Photonic metamaterials for light modulation, energy saving, and sensing applications

1 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 Photonic metamaterials for light modulation, energy saving, and sensing applications Majd Abu Aisheh1,$, Amaljith Chandroth Kalliyadan1,$, Anand M. Shrivastav2*, and Ibrahim Abdulhalim1* Abstract Light manipulation with metamaterial structures is a topic of utmost importance in many fields such as medical diagnostics, monitoring of industrial fabrication processes, environmental and agricultural monitoring, food inspection, sensing of hazardous materials, remote sensing in air and water, night vision, optical communications, integrated quantum photonics, microscopy, solar energy efficient conversion, water desalination, and much more. However, the existing devices often suffer from low speed, narrow dynamic range, wide uncontrolled bandwidth, low light throughput, and the majority are bulky and expensive. A device that combines all these quality parameters does not exist. The recent advances in nanofabrication, photonics, materials science and engineering, electromagnetic simulation tools, algorithms, and computing power, give us extra degrees of freedom to resolve this problem by better understanding and controlling the optical properties of materials and photonic structures. The purpose of this chapter is to expose the status of photonic metamaterials (PMMs) research in three main fields: tunable PMMs in particular with liquid crystals, perfect broadband absorption PMM structures for solar energy conversion, and chemical or biosensing using surface plasmon resonance and surface enhanced spectroscopies. 1Department of Electrooptics and Photonics Engineering and the Ilse Katz Institute for Nanoscale Science and Technology, School of Electrical and Computer Engineering, Ben Gurion University, Beer Sheva 84105, Israel 2Department of Physics and Nanotechnology, College of Engineering and Technology, SRM Institute of Science and Technology Kattankulathur-603 203, Chengalpattu (Dt), Tamil Nadu, India $ Equal contribution *Corresponding authors e-mails: Ibrahim Abdulhalim: ([email protected]), Anand M. Shrivastav: ([email protected]) 2 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 1. Introduction Light manipulation devices require materials that respond to an applied external stimulus such as electric, magnetic, thermal, mechanical, or optical fields. The speed, modulation range, depth, high light throughput, cost, and energy saving, of photonic devices are competing parameters that can lead to bottlenecks in numerous critical applications, such as acquiring images at many wavelengths or polarization states during the heartbeat cycle or from the eye retina before eye movement or blinking, or for improving the yield of production in the nanoelectronics industry, or again in experimental cosmology where fast surveying of galaxies is required to search for extraterrestrial life. One prominent emerging field combining nanotechnology with photonics is the field of photonic metamaterials; however, fast tunable devices exhibiting broadband, large modulation depth, high light throughput at a low cost, ease of scaling, and compact manner, are still lacking. Since these are competing parameters, the existing solutions usually customize the device to a very limited application. Photonic metamaterials (PMMs) offer many possibilities to resolve such problems [ 1 , 2 ]. They are defined as artificial optical materials composed of subwavelength metallic or dielectric building blocks with properties determined mainly by their subwavelength nature rather than their chemical composition. These building blocks or “photonic atoms” (PAs) are structural elements densely packed into an effective material such that the operating wavelength is ideally much larger (typically an order of magnitude or more) than the diameter of the PAs. Highly unusual material properties become accessible, e.g., a negative refraction index that has recently acquired operation wavelengths in the infrared and visible ranges. Tunability of the properties of PMMs broadens their usability for fast light modulation, such as tunable filters, tunable focus flat lenses, spatial light modulators, and frequency reconfigurable antennae. Liquid crystals (LCs) possess strong 3 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 electrooptic effects and can penetrate nanogaps, thus they can be ideal for tunable PMMs. Some preliminary works have demonstrated applications of PMMs using LCs infiltrating the PMM or as a layer adjacent to the PMM. Others have used thermotropic phase change materials, thermochromic materials, semiconductors, electromechanical, magnetooptic, and electrooptic materials. However, until now the majority of the demonstrated tunable PMM device concepts are limited either in their size, switching speed, tunability range, and spectral bandwidth, and usually, they require complex fabrication techniques, thus their practicality is limited. Active control of the wavelength, polarization, or phase, of light - either temporally, spatially, or spatiotemporally - over a wide spectral range, wide field of view, in an achromatic manner, that is fast, has a high light throughput and small form factor, as well as low cost, is always in demand for many important applications. These range from spectral imaging to optical communications, from quantum and optical computing to tunable lasers, from augmented reality devices to autonomous cars, and many other emerging applications 3 , 4 , 5 . Progress is ongoing in light manipulation methods and devices due to their utmost importance in many fields. Nonetheless, the existing solutions, often suffer either from low speed, narrow dynamic range, wide uncontrolled bandwidth, or low light throughput, and the majority are bulky and expensive to manufacture. Narrow resonances of micro and nanostructures tuning can allow fast response and high sensitivity; however, the dynamic range is usually small 6 , 7 , 8 . A device combining all these quality parameters does not yet exist. Here, our main objective is to review recent advances in photonic metamaterial (PMM) structures for light modulation, sensing and energy saving devices. The chapter is divided into three main sections, section 2 is on PMMs for light modulation, section 3 is on PMMs for energy saving, while section 4 is on PMMs for sensing applications. 4 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 2. PMMs for light modulation Photonic metamaterials (PMMs) show great potential for such light manipulation devices 9 , 10 , 11 , 12 , 13 . When the PMM is a 2D surface, in what is called a metasurface, it can be prepared easily with well-established lithography fabrication processes as compared to the 3D PMM. By making the PMM, or part of it, from responsive material such as electrooptic, magnetooptic, thermoscopic, photosensitive, elastomeric, acoustooptic, electromechanical, or nonlinear optical material (see figure 1), it is possible to tune the properties of the PMM as exemplified by many works over the last decade 14 , 15 , 16 , 17 , 18 . In what follows, we present an overview of works done on tunable photonic metamaterials with an emphasis on tunability with liquid crystals. 