Nanophotonics

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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. [1]

Contents

The term "nano-optics", just like the term "optics", usually refers to situations involving ultraviolet, visible, and near-infrared light (free-space wavelengths from 300 to 1200 nanometers).

Background

Normal optical components, like lenses and microscopes, generally cannot normally focus light to nanometer (deep subwavelength) scales, because of the diffraction limit (Rayleigh criterion). Nevertheless, it is possible to squeeze light into a nanometer scale using other techniques like, for example, surface plasmons, localized surface plasmons around nanoscale metal objects, and the nanoscale apertures and nanoscale sharp tips used in near-field scanning optical microscopy (SNOM or NSOM) [2] [3] [4] and photoassisted scanning tunnelling microscopy. [5]

Application

Nanophotonics researchers pursue a very wide variety of goals, in fields ranging from biochemistry to electrical engineering to carbon-free energy. A few of these goals are summarized below.

Optoelectronics and microelectronics

If light can be squeezed into a small volume, it can be absorbed and detected by a small detector. Small photodetectors tend to have a variety of desirable properties including low noise, high speed, and low voltage and power. [6] [7] [8]

Small lasers have various desirable properties for optical communication including low threshold current (which helps power efficiency) and fast modulation [9] (which means more data transmission). Very small lasers require subwavelength optical cavities. An example is spasers, the surface plasmon version of lasers.

Integrated circuits are made using photolithography, i.e. exposure to light. In order to make very small transistors, the light needs to be focused into extremely sharp images. Using various techniques such as immersion lithography and phase-shifting photomasks, it has indeed been possible to make images much finer than the wavelength—for example, drawing 30 nm lines using 193 nm light. [10] Plasmonic techniques have also been proposed for this application. [11]

Heat-assisted magnetic recording is a nanophotonic approach to increasing the amount of data that a magnetic disk drive can store. It requires a laser to heat a tiny, subwavelength area of the magnetic material before writing data. The magnetic write-head would have metal optical components to concentrate light at the right location.

Miniaturization in optoelectronics, for example the miniaturization of transistors in integrated circuits, has improved their speed and cost. However, optoelectronic circuits can only be miniaturized if the optical components are shrunk along with the electronic components. This is relevant for on-chip optical communication (i.e. passing information from one part of a microchip to another by sending light through optical waveguides, instead of changing the voltage on a wire). [7] [12]

Solar cells

Solar cells often work best when the light is absorbed very close to the surface, both because electrons near the surface have a better chance of being collected, and because the device can be made thinner, which reduces cost. Researchers have investigated a variety of nanophotonic techniques to intensify light in the optimal locations within a solar cell. [13]

Controlled release of anti-cancer therapeutics

Nanophotonics has also been implicated in aiding the controlled and on-demand release of anti-cancer therapeutics like adriamycin from nanoporous optical antennas to target triple-negative breast cancer and mitigate exocytosis anti-cancer drug resistance mechanisms and therefore circumvent toxicity to normal systemic tissues and cells. [14]

Spectroscopy

Using nanophotonics to create high peak intensities: If a given amount of light energy is squeezed into a smaller and smaller volume ("hot-spot"), the intensity in the hot-spot gets larger and larger. This is especially helpful in nonlinear optics; an example is surface-enhanced Raman scattering. It also allows sensitive spectroscopy measurements of even single molecules located in the hot-spot, unlike traditional spectroscopy methods which take an average over millions or billions of molecules. [15] [16]

Microscopy

One goal of nanophotonics is to construct a so-called "superlens", which would use metamaterials (see below) or other techniques to create images that are more accurate than the diffraction limit (deep subwavelength). In 1995, Guerra demonstrated this by imaging a silicon grating having 50 nm lines and spaces with illumination having 650 nm wavelength in air. [17] This was accomplished by coupling a transparent phase grating having 50 nm lines and spaces (metamaterial) with an immersion microscope objective (superlens).

