Plasmonics or nanoplasmonics [1] refers to the generation, detection, and manipulation of signals at optical frequencies along metal-dielectric interfaces in the nanometer scale. [2] Inspired by photonics, plasmonics follows the trend of miniaturizing optical devices (see also nanophotonics), and finds applications in sensing, microscopy, optical communications, and bio-photonics. [3] [4]
Plasmonics typically utilizes surface plasmon polaritons (SPPs), [2] that are coherent electron oscillations travelling together with an electromagnetic wave along the interface between a dielectric (e.g. glass, air) and a metal (e.g. silver, gold). The SPP modes are strongly confined to their supporting interface, giving rise to strong light-matter interactions. In particular, the electron gas in the metal oscillates with the electro-magnetic wave. Because the moving electrons are scattered, ohmic losses in plasmonic signals are generally large, which limits the signal transfer distances to the sub-centimeter range, [5] unless hybrid optoplasmonic light guiding networks, [6] [7] [8] or plasmon gain amplification [9] are used. Besides SPPs, localized surface plasmon modes supported by metal nanoparticles are referred to as plasmonics modes. Both modes are characterized by large momentum values, which enable strong resonant enhancement of the local density of photon states, [10] and can be utilized to enhance weak optical effects of opto-electronic devices. [4]
An effort is currently being made to integrate plasmonics with electric circuits, or in an electric circuit analog, to combine the size efficiency of electronics with the data capacity of photonic integrated circuits (PIC). [11] While gate lengths of CMOS nodes used for electrical circuits are ever decreasing, the size of conventional PICs is limited by diffraction, thus constituting a barrier for further integration. Plasmonics could bridge this size mismatch between electronic and photonic components. At the same time, photonics and plasmonics can complement each other, since, under the right conditions, optical signals can be converted to SPPs and vice versa.
One of the biggest issues in making plasmonic circuits a feasible reality is the short propagation length of surface plasmons. Typically, surface plasmons travel distances only on the scale of millimeters before damping diminishes the signal. [12] This is largely due to ohmic losses, which become increasingly important the deeper the electric field penetrates into the metal. Researchers are attempting to reduce losses in surface plasmon propagation by examining a variety of materials, geometries, the frequency and their respective properties. [13] New promising low-loss plasmonic materials include metal oxides and nitrides [14] as well as graphene. [15] Key to more design freedom are improved fabrication techniques that can further contribute to reduced losses by reduced surface roughness.
Another foreseeable barrier plasmonic circuits will have to overcome is heat; heat in a plasmonic circuit may or may not exceed the heat generated by complex electronic circuits. [12] It has recently been proposed to reduce heating in plasmonic networks by designing them to support trapped optical vortices, which circulate light powerflow through the inter-particle gaps thus reducing absorption and Ohmic heating, [16] [17] [18] In addition to heat, it is also difficult to change the direction of a plasmonic signal in a circuit without significantly reducing its amplitude and propagation length. [11] One clever solution to the issue of bending the direction of propagation is the use of Bragg mirrors to angle the signal in a particular direction, or even to function as splitters of the signal. [19] Finally, emerging applications of plasmonics for thermal emission manipulation [20] and heat-assisted magnetic recording [21] leverage Ohmic losses in metals to obtain devices with new enhanced functionalities.
