Basic concepts, advances and emerging applications of nanophotonics

2.1 Confinements methods

Nanotechnology, photonics, photovoltaics, biotechnology, and lasers are just a few of the significant innovation thrust sectors that nanophotonics integrates (Shen and Prasad, 2002; Iqbal et al., 2022; Malik et al., 2022; Conde et al., 2012; Malik et al., 2022; Iqbal et al., 2021; Iqbal et al., 2022; Ikram et al., 2020; Saleh and Teich, 1991; Prasad, 2004). Recent advancements in the knowledge of combining photonics and nanotechnology have made this practice important and challenging, and three categories of confinement methods can be distinguished. By limiting light to dimensions smaller than its wavelength, or in the nanometer range, one can first establish nanoscale links between light and matter. In the method that follows, the matter is constrained to the nanometer range, limiting the range of light-matter interaction to a nanoscopic size as well as describing the world of nanomaterials. The ultimate procedure for producing photonic structures and well-designed nanoscale units requires confinement of a photo-process to the nanoscale via a phase change activated by light or a change in photochemistry. One way to confine light to a nanoscale regime is to employ close-range optical transmission, a concept in which light is captured by a tapered metal-coated optical fiber and then radiated via a tip with a diameter substantially smaller than that of the incident light's wavelength.

In order to condense the magnitudes of specimens and create nanostructures suitable for photonic applications, a variety of techniques were employed, and accordingly, exceptional electrical and photonic properties have been reported. Encouragingly for researchers, these nanoparticles (NPs) are now utilized in nanophotonics attempts, for example, as ultraviolet absorbers for sunscreen lotion. These NPs could be made of organic or inorganic substances like polymers, which are long chains with a lot of repeating units, or nanomers, which are monomeric organic analogs complexed with oligomers with a finite number of identical units and have size-dependent optical properties.

Some specific metallic NPs used in the science of plasmonics have an intriguing optical response that entails a novel electromagnetic field enhancement. Such a strengthening of the field stems from the quantum-cutting phenomenon, which is the down-conversion of a vacuum UV photon into two visible IR photons (Iqbal et al., 2022; Malik et al.). The scale of repeating units in nanomaterial-based photonic crystals is typically comparable to the wavelength of light. A sporadic dielectric structure can also exist in such units. Nanocomposites consist of at least two nanodomains that are phase-isolated from various materials. The bulk media in the nanocomposite will each have a distinct optical characteristic. It is also feasible to regulate the movement of optical energy by using optical communications across different domains. Nanoscale photo-processes can be utilized to construct nanoscale sensors and actuators using nanoscale sensors and actuators created through nanolithography (Malik et al.; Chen et al., 2013).

The ability to confine photo-processes to precisely defined nano-regions is a critical aspect of nanofabrication, allowing the fabrication of objects with precise geometry and arrangement. The subsequent section will serve as an introduction to the core concepts of nanophotonics by outlining the confinement of electrons and photons as well as the limitations on electrons and photons triggered by electronic and optical interactions in the nanoscale range (Saleh and Teich, 1991; Prasad, 2004).

2.2 Confinement of electrons and photons

By reflecting or backscattering photons and electrons along their paths of propagation in areas of changing interaction potential, one can dimensionally confine the propagation of these particles to a single direction or set of directions. In confinement zones, electrons and photons are completely constrained, in accordance with classical physics. Because the amount of electron energy stuck inside due to potential energy limitations is less than the amount of potential energy due to the barrier, they remain completely enclosed within the walls. On the other hand, the wave representation of photons and electrons does not support this. While a waveguide resonator can act as a limiting field, one can visualize the detention of photons by gathering light in a setting with high surface reflectivity and a high refractive index (Saleh and Teich, 1991). The basic example of light detention (trapping) caused by total internal reflection utilizing a beamline is shown in Fig. 3 (Prasad, 2004). In contrast to fiber or channel waveguides, in which confinement occurs in both the × and y directions, a planar waveguide only experiences confinement along its vertical x-axis. By associating the refractive indices of leading and adjoining mediums, a 3D representation of an optical material that blocks light from all angles is called a microsphere. Therefore, disparity n₁/n₂, where n₁ and n₂ are refractive indexes in medium 1 and 2, respectively, acts as a scattering potential and prevents the propagation of light. In contrast, in both the propagation along the z-axis and confinement directions, the spatial contour of the electric field circulation is distinct (Prasad, 2004).

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Fig. 3

Photon and electron detention with z-axis-propagated confinement in various dimensions (Prasad, 2004).

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2.3 Nanoscale constraints on optical interactions

Optical fields in the nanoscale regime can be manipulated by utilizing lateral and axial localization methods, which are further sub-categorized into multiple methods (see Fig. 4). Nanoscale optical interactions will be initiated by the constraining of a photon's accompanying electric field (electromagnetic field) in a variety of geometries.

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Fig. 4

An overview of detaining optical interactions at the nanoscale regime (Iqbal et al., 2021).

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2.3.1

2.3.1 Axial nanoscopic localization

Axial nanoscopic localization uses SP and transitory wave (evanescent wave) techniques, which are covered in more detail below.