Near-field scanning optical microscope (NSOM or SNOM) is a quite different nanophotonic technique that accomplishes the same goal of taking images with resolution far smaller than the wavelength. It involves raster-scanning a very sharp tip or very small aperture over the surface to be imaged. [2]

Near-field microscopy refers more generally to any technique using the near-field (see below) to achieve nanoscale, subwavelength resolution. In 1987, Guerra (while at the Polaroid Corporation) achieved this with a non-scanning whole-field Photon tunneling microscope. [18] In another example, dual-polarization interferometry has picometer resolution in the vertical plane above the waveguide surface.[ citation needed ]

Optical data storage

Nanophotonics in the form of subwavelength near-field optical structures, either separate from the recording media, or integrated into the recording media, were used to achieve optical recording densities much higher than the diffraction limit allows. [19] This work began in the 1980s at Polaroid Optical Engineering (Cambridge, Massachusetts), and continued under license at Calimetrics (Bedford, Massachusetts) with support from the NIST Advanced Technology Program.

Band-gap engineering

In 2002, Guerra (Nanoptek Corporation) demonstrated that nano-optical structures of semiconductors exhibit bandgap shifts because of induced strain. In the case of titanium dioxide, structures on the order of less than 200 nm half-height width will absorb not only in the normal ultraviolet part of the solar spectrum, but well into the high-energy visible blue as well. In 2008, Thulin and Guerra published modeling that showed not only bandgap shift, but also band-edge shift, and higher hole mobility for lower charge recombination. [20] The band-gap engineered titanium dioxide is used as a photoanode in efficient photolytic and photo-electro-chemical production of hydrogen fuel from sunlight and water.

Silicon nanophotonics

Silicon photonics is a silicon-based subfield of nanophotonics in which nano-scale structures of the optoelectronic devices realized on silicon substrates and that are capable to control both light and electrons. They allow to couple electronic and optical functionality in one single device. Such devices find a wide variety of applications outside of academic settings, [21] e.g. mid-infrared and overtone spectroscopy, logic gates and cryptography on a chip etc. [21]

As of 2016 the research of in silicon photonics spanned light modulators, optical waveguides and interconnectors, optical amplifiers, photodetectors, memory elements, photonic crystals etc. An area of particular interest is silicon nanostructures capable to efficiently generate electrical energy from solar light (e.g. for solar panels). [22]

Principles

Plasmons and metal optics

Metals are an effective way to confine light to far below the wavelength. This was originally used in radio and microwave engineering, where metal antennas and waveguides may be hundreds of times smaller than the free-space wavelength. For a similar reason, visible light can be confined to the nano-scale via nano-sized metal structures, such as nano-sized structures, tips, gaps, etc. Many nano-optics designs look like common microwave or radiowave circuits, but shrunk down by a factor of 100,000 or more. After all, radiowaves, microwaves, and visible light are all electromagnetic radiation; they differ only in frequency. So other things equal, a microwave circuit shrunk down by a factor of 100,000 will behave the same way but at 100,000 times higher frequency. [23] This effect is somewhat analogous to a lightning rod, where the field concentrates at the tip. The technological field that makes use of the interaction between light and metals is called plasmonics. It is fundamentally based on the fact that the permittivity of the metal is very large and negative. At very high frequencies (near and above the plasma frequency, usually ultraviolet), the permittivity of a metal is not so large, and the metal stops being useful for concentrating fields.

Scanning electron microscopy (SEM) image of a five-element Yagi-Uda antenna consisting of a feed element, one reflector, and three directors, fabricated by e-beam lithography. SEM-Yagi-text.jpg
Scanning electron microscopy (SEM) image of a five-element Yagi-Uda antenna consisting of a feed element, one reflector, and three directors, fabricated by e-beam lithography.

For example, researchers have made nano-optical dipoles and Yagi–Uda antennas following essentially the same design as used for radio antennas. [25] [26]

Metallic parallel-plate waveguides (striplines), lumped-constant circuit elements such as inductance and capacitance (at visible light frequencies, the values of the latter being of the order of femtohenries and attofarads, respectively), and impedance-matching of dipole antennas to transmission lines, all familiar techniques at microwave frequencies, are some current areas of nanophotonics development. That said, there are a number of very important differences between nano-optics and scaled-down microwave circuits. For example, at optical frequency, metals behave much less like ideal conductors, and also exhibit interesting plasmon-related effects like kinetic inductance and surface plasmon resonance. Likewise, optical fields interact with semiconductors in a fundamentally different way than microwaves do.

Near-field optics

Fourier transform of a spatial field distribution consists of different spatial frequencies. The higher spatial frequencies correspond to the very fine features and sharp edges.