Optimal plasmonic waveguide designs strive to maximize both the confinement and propagation length of surface plasmons within a plasmonic circuit. Surface plasmon polaritons are characterized by a complex wave vector, with components parallel and perpendicular to the metal-dielectric interface. The imaginary part of the wave vector component is inversely proportional to the SPP propagation length, while its real part defines the SPP confinement. [22] The SPP dispersion characteristics depend on the dielectric constants of the materials comprising the waveguide. The propagation length and confinement of the surface plasmon polariton wave are inversely related. Therefore, stronger confinement of the mode typically results in shorter propagation lengths. The construction of a practical and usable surface plasmon circuit is heavily dependent on a compromise between propagation and confinement. Maximizing both confinement and propagation length helps mitigate the drawbacks of choosing propagation length over confinement and vice versa. Multiple types of waveguides have been created in pursuit of a plasmonic circuit with strong confinement and sufficient propagation length. Some of the most common types include insulator-metal-insulator (IMI), [23] metal-insulator-metal (MIM), [24] dielectric loaded surface plasmon polariton (DLSPP), [25] [26] gap plasmon polariton (GPP), [27] channel plasmon polariton (CPP), [28] wedge surface plasmon polariton (wedge), [29] and hybrid opto-plasmonic waveguides and networks. [30] [31] Dissipation losses accompanying SPP propagation in metals can be mitigated by gain amplification or by combining them into hybrid networks with photonic elements such as fibers and coupled-resonator waveguides. [30] [31] This design can result in the previously mentioned hybrid plasmonic waveguide, which exhibits subwavelength mode on a scale of one-tenth of the diffraction limit of light, along with an acceptable propagation length. [32] [33] [34] [35]
The input and output ports of a plasmonic circuit will receive and send optical signals, respectively. To do this, coupling and decoupling of the optical signal to the surface plasmon is necessary. [36] The dispersion relation for the surface plasmon lies entirely below the dispersion relation for light, which means that for coupling to occur additional momentum should be provided by the input coupler to achieve the momentum conservation between incoming light and surface plasmon polariton waves launched in the plasmonic circuit. [11] There are several solutions to this, including using dielectric prisms, gratings, or localized scattering elements on the surface of the metal to help induce coupling by matching the momenta of the incident light and the surface plasmons. [37] After a surface plasmon has been created and sent to a destination, it can then be converted into an electrical signal. This can be achieved by using a photodetector in the metal plane, or decoupling the surface plasmon into freely propagating light that can then be converted into an electrical signal. [11] Alternatively, the signal can be out-coupled into a propagating mode of an optical fiber or waveguide.
The progress made in surface plasmons over the last 50 years has led to the development in various types of devices, both active and passive. A few of the most prominent areas of active devices are optical, thermo-optical, and electro-optical. All-optical devices have shown the capacity to become a viable source for information processing, communication, and data storage when used as a modulator. In one instance, the interaction of two light beams of different wavelengths was demonstrated by converting them into co-propagating surface plasmons via cadmium selenide quantum dots. [38] Electro-optical devices have combined aspects of both optical and electrical devices in the form of a modulator as well. Specifically, electro-optic modulators have been designed using evanescently coupled resonant metal gratings and nanowires that rely on long-range surface plasmons (LRSP). [39] Likewise, thermo-optic devices, which contain a dielectric material whose refractive index changes with variation in temperature, have also been used as interferometric modulators of SPP signals in addition to directional-coupler switches. Some thermo-optic devices have been shown to utilize LRSP waveguiding along gold stripes that are embedded in a polymer and heated by electrical signals as a means for modulation and directional-coupler switches. [40] Another potential field lies in the use of spasers in areas such as nanoscale lithography, probing, and microscopy. [41]
Although active components play an important role in the use of plasmonic circuitry, passive circuits are just as integral and, surprisingly, not trivial to make. Many passive elements such as prisms, lenses, and beam splitters can be implemented in a plasmonic circuit, however fabrication at the nano scale has proven difficult and has adverse effects. Significant losses can occur due to decoupling in situations where a refractive element with a different refractive index is used. However, some steps have been taken to minimize losses and maximize compactness of the photonic components. One such step relies on the use of Bragg reflectors, or mirrors composed of a succession of planes to steer a surface plasmon beam. When optimized, Bragg reflectors can reflect nearly 100% of the incoming power. [11] Another method used to create compact photonic components relies on CPP waveguides as they have displayed strong confinement with acceptable losses less than 3 dB within telecommunication wavelengths. [42] Maximizing loss and compactness with regards to the use of passive devices, as well as active devices, creates more potential for the use of plasmonic circuits.
In physics, a plasmon is a quantum of plasma oscillation. Just as light consists of photons, the plasma oscillation consists of plasmons. The plasmon can be considered as a quasiparticle since it arises from the quantization of plasma oscillations, just like phonons are quantizations of mechanical vibrations. Thus, plasmons are collective oscillations of the free electron gas density. For example, at optical frequencies, plasmons can couple with a photon to create another quasiparticle called a plasmon polariton.
