Bound Exciton States Research Articles

Enhancing Single Photon Emission Purity via Design of van der Waals Heterostructures.

Quantum emitters are essential components of quantum photonic circuitry envisioned beyond the current optoelectronic state-of-the-art. Two dimensional materials are attractive hosts for such emitters. However, the high single photon purity required is rarely realized due to the presence of spectrally degenerate classical light originating from defects. Here, we show that design of a van der Waals heterostructure effectively eliminates this spurious light, resulting in purities suitable for a variety of quantum technological applications. Single photon purity from emitters in monolayer WSe2 increases from 60% to 92% by incorporating this monolayer in a simple graphite/WSe2 heterostructure. Fast interlayer charge transfer quenches a broad photoluminescence background by preventing radiative recombination through long-lived defect bound exciton states. This approach is generally applicable to other 2D emitter materials, circumvents issues of material quality, and offers a path forward to achieve the ultrahigh single photon purities ultimately required for photon-based quantum technologies.

Charge‐Transfer‐Mediated Exciton Dynamics in Van der Waals Antiferromagnet NiPS3

Abstract2D van der Waals antiferromagnets have emerged as excellent candidates for studying novel low‐dimensional magnetism. The recently discovered spin–orbit‐entangled excitonic bound state appearing below the Néel temperature of 150 K in 2D antiferromagnetic NiPS3 (the so‐called Zhang–Rice (ZR) exciton) has drawn considerable attention in terms of exploring the strong correlation of spins, orbitals, and charges in a localized many‐body state. However, the formation mechanism of ZR excitons remains unclear, and its ultrafast dynamics have yet to be fully explored. Here, utilizing broadband transient reflectivity spectroscopy, the strong correlation between the charge‐transfer state excitation and the dynamic evolution of ZR excitons is investigated. Through systematic tuning of the pumping photon energy across the charge‐transfer state in NiPS3, the results reveal a short radiative lifetime of tens of picoseconds and a long nonradiative lifetime of up to nanoseconds for ZR excitons following an ultrafast charge transfer of ≈2 ps from the charge‐transfer state. The findings provide evidence of charge‐transfer‐induced ZR excitonic states in NiPS3, where the intriguing phenomena of strongly coupled spins and charges can be explored.

First-principles calculations into electronic, and excitonic effects of CH3NH3PbX3 (X = Br, I) perovskite solar cells

The orthorhombic phases of CH3NH3PbX3 (X = Br, I) have garnered significant research interest for their potential applications in optoelectronics. This work presents our findings regarding the geometric, electronic, and optical properties of CH3NH3PbX3 utilizing first-principles calculations based on density functions theory (DFT), and many-body perturbation theory (MBPT). Our investigation reveals that CH3NH3PbX3 exhibits direct band gap semiconductors, low effective mass, and high carrier mobility. Furthermore, CH3NH3PbX3 processes interesting optical properties, including a low exciton energy binding, and low reflectivity/high absorption efficiency. Additionally, the identified bound exciton states manifest as bright states in both materials. More importantly, the intimate correlation between optical and electronic characteristics is convincingly established through the orbital hybridization concept. These features suggest that CH3NH3PbX3 materials hold promise as an active layer for perovskite-based solar cells.

Influence of defects induced by plasma-bombarded monolayer WS<sub>2 </sub>on optical properties of bound excitons

