Molecular spin systems based on photoexcited triplet pairs formed via singlet fission (SF) are attractive as carriers of quantum information because of their potentially pure and controllable spin polarization, but developing systems that offer optical routes to readout as well as initialization is challenging. Herein, we characterize the electron spin magnetic resonance change in photoluminescence intensity for a tailored organic molecular crystal while sweeping a microwave drive up to 10 GHz in a broadband loop structure. We observe resonant transitions for both triplet and quintet spin sublevel populations showing their optical sensitivity, and revealing zero-field parameters for each. We map the evolution of these spectra in both microwave frequency and magnetic field, producing a pattern of optically-detected magnetic resonance (ODMR) peaks. Fits to this data using a suitable model suggest significant spin polarization in this system with orientation selectivity. Unusual excitation intensity dependence is also observed, which inverts the sign of the ODMR signal for triplet features, but not for quintet. These observations demonstrate optical detection of the spin sublevel population dictated by SF and intermolecular geometry, and provide unique insight into the dynamics of triplet pairs.
Single-layer WS$_2$ is a direct-gap semiconductor showing strong excitonic photoluminescence features in the visible spectral range. Here, we present temperature-dependent photoluminescence measurements on mechanically exfoliated single-layer WS$_2$, revealing the existence of neutral and charged excitons at low temperatures as well as at room temperature. By applying a gate voltage, we can electrically control the ratio of excitons and trions and assert a residual n-type doping of our samples. At high excitation densities and low temperatures, an additional peak at energies below the trion do…
We address the photoluminescence emission of individual germanium extrinsic centers in Al_0.3Ga0.7As epilayers grown on germanium substrates. Through a thorough analysis of micro-photoluminescence experiments we demonstrate the capability of high temperature emission (70 K) and multiexcitonic features (neutral exciton X, biexciton XX, positive X+ and negative X- charged exciton) of these quantum emitters. Finally, we investigate the renormalization of each energy level showing a large and systematic change of the binding energy of XX and X+ from positive to negative values (from ~+5 meV up to …
We present experimental and theoretical study of single semiconductor quantum dots excited by two non-degenerate, resonantly tuned variably polarized lasers. The first laser is tuned to excitonic resonances. Depending on its polarization it photogenerates a coherent single exciton state. The second laser is tuned to biexciton resonances. By scanning the energy of the second laser for various polarizations of the two lasers, while monitoring the emission from the biexciton and exciton spectral lines, we map the biexciton photoluminescence excitation spectra. The resonances rich spectra of the s…
We investigate the magnetic-field-induced splitting of biexcitons in monolayer WS$_2$ using polarization-resolved photoluminescence spectroscopy in out-of-plane magnetic fields up to 30 T. The observed $g$ factor of the biexciton amounts to $-3.89$, 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 configuration of the biexciton constitu…
The photoluminescence spectrum of a single quantum dot was recorded as a secondary resonant laser optically dressed either the vacuum-to-exciton or the exciton-to-biexciton transitions. High-resolution polarization-resolved measurements using a scanning Fabry-Perot interferometer reveal splittings of the linearly-polarized fine-structure states that are non-degenerate in an asymmetric quantum dot. These splittings manifest as either triplets or doublets and depend sensitively on laser intensity and detuning. Our approach realizes complete resonant control of a multi-excitonic system in emissio…
Engineering semiconductors to enhance carrier multiplication (CM) could lead to increased photovoltaic cell performance and a significant widening of the materials range suitable for future solar technologies. Semiconductor nanocrystals (NCs) have been proposed as a favourable structure for CM enhancement, and recent measurements by transient absorption have shown evidence for highly efficient CM in lead chalcogenide and CdSe NCs. We report here an assessment of CM yields in CdSe and CdTe NCs by a quantitative analysis of biexciton and exciton signatures in transient photoluminescence decays. …
The direct band gap character and large spin-orbit splitting of the valence band edges (at the K and K' valleys) in monolayer transition metal dichalcogenides have put these two-dimensional materials under the spot-light of intense experimental and theoretical studies. In particular, for Tungsten dichalcogenides it has been found that the sign of spin splitting of conduction band edges makes ground state excitons radiatively inactive (dark) due to spin and momentum mismatch between the constituent electron and hole. One might similarly assume that the ground states of charged excitons and biex…
