Polarization-resolved Photoluminescence Spectroscopy Research Articles

Assessment of valley coherence in a high-quality monolayer molybdenum diselenide

We investigate the valley coherence in high and low-quality monolayer MoSe2 by polarization-resolved photoluminescence spectroscopy. The observed valley coherence is on the order of 10% regardless of the sample quality, proving that the suppression of extrinsic effects does not improve the valley coherence. The valley decoherence time estimated based on the valley coherence time and exciton lifetime is sub-picosecond at the longest, which suggests that intrinsic scattering sources, such as phonons, strongly limit the valley coherence.

Influence of defects on the valley polarization properties of monolayer MoS2 grown by chemical vapor deposition

Temperature-dependent polarization-resolved photoluminescence spectroscopy is carried out on as-grown, transferred, and coated 1L-${\mathrm{MoS}}_{2}$ samples grown by the chemical vapor deposition technique to explore the underlying mechanism behind the valley depolarization process. It has been found that the momentum scattering of the excitons due to the sulfur-vacancies-attached-with-air-molecules type of defects has a strong influence in the suppression of valley polarization. Our study reveals that at sufficiently low densities of such defects and temperatures, the long-range electron-hole exchange mediated intervalley transfer of excitons via the Maialle-Silva-Sham (MSS) mechanism, as suggested by a recent theory [Yu and Wu, Phys. Rev. B 89, 205303 (2014)], is indeed the most dominant spin-flip process. In the process, the momentum scattering of the excitons by the defects takes the central stage. Interestingly, the study finds the scattering rate to be proportional to the cube root of the density of the defects. The intervalley transfer process of excitons involving the $\mathrm{\ensuremath{\Gamma}}$ valley also has significance in valley depolarization, especially when the layer is either under a tensile strain or has a high density of ${V}_{S}$ defects, as these perturbations reduces $K$ to $\mathrm{\ensuremath{\Gamma}}$-energy separation. Band-structural calculations carried out within a density functional theory framework validate these findings. The study further suggests that exchange interactions with the physisorbed oxygen molecules can result in the intervalley spin-flip scattering of the excitons and this process gives an important contribution to valley depolarization, especially at the strong scattering regime.

Characterization of polarization- and power-dependent excitons and trions in bulk ReS2

We demonstrate excitons and trions in bulk ReS2 at low temperatures using polarization-resolved photoluminescence (PL) spectroscopy. At 70 K, two excitons (near 1.543 and 1.576 eV) and trions (near 1.517 and 1.570 eV) with anisotropic polarization features are observed, similar with those of a thin layer. We attribute the emergence of excitons and trions in the bulk ReS2 to its weak interlayer coupling, which is distinct from typical 2D semiconductors. Based on the assignment, we further investigate incident-laser-power dependence, providing power exponents for the quasiparticles. We expect that our basic characterization of bulk ReS2 can contribute to understand various quasiparticles and their optical applications.

Electrical control of orbital and vibrational interlayer coupling in bi- and trilayer 2H−MoS2

Manipulating electronic interlayer coupling in layered van der Waals (vdW) materials is essential for designing opto-electronic devices. Here, we control vibrational and electronic interlayer coupling in bi- and trilayer 2H-MoS$_2$ using large external electric fields in a micro-capacitor device. The electric field lifts Raman selection rules and activates phonon modes in excellent agreement with ab-initio calculations. Through polarization resolved photoluminescence spectroscopy in the same device, we observe a strongly tunable valley dichroism with maximum circular polarization degree of $\sim 60\%$ in bilayer and $\sim 35\%$ in trilayer MoS$_2$ that are fully consistent with a rate equation model which includes input from electronic band structure calculations. We identify the highly delocalized electron wave function between the layers close to the high symmetry $Q$ points as the origin of the tunable circular dichroism. Our results demonstrate the possibility of electric field tunable interlayer coupling for controlling emergent spin-valley physics and hybridization driven effects in vdW materials and their heterostructures.

Synthesis of Co-Doped MoS2 Monolayers with Enhanced Valley Splitting.