<sec>Monolayer transition metal dichalcogenides (TMDCs) exhibit exceptional properties including atomic-scale thickness, direct bandgap, and strong spin-orbit coupling, which make them have great potential applications in spintronics, optoelectronics, and other fields. Usually, materials contain various structural defects, which are either formed during preparation and growth or induced by subsequent treatments. These defects can significantly change their physicochemical properties. Consequently, controlling and comprehending defects is an important approach to adjusting the properties of these materials.</sec><sec>Herein, we use Ar<sup>+</sup> plasma to bombard monolayer WS<sub>2,</sub> which is exfoliated mechanically, thereby introducing defects whose density is controlled by changing the bombardment duration. The photoluminescence (PL) and Raman spectroscopic measurements at different temperatures and power values are utilized to investigate the optical properties of the defects. Furthermore, time-resolved photoluminescence is employed to unveil the dynamic behaviors of free and trapped excitons.</sec><sec>The bombardment can introduce different types of defects into typical two-dimensional (2D) TMDCs such as MoS<sub>2</sub> and WS<sub>2</sub>. Single sulfur vacancies are frequently generated, while other defects like double sulfur vacancies or metal atom vacancies can also occur. Exciton effects dominate the optical properties of monolayer TMDCs due to reduced screening and large effective mass. At low temperatures, bound exciton emissions arise from trapped states. Our measurements reveal two types of defect-bound excitons from the PL spectra at around 1.85 eV (X<sup>B1</sup>) and 1.55 eV (X<sup>B2</sup>). Meanwhile, the Raman peaks of the samples before and after treatment exhibit no obvious changes, indicating that the lattice structure remaines unchanged. After the Ar<sup>+</sup> bombardment, the intensity of the free neutral exciton significantly decreases to 1/6 of untreated WS<sub>2</sub>, owing to the free exciton population and the increased non-radiative centers. The dynamic processes of these two bound excitons are considerably slower than the neutral exciton’s, showing the typical dynamic behavior of defect-bound excitons. Furthermore, comparison between the PL under vacuum condition and the PL under atmospheric condition shows that the intensities of the two bound excitons exhibit opposing behaviors. In an atmospheric environment, neutral excitons and bound exciton X<sup>B1</sup> possess higher intensities. In the vacuum, the strength of neutral exciton and X<sup>B1</sup> decrease quickly, while the intensity of deep-level bound exciton X<sup>B2</sup> increases.</sec><sec>In summary, we observe two bound exciton states arising from specific vacancy states in monolayer WS<sub>2</sub> after Ar<sup>+</sup> bombardment. Their energy values are 200 meV and 500 meV lower than those of the neutral exciton, with a splitting energy value being about 300 meV. The detailed evolution of the relative spectral weight with temperature and excitation power are presented. This work provides insights into the generation, control, and characteristic spectra of defects in 2D materials.</sec>

Giant excitonic magneto-optical Faraday rotation in single semimagnetic CdTe/Cd1-xMnxTe quantum ring

Magnetic tuning of the bound exciton states and corresponding giant Zeeman splitting (GZS) between σ+ and σ− excitonic transitions in CdTe/Cd1-xMnxTe quantum ring has been investigated in the Faraday configuration for various concentrations of Mn2+ ions, using the variational technique in the effective mass approximation. The sp-d exchange interaction between the localized magnetic impurity ions and the delocalized charge carriers has been accounted via mean-field theory with the inclusion of a modified Brillouin function. The enhancement of the GZS, and in turn, the effective g-factor with the application of an external magnetic field, is strikingly manifested in type-I – type-II transition in the band structure, which has been well explained by computing the overlap integral between the electron and hole, and the in-plane exciton radius. This highlights the extraordinary magneto-optical properties, including the giant Faraday rotation and associated Verdet constant, which have been calculated using single oscillator model. The oscillator strength has been estimated, and is found to be larger than in the bulk diluted magnetic semiconductors (DMS) and quantum wells, reflecting stronger confinement inside the quantum ring. More extensive Zeeman splitting and a higher oscillator strength in the DMS-based QR lead to an ultra-high Verdet constant of 2.6×109rad/Teslam, which are a few orders of magnitude larger than in the existing quantum systems and magneto-optical materials.

Diffusion of Excitons in a Two-Dimensional Fermi Sea of Free Charges.

Propagation of light-emitting quasiparticles is of central importance across the fields of condensed matter physics and nanomaterials science. We experimentally demonstrate diffusion of excitons in the presence of a continuously tunable Fermi sea of free charge carriers in a monolayer semiconductor. Light emission from tightly bound exciton states in electrically gated WSe2 monolayer is detected using spatially and temporally resolved microscopy. The measurements reveal a nonmonotonic dependence of the exciton diffusion coefficient on the charge carrier density in both electron and hole doped regimes. Supported by analytical theory describing exciton-carrier interactions in a dissipative system, we identify distinct regimes of elastic scattering and quasiparticle formation determining exciton diffusion. The crossover region exhibits a highly unusual behavior of an increasing diffusion coefficient with increasing carrier densities. Temperature-dependent diffusion measurements further reveal characteristic signatures of freely propagating excitonic complexes dressed by free charges with effective mobilities up to 3 × 103 cm2/(V s).

Antisite Defect SiC as a Source of the DI Center in 4H‐SiC

DI center, as a widely existed defect in 4H‐SiC, has attracted much attention in recent years, while the origin of it is still unclear. Herein, by comparing first‐principles calculated potential point defects‐related zero‐phonon lines (ZPLs) and nonradiative capture cross‐sections with the DI center‐related values in the experiment, it is proposed that the transition from the bound exciton states about 57 meV below the CBM to the “+1/+2” level of antisite defect SiC is responsible for the DI center in 4H‐SiC. This work describes the temperature and transition energy level dependence of the hole capture cross‐section of antisite defect in 4H‐SiC, which provides an idea for the optimal design of point defects in semiconductor materials.