1. Evanescent Wave

Evanescent waves, which are less common in optical science, are wavering magnetic or electric fields that do not spread like electromagnetic waves but accumulate spatially localized energy close to their source. Even though the non-zero portions of the pointing vector can have many different forms averaged over one oscillation cycle and they also don't contribute to energy transfer in that direction. Evanescent waves can also be seen in sound and quantum–mechanical waves, among other waveforms. Under similar conditions, a light field can be split into an evanescent half and a spreading half. As the waveguide surface approaches a medium with an evanescent wave having a lower refractive index, the evanescent wave, whose size ranges from 50 to 100 nm, in the axial direction exponentially decreases away from the waveguide.

The evanescent field can be exploited to create nanoscale optical interactions, and it is possible to drive evanescent waves as a strong near-field fluorescence source-detector (Prasad, 2003). Another example of nanoscale optical interaction is shown in Fig. 5, in which an evanescent wave connects two waveguides (see Fig. 5a). As directional couplers in a network of optical communications, these waveguides with evanescent wave coupling can be favorably used to transmit signals by launching photons from one waveguide to the next. Evanescent wave-coupled waveguides are also suggested in sensor applications where their mechanism involves the transfer of photons between waveguide channels (Iqbal et al., 2021; Prasad, 2004).

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Fig. 5

(a) Evanescent wave-coupled waveguides (Prasad, 2004), and (b) Principle of total internal reflection (Martin-Fernandez and Clarke, 2012).

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The propagation of light via a prism with an n₁ refractive index to an environment with an n₂ refractive index is an additional explanation of the geometry that yields an evanescent wave and is referred to as “complete introspection” (Prasad, 2004; Prasad, 2003; Martin-Fernandez and Clarke, 2012). As seen in Fig. 5b, the light beam is reflected off the interface when the incidence angle exceeds a critical angle. Light refracts when the incidence angle is modest enough, partially passing through the second medium.

• Surface Plasmon Resonance

The surface plasmon resonance (SPR) approach is essentially an extension of the evanescent wave interaction that was discussed previously, with the prism being changed to a waveguide or the metal-dielectric interaction. Electromagnetic waves are also known as SPs across the surface of a metal layer and an organic dielectric material (Treebupachatsakul et al., 2021; Fillard, 1996). Since SPs in a metal sheet are spread out as a wave vector using a certain frequency band, no light can pass through any medium, and no immediate SP activation can be accomplished.

Attenuated absolute reflection (ATR) is the method used most frequently to generate an SP wave, as portrayed in Fig. 6. The Kretschmann structure of ATR (see Fig. 6b), which is frequently employed to produce SPs besides Otto geometry (see Fig. 6a), involves incident light passing through a prism with a high index of refraction, causing the light to internally reflect at the metal/prism interface (Treebupachatsakul et al., 2021). Total internal reflection also generates a wave that travels across the metal/prism interface and penetrates a thin layer of metal. The incident light angle is adjusted to match the evanescent wave propagation rate with the surface plasmon polariton (SPP) propagation rate (Kretschmann, 1971; Daghestani and Day, 2010).

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Fig. 6

Excitation of Plasmons using: (a) Otto geometry, and (b) Kretschmann geometry (Treebupachatsakul et al., 2021).

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2.4 Electronic interactions confinement at nanoscale regime

This section illustrates a few fundamental instances of electronic interactions at the nanoscale regime that significantly alter the material's optical characteristics. There are several parameters that regulate electronic interactions and play a key role in the outcome of their confinement in the nanoscale regime. Herein, we discuss quantum confinement and Stark’s effect to understand electronic interactions at the nanoscale regime.

2.4.1

2.4.1 Effects of quantum confinement

Quantum confinement occurs due to the energy difference between bandgap and energy states when one of their dimensions is under 5 nm, which modifies the optical and electrical characteristics of semiconductors in several ways and results in their real-world uses. The main reason for the quantum confinement effect is that a semiconductor's length is constrained to a length of the same order as the exciton radius, approaching the de Broglie wavelength, hence causing energy quantization with the restriction of particles’ motion that follows quantum mechanical motion rather than classical motion. The schematic representation of quantum confinement in a QD and the bulk system is presented in Fig. 7.

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Fig. 7

Schematic representation of quantum confinement, presenting the difference between bulk and QD structure.

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Constraining electrons to a narrow semiconductor surface reduce their dimensionality, which dramatically improves the characteristics and behavior of excitons. Depending on the purpose of confinement, QDs, quantum wells, and quantum wires are three distinct types of quantum-confined structures that have so far been characterized (Prasad, 2004). The optical changes can be brought on by various transitions, such as intra- and interband, and these confinements are listed in Table 1.

Table 1 An optical transition caused by electron confinement in a quantum state in the nanoscale regime.

Luminescence Effects	Absorption Effects
Interband The transition occurs between modified valence and conduction bands	Photoluminescence Optically excited emission
Intraband The transition occurs between quantized sub-bands of a band	Electroluminescence The emission is generated by the recombination of electrically inserted electrons and holes

2.5 Nanoscopic interaction dynamics

It is feasible to optimize complex radiative transitions by managing the dynamics of regional interactions in nanostructured media using a suitable model. Regional interactions reduce multi-phonon excitation energy relaxation and increase the emission efficiency of rare-earth ions. On account of the extreme susceptibility of rare-earth ions to electronic transitions to nanoscale contacts, it is possible to change the presence of electronic interactions by simply adopting a nanocrystalline medium. Such a change makes glasses or plastics suitable for a variety of device technologies. Novel optical transitions resulting in enhanced optical communications can be produced by nanoscale electronic contacts between two electronic centers (Prasad, 2004). This subsection provides a brief overview of the interaction dynamics that took place in the nanoscale regime.