2.1 Overview of tunable PMMs Electromechanical (EMC): EMC-driven metasurface operating in the SWIR was demonstrated by Ou et.al. 19 . It was fabricated by focused ion beam milling on a 50-nm-thick Si3N4 membrane. Under ~ 3 V, the transmittance becomes modulated, however, the depth was only 5%. Tunable plasmonic lattice grating patterned on a flexible and stretchable PDMS substrate was shown by Chen et.al. 20 . It showed a response under external strain varying between 0% – 10%, giving almost a 40% modulation depth in reflectivity by external strain change from 1.6% to 3.5%. The surface plasmon resonance shifted approximately 80 nm in the visible at ~ 780 nm under the same strain variation. A focal-length tunable lens was demonstrated by Arbabi et.al. 21 , using a pair of metalenses based on the high-contrast dielectric arrays. One metalens is on a fixed glass substrate, whereas the other on a movable Si3N4 membrane. The doublet shifted the focal length up to 80 μm. Foland et.al. presented a 2D deformable GMR strain sensor by embedding TiO2 of 210 nm radius in PDMS 22 . The height of the pillars was 200 nm, and they were arranged in arrays of 480 nm period in the x-axis, and 560 nm in the y-axis. Two resonance peaks were obtained, one of 5 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 wavelength around 775 nm for the short-period axis and the second of around 850 nm for the longperiod axis. The resonant wavelength shifted along the long-period axis with a 4.8 nm/%ε sensitivity over a range of 5% strain. Figure 1: PMMs tuning methods and their main characteristics. Optical nonlinearity: Using the optical nonlinear Kerr effect of ITO, Zhu et.al. 23 demonstrated tunable transparency plasmonic metasurface of gold on ITO. An optical transparency window shift Photonic Meta Materials (PMMs) Electromechanical Tuning ➢Resonance shift up to 400 nm ➢Modulation depth up to 40% ➢SWIR range (1.1-1.6 µm) ➢Electrical voltage 3V ➢τdown to 500 ns Optical Non-Linearity Tuning ➢Resonance shift up to 15 nm ➢Modulation depth up to 80% ➢THz range (~ 2250 nm) ➢Pumping fluence of 10mJ/cm2 ➢τdown to picoseconds Thermochromic Tuning ➢Resonance shift up to 200 nm ➢Modulation depth up to 33% ➢SWIR range ➢Temperature increase >320K ➢τ< 10 ms Magnetic Tuning ➢Resonance shift up to 2.2 GHz ➢Up to 100% modulation depth ➢GHz and THz ➢Changing magnetic bias by <1 kOe Tuning with Liquid Crystals ➢Most commonly tuning method ➢Varying tuning range from few nanometers up to hundreds of nanometers. ➢Up to 100% modulation depth ➢Tuned by small voltages ➢Response time depends on LC mode and device thickness ➢Response time can reach nanoseconds 6 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 to the short-wavelength direction in the SWIR range was observed. Optically tunable metasurface was demonstrated by Kim et.al. 24 based on the structure Ag-Al2O3-Ag, called metal–insulatormetal (MIM) nanocavity with a 70 nm-thick Ga:ZnO layer as an active layer. It showed fast switching in the sub-picosecond range with 80% depth at laser pumping fluence of 10mJ/cm2. The cavity resonance red-shifted by 15nm in the SWIR range near the ENZ wavelength. At the ENZ condition, the field becomes drastically enhanced, and this is used to enhance optical nonlinear effects. A polarization switch was demonstrated using plasmonic metasurface and isomeric ethylred polymer 25 based on a 100-nm-thick gold periodic array of L-shaped slots on a 500-μm-thick fused quartz substrate and ~ 300 nm ethyl-red polymer top layer. By irradiating green laser (532nm) the isomeric state of ethyl-red changed from trans to cis state which caused the refractive index to decrease. This induced coupling between the resonant plasmonic modes and the isomeric state, and the resulting polarization change revealed 80% modulation depth at 6Hz. Visible light 5th harmonic generation was observed 26 from the heterostructure of the Indium-doped CdO layer on gold coated with MgO top layer due to the field enhancement at the interface with MgO, although the ENZ condition occurs at the wavelength of 2250nm. Thermochromic: Thermal tuning of metamaterials was done using thermochromic phase change materials, such as germanium-antimony-tellurium (GST) or VO2. All-optical bidirectional metasurface based on the GST was demonstrated 27 using a 15 nm-thick GST layer sandwiched between SiO2 and ZnS/SiO2 layers. Also, the same group 28 demonstrated 10% reflectivity modulation depth in the UV and visible using patterned GST heterostructure with ZnS/SiO2. Plasmonic 50-nm-thick Au trenches support plasmonic resonance and enhanced photo-absorption for a temperature change of GST. As the phase of the GST layer changes from the crystalline to the amorphous phase, the transmission at SWIR spectra rose from 20% to 40%. Electrically 7 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 controllable VO2 metasurface was demonstrated 29 with the VO2 laterally sandwiched by structured gold electrodes and the voltage pulse train changes the phase of VO2 between amorphous and crystalline phases. A modulation depth of 33% at the SWIR spectrum was observed with a response time <10 msec. Thermal tuning of VO2-TiO2 multilayer metamaterial was demonstrated 30 due to the dispersion relation change from the elliptic form to the hyperbolic form as the temperature increases over the critical temperature of VO2 around 325 K. Two types of VO2 metasurfaces were produced by us showing operation as smart windows with improved performance, one by femtosecond pulsed laser 31 and the other by oblique angle deposition technique 32 . Magnetic: Tuning MTM properties with the magnetic field has been done in many works, particularly in the GHz and sub-THz regimes, since in these ranges the metamaterial structure may be considered as a combination of electrical components, such as varactor diodes, capacitors, and inductors with a magnetic response, for instance, the split ring resonator structure 33 . Shifts of SRR resonances were reported 34 , 35 , 36 . Caratenuto 37 et.al. demonstrated, theoretically and analytically, the magnetic field-induced spectral radiative properties of a metamaterial composed of Indium Antimonide (InSb) line-gratings on Tungsten (W) film in terahertz. They showed that the InSb grating has single narrowband emissivity of unity in terahertz, and by applying magnetic field, the light matter interactions are modified by broadening the resonant wavelength by 25 m while still maintaining the near-unity resonance as shown in Figure 2. 8 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 Figure 2: InSb-W grating structure under external magnetic fields of varying magnitudes. Field-induced curves are color-filled to highlight the broadband red or blue emissivity shifts brought on by the magnetic field. Arrows denote the direction of the dominant resonance shift from the original narrowband peak to longer (red) or shorter (blue) wavelengths. The figure was reproduced from [37] with permission from OPTICA Publishing Group. Tuning MTMs with liquid crystals: Some materials change their refractive index when an electric field is applied to them. Some are nonlinear materials following the Kerr effect and others show linear variation with the applied electric field following the Pockels effect. The origin of these effects is electronic and therefore they exhibit a fast response; however, these effects are relatively weak, so a large interaction region with the light is required from millimeters to centimeters and large voltages are needed. Liquid crystals (LCs) are composed of anisotropic molecules, usually rod-shaped, and therefore can rotate under the application of small voltage. It is a strong effect showing birefringence modulation typically in the range 0.1-0.28; however, larger values up to 0.4 were also demonstrated in response to 1-10 V and only a few microns thick layer is required to 9 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 generate 100% modulation. With resonant structures, a very thin LC layer and small index modulation is required to tune the resonance and obtain a large modulation depth. LCs possess a