Exciton Polarization Research Articles

Electronic Properties of Monolayer and van der Waals Bilayer of Janus TiClI.

In this work, the novel electronic properties of the Janus TiClI monolayer (ML) and van der Waals (vdW) bilayers (BLs) have been demonstrated. As a result of the strong spin-orbit coupling (SOC) together with the inversion symmetry breaking, the TiClI ML shows valley spin splitting of 62.67 meV at the K/K' point. In magnetic V- and Cr-doped TiClI MLs, sizable valley polarization of 36.70 and 45.35 meV occurs, respectively. TiClI vdW BLs indicate typical type-II band alignment with a quite large band offset (>500 meV), and interestingly, the interlayer-polarization PH is almost 100% for all considered stacking orders. In addition, the interlayer-polarization is insensitive to the interlayer distance. In this situation, the interlayer exciton and valley polarization lifetimes could be prolonged, and thus, TiClI vdW BLs provide new opportunities for light-energy conversion and valleytronics. As the interlayer distance decreases, the TiClI BLs of AB' and AB stacking indicate a semiconductor-to-metal transition and are characterized by hole-doping, and the doping concentration can be further tuned by changing the interlayer distance.

Anisotropic exciton Stark shift in hemispherical quantum dots

The exciton Stark shift and polarization in hemispherical quantum dots (HQDs) each as a function of strength and orientation of applied electric field are theoretically investigated by an exact diagonalization method. A highly anisotropic Stark redshift of exciton energy is found. As the electric field is rotated from Voigt to Faraday geometry, the redshift of exciton energy monotonically decreases. This is because the asymmetric geometric shape of the hemispherical quantum dot restrains the displacement of the wave function to the higher orbital state in response to electric field along Faraday geometry. A redshift of hole energy is found all the time while a transition of electron energy from this redshift to a blueshift is found as the field is rotated from Voigt to Faraday geometry. Taking advantage of the diminishing of Stark effect along Faraday geometry, the hemispherical shapes can be used to improve significantly the radiative recombination efficiency of the polar optoelectronic devices if the strong internal polarized electric field is along Faraday geometry.

Momentum-resolved spectroscopy for the saddle-point excitons in InSb

We use time- and angle-resolved photoemission spectroscopy to reveal momentum-resolved characteristics of interband transitions in InSb. The transitions along the \ensuremath{\Gamma}-K line are correctly described in terms of the independent-quasiparticle band picture up to 1.9 eV. However, the transitions along the \ensuremath{\Gamma}-L line turn out to be strongly excitonic above 1.7 eV, associated with the saddle-point excitons at the ${E}_{1}$ critical point. The momentum- and energy-resolved spectra of photoionization of the saddle-point excitons show that the wave function is composed of the electron-hole pair states with the wave vector of $0.453\ifmmode\pm\else\textpm\fi{}0.055\phantom{\rule{0.16em}{0ex}}{\AA{}}^{\ensuremath{-}1}$ along the \ensuremath{\Gamma}-L direction of the Brillouin zone. Time-resolved characteristics of the exciton photoionization show that the coherently generated excitonic polarization decays with a time constant of 22 \ifmmode\pm\else\textpm\fi{} 1 fs due to the autoionization from the excitonic state to uncorrelated electron-hole pair states.

Nonradiative recombination channel of dark excitons in colloidal CdSe nanoplatelets

Nonradiative recombination channel of spin-forbidden dark excitons in colloidal 4 ML thick CdSe nanoplatelets synthesized in argon and ambient atmosphere is revealed by theoretical analysis of time-resolved photoluminescence. The nonradiative recombination lifetime τnr ≈ 11 ns is found to be independent on nanoplatelets synthesis atmosphere. It is shown that the presence of the nonradiative recombination channel affects spin polarization of dark excitons and results in the time dependence of the circular polarization degree of photoluminescence in the external magnetic field.