3.1 Devices with active nanophotonics

The most recent techniques in nanophotonics, including lasing and signal amplification, have been realized by utilizing active materials such as metamaterials, NPs, and plasmonic waveguides. It is significant to highlight that the term “active” in nanophotonics refers to the control or reorganization of plasmon propagation by manipulation of the properties of materials, such as in phase-change materials (Jeong et al., 2020). Researchers are interested in nanolasers and SP amplifiers as they allow applying the theory of coherently stimulated emission up to and beyond the diffraction limit. The concept of surface plasmon amplification by stimulated emission of radiation (SPASER) (Stockman, 2010) involves the amplification of oscillating localized surface plasmons (LSPs) within metal NPs as the original goal of the technique of SPASER, which was later expanded to include moving SPPs. They have a significant role in sensing, biology, and super-resolution imaging applications, in addition to being important in the processing of optical and electronic data (Krasnok and Alù, 2020).

3.2 New gain materials

The main methods involving the generation of optical gain in active systems are gain materials and parametric amplification via nonlinear effects. The configuration of a device with active nanophotonics is shown in Fig. 9. In this arrangement, a resonator loss has been reported to be bordered by an active substance (see Fig. 9a-e). To investigate this configuration, bulk gain materials such as halide perovskite, chromosphere, QDs distributed with a gain material layer, dielectric or metallic NPs, or encircling a nucleus core may be employed (Krasnok and Alù, 2020). Material nonlinearity has also been efficiently utilized in metamaterials (Boardman et al., 2011) and nanostructures (Zhang et al., 2016).

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Fig. 9

Various methods involved in active nanophotonics. (a) A resonator (blue) bounded by gain material (red); (below) the Jablonski illustration for molecular fluorescence excitation and energy transfer from gain material to the resonator; (b) intrinsic gain or encapsulated gain material in a resonator; (c) parametric amplification through nonlinear processes; (d) coherent amplification; and (e) the layout of a PT-symmetric nanostructure in the form of two resonators with gain (red) and loss (blue) (Krasnok and Alù, 2020).

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The compensation or correction of energy loss in a cavity can be carried out by optical parametric amplification, whereas in plasmonics it is carried out by coherent amplification. Optical parametric amplification is based on pulse amplification inside a cavity using positive interference, while coherent amplification, which has lately been proposed, yields a good gain (Krasnok and Alù, 2020). Fig. 10 shows the material gain parameters that can be achieved with various prospective nano-laser materials, including QDs, quantum wells, transition metal dichalcogenides (TMDCs), and perovskites (Krasnok and Alù, 2020).

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Fig. 10

Overview of rare-earth doped materials and other materials' achieved gain parameters (Krasnok and Alù, 2020).

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3.3 Metasurfaces in nanophotonics

Metasurfaces are artificially engineered surfaces that enable control of electromagnetic fields, offering functionalities not possible with natural materials. These surfaces enable precise control over the phase, amplitude, and polarization of light at subwavelength dimensions. They are made up of subwavelength structures, such as nanorods and nanoholes that act as scatterers. By creating negative relative permittivity and permeability values, negative refraction can be achieved. When combining double-negative media and positive-indexed materials, metamaterials can be used as a phase compensation medium. A negative refractive index of −1 can be used to create a super-lens that surpasses the limitation of focusing to a square of wavelength by guided wave optics (Zhang and Liu, 2008 Jun).

Additionally, metasurfaces can create a near-zero refractive index, in which one or more parameters, such as relative permittivity or permeability, are close to zero. Finally, metasurfaces are capable of enhancing the nonlinear refractive index (Caspani et al., 2016 Jun 8). Metamaterials can be utilized to achieve an invisibility cloaking effect, which was demonstrated by Galutin et al (Galutin et al., 2017 Sep 21). They showed that waveguides can be used to achieve invisibility cloaking by a metasurface overlayer to alter the scattering fields of an object located on the cloak so that they do not interact with the evanescent field, resulting in the object becoming invisible. The modal distribution and surface intensity were studied in a channel photonic waveguide with a cloaking metasurface overlayer (see Fig. 11), and the composite plasmonic waveguide structure is depicted in Fig. 11a, which is placed on the composite plasmonic waveguide and aids in light confinement in the range of the metasurface boundary and facilitates coupling to the hybrid plasmonic modes. Light manipulation can be achieved through the use of engineered effective permittivity, which reduces the scattering effect. The transformed grids and refractive index of the evanescent field waveguide cloak are shown in Fig. 11b. The surface intensities of the waveguide evanescent field cloak on a chip are depicted in Fig. 11c, with the reference waveguide shown at the top and the cloak with the cylindrical object shown at the bottom (Galutin et al., 2017).

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Fig. 11

Nanophotonic devices and their applications in various fields. (a) Schematic of a plasmonic waveguide, (b) Grids of an evanescent waveguide, (c) Surface intensities of evanescent waveguide cloak (Galutin et al., 2017), (d) Schematic of a silicon-on-insulator waveguide to study the mode conversion effects, and (e) Calculations of mode conversion devices (Greenberg and Karabchevsky, 2019).