wide variety of EO effects depending on their structure determined by the material, the molecular alignment geometry of the device, and the anchoring strength of the molecules on the surfaces. They can flow and fill nanogaps exhibiting strong thermos-optic and magneto-optic responses, therefore they are ideal for tuning metamaterials. Hence, by combining the two fields, a wide range of new devices with high potential can be generated. Recent review articles14,15,32, 38 , 39 , 40 , 41 , 42 on tunable MTMs, contain up-to-date information on tuning using liquid crystals, ref. 37 is dedicated to this topic. A short overview is presented below. Of all the possible tuning methods of liquid crystals, electrical tuning is the most convenient as it requires low voltages and negligible current. Among the first LC tunable MTM devices was the one analyzed by Khoo et al. 43 , consisting of nanospheres immersed in the nematic liquid crystal. LiTaO3 was used as the core material for this purpose, and their effective index properties were calculated using the Maxwell Garnet mixing rule. By using this combination, it was shown that at the frequency of 108 THz, the effective index of the material changes from +1 to -1 as the effective permittivity varied from 2 to 4. Electromagnetically induced transparency (EIT) and absorption resonant structure were demonstrated with the LC layer to shift the resonance up to 0.5THz 44 , 45 with modulation depths of 18.3 dB and 10.5 dB based on different combinations in split ring resonators in the THz range. An all-dielectric metasurface composed of an array of nanodisks was built 46 giving resonance in the SWIR range and the LC layer on the top induced a shift of about 70nm and modulation depth of 75%. Another all-dielectric device is the guided mode resonance (GMR) structure we demonstrated8. It is composed of thin subwavelength grating on top of a waveguide layer and covered with 2000nm LC and showed voltage-induced tuning of the 16 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 by elastic forces, making the relaxation time proportional to the square of the LC layer thickness. The switching times for nematic LCs are generally limited to the msec range 65 , although several complex methodologies have been applied in attempts to effectively reduce the switching time such as: (i) driving voltage waveform manipulation 66 (ii) dividing the device into several thinner (and therefore faster) sub-devices (iii) optimizing material parameters 67 , (iv) optimizing the anchoring strength and pretilt on the surfaces, (v) vertically aligned 68 , in-plane switching mode 69 , twisted alignment, asymmetric alignment 70 , 71 , 72 , (vi) the use of dual frequency mode 73 , blue phase Kerr effect 74 , 75 , the flexoelectro-optic mode 76 , 77 , 78 in short pitch chiral nematic (cholesteric) phase, (vii) using nano or micro structured LC networks 79 such as the nano polymer dispersed LCs (nPDLCs) and stressed LCs 80 , 81 , and (viii) working at elevated temperatures to reduce the LC viscosity. See Figure 4 for some illustrations. In the Kerr effect in the isotropic phase, just above the nematic phase, or when the switching is caused by the electric field induced modification of the order parameters and not due to reorientation of the optic axis, the response becomes closer to the nsec range 82 , 83 , but the effect is very small producing birefringence changes in the order of 0.001-0.01 at applied voltages of the order of kV. LC layer TEC and alignment layers Glass substrate Glass substrate (a) Antiparallel V off V intermediate V large (b) Parallel-Pi cell s w aAlignment direction x z q Glass substrate with interdigitated electrode and alignment layer Glass substrate with alignment layer (c) Vertically standing helix CLC mode-VSH (d) Molecule geometry (e) Interdigitated electrodes structure 17 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 Figure 4: Schematics illustrating some important configurations for: (a) typical nematic LC device in the parallel alignment geometry, (b) parallel alignment geometry which gives slightly faster speed, showing also the deformed structure as the voltage increases, (c) vertically aligned short pitch cholesteric LC with a field applied in the plane of the substrates using interdigitated electrodes, which gives speeds in the sub-msec range in what is called the flexoelectro-optic effect, (d) geometry of LC principal dielectric constants, and (e) interdigitated electrode structure on one substrate, showing also the alignment direction at an angle usually a~10o with respect to the electrode fingers. The cases of submsec switching and below are limited in their applicability because the associated electrooptic effects become weak as in the Kerr effect case: high voltages are needed and usually, heating is required which is not desirable in optical systems as it produces instabilities and noise, and finally increasing the number of LC elements (such as in the Lyot based 84 , 85 LC tunable filter (LCTF) configuration) makes the device bulky and lossy. A relatively faster Lyot-based approach was proposed recently with higher light throughput and more compact, however, it is suitable only for multispectral operation 86 . Nevertheless, some of these improvements are useful and their potential can be evaluated by trying further improvements for the specific PMM devices. Some resonant LC devices, which have been around for a few decades, require smaller LC layer thicknesses such as the tunable Fabry-Perot 87 or guided mode resonance filter8. Nonetheless, the manufacturability of these devices is not easy, their dynamic range is limited to a few tens of nm and the required LC thickness is still in the few mm range, meaning response time in the msec range. In general, resonant structures have a small dynamic range; however, some PMM structures can circumvent this problem by using the broadband nature of lossy surface waves. Like anisotropic layers inside the waveguide, LCs anisotropy can cause coupling between the TE and TM waves. This may result in the appearance of polarization change and additional resonances at the polarization orthogonal to the incident one 88 . Nevertheless, with LCs where the optic axis is rotating in the same plane as a voltage is applied, it is possible to excite only one mode independent 18 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 from the other - see analysis of this problem in our earlier article 89 . The polarization-independent modes such as nano PDLCs and the blue phase Kerr effect can be used; although showing smaller refractive index modulation, with their highly sensitive structure wide tuning range and large modulation depths can still be achieved. These LC phases are also faster, approaching fractions of microseconds response time. It should be noted that in PMM devices the nanoscale LC gaps allow utilizing the full potential of these fast LC phases at relatively small voltages because the Kerrinduced birefringence is proportional to the square of the applied electric field. Ferroelectric Liquid Crystal modes: A more direct, and attractive way to reduce the LC response times is to use chiral (*) polar smectic liquid crystals. Smectic LCs are layered media. In the Figure 5: Schematics of analog (a and b), binary (c), and ternary (d) smectic modes. In the analog EC and DHF modes the index ellipsoid (blue) rotates in a plane normal to E producing an effective ellipsoid (projection onto the substrate plane) depicted as empty ellipses. NOTE: The picture is highly out of scale. The smectic layers are 2-3nm thick, the DHF SmC* pitch is about 300nm, and the LC layer thickness planned is usually <10  m. We are grateful