Control over the dispersion of the refractive index is essential to the performance of most modern optical systems. These range from laboratory microscopes to optical fibres and even consumer products, such as photography cameras. Conventional methods of engineering optical dispersion are based on altering material composition, but this process is time-consuming and difficult, and the resulting optical performance is often limited to a certain bandwidth. Recent advances in nanofabrication have led to high-quality metasurfaces with the potential to perform at a level comparable to their state-of-the-art refractive counterparts. In this Review, we introduce the underlying physical principles of metasurface optical elements (with a focus on metalenses) and, drawing on various works in the literature, discuss how their constituent nanostructures can be designed with a highly customizable effective index of refraction that incorporates both phase and dispersion engineering. These metasurfaces can serve as an essential component for achromatic optics with unprecedented levels of performance across a broad bandwidth or provide highly customized, engineered chromatic behaviour in instruments such as miniature aberration-corrected spectrometers. We identify some key areas in which these achromatic or dispersion-engineered metasurface optical elements could be useful and highlight some future challenges, as well as promising ways to overcome them. Flat metasurface optics provides an emerging platform for combining semiconductor foundry methods of manufacturing and assembling with nanophotonics to produce high-end and multifunctional optical elements. This Review highlights the design of metasurfaces, recent advances in the field and initial promising applications.

Flat optics with dispersion-engineered metasurfaces

Fabrication technology for three-dimensional microstructures with submicrometer accuracy has been needed in the fields of modern optics, such as micro mechanical system driven with photon pressure[1, 2] and laser-trapping near-filed optical microscopy[3]. However, the present accuracy with stereolithography[4] is not yet satisfactory to this purpose. Moreover, it is not so flexible to make a three- dimensional structure with the present technique. In this paper, we propose a new microfabrication method in which a point in three-dimensional volume of UV photopolymerizing resin is photopolymerized through two-photon absorption process. The microfabrication with two-photon absorption drastically improves the depth resolution due to a nonlinearlity between the power of the irradiation and that of the absorption[5].

Three-dimensional microfabrication with two-photon absorbed photopolymerization

Abstract Optical metamaterials have presented an innovative method of manipulating light. Hyperbolic metamaterials have an extremely high anisotropy with a hyperbolic dispersion relation. They are able to support high- k modes and exhibit a high density of states which produce distinctive properties that have been exploited in various applications, such as super-resolution imaging, negative refraction, and enhanced emission control. Here, state-of-the-art hyperbolic metamaterials are reviewed, starting from the fundamental principles to applications of artificially structured hyperbolic media to suggest ways to fuse natural two-dimensional hyperbolic materials. The review concludes by indicating the current challenges and our vision for future applications of hyperbolic metamaterials. 

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Hyperbolic metamaterials: fusing artificial structures to natural 2D materials

Hyperbolic Metamaterials and Metasurfaces: Fundamentals and Applications

As light cannot be stopped or directly stored in any media, optical delay lines are usually used to temporally trap the optical signals. We report a wide-range continuously tunable optical delay line chip consisting of a ring resonator array and a Mach–Zehnder interferometer (MZI) switch array on the 60-nm-thick silicon waveguide platform. The ring delay line provides continuous delay tuning of more than 10 ps with a push–pull differential tuning method. The MZI switchable delay line provides digitally programmable delay tuning with a resolution of 10 ps upon reconfiguration of the MZI switches to establish different optical routing paths. Dual-stage MZI switches are used to ensure low crosstalk with an improved signal-to-noise ratio. The delay line chip can generate a maximum delay of >1  ns with an on-chip insertion loss of 12.4 dB. Optical pulse time-division multiplexing and quasi-arbitrary waveform generation are realized based on the delay line chip. These results represent a significant step towards the realization of highly reconfigurable optical signal processors enabled by optical delay manipulation with broad applications for optical communications and microwave photonics.