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Metasurfaces engraved on a waveguide can be used to exhibit mode conversion effects. Greenberg and Karabchevsky demonstrated mode conversion through the use of dielectric metasurfaces engraved in the silicon waveguide (Greenberg and Karabchevsky, 2019). In order to efficiently couple between the m^th and n^th modes in a waveguide, an effective wavevector k_eff must be provided by the metasurface to overcome the propagation constant mismatch. They proposed a periodic index perturbation along the propagation direction in a silicon-on-insulator (SOI) waveguide strip waveguide, as shown in Fig. 11d. They calculated the mode conversion efficiencies of the TE₀-TE₁ (E_y component) and TE₀-TE₂ modes, as well as the corresponding refractive index profile and mode evolution along the propagation direction, as shown in Fig. 11e. They achieved a high conversion efficiency of 95.4% and 96.4% over interaction distances of 8.91 μm and 6.32 μm, respectively. The coupling coefficient changes sinusoidally as a function of the interaction distance, which is important for efficient energy transfer from one mode to another.

Zhan et al. designed and tested a cubic phase element based on a metasurface and an Alvarez lens. It was the first Alvarez lens known to be built on a metasurface, so they believed that this metasurface platform was practically perfect for both changing present freeform optical components and constructing new classes of arbitrary spatial phase profiles. Furthermore, this platform possesses the hitherto unheard-of capability of integrating freeform optical components at the micrometer scale, resulting in incredibly small optical systems (Zhan et al., 2017).

3.4 Nanophotonics for nanolasers

Optical near-field and nonlinear optical interactions can be greatly improved at the nanoscale using metallic plasmonic structures like NPs, optical diffraction gratings, and nanoapertures. In nonlinear optics, these structures serve three functions: (i) to enhance nonlinearity's effects; (ii) to make nonlinear components shorter; and (iii) to respond extremely quickly, enabling optical signals to be processed quickly (Iqbal et al., 2021; Kauranen and Zayats, 2012).

Surface plasmon polaritons are supported by metal-dielectric surfaces in plasmonics (Zayats et al., 2005). At the interface between the two mediums, these regional or spreading modes are followed by tight geographical constraints and an increase in the electric field. Such constraints and field enhancement produce significantly dispersed surface waves with p-polarization, which are connected to alternations of free electrons in metals. Since SPPs are nearly confined, changing the geometry of the dielectric properties and the plasmonic nanostructure characteristics of embedding optical media will have a significant impact on metal-dielectric interface surface effects. With the development of plasmonics, it has been reported that SPP modes can limit the size of lasers to sub-wavelength dimensions. In these plasmonic-based devices, lasing is accomplished through the population inversion of emitters such as QDs and fluorophores as well as feedback produced by plasmonic resonant structures. A family of nanolasers that piqued the interest of the nanophotonics community was created as a result of the development of novel forms of nano-resonators and active materials (see Fig. 12) (Jeong et al., 2020; Krasnok and Alù, 2020; Wu et al., 2015; Zhang et al., 2014; Yu et al., 2013). Fig. 12a portrays nanolasers based on 2D TMDC, while an organic–inorganic perovskite-based lasing system containing a schematic illustration and output is shown in Fig. 12b. Metal-organic frameworks are also reported to be employed in nanolasers (see Fig. 12c).

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Fig. 12

A description of the nanophotonic materials for nanolasers: (a) 2D TMDC (Wu et al., 2015), (b) organic–inorganic perovskites (Zhang et al., 2014), and (c) metal–organic frameworks (Yu et al., 2013).

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3.5 Plasmonic metamaterials

Plasmonic metamaterial has the ability to enable all-optical switching by tuning the plasmonic resonances of the components and the electromagnetic coupling between them. The embedding dielectric's or substrate's refractive index can influence distinct plasmonic resonances as well as interactions between them. Nonlinear reactions along this line can be improved by changing the refractive index. Plasmonic metamaterial provides a novel way of enhancing nonlinearity by leveraging epsilon-near-zero domain nonlocal effects. Plasmonic excitations can produce efficient all-optical modulation by controlling the binding strength of the molecular exciton (Kauranen and Zayats, 2012). LSPs are essential for a wide variety of exciting applications, including renewable energy (such as solar or water splitting) and photothermal cancer therapy. Such plasmons act as a main factor for estimating metallic NPs' ability to manipulate and harvest light on the subwavelength scale by supporting electronic resonances.

After the NPs are irradiated at the femtosecond period, SPs produce electromagnetic hotspots. These hotspots can enhance nonlinear optical techniques like second harmonic generation and third harmonic generation for high-resolution imaging applications. It is also feasible to produce hot electrons, which can be used to split water and power solar cells. Phonons are generated by lattice heating at the picosecond timescale, enabling nanometal working techniques. The environment is heated later to enable applications for magnetic storage, photothermal therapy, and nanorobotics at the nanosecond timescale. The cycle might then continue as the atmosphere cools (Kuppe et al., 2020). A schematic for temperature-dependent applications of photonic nanostructures is shown in Fig. 13.

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Fig. 13

Temperature dependant plasmonic applications of photonic nanostructures (Kuppe et al., 2020).