to Prof. Per Rudquist for sharing this figure. smectic A* phase the director is parallel to the smectic layer normal z, (Fig. 5a) while in smectic C* and Ca*, the molecules are tilted at the angle q with respect to z (Fig 5b-c). In chiral tilted 19 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 smectics14, the director makes a helix along z (Fig. 5b), and in materials with a period (pitch) smaller than the wavelength of light, the average optic axis is along z. Furthermore, there is a spontaneous polarization density P normal to the director and to z, which allows for polar azimuthal switching of n about z at fixed q under the torque P x E. The synclinic SmC* phase is ferroelectric while anticlinic SmCa* is antiferroelectric. The following three electrooptic modes are attractive for incorporation PMM structures. -Deformed helix ferroelectric mode. In short-pitch FLCs, a field applied normal to z gives a deformation of the helix, causing a tilt of the optical indicatrix in a plane normal to E, cf. Fig 5b. This is the deformed helix ferroelectric (DHF) mode 90 , 91 . When the period is much less than the wavelength p<, it behaves as a uniaxial medium which becomes biaxial when a field is applied 92 , 93 . In figure 5b the mode is called vertically aligned DHF to compare with the standard one where the helix is in the plane of the substrates 94 , 95 . The tilt is analog in the applied field and allows for continuous phase only tuning. In recent years, novel DHF materials with higher molecular tilt and smaller pitch have been developed, providing faster and higher amplitude switching40. Moreover, as p<<, any light scattering from the periodic structure is ruled out. Hence, in the DHF mode the structure behaves effectively as a continuously field-controllable retarder, in a wide region before the helix completely unwinds. Color switching and optically addressed modulators were demonstrated first in 96 . -Surface-stabilized FLC and AFLC modes where the helix is unwound by surface forces, and therefore providing only binary operation, have been successfully commercialized in highresolution microdisplays where both greyscale and color are written in the time domain. In these displays, the molecular tilt is ideally 22.5°. For many purposes, one may focus on FLC materials with 45° tilt, which allows for pure phase-only modulation, cf. Fig 5c and 5d. In the 20 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 antiferroelectric SmCa* phase the structure is anticlinic. Surface-stabilized orthoconic AFLCs 97 , 98 , ideally provide three level phase-only modulation 99 , at a slightly lower speed than FLCs. Alignment of the SSFLC in a twisted-splayed geometry has been shown to reveal an analog response and phase-only modulation 100 . -The electroclinic (EC) effect in SmA* 101 is the fastest useful electrooptic effect in LCs, under certain conditions even providing sub-microsecond switching. This mode has the same geometry as the DHF-mode. The first fast OASLM using this mode was demonstrated by the main PI 102 with a thorough investigation of its electrooptic switching characteristics 103 . The highest electroclinic switching amplitude is found in so-called deVries smectic A* materials 104 , 105 . In some photonic MTM structures, the LC layer is sometimes required to give phase-only modulation, for which the analog vertical alignment FLC modes (DHF and EC) can be used using interdigitated electrodes on one of the substrates to apply the electric field parallel to the plane of the substrate (see figure 4e). For certain applications based on switching between two or more wavelengths, SSFLC and orthoconic AFLCs can be explored. Aligning the LC molecules on the surfaces of the substrates is a crucial issue in obtaining the desired device quality. All the LC tunable PMM devices reported so far mainly used mechanical rubbing and sometimes only on one surface, though no special attention was devoted to the alignment within the nanoapertures. For a device with light-matter interaction mainly occurring inside the nanogaps, a different alignment strategy should be used to obtain the desired LC director orientation and uniform defect-free structures. Since the interaction region in the waveguide case is relatively large (0.1-1mm) the LC layer must be free of defects because scattering and losses in such a large region become significant. Recently54, photoalignment on metasurface made of 21 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 nanodisks showed some improvement over the standard rubbing technique. However, photoalignment using polymer films needs spin coating, a technique that can produce irregularities on the metasurface, particularly when the lines or disks are a few hundred nm or larger in height because of the walls and edges. This can be the reason why the dips obtained in ref.54 are a few tens of nm wide since defects broaden the resonance and reduce its contrast. We identified three alignment strategies that need to be investigated further to resolve the problem as follows: (i) Photoalignment using nano-dimensional chalcogenide glass film: This is based on the permanent photoalignment we discovered 106 , 107 , 108 , 109 a few years ago using 30-50nm films of chalcogenide glass (As2S3, As2Se3,…) deposited using physical vapor deposition. A thin film of 30-60nm is deposited on the surface and then irradiated with linearly polarized blue light. The LC molecules on the surface tend to align along the polarization direction due to the photoinduced anisotropy on the surface. Other photosensitive materials showing high photoinduced anisotropy which should be investigate are a-Se and GST. (ii) Unidirectional surface nanostructuring using ultrashort pulsed lasers: In this methodology, polarized femtosecond laser pulses are shone on the surface at energies just above the ablation threshold (typically few tens of pulses at 1030nm wavelength, pulse width of the order of 100-200fsec and energy of 10-100nJ). A nanograting pattern appears with the lines perpendicular to the polarization direction. We have proved this on the ITO electrode layer showing excellent molecular alignment along the nanotextured lines 110 , 111 . Nanostructuring with ultrafast laser is a well-established methodology on a large group of materials. 22 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 (iii) Oblique angle deposition of oxides: This type of deposition at a large angle (~60o) produces nano-columns on the surface oriented along the same direction, which causes the LC molecules to align along them. SiO2 films and other materials of 20-50nm thickness were shown to give good alignment 112 . Table 1. Overview of the faster LC modes LC mode Typical rise time Typical decay time Comments Pretilt Angle Control Reduces the rise time by up to 50% for small pretilt angle, the effect vanishes at high voltages Increases the decay time by up to 30% Can be applied with other modes, but not for all applications Drive Scheme Hundreds of microseconds up to few msec depending on the overshooting voltage Few msec till tens of msec, depending on thickness as d2 Can be applied with all other modes Dual Frequency LCs Few hundreds of microseconds depending mainly on the LC material >500μs up to few msec, varies with thickness as d2 Nano-Polymer Dispersed LC (Nano-PDLC) From sub-msec up to ten msec, depending on the LC used, the polymer matrix, voltage applied, and thickness of the cell Few msec till hundreds of msec, depend mainly on the composite materials and thickness as d2 Anti Parallel NLC Few msec till tens of msec, decreases as 1/V2 Tens till hundreds of msec, varies with thickness as d2 Blue Phase Sub-msec, can be reduced to 