What is Nanophotonics?

Nanophotonics (also known as nano-optics) describes the study of light-matter interactions at nanoscales (billionths of a meter), encompassing the disciplines of optics, optical engineering, electrical engineering, and nanotechnology.

Crucially, these interactions occur at dimensions smaller than the wavelengths of light — typically in the range of 1-100 nanometers (nm) — in which unique optical properties become apparent that are not present at larger scales.

Therefore, nanophotonics includes a diverse range of radiation-matter interactions that extend far beyond the diffraction limits of light — applied in the near-infrared (IR), visible, and ultraviolet (UV) regions of the electromagnetic spectrum (300-1,200 nm) — opening up a range of opportunities in light harvesting, display technology, optical sensing, nonlinear optics, data transmission, and more.

Physicists, engineers, and materials scientists working to advance the field of knowledge in nanophotonics focus on the interactions of light with nanostructures such as metallic nanoparticles, carbon nanotubes, semiconductor quantum dots, photonic crystals, and organic tissue (e.g., DNA). Their principal aim is to develop nanophotonic devices for the efficient control of light.

Far from being a purely modern phenomenon, humans have sought to manipulate the properties of light for centuries. For instance, the tunable colors in stained glass windows found in medieval churches result from the adding of what we now know to be metallic nanoparticles to the glass.

In the modern era, humans have discovered ingenious ways to control the characteristics of light, including its amplitude, phase, polarization, and localization. This is opening up exciting avenues in optoelectronics, optical communication, solar energy harvesting, and other areas of study.

Nanophotonics, however, has only emerged as a distinct discipline in the last two decades, fueled by an explosion in the development of novel metallic, dielectric, and semiconducting nanomaterials.

These materials are particularly attractive because, when combined with modern machine learning, simulation, and computation tools, they can be assembled in all dimensions with near-atomic precision. Furthermore, engineers can leverage the same methods used to manufacture semiconductor devices, rendering their exploitation cost effective.

As a consequence, nanophotonics drives innovation in, for instance:

• Solar cell technology, enabling the fabrication of metallic nanoparticles that display strong absorption characteristics across a wide range of the solar spectrum, improving light trapping efficiencies while reducing device sizes
• Metalens technology, allowing unprecedented levels of light focusing and polarization while also enabling device miniaturization, with promising applications in spectroscopy and sensing
• Medical photonics, enabling ultrafast light modulation for the detection, prevention, and treatment of diseases, thereby enabling remote, non-invasive diagnostics and therapeutics
• Optical computing, providing ultrafast information processing with little consumption of energy

In high-speed data transmission, existing copper lines exhibit signal degradation as circuit lengths increase.

Photon-based circuits — with the high operating speed of photons comparable to that of electrons, but at a much-reduced energy consumption — offer a promising alternative. In mission-critical data centers, for instance, photonic circuits promise to reduce transmission lines from hundreds of meters to mere meters.

Beyond the diffraction limit, it is possible to constrain light to nanometer scales using novel approaches such as surface plasmon polaritons (i.e., constrained electromagnetic radiation) that form around metal surfaces and structures.

Nanophotonics focuses specifically on single-photon interactions with nanostructures in which field enhancement effects occur as electromagnetic radiation becomes confined to nanometer-scale dimensions. These interactions result in novel optical phenomena that can be used to fashion photonic devices that switch, store, and transmit light at nanoscale dimensions — displaying superior characteristics that extend beyond the limits imposed by classical mechanics.

Nonetheless, manipulating light-matter interactions at nanoscales presents significant challenges, requiring the development of new materials, structures, and processes.

Energy localization and nonlinear interactions are key principles influencing the generation of light at nanoscales — for example, spontaneous emission processes such as photoluminescence, electroluminescence, fluorescence, and Raman scattering.

Optical resonators augment these interactions through enhancements of the electromagnetic field. In particular, plasmonic nanocavities provide efficient resonators for the development of emissions-based sensing technologies. In nonlinear optical applications, bulk metals displaying weak nonlinear responsiveness require high-intensity excitations through the application of pumps or lasers to increase nonlinearity.

To control radiation intensities, either integrated photonic cavities may be employed to increase the presence of pumps, or plasmonic nanostructures may be deployed to achieve highly localized energy densities. Quasi-2D surface plasmons provide substantially increased field intensity enhancements and localizations (exceeding 10⁷ within confinements of 20 nm), enabling second harmonic generation (SHG), ideally applied to high-resolution sensing and imaging.