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Both theoretically and practically, semiconductor optical amplifiers (SOAs) based on QDs show rapid gain dynamics and amplification without pattern effects (Uskov et al., 2004). It is common to use mode-locked (ML) lasers for temporal domain multiplexing for high-frequency applications, such as optical comb generators, because of their low alpha factor and wide range of unprompted emission from QD gain medium (Kuntz et al., 2006). Plasmonic nanostructures can be potentially applied in a variety of applications and Fig. 14 depicts an overview of plasmonic photonic nanostructures for possible applications in light harvesting, photodetection, and optical sensing (Lin et al., 2020).

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Fig. 14

Overview of plasmonic nanostructures for light harvesting and optical sensing (Lin et al., 2020).

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4.1 Biosensors

Nanophotonic biosensors may facilitate more comprehensive, detailed, rapid, and accurate diagnosis of human disease and more precisely targeted healthcare. The field of precision medicine is now growing. New medicines for eradicating diseases and effective personalized therapeutics for malignancies appear to be discovered and put into use almost daily. Precision medicine is the best treatment for the effects of autoimmune, cardiac, and degenerative diseases, as well as cancer. Nanophotonic biosensors take advantage of light's special characteristics to provide some of the most accurate, durable, and dependable sensing systems in the field. Early detection of viral outbreaks is critical for epidemic containment, just as early cancer detection is critical for effective treatment and patient survival.

Biosensors are one of the strongest tools. These self-integrated devices collect and find the presence of a target molecule or analyte in a sample using certain biorecognition components, such as antibodies, DNA strands, or enzymes. This interaction results in a change in a transducer's optical property that can be measured and is directly correlated with the concentration of the analyte. Some of the most sensitive, dependable, and durable sensing systems that are currently available can be made by using nanophotonic biosensors, which utilize the special qualities of light (see Fig. 15) (Soler et al., 2020; Polat et al., 2022).

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Fig. 15

A biosensing design that allows the extracted signal to be viewed or recorded on any portable device with the purpose of tracking health (Polat et al., 2022).

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The biosensing materials offer a low detection limit of analytes by employing high optical sensitivity that has various biomedical applications, wherein the highly exposed surfaces enhance the immobilization rate of various bio-receptors. A biosensor is normally made of bio-receptors, transducers, interfaces, and signal processors, as illustrated in Fig. 16 (Li and Liu, 2017), while optical biosensors include optical transducers and bio-receptors. The bio-receptors are subjected to physical and chemical variations in the transducers that result in varying light properties such as fluorescence, reflection, refraction, absorption, phase, and frequency variations (Bharmoria and Ventura, 2019).

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Fig. 16

Schematic of optical biosensors and variations in light characteristics, (a) without analyte, (b) with an analyte (Bharmoria and Ventura, 2019).

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Due to the great sensitivity of this evanescent field to variations in the medium's refractive index, changes in composition or mass at the metal-dielectric interface are required to immediately affect changes in light's intensity, wavelength, angle, or phase. Real-time analysis of these fluctuations during a biorecognition event such as a contact between an antibody and an antigen yields quantitative data on the concentration of the molecule as well as the affinities and kinetics of the biomolecular interaction. The decay period of the evanescent field in SPR and dielectric waveguide-based biosensors is on the order of 200–400 nm, which is significantly longer than that of the majority of biomolecular analytes. Consequently, interactions between the analyte and the penetrating light are essentially limited by standard evanescent-field-based sensors (Soler et al., 2020). The resonance properties involve controlling the properties of the optical phenomena that cause resonances as well as the design characteristics of nanostructures. Resonances supported by metallic and dielectric nanostructures are widely employed in the majority of nanophotonic biosensors (Altug et al., 2022).

The majority of optical biosensors are based on the evanescent-field concept and typically employ silicon photonics and nanoplasmonics. Evanescent-field biosensors provide a quick, easy, and non-invasive method for analyzing biochemical processes or quantifying analytes since they can detect biomolecular interactions occurring at the sensor surface in real time and without labels or dyes (see Fig. 17) (Soler et al., 2020).

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Fig. 17

A common nanophotonic biosensor has nanostructures on it, which are sensitive to incident light field. Changes in refractive index at the interface brought on by biorecognition events, such as interactions between antibodies and antigens and are then read from the enhanced evanescent field (Soler et al., 2020).

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Nanomaterials are also used as donors and acceptors for biosensors (Shi et al., 2015; Zhao et al., 2013; Zhang et al., 2005), wherein Zhang et al. used CdSe-ZnS QDs coated with streptavidin for DNA detection, making the semiconductor material act as a donor, while Cy5 is used as a dye material in their experiment. This hybridized structure showcases the FRET effect by explaining the very distinct signals owing to the transformation of energy from the CdSe-ZnS semiconductor to the Cy5 dye, as presented in Fig. 18. This nanosensor showed extraordinary sensitivity for DNA, having a detection range of around 4.8 femtomolar (Zhang et al., 2005).

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Fig. 18

Schematic of DNA sensing using QDs via FRET (Zhang et al., 2005).

