Introduction

The following article is derived from a presentation made by the corresponding author, in his capacity as Member, BRICS Photonics Working Group, to a BRICS Photonics meeting in October 2020. Various BRICS representatives presented the research landscape of relevant topics in the respective countries, and this presentation covered the Nanophotonics and Metamaterials (N &M) landscape in India. The work described in this article precedes the meeting by a couple of years. Apropos to the forum of presentation, the selection of results was based on multiple criteria, some of which included: proper representation of various geographical regions, representation of central research institutes, IITs and Universities, sensitivity to gender neutrality, representation of fundamental and applied research, inclusion of theoretical, computational and experimental results etc. Indeed, the authors are aware that many more groups in India work on N &M, and have produced an impressive body of literature. Understandably, not all, or even most, can find a place in a short, time-limited presentation. Therefore, it is imperative to emphasize that this collection of results does not intend to make a value judgement on the nature or quality of work done in any organization in the country, and is merely an aggregate of results chosen according to the aforementioned criteria. Subsequently, in order to bring the reader up to speed with the concurrent proceedings, a short selection of later results are very briefly alluded to in the ending paragraphs, which, again, is only a representative list that is expected to motivate the readers to carry out their own literature survey. With that important disclaimer, we invite the readers to browse through the set of N &M works listed in the following.

Nano-optics/ photonics

Nano-optics and nanophotonics encompass a broad range of research areas focused on manipulating and controlling light at the nanoscale, specifically in the visible and near-visible wavelength range. However, the scope of nano-optics can extend into higher wavelengths, reaching into the microwave frequencies, particularly when electric fields need to be interrogated. In the field of nano-optics, researchers investigate various aspects of light-matter interactions, including the study of the behavior of light confined to or guided within resonators and waveguides. By designing nanostructures with tailored resonant modes, researchers can enhance light-matter interactions, leading to efficient absorption or emission of light at specific frequencies. This has implications for applications such as sensing, energy harvesting, and light-emitting devices. Photonic crystals are periodic structures that exhibit unique optical properties, allowing for the manipulation of light propagation and the generation of nonlinear optical effects. By engineering the periodicity and composition of photonic crystals, researchers can achieve efficient harmonic generation, enabling the generation of light at frequencies that are multiples of the incident light. Overall, the field of nano-optics and nanophotonics continues to uncover intriguing phenomena and develop new devices by structuring samples at the nanoscale. This research not only advances our understanding of light-matter interactions but also paves the way for the development of next-generation optical technologies with enhanced functionalities and improved performance.

As a subdomain of nano-optics, wave transport in disordered media has been the subject of extensive research, with Anderson localization emerging as a significant phenomenon [1,2,3]. However, understanding Anderson localization in dissipating systems poses challenges due to the presence of loss, which compromises the signature of localization [4,5,6,7]. Our group addressed this challenge by investigating Anderson localized modes in a dissipating system operating at THz frequencies, enabling the separate quantification of localization length and loss length [7, 8]. The non-Hermiticity resulting from dissipation gives rise to intriguing transport behavior in complex systems. In particular, the interplay between non-Hermiticity and disorder in a hybrid plasmonic system leads to a new transport regime [9, 10]. This regime was studied numerically using a tight-binding model to examine the modal evolution with respect to disorder strength and its impact on transmission in a 1D hybrid-plasmonic coupled-resonator waveguide, as shown in Fig. 1a. Disorder was introduced by randomly varying the positions of each resonator within a specified range. Upon excitation of the waveguide at a specific frequency \(\omega\), three eigenmodes were excited due to their spatial and spectral superposition, with the latter arising from frequency pinning due to non-Hermiticity. The corresponding intensity distribution is illustrated in Fig. 1b, where individual eigenmodes (labeled I (blue), II (green), and III (red)) exhibit localization. Transmission occurs through hopping via localized states resulting from the spectro-spatial overlap. Figure 1c shows the intensity profile of three disorders, \(\delta =0.05,\delta =0.50~ \& ~\delta =0.95\), for a single configuration at the band edge. In the case of a near-periodic system (green), the mode extends throughout the sample, while at intermediate disorder (blue), the mode becomes localized. At high disorder (red), multiple peaks appear, spreading throughout the sample with an overall extended wave function reminiscent of beads in a necklace. These states are appropriately named necklace states. The inset displays the configurationally averaged effective mode width with respect to disorder, revealing an extended width at the strongest disorder. These observations unveil a novel mechanism of transport in localized disorder.

Fig. 1
figure 1

a Illustration of a hybrid-plasmonic coupled-resonator waveguide. b Distribution of intensity (represented by the black curve) at frequency \(\omega\) under conditions of strong disorder, showcasing three distinct eigenmodes denoted as I (blue), II (olive), and III (red). c Intensity profile at the band edge observed across three levels of disorder. Inset: Variation in the effective width of the intensity distribution (logarithmic Y-axis) at the band edge as disorder is altered. Panels (a), (b)& (C) reproduced from ref.[9] with permission

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Whispering gallery mode resonators (WGMRs) are highly regarded optical devices with a wide range of applications [11], including their use as label-free biomolecular sensors in medical applications [12]. These resonators leverage the interaction between confined electric field modes and analytes to achieve highly sensitive real-time detection in the optical frequency range [13, 14]. Mathai et al. have conducted experimental investigations on a sapphire WGM resonator-based terahertz (THz) sensor, showcasing its potential as an organic molecular sensor with an impressive sensitivity of up to 25 ppm for THz frequencies at 129.49 GHz[15]. Figure 2 illustrates the measured transmission phase response curve for WGM coupling in the sapphire WGMR, while the inset demonstrates the electric field distribution of the fundamental resonant mode at 129.49 GHz. This characteristic, combined with an improved analytical approach based on coupled-mode theory between the waveguide mode and the spherical resonator mode, enables the determination of the optical constants of adsorbed molecules. Consequently, this approach holds promise for refractometric sensing of organic polar molecules, including explosive chemicals, biomolecular proteins, and more.