100 μs if doped with orientation polymer-stabilizing dopants Sub-msec Have narrow temperature range Twisted nematic Few msec till few tens of msec, decreases as 1/V2, and depends on thickness d2 and the LC molecules Tens of msec, varies with thickness as d2 23 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 3. PMMs for energy applications 3.1 Introduction The need for efficient energy conversion is very necessary to meet the increase in environmentally friendly, renewable energy requirements. From the past decade we have seen many techniques to improve the energy efficiency by utilizing the advancements in the field of plasmonics and various fabrication 113 , 114 , 115 , 116 , 117 . Solar energy conversion technique paves the way for various applications including solar absorption, water desalination, distillation, wastewater management etc 118 , 119 ,120. However, the main snag in developing solar absorbers is to generate strong spectral selectivity, near-perfect solar absorption, angular independence, and polarization independence. Achieving near-ideal solar thermal energy conversion requires absorbers that exhibit near-perfect absorption from the ultraviolet (UV) to the near-infrared (NIR) regions while minimizing midFerroelectric LCs Can reach down to microseconds even at law voltage if the pitch is much smaller than the cell thickness Decay time can be shorter than rise time and reaches down to few microseconds Operate under DC voltage Electroclinic LCs Reaching down to submicroseconds. Larger than the rise time by a factor of 2. Strong temperature dependence Flexoelectooptic effect in shortpitch cholesterics Few microseconds. Few tens/hundreds of microseconds (~ten times slower than rise time) Optic axis rotates in a plane perpendicular to the applied field Nanosecond EMOP mode Nanoseconds. Tens of nanoseconds. Based on electrically induced modification of the order parameters (EMOP) of NLCs rather than on the Frederiks reorientation of 𝑛 󰇍 . 24 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 and-far infrared (IR) emissions. Traditional PMM’s relay on localized surface plasmon resonance (LSPR) at metal/dielectric interfaces 120 , 121 , which lacks the broadband operating wavebands. Recent advancements have demonstrated that having impedance matching, slow light PMMs can greatly enhance the broadband spectral absorption Metamaterial nanostructures, particularly those utilizing metal/dielectric stacks, have shown promise in realizing broadband high absorption. These structures, known as hyperbolic metamaterials (HMM), can achieve high absorption efficiencies across a wide spectral range. 122 , 123 , 124 3.2 Design physics of Metamaterial Photothermal Converters Metamaterials can either be of periodic or aperiodic structures that are subwavelength, enabling the device to manipulate light in a very peculiar way compared to naturally occurring materials. This property arises solely due to the structural composition rather than material composition. For photothermal applications, metamaterials can be designed to have high absorption across a broad range of wavelengths. This can be achieved by various techniques such as: 3.21 Localized Surface Plasmon Resonances (LSPR) PMM: LSPR can significantly enhance the local electromagnetic field, leading to increased absorption of light. Earlier PMM absorbers were developed using LSPR phenomena. The excitation of LSPR at metal/dielectric interfaces results in strong absorption peaks, which can be tuned by adjusting the size, shape, and material composition of the nanostructures. In order to attain broadband absorption a simultaneous excitation of magnetic and electric resonances resulting from localized surface plasmon resonance at metal/dielectric interfaces is required 125 ,126. In one of the promising works, a near-ideal total solar-thermal conversion efficiency of up to 90.32% at 373.15 K was reported for solar absorbers, with an ideal efficiency of 95.6% 126 . According to the results obtained, it shows that geometric 25 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 parameter adjustment can sustain efficient solar thermal conversions at greater operating temperatures. The structure has shown near perfect absorption at broader angle range of up to ±60 degrees, hence the absorber has a huge potential to be used for solar energy harvesting applications irrespective of the incident polarization. There have been published works that combine the additional MM layer with these multilayer topologies. One such design has shown broadband ideal absorption in the solar spectrum region 127 .The design of the ultra-wideband (UWB) ideal solar energy absorber is based on a Ti ring with a SiO2-Si3N4-Ti thin film layered structure. This absorber has a total thickness of 620 nm and a basic structure (in this case a ring shape), making it relatively straightforward to build. It can also overcome the constraints of typical solar energy absorbers, such as poor average absorption rate and perfect absorption bandwidth. The absorptivity at 3683 nm is greater than 90% in the 300–4000 nm range; the average absorptivity is 95.0%, and the weighted absorptivity under air mass (AM) 1.5 is 97.0%. Perfect absorption values of 99.9% and 99.7% were specifically attained at wavelengths of 483 and 2380 nm. The metal used in this work is very ideal for the photothermal conversion studies as Ti is very stable at room temperatures and its boiling point is as high as 1668o. similar to typical lossy metals Ti also has large dielectric loss term, which also assists in the absorption efficiency. 32 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 technologies have the potential to become more significant components of renewable energy systems. 4. PMMs for sensing applications In the above sections, we have seen that using metamaterial surface one can easily modulate the light properties and can be used to concentrate the electric field for energy applications. In addition, the meta-surfaces provide the feasibility to significantly tune the resonant electromagnetic (EM) spectra, which are influenced by the surrounding environment. Due to these resonant properties, the refractive index (RI) of nearby biomolecular analytes can be assessed by observing changes in the scattered output spectra. Therefore, designing sensitive MAs for specific target wavelengths and configurations is crucial. Additionally, photonic metamaterial (PMM) based RI sensing platforms offer several advantages over traditional optical biosensors including surface plasmon resonance (SPR), lossy mode resonance-based biosensors 132 , 133 , 134 , 135 .First, since RI changes are detected through macroscopic optical responses, primarily reflection or transmission of focused input beams, PMM based sensors generally provide better fabrication tolerance and signal stability compared to SPR-based sensors. Second, the periodic arrangement of photonic unit cells leads to reduced radiative damping and higher quality factors, facilitated by phenomena such as plasmoninduced transparency and Fano resonances 136 . Finally, the functionality of a single nanophotonic RI sensor can be enhanced by incorporating PMMs. Carefully designed arrays of various photonic unit cells or supercells can produce multiple resonances and broadband slow light effects, which are difficult to achieve with SPR-based sensors. In addition, by analyzing interference between multiple beams, anisotropic scattering, and dispersive responses from certain EM devices, fundamental properties of incident EM waves can be detected and analyzed. Key characteristics of interest for PMMs include polarization and spectral composition, which are commonly analyzed 33 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 using commercial bulk-optic devices such as polarimeters and spectrometers. As the demand for compact and integrated light-sensing components in electronic devices has surged, PMMs are emerging as promising platforms for implementing these functions in a compact and lightweight form 137 , 138 . In this section of the chapter, we will discuss a few examples of using PMMs for sensing applications showing their potential for real world applications along with challenges and future directions. 