Importance of liquid phase epitaxy on achieving near-lattice-matched growth of In0.145Ga0.855As0.132Sb0.868 layers on GaSb(100) substrates

We report the growth of In0.145Ga0.855As0.132Sb0.868 layers on GaSb(100) substrates by the liquid phase epitaxy (LPE) technique using the ramp-cooling method. We achieved a near-lattice-matched epitaxial growth with a lattice mismatch of between the quaternary layer and GaSb(100) substrate due to optimal growth parameters. Aberration-corrected scanning transmission electron microscope (AC-STEM) confirmed the high crystalline quality of the quaternary layer and the low lattice mismatch in the heterostructure, without the presence of linear or planar defects. Also, the Secondary Ion Mass Spectrometry (SIMS) technique evidenced a uniform distribution of the atomic elements along the quaternary layer and an abrupt interface between the In0.145Ga0.855As0.132Sb0.868 layer and the GaSb substrate. Plasmon-phonon interactions were observed by Raman spectroscopy indicating that the crystalline quality increases at greater depth in the sample with respect to the surface. The quaternary layer presented a uniform and flat morphology, and luminescence emission attributed to the recombination of bound exciton states at 641 meV. The structural, chemical, and optical properties of the In0.145Ga0.855As0.132Sb0.868 layer demonstrated that it could be auspicious material for infrared range optoelectronic applications. Likewise, the LPE technique successfully shows that it should be used to grow near-lattice-matched heterostructures.

Anisotropic Interlayer Exciton in GeSe/SnS van der Waals Heterostructure.

Stacking two or more two-dimensional materials to form a heterostructure is becoming the most effective way to generate new functionalities for specific applications. Herein, using GW and Bethe-Salpeter equation simulations, we demonstrate the generation of linearly polarized, anisotropic intra- and interlayer excitonic bound states in the transition metal monochalcogenide (TMC) GeSe/SnS van der Waals heterostructure. The puckered structure of TMC results in the directional anisotropy in band structure and in the excitonic bound state. Upon the application of compressive/tensile biaxial strain dramatic variation (∓3%) in excitonic energies, the indirect-to-direct semiconductor transition and the red/blue shift of the optical absorption spectrum are observed. The variations in excitonic energies and optical band gap have been attributed to the change in effective dielectric constant and band dispersion upon the application of biaxial strain. The generation and control over the interlayer excitonic energies can find applications in optoelectronics and optical quantum computers and as a gain medium in lasers.

Optically Detected Magnetic Resonance in Neutral Silicon Vacancy Centers in Diamond via Bound Exciton States.

Neutral silicon vacancy (SiV^{0}) centers in diamond are promising candidates for quantum networks because of their excellent optical properties and long spin coherence times. However, spin-dependent fluorescence in such defects has been elusive due to poor understanding of the excited state fine structure and limited off-resonant spin polarization. Here we report the realization of optically detected magnetic resonance and coherent control of SiV^{0} centers at cryogenic temperatures, enabled by efficient optical spin polarization via previously unreported higher-lying excited states. We assign these states as bound exciton states using group theory and density functional theory. These bound exciton states enable new control schemes for SiV^{0} as well as other emerging defect systems.

Hybrid exciton-plasmon-polaritons in van der Waals semiconductor gratings

Van der Waals materials and heterostructures that manifest strongly bound exciton states at room temperature also exhibit emergent physical phenomena and are of great promise for optoelectronic applications. Here, we demonstrate that nanostructured, multilayer transition metal dichalcogenides (TMDCs) by themselves provide an ideal platform for excitation and control of excitonic modes, paving the way to exciton-photonics. Hence, we show that by patterning the TMDCs into nanoresonators, strong dispersion and avoided crossing of exciton, cavity photons and plasmon polaritons with effective separation energy exceeding 410 meV can be controlled with great precision. We further observe that inherently strong TMDC exciton absorption resonances may be completely suppressed due to excitation of hybrid light-matter states and their interference. Our work paves the way to the next generation of integrated exciton optoelectronic nano-devices and applications in light generation, computing, and sensing.

Photon Energy-Dependent Ultrafast Photoinduced Terahertz Response in a Microcrystalline Film of CH3NH3PbBr3.

