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

This tutorial reviews various noncontact optical sensing techniques that can be used to measure distances to objects, and related parameters such as displacements, surface profiles, velocities and vibrations. The techniques that are discussed and compared include intensity-based sensing, triangulation, time-of-flight sensing, confocal sensing, Doppler sensing, and various kinds of interferometric sensing with both high- and low-coherence sources.

Optical methods for distance and displacement measurements

DOI: 10.1002/adom.201801433 Scientific American selects plasmonic sensing as the top 10 emerging technologies of 2018.[15] Almost every single new plasmonic or photonic structure would be explored to test its sensing ability.[16–29] These works tend to report the sensing performance of their own structure. Some declare that their sensitivity breaks the world record. However, there is still a missing literature on what the world record really is, the gap between the experiments and the theoretical limit, as well as the differences between metal-based plasmonic sensors and dielectric-based photonic sensors. To push plasmonic and photonic sensors into industrial applications, an optical sensing technology map is absolutely necessary. This review aims to cover a wide range of most representative plasmonic and photonic sensors, and place them into a single map. The sensor performances of different structures will be distinctly illustrated. Future researchers could plot the sensing ability of their new sensors into this technology map and gauge their performances in this field. In this review, we focus on optical refractive index (RI) sensors with no fluorescent labeling required. We will utilize two parameters to characterize and compare the performance of optical RI sensors: sensitivity to RI change (denoted by symbol SRI) and figure of merit (in short, FoM). For simplicity, we restrict our discussions to bulk RI change, where the change in RI occurs within the whole sample. There is another case where the RI variation occurs only within a very small volume close to the sensor surface. This surface RI sensitivity is proportional to the bulk RI sensitivity, the ratio of the thickness of the layer within which the surface RI variation occurs, and the penetration depth of the optical mode.[6] The bulk RI sensitivity defines the ratio of the change in sensor output (e.g., resonance angle, intensity, or resonant wavelength) to the bulk RI variations. Here, we limit our discussions to the spectral interrogations and the bulk RI sensitivity SRI is given by[3,5–7,30]

Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth

Recently, there has been an explosion of interest in metalenses for imaging. The interest is primarily based on their subwavelength thicknesses. Diffractive gratings have been used as thin optical elements since the late 19th century. Here, we show that multilevel diffractive lenses (MDLs), when designed properly, can exceed the performance of metalenses. Furthermore, MDLs can be designed and fabricated with larger constituent features, making them accessible to low-cost, large-area volume manufacturing, which is generally challenging for metalenses. The support substrate will dominate overall thickness for all flat optics. Therefore, the advantage of a slight decrease in thickness (from ∼2λ to ∼λ/2) afforded by metalenses may not be useful. We further elaborate on the differences between these approaches and clarify that metalenses have unique advantages when manipulating the electromagnetic fields, rather than intensity.

Imaging with flat optics: metalenses or diffractive lenses?

a b s t r a c t We demonstrated a refractive index (RI) sensor based on optical fiber Michelson interferometer (MI), which was fabricated by splicing a section of thin core fiber (TCF) to a standard single mode fiber with a core offset. Experimentally, such a MI-based RI sensor with a core offset of 8 m and a TCF length of 3 mm exhibits a high resolution of 4.9 × 10−6 RIU and sensitivity of −202.46 dB/RIU, which is two or three times higher than that of intensity-modulated RI sensors reported previously. In contrast, our MIbased RI sensor is insensitive to temperature, thus overcoming the cross-sensitivity problem between surrounding RI and temperature. Moreover, intensity modulation, rather than wavelength modulation, was used in the proposed MI-based RI sensor, and the sensor also has the advantages of compact size (8 mm), simple structure, easy fabrication, and good repeatability. © 2014 Published by Elsevier B.V.

Intensity modulated refractive index sensor based on optical fiber Michelson interferometer

An all-fiber sensor for simultaneous measurement of refractive index and temperature in solutions is proposed and demonstrated. The sensing head contains a core-offset Mach–Zehnder interferometer (MZI) and a fiber Bragg grating (FBG). The interference fringe of the MZI and the Bragg wavelength of the FBG would shift with the variation of the ambient refractive index (RI) and/or temperature. The experimental results show that the RI sensitivity and the temperature sensitivity for the sensor are 13.7592 nm/RI and 0.0462 nm/°C, respectively. Its low fabrication cost, simple configuration and high sensitivity will have attractive potential applications in chemical and biological sensing.

Simultaneous measurement of refractive index and temperature based on a core-offset Mach–Zehnder interferometer combined with a fiber Bragg grating

A simple sensor for simultaneous measurement of refractive index and temperature in solutions based on fiber Bragg grating (FBG) is proposed. The sensor contains only a normal FBG with a pigtail fiber. When the FBG is immersed in different liquids of refractive index, the side mode suppression ratio of the FBG varies due to the Fresnel reflection of the pigtail fiber. The sensor is also capable of simultaneous measurement refractive index and temperature combining the thermo-optic effect of the FBG. Its ease of fabrication and low-cost offer the attractive applications in chemical and biological sensing.

Fiber Bragg grating-based fiber sensor for simultaneous measurement of refractive index and temperature

In this paper, we demonstrated a low-loss and high-transmission analogy of electromagnetically induced transparency based on all-dieletric metasurface. The metamaterial unit cell structure is composed of two mutually perpendicular silicon nanoscale bars. Under the joint effects of the neighboring meta-atoms’ coherent interaction and significant low absorption loss, the transmission and the Q-factor can reach up to 93 % and 139, respectively. Moreover, we use the coupled harmonic oscillator model to analyze the near field interaction between the two elements in the electromagnetically induced transparency (EIT) metamaterial unit cell qualitatively and the effects of parameters on EIT. The figure-of-merit of 42 and the group delay of 0.65 ps are obtained. These characteristics make the metamaterial structure with potential to apply for ultrafast switches, sensor, and slow-light devices.

Analogue Electromagnetically Induced Transparency Based on Low-loss Metamaterial and its Application in Nanosensor and Slow-light Device

Simultaneous measurement of refractive index and temperature based on asymmetric structures modal interference

Active plasmonic band-stop filters based on single- and double-layer doped graphene metamaterials at the THz wavelengths are proposed and investigated numerically by using the finite-difference time-domain (FDTD) method. The metamaterial unit cell structure is composed of two parallel graphene nanoscale ribbons. Simulated results exhibit that significant resonance wavelength shifts can be achieved with a slight variation of the doping concentration of the graphene ribbons. Besides, the asymmetry double-layer graphene metamaterial device has two apparent filter dips while the symmetry single-, double-layer and asymmetry single-layer graphene metamaterial devices just only one. The metamaterials with symmetry single-layer and asymmetry double-layer graphene can be used as a high-sensitivity refractive sensor with the sensitivity up to 5100 nm/RIU and a two-circuit switch, respectively. These prospects pave the way towards ultrafast active graphene-based plasmonic devices for THz applications.

Hongyun Meng

Papers

Simultaneous measurement of refractive index and temperature based on a core-offset Mach–Zehnder interferometer combined with a fiber Bragg grating

Fiber Bragg grating-based fiber sensor for simultaneous measurement of refractive index and temperature

Analogue Electromagnetically Induced Transparency Based on Low-loss Metamaterial and its Application in Nanosensor and Slow-light Device

Simultaneous measurement of refractive index and temperature based on asymmetric structures modal interference

Active plasmonic band-stop filters based on graphene metamaterial at THz wavelengths