Optical mode localization sensing based on fiber-coupled ring resonators.

Mode localization is widely used in coupled micro-electro-mechanical system (MEMS) resonators for ultra-sensitive sensing. Here, for the first time to the best of our knowledge, we experimentally demonstrate the phenomenon of optical mode localization in fiber-coupled ring resonators. For an optical system, resonant mode splitting happens when multiple resonators are coupled. Localized external perturbation applied to the system will cause uneven energy distributions of the split modes to the coupled rings, this phenomenon is called the optical mode localization. In this paper, two fiber-ring resonators are coupled. The perturbation is generated by two thermoelectric heaters. We define the normalized amplitude difference between the two split modes as: (T M1-T M2)/T M1×100%. It is found that this value can be varied from 2.5% to 22.5% when the temperature are changed by the value from 0K to 8.5K. This brings a ∼ 2.4%/K variation rate, which is three orders of magnitude greater than the variation rate of the frequency over temperature changes of the resonator due to thermal perturbation. The measured data reach good agreement with theoretical results, which demonstrates the feasibility of optical mode localization as a new sensing mechanism for ultra-sensitive fiber temperature sensing.

Asymmetric vortex beam emission from a metasurface-integrated microring with broken conjugate symmetry.

Vortex beams that carry orbital angular moment (OAM) have recently attracted a great amount of research interest, and metasurfaces and planar microcavities have emerged as two prominent, but mostly separated, methods for Si chip-based vortex beam emission. In this work, we demonstrate in numerical simulation for the first time the hybridization of these two existing methods in a Si chip-based passive emitter (i.e., a light coupler). A unique feature of this device is its broken conjugate symmetry, which originates from introducing a metasurface phase gradient along a microring. The broken conjugate symmetry creates a new phenomenon that we refer to as asymmetric vortex beam emission. It allows two opposite input directions to generate two independent sets of OAM values, a capability that has never been reported before in Si chip-based passive emitters. In addition, we have also developed here a new analytical method to extract the OAM spectrum from a vector vortex beam. This analytical method will prove to be useful for vector vortex beam analysis, as mode purity analysis has rarely been reported in literature due to the complexity of the full-vector nature of such beams. This study provides new approaches for both the design and the analysis of integrated vortex beam emission, which could be utilized in many applications such as free-space optical communications and microfluidic particle manipulation.

Metasurfaces integrated with a single-mode waveguide array for off-chip wavefront shaping.

Integration of metasurfaces and SOI (silicon-on-insulator) chips can leverage the advantages of both metamaterials and silicon photonics, enabling novel light shaping functionalities in planar, compact devices that are compatible with CMOS (complementary metal-oxide-semiconductor) production. To facilitate light extraction from a two-dimensional metasurface vertically into free space, the established approach is to use a wide waveguide. However, the multi-modal feature of such wide waveguides can render the device vulnerable to mode distortion. Here, we propose a different approach, where an array of narrow, single-mode waveguides is used instead of a wide, multi-mode waveguide. This approach tolerates nano-scatterers with a relatively high scattering efficiency, for example Si nanopillars that are in direct contact with the waveguides. Two example devices are designed and numerically studied as demonstrations: the first being a beam deflector that deflects light into the same direction regardless of the direction of input light, and the second being a light-focusing metalens. This work shows a straightforward approach of metasurface-SOI chip integration, which could be useful for emerging applications such as metalens arrays and neural probes that require off-chip light shaping from relatively small metasurfaces.

Coherent generation of arbitrary first-order Poincaré sphere beams on an Si chip.

Generalized vector vortex light beams possess spatially variant polarization states, and higher-order Poincaré spheres represent a powerful analytical tool for analyzing these intriguing and complicated optical fields. For the generation of these vortex beams, a range of different methods have been explored, with an increasing emphasis placed on compact, integrated devices. Here, we demonstrate via numerical simulation, for the first time, an on-chip light emitter that allows for the controllable generation of all points on a first-order Poincaré sphere (FOPS). The FOPS beam generator consists of a waveguide-coupled, nanostructured Si microring resonator that converts two guided, coherent light waves into freely propagating output light. By matching their whispering gallery modes with the nanostructures, the fundamental TE (transverse electric) and TM (transverse magnetic) input modes produce radial and azimuthal polarizations, respectively. These two linear polarizations can form a pair of eigenstates for the FOPS. Consequently, tuning the phase contrast and the intensity ratio of these two coherent inputs allows for the generation of an arbitrary point on the FOPS. This result indicates a new way for on-chip vector vortex beam generation, which may be applied for integrated optical tweezers and high-capacity optical communications.

Coherently switching the focusing characteristics of all-dielectric metalenses.

Flat, gradient index, metasurface optics - in particular all-dielectric metalenses - have emerged and evolved over recent years as compact, lightweight alternative to their conventional bulk glass/crystal counterparts. Here we show that the focal properties of all-dielectric metalenses can be switched via coherent control, which is to say by changing the local electromagnetic field in the metalens plane rather than any physical or geometric property of the nanostructure or surrounding medium. The selective excitation of predominantly electric or magnetic resonant modes in the constituent cells of the metalens provides for switching, by design, of its phase profile enabling binary switching of focal length for a given lens type and, uniquely, switching between different (spherical and axicon) lens types.

On-chip optical pulse train generation through the optomechanical oscillation.

This paper proposes a novel on-chip optical pulse train generator (OPTG) based on optomechanical oscillation (OMO). The OPTG consists of an optical cavity and mechanical resonator, in which OMO periodically modulates the optical cavity field and consequently generates optical pulse trains. The dimensionless method are introduced to simulate the OMO-based OPTG with reduced analysis complexity. We investigate the optomechanical coupling and the dynamic back-action processes, by which we found a dead zone that forbids the OMO, and derived the optimal laser detuning and the minimum threshold power. We analysed the OMO-based OPTG in terms of the pulse shape distortion, extinction ratio (ER) and duty-cycle (DC). Increasing input power, mechanical and optical Q-factors will increase ER, reduce DC and produce sharper and shorter optical pulses. We also discuss the design guidance of OMO-based OPTG and explore its application in distributed fibre optical sensor (DFOS).