Temperature dependence of radiative recombination in CdSe quantum dots with enhanced confinement

Tunable exciton–photon couplings are of great importance for cavity quantum electrodynamics (QED), polariton chemistry, optical computing, and bosonic lasing. In this regard, two-dimensional (2D) excitons in transition metal dichalcogenides (TMDs) have attracted tremendous interest─the huge oscillator strengths endow sensitive responses to optical modes, and their excitonic properties can be actively tuned by external stimuli. However, tunable coupling with spin-forbidden dark excitons in monolayer TMDs has rarely been demonstrated, and how the bright/dark-exciton–photon couplings coexist in one system is still a mystery. Herein, we utilize waveguide and antenna modes in a gold nanosphere-on-mirror cavity to match the in-plane and out-of-plane dipole moments of bright and dark excitons in monolayer WS2, respectively, and simultaneously. Strong bright-exciton–photon couplings with a coupling strength of up to 60 meV are observed at room temperature. We show that the waveguide mode can be tuned in wavelengths by controlling the spacer thickness and demonstrate the anti-crossing feature of polaritons. Next, by increasing the excitation angle, we dynamically enlarge the relative contribution from the antenna mode coupled with dark excitons and suppress the waveguide mode coupled with bright states. Moreover, a tunable bright exciton linewidth is achieved as a result of suppressed homogeneous broadening caused by the dominant dark exciton decays under strong dark-exciton–photon couplings. This is not only proof of tunable couplings with dark excitons but also an insightful finding that reveals the novel interactions between bright- and dark-exciton–photon hybrids in a single optical cavity, opening new possibilities in tunable QED.

We show that strong light-matter coupling can be used to overcome a long-standing problem that has prevented efficient optical emission from carbon nanotubes. The luminescence from the nominally bright exciton state of carbon nanotubes is quenched due to the fast nonradiative scattering to the dark exciton state having a lower energy. We present a theoretical analysis to show that by placing carbon nanotubes in an optical microcavity the bright excitonic state may be split into two hybrid exciton-polariton states, while the dark state remains unaltered. For sufficiently strong coupling between the bright exciton and the cavity, we show that the energy of the lower polariton may be pushed below that of the dark exciton. This overturning of the relative energies of the bright and dark excitons prevents the dark exciton from quenching the emission. Our results pave the way for a new approach to band-engineering the properties of nanoscale optoelectronic devices.

We studied exciton structures and the Aharonov-Bohm effect in a single carbon nanotube using microphotoluminescence (PL) spectroscopy under a magnetic field at low temperatures. A single sharp PL peak from the bright exciton state of a single carbon nanotube was observed under zero magnetic field, and the additional PL of dark exciton state appeared below the bright exciton peak under high magnetic fields. It was found that the split between the bright and dark exciton states is several millielectron volts at zero field. The tube diameter dependence of the splitting arises from the intervalley short-range Coulomb interaction.

We investigated the magnetic and temperature dependence of the bright and dark exciton luminescence spectra in single carbon nanotubes. We found that the phonon-induced exciton scattering rate from the bright to the dark state is only one order of magnitude larger than the dark exciton recombination rate below 10 K at zero magnetic field. Our results indicate that excitons are nonequilibriumly distributed between the bright and dark states due to the different parities of the wave functions and that Aharonov-Bohm flux enhances the phonon-induced exciton scattering between these two states. Our nonequilibrium exciton distribution model can also explain the nonzero photoluminescence intensity at very low temperatures.

In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright–dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.

The intricate interplay between optically dark and bright excitons governs\nthe light-matter interaction in transition metal dichalcogenide monolayers. We\nhave performed a detailed investigation of the "spin-forbidden" dark excitons\nin WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field\nBz. In agreement with the theoretical predictions deduced from group theory\nanalysis, magneto-photoluminescence experiments reveal a zero field splitting\n$\\delta=0.6 \\pm 0.1$ meV between two dark exciton states. The low energy state\nbeing strictly dipole forbidden (perfectly dark) at Bz=0 while the upper state\nis partially coupled to light with z polarization ("grey" exciton). The first\ndetermination of the dark neutral exciton lifetime $\\tau_D$ in a transition\nmetal dichalcogenide monolayer is obtained by time-resolved photoluminescence.\nWe measure $\\tau_D \\sim 110 \\pm 10$ ps for the grey exciton state, i.e. two\norders of magnitude longer than the radiative lifetime of the bright neutral\nexciton at T=12 K.\n

The optical properties of transition metal dichalcogenide monolayers such as the two-dimensional semiconductors MoS$_2$ and WSe$_2$ are dominated by excitons, Coulomb bound electron-hole pairs. The light emission yield depends on whether the electron-hole transitions are optically allowed (bright) or forbidden (dark). By solving the Bethe Salpeter Equation on top of $GW$ wave functions in density functional theory calculations, we determine the sign and amplitude of the splitting between bright and dark exciton states. We evaluate the influence of the spin-orbit coupling on the optical spectra and clearly demonstrate the strong impact of the intra-valley Coulomb exchange term on the dark-bright exciton fine structure splitting.

Optical alignment and optical orientation of excitons are studied experimentally on an ensemble of core/shell CdSe/CdS colloidal nanoplatelets. Linear and circular polarization of photoluminescence during resonant excitation of excitons is measured at cryogenic temperatures and with magnetic fields applied in the Faraday geometry. The developed theory addresses the optical alignment and optical orientation of excitons in colloidal nanocrystals, taking into account both bright and dark exciton states in the presence of strong electron–hole exchange interaction and the random in-plane orientation of nanoplatelets within the ensemble. Our theoretical analysis of the obtained experimental data allows us to evaluate the exciton fine structure parameters, the g-factors, and the spin lifetimes of the bright and dark excitons. The optical alignment effect enables the identification of the exciton and trion contributions to the emission spectrum, even in the absence of their clear separation in the spectra.