An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber waveguides, transparent dielectric waveguides made of plastic and glass, liquid light guides, and liquid waveguides.
Nanophotonics or nano-optics is the study of the behavior of light on the nanometer scale, and of the interaction of nanometer-scale objects with light. It is a branch of optics, optical engineering, electrical engineering, and nanotechnology. It often involves dielectric structures such as nanoantennas, or metallic components, which can transport and focus light via surface plasmon polaritons.
Extraordinary optical transmission (EOT) is the phenomenon of greatly enhanced transmission of light through a subwavelength aperture in an otherwise opaque metallic film which has been patterned with a regularly repeating periodic structure. Generally when light of a certain wavelength falls on a subwavelength aperture, it is diffracted isotropically in all directions evenly, with minimal far-field transmission. This is the understanding from classical aperture theory as described by Bethe. In EOT however, the regularly repeating structure enables much higher transmission efficiency to occur, up to several orders of magnitude greater than that predicted by classical aperture theory. It was first described in 1998.
Surface plasmons (SPs) are coherent delocalized electron oscillations that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface. SPs have lower energy than bulk plasmons which quantise the longitudinal electron oscillations about positive ion cores within the bulk of an electron gas.
A spaser or plasmonic laser is a type of laser which aims to confine light at a subwavelength scale far below Rayleigh's diffraction limit of light, by storing some of the light energy in electron oscillations called surface plasmon polaritons. The phenomenon was first described by David J. Bergman and Mark Stockman in 2003. The word spaser is an acronym for "surface plasmon amplification by stimulated emission of radiation". The first such devices were announced in 2009 by three groups: a 44-nanometer-diameter nanoparticle with a gold core surrounded by a dyed silica gain medium created by researchers from Purdue, Norfolk State and Cornell universities, a nanowire on a silver screen by a Berkeley group, and a semiconductor layer of 90 nm surrounded by silver pumped electrically by groups at the Eindhoven University of Technology and at Arizona State University. While the Purdue-Norfolk State-Cornell team demonstrated the confined plasmonic mode, the Berkeley team and the Eindhoven-Arizona State team demonstrated lasing in the so-called plasmonic gap mode. In 2018, a team from Northwestern University demonstrated a tunable nanolaser that can preserve its high mode quality by exploiting hybrid quadrupole plasmons as an optical feedback mechanism.
Plasmonic nanolithography is a nanolithographic process that utilizes surface plasmon excitations such as surface plasmon polaritons (SPPs) to fabricate nanoscale structures. SPPs, which are surface waves that propagate in between planar dielectric-metal layers in the optical regime, can bypass the diffraction limit on the optical resolution that acts as a bottleneck for conventional photolithography.
A zero-mode waveguide is an optical waveguide that guides light energy into a volume that is small in all dimensions compared to the wavelength of the light.
A nanolaser is a laser that has nanoscale dimensions and it refers to a micro-/nano- device which can emit light with light or electric excitation of nanowires or other nanomaterials that serve as resonators. A standard feature of nanolasers includes their light confinement on a scale approaching or suppressing the diffraction limit of light. These tiny lasers can be modulated quickly and, combined with their small footprint, this makes them ideal candidates for on-chip optical computing.
A photonic metamaterial (PM), also known as an optical metamaterial, is a type of electromagnetic metamaterial, that interacts with light, covering terahertz (THz), infrared (IR) or visible wavelengths. The materials employ a periodic, cellular structure.
Surface plasmon polaritons (SPPs) are electromagnetic waves that travel along a metal–dielectric or metal–air interface, practically in the infrared or visible-frequency. The term "surface plasmon polariton" explains that the wave involves both charge motion in the metal and electromagnetic waves in the air or dielectric ("polariton").
A plasmonic metamaterial is a metamaterial that uses surface plasmons to achieve optical properties not seen in nature. Plasmons are produced from the interaction of light with metal-dielectric materials. Under specific conditions, the incident light couples with the surface plasmons to create self-sustaining, propagating electromagnetic waves known as surface plasmon polaritons (SPPs). Once launched, the SPPs ripple along the metal-dielectric interface. Compared with the incident light, the SPPs can be much shorter in wavelength.
