The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach toward measuring the exciton binding energy of monolayer WS2 with linear differential transmissio
n spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71eV around K valley in the Brillouin zone. The trion binding energy of 34meV, two-photon absorption cross section 4X10^{4}cm^{2}W^{-2}S^{-1} at 780nm and exciton-exciton annihilation rate around 0.5cm^{2}/s are experimentally obtained.
Motivated by the triumph and limitation of graphene for electronic applications, atomically thin layers of group VI transition metal dichalcogenides are attracting extensive interest as a class of graphene-like semiconductors with a desired band-gap
in the visible frequency range. The monolayers feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. Here we report the first optical studies of WS2 and WSe2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at mutilayers to a direct-gap one at monolayers. The PL spectra and first-principle calculations consistently reveal a spin-valley coupling of 0.4 eV which suppresses interlayer hopping and manifests as a thickness independent splitting pattern at valence band edge near K points. This giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials.
We report experimental evidences on selective occupation of the degenerate valleys in MoS2 monolayers by circularly polarized optical pumping. Over 30% valley polarization has been observed at K and K valley via the polarization resolved luminescence
spectra on pristine MoS2 monolayers. It demonstrates one viable way to generate and detect valley polarization towards the conceptual valleytronics applications with information carried by the valley index.
We report observation of magneto-electric photocurrent generated via direct inter-band transitions in an InGaAs/InAlAs two-dimensional electron gas excited by a linearly polarized incident light.The electric current is proportional to the in-plane ma
gnetic field which unbalances the velocities of the photoexcited carriers with opposite spins and consequently generates electric current from a spin photocurrent. The observed light polarization dependence of the electric current is explained microscopically by taking into account of the anisotropy of the photoexcited carrier density in wave vector space. The spin photocurrent can be extracted from the measured current and the conversion coefficient of spin photocurrent to electric current is estimated to be $10^{-3}$$sim$$10^{-2}$ per Tesla.
The knowledge of electron g factor is essential for spin manipulation in the field of spintronics and quantum computing. While there exist technical difficulties in determining the sign of g factor in semiconductors by the established magneto-optical
spectroscopic methods. We develop a time resolved Kerr rotation technique to precisely measure the sign and the amplitude of electron g factor in semiconductors.
Electron spin dynamics in InAs/GaAs heterostructures consisting of a single layer of InAs (1/3$sim$1 monolayer) embeded in (001) and (311)A GaAs matrix was studied by means of time-resolved Kerr rotation spectroscopy. The spin relaxation time of the
sub-monolayer InAs samples is significantly enhanced, compared with that of the monolayer InAs sample. We attributed the slowing of the spin relaxation to dimensionally constrained Dtextquoteright{}yakonov-Pereltextquoteright{} mechanism in the motional narrowing regime. The electron spin relaxation time and the effective g-factor in sub-monolayer samples were found to be strongly dependent on the photon-generated carrier density. The contribution from both Dtextquoteright{}yakonov-Pereltextquoteright{} mechanism and Bir-Aronov-Pikus mechanism were discussed to interpret the temperature dependence of spin decoherence at various carrier densities.
We report a measurement on quantum capacitance of individual semiconducting and small band gap SWNTs. The observed quantum capacitance is remarkably smaller than that originating from density of states and it implies a strong electron correlation in SWNTs.
