Valley-addressable monolayer lasing through spin-controlled Berry phase photonic cavities.

The spin-valley coupling between circularly polarized light and valley excitons in transition metal dichalcogenides provides the opportunity to generate and manipulate spin information by exploiting the valley degree of freedom. Here, we demonstrate a room-temperature valley-addressable tungsten disulfide monolayer laser in which the spin of lasing is controlled by the spin of pump without magnetic fields. This effect was achieved by integrating a tungsten disulfide monolayer into a photonic cavity that supports two orthogonal spin modes with high quality factors. The spin-pumped lasing effectively broke the population symmetry of valley excitons, resulting in highly coherent emission with valley-switchable radiation modes due to distinct laser thresholds. Our scheme provides a nanophotonic platform to develop versatile coherent spin-light sources operating at room temperature by actively manipulating spin-valley coupling in light-matter interactions.

Spin-valley Rashba monolayer laser.

Direct-bandgap transition metal dichalcogenide monolayers are appealing candidates to construct atomic-scale spin-optical light sources owing to their valley-contrasting optical selection rules. Here we report on a spin-optical monolayer laser by incorporating a WS2 monolayer into a heterostructure microcavity supporting high-Q photonic spin-valley resonances. Inspired by the creation of valley pseudo-spins in monolayers, the spin-valley modes are generated from a photonic Rashba-type spin splitting of a bound state in the continuum, which gives rise to opposite spin-polarized ±K valleys due to emergent photonic spin-orbit interaction under inversion symmetry breaking. The Rashba monolayer laser shows intrinsic spin polarizations, high spatial and temporal coherence, and inherent symmetry-enabled robustness features, enabling valley coherence in the WS2 monolayer upon arbitrary pump polarizations at room temperature. Our monolayer-integrated spin-valley microcavities open avenues for further classical and non-classical coherent spin-optical light sources exploring both electron and photon spins.

Regulated Photon Transport in Chaotic Microcavities by Tailoring Phase Space.

Manipulating light dynamics in optical microcavities has been made mainly either in real or momentum space. Here we report a phase-space tailoring scheme, simultaneously incorporating spatial and momentum dimensions, to enable deterministic and in situ regulation of photon transport in a chaotic microcavity. In the time domain, the chaotic photon transport to the leaky region can be suppressed, and the cavity resonant modes show stronger temporal confinement with quality factors being improved by more than 1 order of magnitude. In the spatial domain, the emission direction of the cavity field is controlled on demand through rerouting chaotic photons to a desired channel, which is verified experimentally by the far-field pattern of a quantum-dot microlaser. This work paves a way to in situ study of chaotic physics and promoting advanced applications such as arbitrary light routing, ultrafast random bit generation, and multifunctional on-chip lasers.

Photonic Rashba effect from quantum emitters mediated by a Berry-phase defective photonic crystal.

Heterostructures combining a thin layer of quantum emitters and planar nanostructures enable custom-tailored photoluminescence in an integrated fashion. Here, we demonstrate a photonic Rashba effect from valley excitons in a WSe2 monolayer, which is incorporated into a photonic crystal slab with geometric phase defects, that is, into a Berry-phase defective photonic crystal. This phenomenon of spin-split dispersion in momentum space arises from a coherent geometric phase pickup assisted by the Berry-phase defect mode. The valley excitons effectively interact with the defects for site-controlled excitation, photoluminescence enhancement and spin-dependent manipulation. Specifically, the spin-dependent branches of photoluminescence in momentum space originate from valley excitons with opposite helicities and evidence the valley separation at room temperature. To further demonstrate the versatility of the Berry-phase defective photonic crystals, we use this concept to separate opposite spin states of quantum dot emission. This spin-enabled manipulation of quantum emitters may enable highly efficient metasurfaces for customized planar sources with spin-polarized directional emission.

Experimental demonstration of nanophotonic devices and circuits with colloidal quantum dot waveguides.

Colloidal quantum dots (CQDs) have been widely used as absorption or emission materials due to their large-absorption and high-gain properties. However, they are seldom used as low-loss materials in passive nanophotonic devices. Moreover, combinations of two or more properties of CQDs are difficult owing to miscibility of different CQDs. Here, low-loss CQD waveguides are experimentally achieved at wavelengths longer than their fluorescence wavelengths. By using the low-loss and uniform CQD waveguides, various passive nanophotonic devices and a nanophotonic circuit are successfully demonstrated. Furthermore, by employing both of a pattern-assisted stacking and a transfer-printing approach, the miscible problem of different CQDs is addressed, and a low-loss CQD waveguide and a high-gain CQD laser are experimentally integrated on a single chip.

Probing nanoscale fluctuation of ferromagnetic meta-atoms with a stochastic photonic spin Hall effect.

The photonic spin Hall effect, a deep subdiffraction-limited shift between the opposite spin components of light, emerges when light undergoes an evolution of polarization or trajectory that induces the geometric phase. Here, we study a stochastic photonic spin Hall effect arising from space-variant Berry-Zak phases, which are generated by disordered magneto-optical effects. This spin shift is observed from a spatially bounded lattice of ferromagnetic meta-atoms displaying nanoscale disorders. A random variation of the radii of the meta-atoms induces the nanoscale fluctuation. The standard deviation of the probability distribution of the spin shifts is proportional to the fluctuation of the meta-atoms. This enables us to detect a five-nanometre fluctuation by measuring the probability distribution of the spin shifts via weak measurements. Our approach may be used for sensing deep-subwavelength disorders by actively breaking the photonic spin symmetry and may enable investigations of fluctuation effects in magnetic nanosystems.