4.1. Surface Enhanced Raman Scattering using PMMs The inelastic scattering of electromagnetic waves with the molecules produces the signature of molecular vibrational modes, that occur due to the emission of new photons through a process called Raman scattering. It shows the broad range of advantages (i) unique fingerprint signature of the analyte causing high selectivity, (ii) easy sample preparation method, (iii) no signal interference from the water, (iv) single molecule detection, (v) feasibility of the multiplexed sensing with a single laser beam and (vi) high throughput and point of care (POC) applicability by using commercially available portable Raman microscope. Raman scattering typically produces very weak signals due to low scattering cross-section (1/1012), which makes it challenging to detect with standard experimental methods. To address this challenge, Surface-Enhanced Raman Scattering (SERS) has been developed as a technique to significantly amplify the Raman signal. SERS relies on two primary mechanisms to achieve this enhancement: electromagnetic enhancement and chemical enhancement 139 . In the first method, the enhancement of the electromagnetic field at the nanometallic surface is utilized to improve the signal intensity up to the orders of 108. This enhancement occurs when the wavelength of the incident light matches with the resonance wavelength corresponding to the Localized Surface Plasmon (LSP) resonance of the nanometallic substrate. LSP resonance refers to the collective oscillation of electrons at the 34 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 surface of metallic nanoparticles, which intensifies the local electromagnetic field. The geometry of the nanoparticles plays a critical role here such as nanoparticles with sharp edges or tips can produce particularly strong SERS signals because these geometrical features enhance the local electromagnetic field more effectively 140 , 141 . The later factor, which plays a significant role for enhancing the Raman signal is chemical enhancement. This mechanism involves interactions between the metal surface and the analyte molecules and occurs due to the charge transfer or the formation of chemical bonds between the metal and the molecules being studied. These interactions increase the polarizability of the molecules, which enhances the Raman scattering signal 142 , 143 . Essentially, the nanometallic surface can modify the electronic properties of the analyte molecules, making them more responsive to the incident light. Numerous studies have been reported in literature utilizing specially designed PMMs for enhanced electromagnetic field provided SERS based applications136 144 . In addition, configurations that enable coupling between extended surface plasmons (ESP) and localized SP (LSP) can result in even greater signal enhancements. Studies have demonstrated that certain geometries, such as those with closely spaced metal structures or nanoparticles with specific shapes, can significantly boost the Raman signal through this coupling effect 145146 . A simplified model for understanding Raman scattering enhancement involves calculating the field enhancement near a single isolated particle. This model provides a way to estimate the total enhancement factor by multiplying the enhancements of both the incident and scattered fields. An example of this calculation for a SERS enhancement factor per molecule can be found in the supplementary materials of Ref. [146]. For a comprehensive overview of current developments and future directions in SERS technology, refer to the recent review in Ref. [140]. This review discusses the latest advancements in the field, including new 35 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 materials, techniques, and applications. A few studies using PMMs for SERS applications will be discussed in section 4.3. 4.2 Surface Enhanced Fluorescence (SEF) using PMMs Like SERS, SEF is one of the important applications where enhanced electromagnetic field is used to improve the fluorescence intensity of the fluorophore when kept near PMMs. Fluorescence is a phenomenon observed in molecules known as fluorophores, where the molecular structures are capable of absorbing light at one wavelength (the excitation wavelength) and emitting it at a longer wavelength (the emission wavelength). When fluorophores are placed near metallic nanostructures, their fluorescence properties can change significantly. Appropriate selection of fluorophore is very important to ensure that optical absorption of the fluorophore and metal overlaps. The energy transfer between fluorophore and enhanced electromagnetic field due to PMMs is dominated by dipole-dipole interaction as follows if the distance between the PMMs and the fluorophore lies within 1-10 nm, the non-radiative localized field of the plasmon dipole can excite that of fluorophore 147 , 148 . In addition, the lifetime of the excited state can be shortened in the vicinity of the PMMs. This reduction occurs because the metal can facilitate faster energy transfer processes, such as enhanced radiative decay, making the fluorophore return to its ground state more quickly. This phenomenon is also called Főrster resonance energy transfer (FRET). Understanding and controlling these interactions between fluorophores and PMMs are crucial for applications in fields such as biosensing, imaging, and molecular detection, where fluorescence signals are used to obtain detailed information about molecular interactions and concentrations. In the next sections, we will be discussing a few studies reported in literature applying PMMs for SERS and SEF based sensing applications. 4.3 Studies reported using PMMs for sensing applications 36 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 4.3.1 Ag nanosculptured thin films (nSTFs) for SERS applications: Nano-sculptured thin films (nSTFs) are the group of meterials prepared by the oblique or the glancing angle deposition technique in forms of nanocolumns, nanoscrews, nanozigzags and many other nanoshapes. Silver (Ag) nSTFs, exhibit remarkable localized plasmonic properties that make them a promising substrate for enhanced spectroscopies such as SERS, SEF etc 149 , 150 . These enhanced signals strongly depend on the morphological nature of n-STF metasurfaces such as nanorod diameter, height, pore size etc, that depends upon the substrate properties, preparation conditions etc. Their long-term stability in the water environment makes them suitable candidates for biosensing in water as it is already demonstrated for several water pollutants. are highlighted with emphases on the unresolved issues and future trends. A nanobiosensor chip leveraging surface-enhanced Raman spectroscopy (SERS) on Ag nSTFs was developed to detect Escherichia coli (E. coli) bacteria at concentrations as low as a single bacterium 151 . The sensor was based on capitalizing highly enhanced plasmonic properties of silver nSTFs on a silicon substrate, significantly amplifying the Raman signals, as verified with adsorbed 4-aminothiophenol molecules. To ensure specificity, T-4 bacteriophages were immobilized on the sensor’s surface, enabling the targeted capture of E. coli bacteria. Figure 9 represents the used Raman setup and the corresponding enhanced Raman signal with varying bacterial concentrations on the Ag n-STF substrate. 