Internal magnetic moments induced by magnetic dopants in MoS2 monolayers are shown to serve as a new means to engineer valley Zeeman splitting (VZS). Specifically, successful synthesis of monolayer MoS2 doped with the magnetic element Co is reported, and the magnitude of the valley splitting is engineered by manipulating the dopant concentration. Valley splittings of 3.9, 5.2, and 6.15 meV at 7 T in Co-doped MoS2 with Co concentrations of 0.8%, 1.7%, and 2.5%, respectively, are achieved as revealed by polarization-resolved photoluminescence (PL) spectroscopy. Atomic-resolution electron microscopy studies clearly identify the magnetic sites of Co substitution in the MoS2 lattice, forming two distinct types of configurations, namely isolated single dopants and tridopant clusters. Density functional theory (DFT) and model calculations reveal that the observed enhanced VZS arises from an internal magnetic field induced by the tridopant clusters, which couples to the spin, atomic orbital, and valley magnetic moment of carriers from the conduction and valence bands. The present study demonstrates a new method to control the valley pseudospin via magnetic dopants in layered semiconducting materials, paving the way toward magneto-optical and spintronic devices.

Ultrafast dynamics of bright and dark positive trions for valley polarization in monolayer WSe2

Optical excited-state dynamics of bright and dark positively charged excitons (positive trions) in one-layer (1L-) $\mathrm{WS}{\mathrm{e}}_{2}$ were studied by a combination of time- and polarization-resolved photoluminescence and pump-probe spectroscopy. Bright positive trions showed ultrafast relaxation to dark positive trions (\ensuremath{\sim}1 ps) and to ground state (10 \ifmmode\pm\else\textpm\fi{} 5 ps), while dark positive trions showed slow decay to the ground state (\ensuremath{\sim}300 ps) at 10 K. Under valley selective excitation with circularly polarized light, the valley relaxation time of the bright trions was 1.5 \ifmmode\pm\else\textpm\fi{} 0.5 ns at 10 K, which is much longer than typical exciton valley relaxation times of 10 ps. This insight into the slower valley relaxation dynamics of positive trions suggests that the electron-hole exchange interactions that are dominant in neutral exciton valley relaxation, are suppressed by the presence of additional holes.

Strain sensitivity and symmetry of 2.65 eV color center in diamond nanoscale needles

Color centers in diamond have unique applications as nanoscale field sensors. In particular, in the case of strain field, they are key components for the realization of strain-coupled hybrid spin-oscillator systems. Here, we report on the strain sensitivity of the color center emitting at 2.65 eV in diamond nanoscale needles. By contactless piezo-spectroscopy, we compare the strain sensitivity of this center with that of the well-known neutral nitrogen-vacancy (NV0) center. We demonstrate that the 2.65 eV center has a higher strain sensitivity than the NV0 center and can be explored as a strain sensor and/or for strain-coupled systems. Moreover, we perform polarization-resolved photoluminescence spectroscopy under a high uniaxial tensile stress and the polar behavior reported for the 2.65 eV center points out to a defect symmetry which is different from that of the NV0 center.

Zeeman Splitting and Inverted Polarization of Biexciton Emission in Monolayer WS_{2}.

Atomically thin semiconductors provide an ideal testbed to investigate the physics of Coulomb-bound many-body states. We shed light on the intricate structure of such complexes by studying the magnetic-field-induced splitting of biexcitons in monolayer WS_{2} using polarization-resolved photoluminescence spectroscopy in out-of-plane magnetic fields up to 30T. The observed g factor of the biexciton amounts to about -3.9, closely matching the g factor of the neutral exciton. The biexciton emission shows an inverted circular field-induced polarization upon linearly polarized excitation; i.e., it exhibits preferential emission from the high-energy peak in a magnetic field. This phenomenon is explained by taking into account the hybrid configuration of the biexciton constituents in momentum space and their respective energetic behavior in magnetic fields. Our findings reveal the critical role of dark excitons in the composition of this many-body state.

Determination of the Excitation and Coupling Rates Between Light Emitters and Surface Plasmon Polaritons.

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising from metallic periodic arrays without involving time-resolved techniques. We have formulated the rates by quantities that can be measured by simple optical measurements. The instrumentation based on angle- and polarization-resolved reflectivity and photoluminescence spectroscopy will be described in detail here. Our approach is intriguing due to its simplicity, which requires routine optics and several mechanical stages, and thus is highly affordable to most of the research laboratories.

Determination of the Excitation and Coupling Rates Between Light Emitters and Surface Plasmon Polaritons

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising from metallic periodic arrays without involving time-resolved techniques. We have formulated the rates by quantities that can be measured by simple optical measurements. The instrumentation based on angle- and polarization-resolved reflectivity and photoluminescence spectroscopy will be described in detail here. Our approach is intriguing due to its simplicity, which requires routine optics and several mechanical stages, and thus is highly affordable to most of the research laboratories.

