A single event has never defined the emergence of a new and emerging field of science. This is also true for metamaterials, a field that has gradually accumulated knowledge through consistent and dedicated research over the past century. A major factor in the development of antenna was technologies related to wireless communication. The scalability and efficiency of these antennas and the simplification of underlying physical modelling have great advantages over isolated antennas, such as reducing their size to that of the wavelength of the light. Natural optical devices control the wave front of light such as polarization, phase and amplitude. According to classical optics, atoms and molecules composing the medium shape the behaviour of light in naturally occurring materials. As a result of refractive index differences in the media, refraction, reflection and diffraction can all be controlled. However, natural materials tend to have small deviations in their properties when manipulated optically ^{[1][2][3][4][5]}[1,2,3,4,5]. Various types and configurations of chemical, bio, gas and refractive index optical sensors have already been reported. There are advantages to both fibre-based and waveguide-based sensors. Some SPR and LSPR sensors are growing rapidly and opening up a lot of possibilities ^{[6][7][8][9][10][11][12]}[6,7,8,9,10,11,12]. Through the integration of metasurface, a whole new world of senses can be opened up. Sensitivity can be enhanced, detection accuracy can be improved and the size can be compacted.

Metamaterials are subwavelength periodic metallic and dielectric structures, exhibiting properties that cannot be found in nature, which couple to the electric and magnetic components of incident electromagnetic fields. Over the past 15 years, this micro- and nano-structured artificial media class has attracted considerable attention and produced ground-breaking electromagnetic and photonic phenomena. Despite their potential, however, the high losses and strong dispersion associated with resonant responses and the use of metallic structures and the difficulties of fabricating 3D structures at the micro-and nanoscale have largely prevented the effective use of metamaterials. Through lithography and nanoimprinting, it is possible to manufacture planar metamaterials and metasurfaces with subwavelength thickness. Wave reflection losses can be greatly reduced by applying a very thin layer in the wave propagation direction. With metasurfaces, optical wavefronts can be modulated into any desired shape, and functional materials can be integrated to accomplish various objectives (e.g., altering amplitude, phase, polarisation). Moreover, nonlinearity is greatly enhanced and enables active control. There has been increasing interest in 2D planar metamaterials, namely metasurfaces. They can provide many of the same phenomena as metamaterials, except that they are a fraction of a wavelength thin, easier to fabricate, theoretically simpler to realise, and have negligible losses. It has been used to realise many optical devices. Usually, metasurfaces engineer the wavefront of light by abrupt phase changes ^{[13][14][15][16][17][18]}[13,14,15,16,17,18].

Aside from superlensing, slow light and cloaking devices, refractive index (RI) bio-sensing is the most realistic and representative application of them all. A change in the RI results from biomolecular interactions occurring in analyte layers. Sensors such as the electromagnetic (EM) RI can be used in a variety of chemical and biological sensing applications due to their unique capabilities for sensitive and label-free biochemical assays. The resonant EM spectrum that is dominated by the environment can be vastly tuned by engineering individual MAs (meta-atoms) and their arrangements. This resonant property allows variation in the scattering output spectrum, which is used to measure the RI of the surrounding biomolecular analytes. Therefore, certain wavelengths and certain sensitivity levels have to be designed in mass setups. Additionally, RI sensors based on metamaterial (MM)- and metastructure (MS)-based sensing platforms have several advantages over conventional surface plasmon polariton (SPP)-based biosensors. MM- and MS-based RI sensors have superior performance than SPP-based sensors, primarily due to fabrication tolerance and signal stability, as RI variation is detected through macroscopic optical responses, mainly reflection or transmission of focused input beams ^[13][14][13,14]. The second advantage of periodic MAs is lower radiative damping and a higher quality factor, provided by interesting physical mechanisms such as plasmonically induced transparency or Fano resonances. A single nanophotonic RI sensor can expand its capabilities if it is combined with MM or MS. Combining multiple MAs in a unit cell or supercell can result in multiple resonances and a broad range of slow light effects, which are difficult to achieve in SPP sensors ^{[19][20][21][22][23][24][25][26][27]}[19,20,21,22,23,24,25,26,27].

