We have investigated the exciton dynamics in transition metal
Optical interband transitions in monolayer transition metal dichalcogenides such as WSe 2 and MoS 2 are governed by chiral selection rules. This allows efficient optical initialization of an electron in a specific K-valley in momentum space. Here we probe the valley dynamics in monolayer WSe 2 by monitoring the emission and polarization dynamics of the well separated neutral excitons (bound electron hole pairs) and charged excitons (trions) in photoluminescence. The neutral exciton photoluminescence intensity decay time is about 4ps, whereas the trion emission occurs over several tens of ps. The trion polarization dynamics shows a partial, fast initial decay within tens of ps before reaching a stable polarization of ≈ 20%, for which a typical valley polarization decay time larger than 1ns can be inferred. This is a clear signature of stable, optically initialized valley polarization.
The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the "spin-forbidden" dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field Bz. In agreement with the theoretical predictions deduced from group theory analysis, magneto-photoluminescence experiments reveal a zero field splitting δ=0.6 ± 0.1 meV between two dark exciton states. The low energy state being strictly dipole forbidden (perfectly dark) at Bz=0 while the upper state is partially coupled to light with z polarization («grey» exciton). The first determination of the dark neutral exciton lifetime τ D in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure τ D~1 10 ± 10 ps for the grey exciton state, i.e. two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T=12 K.
Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strength. Encapsulation of monolayers such as MoSe2 in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect. The time-resolved photoluminescence measurements show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizeable enhancement of the exciton radiative decay rate. Understanding the role of these electrodynamical effects allow us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.
The electron spin dynamics in (111)-oriented GaAs/AlGaAs quantum wells is studied by timeresolved photoluminescence spectroscopy. By applying an external field of 50 kV/cm a two-order of magnitude increase of the spin relaxation time can be observed reaching values larger than 30 ns; this is a consequence of the electric field tuning of the spin-orbit conduction band splitting which can almost vanish when the Rashba term compensates exactly the Dresselhaus one. The measurements under transverse magnetic field demonstrate that the electron spin relaxation time for the three space directions can be tuned simultaneously with the applied electric field.The control of the electron spins in semiconductors for potential use in transport devices or quantum information applications has attracted a great attention in recent years [1][2][3]. In 2D nanostructures made of III-V or II-VI semiconductors, the dominant loss of electron spin memory is related to the spin relaxation mechanism known under the name Dyakonov-Perel (DP) [4,5]. In these materials, the absence of inversion symmetry and the spin-orbit (SO) coupling are responsible for the lifting of degeneracy for spin | 1/2 and | −1/2 electrons states in the conduction band (CB). This splitting plays a crucial role for the spin manipulation and spin transport phenomena [6,7]. As it depends strongly on the crystal and nanostructure symmetry [8][9][10], it can be efficiently tailored as explained below. The SO splitting can be viewed as the result of the action on the electron spin of an effective magnetic field whose amplitude and direction depend on the wave vector k of the electron. The electronic spin will precess around this field with an effective, momentum dependent, Larmor vector Ω whose magnitude corresponds to the CB spin splitting. This effective magnetic field changes with time since the direction of electron momentum varies due to electron collisions. As a consequence, spin precession around this field in the intervals between collisions gives rise to spin relaxation. In the usual case of frequent collisions, the relaxation time of an electron spin oriented along the direction i can be written [4]:where Ω 2 ⊥ is the mean square precession vector in the plane perpendicular to the direction i (i=x, y, z) and τ * p the electron momentum relaxation time. This yields the loss of the electron spin memory in a few tens or hundreds of picoseconds [11,12]. As the driving force in the DP spin relaxation is the SO splitting, its reduction is expected to lead to an increase of the spin relaxation time [13,14]. In bulk zinc blende semiconductor, the Bulk Inversion Asymmetry (BIA) spin splitting, also called Dresselhaus term, is determined by [1,8]:where γ is the Dresselhaus coefficient and k = (k x , k y , k z ) the electron wavector. In a quantum well (QW) where the momentum component along the growth axis z is quantized, the vector Ω due to the BIA for the lowest electron sub-band writes:where k 2 z is the averaged squared wavevector along the growth direction and k the...
