser, J Schaibley, E J Seifert, D Pleskot, N M Gabor, J Yan, D G Mandrus, M Richter, E Malic, X Xu, X Li, (2016) “Trion Formation Dynamics in Monolayer Transition Metal Dichalcogenides”, Physical Review B Rapid Communications, 93, 041401(R)
S Anghel, A Singh, F Passmann, H Iwata, N Moore, G Yusa, X Li, M Betz, (2016) “Enhanced spin lifetimes in a two dimensional electron gas in a gate-controlled GaAs quantum well”, Physical Review B 94, 035303
A Singh, K Tran, M Kolarczik, J Seifert, Y Wang, K Hao, D Pleskot, N M Gabor, S Helmrich, N Owschimikow, U Woggon, X Li “Long-Lived Valley Polarization of Intra-Valley Trions in Monolayer WSe2”, Under Review
G Moody, C K Dass, K Hao, C-H Chen, L-J Li, A Singh, K Tran, G Clark, X Xu, G Berghäuser, E Malic, A Knorr & X Li, (2015) “Intrinsic homogeneous linewidth and broadening mechanisms of excitons in monolayer transition metal dichalcogenides”, Nature Communications 6, 8135