Fig. 2
figure 2

a Experimental measurement of the transmission phase response curve for sapphire Whispering Gallery Mode Resonator (WGMR). The resonant frequency of the WGMR is observed at 129.49 GHz. The inset illustrates the electric field distribution of the fundamental resonant mode at 129.49 GHz. The associated waveguide (WG) modes (q, p, m) obtained through simulation are indicated. The intrinsic quality factor (\(Q_i\)) obtained is also depicted. Figure reproduced from ref.[15] with permission

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In the domain of open quantum systems, due to inherent non-Hermiticity, the eigenfunctions are non-orthogonal and their eigenvalues are complex. The interaction between states of such complex systems leads to crossing/anticrossing behavior in the dispersion curves. The level repulsion of of the states results in avoided resonance crossings (ARCs). Exceptional points (EPs) in non-Hermitian systems are degenerate points in the complex energy plane where two coupled eigenstates and their eigenvalues coalesce simultaneously. Such EPs have manifested many exciting phenomena like asymmetric mode conversion, unidirectional light transmission, resonance-assisted tunneling, manipulating the lasing-absorbing modes, parametric instability etc.[16,17,18,19]. In many non-Hermitian systems like topologically protected systems, the physical impacts of an EP are related to broken PT-symmetry. However, in some specially configured systems, EPs are encountered without violation of PT-symmetry. Laha et al. have proposed a unique way of manipulating a second-order EP without breaking PT-symmetry [20]. The configured system is a two-port Fabry-Pérot type trilayer optical microcavity. The cavity has a non-uniform refractive index profile along the direction of transmission and is made of three regions: an air region (\(n = 1\)) is inserted between two regions of a high refractive index medium (\(n = 1.5\)). Non-Hermiticity is created by introducing a balanced gain(G)-loss(L) in two high refractive index media with similar gain coefficient. The EPs are manipulated by two parameters: gain coefficient and tapering parameter(w). The intermediate regions between low and high refractive index media are tapered by optimizing w(x), which is a parabolic optimization function in vertical direction to x. In order to encounter the EP, scattering matrix (S-matrix) formalism was used in the numerical study. The S-matrix relates the incoming and outgoing waves. The boundaries of the intermediate index layer are appropriately tapered. The cavity is partially pumped by a transverse distribution of a balanced gain-loss profile. By controlling infinitesimally small variations of gain-coefficient with simultaneous tapering parameters, an EP is encountered in cavity parameter space to study the dynamics of the coupled states around the EP. Such a trilayer microcavity with tunable width turns out to be a specific approach to achieve maximum efficiency in conversion with minimum asymmetry, which can open a platform for device-level implementation of system control exploiting EPs.

Recent development in optoelectronics has led to extensive research on emission from silicon nanowires (NW’s), especially in low-dimensional systems [21]. The origin of photoluminescence (PL) emission and lifetime statistics from Silicon nanostructures (Si-NSs) have uncovered many physical phenomena such as quantum confinement effect, surface states, and defect-centre luminescence. A fast lifetime of few nanoseconds was observed in the carbon terminated water soluble Si nano-crystals and attributed to the direct radiative recombination in Si nanocrystals while the size dependence of Si-NSs has shown the lifetime of few tens of microseconds [22,23,24,25]. Yogi et al. analysed the optical properties of porous silicon nanowires at room temperature. The visible PL emission, its lifetime and their effect on the size of Si NSs at room temperature have been observed [26]. Figure 3 shows three different samples of Si nanowires (labeled by S1, S2 and S3), fabricated using metal-induced etching (MIE) of silicon wafers [27, 28]. Left and right panels of the figure represent SEM image of the top and cross section view. The top and cross sectional view confirms the pores like structures spreading over the entire surface and parallel aligned nanowires(NWs) with few tens of micrometers. The size measurements were done using Raman spectroscopy and the measured sizes obtained are 3, 6 and 7 nm for samples S1, S2 and S3 respectively. In order to see the size dependence behavior of absorption and photoluminescence (PL) emission in these Si nanowire samples, diffuse reflectance absorption spectroscopy (DRA) and PL emission studies had been carried out by exciting the samples with 3.06 eV (405 nm) diode laser respectively. From the DRA spectra, the bandgaps of the samples S1, S2 and S2 were measured at 3.0 eV, 2.9 eV and 2.6 eV respectively. Such strong increase in bandgap with the reduction in the size of Si NWs clearly shows that the absorption of light takes place in Si-NSs and establishes the existence of quantum confinement effect. The PL emission originates from the surrounding porous SiO\(_x\) and the spectra show a broad band emission in the range of 1.5 eV\(-\)2.2 eV.

Fig. 3
figure 3

Scanning Electron Microscopy (SEM) morphology of three distinct samples, namely S1, S2, and S3, arranged vertically in descending order. The left panel displays a top view of the sample surface, while the right panel presents a cross-section view. Figure reproduced from ref.[26] with permission

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Nitrogen-vacancy (NV) centers embedded in diamond have emerged as a highly promising contender for a diverse array of applications, spanning from quantum sensing to bio-imaging. These NV centers represent imperfections within natural or lab-grown diamonds, either in bulk form or on a nano-scale. They form when two adjacent carbon atoms in a diamond crystal undergo alteration, whereby one atom is replaced by a nitrogen atom while the other site remains vacant. The resulting NV center exhibits emissions characterized by two distinct resonant peaks at 575 nm and 637 nm, with each peak corresponding to the zero-phonon lines (ZPLs) associated with \(NV^{0}\) and \(NV^{-}\), respectively. Leveraging the interaction between nanodiamonds housing NV centers and various photonic structures such as photonic crystals, photonic cavities, and gap plasmon waveguides yields a remarkable enhancement in the intensity of these ZPLs [29,30,31,32].

