Increasing efficiency of high numerical aperture metasurfaces using the grating averaging technique

One of the important advantages of optical metasurfaces over conventional diffractive optical elements is their capability to efficiently deflect light by large angles. However, metasurfaces are conventionally designed using approaches that are optimal for small deflection angles and their performance for designing high numerical aperture devices is not well quantified. Here we introduce and apply a technique for the estimation of the efficiency of high numerical aperture metasurfaces. The technique is based on a particular coherent averaging of diffraction coefficients of periodic blazed gratings and can be used to compare the performance of different metasurface designs in implementing high numerical aperture devices. Unlike optimization-based methods that rely on full-wave simulations and are only practicable in designing small metasurfaces, the gradient averaging technique allows for the design of arbitrarily large metasurfaces. Using this technique, we identify an unconventional metasurface design and experimentally demonstrate a metalens with a numerical aperture of 0.78 and a measured focusing efficiency of 77%. The grating averaging is a versatile technique applicable to many types of gradient metasurfaces, thus enabling highly efficient metasurface components and systems.

Micro-lensed single-mode optical fiber with high numerical aperture

We show that the output mode of a single-mode optical fiber can be directly focused to a sub-wavelength waist with a finite working distance by tapering the fiber to a diameter of the order of the wavelength and terminating it with a spherically/hemispherically shaped tip. Numerical simulations show that a beam waist with a width of as small as 0.62lambda can be formed. We fabricate micro-lensed fibers and construct a probe-scanning confocal reflection microscope. Measurements on gold nano-particles show a spatial profile with a width of 0.29lambda for lambda = 850 nm, which is in good agreement with the numerical simulations. Due to their monolithic structures, these micro-lensed fibers will be flexible substitutes for conventional compound lenses in various experimental conditions such as cryogenic temperature and ultra-high vacuum.

Time reversal symmetry in optics

The utilization of time reversal symmetry in designing and implementing (quantum) optical experiments has become more and more frequent over the past years. We review the basic idea underlying time reversal methods, illustrate it with several examples and discuss a number of implications.

Near-field imaging beyond the probe aperture limit

Near-field scanning optical microscopy has been an indispensable tool for designing, characterizing and understanding the functionalities of diverse nanoscale photonic devices. As the advances in fabrication technology have driven the devices smaller and smaller, the demand has grown steadily for improving its resolving power, which is determined mainly by the size of the probe attached to the scanner. The use of a smaller probe has been a straightforward approach to increase the resolving power, but it cannot be made arbitrarily small in practice due to the steep reduction of the collection efficiency. Here, we develop a method to enhance the resolving power of near-field imaging beyond the limit set by the physical size of the probe aperture. The main working principle is to unveil high-order near-field eigenmodes invisible with conventional near-field microscopy. The destructive interference of near-field waves is induced in these high-order eigenmodes by the locally varying phases, which can reveal subaperture-scale fine structural details. To extract these eigenmodes, we construct a self-interference near-field microscopy system and measure a fully phase-referenced far- to near-field transmission matrix (FNTM) composed of near-field amplitude and phase maps recorded for various angles of far-field illumination. By the singular value decomposition of the measured FNTM, we could extract the antisymmetric mode, quadrupole mode, and other higher-order modes hidden under the lowest-order symmetric mode. This enables us to resolve double and triple nano-slots whose gap size (50 nm) is three times smaller than the diameter of the probe aperture (150 nm). The subaperture near-field mode mapping by the FTNM can be potentially combined with various existing near-field imaging modalities and promote their ability to interrogate local near-field optical waves of nanoscale devices.

Quantum optics of soliton microcombs

Soliton microcombs -- phase-locked microcavity frequency combs -- have become the foundation of several classical technologies in integrated photonics, including spectroscopy, LiDAR, and optical computing. Despite the predicted multimode entanglement across the comb, experimental study of the quantum optics of the soliton microcomb has been elusive. In this work, we use second-order photon correlations to study the underlying quantum processes of soliton microcombs in an integrated silicon carbide microresonator. We show that a stable temporal lattice of solitons can isolate a multimode below-threshold Gaussian state from any admixture of coherent light, and predict that all-to-all entanglement can be realized for the state. Our work opens a pathway toward a soliton-based multimode quantum resource.