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SPR biosensors also lie in the category of optical biosensors that operate on the linear optics principle, dealing with the oscillation electrons’ resonance with opposite permittivity that generates a non-radiative electromagnetic wave at the material interface, which is further propagated in the same direction of permittivity, possessing a conducting material interface, and eventually traveling parallel to the interface, as depicted in Fig. 19 (Sabban, 2011). The SPR oscillations are very sensitive to the variations occurring at the material interface, which is a result of the conducting nature of the material that can be eventually employed in biosensing applications. Moreover, SPR-based biosensors normally measure variations in the refractive index of the material. When the sensors are irradiated, the incident light reflects at an SPR angle, where the light is reduced to its minimum level by passing its energy to the NP surface, which causes the oscillation of electrons as a result of excitations called plasmons. The field vector of this excited plasmon is sensitive to environmental variations due to biomedical adsorptions; hence, upon the binding of biomedical to the sensor, the refractive index of the sensor surface changes, which eventually changes the SPR angle owing to the variations in the electric field vector of plasmons (Sabban, 2011).

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Fig. 19

Schematic description of an SPR-based biosensor (Sabban, 2011).

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4.2 Optical nanoparticles and photonic devices

Optical NPs are used most frequently in relatively low-tech items like sunscreen creams and optical coatings owing to their tuned properties (Zvyagin et al., 2008; Krogman et al., 2005; Moghal et al., 2012; Moiseev, 2011). The manipulation of optical nanostructures for desired applications is an intriguing innovation for the future. Despite the recent development of numerous high-tech industrial solutions, such as sensors for identifying and reacting to biological and chemical concerns, photonic crystals are still used in signal processing and intricate optical circuitry. The most widely used optical nanomaterial applications at the moment are in these low-tech products (Malik et al., 2022; Prasad, 2004). The development of advanced solar cells based on the NP concept in the future may bring about several profitable businesses (Khan et al., 2019). Nevertheless, they are still in the initial phases of development. Fig. 20a–h shows various 2D nanostructure-based devices that have been potentially useful for photonic and optical applications (Malik et al.; Iqbal et al., 2023; Yu et al., 2018; Zhang et al., 2018; Deng et al., 2018; Xu et al., 2019).

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Fig. 20

2D heterostructure photodetectors. (a) schematic of hybrid graphene/Ti₂O₃ photodetector (Yu et al., 2018) and, (b) responsivity spectra, (c) PbI₂-photodetector, (d) photoresponse spectra (Zhang et al., 2018), (e) Graphene-MoS₂ photodetector, (f) photoresponsivity (Deng et al., 2018), (g) schematics of PtTe₂ photodetector, and (h) responsivity spectra (Xu et al., 2019).

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The current atomic trap mentions the potential of tailored light to trap atoms in the material risen by evanescent fields as a result of photonic waveguides as the basic building block of the advanced technology of ultra-cold atoms used for quantum devices. Evanescent fields that are responsible for optical lattices and atomic trapping already have the potential to be used in optical fiber (Vetsch et al., 2010) as well as waveguides (Meng et al., 2015). The dense lattice having atomic trapping at the nanoscale range was not practically done in the past and from an experimental point of view, this will allow atomic and photonic interactions. Moreover, the waveguides can also be integrated on the same chip with the other components that are used for other atomic properties, such as atomic detections (Aoki et al., 2006), manipulations, and single photon operations (Rosenblum et al., 2016).

Integrated waveguides offer inherent properties that are not achievable through other atom-trapping methods. For instance, they allow for subwavelength-scale separation between adjacent traps and the design of curved structures, which is not feasible with atom trapping in optical lattices or magnetic trapping and guiding using current-carrying wires on atom chips (Haller et al., 2015). In integrated waveguides, atoms are trapped using evanescent, homogeneous light fields, eliminating fragmentation or reduced lifetime caused by technical noise from current sources. Furthermore, atoms can be brought closer to the chip surface without experiencing the Johnson noise generated by the metallic surface (Henkel et al., 2003). A proposed platform for collective interaction between cold atoms and light utilizes a nano-waveguide composed of a silicon nitride core that utilizes evanescent field coupling with mirrors. Bi-chromatic evanescent field atom traps are used to secure atoms at a minimal distance of 150 nm above the waveguide.

Another similar system was proposed in which a silicon nitride ridge waveguide with a nano-antenna on top is used to trap atoms using an all-dielectric device. The toroidal nanoantenna allows for the trapping of atoms at a distance of approximately 70 nm from the surface. These findings are shown in Fig. 21a–c (Ang et al., 2020). Additionally, it has been demonstrated that NPs are helpful in projects involving optical communication, light-activated therapies, and optical diagnostics (Malik et al., 2022; Prasad, 2004). Fig. 21d displays a glimpse of optical nanostructures for possible applications in diagnostics and therapy (Iqbal et al., 2021).

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Fig. 21

(a) Presentation of a rigid waveguide, (b) Iso-surface at zero-point potential, and (c) Potential dependence on z (Ang et al., 2020). (d) A glimpse of the potential applications of optical NPs (Iqbal et al., 2021).

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4.3 Photovoltaic technology

In recent decades, the use of renewable energy has increased dramatically. The main causes of this are global warming and the depletion of fossil fuels. There are many different types of renewable energy sources, but some of the most well-known are solar power, hydropower, tidal power, wind power, and nuclear power. Among these, solar energy has emerged as one of the most popular. This energy is environmentally friendly, safe, involves very little site modification, has little or no impact on the environment, and has many other benefits. The use of nanophotonics in the production of solar energy is one of the advancements (Bharmoria and Ventura, 2019). Nanophotonics in solar power involves either directly generating electricity from the sun's energy or using it to heat water. Photovoltaic cells are used to collect solar radiation and turn it into electrical energy. These cells use electromagnetic radiation, often light, to generate electricity and are completely mechanical. They operate on the basis of the photovoltaic effect, which is the mechanism by which voltage and current are produced in a solar cell, also known as a PV cell (Williams, 1960).