Sharma et al have reported enhancement and suppression of NV center emission lifetimes at room temperature by modifying the local density of optical states (LDOS) around them in photonic crystals [33]. Here, photonic crystals with different lattice constants are synthesized using polystyrene spheres of diameters 196 nm (S1 sample), 287 nm (S2 sample), and 295 nm (S3 sample). The nanodiamonds containing ensembles of NV centers are drop cast over the surface of photonic crystals that are prepared using the convective self-assembly method. Time-resolved emission setup is done in which a 532 nm (10 MHz; 52 ps) pulsed diode laser is used to excite the samples using a microscope objective of 4X magnification. The modification of the local density of optical states (LDOS) surrounding the NV centers, leads to a wavelength-selective enhancement or suppression of their emission lifetime at room temperature. This modification is achieved through the induction of a wavelength-dependent photonic stop gap via Bragg diffraction. The observed results are explained through the application of the Barnett-Loudon sum rule and further supported by LDOS calculations. To investigate the emission behavior dependent on direction, two complementary measurement geometries are employed, examining the nanodiamonds adorned atop photonic crystals. In order to validate the distinct nature of lifetime distributions, the obtained distributions are compared with an appropriate reference sample using the Kolmogorov-Smirnov test. The achieved wavelength-selective enhancement or suppression of emission rates holds promising potential for high-resolution imaging based on NV centers and efficient readout of their charge states.

Nano-opto-mechanics

Nano-opto-mechanics is an emerging field at the intersection of nanotechnology, optics, and mechanics, where the manipulation and control of mechanical motion are achieved using light at the nanoscale. The advancement in nanotechnology has led to active research towards colloidal manipulation, and in this context, optical tweezers have been developed to control and manipulate submicron objects in fluidic environments. However, measuring and understanding the nanoscale optomechanical dynamics of fluids with high precision has remained a subject of fundamental interest. This pursuit aims to quantify the mechanical action of light and explore the intricate interplay between light and mechanical motion at the nanoscale. Recent years have witnessed significant progress in this field, with researchers investigating the optomechanical properties of fluidic systems and exploring novel approaches for high-precision manipulation and characterization. Here, we mention a couple of recent advancements in nano-opto-mechanics that unravel the intricate nanoscale dynamics and develop cutting-edge techniques for optomechanical control and measurement.

Ghosh et al. presented a class of mobile nanotweezers (MNT) based on the technology of plasmonically enhanced trapping integrated with artificial microrobots that can be externally controlled with small magnetic fields[34]. They can be operated remotely and noninvasively to trap, transport, release, and position the cargo in fluidic environments and can be applied to both thermophilic (silica) and thermophobic (PS) materials, despite their opposite response to thermal gradients [34,35,36,37,38]. They designed two MNTs namely, MNT-D1 and MNT-D2. Both designs contained iron as the magnetic element integrated with the helical structure. Whereas D1 contained small plasmonic (silver) nanoparticles distributed across its surface, alternating layers of silver (plasmonic) and iron (magnetic) were incorporated within the structure of D2. The MNTs provide a significant advantage over conventional plasmonic-based trappings that are limited by their speed and location due to colloidal diffusion. These limitations are overcome by integrating plasmonic nanostructures with magnetically driven helical microrobots and maneuvering the resultant mobile nanotweezers (MNTs) under optical illumination. The MNTs can be mass-produced and are easily integrable with the standard lab-on-chip systems and are quite versatile. Maneuvering multiple MNTs along independent directions is also possible using different magnetic field configurations. They can find applications in carrying out colloidal manipulation tasks in biological materials, nanodiamonds and could be particularly useful in hybrid nanoscale assembly applications.

Chaudhary et al. have reported significant progress in the field of optofluidic deformation measurement by achieving picometer precision in their experiments [39]. They employed a sessile fluid drop as an ultrastable interferometer to capture and analyze the dynamics of its interface under the influence of a second laser (pump laser). Their technique allowed them to measure interface deformations in real-time with a precision of 630 pm and 20 pm with a modulated beam, surpassing the thermal limit. The experimental setup of their picometer-resolved liquid-drop interferometer is depicted in Fig. 4. One remarkable achievement of their work was the ability to isolate and study thermally induced nano dimples on the fluid drop’s surface. These nano dimples arise due to the surface-tension gradient caused by nonuniform heating. The researchers successfully measured a precision of 600 nK in local temperature changes associated with these dimples. They further investigated the heating and momentum transfer characteristics in a wide range of simple and complex biological fluids, demonstrating universal features. Their study also presents a generic framework for distinguishing ultra-low thermal effects from optical momentum effects. The implications of their findings are far-reaching. The precise measurement of fluid properties, exploration of Abraham momentum in fluids, and the understanding of intriguing properties in complex biological fluids are among the potential applications. Furthermore, their work provides a foundation for extending similar approaches to investigate gels, foams, and opaque fluids with picometer-level precision. The high-precision data obtained from their experiments can also serve as a valuable benchmark for detailed theoretical modeling of thermal effects at various interfaces [40,41,42,43]. The achievement of picometer precision in optofluidic deformation measurement opens up new possibilities for studying fluid dynamics and interface phenomena with unprecedented accuracy and has wide-ranging applications in various fields.

Fig. 4
figure 4

Schematic diagram of a picometer-resolved liquid-drop interferometer. A collimated He-Ne laser generates high-contrast fringes, with the central fringe locally detected using an iris and photodiode D1. The liquid drop is excited by a focused Gaussian pulsed laser (olive laser). Inset: Ray diagram depicting quasinormal incidence and the illustration of the displacement \(\delta h\). Photodiodes D1 and D2, along with mirror M, are also indicated in the diagram. Figure reproduced from ref.[39] with permission

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Photonic device design & analysis

Confinement of light within nanoscale in waveguides offers the possibility of propagation control, and can be used in a large number of applications, like optical signal processing and modulation, biosensing, etc. However, scattering losses associated with waveguides can be detrimental to signal processing and sensing applications. The field of device design pertains to optimizing guiding structures towards specifically envisaged applications.

In the field of slot waveguide-based integrated photonic devices [44,45,46,47], the importance of strip-to-slot waveguide mode converters cannot be overstated. These mode converters play a crucial role in the overall performance of such devices, as they enable efficient coupling of the fundamental mode between strip and slot waveguides. Dhingra et al. propose a design based on adiabatic mode evolution to achieve highly efficient fundamental mode coupling [48]. The design utilizes a fast quasi-adiabatic approach to obtain optimized taper profiles that maintain a consistently low adiabaticity parameter along the length of the taper. These optimized taper profiles result in an exceptional conversion efficiency of 99.4%, achieved with remarkably short taper lengths. As a result, the mode converter has an ultra-compact footprint of only \(1.23 \times 3.7~\upmu\)m\(^{2}\). One notable aspect of this design is its ultra-broadband performance. The conversion efficiency variation is less than 1.5% over a bandwidth of 100 nm and less than 5% over a bandwidth of 200 nm. This wide bandwidth makes the mode converter suitable for various applications requiring broad spectral operation. Furthermore, the design exhibits a remarkable tolerance to fabrication inaccuracies, enhancing its practicality and ease of manufacturing. The proposed design of the strip-to-slot waveguide mode converter offers efficient and broadband fundamental mode coupling, compact size, and robustness, making it highly promising for integration in slot waveguide-based photonic devices.