Ultrafast sublattice pseudospin relaxation in graphene probed by polarization-resolved photoluminescence

Electronic states in 2D materials can exhibit pseudospin degrees of freedom, which allow for unique carrier-field interaction scenarios. Here, we investigate ultrafast sublattice pseudospin relaxation in graphene by means of polarization-resolved photoluminescence spectroscopy. Comparison with microscopic Boltzmann simulations allows to determine a lifetime of the optically aligned pseudospin distribution of $12\pm 2\,\text{fs}$. This experimental approach extends the toolbox of graphene pseudospintronics, providing novel means to investigate pseudospin dynamics in active devices or under external fields.

Poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] Oligomer Single-Crystal Nanowires from Supercritical Solution and Their Anisotropic Exciton Dynamics.

Supercritical fluids, exhibiting a combination of liquid-like solvation power and gas-like diffusivity, are a relatively unexplored medium for processing and crystallization of oligomer and polymeric semiconductors whose optoelectronic properties critically depend on the microstructure. Here we report oligomer crystallization from the polymer organic semiconductor, poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) in supercritical hexane, yielding needle-like single crystals up to several microns in length. We characterize the crystals' photophysical properties by time- and polarization-resolved photoluminescence (TPRPL) spectroscopy. These techniques reveal two-dimensional interchromophore coupling facilitated by the high degree of π-stacking order within the crystal. Furthermore, the crystals obtained from supercritical fluid were found to be similar photophysically as the crystallites found in solution-cast thin films and distinct from solution-grown crystals that exhibited spectroscopic signatures indicative of different packing geometries.

Highly Anisotropic in-Plane Excitons in Atomically Thin and Bulklike 1T'-ReSe2.

Atomically thin materials such as graphene or MoS2 are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe2. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness. The experimental results are well reproduced by ab initio calculations based on the GW-BSE approach within LDA+GdW approximation. The excitons have high binding energies of 860 meV for the monolayer and 120 meV for bulk. They are strongly confined within a single layer even for the bulk crystal. In addition, we find in our calculations a direct band gap in 1T'-ReSe2 regardless of crystal thickness, indicating weak interlayer coupling effects on the band gap characteristics. Our results pave the way for polarization-sensitive applications, such as optical logic circuits operating in the infrared spectral region.

Determination of the excitation rate of quantum dots mediated by momentum-resolved Bloch-like surface plasmon polaritons.

When light emitters are being placed in close proximity to a plasmonic system, not only the emission but also the excitation can be strongly enhanced and both yield the surface plasmon polariton (SPP) mediated fluorescence enhancement. Here, we combine the rate equation model and coupled mode theory to formulate the excitation rate of light emitters located on a periodic metallic array. The rate is expressed in terms of quantities that can be measured by angle- and polarization-resolved reflectivity and photoluminescence spectroscopy. As a demonstration, we have studied the excitation rate of CdSeTe quantum dots deposited on a 2D Au nanohole array as a function of the propagation direction of the (-1,0) Bloch-like SPPs. At the excitation wavelength of 633 nm, we find the rate remains almost constant at ~44 ps-1 regardless of the propagation direction of SPPs, which move from the Γ-X towards the Γ-M direction in the first Brillouin zone, and the crossing of the (-1,0) and (0,-1) SPPs along the Γ-M direction where two bright and dark modes are formed. The results are supported by the finite-difference time-domain simulations. We conclude the excitation rate is an intrinsic parameter and the enhanced excitation of the quantum dots arises entirely from field enhancement.

Coherent transport and manipulation of spins in indirect-exciton nanostructures

We report on the coherent control and transport of indirect exciton (IX) spins in GaAs double quantum well (DQW) nanostructures. The spin dynamics was investigated by optically generating spins using a focused, circularly polarized light spot and by probing their spatial distribution using spatially and polarization resolved photoluminescence spectroscopy. Optically injected exciton spins precess while diffusing over distances exceeding 20 {\mu}m from the excitation spot with a spatial precession frequency that depends on the spin transport direction as well as on the bias applied across the DQW structure. This behavior is attributed to the spin precession in the effective magnetic field induced by the spin-orbit interaction. From the dependence of the spin dynamics on the transport direction, bias and external magnetic fields we directly determined the Dresselhaus and Rashba spin splitting coefficients for the structure. The precession dynamics is essentially independent on the IX density, thus indicating that the long spin lifetimes are not associated with IX collective effects. The latter, together with the negligible contribution of holes to the spin dynamics, are rather attributed to spatial separation of the electron and hole wave functions by the electric field, which reduces the electron-hole exchange interaction. Coherent spin precession over long transport distances as well as the control of the spin vector using electric and magnetic fields open the way for the application of IX spins in the quantum information processing.

