Exciton–Photon Interactions in Two-Dimensional Semiconductor Microcavities

Two-dimensional (2D) semiconductors possess strongly bound excitons, opening novel opportunities for engineering light–matter interaction at the nanoscale. However, their in-plane confinement leads to large non-radiative exciton–exciton annihilation (EEA) processes, setting a fundamental limit for their photonic applications. In this work, we demonstrate suppression of EEA via enhancement of light–matter interaction in hybrid 2D semiconductor–dielectric nanophotonic platforms, by coupling excitons in WS2 monolayers with optical Mie resonances in dielectric nanoantennas. The hybrid system reaches an intermediate light–matter coupling regime, with photoluminescence enhancement factors up to 102. Probing the exciton ultrafast dynamics reveal suppressed EEA for coupled excitons, even under high exciton densities >1012 cm−2. We extract EEA coefficients in the order of 10−3, compared to 10−2 for uncoupled monolayers, as well as a Purcell factor of 4.5. Our results highlight engineering the photonic environment as a route to achieve higher quantum efficiencies, for low-power hybrid devices, and larger exciton densities, towards strongly correlated excitonic phases in 2D semiconductors.

Layered semiconductors based on transition-metal chalcogenides usually cross from indirect bandgap in the bulk limit over to direct bandgap in the quantum (2D) limit. Such a crossover can be achieved by peeling off a multilayer sample to a single layer. For exploration of physical behavior and device applications, it is much desired to reversibly modulate such crossover in a multilayer sample. Here we demonstrate that, in a few-layer sample where the indirect bandgap and direct bandgap are nearly degenerate, the temperature rise can effectively drive the system toward the 2D limit by thermally decoupling neighboring layers via interlayer thermal expansion. Such a situation is realized in few-layer MoSe(2), which shows stark contrast from the well-explored MoS(2) where the indirect and direct bandgaps are far from degenerate. Photoluminescence of few-layer MoSe(2) is much enhanced with the temperature rise, much like the way that the photoluminescence is enhanced due to the bandgap crossover going from the bulk to the quantum limit, offering potential applications involving external modulation of optical properties in 2D semiconductors. The direct bandgap of MoSe(2), identified at 1.55 eV, may also promise applications in energy conversion involving solar spectrum, as it is close to the optimal bandgap value of single-junction solar cells and photoelechemical devices.

All-dielectric photonic crystals or metasurfaces supporting optical bound states in the continuum (BIC) are emerging as a promising platform for the study of light-matter interactions in the strong coupling regime. However, coupling of BIC to other optical excitations is generally weak, with the coupling strength limited in the range of several tens of milli electronvolts with short decoherence time. This greatly hinders their applications in solid-base quantum information devices that require a fast information exchange rate and long coherence time. Herein, a cavity-coupling strategy is used to extend the BIC-based coupling platforms. We propose a hybrid system containing an all-dielectric metasurface embedded in an optical microcavity, demonstrating the formation of BIC-cavity polaritons in the strong coupling regime with a surprisingly large Rabi splitting over 220 meV, far beyond the strong coupling regime. A flexible tuning of BIC-cavity coupling from weak to strong coupling regime is realized by controlling the shape of dielectric element of the metasurface. Importantly, a full quantum model based on Heisenberg-Langevin formalism reveals the temporal population dynamics of the coupling system, demonstrating the ultrafast Rabi socillation within 19 fs with a long decoherence time over 200 fs. Utilizing the multipolar expansion approach combined with full wave simulations, we also investigate the coupling-induced polariton characteristics in both the weak and strong coupling regimes. We find that the electric dipole component of the BIC mode dominates in the polariton states in weak coupling regime, which dramatically changes as the system is tuned into strong coupling regime. Such a cavity-coupling strategy in the present hybrid system is expected to be of great use for enhancing BIC-based light-matter interactions for more complex hybrid nanostructures such as three-mode systems with mixing BIC-cavity-exciton interactions.

Anisotropic polariton scattering and spin dynamics of cavity polaritons

Polariton Lasers do not rely on stimulated emission of photons, a criterion that sets stringent conditions on the threshold current in a conventional laser. Therefore, they have the capability to outperform photon lasers in the weak coupling regime in terms of the threshold power consumption. We present the first successful realization of an electrically pumped polariton laser based on a GaAs/AlAs distributed Bragg reflector cavity. We have furthermore identified the system’s response to an applied magnetic field as a sensitive tool to distinguish a polariton laser from a standard VCSEL device in the weak light-matter coupling regime

The concept of Bose–Einstein condensation (BEC) critical temperature and volume are introduced for atomic systems. The possibility of observation of BEC at high temperature due to small mass is discussed considering exciton-polaritons in bulk CuCl, CdSe, and Cu2O, indirect excitons in coupled QWs, and mostly polaritons in semiconductor microcavities, the last system being most effective for observing BEC at room temperature. The dispersion relations of exciton-polaritons for both upper and lower polariton branches, effective masses, scattering processes, and conditions for observing BEC in microcavities are discussed. Experimental results for BEC in such structures, pumping and detection mechanisms, and characteristics are then presented. Novel features of polariton lasers, progress in developing room temperature electrically pumped lasers with structures are then presented. A model for electrically driven polariton laser considering formation and scattering of excitons is developed and a steady state rate equation model is used to obtain useful characteristics.