Photovoltaics is the conversion of light into electricity by employing semiconductor materials with the involvement of a physiochemical process where electrical current is generated along the p-n junction of a semiconductor due to the light absorption, as depicted in Fig. 22 (Williams, 1960). Furthermore, Edmund Becquerel discovered an electrolytic cell that consists of two metal electrodes, while further on, the photovoltaic system is considered to be composed of solar cells for the generation of electrical power (see Fig. 22a). With the discovery of silicon solar cells in 1954 (Chapin et al., 1991), this kind of solar energy gathered scientists’ interest because 90% of the solar cells around the globe started using the same silicon-based technology. Moreover, the abundance of silicon on earth is also a positive aspect of using silicon as an efficient material. However, apart from its various advantages, the contribution of solar energy to electricity production was around 1.8% by 2016, which is anticipated to be 2.8% by 2018, as illustrated in Fig. 22b (Renewables, 2018; Bharmoria and Ventura, 2019).

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Fig. 22

(a) A schematic description of photovoltaic effects in solar cells, and (b) A glimpse of solar photovoltaic generation by region from 2017 to 2023 (International Energy Agency 2018) (Bharmoria and Ventura, 2019).

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In high-performance solar cells, light management is crucial. Light-trapping nanostructures have the potential to significantly improve solar cell conversion efficiency, which has significantly decreased the use of materials. Creating affordable, large-scale nanostructures that can be integrated with solar cells provides new opportunities for high-efficiency and affordable solar energy production (Chapin et al., 1991). A challenging issue in nanophotonics is increasing light absorption while simultaneously shrinking the size of PV cells to the nanoscale. Compared to such thin cells, conventional cells are larger and absorb more light. However, as the amount of material utilized increases with size, the cost of the cell does as well. The maximum amount of light that can theoretically be absorbed by a substance is 4n², where n is the substance's refractive index. This can be determined by modeling light as a straight ray. However, trapping light beyond its critical angle can cause it to remain trapped inside the material for a longer period of time, boosting the material's ability to absorb light, known as “light trapping” (Yu et al., 2010; Peters et al., 2009). Using thin-layer photovoltaic cells, light trapping can be accomplished using nanophotonics. The duration of the photon’s interactions with the substance determines how much light is absorbed by semiconductor electrons. The refractive index and absorption coefficient of the substance affects how long absorption takes. Light trapping can extend the time that light has to travel through the material (see Fig. 23) (Yu et al., 2010).

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Fig. 23

Light trapping using a grating structure and random texturing: (a) light trapping caused by a surface with an irregular texture, (b) a periodic grating on a back-reflector is used to capture light, and (c) absorption spectra of the structure's waveguide modes (transverse magnetic mode, normal incidence) and their relation to dispersion (Yu et al., 2010).

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Nanomaterials applications are not limited to silicon solar cells. They are also used for dye-sensitized solar cells (DSSC), which are reported by Gratzel for the very first time to present a suitable alternative to conventional silicon-based solar cells (O'regan and Grätzel, 1991). DSSCs are known as potential alternatives to normal solar cells because of their remarkable physical and optical properties, as well as their easy-to-do fabrication, coloring, transparency, and ability to operate in minimal light conditions, such as indirect light irradiations. Typically, a photosensitizer dye is adsorbed on the material surface, on which the irradiated photon absorbs, which causes the dye electrons to excite to a higher energy state, which is then injected into the material’s conduction band, hence causing the oxidation of the photosensitizer dye. Moreover, the conduction electron is transported to the conducting glass that is attached to the semiconductor material through the circuit system. The oxidized photosensitizer accepts electrons from the redox mediator to regenerate the ground state. Fig. 24a depicts the working mechanism of DSSC (Carella et al., 2018). However, improving the DSSC efficiency is considered a challenging task because of the low absorption of these solar cells due to their high bandgap, which is higher than 1.8 eV with an absorption light limit of < 700 nm, wasting 52% of solar energy in the near IR range. The unemployed IR photons are used to be converted into high-energy photons to be absorbed by the dye using the photon upconversion technique, demonstrated in Fig. 24b (Shang et al., 2015). Our group also recently reported enhanced efficiency of DSSCs using rare-earth metals, including neodymium, praseodymium, and gadolinium doped with bismuth ferrites (Khan et al., 2023).

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Fig. 24

(a) Typical model and working principle of DSSC (Carella et al., 2018), and (b) DSSC fabricated by upconversion nanomaterials (Shang et al., 2015).

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4.4 Biomaterials

Material scientists discovered that biological structures provide a rich environment for the development of novel nanotechnologies with a wide variety of application domains. It is possible to create nearly perfect nanostructures using bioprocesses, which are recyclable and good at filtering, low-threshold lasing, high-density data storage, and optical switching. In a variety of biomaterial-based systems and an extensive array of photonic systems, nanophotonic devices, both active and passive ones, can be used (Prasad, 2004).

