Cavity quantum electrodynamics, a central research field in optics and solid-state physics, addresses properties of atom-like emitters in cavities and can be divided into a weak and a strong coupling regime. For weak coupling, the spontaneous emission can be enhanced or reduced compared with its vacuum level by tuning discrete cavity modes in and out of resonance with the emitter. However, the most striking change of emission properties occurs when the conditions for strong coupling are fulfilled. In this case there is a change from the usual irreversible spontaneous emission to a reversible exchange of energy between the emitter and the cavity mode. This coherent coupling may provide a basis for future applications in quantum information processing or schemes for coherent control. Until now, strong coupling of individual two-level systems has been observed only for atoms in large cavities. Here we report the observation of strong coupling of a single two-level solid-state system with a photon, as realized by a single quantum dot in a semiconductor microcavity. The strong coupling is manifest in photoluminescence data that display anti-crossings between the quantum dot exciton and cavity-mode dispersion relations, characterized by a vacuum Rabi splitting of about 140 microeV.

Photonic molecules have been fabricated by coupling pairs of micrometer-sized semiconductor cavities via a narrow channel. The optical modes in these structures have been studied spectroscopically as a function of the coupling and the mode energies are compared to detailed calculations. These results provide a rich picture of photonic modes in these molecules. [S0031-9007(98)

The influence of quantum dot (QD) asymmetry on the emission of single three-dimensionally confined biexcitons in II-VI semiconductor nanostructures has been studied by magnetophotoluminescence spectroscopy. Investigating both the biexciton and the single-exciton transition in the same single QD, we obtain a unified picture of the impact of electron-hole exchange interaction on the fine structure and the polarization properties of optical transitions in QDs. The exchange splitting is demonstrated to have a strong influence on the derivation of the biexciton binding energy, which we determine to be about 17 meV, much less than the separation between exciton and biexciton lines (ഠ24 meV) in the spectra.[S0031-9007 (99)08536-1] PACS numbers: 78.66. -w, 71.35.Cc, 71.70.GmIn recent years, optical investigations on single semiconductor quantum dots (QDs), often designated as "artificial atoms," opened a new and exciting field of basic physics studies. In contrast to "real" atoms or molecules, a unique feature of solid state quantum dots is the formation of Wannier excitons giving experimental access to both the Coulomb and the electron-hole (e-h) exchange interaction in three-dimensionally confined solid state systems. Therefore, semiconductor QDs with geometries smaller than or comparable to the bulk exciton Bohr radius can be regarded as a model system in order to study the impact of Coulomb and exchange interaction on the optical properties of zero-dimensional excitons and excitonic complexes [1][2][3][4][5]. Several techniques have been developed to realize semiconductor QDs with high quantum efficiencies. This includes chemically prepared QDs embedded in a matrix [1,6] as well as ensembles of QDs fabricated by means of epitaxy and/or lithography [2,[7][8][9][10][11][12][13][14][15]. A drawback of such QD arrays is a broadening of the optical transitions due to the size dispersion of the dots, which prevents the investigation of, e.g., the fine structure of exciton states [16,17]. In order to suppress the influence of inhomogeneous broadening effects, spectroscopic techniques with a high spatial resolution have been introduced as a powerful experimental tool. This allows one to investigate single quantum dots (SQDs) by means of photoluminescence (PL) spectroscopy [2][3][4][5]10,14,18].In II-VI nanostructures, the e-h exchange interaction is significantly enhanced as compared to the (Ga,In)As system [1,6]. This allows studies of the optical transitions of excitons and multiexcitons without any significant mixing of radiative ("bright") and nonradiative ("dark") excitonic states. Therefore, II-VI materials such as, e.g., CdSe͞ZnSe are ideal systems to study the impact of the dot symmetry and the e-h exchange interaction on the fine structure and the polarization properties of excitons and excitonic complexes in SQDs. Until now, the work on e-h exchange interaction in quantum dots has concentrated on single-exciton states, where, e.g., the energy splitting between dark and bright excitons [1,6] or the splitting of the ...

The decay characteristics of excitons and biexcitons in one single semiconductor quantum dot (QD) are directly monitored using time-and spatially resolved photoluminescence spectroscopy. The experiments are performed on a CdSe͞ZnSe QD, occupied by either one or two excitons at a time, allowing a direct comparison between the radiative lifetime of a biexciton and an exciton confined in the same QD. The rather surprising result of comparable recombination rates for both states is related to the spatial wave function distribution and the spin structure of the particles and their coupling to the photon field, i.e., the superradiance effect.

Optical bound states in the continuum (BICs) provide a way to engineer resonant response in photonic crystals with giant quality factors. The extended interaction time in such systems is particularly promising for enhancement of nonlinear optical processes and development of a new generation of active optical devices. However, the achievable interaction strength is limited by the purely photonic character of optical BICs. Here, 1 arXiv:1905.13505v1 [cond-mat.mes-hall] 31 May 2019 we mix optical BIC in a photonic crystal slab with excitons in atomically thin transition metal dichalcogenide MoSe 2 via strong coupling to form exciton-polaritons with Rabi splitting exceeding 27 meV. We experimentally show BIC-like behavior of both upper and lower polariton branches, with complete suppression of radiation into far-field at the BIC wavevector and strongly varying Q-factor in its vicinity. Owing to an effective disorder averaging through motional narrowing, we achieve small polariton linewidth of 2 meV and demonstrate linewidth control via angle and temperature tuning. Our results pave the way towards developing tunable BIC-based polaritonic devices for sensing, lasing, and nonlinear optics. Optical bound states in the continuum (BICs), supported by photonic crystal structures of certain geometries, have received much attention recently as a novel approach to generating extremely spectrally narrow resonant responses. 1,2 Since BICs are uncoupled from the radiation continuum through symmetry protection 3 or resonance trapping, 4 their high quality factors, while reaching 10 5 − 10 6 , can be robust to perturbations of photonic crystal geometric parameters. This enables a broad range of practical applications, including recently demonstrated spectral filtering, 5 chemical and biological sensing, 6,7 and lasing. 4Providing an efficient light-trapping mechanism, optical BICs are particularly attractive for enhancing nonlinear optical effects, with recent theoretical proposals discussing enhanced bistability 8 and Kerr-type focusing nonlinearity. 9 However, for practical realization of these proposals, a significantly stronger material nonlinear susceptibility is needed than generally available in dielectric-based photonic crystals.An attractive approach to the enhancement of effective nonlinearity is through the use of exciton-polaritons -hybrid quasi-particles that inherit both the coherent properties of photonic modes and interaction strength of excitons. 10,11 Hybrid nanophotonic systems incorporating atomically thin transition metal dichalcogenides (TMDs) have proven to be a particularly promising platform owing to their ease of fabrication and possibility of room temperature operation. [12][13][14] In addition to conventional microcavity-based designs, TMD