Interlayer exciton valleytronics in bilayer heterostructures interfaced with a phase gradient metasurface

Interlayer excitons in bilayer heterostructures of two-dimensional semiconductors have opened a new platform for valley physics and optoelectronic applications owing to several unique properties. Here, we theoretically demonstrate the coherent manipulation of the interlayer excitonic valley degree of freedom in a transition metal dichalcogenide heterobilayer interfaced with a phase gradient metasurface. We harness the peculiar out-of-plane component of polarization of interlayer excitons in conjunction with the anisotropy of the vacuum of the electromagnetic field enabled by the metasurface to induce coherent coupling between mutually orthogonal interlayer excitonic optical selection rule. This coupling leads to a much larger domain within the valley psuedo-spin Bloch sphere being accessible in these heterostructures. Our proposed route to coherent manipulation of interlayer excitons in bilayer heterostructures with structured interfaces such as a metasurface may pave the way for engineering an array of interacting quantum emitters in Moiré heterostructures for chip-scale quantum information science.

Valley-polarized local excitons in WSe2/WS2 vertical heterostructures.

Two-dimensional van der Waals heterostructures (vdWHs) are drawing growing interest in the investigation of their valley polarization properties of localized excitons. However, most of the reported vdWHs were made by micro-mechanical peeling, limiting their large-scale production and practical applications. Furthermore, the circular polarization characters of localized excitons in WSe2/WS2 heterostructures remain elusive. Here, a bidirectional-flow physical vapor deposition technique was employed for the synthesis of the WSe2/WS2 type-II vertical heterostructures. The interfaces of such heterojunctions are sharp and clean, making the neutral excitons of the constituent layers quenched, which significantly highlights the luminescence of the local excitons. The circular polarization of localized excitons in this WSe2/WS2 heterostructure was demonstrated by circularly-polarized PL spectroscopy. The degree of the circular polarization of the localized excitons was determined as 7.17% for σ- detection and 4.78% for σ+ detection. Such local excitons play a critical role in a quantum emitter with enhanced spontaneous emission rate that could lead to the evolution of LEDs. Our observations provide valuable information for the exploration of intriguing excitonic physics and the applications of innovative local exciton devices.

Size effect of semiconductor quantum wells in excitonic spin generation under drift

The excitonic spin polarization in dependence of the size of semiconductor quantum well (QW) was investigated by observing the two different circular polarizations of photoluminescence (PL). From the measurements of PL in QWs, it was found that there is a difference between the two different polarization conditions, which is caused by spin-dependent phase-space filling. The PL spin polarization was estimated from the signals of the left and right circularly polarized PL and was found to depend on the size of the wells as well as on the strength of the bias field. The effects of the size of the well and applied electric field on the excitonic PL spin polarization were studied.

Dynamics of Intermittent Delayed Emission in Single CdSe/CdS Quantum Dots

Bright and fast fluorescence makes semiconductor nanocrystals, or quantum dots (QDs), appealing for applications ranging from biomedical research to display screens. However, a few percent of their fluorescence intensity is surprisingly slow. Research into this “delayed emission” has been scarce, despite undesired consequences for some applications and potential opportunities for others. Here, we characterize the dynamics of delayed emission exhibited by individual CdSe/CdS core/shell QDs and correlate these with changes in the emission spectrum. The delayed-emission intensity from a single QD fluctuates strongly during an experiment of several minutes and is thus not always “on”, implying that control over delayed emission may be possible. Periods of bright delayed emission correlate with red-shifted emission spectra. This behavior is consistent with exciton polarization by fluctuating electric fields due to diffusing surface charges, which have been known to cause spectral diffusion in QDs. Our findings thus provide a stepping stone for future efforts to control delayed emission.

CdSe/CdMnS Nanoplatelets with Bilayer Core and Magnetically Doped Shell Exhibit Switchable Excitonic Circular Polarization: Implications for Lasers and Light-Emitting Diodes

We have utilized time-resolved photoluminescence (TRPL) spectroscopy to study the excitonic circular polarization (PX) from CdSe/CdMnS core/shell nanoplatelets (NPLs) with a bilayer core. This allo...

Valley-Layer Coupling: A New Design Principle for Valleytronics.

The current valleytronics research is based on the paradigm of time-reversal-connected valleys in two-dimensional (2D) hexagonal materials, which forbids the fully electric generation of valley polarization by a gate field. Here, we go beyond the existing paradigm to explore 2D systems with a novel valley-layer coupling (VLC) mechanism, where the electronic states in the emergent valleys have a valley-contrasted layer polarization. The VLC enables a direct coupling between a valley and a gate electric field. We analyze the symmetry requirements for a system to host VLC, demonstrate our idea via first-principles calculations and model analysis of a concrete 2D material example, and show that an electric, continuous, wide-range, and switchable control of valley polarization can be achieved by VLC. Furthermore, we find that systems with VLC can exhibit other interesting physics, such as valley-contrasting linear dichroism and optical selection of the valley and the electric polarization of interlayer excitons. Our finding opens a new direction for valleytronics and 2D materials research.