Quantum optical effects in semiconductor microcavities

We study Monte Carlo calculations of the effective potential for a scalar field theory using three techniques. One of these is a new method proposed and tested for the first time. In each case we extract the renormalised quantities of the theory. The system studied in our calculations is a one component $\phi^4$ model in two dimensions. We apply these methods to both the weak and strong coupling regime. In the weak coupling regime we compare our results for the renormalised quantities with those obtained from two-loop lattice perturbation theory. Our results are verified in the strong coupling regime through comparison with the strong coupling expansion. We conclude that effective potential methods, when suitably chosen, can be accurate tools in calculations of the renormalised parameters of scalar field theories.

Cavity polaritons which are the elementary optical excitations in semiconductor microcavities may be viewed as a superposition of excitons and cavity photons. The major feature of cavity polariton technology centers on large and unique optical nonlinearities which would lead to a new class of optical devices such as polariton lasers exhibiting very low thresholds and polariton parametric amplifiers with ultrafast response. Among the wide bandgap semiconductors, GaN and ZnO are promising candidates for low-threshold polariton lasers operating at room temperature because of their large oscillator strengths and large exciton binding energies, particularly ZnO with its unmatched exciton binding energy of 60 meV. In this chapter, the recent progress on polariton devices based on wide bandgap semiconductor microcavities is reviewed.

Exciton quantum phases of Bose-Einstein condensation, exciton insulators, and superfluidity are largely governed by exciton-exciton interactions. Manipulating these interactions has sparked tremendous efforts to investigate quantum-confined excitons in two-dimensional (2D) semiconductors. In typical 2D exciton systems, however, strong dipolar interactions can be accessible at the cost of optical deactivation with significantly reduced oscillator strengths. Here we observe an emergent dipolar exciton with strong interactions in 2D perovskites with a surface lattice reconstruction. To unravel the microscopic nature of this exciton, we synergistically combine extensive optical spectroscopy, quantum-confined Stark effect, and density functional theory calculations. The dipolar exciton features dominant in-plane (IP) transition dipole moments, alongside coexisting out-of-plane (OP) electric dipole moments, giving rise to an exciton-exciton interaction strength of 38.1 ± 6.7 μeV·μm2 as one of the highest reported to date. The observation promises great potential to explore quantum phenomena and optoelectronic applications based on strongly interacting dipolar excitons in 2D semiconductors.

The field of topological photonics has seen tremendous and wide‐ranging developments in recent years. Evolving from the broader field of topological insulators, topological photonics systems today harness a variety topological phases. These include the Su–Schreifer–Heeger, quantum Hall, quantum valley Hall and quantum spin Hall topologies. Importantly, the latter two generate edge states with opposite group velocities and opposite spin, respectively, allowing unidirectional light propagation and advanced photonic routing to occur. Amongst these exciting developments is a subset of advancements made in topological silicon photonics, which could potentially lend its appeal to complementary metal–oxide–semiconductor (CMOS) photonics applications, including telecommunications, data communications, quantum photonics, future exascale supercomputers, photonic neuromorphic computing, and infrared sensing. The fundamental underpinnings of these topological phases lead to interesting features, including chirality, scatter‐free light propagation around sharp bends, and importantly topological protection against defects, disorder, and scattering. This topological protection may be harnessed toward tunable light propagation, photon‐pair generation, quantum spatial entanglement, robust photonic routing, and beyond. Herein, the recent advancements made in the burgeoning field of topological silicon photonics are discussed.

We describe recent continuous wave studies of stimulated polariton scattering in semiconductor microcavities, with emphasis on the variety of new physics which has been observed in the last few years. The ability to manipulate both exciton and photon properties, and how they interact together to form strongly coupled exciton–photon coupled modes, exciton polaritons, leads to a number of novel phenomena, previously unreported in bulk semiconductors or quantum wells. The unusual dispersion of the lower polariton branch, which allows energy and momentum conserving polariton–polariton parametric scattering under resonant excitation, is shown to be a key factor enabling much of the new physics. A classical theoretical treatment of the phenomena which draws parallels between the equivalent pictures of stimulated polariton scattering and those of a triply resonant optical parametric oscillator is discussed. Higher order parametric phenomena and the factors which control the threshold for stimulation are also described. The results obtained under resonant excitation are contrasted with phenomena which occur under conditions of non-resonant excitation. Prospects to achieve ‘polariton lasing’ under non-resonant excitation are discussed.

