What Is Plasmonics?

The last few decades have seen substantial advances in electronics and photonics, bringing vast improvements in data processing technologies that have significantly improved our lives.

Plasmonics describes the nanoscale (billionth of a meter) manipulation of optical signals at metal-dielectric interfaces. Inspired by photonics, plasmonics exploits the distinctive properties of metallic nanostructures that allow the routing of light signals at near-atomic scales.

Integrating conventional photonics and electronics with plasmonics on the same semiconductor chip offers significant advantages, yielding superfast computer chips and optical communication devices, and powering ultrasensitive sensors and microscopes.

When Professor Atwater of Caltech first introduced the concept of plasmonics in 2007, he predicted that the technology would spawn an array of applications, from ultrasensitive biosensing to invisibility cloaking.

Whatever the application, plasmonics relies on manipulating interactions between electromagnetic fields and free electrons at metal-dielectric interfaces — a dielectric being an insulator (such as glass or air) that can be polarized by the application of an electric field. Free electrons, which govern electrical and optical properties in metal, oscillate in the presence of electromagnetic fields (i.e., light), resulting in a phenomenon called surface plasmons. 

What Is Surface Plasmon Resonance?

At nanometer scales, free electrons are confined to tiny regions of space, thereby limiting the range of frequencies at which they vibrate. As they interact with light, they absorb the light that matches their vibrational frequencies (reflecting the rest of the light), meaning they are at resonance — hence the term “surface plasmon resonance” (SPR). SPR can be exploited in nanorods, nanowires, nanophotonics, and other forms of nanotechnology.

Technological Drivers of Plasmonics

Our data-driven society has come a long way since the first chip-based semiconductors, producing ever smaller and faster processors. Nonetheless, the ever-decreasing size of these devices has created its own challenges, with limitations imposed by thermal issues and processing speeds.

Optical interconnects, with their large bandwidth (data-carrying capacity), offer a promising solution. However, the diffraction limit of light plays an important role in limiting the downsizing of photonic components — to about half the wavelength of light. Therefore, photonic devices are generally one to two orders of magnitude greater than their electronic counterparts.

Significant efforts are being made to exploit the unique properties of surface plasmons to combine the size efficiency of electronics with the data efficiency of photonics.

Challenges of Plasmonics

Since Ohmic losses dampen the propagation of surface plasmons only after a few millimeters of travel, plasmonic nanostructures — built from plasmonic nanoparticles such as graphene, metal oxides, and nitrides — are being investigated.

Heat presents another challenge. It can affect the propagation length and amplitude of the plasmonic signals.

Metal nanostructures and geometries displaying the right combination of electrical and optical properties may solve these challenges. This is because metallic nanostructures in copper, silver, aluminum, gold, and other materials allow the propagation of surface plasmon polaritons (SPPs).

SPPs are resonant electron oscillations that propagate at metal-dielectric interfaces. They give rise to strong light-matter interactions that can enhance weak optical effects for opto-electronic applications.

Plasmonic Waveguides

SPPs can be thought of as special types of light waves. Therefore, metallic interconnects that support the propagation of these waves at dielectric-metal interfaces behave as optical waveguides, or plasmonic waveguides.

SPPs are represented by a complex wave vector. The imaginary part of this vector is inversely proportional to SPP propagation length, while the real part is proportional to confinement.

The practical integration of surface plasmons in circuit design depends on balancing the inverse relationship between propagation length and confinement. Ideally, plasmonic waveguides maximize both the confinement and propagation length of surface plasmons for optimal effect.

Dissipation losses that naturally result from the propagation of surface plasmon polaritons can be countered with gain amplification or by integrating photonic elements such as fibers, resulting in hybrid plasmonic waveguides.

Plasmonic waveguides exhibit subwavelength modalities, less than the diffraction limit of light. The idea that propagation modalities of SPPs at wavelengths smaller than light is possible has generated tremendous excitement, opening up the possibility of chip-scale devices capable of nanoscale information processing at optical frequencies.

Common types of plasmonic waveguides include metal-insulator-metal (MIM), insulator-metal-insulator (IMI), channel plasmon polariton (CPP), and gap plasmon polariton (GPP) waveguides.

A metamaterial is a composite material engineered to exhibit properties not seen in its constituent materials. Metamaterials derive their properties from their unique sizes, shapes, geometries, and orientations that give them the ability to bend, block, absorb, or enhance electromagnetic waves in new and beneficial ways. Metamaterials are arranged in repeating patterns at wavelengths smaller than the phenomena they seek to influence.

