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Many breakthroughs in technologies are closely associated with the deep understanding and development of new material platforms. As the main material used in microelectronics, Si also plays a leading role in the development of integrated photonics. The indirect bandgap, absence of χ (2) nonlinearity and the parasitic nonlinear absorptions at the telecom band of Si imposed technological bottlenecks for further improving the performances and expanding the functionalities of Si microcavities in which the circulating light intensity is dramatically amplified. The past two decades have witnessed the burgeoning of the novel material platforms that are compatible with the complementary metal-oxide-semiconductor (COMS) process. In particular, the unprecedented optical properties of the emerging materials in the thin film form have resulted in revolutionary progress in microcavity photonics. In this review article, we summarize the recently developed material platforms for integrated photonics with the focus on chip-scale microcavity devices. The material characteristics, fabrication processes and device applications have been thoroughly discussed for the most widely used new material platforms. We also discuss open challenges and opportunities in microcavity photonics, such as heterogeneous integrated devices, and provide an outlook for the future development of integrated microcavities.

Microlasers in near-degenerate supermodes lay the cornerstone for studies of non-Hermitian physics, novel light sources, and advanced sensors. Recent experiments of the stimulated scattering in supermode microcavities reported beating phenomena, interpreted as dual-mode lasing, which, however, contradicts their single-mode nature due to the clamped pump field. Here, we investigate the supermode Raman laser in a whispering-gallery microcavity and demonstrate experimentally its single-mode lasing behavior with a side-mode suppression ratio (SMSR) up to 37 dB, despite the emergence of near-degenerate supermodes by the backscattering between counterpropagating waves. Moreover, the beating signal is recognized as the transient interference during the switching process between the two supermode lasers. Self-injection is exploited to manipulate the lasing supermodes, where the SMSR is further improved by 15 dB and the laser linewidth is below 100 Hz.

This paper addresses part of a wider project aiming at the development of a microchannel based heat sink to cool photovoltaic panels. The work presented here gives emphasis to working conditions leading to flow boiling in a microchannel and focuses on flow instabilities and on the potential effect of surface microstructuring in heat transfer enhancement and in controlling such instabilities. After identifying the flow boiling regimes, the observed phenomena are qualitatively and quantitatively described. The results show that a regular pattern of microcavities on the surface to cool enhances heat transfer and promotes the occurrence of a more stable flow.

An optimal single-photon source should deterministically deliver one, and only one, photon at a time, with no trade-off between the source’s efficiency and the photon indistinguishability. However, all reported solid-state sources of indistinguishable single photons had to rely on polarization filtering, which reduced the efficiency by 50%, fundamentally limiting the scaling of photonic quantum technologies. Here, we overcome this long-standing challenge by coherently driving quantum dots deterministically coupled to polarization-selective Purcell microcavities. We present two examples: narrowband, elliptical micropillars and broadband, elliptical Bragg gratings. A polarization-orthogonal excitation–collection scheme is designed to minimize the polarization filtering loss under resonant excitation. We demonstrate a polarized single-photon efficiency of 0.60 ± 0.02 (0.56 ± 0.02), a single-photon purity of 0.975 ± 0.005 (0.991 ± 0.003) and an indistinguishability of 0.975 ± 0.006 (0.951 ± 0.005) for the micropillar (Bragg grating) device. Our work provides promising solutions for truly optimal single-photon sources combining near-unity indistinguishability and near-unity system efficiency simultaneously.

We developed a mathematical model to examine the scattering of radiation by a periodic structure of circular and elliptical microcavities formed in a planar optical waveguide. The waveguide simulates the behaviour of a 62.5/125 µm multimode optical fibre. The calculations focused on the intensity distribution of scattered light with a wavelength of 1310 nm along the periodic structure, i.e., along the side surface of the waveguide, as a function of the microcavity dimensions and their spatial arrangement within the waveguide core. The optimal geometrical parameters of the microstructure, ensuring the most uniform light scattering, were identified. The model is valid for multimode optical fibres containing strictly periodic structures of microcavities with spherical or elliptical cross-sections that scatter laser radiation in all directions. One potential application of such fibres is as light sources in medical probes for surgical procedures requiring additional illumination and uniform irradiation of affected tissues. Furthermore, the findings of this study offer significant potential for the development of sensing elements for fibre-optic sensors. The findings of this study will facilitate the design of scattering structures with microcavities that ensure a highly uniform scattering pattern.

