Observation of intrinsic chiral bound states in the continuum.

Photons with spin angular momentum possess intrinsic chirality, which underpins many phenomena including nonlinear optics1, quantum optics2, topological photonics3 and chiroptics4. Intrinsic chirality is weak in natural materials, and recent theoretical proposals5-7 aimed to enlarge circular dichroism by resonant metasurfaces supporting bound states in the continuum that enhance substantially chiral light-matter interactions. Those insightful works resort to three-dimensional sophisticated geometries, which are too challenging to be realized for optical frequencies8. Therefore, most of the experimental attempts9-11 showing strong circular dichroism rely on false/extrinsic chirality by using either oblique incidence9,10 or structural anisotropy11. Here we report on the experimental realization of true/intrinsic chiral response with resonant metasurfaces in which the engineered slant geometry breaks both in-plane and out-of-plane symmetries. Our result marks, to our knowledge, the first observation of intrinsic chiral bound states in the continuum with near-unity circular dichroism of 0.93 and a high quality factor exceeding 2,663 for visible frequencies. Our chiral metasurfaces may lead to a plethora of applications in chiral light sources and detectors, chiral sensing, valleytronics and asymmetric photocatalysis.

Ultracompact Orbital Angular Momentum Sorter on a CMOS Chip.

On-chip integrated orbital angular momentum (OAM) sorting is of great importance in tackling the severe challenge of exponential growth in data traffic. Despite the continuous success, current demultiplexing techniques either scarify efficiency dramatically or lose the compactness of a system. Here we experimentally demonstrate an ultracompact OAM sorter using TiO2 metasurfaces integrated onto a complementary metal-oxide-semiconductor (CMOS) camera. By utilizing the propagation phases, we transfer the unitary transformation theory in bulky systems into two TiO2 metasurfaces, responsible for the functions of log-polar transformation and fan-out beam copying and focusing as well as the functions of phase correction and Fourier transform. The flatform metasurface doublet enables one to integrate the OAM sorter onto a camera chip. Consequently, OAM beams with topological charges of m = -3 to 3 were separated by a CMOS camera with an average crosstalk of -6.43 dB. This approach shall shed light on next-generation OAM modes processing.

Chirality tuning and reversing with resonant phase-change metasurfaces.

Dynamic control of circular dichroism in photonic structures is critically important for compact spectrometers, stereoscopic displays, and information processing exploiting multiple degrees of freedom. Metasurfaces can help miniaturize chiral devices but only produce static and limited chiral responses. While external stimuli can tune resonances, their modulations are often weak, and reversing continuously the sign of circular dichroism is extremely challenging. Here, we demonstrate the dynamically tunable chiral response of resonant metasurfaces supporting chiral bound states in the continuum combining them with phase-change materials. Phase transition between amorphous and crystalline phases allows for control of chiral response and varies chirality rapidly from -0.947 to +0.958 backward and forward via the chirality continuum. Our demonstrations underpin the rapid development of chiral photonics and its applications.

Cascaded metasurfaces for high-purity vortex generation.

We introduce a new paradigm for generating high-purity vortex beams with metasurfaces. By applying optical neural networks to a system of cascaded phase-only metasurfaces, we demonstrate the efficient generation of high-quality Laguerre-Gaussian (LG) vortex modes. Our approach is based on two metasurfaces where one metasurface redistributes the intensity profile of light in accord with Rayleigh-Sommerfeld diffraction rules, and then the second metasurface matches the required phases for the vortex beams. Consequently, we generate high-purity LGp,l optical modes with record-high Laguerre polynomial orders p = 10 and l = 200, and with the purity in p, l and relative conversion efficiency as 96.71%, 85.47%, and 70.48%, respectively. Our engineered cascaded metasurfaces suppress greatly the backward reflection with a ratio exceeding -17 dB. Such higher-order optical vortices with multiple orthogonal states can revolutionize next-generation optical information processing.

All-Dielectric Metasurface-Enabled Multiple Vortex Emissions.

On-chip integrated micro- and nanoscale vortex lasers are key elements for addressing the exponentially growing demand for information capacity. Although tunable vortex microlasers have been reported, each laser pulse still possesses one particular topological charge and requires additional multiplexing. In this study, the simultaneous generation of coherent laser arrays with different topological charges by combining metalenses with semiconductor microlasers is demonstrated. A TiO2 vortex metalens converts two orbital angular momentum beams into the same diffraction-limit spot with opposite circular polarizations through spin-to-orbital conversion. Consequently, the microlaser emission at the focal spot, which can be decomposed into two circularly polarized beams, is collimated into vortex microlaser beams with two different topological charges through a time-reversal process. This concept is extended to a 2 × 2 metalens array by introducing off-axis terms, and eight topological charges are produced simultaneously. This research is a significant step toward the on-chip integration of micro- and nanolasers.