In plasmonic metamaterials, it is surface plasmons that give these materials their unique properties. Under certain conditions, incident light couples with surface plasmons at metal-dielectric interfaces to form self-sustaining, propagating electromagnetic waves known as surface plasmon polaritons (SPPs).

These SPPs derive their properties from the structure of the underlying metal nanoparticles. SPPs display tunable features at wavelengths shorter than the incident light. Examples of plasmonic metamaterials include gold nanoparticles (nanocubes) and silver and gold nanoshells in periodic arrangements.

Because plasmonic metamaterials derive their properties from the arrangements of metal nanoparticles at subwavelength scales, engineers can manipulate properties such as dispersion, permittivity, permeability, and refractive index to achieve a range of novel applications.

Negative-Index Plasmonic Metamaterials

As light travels from one medium to another, say from air to water, it bends as it crosses the normal, being a plane perpendicular to the surface. In negative-index materials, this bending occurs in the opposite direction, meaning that the electromagnetic energy from the light is transported in a direction opposite to its propagating wavefront. 

As the material’s refractive index relates to its permittivity, which in turn influences its electromagnetic propagation length, negative-index metamaterials offer tunable optical properties that extend beyond the capabilities of conventional lenses, mirrors, and optical devices.

Gradient Index Plasmonic Metamaterials

Plasmonic metamaterials can also be configured to exhibit varying refractive indices across their lengths or surfaces. These can be fabricated, for example, by depositing a synthetic polymer, such as PMMA, onto a gold nanosurface using electron beam lithography.

Gradient index plasmonic metamaterials were used to fabricate Luneburg and Eaton lenses that interact with surface plasmon polaritons instead of conventional photons from light.

Three-dimensional negative-index metamaterials have also been proposed, potentially fabricated via self-assembly, multilayer thin film deposition, and focused ion beam milling.

Negative Radiation Pressure Metamaterials

Shining light on conventional materials — i.e., displaying a positive refractive index — results in positive radiation pressure, meaning the material is pushed away from the light source. The opposite effect occurs on a negative-index material, meaning the material is pulled towards the light source.

This could be applied, for example, in increasing energy transfer efficiency and light absorption in the operation of light sources and lasers or improving light absorption in thin-film solar cells.

Hyperbolic Metamaterials

Hyperbolic metamaterials behave as a metal or dielectric depending on the direction of travel of light. In this instance, the material’s dispersion relation forms a hyperboloid, resulting (theoretically) in infinitely small propagation wavelengths.

Hyperbolic metasurfaces have been demonstrated on silver and gold nanostructures. These structures exhibit enhanced capabilities (negative refraction, diffraction-free, and more) for sensing and imaging. As such, these structures offer promising applications in quantum information processing inside optical integrated circuits.

Furthermore, hyperbolic superlattices can form from the combination of compatible crystal structures, such as titanium nitride and aluminum scandium nitride. Unlike gold and silver, these materials are compatible with existing CMOS components and are thermally stable at higher temperatures. As they exhibit higher photonic densities (when compared to gold or silver), they are also efficient light absorbers.

Hyperbolic metamaterials open up possibilities such as planar lenses that provide advanced sensing capabilities, diffraction-free imaging, ultrasensitive optical microscopes, nanoresonators, and more.

Resonant Nanostructures

Resonant nanostructures exhibit the strength required for light-matter interactions, high localization of electromagnetic interactions, and large cross-sections for scattering and absorption. They may serve as highly effective superlenses, light concentrators, nanoresonators, and subwavelength guides.

Plasmonics rely on optical processes occurring in nanostructures at metal-dielectric interfaces. Surface plasmon polaritons are highly confined electromagnetic waves at these interfaces that result from the interactions of free carrier electrons and photons.

Tunable properties of SPPs allow for the nanoscale control of light-matter interactions, forming a bridge between diffraction-limited photonic devices and nanoscale electronics for next-generation integrated circuits.

The generation, amplification, processing, and routing of optical signals at nanometer scales present numerous opportunities for applications in fields as varied as telecommunications, biochemistry, energy harvesting, and sensing.

Following are prominent examples of potential applications of hybrid plasmonic-electronic-photonic integrated circuits.