A localized surface plasmon (LSP) is the result of the confinement of a surface plasmon in a nanoparticle of size comparable to or smaller than the wavelength of light used to excite the plasmon. When a small spherical metallic nanoparticle is irradiated by light, the oscillating electric field causes the conduction electrons to oscillate coherently. When the electron cloud is displaced relative to its original position, a restoring force arises from Coulombic attraction between electrons and nuclei. This force causes the electron cloud to oscillate. The oscillation frequency is determined by the density of electrons, the effective electron mass, and the size and shape of the charge distribution. The LSP has two important effects: electric fields near the particle's surface are greatly enhanced and the particle's optical absorption has a maximum at the plasmon resonant frequency. Surface plasmon resonance can also be tuned based on the shape of the nanoparticle. The plasmon frequency can be related to the metal dielectric constant. The enhancement falls off quickly with distance from the surface and, for noble metal nanoparticles, the resonance occurs at visible wavelengths. Localized surface plasmon resonance creates brilliant colors in metal colloidal solutions.
In nano-optics, a plasmonic lens generally refers to a lens for surface plasmon polaritons (SPPs), i.e. a device that redirects SPPs to converge towards a single focal point. Because SPPs can have very small wavelength, they can converge into a very small and very intense spot, much smaller than the free space wavelength and the diffraction limit.
Ortwin Hess is a German-born theoretical physicist at Trinity College Dublin (Ireland) and Imperial College London (UK), working in condensed matter optics. Bridging condensed matter theory and quantum optics he specialises in quantum nanophotonics, plasmonics, metamaterials and semiconductor laser dynamics. Since the late 1980s he has been an author and coauthor of over 300 peer-reviewed articles, the most popular of which, called "'Trapped rainbow' storage of light in metamaterials", was cited more than 400 times. He pioneered active nanoplasmonics and metamaterials with quantum gain and in 2014 he introduced the "stopped-light lasing" principle as a novel route to cavity-free (nano-) lasing and localisation of amplified surface plasmon polaritons, giving him an h-index of 33.
A hybrid plasmonic waveguide is an optical waveguide that achieves strong light confinement by coupling the light guided by a dielectric waveguide and a plasmonic waveguide. It is formed by separating a medium of high refractive index from a metal surface by a small gap.
An electromagnetic metasurface refers to a kind of artificial sheet material with sub-wavelength thickness. Metasurfaces can be either structured or unstructured with subwavelength-scaled patterns in the horizontal dimensions.
Graphene is a 2D nanosheet with atomic thin thickness in terms of 0.34 nm. Due to the ultrathin thickness, graphene showed many properties that are quite different from their bulk graphite counterparts. The most prominent advantages are known to be their high electron mobility and high mechanical strengths. Thus, it exhibits potential for applications in optics and electronics especially for the development of wearable devices as flexible substrates. More importantly, the optical absorption rate of graphene is 2.3% in the visible and near-infrared region. This broadband absorption characteristic also attracted great attention of the research community to exploit the graphene-based photodetectors/modulators.
Prof. R K Sinha is the Vice Chancellor of Gautam Buddha University, Greater Noida, Gautam Budh Nagar Under UP Government. He was the director of the CSIR-Central Scientific Instruments Organisation (CSIR-CSIO) Sector-30C, Chandigarh-160 030, India. He has been a Professor - Applied Physics, Dean-Academic [UG] & Chief Coordinator: TIFAC-Center of Relevance and Excellence in Fiber Optics and Optical Communication, Mission REACH Program, Technology Vision-2020, Govt. of India Delhi Technological University Bawana Road, Delhi-110042, India.
Spoof surface plasmons, also known as spoof surface plasmon polaritons and designer surface plasmons, are surface electromagnetic waves in microwave and terahertz regimes that propagate along planar interfaces with sign-changing permittivities. Spoof surface plasmons are a type of surface plasmon polariton, which ordinarily propagate along metal and dielectric interfaces in infrared and visible frequencies. Since surface plasmon polaritons cannot exist naturally in microwave and terahertz frequencies due to dispersion properties of metals, spoof surface plasmons necessitate the use of artificially-engineered metamaterials.