The investigation of photonic nanostructures constitutes the driving force behind the development of nanophotonics, fueling applications in nanomedicine, optical diagnostics, remote sensing, biotechnology, biomaterials, and solar cells.

Confinement Methods in Nanophotonics

Researchers use one of three ways to confine light-matter interactions to nanoscale dimensions:

1. Confinements of light far below the diffraction limit of light
2. Confinement of matter
3. Confinement through photochemistry or a photoinduced phase transition (PIPT)

In terms of matter, different methods are used, leading to exotic structures like nanomers (nano-sized oligomers with size-dependent optical properties) and nanoparticles with unique electronic and photonic properties.

In plasmonics, for instance, metallic nanoparticles have an enhanced electromagnetic field, displaying unique properties — for example, absorbing two infrared photons and converting them into a visible ultraviolet photon.

Elsewhere, photonic crystals are dielectric structures that repeat periodically at a length in the order of the wavelength of light. Further, nano composites are formed from phase-isolated domains of dissimilar materials and are used in optical communications.

Confinement Effects in Nanophotonics

Researchers use a variety of confinement geometries, including:

• Axial localization that relies on the formation of evanescent waves and their interactions with surface plasmons
• Lateral localization, in which a sample is placed within a fraction of the wavelength of incident light emanating from a light source

Evanescent Waves in Nanophotonics

Evanescent waves are oscillating electric or magnetic fields that do not propagate like conventional electromagnetic waves. Instead, they concentrate their energy near their source, without contributing to energy propagation in any direction.

Evanescent waves form when light undergoes total internal reflection at an interface between two media with different refractive indices (such as at a prism-sample interface).

Prisms are typically used to generate evanescent waves that interact with samples to enable measurements. Interestingly, an electromagnetic field can also disintegrate into evanescent and propagating components under certain circumstances.

One advantage of evanescent waves is that they facilitate nanoscale optical interactions — especially in sensing — enabling, for example, strong near-field fluorescence source detection.

Evanescent-wave-coupled waveguides have also been proposed for sensing applications involving energy transfer between waveguide channels. These waveguides may also be used as directional wave couplers in optical communication networks.

Surface Plasmon Resonance (SPR) in Nanophotonics

Surface plasmons (SPs) are collective oscillations of free electrons at metal surfaces. Resonance occurs when the momentum of the incident light matches that of the surface plasmons. In SPR, evanescent waves are formed and coupled to surface plasmons at metal-dielectric interfaces, significantly enhancing light-matter interactions.

Total internal light reflection is achieved with the use of a waveguide (typically a thin metal film on a dielectric substrate) instead of a prism. Attenuated absolute reflection (ATR) is the favored method for generating SP waves.

Surface plasmon polaritons (SPPs) form once light couples strongly to SPs, propagating along a metal-dielectric interface. They are particularly favored for their ability to confine electromagnetic fields to dimensions significantly smaller than the wavelengths of light in free space.

Metal Optics in Nanophotonics

As it happens, metals provide an effective way to confine light below the diffraction limit. This is due to the fact that metals display large negative permittivity at optical frequencies (in the visible and near-infrared regions of the spectrum).

The permittivity (also known as the dielectric constant) is frequency dependent. At frequencies approaching and exceeding the plasma frequency (which lies in the ultraviolet range), it becomes less large and negative, thus becoming ineffective at supporting surface plasmons.

Metals have routinely been used in radio and microwave engineering where, for example, subwavelength metal antennas and waveguides (hundreds of times smaller than the free-space wavelength of light) successfully capture electromagnetic radiation. Following similar principles, light can also be confined to nanometer lengths via the application of nanosized metallic structures such as nanoantennas, nanowires, and nanorods.

In fact, many nano-optics designs resemble microwave or radio wave circuits by employing similar design techniques, such as lumped-constant circuit elements (e.g., inductance and capacitance), metallic parallel-plate waveguides (striplines), and impedance-matching of dipole antennas to transmission lines.

However, important differences remain between nano-optics and microwave circuits: At nanoscale (and at optical frequencies), metals behave much less like ideal conductors, also exhibiting many interesting properties, including surface plasmon resonance and kinetic inductance. Additionally, at nanoscales, electromagnetic fields interact with semiconductors in radically different ways.

