arXiv subjects

Chih-Wei Lai

Publications and source records attributed to Chih-Wei Lai.

8 recordsLinked to original sources

arXiv2015

Exciton spin dynamics in GaSe

We analyze exciton spin dynamics in GaSe under nonresonant circularly polarized optical pumping with an exciton spin-flip rate-equation model. The model reproduces polarized time-dependent photoluminescence measurements in which the initial circular polarization approaches unity even when pumping with 0.15 eV excess energy. At T = 10 K, the exciton spin relaxation exhibits a biexponential decay with a sub-20 ps and a >500 ps time constants, which are also reproduced by the rate-equation model assuming distinct spin-relaxation rates for hot (nonequilibrium) and cold band-edge excitons.

cond-mat.mes-hall↗

arXiv2015

Transient dual-energy lasing in a semiconductor microcavity

We demonstrate sequential lasing at two well-separated energies in a highly photoexcited planar microcavity at room temperature. Two spatially overlapped lasing states with distinct polarization properties appear at energies more than 5 meV apart. Under a circularly polarized nonresonant 2 ps pulse excitation, a sub-10-ps transient circularly polarized high-energy (HE) state emerges within 10 ps after the pulse excitation. This HE state is followed by a pulsed state that lasts for 20--50 ps at a low energy (LE) state. The HE state is highly circularly polarized as a result of a spin-preserving stimulated process, while the LE state shows a significantly reduced circular polarization because of a diminishing spin imbalance.

physics.optics↗

arXiv2015

Linearly polarized remote-edge luminescence in GaSe nanoslabs

We report highly linearly polarized remote luminescence that emerges at the cleaved edges of nanoscale gallium selenide slabs tens of micrometers away from the optical excitation spot. The remote-edge luminescence (REL) measured in the reflection geometry has a degree of linear polarization above 0.90, with polarization orientation pointing toward the photoexcitation spot. The REL is dominated by an index-guided optical mode that is linearly polarized along the crystalline $c$-axis. This luminescence is from out-of-plane dipoles that are converted from in-plane dipoles through a spin-flip process at the excitation spot.

cond-mat.mes-hall↗

arXiv2014

Near Unity Optical Spin Polarization in GaSe Nanoslabs

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 ($ρ_0$) 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, $ρ_0$ as large as 0.7 is observed, while pumping 1 eV above the band edge yields $ρ_0$ = 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 TMDs, here optical spin polarization is preserved in nanoslabs of 100 layers or more of GaSe.

cond-mat.mes-hall↗

arXiv2013

A Room-temperature Spin-polarized Polariton Laser

In the field of spin-controlled semiconductor lasers, massive effort has been focused upon materials with long spin relaxation times (~ns). In contrast, we demonstrate room-temperature spin-polarized ultrafast pulsed lasing in InGaAs quantum wells (~10 ps) embedded within a GaAs microcavity. The microcavity studied here is similar to vertical-cavity surface-emitting lasers (VCSEL) used in optical communication. Unlike a VCSEL, the present polariton laser has nonlinear output and energy shifts owing to the mixing of the free-carrier polarization and cavity light field. At room temperature, we observe features resembling those in exciton-polariton condensates at cryogenic temperatures, including the spontaneous build-up of spatial coherence, macroscopic occupation, and spin polarization. Our results should stimulate activities to exploit spin-orbit interaction and many-body effects for fundamental studies of quantum light-matter fluids and developments of spin-dependent optoelectronic devices.

cond-mat.mes-hall↗

arXiv2013

Angle-Resolved Spectroscopy of Parametric Fluorescence

The parametric fluorescence from a nonlinear crystal forms a conical radiation pattern. We measure the angular and spectral distributions of parametric fluorescence in a beta-barium borate crystal pumped by a 405-nm diode laser employing angle-resolved imaging spectroscopy. The experimental angle-resolved spectra and the generation efficiency of parametric down conversion are compared with a plane-wave theoretical analysis. The parametric fluorescence is used as a broadband light source for the calibration of the instrument spectral response function in the wavelength range from 450 to 1000 nm.

physics.optics↗

arXiv2009

Signature of the microcavity exciton-polariton relaxation mechanism in the polarization of emitted light

We have performed real and momentum space spin-dependent spectroscopy of spontaneously formed exciton polariton condensates for a non-resonant pumping scheme. Under linearly polarized pump, our results can be understood in terms of spin-dependent Boltzmann equations in a two-state model. This suggests that relaxation into the ground state occurs after multiple phonon scattering events and only one polariton-polariton scattering. For the circular pumping case, in which only excitons of one spin are injected, a bottleneck effect is observed, implying inefficient relaxation.

cond-mat.mes-hall↗

arXiv2008

GaAs Microcavity Exciton-Polaritons in a Trap

We present a simple method to create an in-plane lateral potential in a semiconductor microcavity using a metal thin-film. Two types of potential are produced: a circular aperture and a one-dimensional (1D) periodic grating pattern. The amplitude of the potential induced by a 24 nm-6 nm Au/Ti film is on the order of a few hundreds of ueV measured at 6 ~ 8 K. Since the metal layer makes the electromagnetic fields to be close to zero at the metal-semiconductor interface, the photon mode is confined more inside of the cavity. As a consequence, the effective cavity length is reduced under the metal film, and the corresponding cavity resonance is blue-shifted. Our experimental results are in a good agreement with theoretical estimates. In addition, by applying a DC electric voltage to the metal film, we are able to modify the quantum well exciton mode due to the quantum confined Stark effect, inducing a ~ 1 meV potential at ~ 20 kV/cm. Our method produces a controllable in-plane spatial trap potential for lower exciton-polaritons (LPs), which can be a building block towards 1D arrays and 2D lattices of LP condensates.

cond-mat.mes-hall↗