$J$-coupled nuclear magnetic resonance (NMR) spectroscopy in the strong coupling regime at low magnetic field ($10{}^{\ensuremath{-}7}$ T $lBl{10}^{\ensuremath{-}3}$ T) is more complex than at high field ($Bg{10}^{\ensuremath{-}3}$ T) and at ultralow field ($Bl{10}^{\ensuremath{-}7}$ T). We show that several upper and lower boundaries ${B}_{i}^{\mathrm{up}}$ and ${B}_{i}^{\mathrm{low}}$ of the magnetic field $B$ exist, where the complexity of $J$-coupled NMR spectra changes in terms of the number of lines. The index $i=1,2,\dots{}$ for ${B}_{i}^{\mathrm{up}}$ at high field specifies the perturbation order of the dominating Zeeman interaction and for ${B}_{i}^{\mathrm{low}}$ at ultralow field the perturbation order of the dominating $J$-coupling interaction. Mathematical expressions for these boundaries are derived for the case of a $J$-coupled $S$-$I$${}_{N}$ group where $S$ and $I$ are rare and abundant spins $\frac{1}{2}$ and $N$ counts the abundant spins $I$. The entire $B$-field range can further be delineated into two weak coupling regimes, one at high field with ${B}_{2}^{\mathrm{up}}lBl{B}_{1}^{\mathrm{up}}$ ($10{}^{\ensuremath{-}3}$ T $lBl{10}^{2}$ T), one at low field with ${B}_{1}^{\mathrm{low}}lBl{B}_{2}^{\mathrm{low}}({10}^{\ensuremath{-}8}$ T $lBl{10}^{\ensuremath{-}7}$ T), and a strong coupling regime with ${B}_{2}^{\mathrm{low}}lBl{B}_{2}^{\mathrm{up}}$ ($10{}^{\ensuremath{-}7}$ T $lBl{10}^{\ensuremath{-}3}$ T). The corresponding NMR spectra for the $S\ensuremath{-}{I}_{N}$ group are investigated by experiment and by simulation. In the strong coupling regime, the maximum number of lines is $(N+1){}^{2}$. In the weak coupling regime ${B}_{1}^{\mathrm{low}}lBl{B}_{2}^{\mathrm{low}}$ at low field, symmetric multiplet structures group around the frequencies $0$, $J$, ($3/2$)$J$, $2J$, ($5/2$)$J$, etc. These spectra determine the structure of the $S\ensuremath{-}{I}_{N}$ group unambiguously and are in dual correspondence to the weakly coupled spectra at high field. High-resolution NMR spectroscopy at ultralow field may open up new ways for chemical analysis by small and mobile instruments with many applications in science and technology.

We have studied dispersion of surface plasmon polaritons (SPPs) in the Kretcshmann’s geometry (prism/Ag/dye-doped polymer) in weak and strong exciton-plasmon coupling regimes. In reflection, the dispersion curves were in a good agreement with simple model predictions at small concentrations of the dye (R6G) in the polymer (PMMA). At the same time, highly unusual multi-segment ladder-like dispersion curves were observed at large dye concentrations. To describe this behavior, we upgraded the theoretical model taking into account strong exciton-plasmon coupling. The dispersion curves obtained in emission experiments were strongly different from those in reflection and did not seem to be affected by the strong coupling.

This work aimed to investigate the regimes and coupling in hybrid systems composed of a molybdenum disulfide (MoS2) monolayer coupled to different metallic gold nanostructures, using numerical simulations based on the Finite-Difference Time-Domain (FDTD) method, complemented by experimental differential reflectance measurements. The MoS2 monolayer, a two-dimensional semiconductor with a direct bandgap in the visible region, exhibits well-defined excitonic excitations, notably the A and B excitons. By engineering the geometric parameters of the metallic gratings such as periodicity (Λ), depth (d), and slit width (w) it was possible to tune the plasmonic resonances to coincide with the excitonic energies of the MoS2 monolayer. The optimal parameters were determined as Λ = 400 nm, d = 70 nm, and w = 50 nm, with the most favorable excitation condition occurring at an incidence angle of 50°. Among the most relevant indications of a possible strong-coupling regime is the observation of behavior consistent with the anti-crossing phenomenon in the simulated spectra, which may be related to Rabi splitting and the formation of hybrid states, known as plexcitons. Although these results are promising, additional studies are needed to unequivocally confirm the occurrence of strong coupling. These findings contribute to the understanding and development of hybrid plasmon-exciton platforms for applications in nanophotonics, with potential impact on the design of photodetectors, high-sensitivity optical sensors, ultrafast modulators, nanoscale coherent light sources, and the enhancement of nonlinear optical processes, such as Second Harmonic Generation (SHG), in two-dimensional materials.