Continuously tunable ultra-thin silicon waveguide optical delay line

In order to manufacture large-scale photonic devices of various dimensions at a low cost, a number of patterning techniques have been developed. Nanoimprint lithography is among the most promising given its unique advantages, such as high resolution, fast processing speed, high throughput, compatibility with diverse materials, and low cost. This review covers various aspects of nanoimprint lithography, including its operational principles, material requirements, and different ways of implementation. Nanoimprint lithography facilitates numerous high-performance and low-cost photonic elements, including optical interconnects, sensors, solar cells, and metamaterials. In addition, other related patterning techniques, together with their utilization for photonic device fabrication and their integration with nanoimprint lithography, are briefly discussed.

Printed photonic elements: nanoimprinting and beyond

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Efficient Photoacoustic Conversion in Optical Nanomaterials and Composites

Beyond the implementation of the photoacoustic effect to photoacoustic imaging and laser ultrasonics, this study demonstrates a novel application of the photoacoustic effect for high-precision cavitation treatment of tissue using laser-induced focused ultrasound. The focused ultrasound is generated by pulsed optical excitation of an efficient photoacoustic film coated on a concave surface, and its amplitude is high enough to produce controllable microcavitation within the focal region (lateral focus <100 µm). Such microcavitation is used to cut or ablate soft tissue in a highly precise manner. This work demonstrates precise cutting of tissue-mimicking gels as well as accurate ablation of gels and animal eye tissues.

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Laser-Induced Focused Ultrasound for Cavitation Treatment: Toward High-Precision Invisible Sonic Scalpel

In this work, a special hyperbolic metamaterial (HMM) metamaterial is investigated for plasmonic lithography of period reduction patterns. It is a type II HMM (ϵ∥ 0) whose tangential component of the permittivity ϵ∥ is close to zero. Due to the high anisotropy of the type II epsilon-near-zero (ENZ) HMM, only one plasmonic mode can propagate horizontally with low loss in a waveguide system with ENZ HMM as its core. This work takes the advantage of a type II ENZ HMM composed of aluminum/aluminum oxide films and the associated unusual mode to expose a photoresist layer in a specially designed lithography system. Periodic patterns with a half pitch of 58.3 nm were achieved due to the interference of third-order diffracted light of the grating. The lines were 1/6 of the mask with a period of 700 nm and ∼1/7 of the wavelength of the incident light. Moreover, the theoretical analyses performed are widely applicable to structures made of different materials such as silver as well as systems working a...

Plasmonic Lithography Utilizing Epsilon Near Zero Hyperbolic Metamaterial

Photoacoustic spectrum analysis (PASA) has been demonstrated as a new method for quantitative tissue imaging and characterization. The ability of PASA in evaluating micro-size tissue features was limited by the bandwidth of detectors for photoacoustic (PA) signal acquisition. We improve upon such a limit, and report on developments of PASA facilitated by an optical ultrasonic detector based on micro-ring resonator. The detector’s broad and flat frequency response significantly improves the performance of PASA and extents its characterization capability from the tissue level to cellular level. The performance of the system in characterizing cellular level (a few microns) stochastic objects was first shown via a study on size-controlled optically absorbing phantoms. As a further demonstration of PASA’s potential clinical application, it was employed to characterize the morphological changes of red blood cells (RBCs) from a biconcave shape to a spherical shape as a result of aging. This work demonstrates that PASA equipped with the micro-ring ultrasonic detectors is an effective technique in characterizing cellular-level micro-features of biological samples.

Qiaochu Li

Papers

Printed photonic elements: nanoimprinting and beyond

Efficient Photoacoustic Conversion in Optical Nanomaterials and Composites

Laser-Induced Focused Ultrasound for Cavitation Treatment: Toward High-Precision Invisible Sonic Scalpel

Plasmonic Lithography Utilizing Epsilon Near Zero Hyperbolic Metamaterial

Characterizing cellular morphology by photoacoustic spectrum analysis with an ultra-broadband optical ultrasonic detector.