Effect of Langmuir coating on the localization of excitonic polarization in CdS surface layer

Low temperature (T=2K) optical reflection spectra have been studied for the organic-semiconductor structures prepared by deposition of Langmuir-Blodgett films on CdS surface. The spectra are measured in the resonant spectral range of the An=1 exciton state in CdS. Theoretical analysis of the spectra is carried out with an account of the spatial dispersion effects and the exciton-free surface “dead” layer. The conclusion has been drawn of the interface localization of the Wannier-Mott exciton due to organic film deposition on the crystal surface.

Robust circular polarization of indirect Q-K transitions in bilayer 3R−WS2

Valley-contrasting Berry curvature and orbital magnetic moment have led to highly selective circular polarization of direct excitons at the K valleys in transition-metal dichalcogenides. In addition to K valleys, Q valleys, another critical point in the conduction band, also possess well-defined but distinct magnetic moment. Being akin to the direct excitons at K valleys, indirect excitons associated with Q (K) valleys in the conduction (valence) band could allow circular polarization in principle. Here, we report an experimental observation of the circular polarization of indirect Q-K transitions in noncentrosymmetric bilayer $3R\ensuremath{-}\mathrm{W}{\mathrm{S}}_{2}$. In stark contrast to the circular polarization of direct excitons which depolarizes with increasing lattice temperature, the circular polarization of indirect Q-K excitons is extremely robust and independent on the temperature. Such robust circular polarization can be understood as follows: the spin-orbit coupling in the Q valley is much stronger than that in the K point of the conduction band, significantly suppressing the temperature induced valley depolarization. Our results open up opportunities for exotic valleytronics and quantum information processing applications.

Exciton Polarization and Renormalization Effect for Optical Modulation in Monolayer Semiconductors

The ideal quantum confinement structure of monolayer semiconductors offers prominent optical modulation capabilities that are mediated by enhanced many-body interactions. Herein, we establish an electrolyte-gating method for tuning the luminescence properties that are in transition metal dichalcogenide (TMDC) monolayers. We fabricate electric double-layer capacitors on TMDC/graphite heterostructures to investigate electric-field- and carrier-density-dependent photoluminescence. The exciton peak energy initially shows a slight quadratic red shift of ∼1 meV without carrier accumulations, which is caused by the quantum-confined Stark effect. In contrast, the exciton resonance exhibits a larger red shift up to 10 meV with the accumulated carrier density above 1013 cm-2. These results indicate that the optical transitions can be largely modulated by the carrier density control in S- and Se-based TMDCs, as triggered by the doping-induced band gap renormalization effect. To further inspire this modulation capability, we also apply our method to electrolyte-based TMDC light-emitting devices. Biasing solely in electrolyte-induced p-i-n junctions yields pronounced red shifts up to 40 meV for exciton and trion electroluminescence. Consequently, our approach reveals that the doping effects in the high-carrier-density regimes are potentially significant for efficient optical modulation in monolayer semiconductors.

Detection of both optical polarization and coherence transfers to excitonic valley states in CVD-grown monolayer MoS2

We experimentally demonstrate the generation and detection of both excitonic valley polarization and coherence in CVD-grown monolayer MoS2 under nearly resonant excitation in polarized photoluminescence. By circular and linear photoexcitation close to the A-exciton energy, where the excitation energy is 37 meV above the A-exciton energy, 54% and 32% of valley polarization and coherence, respectively, are observed at T = 20 K. By tuning the optical polarization and the polarization plane of photoexcitation, valley polarization and its coherence are systematically controlled. This indicates the coherent control of an excitonic valley two-level system, an important step for valley quantum information.

Continuous Control and Enhancement of Excitonic Valley Polarization in Monolayer WSe2 by Electrostatic Doping

AbstractEnhancement and continuous control of the excitonic valley polarization in electrostatically doped monolayer WSe2 are demonstrated. Under excitation with circularly polarized light, 20% valley polarization of excitons around the charge neutrality condition at 70 K is increased to 40% by modulating the electron/hole density up to 2 × 1012 cm−2. This increase originates from slow valley relaxation for neutral exciton between the K and −K valleys owing to screening of long‐range e–h exchange interactions by doped carriers. The gate‐dependences of the exciton valley polarization at various temperatures are reproduced by theoretical calculations, which holds potential for next‐generation valleytronic devices continuously controlled by an applied bias voltage.

