Van der Waals materials can be placed on a variety of substrates with minimal degradation of their optical properties. They therefore open the opportunity to create polaritons in nearly arbitrary photonic structures. We will discuss a few different types of cavity systems integrated with van der Waals materials to realize polaritons with unusual properties, potentially enabling novel manybody phenomena.
Light-matter interactions are at the heart of quantum electrodynamics and underpin modern photonic technologies. As we develop means to control the properties of light, matter and their interactions, intriguing new phenomena emerge. We will discuss a few examples ranging from conventional semiconductors to two-dimensional materials and molecular systems.
Van der Waals semiconductors provide a platform for creating two-dimensional crystals layer-by-layer and engineering excitonic states therein with exceptional properties. We discuss here a few interesting opportunities enabled by interlayer excitons in bilayer transitional metal dichalcogenides (TMDs), including high valley polarizations, lasing in 2D cavities, and tunable interlayer excitons in homo-bilayers.
Monolayer TMDs feature spin-valley locking, enabling valleytronic phenomena and applications. However, strong inter-valley scattering due to electron hole exchange interactions leads to rapid valley depolarization in picoseconds, making it difficult to achieve a high degree of valley polarization. The electron-hole exchange interaction becomes suppressed for interlayer excitons in heterobilayers with type II band alignment. We show highly polarized interlayer excitons with a long valley lifetimein in both spin singlet and brightened triplet states in hetero-bilayers.
TMDs have also garnered intense interest as an active medium, for they feature very strong exciton-photon interactions in a monolayer. However, the rapid radiative decay makes it challenging to establish population inversion. Interlayer excitons decay much more slowly, comparable to excitons in quantum wells of conventional semiconductors. When the interlayer exciton are integrated on a cavity, lasing was established at the cavity resonance accompanied by increased temporal and spatial coherence.
Lastly, due to the strong intra-layer localization of the carriers in 2D materials, interlayer excitons co-exist with intra-layer excitons as meta-stable states even in homo-bilayers without artificial interfaces. These interlayer excitons also feature oscillator strengths between intra-layer excitons and inter-layer ones in hetero-structures, offering a potentially highly tunable system for 2D optoelectronics.
Indium gallium nitride (InGaN) semiconductor quantum dots are an attractive candidate for scalable room temperature quantum photonics applications owing to their large exciton binding energy and large oscillation strength. Previously, we reported single photon emission from site-controlled InGaN quantum dot structures. However, large homogeneous linewidth and significant non-radiative recombination were thought to be linked to the nearby surface charge centers. These charge centers can lead to spectral diffusion and excessive non-radiative recombinations at high temperature. In this work, ammonium sulfide passivation was investigated. Nitrogen vacancies were successfully passivated by ammonium sulfide ((NH4)2Sx) treatment, and the emission linewidth of a single quantum dot was reduced by 5 meV. Furthermore, the linewidth broadening with an increasing temperature was suppressed in the temperature range from 9 K to 95 K in this study. Satellite emission peak believed to be associated with the nitrogen vacancy was observed for un-passivated quantum dots. The satellite peak was 55 ~ 80 meV away from the main InGaN emission peak and was eliminated after sulfide passivation.
The characteristics of exciton-polaritons in ZnO-based microcavities (MCs) are demonstrated with a large vacuum Rabi
splitting due to large exciton binding energy and oscillator strength. The lower polariton branches (LPBs) can be clearly
observed. For low temperature and large negative detuning conditions, a clear polariton relaxation bottleneck in bulk
ZnO-based MCs has been observed in angle-resolved photoluminescence measurements from 100 to 353 K at different
cavity-exciton detunings. The bottleneck is strongly suppressed with increasing the temperature and pumping power and
reducing detuning. This observed results supposed to be due to more efficient phonon-assisted relaxation and a longer
radiative lifetime of the polaritons. In addition, the linewidth broadening, blue-shift of the emission peak, and
polarization of polariton lasing from below threshold to up threshold are also discussed.
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