37 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 Figure 9: (a) SEM image of fabricated n-STF substrate (top view), and (b) SERS spectra with varying E.coli. concentration over the Ag n-STF substrate. Reproduced with permission from Ref. [151]. Results shown in figure 9 revealed that the sensor facilitates rapid, accurate, and stable detection of E. coli, even at ultra-low concentrations, down to the level of a single bacterium within a 10 μl sample volume. This level of sensitivity and specificity positions the sensor as a powerful tool for detecting E. coli in various applications, offering both precision and reliability. 4.3.2 LSPR-SPR coupled plasmonic metasurface for SERS applications: As mentioned earlier, SERS is usually achieved when the Raman active molecule is brought near metal nanostructures mediated plasmonic hot-spots, it enhances the Raman signal up to a million times (even more, depending on nanostructure morphology). In addition, the signal can further be enhanced when the plasmonic nanostructure is kept near the metallic thin film causing the coupling of the propagating surface plasmon resonance (SPR) and localized SPR (LSPR) 152 , 153154 . For example, in a study reported by Srivastava et al., an approach is demonstrated to create extremely high electromagnetic hot spots using a configuration that couples propagating (or extended) and localized SPR [146]. In the study conventional Kretschmann-Raether configuration is employed as shown in Figure 10. 38 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 Figure 10: Schematic of the experimental-setup for ESP-LSP coupled metasurface. Reproduced with permission from Ref. [146]. The metasurface chip is fabricated by spin coating of Au nanoparticle over the Ag (silver) coated glass surface that enables the coupling the extended plasmon wave due to Ag thin film with localized plasmons in gold nanoparticles that are dispersed on top of the silver film. When the extended plasmon wave interacts with the gold nanoparticles, it excites localized plasmons within them. This interaction between the extended and localized plasmons leads to a substantial increase in the electromagnetic field, creating the extremely high hot spots necessary for enhanced sensing. A critical factor in achieving maximum enhancement is the interparticle gap—the distance between neighboring gold nanoparticles. The study highlights that the SERS enhancement is highly dependent on this gap ensuring the generation of the strongest possible hot spots. To test the SERS efficacy of the proposed configuration, a monomolecular layer of 4-aminothiophenol as a test molecule is placed between the silver film and gold nanoparticles. Figure 4(a) shows the electromagnetic enhancement with respect to interparticle gap while 4(b) represents the SERS 39 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 spectra obtained for 4-ATP molecule using varying concentrations of Au nanoparticles over the Ag surface. Figure 11 (a): Maximum electric field intensity vs interparticle gap of optimization of ESP-LSP enhancement and (b) SERS spectra obtained for 4-ATP biomolecules with varying nanpoparticle concentrations over Ag surface. Reproduced with permission from Ref. [146]. From figure 4(b), it is evident that an extraordinary SERS enhancement factor of the order of 10¹⁰ per molecule was found allowing for the detection of extremely low concentrations of molecules, which is vital for sensitive analytical techniques. The study's findings have broad implications beyond just SERS. The ability to create ultra-high electromagnetic hot spots can enhance the performance of optoelectronic devices, such as solar cells and photodetectors, by increasing their interaction with light. Additionally, this technology could be applied in energy systems, where plasmonic enhancements might improve the efficiency of energy conversion and storage devices, such as in plasmon-enhanced photocatalysis. In continuation, several studies have been reported to develop plasmonic metasurfaces leveraging the benefits of extended surface plasmons and localized surface plasmon coupling for metal enhanced spectroscopies and their applications for sensing. 40 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 4.3.3. Self-referenced refractive index sensor using thin dielectric grating on thin metal film (TDGTMF) meta surface: Recently, our group has been pioneering a self-referenced PMM, featuring a thin dielectric grating with a thickness of less than 200 nm atop a thin metal film measuring under 50 nm 155 , 156 , 157 . These studies highlighted that employing a very thin metallic grating, in the range of approximately 20– 40 nm, can significantly enhance optical transmission, resulting in the emergence of two distinct transmission peaks 158 . The TDGTMF geometry, as illustrated in Figure 12, was found to support the excitation of two distinct optical modes155,156. The first mode arises from the guided mode resonance induced by the dielectric grating. The observed dip in the reflection spectrum, rather than a peak, is attributable to the presence of the metal film beneath the grating. This mode predominantly manifests in the analyte medium and is referred to as the analyte mode. In contrast, the second optical mode is the ESP mode, which is excited at the interface between the metal film and the substrate, known as the substrate mode156,157. Simulations of field distributions reveal how the presence and behavior of these fields relate to the sensitivity of each resonance to changes in the refractive index (RI) of the surrounding material. Since the substrate mode is minimally sensitive to the refractive index (RI) of the analyte, it can serve as a reference for detecting changes in the analyte RI. This is illustrated in the reflection RI-λ map of the TDGTMF geometry, with a grating thickness of h = 175 nm and a metal film thickness of d = 40 nm (see Fig. 13a). Notably, as the analyte RI increases, the substrate mode begins to diminish and nearly vanishes when the analyte RI approaches the RI of the SiO₂ substrate (1.443-1.445 within the specified spectral range) 155. The substrate mode, which has a greater penetration depth compared to the analyte mode156,157, is associated with localized resonant surface plasmon (LRSP) excitation, making it suitable for 41 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 detecting larger biological entities such as cells and bacteria. The thickness of the metal film plays a crucial role in the excitation of the substrate mode (Fig. 13b). Fig. 12 Diagram of the self-referenced RI sensor utilizing the TDGTMF geometry. Reproduced with permission from Ref. [4]. Fig. 13 (a) Reflection map of analyte RI versus wavelength (λ) for the TDGTMF geometry with a grating thickness h=175 nm and a metal film thickness d=40 nm. (b) Reflection map of grating thickness d versus wavelength (λ) with a grating thickness h=175 nm. Simulations used TM-polarized light at normal incidence, applied to a Si₃N₄ grating with a periodicity Λ=1000 nm and grating spacings of 450 nm in both (a) and (b). Water and SiO₂ were used as the superstrate (analyte) and substrate materials, respectively. Reproduced with permission from Ref. [155]. 