The molecular-level control that bio-derived materials have over their characteristics, both at single-molecule and bulk scales, is unmatched. Designing and creating bio-derived materials that are made of proteins, carbohydrates, and lipids requires the application of interdisciplinary techniques. The discipline of biomimicry, which is expanding, focuses on producing multifunctional ordered materials as well as morphologies that are inspired by nature and can operate as a crystalline structure for collecting light. The synthesis of natural biological materials is based on emulating biological principles to make artificial nanostructures.

Bio-templates are naturally occurring nanostructures with appropriate surface interactions as well as morphologies that can serve as models for the development of materials that are multiscale and multi-component for photonics. They offer reinforcing interfaces for optically active structures that can themselves self-assemble. Metabolically manufacturing photonic polymers in bacteria-based bioreactors, which modify the organically produced bacterial biosynthesis pathway to create an assortment of helical polymers with various optical characteristics.

The progress of implanted biophotonic devices for biomedical applications is still in its early stages. Light is now routinely used in surgery, therapy, and diagnostics. The pulse oximeter, which calculates the patient's arterial oxygen saturation, is always employed in emergency medicine. To obtain images of the upper gastrointestinal system, patients frequently swallow the wireless capsule endoscope. Wearable health monitoring can be enabled by emerging flexible optoelectronic technologies like quantitative imaging of skin temperature and thermal characteristics (Peter Amalathas and Alkaisi, 2019). The great majority of photonic applications in medical settings, however, call for the utilization of external light sources, restricting their usage to superficial tissues like the skin and eyes. Implantable photonic devices for human use are likely to develop as biocompatible photonic materials continue to be developed. These devices will allow for long-term sensing and imaging of hard-to-reach areas, like the brain (Gao et al., 2014; Kozai et al., 2012), as well as the expansion of therapeutic applications like blue-light antimicrobial treatment, low-level light therapy, and photodynamic therapy beyond the surface of the skin to deep tissues.

Opportunities for light-based cell therapy and sensing with an apparatus, such as optogenetics and intracellular lasers, have been made possible by the capacity to make cells photoactive. Manipulation of light-tissue interactions is noticeably more varied and involves a variety of photonic materials, from semiconductors and polymers to organic molecules and cells, as opposed to electrical stimulation of cells. There are many factors that should be considered while designing biophotonic devices, as all these factors are crucial for determining the performance of these devices. Fig. 25 summarizes these factors that should be considered while designing biophotonic devices (Humar et al., 2017).

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Fig. 25

Overview of factors to be considered while designing biophotonic devices (Humar et al., 2017).

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4.5 Nanomedicine and biotechnology

Research into the fundamental processes and interactions occurring at the single cell or molecule level, in addition to uses of light-energy and light-activated technologies in nanomedicine analysis, are just a few of the many biomedical research and technological uses for nanophotonics. In the rapidly developing field of nanomedicine, NPs are employed to create novel noninvasive diagnostics for the early identification of disease, in addition to enabling targeted drug administration, treatment effectiveness, and real-time drug monitoring.

A comprehensive knowledge of how drugs interact with cells is necessary with a focus on molecular changes within a single cell brought on by the early stages of the disease (Liu et al., 2007). This will allow for the development of personalized therapeutic management for disease control, which focuses on molecular identification. To map drug ingestion, describe a cellular function, and track future cytosolic interactions, nanophotonics uses optical methods. This increases the value of optical probe-based biosensing, bioimaging, and single-cell bio function research. The use of light-guided and light-activated therapies has considerably increased molecular disease detection in nanomedicine. Modern technologies for processing NPs require sophisticated carrier groups, optical probes, and light-activated therapeutics that can guide them to diseased tissues or cells for targeted drug administration in real-time drug efficiency monitoring (Malik et al., 2022; Malik et al., 2022; Prasad, 2004). A summary of nanophotonics for the theragnostic is portrayed in Fig. 26 (Conde et al., 2012).

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Fig. 26

A summary of nanophotonics for theragnostic (Conde et al., 2012).

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Maria et al. (Malik et al., 2022) and Rija et al. (Abaid et al., 2023) also worked on the antibacterial and anticancer activities of optical NPs, respectively, that were synthesized with the natural phytochemicals present in the plant extracts. Fig. 27 presents the mechanism employed as a result of synthesized NPs to showcase the zone of inhibition of the active bacterial and cancer cell lines, which ultimately refers to the potential of NPs as an effective entity in the field of biomedicine (Malik et al., 2022; Abaid et al., 2023). Fig. 27a shows that NPs can strike the walls of active bacteria and disrupt their cell walls as well as cell membranes to cause bacterial death. Also, reactive oxygen species generated inside the active bacteria can also lead to bacterial death. Similarly, the almost same mechanism works for anticancer activity in which NPs affect the cancer cell lines’ DNA structure or their wall disruption that causes apoptosis, as depicted in Fig. 27b (Abaid et al., 2023).

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Fig. 27

Schematic mechanism of optical NPs for applications in the biomedical domain. (a) Antibacterial activity (Malik et al., 2022), and (b) Anticancer activity (Abaid et al., 2023).

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The most recent study discusses current advances in image sensing employing nanostructured emerging materials and tackles the integration of nanofabrication as well as the development of technology on both conventional and contemporary technology platforms (Iqbal et al., 2023). It is expected that photonic nanostructures, with proper manipulation and regulation of their size and shape, can further explore a plethora of novel applications in innovative science and technology in the future.