Nonlinear Optics in Nanophotonics

As light propagates through nonlinear media — in which the dielectric polarization responds nonlinearly to electric fields — it induces unusual optical effects resulting in phenomena not ordinarily observed. Nonlinear optical effects can be induced with the introduction of metallic metamaterials, with the aim of reducing component sizes and speeding up signal processing.

Particularly at high field intensities (such as those generated by lasers) nonlinear optical effects become significant. This yields new functionalities with important applications to nanophotonics, including:

• Frequency conversion: This occurs when nonlinear optical processes like second-harmonic generation, optical parametric oscillation, optical phase conjugation, and four-wave mixing allow the generation of new frequencies of light, useful in high-resolution sensing and imaging.
• Multiphoton processes: Nonlinear multi-photon processes may yield new ways of controlling light-matter interactions.
• Ultrafast processing: Nonlinear optical effects occur on ultrafast timescales, allowing the high-speed modulation and switching of light.
• Device miniaturization: Nonlinear photonics facilitate device miniaturization, crucial to advancements in on-chip functionalities.
• New materials: In the pursuit of nonlinear enhancements, new materials are being discovered, including glasses, crystals, semiconductors, and, more recently, nanostructured materials. All-dielectric metamaterials are also generating interest, exhibiting lower losses compared to their metallic counterparts.
• Quantum emitters: Nonlinear interactions occur between quantum emitters (such as quantum dots) and nanophotonic waveguides, yielding novel quantum nanophotonic effects with applications in emerging quantum technologies.

As researchers learn to control the flow, phase, amplitude, and polarization of light at subwavelength dimensions, they are able to scatter, refract, confine, and filter this light in fascinating new ways. This opens up new avenues in integrated circuitry, optical computing, biochemistry, medicine, fuel cell technology, solar cell technology, and more.

Below is a summary of the prominent applications in nanophotonics.

Nanolasers

At metal-dielectric interfaces, surface plasmon polaritons can be used to constrain lasers to subwavelength scales. Nanolasing is achieved through a population inversion of emitters such as quantum dots and fluorophores combined with feedback generated by plasmonic resonant structures.

Nanolasers display a variety of desirable properties useful in optical communication, including fast modulation (improving data transmission) and low threshold current (improving power efficiency).

Spasers (surface plasmon amplification by stimulated emission of radiation) are the surface plasmon version of lasers (light amplification by stimulated emission of radiation), involving the amplification of oscillating localized surface plasmons (LSPs) within metal nanoparticles.

Nanolasers and surface plasmon amplifiers are of interest to researchers because they enable coherent stimulated emission up to and beyond the diffraction limit, with applications in high-resolution sensing and imaging, and optical and electronic data processing.

Photodetectors

Photodetectors  play a central role in both optoelectronic and microelectronic circuits due to their ability to detect and convert light into electrical signals, thereby instigating a wide range of applications in devices, including:

• Single photon avalanche detectors (SPADs) able to detect low-intensity signals down to individual photons (with high spatio-temporal resolution) used, for example, in lidar, 3D imaging, and PET scanning
• Fiber-optic communication devices such as indium gallium arsenide (InGaAs) diodes that are able to detect high-speed infrared signals transmitted through optical fibers, enabling data communication rates of up to 2.5 Gigabits per second
• Optical networking devices such as waveguide-integrated near-infrared detectors
• Remote sensing instruments used in astronomy to detect a wide range of wavelengths from the far-infrared to gamma rays
• Environmental monitoring devices used to detect traces of chemical and biological agents
• Photovoltaic cells that convert sunlight into electrical energy
• CCD (charge-coupled device) and CMOS (complementary metal-oxide-semiconductor) sensors, used in digital cameras and other imaging devices
• Access control systems for buildings

All-optical Switching

In plasmonic metamaterials, the tuning of the plasmonic resonances of individual components in integrated circuits — and of the electromagnetic coupling between them — can enable all-optical switching. Plasmonic resonances and couplings can be tuned by altering the refractive indices of the embedding dielectric or substrate, yielding improved nonlinear responses. Controlling the binding power of molecular excitons, plasmonic excitations results in effective all-optical modulation.

Data Storage

In optical data storage, subwavelength near-field optical structures, either embedded into or separated from the recording media, can be used to achieve optical recording densities far below the diffraction limit.