48 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 Further, the sensing application of the proposed metasurface is based on the detection of shifts in the toroidal resonance frequency as a result of biomarker binding. When the target biomarker, ZIKV-EPs, binds to the functionalized surface of the metamolecules, it induces a change in the local refractive index around the metamolecules. This change in refractive index perturbs the electromagnetic environment of the toroidal resonance, leading to a measurable shift in the resonance frequency, as shown in figure 18. Figure 18: (a) The transmission amplitude spectra for the fabricated metasurfaces for with and without gold nanoparticles (GNPs) for varying concentrations of ZIKV-AB and ZIKV-EPs variants. (b) Calibration curve representing the toroidal resonance shift vs ZIKV-EPs concentration for with and without GNPs. (c) The magnified transmission spectra as a function of frequency. Reproduced with permission from Ref. [163]. 49 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 It is worth mentioning, when the GNPs are introduced into the medium surrounding the metasurface, they increase the local concentration of the biomarker near the metamolecules. This results in a more pronounced perturbation of the toroidal resonance, leading to a larger shift in the resonance frequency. By measuring the shift of the toroidal dipolar momentum (up to Δω~0.35 cm−1) for different concentrations of the biomarker, it was possible to analyze the sensitivity, repeatability, and limit of detection (LoD) of the enhanced metasensor. In the initial experiments, the metasensor was used to sense and quantify ZIKV-EPs by measuring the spectral shifts of the toroidal resonances as the concentration of the biomarker varied. The results showed that the metasensor was capable of detecting very low concentrations of the biomarker, with a high degree of sensitivity and specificity. The detection process is rapid, making the metasensor suitable for real-time monitoring of biomolecular interactions. In conclusion, the THz metasensor based on toroidal plasmonic metamaterials represents a powerful and versatile tool for biosensing, with the potential to revolutionize various fields by enabling the detection of low concentrations of biomarkers with unprecedented sensitivity. 4.4 Concluding remarks In conclusion, metasurface sensors represent a significant leap forward in optical sensing technology. Unlike conventional optical sensors, which often face challenges related to sensitivity, size, and complexity, metasurfaces offer a solution through their unique design and material properties. Metasurfaces are engineered materials with nanostructures that allow for unprecedented manipulation of light. By precisely tailoring these nanostructures, metasurfaces can achieve exceptional levels of sensitivity, enabling the detection of subtle environmental changes or minute quantities of analytes. This chapter has summarized various metal sensors, including those based on STFs, ESP-LSP coupled metasurfaces, thin gratings, and toroidal metasurfaces, all 50 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 of which demonstrate significant potential for developing sensors that are miniaturized, costeffective, and capable of lower detection limits. However, challenges such as precise fabrication, stability, and signal variability still exist. With ongoing technological advancements, these limitations can be addressed, paving the way for the practical applications of metasurface sensors in real-world scenarios. Acknowledgements: The funding from the following sources was very helpful: NATO Science for Peace and Security Programme under contract number MYP G5814 for supporting the project titled: “Nanomaterials for explosive traces detection with SERS” (NOOSE); the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 872662; the Israel Ministry of Innovation, Science and Technology, and the Ministry of Energy. Biography Majd Abu Aisheh graduated from the Department of Mechanical Engineering of Middle East Technical University in Ankara, Turkey. During his MSc studies from 2016 to 2018 he worked as a research assistant and focused on the study of fires in tunnels to develop fire extinguishing systems. He is a PhD candidate in the Department of Electro-Optics and Photonics Engineering at Ben-Gurion University, where his research focuses on enhancing the response time of photonic devices. 51 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 Amaljith C. K. graduated from the Department of Nuclear Physics at the University of Madras, Chennai, India, in 2018. From 2018 to 2021, he served as a research assistant in the same department. During this time, he worked on the development of THz metamaterial devices. He is pursuing a PhD in the Department of Electro-Optics and Photonics Engineering at Ben-Gurion University, where his research focuses on tunable metamaterial devices for various electro-optic applications. Dr. Anand M. Shrivastav obtained his Ph.D. Degree form Physics Department, Indian Institute of Technology Delhi in 2018. He worked as postdoctoral fellow in The HongKong Polytechnique University (Hong Kong), Ben Gurion University of Negev (Israel) and Public University of Navarra (Spain). He has received several awards and fellowships such as PBC postdoc fellowship (Israel) and Marie Curie Postdoc Fellowship (Spain), Distinction in doctoral thesis award etc. He has authored 35+ research articles in peer reviewed journals, one book (Optical Sensors for Environmental Monitoring and Biomedical Diagnostics) and about 14+ conference papers. He holds the memberships of SPIE, The Optical Society (OSA) and Optical Society of India (OSI). Current research area is focused on plasmonics and nanophononics for biosensing applications. Currently, he is working as research track Assistant Professor in from SRM Institute of Science and Technology, Kattankulathur, Chennai, India. Prof. Ibrahim Abdulhalim: is a professor in the Department of Electrooptics and Photonics Engineering at Ben Gurion University. He has worked in research and development in variety of academic institutions and industrial companies such as: the Optoelectronic Computing Systems Center in the University of Colorado at Boulder, USA, the Optoelectronics Research Center of Southampton University, England, the Thin Films Center of the University of Western Scotland, KLA-Tencor and Nova measuring instruments, and in GWS-Photonics. In October 2005 he joined the Department of Electrooptic Engineering at Ben Gurion University and acted as the department head between 2006-2014. His current research activities involve liquid crystal devices for photonic applications, nanophotonic and plasmonic structures for biosensing, biomedical optical imaging techniques such as spectropolarimetric imaging and full-field optical coherence tomography. Prof. Abdulhalim has published over 220 journal articles, 70 conference proceedings papers, 11 book chapters, coauthored one book titled: Integrated Nanophotonic Devices (Micro and Nano Technologies), co-edited a book titled: Signal amplification in optical biosensing, and has 22 granted patents. He became a fellow of the Institute of Physics, UK in 2004, SPIE fellow in 2010 and senior member of Optica in 2016. He was an associate editor of the SPIE Journal of NanoPhotonics, and presently acting for the Journals of Sensors, and 52 Chapter 5, pages 87-132 in: More Adventures in Contemporary Electromagnetic, F. Chiadini, V. Fiumara (eds.), Springer Nature Switzerland AG 2025. https://link.springer.com/book/10.1007/978-3-031-83131-7 Biosensors, and a topical editor for Applied Optics. In 2014 he founded a company called Photonicsys, specialized in optical and photonic devices for biosensing, and in 2019 a company called Photoliqsys, specialized in liquid crystal devices and applications. 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