In nanophotonics, strongly localized radiation sources (dipolar emitters such as fluorescent molecules) are often studied. These sources can be decomposed into a vast spectrum of plane waves with different wavenumbers, which correspond to the angular spatial frequencies. The frequency components with higher wavenumbers compared to the free-space wavenumber of the light form evanescent fields. Evanescent components exist only in the near field of the emitter and decay without transferring net energy to the far field. Thus, subwavelength information from the emitter is blurred out; this results in the diffraction limit in the optical systems. [27]

Nanophotonics is primarily concerned with the near-field evanescent waves. For example, a superlens (mentioned above) would prevent the decay of the evanescent wave, allowing higher-resolution imaging.

Metamaterials

Metamaterials are artificial materials engineered to have properties that may not be found in nature. They are created by fabricating an array of structures much smaller than a wavelength. The small (nano) size of the structures is important: That way, light interacts with them as if they made up a uniform, continuous medium, rather than scattering off the individual structures.

See also

Related Research Articles

<span class="mw-page-title-main">Plasmon</span> Quasiparticle of charge oscillations in condensed matter

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.

<span class="mw-page-title-main">Metamaterial</span> Materials engineered to have properties that have not yet been found in nature

A metamaterial is any material engineered to have a property that is rarely observed in naturally occurring materials. They are made from assemblies of multiple elements fashioned from composite materials such as metals and plastics. These materials are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence. Metamaterials derive their properties not from the properties of the base materials, but from their newly designed structures. Their precise shape, geometry, size, orientation and arrangement gives them their smart properties capable of manipulating electromagnetic waves: by blocking, absorbing, enhancing, or bending waves, to achieve benefits that go beyond what is possible with conventional materials.

A superlens, or super lens, is a lens which uses metamaterials to go beyond the diffraction limit. The diffraction limit is a feature of conventional lenses and microscopes that limits the fineness of their resolution depending on the illumination wavelength and the numerical aperture (NA) of the objective lens. Many lens designs have been proposed that go beyond the diffraction limit in some way, but constraints and obstacles face each of them.

Contact lithography, also known as contact printing, is a form of photolithography whereby the image to be printed is obtained by illumination of a photomask in direct contact with a substrate coated with an imaging photoresist layer.

<span class="mw-page-title-main">Extraordinary optical transmission</span>

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.

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 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.

<span class="mw-page-title-main">Photonic metamaterial</span> Type of electromagnetic metamaterial

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.

A plasmonic-enhanced solar cell, commonly referred to simply as plasmonic solar cell, is a type of solar cell that converts light into electricity with the assistance of plasmons, but where the photovoltaic effect occurs in another material.

A metamaterial absorber is a type of metamaterial intended to efficiently absorb electromagnetic radiation such as light. Furthermore, metamaterials are an advance in materials science. Hence, those metamaterials that are designed to be absorbers offer benefits over conventional absorbers such as further miniaturization, wider adaptability, and increased effectiveness. Intended applications for the metamaterial absorber include emitters, photodetectors, sensors, spatial light modulators, infrared camouflage, wireless communication, and use in solar photovoltaics and thermophotovoltaics.

<span class="mw-page-title-main">Surface plasmon polariton</span> Electromagnetic waves that travel along an interface

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.

<span class="mw-page-title-main">Localized surface plasmon</span>

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.

<span class="mw-page-title-main">Hybrid plasmonic waveguide</span>

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.

<span class="mw-page-title-main">Plasmonics</span>

Plasmonics or nanoplasmonics refers to the generation, detection, and manipulation of signals at optical frequencies along metal-dielectric interfaces in the nanometer scale. Inspired by photonics, plasmonics follows the trend of miniaturizing optical devices, and finds applications in sensing, microscopy, optical communications, and bio-photonics.

A nanophotonic resonator or nanocavity is an optical cavity which is on the order of tens to hundreds of nanometers in size. Optical cavities are a major component of all lasers, they are responsible for providing amplification of a light source via positive feedback, a process known as amplified spontaneous emission or ASE. Nanophotonic resonators offer inherently higher light energy confinement than ordinary cavities, which means stronger light-material interactions, and therefore lower lasing threshold provided the quality factor of the resonator is high. Nanophotonic resonators can be made with photonic crystals, silicon, diamond, or metals such as gold.

<span class="mw-page-title-main">Electromagnetic metasurface</span>

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.

<span class="mw-page-title-main">Ravindra Kumar Sinha (physicist)</span> Indian physicist and administrator

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.

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