Detecting the sign reversal of the g-factor of two-dimensional electrons in narrow GaAs/Al x Ga1 − x As quantum wells

By means of CW polarization-resolved photoluminescence spectroscopy we demonstrate the sign inversion of $g_z$-component of 2D electron g-factor with narrowing of a hosting GaAs/Al$_{0.33}$Ga$_{0.67}$As quantum well (QW). The energy splitting between spectral lines correspoding to recombination from the electron lowest Landau level and possessing opposite curcular polarization is analyzed. Due to exchange interaction, this value reveals maximum or minimum as a function of magnetic field strength in the vicinity of electron filling factor $\nu=3$. The actual type of extremum depends on the electron $g_z$-component sign. The method allows to detect the sign inversion of electron Lande g-factor with its absolute value changing by less than $\Delta g \approx 0.06$. The reversal happens at a QW width of ($65\pm 3$ \AA).

Breaking of valley degeneracy by magnetic field in monolayer MoSe2.

Using polarization-resolved photoluminescence spectroscopy, we investigate the breaking of valley degeneracy by an out-of-plane magnetic field in back-gated monolayer MoSe2 devices. We observe a linear splitting of -0.22 meV/T between luminescence peak energies in σ+ and σ- emission for both neutral and charged excitons. The optical selection rules of monolayer MoSe2 couple the photon handedness to the exciton valley degree of freedom; so this splitting demonstrates valley degeneracy breaking. In addition, we find that the luminescence handedness can be controlled with a magnetic field to a degree that depends on the back-gate voltage. An applied magnetic field, therefore, provides effective strategies for control over the valley degree of freedom.

Leaky mode analysis of luminescent thin films: The case of ZnO on sapphire

Zinc oxide (ZnO) epitaxial thin films grown on c-sapphire substrates by pulsed laser deposition were investigated using angle and polarization-resolved photoluminescence spectroscopy. Side-emission spectra differed significantly from surface-emission spectra in exhibiting dominant, narrow, polarization-resolved peaks. These spectral features were attributed to leaky substrate modes in the layers. Observations were first verified using transmission calculations with non-adjustable parameters, which took into account the dispersion, the anisotropy of the ZnO refractive index, and the dependence on film thickness. Results were consistent with Fabry-Perot-like interference being the origin of the distinctive ZnO luminescence observed at grazing incidence angles. A second analysis, based on the source terms method, was used in order to retrieve the bulk emission properties, including the wavelength-dependent quantum yield and the emission anisotropy. While ZnO thin films were considered here, this analysis method can be extended to any luminescent thin film of similar geometry, demonstrating the potential of leaky mode analysis for probing passive and active material properties.

Vertical-Electrical-Field-Induced Control of the Exciton Fine Structure Splitting in GaAs Island Quantum Dots for the Generation of Polarization-Entangled Photons

We report on the polarization-resolved photoluminescence spectroscopy of a single GaAs island quantum dot (QD) formed by the monolayer thickness fluctuation of a GaAs/Al0.3Ga0.7As quantum well and embedded in a Schottky device. By applying a forward vertical electric field between the top metallic contact and the sample substrate we suppress the QD excitonic fine structure splitting to ≈1.5 µeV. These results open the door toward the possible generation of visible entangled photon pairs using GaAs island QDs.

Phonon-Mediated Coupling ofInGaAs/GaAsQuantum-Dot Excitons to Photonic Crystal Cavities

We demonstrate that the emission characteristics of site-controlled InGaAs/GaAs single quantum dots embedded in photonic crystal slab cavities correspond to single confined excitons coupled to cavity modes, unlike previous reports of similar systems based on self-assembled quantum dots. By using polarization-resolved photoluminescence spectroscopy at different temperatures and a theoretical model, we show that the exciton-cavity interaction range is limited to the phonon sidebands. Photon-correlation and pump-power dependence experiments under nonresonant excitation conditions further establish that the cavity is fed only by a single exciton.