Using the composite boson many-body formalism that takes single-exciton states rather than free carrier states as a basis, we derive the integral equation fulfilled by the exciton-exciton effective scattering from which the role of fermion exchanges can be unraveled. For excitons made of -spin electrons and -spin holes, as in GaAs heterostructures, one major result is that most spin configurations lead to brightness-conserving scatterings with equal amplitude Δ, despite differences in the carrier exchanges involved. A brightness-changing channel also exists when two opposite-spin excitons scatter: dark excitons can end either in the same dark states with an amplitude , or in opposite-spin bright states , with a different amplitude , the number of carrier exchanges involved in these scatterings being even or odd, respectively. Another major result is that these amplitudes are linked by a striking relation, , which has decisive consequence on exciton Bose-Einstein condensation. By using Born values, we show that the exciton condensate can be optically observed through a bright part when excitons have large dipole only, that is, when the electrons and holes are in two well-separated layers, as in current experiments.

We discuss near-field wave function imaging, introducing a model for high spatial resolution photoluminescence imaging of semiconductor nanostructures. The model is applied to optically bright and dark exciton and biexciton states in different quantum dot systems, explicitly taking the experimental imaging configuration into account. Our results show that direct imaging of the exciton density is only possible in collection mode experiments with nonresonant excitation in the high-resolution limit. For other geometries and for biexcitonic states, the images reflect not only the size and shape of the wave function and the spatial resolution of the near-field probe but also in particular the inherent optical nonlinearity of the imaging process. Different examples for the effects of this nonlinearity are discussed, providing new insight into the interpretation of existing experiments, and guidelines for designing novel experiments.

The coupling efficiency of a micro-disk with the dark excitons in an atomically thin semiconductor is strengthened by Purcell enhanced out-of-plane resonator modes. This work proposes the engineering of these modes through the tapered disks.

Dark excitons in transition metal dichalcogenide (TMDC) monolayers (MLs) are promising candidates for Bose–Einstein condensation and quantum information processing owing to their long lifetime. Previous studies employing nanoparticle‐on‐mirror and tip‐on‐mirror cavities have successfully enabled room‐temperature dark exciton emissions through the Purcell effect. However, these approaches suffer from mode complexity, the geometric variability of the contact facet, and the extreme precision of the tip spacing, which limit further control over emission intensities. Herein, the switching‐on of dark excitons in WSe 2 ML at room temperature is demonstrated using plasmonic Au nanodisk‐on‐mirror cavities with the ML sandwiched in between. The hybrid structures combine simplicity, reproducibility, and flexible control over the dark exciton emission intensity. The dark exciton emissions are controlled by two orthogonal plasmon modes with strongly confined out‐of‐plane‐polarized electric field and can be switched on and off. The emission intensity ratio between dark excitons and bright excitons is enhanced up to 22 times by flexibly controlling the nanodisk diameter to match the plasmon energy to the dark exciton energy. The plasmonic nanodisk‐on‐mirror cavities feature enlarged field enhancement, narrowed mode linewidths, and precise geometry, providing a facile platform for switching on and controlling dark exciton emissions in TMDC MLs under readily accessible conditions.

We investigated the magnetic effect of Mn2+ ions on an exciton of Mn-doped CsPbI3 quantum dots (QDs), where we looked for the signatures of an exciton magnetic polaron known to produce a large effective magnetic field in Mn-doped CdSe QDs. In contrast to Mn-doped CdSe QDs that can produce ∼100 T of magnetic field upon photoexcitation, manifested as a large change in the energy and relaxation dynamics of a bright exciton, Mn-doped CsPbI3 QDs exhibited little influence of a magnetic dopant on the behavior of a bright exciton. However, a μs-lived dark exciton in CsPbI3 QDs showed 40% faster decay in the presence of Mn2+, equivalent to the effect of ∼3 T of an external magnetic field. While further study is necessary to fully understand the origin of the large difference in the magneto-optic property of an exciton in two systems, we consider that the difference in antiferromagnetic coupling of the dopants is an important contributing factor.

Recent research in twistronics, particularly in small-angle twisted bilayers of transition metal dichalcogenides, has uncovered exciting phenomena like periodic arrays of excitonic quantum emitters, exotic many-body states, and long-lived interlayer excitons. However, less explored has been the physics of large-angle, incommensurate bilayers, where periodicity breaks down. In this study, we demonstrate the emergence of a brightened dark intralayer exciton in a twisted n-doped molybdenum diselenide homobilayer. This dark exciton diffuses more efficiently than bright excitons or trions, with diffusion lengths over 4 μm. Temperature-dependent spectra show a brightened dark trion, and we observe a robust valley coherence. This unique behavior is attributed to a small mixing of spin-resolved conduction bands, caused by a lack of out-of-plane reflection symmetry and strong dielectric contrast. Our findings open new possibilities for valleytronic devices using valley-robust "mixed" dark excitons.

Effects of environmental dielectric materials on cross-polarized excitons of semiconducting carbon nanotubes are theoretically studied in an effective-mass scheme. When a nanotube is surrounded by a medium, the bright exciton, shifted to the high-energy side by depolarization effect, becomes weaker and disappears with the increase of the dielectric constant. When a nanotube contains a medium inside, the bright exciton becomes more eminent. The dark exciton, brightened due to electron-hole asymmetry, is enhanced with the increase of the dielectric constant both for outside and inside materials. Consequently, the relative intensity of the brightened dark exciton normalized by the bright exciton is strongly enhanced by environmental screening, in particular, for outside materials.