The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous meta-waveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems, by enhancing light-matter interaction strength to drastically boost device performance, or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities. We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits, biomedical sensing, artificial intelligence and beyond.

Metasurfaces have found broad applicability in free-space optics, while its potential to tailor guided waves remains barely explored. By synergizing the Jones matrix model with generalized Snell’s law under the phase-matching condition, we propose a universal design strategy for versatile on-chip mode-selective coupling with polarization sensitivity, multiple working wavelengths, and high efficiency concurrently. The coupling direction, operation frequency, and excited mode type can be designed at will for arbitrary incident polarizations, outperforming previous technology that only works for specific polarizations and lacks versatile mode controllability. Here, using silicon-nanoantenna-patterned silicon-nitride photonic waveguides, we numerically demonstrate a set of chip-scale optical couplers around 1.55 μm, including mode-selective directional couplers with high coupling efficiency over 57% and directivity about 23 dB. Polarization and wavelength demultiplexer scenarios are also proposed with 67% maximum efficiency and an extinction ratio of 20 dB. Moreover, a chip-integrated twisted light generator, coupling free-space linear polarization into an optical vortex carrying 1 ℏ orbital angular momentum (OAM), is also reported to validate the mode-control flexibility. This comprehensive method may motivate compact wavelength/polarization (de)multiplexers, multifunctional mode converters, on-chip OAM generators for photonic integrated circuits, and high-speed optical telecommunications.

A new sawtooth control mechanism relying on toroidally propagating ion cyclotron resonance frequency waves: Theory and Joint European Torus tokamak experimental evidencea) Phys. Plasmas 17, 056118 (2010); 10.1063/1.3363201 Sawtooth control using beam ions accelerated by fast waves in the DIII-D tokamakClosed and open loop control techniques were applied to growing m/nϭ2/1 rotating islands in wall-stabilized plasmas in the High Beta Tokamak-Extended Pulse ͑HBT-EP͒ ͓J. Fusion Energy 12, 303 ͑1993͔͒. HBT-EP combines an adjustable, segmented conducting wall ͑which slows the growth or stabilizes ideal external kinks͒ with a number of small ͑6°wide toroidally͒ driven saddle coils located between the gaps of the conducting wall. Two-phase driven magnetic island rotation control from 5 to 15 kHz has been demonstrated powered by two 10 MW linear amplifiers. The phase instability has been observed and is well modeled by the single-helicity predictions of nonlinear Rutherford island dynamics for 2/1 tearing modes including important effects of ion inertia and finite Larmor radius, which appear as a damping term in the model equations. The closed loop response of active feedback control of the 2/1 mode at moderate gain was observed to be in good agreement with the theory. Suppression of the 2/1 island growth has been demonstrated using an asynchronous frequency modulation drive which maintains the inertial flow damping of the island by application of rotating control fields with frequencies alternating above and below the natural mode frequency. This frequency modulation control technique was also able to prevent disruptions normally observed to follow giant sawtooth crashes in the plasma core.

The High Beta Tokamak-Extended Pulse ͑HBT-EP͒ experiment ͓J. Fusion Energy 12, 303 ͑1993͔͒ combines an internal, movable conducting wall with a high-power, modular saddle coil system to provide passive and active control of long wavelength magnetohydrodynamic ͑MHD͒ instabilities. Systematic adjustment of the radial position, b, of the conducting wall elements in relation to the surface of the plasma ͑minor radius a͒ resulted in the suppression of ␤-limiting disruptions for discharges in which b/aϽ1.2 and a positive plasma current ramp was maintained. Conducting wall stabilization of kink instabilities was observed in discharges with strong current ramps and in plasmas with ␤ values near the Troyon stability boundary. The frequency of slowly growing modes that persisted in wall-stabilized discharges was controlled by applying oscillating mϭ2, nϭ1 resonant magnetic perturbations. A compact, single-phase saddle coil system permitted modulation of the rotation velocity of internal m/nϭ2/1 instabilities by a factor of 2.

This paper presents a 3.89 kW 1123 nm Raman all-fiber laser with an overall optical-to-optical efficiency of 70.9%. The system consists of a single-wavelength (1070nm) seed and one-stage bidirectional 976 nm non-wavelength-stabilized laser diodes (LDs) pumped Yb-doped fiber amplifier. The unique part of this system is the application of non-wavelength-stabilized LDs in high power bidirectional pumping configuration fiber amplifier via refractive index valley fiber combiners. This approach not only increases the pump power, but also shortens the length of fiber by avoiding the usage of multi-stage amplifier. Through both theoretical research and experiment, the bidirectional pumping configuration presented in this paper proves to be able to convert 976 nm pump laser to 1070 nm laser via Yb³⁺ transfer, which is then converted into 1123 nm Raman laser via the first-order Raman effect without the appearance of any higher-order Raman laser.

The purpose of this study was to investigate the interaction between internal representations of invisible moving targets and visual responses of neurons in frontal eye fields (FEFs). Monkeys were trained to make saccades to the extrapolated position of a target that was temporarily rendered invisible for variable durations as if it had passed behind an occluder. Flashed, task-irrelevant visual probe stimuli were used to study the visual responsiveness of FEF neurons during this task. Probes were flashed at various times and locations during the occlusion interval. Net changes in neuronal activity were obtained by comparing the activity on trials with probes with randomly interleaved trials without any probe. Most neurons showed an increase in firing rate in response to the probe, but some showed a decrease. Both types of responses were enhanced when the invisible target moved toward the receptive field (RF) as compared with trials on which the target moved away from the RF. Some neurons showed a spatial shift in the visual response during the occlusion interval. For cells that were excited by the probe, the shift tended to be correlated with the direction of motion of the target, whereas for cells that were inhibited the shift tended to be in the opposite direction. These results suggest that the role of FEF in predicting invisible target motion includes a sensory/perceptual component.

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