Saini et al. conducted a numerical analysis and design of a highly nonlinear optical rib waveguide for on-chip mid-infrared supercontinuum sources [49]. They achieved ultra-broadband supercontinuum generation in the mid-infrared regime using a triangular-core As\(_{2}\)Se\(_{3}\) glass waveguide structure, which is the first of its kind. A parameter q was used to characterize the core shape, with q values of 0.25, 0.5, and infinity corresponding to triangular, parabolic, and rectangular core profiles, respectively. The simulation provided the normal electric field pattern of the fundamental TE mode at 2800 nm. By tuning the dispersion characteristics of the waveguide structure using the core shape profile parameter, the researchers observed that the broadening of the supercontinuum spectrum at the output increased with the length of the sample. Through numerical modeling, they determined that a 6 mm long triangular waveguide was the optimal choice for generating a broadband mid-infrared supercontinuum spanning 1000–11,880 nm. This was achieved by pumping the waveguide with 497 fs laser pulses with a peak power of 6.4 kW at 2800 nm. The on-chip supercontinuum generation approach has several advantages, including its low cost, robustness, and power efficiency, making it a promising alternative to the photonic crystal fiber approach. The generated supercontinuum source has potential applications in various fields such as optical imaging, early cancer diagnostics, optical coherence tomography, food quality control, security, and sensing [50,51,52,53,54].

Plasmonics

Plasmonics is a field at the intersection of metal optics and nanotechnology that explores the unique properties of surface plasmons. Surface plasmons arise from the coupling of photons with charges at metal interfaces, enabling the localization of light beyond the diffraction limit under relevant conditions. This understanding has vitalized the field of surface plasmons, as it allows for the confinement and enhancement of light in nanoscale structures. One of the many advantages of plasmonics is the ability to create high-density arrays of subwavelength sensor elements within micrometer-sized structures. This capability opens up exciting possibilities for various applications, including enhanced light-matter interactions, subwavelength imaging, and ultra-sensitive sensing. In this section, we delve into the field of plasmonics, focusing on recent advancements and breakthroughs in understanding the fundamental principles, designing novel plasmonic structures, and exploring their applications in areas such as biosensing, imaging, and energy harvesting.

In one study, Joseph et al. propose and demonstrate an innovative fabrication technique for creating a miniaturized “lab-on-chip" surface plasmon resonance (SPR) sensor with a flat metal-analyte interface [55]. Figure 5 illustrates the schematic of the resonant sensing element, which consists of a high-indexed grating embedded in a substrate with a lower index and a flat metallic (Au) surface on top, resembling a prism-coupling configuration. The incident light interacts with the structure from the substrate side, and its reflectance characteristics and resonances are obtained using finite-difference time-domain simulations. The results reveal that the excited surface plasmon polariton (SPP) mode on the flat metal-analyte interface exhibits a remarkable propagation length of \(270~\upmu\)m, which is significantly larger than that of a general grating-coupled configuration. This extended propagation length allows for increased interaction time with the analyte medium, leading to improved sensor detection. The electric field distribution analysis shows that the evanescent field penetrates the analyte medium up to a depth of 1284 nm. Experimental demonstrations of the reflectance dip using two different analyte media confirm the sensor’s performance, with an enhanced bulk sensitivity of 1133 nm/RIU (refractive index unit). Notably, this resonant mode exhibits sensing capabilities over a wide range of analyte refractive indices. The miniaturized and compact grating-coupled SPR sensor offers high sensitivity, deep penetration depth, and a prolonged interaction time, making it a promising candidate for the design of ultracompact biosensors capable of detecting and dynamically monitoring large biomolecules or cells in situ.

Fig. 5
figure 5

Schematic diagram of the resonant sensing element consisting of a high-indexed grating embedded in a substrate with a lower index, featuring a flat metallic (Au) surface on top. Incident light interacts with the structure from the substrate side, leading to the formation of an excited surface plasmon polariton (SPP) at the interface between the Au layer and the analyte. Figure reproduced from ref.[55] with permission

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In another work, Maurya et al. propose a novel approach for device applications using epitaxial single-crystalline TiN thin films deposited via ultra-high vacuum plasma-assisted molecular beam epitaxy (UHV-PEMBE). The authors demonstrate that these TiN thin films exhibit reduced optical loss compared to noble metals such as Au and Ag [56]. The study involves the structural and temperature-dependent optical characterization of MBE-deposited single-crystalline TiN thin films on MgO and Al\(_2\)O\(_3\) substrates. Figure 6 presents a comparison of the optical constants between the MBE-deposited TiN thin film on Al\(_2\)O\(_3\) and MgO substrates at room temperature and those of Au and Ag. It is observed that TiN has a smaller \(\epsilon _1\) (real part of the dielectric constant) in the visible wavelength range, but it increases rapidly and monotonically in the near-IR range. On the other hand, the optical losses (\(\epsilon _2\), imaginary part of the dielectric constant) of TiN remain lower than those of Au for both visible and near-IR wavelengths. In Fig. 6c and d, the propagation length (L) of surface plasmon polaritons (SPPs) and the plasmon decay length (\(\delta\)) inside TiN, Au, and Ag are shown. The TiN thin film exhibits a larger plasmon decay length, particularly in TiN/Al\(_2\)O\(_3\) structures. This characteristic makes it a suitable candidate for various photonic applications, including integration with III-V semiconductors such as GaN and InN, TiN-based photonic/plasmonic on-chip devices, broadband light absorbers, emitters for solar-thermophotovoltaics, and hot-carrier-assisted devices with improved efficiencies [57,58,59,60].

