Nano-optics
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Nanophotonics or nano-optics is the study of the behavior of
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
on the
nanometer 330px, Different lengths as in respect to the Molecule">molecular scale. The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm), or nanometer (American spelling Despite the va ...
scale, and of the interaction of nanometer-scale objects with light. It is a branch of
optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...
,
optical engineering Optical engineering is the field of engineering encompassing the physical phenomena and technologies associated with the generation, transmission, manipulation, detection, and utilization of light. Optical engineers use the science of optics to ...
,
electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems that use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
, and
nanotechnology Nanotechnology is the manipulation of matter with at least one dimension sized from 1 to 100 nanometers (nm). At this scale, commonly known as the nanoscale, surface area and quantum mechanical effects become important in describing propertie ...
. It often involves dielectric structures such as nanoantennas, or metallic components, which can transport and focus light via
surface plasmon polariton Surface plasmon polaritons (SPPs) are electromagnetic waves that travel along a metal–dielectric or metal–air interface, practically in the infrared or visible spectrum, visible-frequency. The term "surface plasmon polariton" explains that the ...
s. The term "nano-optics", just like the term "optics", usually refers to situations involving
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
, visible, and
near-infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...
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 In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same '' phase'' on ...
) scales, because of the
diffraction limit In optics, any optical instrument or systema microscope, telescope, or camerahas a principal limit to its resolution due to the physics of diffraction. An optical instrument is said to be diffraction-limited if it has reached this limit of res ...
(
Rayleigh criterion Rayleigh criterion may refer to: * , optical angular resolution * , instability criterion in Taylor–Couette flow * Rayleigh roughness criterion, surface roughness criterion in optics * Rayleigh criterion (thermo-acoustic instability), criterion ...
). Nevertheless, it is possible to squeeze light into a nanometer scale using other techniques like, for example,
surface plasmon 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 (e.g. a metal-dielectric interface, such ...
s,
localized surface plasmon 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 irr ...
s around nanoscale metal objects, and the nanoscale apertures and nanoscale sharp tips used in
near-field scanning optical microscopy Near-field scanning optical microscopy (NSOM) or scanning near-field optical microscopy (SNOM) is a microscopy technique for nanostructure investigation that breaks the far field resolution limit by exploiting the properties of evanescent waves ...
(SNOM or NSOM) and photoassisted
scanning tunnelling microscopy A scanning tunneling microscope (STM) is a type of scanning probe microscope used for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer, then at IBM Zürich, the Nobel Prize in ...
.


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
photodetector Photodetectors, also called photosensors, are devices that detect light or other forms of electromagnetic radiation and convert it into an electrical signal. They are essential in a wide range of applications, from digital imaging and optical ...
s tend to have a variety of desirable properties including low noise, high speed, and low voltage and power. Small
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
s have various desirable properties for
optical communication Optical communication, also known as optical telecommunication, is communication at a distance using light to carry information. It can be performed visually or by using electronic devices. The earliest basic forms of optical communication date ...
including low threshold current (which helps power efficiency) and fast modulation (which means more data transmission). Very small lasers require
subwavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same '' phase'' on ...
optical cavities. An example is
spaser A spaser or plasmonic laser is a type of laser which aims to confine light at a subwavelength scale far below John William Strutt, 3rd Baron Rayleigh, Rayleigh's Diffraction limited beam, diffraction limit of light, by storing some of the light ene ...
s, the
surface plasmon 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 (e.g. a metal-dielectric interface, such ...
version of lasers. Integrated circuits are made using
photolithography Photolithography (also known as optical lithography) is a process used in the manufacturing of integrated circuits. It involves using light to transfer a pattern onto a substrate, typically a silicon wafer. The process begins with a photosensiti ...
, 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 Immersion lithography is a technique used in semiconductor manufacturing to enhance the resolution and accuracy of the lithographic process. It involves using a liquid medium, typically water, between the lens and the wafer during exposure. By ...
and phase-shifting
photomask A photomask (also simply called a mask) is an opaque plate with transparent areas that allow light to shine through in a defined pattern. Photomasks are commonly used in photolithography for the production of integrated circuits (ICs or "chips") ...
s, it has indeed been possible to make images much finer than the wavelength—for example, drawing 30 nm lines using 193 nm light. Plasmonic techniques have also been proposed for this application.
Heat-assisted magnetic recording Heat-assisted magnetic recording (HAMR) (pronounced "''hammer''") is a magnetic storage technology for greatly increasing the amount of data that can be stored on a magnetic device such as a hard disk drive by temporarily heating the disk materia ...
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 Optoelectronics (or optronics) is the study and application of electronic devices and systems that find, detect and control light, usually considered a sub-field of photonics. In this context, ''light'' often includes invisible forms of radi ...
, for example the miniaturization of transistors in
integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...
s, has improved their speed and cost. However,
optoelectronic Optoelectronics (or optronics) is the study and application of electronic devices and systems that find, detect and control light, usually considered a sub-field of photonics. In this context, ''light'' often includes invisible forms of radia ...
circuits can only be miniaturized if the optical components are shrunk along with the electronic components. This is relevant for on-chip
optical communication Optical communication, also known as optical telecommunication, is communication at a distance using light to carry information. It can be performed visually or by using electronic devices. The earliest basic forms of optical communication date ...
(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).