In heat-assisted magnetic recording, a laser heats a subwavelength area of magnetic material before encoding data, thereby increasing the amount of data stored per unit area. The magnetic write-head also incorporates metal optical components that concentrate light.

Silicon Photonics

Silicon photonics involve nanoscale optoelectronic devices embedded on silicon substrates that are capable of directing both light and electrons, enabling the coupling of electronic and optical functionality on a single on-chip device. Silicon photonics fuels innovations in optical waveguides and interconnectors, optical amplifiers, light modulators, photodetectors, memory elements, photonic crystals, and more.

Integrated Circuits

Manufacturers use photolithography to fabricate integrated circuits such as microprocessors and memory chips. During photolithography, various types of light — including ultraviolet, extreme ultraviolet, and X-ray light — are used to transfer nanosized geometric patterns from a photomask onto a photosensitive material called a photoresist (a photosensitive material applied to a substrate, typically a silicon wafer).

Furthermore, the miniaturization of electronic components (such as transistors) within integrated circuits is critical to improving speed and cost efficiencies. However, in optoelectronic circuits — in which on-chip communication involves the transfer of optical signals from one part of the chip to another with the help of waveguides — this is only possible if optical components are also miniaturized.

Biosensors

Nanophotonic biosensors provide some of the most dependable and accurate sensing systems available. These biosensors incorporate optical transducers and receptors. Receptors respond to physical and chemical variations in the transducer, resulting in absorption, reflection, refraction, fluorescence, phase, and frequency variations in light signals.

These self-contained devices detect minute amounts of molecules (or analytes) with the help of biorecognition components such as DNA, antibodies, or enzymes. These interactions result in alterations to the transducer's optical properties that can be correlated to analyte concentrations.

Optical biosensors rely on evanescent fields, integrating silicon photonics and nanoplasmonics. In SPR and dielectric waveguide-based biosensors, the decay period of the evanescent field is substantially longer compared to the majority of biomolecular analytes (on the order of 200-400 nm).

Optical biosensors provide a noninvasive and reliable way of detecting biochemical agents, relying on real-time interactions at the sensor’s surface without requiring the application of labels or dyes.

Metasurfaces

In the pursuit of nonlinear field enhancements, metasurfaces are artificially engineered nanosurfaces formed from subwavelength nanostructures such as nanorods and nanoholes that scatter light. Thus, they provide precise control over the phase, amplitude, and polarization of light at nanoscales, for example enabling the formation of:

• Superlenses formed from metasurfaces displaying negative permittivity and permeability (negative refractive index) that can focus light beyond the diffraction limit, especially useful in microscopy
• Phase compensation media, which combine double-negative media with positive-indexed materials
• Cloaked surfaces formed from near-zero refractive index metasurfaces, in which relative permittivity or permeability are close to zero

Nanophotonics holds significant promise in the quest for compact, energy-efficient technologies that are capable of delivering multimodal functionalities at ever-decreasing scales.

Photonic devices, such as fiber optic cables, convey vast amounts of data, but they are limited by their larger size in comparison to electronic devices. The next frontier lies in bridging the vast data carrying capabilities of photonics with the rapid signal processing capabilities of electronics.

Ansys Lumerical FDTD^TM software is the gold-standard software for modeling nanophotonic devices, processes, and materials. FDTD software allows advanced photonic designs of a wide range of devices, including gratings, multi-layered stacks, microLEDs, image sensors, and metalenses, as well as rapid prototyping through thousands of iterations; integrating, among others:

• FDTD, rigorous coupled-wave analysis (RCWA), and STACK solvers in a single design environment
• Rapid analysis of periodic structures with the RCWA solver
• Rapid analysis of multilayer thin films
• Automated S-parameter extraction
• Automation API (Lumerical scripting, Python, and MATLAB)
• Foundry compatibility

Furthermore, FDTD software integrates seamlessly with Ansys Lumerical CML Compiler^TM software, Ansys Multiphysics solvers, Ansys Speos® software, Ansys Zemax OpticStudio^TM software, and third-party electronic-photonic design automation (EPDA) software, ensuring fast, accurate, and scalable photonic designs.

Leverage the FDTD best-in-class solver on high-performance computing (CPU and GPU) solutions and the cloud to excel in tackling the most difficult nanophotonics challenges swiftly and efficiently.

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