Fig. 6
figure 6

a and b: A comparison of the real (\(\epsilon _1\)) and imaginary (\(\epsilon _2\)) parts of the dielectric permittivity is made between a TiN thin film deposited by Molecular Beam Epitaxy (MBE) on both Al\(_2\)O\(_3\) and MgO substrates at room temperature. The comparison is performed with the optical constants of Au and Ag. The results demonstrate that TiN/Al\(_2\)O\(_3\) exhibits lower optical loss compared to Au. c: The propagation length of surface plasmon polaritons (SPPs) is investigated for TiN, Au, and Ag. d: The plasmon decay length (\(\delta\)) inside TiN, Au, and Ag is evaluated, revealing higher values for TiN. Figure reproduced from ref.[56] with permission

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The manipulation of colloidal particles through optical means is a vibrant field of research with diverse applications in the study of light-matter interactions. These applications include optical trapping, optical binding, and various aspects of soft matter physics such as light-induced assembly, swarming, and locomotion of biological entities in fluidic environments. In a recent study by Sharma et al., they present a method for the large-scale assembly of colloidal particles, ranging up to hundreds of microns in size, by utilizing the opto-thermal effects of a single evanescently excited gold triangular microplate. The organization of the assembled particles was quantified in terms of coordination number and hexagonal order parameter [61]. Further investigation into the assembly process examined the influence of excitation polarization and the surface-to-volume ratio of the microplate. The results demonstrated that efficient heat generation and assembly required volumetric excitation and polarization directed perpendicular to the structure. The researchers also observed a material-selective assembly behavior, where silica particles exhibited a preference for accumulation towards heat, while polystyrene particles with a size of 0.97 \(\upmu\)m did not exhibit such a trend. This suggests potential applications for colloidal sorting based on material properties. The authors highlight the advantages of their approach over traditional optical tweezers, as their system enables the creation of large-scale assemblies and complements existing thermoplasmonic platforms. This work promotes the utilization of metallic microstructures for opto-thermal colloidal crystal assembly and swarming studies. The experimental system holds promise for exploring optically driven interactions in soft matter systems, including biological entities such as cells and microorganisms [62,63,64,65].

Plasmons/2D Materials/Excitons

The field of plasmons, two-dimensional (2D) materials, and excitons has witnessed significant interest due to the intriguing combination of surface plasmons with the unique properties of 2D materials and excitons. Plasmonic modes offer the potential to overcome the diffraction limit, enabling the miniaturization of integrated optical devices and enhancing light-matter interactions. The synergy between plasmons, 2D materials, and excitons holds great promise for applications in quantum information processing, low-power lasers, and micro-nano processing devices. In this section, we explore a few recent developments in the realm of plasmons, 2D materials, and excitons, shedding light on their fundamental properties, novel device architectures, and potential applications.

Plasmonic nanostructures have the ability to sustain localized surface plasmon resonances (LSPRs), which play a crucial role in charge transfer and localization phenomena. These LSPRs can relax through either radiative relaxation via photon re-emission or nonradiative relaxation through Landau damping, which involves the injection of hot carriers. The nonradiative plasmon relaxation in plasmonic nanostructures leads to the transfer of energy to neighboring materials, influencing the carrier distribution in 2D materials. This unique feature of metal-induced hot-electron injection provides opportunities for active control and manipulation of the properties of 2D semiconductors, including lattice strain [66,67,68,69].

The lattice distortion in a monolayer MoS\(_2\) film induced by charge transfer between MoS\(_2\) and physisorbed gold nanoparticles has been investigated by Singh et al.[70]. Figure 7a depicts a high-resolution transmission electron microscopy (HRTEM) image of a monolayer MoS\(_2\) sample without gold nanoparticles, showing the atomic arrangement and hexagonal lattice structure. The inset shows a lower resolution image for reference. Figure 7b displays transmission electron microscopy (TEM) images of the spherical, elliptical, and rod-shaped gold nanoparticles used in the study. The findings reveal that the presence of gold nanoparticles induces approximately 10% biaxial compressive strain in the monolayer MoS\(_2\), with the strain being localized in the vicinity of the gold nanoparticles. The relaxation of strain occurs within a narrow region of approximately 50 nm from the edge of the gold nanoparticle. The injection of hot electrons via photonic excitation of LSPRs at the metal interface and the resulting lattice distortion manifest in shifts and broadening of Raman modes in the MoS\(_2\) monolayer.

Fig. 7
figure 7

a High-resolution image of a 2D hexagonal patterned monolayer of MoS\(_2\), featuring a lattice constant of 3.12Å, without the presence of gold nanoparticles. The main image provides a detailed view of the monolayer structure. Inset: lower resolution image for reference. b Transmission Electron Microscopy (TEM) images showcasing three different types of gold nanoparticles. The images include spherical, elliptical, and rod-shaped gold nanoparticles, providing visual representations of their respective morphologies. Figure reproduced from ref.[70] with permission

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The interaction between localized surface plasmon resonance (LSPR) modes and excitons gives rise to hybrid plexciton modes, which exhibit anticrossing behavior as the LSPR detuning is varied. The broadening of the LSPR linewidth can occur due to plasmon excitation damping through various decay channels such as radiation damping and bulk dephasing. Understanding the specific role of each plasmon decay channel in plasmon-exciton coupling is crucial for unlocking the potential applications of plexcitonic nanostructures. In an experimental study by Kumar et al., the focus was on investigating one specific plasmon dephasing channel, radiation damping, in gold nanorod-J-aggregate hybrids [71]. The researchers also examined the influence of the plasmonic cavity’s mode volume on plasmon-exciton interaction by studying the roles of radiative damping-induced plasmon linewidth and plasmon mode volume. By systematically varying the nanorod diameter, they were able to effectively control the contribution of other damping channels, which were found to be negligible. The optical response of plexcitons was investigated using single-particle spectroscopy, and LSPR detuning was achieved through the inherent size heterogeneity of the nanorod sample [72,73,74,75].