Pattern-assisted stacking colloidal quantum dots for photonic integrated circuits.

In photonic integrated circuits (PICs), on-chip light sources and other photonic devices are usually made of different materials. The complexity and compatibility brought about by different materials and various structures in a single chip considerably increase the fabrication and integration difficulties. Here, we propose to stack the same nanoscale building blocks [colloidal quantum dots (CQDs) with both large gains and high refractive indices] in predefined trench patterns to address the fabrication and integration problems of PICs. By employing this simple approach of using the same material (CdSe/ZnS CQDs), the on-chip integration of more than 10 CQD-based photonic components (including the laser, low-noise amplifier, bending waveguide, Y-splitter, Mach-Zehnder interferometer, and grating) is experimentally demonstrated. In particular, the integrated low-noise amplifier (net gain coefficient >600 cm-1) addresses the absorption loss problem brought about by the utilization of the same material. Moreover, the little influence of the CQD layer on the CQD nanophotonic components facilitates the fabrication and is beneficial for large-scale integration. This simple fabrication approach with a flexible integration strategy may provide a possible platform to construct functional PICs.

Photonic Topological Spin Hall Effect Mediated by Vortex Pairs.

Over the past decades, topology has provided unique insight into numerous physical phenomena. Here, we report on a topological mechanism for spin-dependent photonic transport. We observe photonic topological defects of bound vortex pairs and unbound vortices generated from a two-dimensional array of nanoantennas, i.e., a metasurface, which is achieved by randomly inserting local deformations in the metasurfaces, inducing the Pancharatnam-Berry phase. The observed spin-dependent bound vortex pairs are established as the origin of the photonic topological spin Hall effect-a subdiffraction-limited spin-split mode in momentum space, while the spin-dependent unbound vortices induce random spin-split modes throughout the entire momentum space as a random Rashba effect. The topological phenomena-creation of bound vortex pairs and unbound vortices-indicate the universality of the topological effect for particles of different natures.

Configurable Integration of On-Chip Quantum Dot Lasers and Subwavelength Plasmonic Waveguides.

The integration of on-chip dielectric lasers and subwavelength plasmonic waveguides has attracted enormous attention because of the combination of both the advantages of the high performances of the small dielectric lasers and the subwavelength plasmonic waveguides. However, the configurable integration is still a challenge owing to the complexity of the hybrid structures and the damageability of the gain media in the multistep micro/nanofabrications. By employing the dark-field optical imaging technique with a position uncertainty of about 21 nm and combining the high-resolution electron beam lithography, the small colloidal quantum dot (CQD) lasers without any damages are accurately aligned with the silver nanowires. As a result, the integration of the CQD lasers and the silver nanowires can be flexibly configured on chips. In the experiment, the tangential coupling, radial coupling, and complex coupling between the high-performance CQD lasers and the subwavelength silver nanowires are demonstrated. Because of the subwavelength field confinements of the silver nanowires, the deep-subwavelength coherent sources (multimode, one-color single-mode, or two-color single-mode) with a mode area of only 0.008λ2 are output from these hybrid structures. This configurable on-chip integration with high flexibility and controllability will greatly facilitate the developments of the complex functional hybrid photonic-plasmonic circuits.

Plasmonic ridge waveguides with deep-subwavelength outside-field confinements.

Subwavelength plasmonic waveguides are the most promising candidates for developing planar photonic circuitry platforms. In this study a subwavelength metallic ridge waveguide is numerically and experimentally investigated. Differing from previous plasmonic waveguides, the metallic strip of the subwavelength ridge waveguide is placed on a thick metal film. It is found that the surface-plasmon-polariton (SPP) waveguide modes result from the coupling of the corner modes in the two ridge corners. The bottom metal film has a great influence on the SPP modes, and nearly all the evanescent fields of the SPP modes are tightly confined outside the ridge waveguide. Simulations show that 50% of the total power flow in the SPP mode can be confined outside the ridge waveguide with an area of only about λ (2)/20. The propagation length is still about 10 times the plasmon wavelength. Through comparison with a metallic strip placed directly on the dielectric substrate, the proposed ridge waveguide exhibits a much higher sensing performance. This plasmonic ridge waveguide with deep-subwavelength outside-field confinements is of significance in a range of nano-optics applications, especially in nanosensing.

An ultrahigh-contrast and broadband on-chip refractive index sensor based on a surface-plasmon-polariton interferometer.

Using a double-slit structure fabricated on a gold film or a subwavelength (300 nm) plasmonic waveguide, high-contrast and broadband plasmonic sensors based on the interference of surface plasmon polaritons (SPPs) are experimentally demonstrated on chips. By adjusting the focused spot position of the p-polarized incident light on the double-slit structure to compensate for the propagation loss of the SPPs, the interfering SPPs from the two slits have nearly equal intensities. As a result, nearly completely destructive interference can be experimentally achieved in a broad bandwidth (>200 nm), revealing the robust design and fabrication of the double-slit structure. More importantly, a high sensing figure of merit (FOM*) of >1 × 10(4) RIU(-1) (refractive index unit), which is much greater than the previous experimental results, is obtained at the destructive wavelength because of a high contrast ratio (C = 0.96). The high-contrast and broadband on-chip sensor fabricated on the subwavelength plasmonic waveguide may find important applications in the real-time sensing of particles and molecules.