Optical and spin polarization dynamics in GaSe nanoslabs

Abstract

We report nearly complete preservation of "spin memory" between optical absorption and photoluminescence (PL) in nanometer slabs of GaSe pumped with 0.2 eV excess energy. At cryogenic temperatures, the initial degree of circular polarization (ρ0) of PL approaches unity, with the major fraction of the spin polarization decaying with a time constant >500 ps in sub-100-nm GaSe nanoslabs. Even at room temperature, ρ0 as large as 0.7 is observed, while pumping 1 eV above the band edge yields ρ0 = 0.15. Angular momentum preservation for both electrons and holes is due to the separation of the non-degenerate conduction and valence bands from other bands. In contrast to valley polarization in atomically thin transition metal dichalcogenides, here optical spin polarization is preserved in nanoslabs of 100 layers or more of GaSe.Solid-state systems exhibiting high spin polarization and long spin relaxation time are desirable for spintronic applications. Various semiconductors have been studied for creation of long-lived non-equilibrium spin populations and coherences [1][2][3][4] . The most extensively studied system is gallium arsenide (GaAs). However, optically pumped electron spin polarization is limited to 1/2, while the maximal degree of circular polarization of photoluminescence is 1/4 1,3 , owing to the degenerate heavy-and light-hole valence bands and sub-ps hole spin relaxation. Doping 5 or quantum confinement 6 has been used to quench electron spin relaxation. Unity electron spin polarization can be achieved in heterostructures where heavy-and light-hole energy degeneracy is lifted by quantum confinement or strain. Still, nearresonant optical excitation is necessary to avoid transitions involving both heavy-and light-hole bands. In analogy to spin polarization, valley polarization has been demonstrated in monolayer transition metal dichalcogenides (TMDs) with potential applications exploiting both spin and valley degrees of freedom.In monolayer TMDs with broken inversion symmetry, a direct gap emerges at the corners (K points) of the Brillouin zone, enabling valley-dependent inter-band transitions under circularly polarized optical excitation 7-9 . Furthermore, the substantial spin-splitting of valence bands at the band edges due to spin-orbit interaction originating from the d orbitals of TM ions has led to recent reports of long hole spin and valley lifetimes 10,11 , valley exciton polarization and coherence 12,13 , circularly polarized electroluminescence 14 , and valley Hall effect 15 . Indeed, circularly polarized PL was observed in single-and bi-layer TMDs 8,9,11-13 with steady-state near-resonant circularly polarized excitation. However, time-and polarization-resolved PL measurements suggest that, at least in MoS 2 , circularly polarized PL can result from sub-10-ps recombination and valley (spin) lifetimes rather than an intrinsically long-lived valley or hole spin polarization 8,16,17 . Additionally, emission at the direct gap becomes dominant only at the monolayer level [18][19][20] .Here, we demonstr...

Exciton spin dynamics in GaSe

“…3 shows time-integrated PL spectra under co-and cross-circularly polarized excitation and detection at T = 10 K. The PL is peaked at about 2.07 eV with a lowenergy tail that is attributed to localized excitons. As in samples described previously 12 , the degree of steadystate spin polarization (ρ c ) is highest at high energies, as expected given the reduced time spent at such energies. There is also a pronounced increase inρ c as the excitation photon energy decreases towards the band gap.…”

“…In a recent report 12 , we demonstrated that generation and preservation of a high degree of optical spin polarization, ρ, is possible in nanoscale slabs of GaSe even when optically pumping 0.1 to 0.2 eV above the gap. This is a consequence of the unique bandstructure ( Fig.…”

Symmetry, distorted band structure, and spin-orbit coupling of group-III metal-monochalcogenide monolayers

“…This dynamic upper valence band hole polarization can be modeled via a rate-equation approach, similar to the simulation of recent time-resolved luminescence studies in thin but still 3D GaSe flakes. 37,38 A simplified equivalent three-level system is shown in Fig. 7(a), where spin-conserving interband transitions (Γ − 3 ↔ Γ − 2 optical orientation with generation rate constant G ↑ and spontaneous relaxation rate constant R ± , and Γ + 1 → Γ − 2 bandgap radiative relaxation with rate constant R z ) appear in solid arrows, while spin-mixing intraband spin relaxation (with timescales τ c,v, for conduction, valence and lower-valence bands, respectively, indicated by subscripts) appears in dashed arrows.…”

Optical Generation of High Spin Polarization in GaSe Nanoslabs

“…2a). Meanwhile, time-dependent PL decay becomes bi-exponential with time constants ′ 0 ≈ 20 − 50 ps and ′′ 0 ≈ 150 − 200 ps in all samples [4]. Time-dependent circular polarization ̅ (t) is also found to be bi-exponential, with decay time constants ′ ≈ 30 − 40 ps and ′′ ≳ 500 ps (Fig.…”

“…Time-integrated PL shows a pronounced increase of degree of circular polarization ( ̅ ) from ~0.05 in bulk to 0.3 in sub-100-nm nanoslabs (Fig. 1a) [4]. To explore the mechanism of the increase of ̅ , we carried out polarized time-resolved PL measurements as shown in Fig 1b. The degree of circular polarization ̅ (t) decays within 10 ps, independent of the slab thickness and photoexcited carrier density (Fig 1c).…”