Zhanghai Chen’s research while affiliated with Xiamen University and other places

We propose a scheme for producing and manipulating quantized exciton-polariton vortices in the higher-order topological corner modes of a two-dimensional array of micropillars. By nonresonantly exciting p-orbital condensates with different orientations at two input corners, polariton vortices carrying the required topological charges can be controllably created at output corners away from the pumping spots. Besides, polariton vortices formed at input corners can be copied to the output corners through the topological edge states. Our scheme provides topological double insurance for intrinsic binary information memory and holds potential applications in remote information processing.

Exciton polaritons—quasi-particle excitations consisting of strongly coupled photons and excitons—present fascinating possibilities for photonic circuits, owing to their strong nonlinearity, ultrafast reaction times and their ability to form macroscopic quantum states at room temperature via non-equilibrium condensation. Past implementations of transistors and logic gates with exciton polaritons have been mostly realized using the spatial propagation of polariton fluids, which place high demands on the fabrication of the microcavities and typically require complex manipulations. In this work we have implemented the full set of logical gate functionalities (that is, temporal AND, OR and NOT gates) in localized exciton polaritons at room temperature, on the basis of precisely controlling the interplay between polariton condensate and exciton reservoir dynamics, using a two-pulse excitation scheme. The dynamics intrinsically covers the cascadability required by the logical operations, enabling efficient information processing without the need for spatial flow. The temporal polariton logic gates demonstrate advantages in ultrafast switching, universality and simplified compatibility with other dimensional controls, showing great potential for building polariton logic networks in strongly coupled light–matter systems.

Two-dimensional transition metal dichalcogenide heterostructures provide a unique opportunity for quantum engineering of electronic and excitonic states at the nanoscale. Critical optical properties of interlayer excitons, including transition energy, optical selectivity, and quantum yield, are strongly correlated to the stacking orders. However, these optical properties could vary from sample to sample, setting an obstacle to extracting the intrinsic stacking order dependence experimentally. We report an effective method to fabricate heterobilayers with both stacking orders obtained on a single device. The sharp difference of interlayer excitons induced by the stacking orders was unambiguously identified, including emission wavelength, valley polarization, and temperature dependence of quantum yield. This method provides a flexible platform to study stacking order dependence of heterobilayer excitons, and can be readily applied to explore the layer hybridization, strong correlations, and exciton diffusion that are sensitive to stacking order.

ZnO nanowire (NW) lasing driven by mid-infrared (MIR) laser pulses has attracted significant attention owing to its remarkable wavelength-independent lasing threshold and potential applications in diverse situations. However, the properties of MIR laser-driven ZnO microwire (MW) lasing are rarely studied when the wire diameter is increased from nanoscale to microscale, comparable to the wavelength of the driving laser. Here we experimentally measured the ZnO MW lasing driven by MIR laser with different polarizations. The measurements show that the laser polarized along the c axis was more efficient for the lasing in MW with diameter smaller than the driving laser wavelength, while the polarization dependence was ambiguous when the MW diameter was greater than the driving laser wavelength. Through the modeling of the lasing process in ZnO MW, the observed polarization dependence is reproduced and can be attributed to the combined effect of the optical interference of the driving laser in the MW and the varying absorption properties of different MWs. The former is related to the MW diameter and the latter is sensitive to the sample growth condition. Our findings shed light on the feasibility of manipulation of the ZnO MW lasing.

The optical spin Hall effect, as an optical analogue of spin-orbit coupling of fermionic electrons, has attracted increasing interest in fundamental physics and nanophotonic applications. In addition to spin, charge of particles, as the other degree of freedom, plays a significant role in spin-orbit interaction as well. However, the optical counterpart of charge conjugation has been absent due to the charge-free nature of photons. Here, in a one-dimensional photonic crystal, we report reversible optical spin currents from the opposite band edges of the photonic band gap, which can provide an optical analogue of charge conjugation in spin-orbit coupling. The effective magnetic field acting on the photons is controlled by the tunable splitting of the transverse electric and transverse magnetic optical modes, which is confirmed by both experiment and simulation. Based on the tunable effective magnetic field, the optical spin current can be controlled by the tunable cavity length in an open cavity. Our results will facilitate the development of optical spintronic devices and guide the understanding of analogies and generalizations involving quantum and classical wave theories.

Coding metasurfaces bring photonic design into a new era for their convenience in manipulating electromagnetic waves in a programmable way. Different coding patterns can be further convoluted to give rise to nontrivial effects such as multiple beam steering, simultaneous control of surface and space waves, and the integration of multiple functionalities. However, previous experimental works have been limited to low frequencies. Extending convoluted coding metasurfaces to optical frequency can significantly reduce the size of structures, and therefore facilitate their applications in optical circuits. Here, we experimentally demonstrate a Pancharatnam-Berry metasurface designed by convoluting two distinct coding patterns. Such a metasurface exhibits integrated functionalities: the optical spin Hall effect and spin to orbital angular momentum conversion, which are robust over a broad visible band and a wide range of incident angles. Our work represents an important step towards multifunctional coding metasurfaces at optical frequency based on convolution operation.

The past 30 years have witnessed remarkable developments of microcavity exciton–polaritons, which have made a great impact on photonics and optoelectronics from fundamental physics to device applications. New materials and optical structures have been developed for novel polariton lasers for the sake of room temperature operation, flexible mode engineering, and high power efficiency. More powerful spectroscopic techniques have also promoted the understanding of polariton dynamics, coherence, nonlinearity, and topology. In this review, we start with a brief introduction to the picture of polaritons, and various polariton systems based on different microcavity structures and semiconductor materials. Then, we present several important spectroscopic techniques and numerical tools for characterizing polaritons experimentally and theoretically. Next, we address the macroscopic quantum phenomena observed in the polariton systems and review the physics and applications of polariton nonlinearity. Moreover, we highlight the new emerging fields of topological and non-Hermitian polaritons. In the end, we conclude with the future perspectives of microcavity exciton–polaritons.

Parametric scattering dynamics are general and of crucial importance for cavity exciton polaritons. Here, parametric scattering process driven by exciton polariton condensates has been revealed in a 1D ZnO microcavity between the whispering-gallery mode and quasiwhispering-gallery mode. When the occupation of the produced polariton condensate is dense enough, polariton condensates formed on quasiwhispering-gallery mode can be scattered towards the ground state of the adjacent whispering-gallery modes at higher and lower energies. By using the femtosecond angle-resolved spectroscopic imaging technique, the ultrafast dynamics of this intermode polariton parametric scattering have been explicitly observed. The scattering towards a higher mode occurs faster than that to a lower-energy mode by less than a picosecond. The revealed dynamics can not only expand the present investigations on polariton parametric scattering, but also promote the potential applications in, e.g., quantum information processing.

We propose an all-optical scheme of topological lasing and switching based on the Aubry-André-Harper (AAH) model of an exciton-polariton chain. We theoretically show that the phase parameter of the optical potential, with a tunable effective quasimomentum, allows the system to exhibit nontrivial topological properties which are attributed to higher dimensions. The topological modes emerging within the bulk band gaps are spatially localized at the edges of the polariton lattice, and their topological properties are characterized by the nonzero Chern numbers of the bulk bands. Polariton lasing in topological edge modes exhibits a higher efficiency and better robustness than in bulk modes, and can be switched between two opposite edges of the lattice by nonresonant excitation, which paves a way for topologically protected optical circuits.