Time-resolved photoluminescence was performed on as-grown and annealed bulk GaAsBi samples. Rapid thermal annealing was carried out at a temperature of 750 • C. With annealing, we observed a significant change in the photoluminescence decay time at low temperature and low excitation power, which is likely due to a reduction of localized states. Although the time-integrated photoluminescence intensity did not show a large variation, this enhancement was confirmed by the observed removal after annealing of the S-shape behaviour present in the as-grown sample.
The carrier spin dynamics in ZnO is investigated by time-resolved optical orientation experiments. We evidence a clear circular polarization of the donor-bound exciton luminescence in both ZnO epilayer and nonintentionally doped bulk ZnO. This allows us to measure the localized hole spin relaxation time. We find h s ϳ 350 ps at T = 1.7 K in the ZnO epilayer. The strong energy and temperature dependences of the photoluminescence polarization dynamics are well explained by the fast free exciton spin relaxation time and the ionization of bound excitons.Wide band gap oxide semiconductor ZnO and its related heterostructures have raised substantial interest in the optoelectronics-oriented research field in the blue/ultraviolet ͑UV͒ range. 1 Besides, with a small spin-orbit coupling and a very large exciton binding energy, ZnO represents a potential candidate for room-temperature ͑RT͒ spintronic applications. However, only few measurements on the carrier spin dynamics in bulk or even nanostructured ZnO have been published to date compared to GaAs-based structures. 2-4 Ghosh et al. 5 have investigated the electron spin properties in n-type ZnO structures and found an electron spin relaxation time varying from 20 ns to 190 ps when the temperature increases from T = 10 to 280 K. RT electron spin relaxation as long as 25 ns has also been measured by electron paramagnetic resonance ͑EPR͒ spectroscopy in colloidal n-doped ZnO quantum dots. 6 To the best of our knowledge, neither the exciton nor the hole spin dynamics in ZnO have been measured yet. Indeed two experimental issues arise: ͑i͒ the small value of the spinorbit coupling energy ͑9-16 meV͒ ͑Refs. 7 and 8͒ imposes resonant optical excitation conditions 9 in the near UV to create an exciton spin polarization; ͑ii͒ the direct measurement of the free hole spin relaxation by pump-probe experiments would require the fabrication of stable p-doped samples, 10,11 which remains a challenge in ZnO ͑Ref. 12͒. In order to investigate the hole spin dynamics in ZnO, we have studied the polarization properties of the exciton bound to neutral donors. Since this complex consists of a singlet of electrons and a hole, its spin polarization is directly determined by the orientation of the hole bound in the complex. [13][14][15] ZnO crystallizes in the wurtzite phase, where the hexagonal crystal field ⌬ cr and the spin-orbit coupling ⌬ so give rise to three doubly degenerated valence bands, labeled A, B, and C. The optical selection rules and oscillator strengths impose that only the transitions from the A and B valence bands are optically allowed when the light propagates along the c axis of the crystal. 7,8,13 We present in this paper a detailed investigation of the optical orientation of excitons and holes in ZnO bulk and epilayer samples. By time-resolved photoluminescence ͑PL͒ experiments, we evidence the fast free exciton spin relax-ation ͑ FX s Ͻ 10 ps͒ and we measure the hole spin relaxation time ͑up to h s ϳ 350 ps͒ in the donor-bound exciton complex.The samples under investigatio...
Electron spin dynamics in elastically strained bulk GaAsBi epilayer with 2.2% Bi concentration has been measured by time resolved photoluminescence spectroscopy. Under external transverse magnetic field, the measurement of the photoluminescence polarization oscillations resulting from the Larmor precession of electron spins yields an accurate determination of the Landé g-factor. We find that the value of g increases from −0.81 to −0.68 when the temperature rises from T = 100 K to T = 300 K. This is typically double the value of GaAs, in agreement with the larger spin-orbit interaction in GaAsBi. In this temperature range, the electron spin lifetime decreases from 370 to 100 ps.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.