2. Fundamentals of Metasurfaces

MMs and MSs have centred attention of research fraternity due to their anomalous and tuneable properties. MMs are made up of periodic subwavelength metal/dielectric structures. These structures resonantly couple to electric and magnetic fields of the incident electromagnetic waves. Optical properties of MMs and MSs are decided by geometrical parameters of their constituents, called MAs. MA can be composed of one or more subwavelength sized nanostructures of noble metals or high index dielectrics. Smith [28] and Pendry [29] designed first artificial materials predicted theoretically far earlier in 1968 by Veselago [30]. After that many new exciting functionalities have been achieved in MMs such as negative refractive index, nearly perfect absorption, transmission and reflection which have potential applications in superlensing, electromagnetic cloaking etc. At present, MSs (subwavelength thick metamaterials) are replacing MMs that make it possible to achieve new applications such as planar lenses, generalisation of Snell’s law, ultrathin invisibility cloaks to name a few ^[31][32][33][31,32,33]. They are easy to fabricate and cost effective in comparison to MMs. They can give spatially varying optical responses (e.g., amplitude, phase, polarisation), which are used to manipulate wavefronts into desirable shapes. Due to their strong wavefront modulation capability in the sub-wavelength domain, various meta-devices have been introduced in recent years, such as meta-lens, absorber, vortex beam generator, holograms and many more. Generally, MSs are characterised into two classes: plasmonic (metallic) and dielectric MS. In plasmonic MSs, collective oscillations of electrons in a metal give rise to resonance, called localised surface plasmon resonance (LSPR). Plasmonic MSs have advantages such as the ability to sense analytes directly at the metal surface where field is confined strongly. This intense field confinement enhances the light matter interaction with the analyte which strongly alters the spectral response. These exciting properties make MSs a prominent candidate for sensing applications. However, metals offer significant joule heating which can alter the property of the analyte. Furthermore, high dissipation can also give rise to low quality factor (Q-factor) in a resonator. Q-factor is a measure of the energy stored in the resonator relative to the energy lost in radiation or joule heating. Low Q-factor limits the detection sensitivity. To resolve the loss issue, MSs are designed using dielectric nanoparticles which support electric and magnetic modes based on the Mie theory. Dielectric MSs have larger Q-factor in comparison to plasmonic MSs due to the absence of joule heating. However, the modes supported by dielectric MAs are less localised and have larger mode volume. For the sensing applications, dielectric MSs could be advantageous if large analyte volume is being used. Whenever MS is illuminated with a broad light source, the wavelength corresponding to the resonant wavelength is reflected due to the strong scattering, while the other wavelengths will pass through. When the incident light coincides with the resonant wavelength, the near fields of the MSs are increased in accordance with the Q-factor of the resonance. Therefore, the interaction between incident light and analyte will enhance. Q-factor can be improved via Fano resonance. Fano resonance is a type of resonance which results in asymmetric line-shape. This asymmetric line-shape is due to interference between two scattering amplitudes, one lies in the continuum state and the other lies in the discrete state. In 2007, N. I. Zheludev’s group observed Fano resonance for the first time in the microwave frequency range using asymmetric split rings (acting as resonator) for MS ^{[34][35][36][37][38][39][40][41][42][43][44]}[34,35,36,37,38,39,40,41,42,43,44]. Here, Fano resonance is achieved by breaking the symmetry of nanostructures. In subwavelength nanostructures, dipole moments are excited which usually have broad spectral response. By breaking symmetry, narrow ‘dark’ modes, which exist due to the higher order oscillations, are excited and they interact with broad ‘bright’ mode [32]. Fano resonance depends on the degree of asymmetry of the MSs and refractive index of surrounding materials. In Fano resonance, sharp resonance peaks with high Q-factor are observed. Due to high Q-factor, Fano resonance MSs are seeing immense research attention. The above discussed mechanism of MSs can be used for various optical sensing applications such as refractive index sensing, chemical sensing, bio sensing and gas sensing.

3. Application of Metasurfaces in Analyte Sensing

3.1. Bio Sensing

MMs and MSs have opened new frontiers in many research areas. In particular, in the sensing field, sensors based on these artificially engineered materials have an edge due to high sensitivity and selective detection and measurement of biomarkers exploited majorly for accurate and early diagnosis of disease conditions. MSs and MMs introduce novel functionalities to conventional plasmonic sensors by enhancing sensitivity, limit of detection and allowing low-cost fabrication, giving rise to hybrid sensing paradigm. There are two primary types of plasmonic excitations, surface plasmon polariton (SPP) and localised surface plasmon resonance (LSPR). The surface plasmon resonance (SPR) sensors have been extensively investigated over the past few decades, resulting in many research articles and several commercial implementations ^[45][46][45,46]. LSPR is produced by the oscillation of free electrons at confined metal (Au, Ag, Cu, Pt, etc.)–dielectric interface, such as in metal nanoparticles, upon excitation by p-polarised light [47]. Some salient features of plasmonic sensors include real-time monitoring of binding dynamics of biomarkers on the device surface, reusability, fast response, straightforward sample treatments and label-free detection at the point of care. However, conventional SPR instruments have several limitations, including a lack of multiplexing capability and hence low throughput, dependence on the specific binding surface, chemical inertness to metal surfaces leading to reduced sensitivity, lack of wireless operation and risk of data misinterpretation [48]. Typically, despite the availability of different combinations of metal and dielectric materials, substantial modulation of optical properties is not feasible, thereby lacking manoeuvrability. In contrast, composite structures such as metamaterial and MS-based structures with negative permittivity, permeability and perfect absorption, can be utilised to tailor the optical properties near the metal-dielectric interface [49]. Plasmonic MS-based sensors follow the fundamentals of optical properties near the MS-dielectric boundary. Russian physicist Victor Veselago first introduced the theoretical approach of negative refractive index (RI) material in 1968 [28]. The MM-based RI sensor was experimentally demonstrated at microwave frequency in 2000 [31]. The MM-based plasmonic biosensor has been successfully implemented in 2D and 3D nanostructures for different bio-analyte detection. These sensors have drawn much attention due to their ultrahigh sensitivity compared to conventional plasmonic biosensors [50]. Plasmonics and their meta configurations have been utilised to detect a variety of viruses, including hepatitis B [51], Zika Virus [52], HIV DNA [53], SARS-CoV-2 [54] and malaria [55]. The advantages and disadvantages (where applicable) of various biosensors are then compared in Table 1.