Sensors and Biosensors

Plasmonic materials supporting localized surface plasmon resonance (LSPR) lead to strong local electromagnetic field enhancements, thereby significantly improving spectroscopy and sensing applications.

Plasmon-induced resonance energy transfer (PIRET), for example, can be employed to improve the efficiency of light-emitting diodes (LEDs), as well as the performance of fluorescence-based sensors.

One powerful application of plasmonics includes sensors for detecting minute traces of biological or chemical agents. In one instance, researchers coated a plasmonic nanomaterial with a substance easily bound to a bacterial toxin. The presence of this toxin altered the frequency of the surface plasmons and, thereby, the angle of the reflected light — an effect that is measured with great precision, allowing the detection of even the smallest traces.

Other applications of plasmonic technology for sensing include distinguishing viral from bacterial infections and internal sensors for batteries to monitor charge rate and power density.

Surface Plasmon Resonance (SPR) Sensors

SPR sensors effectively replace chromatography-based techniques for detecting environmental pollutants. SPR sensing was shown to be as accurate as chromatography in detecting chloroprene while also producing faster results.

Elsewhere, fiber-optic SPR technology describes the use of SPR sensors at the ends of optical fibers, facilitating the coupling of light with surface plasmons. This enables ultrasensitive, compact sensing devices, which is especially useful in remote sensing applications.

Graphene Plasmonics

It has been shown that layering graphene over gold nanostructures improves the performance of SPR sensors. Graphene’s low refractive index minimizes interference while its large surface area facilitates the trapping of biomolecules.

Thus, incorporating graphene expands the range of applications of SPR sensors. Graphene has also been shown to improve the resistance of SPR sensors to high-temperature annealing during fabrication.

Photovoltaics

Gold group plasmonic materials—which include gold, copper, and silver—have been used in photovoltaic and solar cells. These materials, acting as electron and hole donors, play important roles in powering smart sensors in IoT networks.

Plasmonic nanomaterials can also improve light extraction from LEDs, increasing their brightness and efficiency, and yielding low-cost, flexible, and lightweight LED displays.

Optical Computing

Optical computing seeks to leverage the high bandwidth of optical signals by swapping electronic devices with light processing devices.

For example, in 2014, researchers produced a 200 nm terahertz optical switch manufactured from vanadium dioxide plasmonic materials. Vanadium dioxide displays the ability to switch between an opaque, metallic phase and a transparent, semiconducting phase.

Vanadium dioxide nanoparticles were deposited on a glass substrate and overlain with gold nanoparticles that acted as a plasmonic photocathode. Subsequently, short laser pulses were applied, causing free electrons to jump from the gold nanoparticles onto the vanadium dioxide metamaterial, creating short-lived phase changes.

Vanadium dioxide switches are compatible with existing silicon-based chips and operate in the near-infrared and visible regions of the spectrum. Near-infrared light is essential for telecommunications and optical communications, while visible light is essential for sensors and microscopes.

Plasmonic metamaterials may also assist heat-assisted magnetic memory storage on disks — where memory storage is increased by heating tiny spots on a disk during writing.

Microscopy

An obvious application of subwavelength plasmonics is in the application to microscopy beyond the diffraction limit of light. This limit prevents conventional microscopes (displaying positive refractive index) from resolving objects smaller than a half wavelength of light.

Lenses fabricated from negative-index plasmonic materials could circumvent the diffraction limit, yielding superlenses capable of capturing spatial information beyond the view of conventional microscopes, with applications in optical switches, photodetectors, modulators, and directional light emitters. 

The semiconductor industry has made enormous strides in reducing electronic devices to nanometer scales in the last few decades. However, signal delay issues present significant challenges in pursuing 10GHz+ circuits.

While photonic devices provide enormous bandwidth, diffraction limits the size of photonic components. Plasmonic nanotechnology forms the bridge between the microscale (millionths of a meter) world of photonics and the nanoscale (billionths of a meter) world of electronics.

The future looks bright for plasmonics, with researchers able to work with novel metamaterials like graphene. As long as companies can produce robust, reliable, and reasonably-priced plasmonic devices, plasmonic nanotechnology will be the linchpin that provides essential synergy in the next generation of 10 GHz+ integrated circuit boards.

The plasmonic materials market will be worth nearly $40 billion by 2031, from just shy of $11 billion in 2023, an annual growth rate of around 15.5%.

For further information on the applications of plasmonics, visit our dedicated plasmonics applications page.

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