Time-resolved terahertz (THz) spectroscopy is applied for a microcrystalline film of methylammonium lead bromide perovskite, CH3NH3PbBr3, to observe the carrier dynamics around the band edge. The ultrafast response of the transmitted THz electric field amplitude after carrier generation is modeled with a biexponential curve with ∼5 and 180 ps time constants, which are ascribed to Auger and electron-hole recombination processes, respectively. From the pump photon energy dependence of the time evolution of the THz electric field amplitude, it is shown that the bound exciton states and free interband excited carrier states show a clearly different temporal response. These measurements support the idea that the bound excitons generated in CH3NH3PbBr3 remain as stable excitons even at room temperature (RT). This is in clear contrast to the cases in CH3NH3PbI3 in which the excitons and band-edge free carriers are interchangeable at RT.

Temperature dependence of photoluminescence properties of water-soluble ZnSe quantum dots

We have investigated the optical properties of ZnSe quantum dots (QDs) prepared by a hydrothermal method. The photoluminescence (PL)-decay profiles become slower with an increase in temperature up to 160 K, contrary to an ordinary behaviour due to thermal quenching. The temperature dependence of the PL-decay profile is explained by a three-state model consisting of a ground state and two excited states of the lower-lying bound-exciton and higher-lying dark-exciton states. The analysis of the temperature dependence of the decay time indicates that the dark-exciton state contributes to the PL-decay process in the ZnSe QDs.

Exciton states and oscillator strength in few-layer α-tellurene

Tellurene is an elementary two-dimensional (2D) semiconductor material that has attracted intensive attention. Here, first-principles methods are used to explore the effects of excitons on the optical properties of few-layer α-tellurene by considering the thickness and strain effects. Compared to other 2D materials, an α-tellurene monolayer possesses the characteristics of bound exciton states with a binding energy of 0.18 eV and a high optical absorption and oscillator strength in the infrared region. In addition, increasing the thickness and biaxial strain can cause a red-shift of the absorption spectra. The obtained results enrich the current understanding of the underlying physical mechanisms of α-tellurene, which are useful when designing related optoelectronic nanodevices.

Two-dimensional excitons in monolayer transition metal dichalcogenides from radial equation and variational calculations

Exciton energy spectra of monolayer transition metal dichalcogenides (TMDs) in various dielectric environments are studied with an effective mass model using the Keldysh potential for the screened electron–hole interaction. Two-dimensional (2D) excitons are calculated by solving a radial equation (RE) with a shooting method, using boundary conditions that are derived by applying the asymptotic properties of the Keldysh potential. For any given main quantum number n, the exciton Bohr orbit shrinks as becomes larger (m is the orbital quantum number) resulting in increased strength of the electron–hole interaction and a decrease of the exciton energy. Further, both the exciton energy and its effective radius decrease linearly with . The screened hydrogen model (SHM) (Olsen et al 2016 Phys. Rev. Lett. 116 056401) is examined by comparing its exciton energy spectra with our RE solutions. While the SHM is found to describe the nonhydrogenic exciton Rydberg series (i.e. the energy’s dependence on n) reasonably well, it fails to account for the linear dependence of the exciton energy on the orbital quantum number. The exciton effective radius expression of the SHM can characterize the exciton radius’s dependence on n, but it cannot properly describe the exciton radius’s dependence on m, which is the cause of the SHM’s poor description of the exciton energy’s m-dependence. Analytical variational wave-functions are constructed with the 2D hydrogenic wave-functions for a number of strongly bound exciton states, and very close exciton energies and wave-functions are obtained with the variational method and the RE solution (exciton energies are within a 6% of deviation). The variational wave-functions are further applied to study the Stark effects in 2D TMDs, with an analytical expression derived for evaluating the redshift the ground state energy.

Nonlinear optical absorption of beryllium isoelectronic centers doped in silicon waveguides

Impurities provide host materials with additional optical functionalities. In this study, we observed the nonlinear optical absorption of beryllium isoelectronic centers (Be-IECs) doped in silicon waveguides (WGs) with optical population control of their bound exciton states. The optimized fabrication based on ion implantation and rapid thermal annealing achieved Be-IEC doping with a high concentration. The bound exciton state localized at the doped Be-IECs shows a photoluminescence peak and optical absorption simultaneously at a wavelength of 1150 nm. Nonresonant optical pumping at a power of ∼70 μW reduces the optical absorption coefficient of a Be-doped WG by 1.3 cm−1, which is one third of the intrinsic absorption. This significant reduction is attributed to the suppression of the absorption transition to the discrete bound exciton state filled by optical pumping. The nonlinear optical absorption of these impurity centers makes it possible to expand the potential application of Si-based photonic devices for enabling all-optical switching with lower optical power.