Intrinsic and magnetic-field-induced linear polarization of excitons in ultrathin indirect-gap type-II GaAs/AlAs quantum wells

Intrinsic and magnetic-field-induced linear polarization of excitons in ultrathin indirect-gap type-II GaAs/AlAs quantum wells

Crystallization, cyanamide defect and ion induction of carbon nitride: Exciton polarization dissociation, charge transfer and surface electron density for enhanced hydrogen evolution

Exciton polarization dissociation, charge transfer and surface electron density in photocatalysts are crucial for photocatalytic hydrogen evolution reactions (HERs). In this study, crystalline carbon nitride with cyanamide defect edges (crystalline CCN) was synthesized by one step polymerization of urea in the presence of KCl. The texture and electronic band structure of carbon nitrides could be facilely tailored by changing KCl dosage. The light absorption edge of crystalline CCN extended to 736 nm due to n→π* electron transition. The enhanced dielectric constants of crystalline carbon nitrides promoted exciton polarization dissociation. The small effective electron mass (me*) in crystalline CCN facilitated me* diffusion. The efficient separation of electrons and holes benefited the formation of internal electric field, showing an 8.56-fold promotion in electron transfer compared to pristine CN. Significantly, femtosecond time-resolved transient absorption demonstrated that the surface electron density on crystalline CCN was enhanced in the presence of salt ions (NaCl). As a result, crystalline CCN exhibited 14.9 times higher HER rate than pristine CN under visible light irradiation. The apparent quantum yield for H2 evolution on crystalline CCN reached to 42% at 420 nm and 9% at 500 nm. This study gets a comprehensive understanding of photocatalytic HERs using carbon nitride photocatalysts.

Zeeman-Induced Valley-Sensitive Photocurrent in Monolayer MoS_{2}.

The control of the valley degree of freedom lies at the core of interest in monolayer transition metal dichalcogenides, where specific valley-spin excitation can be created using circularly polarized light. Measurement and manipulation of the valley index has also been achieved, but mainly with purely optical methods. Here, in monolayer MoS_{2}, we identify a response to the valley polarization of excitons in the longitudinal electrical transport when the valley degeneracy is broken by an out-of-plane magnetic field B_{z}. The spin information is also simultaneously determined with spin-sensitive contacts. In the presence of B_{z}, a significant modulation of the photocurrent is observed as a function of the circular polarization state of the excitation. We attribute this effect to unbalanced transport of valley-polarized trions induced by the opposite Zeeman shifts of two (K and K^{'}) valleys. Our interpretation is supported by the contrasting behavior in bilayer MoS_{2}, as well as the observed doping and spatial dependence of the valley photocurrent.

Electrically tunable valley polarization and valley coherence in monolayer WSe 2 embedded in a van der Waals heterostructure

The valley degree of freedom possessed by electronic excitations in transition metal dichalcogenides is providing new opportunities for information processing and optoelectronics. Valley contrasting polarization selection rules present unique opportunities for optical control in valleytronic devices. Critical to devices leveraging the valley degree of freedom is the ability to tailor optical valley polarizability and its degree of coherence. In this manuscript, we demonstrate the electric field control of both valley polarization and valley coherence in a monolayer of tungsten diselenide that has been incorporated into a van der Waals heterostructure. We find the competition between electron-hole exchange, radiative decay and pure dephasing determines the ability to control valley polarization and coherence of both excitons and trions.

Valley polarization of exciton-polaritons in monolayer WSe2 in a tunable microcavity.

Monolayer transition metal dichalcogenides, known for exhibiting strong excitonic resonances, constitute a very interesting and versatile platform for the investigation of light-matter interactions. In this work, we report on a strong coupling regime between excitons in monolayer WSe2 and photons confined in an open, voltage-tunable dielectric microcavity. The tunability of our system allows us to extend the exciton-polariton state over a wide energy range and, in particular, to bring the excitonic component of the lower polariton mode into resonance with other excitonic transitions in monolayer WSe2. We can retain up to 40% of initial circular polarization of the laser or loose it completely if polariton modes are brought into resonances with low energy excitonic modes.