Defect centres in diamond are promising building blocks for quantum networks thanks to a long-lived spin state and bright spin-photon interface. However, their low fraction of emission into a desired optical mode limits the entangling success probability. The key to overcoming this is through Purcell enhancement of the emission. Open Fabry-Perot cavities with an embedded diamond membrane allow for such enhancement while retaining good emitter properties. To guide the focus for design improvements it is essential to understand the influence of different types of losses and geometry choices. In particular, in the design of these cavities a high Purcell factor has to be weighed against cavity stability and efficient outcoupling. To be able to make these trade-offs we develop analytic descriptions of such hybrid diamond-and-air cavities as an extension to previous numeric methods. The insights provided by this analysis yield an effective tool to find the optimal design parameters for a diamond-air cavity.

Microlaser with multiple lasing bands is critical in various applications, such as full-color display, optical communications, and computing. Here, we propose a simple and efficient method for homogeneously doping rare earth elements into a silica whispering-gallery microcavity. By this method, an Er-Yb co-doped silica microsphere cavity with the highest quality (Q) factor (exceeding 108) among the rare-earth-doped microcavities is fabricated to demonstrate simultaneous and stable lasing covering ultraviolet, visible, and near-infrared bands under room temperature and a continuous-wave pump. The thresholds of all the lasing bands are estimated to be at the submilliwatt level, where both the ultraviolet and violet continuous wave upconversion lasing from rare earth elements has not been separately demonstrated under room temperature until this work. This ultrahigh-Q doped microcavity is an excellent platform for high-performance multiband microlasers, ultrahigh-precision sensors, optical memories, and cavity-enhanced light–matter interaction studies.

Semiconductor excitations can hybridize with cavity photons to form exciton-polaritons (EPs) with remarkable properties, including light-like energy flow combined with matter-like interactions. To fully harness these properties, EPs must retain ballistic, coherent transport despite matter-mediated interactions with lattice phonons. Here we develop a nonlinear momentum-resolved optical approach that directly images EPs in real space on femtosecond scales in a range of polaritonic architectures. We focus our analysis on EP propagation in layered halide perovskite microcavities. We reveal that EP–phonon interactions lead to a large renormalization of EP velocities at high excitonic fractions at room temperature. Despite these strong EP–phonon interactions, ballistic transport is maintained for up to half-exciton EPs, in agreement with quantum simulations of dynamic disorder shielding through light-matter hybridization. Above 50% excitonic character, rapid decoherence leads to diffusive transport. Our work provides a general framework to precisely balance EP coherence, velocity, and nonlinear interactions. Exciton-polaritons are part-light part-matter states in semiconductors. Here the authors leverage momentum-resolved optical microscopy to image ballistic and diffusive propagation of exciton-polaritons on femtosecond scales.

Exceptional points are singularities of open systems, and among their many remarkable properties, they provide a way to enhance the responsivity of sensors. Here we show that the improved responsivity of a laser gyroscope caused by operation near an exceptional point is precisely compensated by increasing laser noise. The noise, of fundamental origin, is enhanced because the laser mode spectrum loses the oft-assumed property of orthogonality. This occurs as system eigenvectors coalesce near the exceptional point and a bi-orthogonal analysis confirms experimental observations. While the results do not preclude other possible advantages of the exceptional-point-enhanced responsivity, they do show that the fundamental sensitivity limit of the gyroscope is not improved through this form of operation. Besides being important to the physics of microcavities and non-Hermitian photonics, these results help clarify fundamental sensitivity limits in a specific class of exceptional-point sensor. Operating a laser gyroscope near an exceptional point has been shown to enhance its responsivity. However, here the authors demonstrate in theory and experiment that the enhanced responsivity is exactly compensated by increased noise that is inherent to this system near the exceptional point.

The optical properties of transition metal dichalcogenide monolayers are widely dominated by excitons, Coulomb-bound electron–hole pairs. These quasi-particles exhibit giant oscillator strength and give rise to narrow-band, well-pronounced optical transitions, which can be brought into resonance with electromagnetic fields in microcavities and plasmonic nanostructures. Due to the atomic thinness and robustness of the monolayers, their integration in van der Waals heterostructures provides unique opportunities for engineering strong light-matter coupling. We review first results in this emerging field and outline future opportunities and challenges. Excitons, quasi-particles of tightly bound electron-hole pairs, dominate the optical response of atomically thin transition metal dichalcogenides. Here, the authors review strong light-matter coupling in two-dimensional semiconductors arising from confined excitons interacting with trapped photons or localized plasmons.