Singlet fission (SF) process, where a singlet exciton decays into a pair of spin one exciton states which are in the total spin singlet state, is one of the possible channels for multiple exciton generation (MEG). In chiral single-wall carbon nanotubes (SWCNTs) efficient SF is present within the solar spectrum energy range which is shown by the many-body perturbation theory (MBPT) calculations based on the density functional theory (DFT) simulations. We calculate SF exciton-to-biexction decay rates ${\rm R}_{1\to 2}$ and biexciton-to-exction rates ${\rm R}_{2\to1}$ in the (6,2), (6,5), (10,5) …
We performed spectroscopic studies of a single GaAs quantum ring with an anisotropy in the rim height. The presence of an asymmetric localised state was suggested by the adiabatic potential. The asymmetry was investigated in terms of the polarization dependence of excitons and biexcitons, where a large energy di erence (0.8 meV) in the exciton emission energy for perpendicular polarizations was observed and the oscillator strengths were also compared using the photoluminescence decay rate. For perpendicular polarizations the biexciton exhibits twice the energy di erence seen for the exciton, a…
A recent single quantum wire is of sufficient quality to reveal new details of the photoluminescence (PL) evolution with increasing electron--hole (e--h) pair density. At a pair density of 3.6 $\times$ 10$^{3}$ cm$^{-1}$, the PL is characteristic of biexcitons shifted below the exciton peak by the 2.8-meV biexciton binding. At the pair density of 1.2 $\times$ 10$^{5}$ cm$^{-1}$, the biexciton peak broadens without energy shift to an e--h plasma. At all pair densities up to 30 K, neither the exciton peak nor the one-dimensional (1D) continuum edge shows any shift. In contrast to prevailing theo…
We theoretically examine the magnetoluminescence from a trion and a biexciton in a type-II semiconductor quantum dot, in which holes are confined inside the quantum dot and electrons are in a ring-shaped region surrounding the quantum dot. First, we show that two electrons in the trion and biexciton are strongly correlated to each other, forming a Wigner molecule: Since the relative motion of electrons is frozen, they behave as a composite particle whose mass and charge are twice those of a single electron. As a result, the energy of the trion and biexciton oscillates as a function of magnetic…
We consider quantum phase transitions in a system of bright dipolar excitons which can form bound pairs (dipolar biexcitons). We assume a narrow resonance in the interaction of excitons with opposite spins. At sufficiently large density a resonant exciton superfluid transforms into a superfluid of biexcitons. The transition may be either of the first or the second kind. The average relative momenta of excitons in the pairs being beyond the light cone, the transition should be accompanied by reduction of the photoluminescence intensity. Effective magnetic fields due to the long-range exchange s…
We report on molecular beam epitaxy growth of symmetric InAs/InP quantum dots (QDs) emitting at telecom C-band (1.55 $μ$m) with ultra-small excitonic fine-structure splitting of ~2 $μ$eV. The QDs are grown on distributed Bragg reflector and systematically characterized by micro-photoluminescence ($μ$-PL) measurements. One order of magnitude of QD PL intensity enhancement is observed in comparison with as-grown samples. Combination of power-dependent and polarization-resolved measurements reveal background-free exciton, biexciton and dark exciton emission with resolution-limited linewidth below…
Cd(Se,Te) Quantum Dots (QD) in ZnSe barrier typically exhibit a very high spectral density, which precludes investigation of single dot photoluminescence. We design, grow and study individual Cd(Se,Te)/ZnSe QDs of low spectral density of emission lines achieved by implementation of a Mn-assisted epitaxial growth. We find an unusually large variation of exciton-biexciton energy difference (3 meV $\leq$ $Δ\mathrm{E_{X-XX}}$ $\leq$ 26 meV) and of exciton radiative recombination rate in the statistics of QDs. We observe a strong correlation between the exciton-biexciton energy difference, exciton …
We report on the observation of spin dependent optically dressed states and optical Stark effect on an individual Mn spin in a semiconductor quantum dot. The vacuum-to-exciton or the exciton-to-biexciton transitions in a Mn-doped quantum dot are optically dressed by a strong laser field and the resulting spectral signature is measured in photoluminescence. We demonstrate that the energy of any spin state of a Mn atom can be independently tuned using the optical Stark effect induced by a control laser. High resolution spectroscopy reveals a power, polarization and detuning dependent Autler-Town…
We demonstrate a one to one correspondence between the polarization state of a light pulse tuned to neutral exciton resonances of single semiconductor quantum dots and the spin state of the exciton that it photogenerates. This is accomplished using two variably polarized and independently tuned picosecond laser pulses. The first "writes" the spin state of the resonantly excited exciton. The second is tuned to biexcitonic resonances, and its absorption is used to "read" the exciton spin state. The absorption of the second pulse depends on its polarization relative to the exciton spin direction.…