Interfacially Bound Exciton State in a Hybrid Structure of Monolayer WS2 and InGaN Quantum Dots.

van der Waals heterostructures that are usually formed using atomically thin transition-metal dichalcogenides (TMDCs) with a direct band gap in the near-infrared to the visible range are promising candidates for low-dimension optoelectronic applications. The interlayer interaction or coupling between two-dimensional (2D) layer and the substrate or between adjacent 2D layers plays an important role in modifying the properties of the individual 2D material or device performances through Coulomb interaction or forming interlayer excitons. Here, we report the realization of quasi-zero-dimensional (0D) photon emission of WS2 in a coupled hybrid structure of monolayer WS2 and InGaN quantum dots (QDs). An interfacially bound exciton, i.e., the coupling between the excitons in WS2 and the electrons in QDs, has been identified. The emission of this interfacially bound exciton inherits the 0D confinement of QDs as well as the spin-valley physics of excitons in monolayer WS2. The effective coupling between 2D materials and conventional semiconductors observed in this work provides an effective way to realize the 0D emission of 2D materials and opens the potential of compact on-chip integration of valleytronics and conventional electronics and optoelectronics.

Orbital Symmetry and the Optical Response of Single-Layer MX Monochalcogenides.

We show that the absorption spectra of single-layer GaSe and GaTe in the hexagonal phase feature exciton peaks with distinct polarization selectivity. We investigate these distinct features from first-principles calculations using the GW-BSE formalism. We show that, because of the symmetry of the bands under in-plane mirror symmetry, the bound exciton states selectively couple to either in-plane or out-of-plane polarization of the light. In particular, for a p-polarized light absorption experiment, the absorption peaks of the hydrogenic s-like excitons emerge at large angle of incidence, while the overall absorbance reduces over the rest of the spectrum.

(Invited) Strongly-Bound Interlayer Exciton States in Twisted Bilayer Materials

When two sheets of a 2D material such as graphene are stacked at a twisted angle, the 2p orbitals rehybridize, giving angle-tunable absorption resonances. By comparing the ultrafast intra-band and multi-photon transient absorption spectra, our results agree best with a recent theoretical model that predict that overlapping interlayer 2p orbitals interfere to produce symmetrized bound exciton states that are decoupled from lower lying continuum states. We further map out the excited state manifold of exciton fine states using 2-photon photoluminescence microscopy. Our spectral analysis suggests the observed photoluminescence emission is a bright exciton state that is thermally populated by a lower-lying, long lived dark-exciton state. For this dark state, both our ultrafast transient absorption and two-photon photoluminescence studies support a large binding energy of the order of ~0.6 eV. If resonantly excited, we believe twisted bilayer graphene is the first 2D metallic material that can form stable, strongly bound excitons. Our results support that twisted bilayer graphene and similar materials are novel example of 2D hybrid material, where resonantly excited excitons are strongly-bound and quasi-stable from inherently weak coupling to the continuum metallic states. The formation mechanism of this dark state is very general, and holds exciting promise for other twisted stacked 2D materials. [1] R. W. Havener, Y. Liang, L. Brown, L. Yang, J. Park, Van Hove Singularities and Excitonic Effects in the Optical Conductivity of Twisted Bilayer Graphene, Nano Letters, 14, 3353-3357 (2014). [2] Y. Liang, R. Soklaski, S. Huang, M. W. Graham, R. Havener, J. Park, L. Yang, Strongly Bound Excitons in Gapless Two-dimensional Structures, Phys Rev. B, 90, 115418-3 (2014). [3] H. Patel, R. Havener, L. Brown, Y. Liang, L. Yang, J. Park, M.W. Graham, Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons, Nano Letters, 15, 5932-7 (2015). Figure 1

Study of electronic and optical properties of two-layered hydrogenated aluminum nitrate nanosheet

First principle calculations based on density functional theory using GW approximation and two particle Bethe–Salpeter equation with electron-hole effect were performed to investigate electronic structure and optical properties of two-layered hydrogenated AlN. According to many body green function due to decrease in dimension and considering electron-electron effect, direct (indirect) band gap change from 2 (1.01)eV to 4.83 (3.62)eV. The first peak in imaginary part of dielectric function was observed in parallel direction to a plane obtaining 3.4 was achieved by bound exciton states possess 1.39eV. The first absorption peak was seen in two parallel and perpendicular directions to a plane which are in UV region.