To analyze the plexcitonic behavior, the upper and lower plexciton mode frequencies were fitted with a classical coupled harmonic oscillator model. The results revealed clear anticrossing between the two plexcitonic branches at zero detuning, exhibiting a Rabi splitting of approximately 205 meV. This comprehensive study sheds light on the intricate interplay between plasmons and excitons in plexcitonic systems, providing valuable insights for future applications in fields such as nanophotonics and quantum technologies.

Fano resonances are fascinating quantum mechanical interference phenomena that occur when discrete states overlap energetically with a continuum. The signature asymmetrical line shapes of Fano resonances arise from the constructive and destructive interference around the resonant energy of the discrete state. In two-dimensional layered transition-metal dichalcogenides (TMDs) such as MoS\(_2\), the strong spin-orbit coupled band splitting allows for the coexistence of two discrete excitonic states. By harnessing the strong coupling between these discrete excitons and a plasmon-continuum in carefully designed metal-TMD hybrid nanostructures, asymmetrical Fano features can be achieved even at room temperature [76,77,78,79,80,81]. In an experimental study by Chowdhury et al., the time-resolved evolution of double Fano resonances in an exciton (MoS\(_2\))-plasmon (Au) hybrid was demonstrated using femtosecond transient spectroscopy at room temperature [82]. The setup involved Au nanodisks placed on layered MoS\(_2\), as depicted in Fig. 8. The MoS\(_2\) layers were grown through chemical vapor deposition on a SiO2/Si wafer, and self-assembled Au nanodisks were fabricated on top of the MoS\(_2\) layers using rapid thermal dewetting. The Au nanodisks acted as the continuum for surface plasmons, while MoS\(_2\) provided the paired discrete states in the form of two spin-resolved excitons. The resonant coupling regime between the strongly bound discrete excitons and plasmons was demonstrated to be spectrally correlated (600–700 nm) and temporally correlated, resulting in the formation of double Fano line shapes in the time domain. This study on time-resolved double Fano resonances provides crucial evidence of the dynamics of strongly coupled exciton-plasmon interactions in metal-2D semiconductor hybrid platforms. It opens up new possibilities for ultrafast two-level all-optical sensors and switches, showcasing the potential of these systems in various applications.

Fig. 8
figure 8

a Schematic representation of an Au-MoS\(_2\) hybrid structure, illustrating the arrangement of Au nanodisks on layered MoS\(_2\). b In-plane electric-field coupling (wavelength of 675 nm) at the interface between Au nanodisks and MoS\(_2\). c, d, and e: Out-of-plane E-field distribution observed under off-resonant and resonant conditions (indicated by a rectangle with a white-colored dashed line). These panels suggest that the coupling between the metal (Au) and the semiconductor (MoS\(_2\)) becomes active within the range of 600–700 nm, near the excitonic absorption of MoS\(_2\). The input wave propagates along the k-direction. Figure reproduced from ref.[82] with permission

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Plasmons/Q-dots

Metal nanostructures exhibiting surface plasmon excitations and semiconductor quantum dots (QDs) with long-lived excitations complement each other, making them highly suitable for studying optical coupling at the nanoscale. Plasmonic devices enable the confinement of light in nanometer-sized regions, effectively creating cavities for quantum emitters. QDs possess high photostability and large oscillator strengths, making them ideal for investigations at the single-particle level. The coupling between QD excitons and surface plasmons can vary from weak to strong, depending on the specific structures and energy scales involved, leading to distinct optical properties. In the weak coupling regime, plasmonic cavities primarily enhance the radiative rate of an emitter, while in the strong coupling regime, the energy levels of the two systems hybridize, resulting in the formation of coupled matter-light states. The interaction between QD excitons and plasmonic cavities has been extensively studied experimentally and theoretically, with potential applications ranging from sensing to quantum information technology. Prajapati et al. conducted a comparative study on the photoluminescence properties of ZnO nanorods using plasmonic metal nanoparticles, specifically Ag and Au, in conjunction with CdSe semiconducting quantum dots that exhibit similar absorption energies[83]. Figure 9a illustrates the model geometry of a ZnO nanorod decorated with Au nanoparticles, along with embedded dipole emitters. Figure 9b depicts the variation of the emission intensity ratio from Au-ZNR to bare ZNR as a function of wavelength, as obtained from both experimental measurements and simulations. The incorporation of plasmonic nanoparticles resulted in the enhancement of near-band-edge (NBE) UV emission. Through the use of a theoretical model based on finite element modelling of the metal nanoparticles, it was determined that the enhanced emission arises from charge transfer between ZnO and the metal nanoparticles at sub-conduction band energy states. The plasmonic absorption leads to a reduction in visible emission. In the presence of CdSe quantum dots, the luminescence of ZnO demonstrated NBE emission, but only when there was suitable band alignment between CdSe and ZnO. Quantum dots with a conduction band edge higher than that of ZnO exhibited NBE enhancement, while those with a lower band edge compared to ZnO showed a decrease in NBE emission. This investigation not only provides insights into the role of plasmonic nanoparticles in achieving spectrally selective emission tuning but also presents a theoretical model to predict the luminescence properties of novel metal–semiconductor nanocomposites for applications in light harvesting, particularly in nanophotonics [83,84,85,86,87].

Fig. 9
figure 9

a Geometry of the model depicting a ZnO nanorod decorated with Au nanoparticles, accompanied by embedded dipole emitters. b Plot illustrating the variation of the emission intensity ratio from Au-decorated ZnO nanorods (Au-ZNR) to bare ZnO nanorods (ZNR) as a function of wavelength. The data includes results from both experimental measurements and simulations. Figure reproduced from ref.[83] with permission

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Metal halide perovskite crystal structures have emerged as a promising class of optoelectronic materials, offering excellent optical absorption and emission properties combined with the advantages of solution processability. However, the presence of lead as a cationic species in the most promising perovskite structures hampers their commercial viability. Although the substitution of lead with non-toxic alternatives like tin has been explored in bulk materials, the investigation of tin-based nanocrystals is still limited [88,89,90,91]. In a recent study by Pradeep et al., Sn and Pb-based alloy perovskite nanocrystals were successfully synthesized using a direct synthesis method. By incorporating a mixture of Pb and Sn precursors in the desired ratio, they obtained quantum dots (QDs) of CsPb\(_{1-x}\)Sn\(_{x}\)Br\(_{y}\)I\(_{3-y}\) with effective Sn incorporation into the perovskite lattice [92]. The colloidal stability of these QDs is of great importance for their practical applications. The researchers investigated the stability of Sn-based QDs under various conditions and observed faster degradation when anti-solvents were used during the washing process. To address this issue, they proposed a purification method, which is discussed in detail. Additionally, while the optical and crystal structure stability of inorganic perovskites can be improved, there is a lack of studies exploring the structure–property correlation. The researchers examined the structural purity and optical stability of CsPbI\(_{3}\) and CsPbBr\(_{3}\) perovskite structures, providing insights into the structure–property relationship. Remarkably, the Sn-doped perovskites exhibited exceptional stability across the series of compounds for a period of up to three months, demonstrating the significance of understanding the structure–property correlation in achieving improved stability. This study highlights the synthesis, stability, and structure–property correlation of Sn-based alloy perovskite nanocrystals, providing valuable insights for the development of stable and non-toxic perovskite materials for various applications.

Bio-relevant plasmonics

Surface plasmon resonance (SPR) sensors, which are the current benchmark in label-free biosensing, have the potential to fulfill the requirements of future biosensor technologies. The field is now witnessing remarkable enthusiasm for metallic nanostructures with well-defined shapes and arrangements, featuring dimensions smaller than the wavelength of light. These nanostructures possess exceptional capabilities to scatter and absorb light, surpassing the efficiency of dielectric nanoparticles and dye molecules by several orders of magnitude. Additionally, they can confine and amplify electromagnetic radiation at the nanoscale. Metallic nanoparticles are already being employed in various applications such as pregnancy tests and photothermal ablation therapy for cancer treatment.

Metal-Enhanced Fluorescence (MEF) technique has found broad applications in various materials, including quantum dots, dyes, lanthanide nanocrystals, and carbon dots. MEF takes advantage of the plasmon-enhanced optical fields provided by metallic nanostructures when fluorophores are positioned at an optimal distance from them [93,94,95,96]. Pawar et al. demonstrated the use of gold nanoparticles (AuNPs) to induce MEF in the fluorophore 4-(pyridine-2-yl)-3 H-pyrrolo[2,3-c]quinoline (PPQ) and its Zn\(^{2+}\) complex within a bilayer structure of a niosome under aqueous conditions [97]. Figure 10 depicts the schematic representation of the possible MEF interaction between fluorophore molecules and gold nanoparticles (AuNPs). The AuNPs are positioned outside the bilayer at an optimal distance through electrostatic interactions between the positively charged surfaces of the niosome and the negatively charged surfaces of the AuNPs. The study found that the optimal distance between the metal and the fluorophore is less than 10 nm. The experimental measurements utilized three different sizes of AuNPs, and MEF enhancement was observed only with the 33 nm-sized particles, while the 18 nm and 160 nm sizes did not exhibit any enhancement. The fluorescence enhancement was accompanied by a reduction in lifetime components. These findings provide insights for designing fluorophore-metal configurations with desired emissive properties and lay the foundation for nanophotonic technology in biological conditions.

Fig. 10
figure 10

Schematic representation of Metal-Enhanced Fluorescence (MEF) occurring between fluorophore molecules and gold nanoparticles (AuNPs) within a niosome. Figure reproduced from ref.[97] with permission

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Singh et al. conducted a study on the strong radiative dipole coupling observed between molecular excitons in Chl-a (Chlorophyll-a) molecules and both propagating and localized Surface Plasmon Polariton (SPP) excitations in Chl-a-gold hybrid nanostructures [98]. Figure 11 illustrates the schematic representation of the sample, which consists of a Chl-a-coated gold film on a glass substrate attached to a hemispherical glass prism. The reflectivity spectrum of the Chl-a film coated on the gold film is shown by the red curve, covering the spectral range of 600–720 nm at near-normal incidence. The strong coupling effect enhances the absorption of Chl-a and provides tunability over a broad spectral range, resulting in the emergence of a polariton emission spectrum. This coupling effect significantly impacts the energy relaxation dynamics of Chl-a molecules, leading to improved light-harvesting and conversion efficiencies within the hybrid structure. The ability to achieve such strong interactions at room temperature opens up opportunities for tailoring the optical response to suit various applications, such as solar cells and other optical devices. Further investigations into the strong coupling phenomena occurring in naturally occurring molecules hold great scientific and technological potential [99,100,101].

Fig. 11
figure 11

Illustration of the Chl-a (Chlorophyll-a) gold hybrid nanostructure. The configuration comprises a 50nm-thick gold film coated with Chl-a on a glass substrate, which is affixed to a hemispherical glass prism using index-matching oil. The reflectivity spectrum of the Chl-a coated gold film reveals a pronounced dip at 656nm, corresponding to the \(Q_{y}\)(0-0) band of the Chla monomer. Figure reproduced from ref.[98] with permission

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Metamaterials

Metamaterials are engineered periodic composites that can manipulate electromagnetic properties to achieve responses not found in natural materials. Certain metamaterials, such as negative-index metamaterials, rely on the Localized Surface Plasmon Resonance (LSPR) phenomenon. Among plasmonic materials, noble metals exhibit the best performance, followed by metal nitrides and Transparent Conductive Oxides (TCOs). On the other hand, non-LSPR metamaterials exploit the effective medium behavior of composite materials. Examples of non-LSPR metamaterials include cloaks, hyperbolic metamaterials, and epsilon-near-zero (ENZ) materials. In the field of transformation optics, most metamaterial implementations fall into the non-LSPR category. Alternative plasmonic materials offer significant advantages for metamaterials due to their lower imaginary part of the permittivity, \(Im(\epsilon _{m})\), which reduces energy losses and improves overall performance.

Kumar et al., [102] employed a metamaterial in the form of periodically patterned Ag-columnar thin film metamaterials infiltrated with rhodamine 6 G dye to achieve random lasing. Random lasers are an exciting class of optical sources that combine amplification with multiple scattering. They have attracted significant attention due to their unique optical properties and a wide range of potential applications, including advanced lighting systems, speckle-free imaging, medical diagnostics, and super-resolution spectroscopy [103,104,105,106]. In the study by Kumar et al., the necessary feedback for random lasing is provided by the plasmonic scattering from the silver columnar thin films, while the amplification is achieved through the presence of Rhodamine 6 G dye. The fabrication of periodic one-dimensional gratings was accomplished using a two-beam laser interference lithography (LIL) technique. The silver columnar thin film (CTF) was fabricated using the oblique angle deposition (OAD) technique. The topography of the photoresist grating was examined using an Atomic Force Microscope (AFM). The slanted nanocolumns in the metamaterial structure introduce angular asymmetry, resulting in selective coupling of incident light to specific surface plasmon modes on the Ag CTF. Angle-resolved transmission spectra of the periodically patterned Ag CTF reveal this selective coupling behavior. When the pump laser is appropriately oriented to couple with a surface plasmon mode, the system exhibits a lower lasing threshold and higher efficiency in random lasing. This highlights the crucial role of surface plasmons in the system and offers precise control over the laser action through the polarization and incidence angle of the pump laser. The combination of metamaterials and random lasers offers exciting opportunities for achieving tailored optical responses and novel functionalities. The unique properties of metamaterials, such as their ability to manipulate electromagnetic properties, coupled with the fascinating behavior of random lasers, pave the way for advancements in various fields, including photonics, sensing, and energy conversion.

The above set of results represent part of the N &M landscape presented to the BRICS representatives. Naturally, subsequent to the meeting, several research reports have appeared in the literature marking further strides in research. These recent publications underscore the immense potential of nanophotonics in contributing to scientific advancements. We list a select few in very brief. Noteworthy developments include the experimental identification of anomalous transport regimes in non-Hermitian, Anderson-localizing environments[10]. Wavefront shaping through digital micromirror devices (DMD) in complex photonics[107] and the study of geometry-controlled oscillations in Liquid Crystal Polymer Films for thermally actuated optical chopper applications in optomechanics[108] mark important development. A novel metal-free fluorescent sensor, utilizing nitrogen-passivated carbon dots within a molecularly imprinted polymer, has emerged, enabling highly selective detection of 2,4,6-trinitrophenol (TNP). This synthesized fluorescent probe proves effective in analyzing TNP in regular tap and lake water samples[109]. A distinctive plasmonic response is observed in a stratified multistack structure containing high refractive index (HRI) graphene oxide (GO) and low refractive index (LRI) polymethyl methacrylate (PMMA), with Graphene oxide plasmon-coupled emission (GraPE) revealing robust surface states driven by chemical defects, and the metal-dielectric-metal (MDM) arrangement exhibiting unique properties such as zero-normal steering emission and directional GraPE, as supported by both theoretical analysis and experimental validation[110]. In addition, a plasmonic grating, fabricated through colloidal self-assembly and an ultrathin injection layer, demonstrates the potential for selectively guiding resonant modes. This structure holds promise for applications in refractive index sensing, where the sensitivity of guided-mode resonance (GMR) devices can be enhanced by introducing plasmonic signatures without significant resolution loss[111]. A comprehensive investigation of the ultrafast behavior of excited carriers in monolayer molybdenum disulfide (MoS2) is made, crucial for its optoelectronic applications. The study, conducted under high excitation densities, focuses on charge carrier dynamics using transient transmission techniques, revealing processes like exciton dissociation and bandgap renormalization[112]. Recent advancements in nanophotonics have paved the way for innovative nanomaterial-based biosensing systems capable of detecting disease-specific biomarkers, including circulating long noncoding RNAs (lncRNAs) which demonstrate rapid and specific capture of lncRNAs with high selectivity, offering a potential method for assessing lncRNA expression in the context of disease initiation and progression[113]. To enhance concentrated solar power (CSP) systems, refractory epitaxial hafnium nitride (HfN) and zirconium nitride (ZrN) thin films are introduced as promising alternatives to silver, exhibiting solar reflectivity of \(\sim 90.3\%\) and infrared reflectivity of \(\sim 95\%\), making them suitable for applications in CSP and daytime radiative cooling[114]. In a recent study, ZnS nanostructures, particularly hexagonal wurtzite nanowires (NWs), were found to exhibit distinctive Raman spectral modes, with surface optical (SO) modes activated by surface modulation confirmed through XRD and TEM. This activation contributed to frequency downshift in different dielectric media, elucidated via a dielectric continuum (DC) model and TEM image comparisons[115]. In the domain of meta-polymers, a straightforward method was demonstrated to fabricate V-shaped meta-polymers on substrates, enabling a periodic arrangement of NP junction arrays and showcasing their efficacy through polarized SERS response and electromagnetic field enhancement along the inter-arm angle, thus offering potential for diverse subwavelength assemblies in plasmonics and optical metamaterials exploration[116]. In another study, the demonstration of dynamic multi-color switching with a large color gamut using a simple, unpatterned, ultrathin asymmetric Fabry-Perot structure of VO2 on aluminum was done, achieving consistent and reversible color-switching across a wide temperature range and offering potential applications in display, thermochromic structures, and visible camouflage[117]. The overlap between N &M and Metamaterials and Quantum Photonics is beginning to emerge as an intriguing research area, such as, for instance, recent efforts focusing on the utilization of gold nanoparticles and silver nanowires to trap individual fluorescent nanodiamonds (FNDs) in fluids [118], as well as leveraging ensemble nitrogen vacancy centers (NVCs) in diamond for magnetic field sensing through photoluminescence [119].

In conclusion, the landscape of Nanophotonics and Metamaterials in India has witnessed significant advancements over the last few years. The thorough exploration of prominent journals has revealed a substantial increase in research output. This surge in publications reflects the dynamic and evolving nature of the field, underscoring the continued dedication and innovation of Indian researchers in the realm of nanophotonics. The wealth of knowledge presented in this review serves as a valuable foundation, offering a holistic understanding of the progress made in N &M in India while also acting as a springboard for future endeavors in this rapidly evolving scientific domain. Readers are urged to access output of similar nature emanating from fellow BRICS countries, and identify possible avenues of collaborative endeavours to strengthen the foundations of BRICS Scientific policies.