Solar cells

Solar cells A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.
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.


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.


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 Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in Nonlinearity, nonlinear media, that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity ...
; an example is
surface-enhanced Raman scattering Surface-enhanced Raman spectroscopy or surface-enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on rough metal surfaces or by nanostructures such as plasmonic-magnetic silic ...
. It also allows sensitive
spectroscopy Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectro ...
measurements of even single molecules located in the hot-spot, unlike traditional spectroscopy methods which take an average over millions or billions of molecules.


Microscopy

One goal of nanophotonics is to construct a so-called "
superlens 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 ...
", which would use
metamaterial A metamaterial (from the Greek word μετά ''meta'', meaning "beyond" or "after", and the Latin word ''materia'', meaning "matter" or "material") is a type of material engineered to have a property, typically rarely observed in naturally occu ...
s (see below) or other techniques to create images that are more accurate than the diffraction limit (deep
subwavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same '' phase'' on ...
). In 1995, Guerra demonstrated this by imaging a silicon grating having 50 nm lines and spaces with illumination having 650 nm wavelength in air. 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. 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. In another example,
dual-polarization interferometry Dual-polarization interferometry (DPI) is an analytical technique that probes molecular layers adsorbed to the surface of a waveguide using the evanescent wave of a laser beam. It is used to measure the conformational change in proteins, or ot ...
has picometer resolution in the vertical plane above the waveguide surface.


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. 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. 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 Silicon photonics is the study and application of photonic systems which use silicon as an optical medium. The silicon is usually patterned with sub-micrometre precision, into microphotonic components. These operate in the infrared, most commo ...
is a
silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
-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, e.g. mid-infrared and overtone spectroscopy, logic gates and cryptography on a chip etc. As of 2016 the research of in silicon photonics spanned light modulators,
optical waveguide 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 ligh ...
s and interconnectors,
optical amplifier An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback fro ...
s,
photodetector Photodetectors, also called photosensors, are devices that detect light or other forms of electromagnetic radiation and convert it into an electrical signal. They are essential in a wide range of applications, from digital imaging and optical ...
s, memory elements,
photonic crystal A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of Crystal structure, natural crystals gives rise to X-ray crystallograp ...
s etc. An area of particular interest is silicon nanostructures capable to efficiently generate electrical energy from solar light (e.g. for
solar panel A solar panel is a device that converts sunlight into electricity by using photovoltaic (PV) cells. PV cells are made of materials that produce excited electrons when exposed to light. These electrons flow through a circuit and produce direct ...
s).


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 Microwave engineering pertains to the study and design of microwave circuits, components, and systems. Fundamental principles are applied to analysis, design and measurement techniques in this field. The short wavelengths involved distinguish this ...
, where metal antennas and
waveguide A waveguide is a structure that guides waves by restricting the transmission of energy to one direction. Common types of waveguides include acoustic waveguides which direct sound, optical waveguides which direct light, and radio-frequency w ...
s 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. 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 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 op ...
. It is fundamentally based on the fact that the
permittivity In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter (epsilon), is a measure of the electric polarizability of a dielectric material. A material with high permittivity polarizes more ...
of the metal is very large and negative. At very high frequencies (near and above the
plasma frequency Plasma oscillations, also known as Langmuir waves (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metals in the ultraviolet region. The oscillations can be described as an instability ...
, usually ultraviolet), the permittivity of a metal is not so large, and the metal stops being useful for concentrating fields. For example, researchers have made nano-optical dipoles and
Yagi–Uda antenna A Yagi–Uda antenna, or simply Yagi antenna, is a directional antenna consisting of two or more parallel Antenna (radio)#Resonant antennas, resonant antenna elements in an Antenna array#Types, end-fire array; these elements are most often metal ...
s following essentially the same design as used for radio antennas. Metallic parallel-plate waveguides (striplines), lumped-constant circuit elements such as
inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The electric current produces a magnetic field around the conductor. The magnetic field strength depends on the magnitude of the ...
and
capacitance Capacitance is the ability of an object to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related ...
(at
visible light Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm ...
frequencies, the values of the latter being of the order of femtohenries and attofarads, respectively), and impedance-matching of
dipole antenna In radio and telecommunications a dipole antenna or doublet is one of the two simplest and most widely used antenna types, types of antenna; the other is the monopole antenna, monopole. The dipole is any one of a class of antennas producin ...
s to
transmission lines In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
, all familiar techniques at
microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...
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 Kinetic inductance is the manifestation of the inertial mass of mobile charge carriers in alternating electric fields as an equivalent series inductance. Kinetic inductance is observed in high carrier mobility conductors (e.g. superconductors) and ...
and
surface plasmon resonance Surface plasmon resonance (SPR) is a phenomenon that occurs where electrons in a thin metal sheet become excited by light that is directed to the sheet with a particular angle of incidence (optics), angle of incidence, and then travel parallel to ...
. Likewise, optical fields interact with
semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...
s in a fundamentally different way than microwaves do.


Near-field optics

Fourier transform In mathematics, the Fourier transform (FT) is an integral transform that takes a function as input then outputs another function that describes the extent to which various frequencies are present in the original function. The output of the tr ...
of a spatial field distribution consists of different
spatial frequencies In mathematics, physics, and engineering, spatial frequency is a characteristic of any structure that is periodic across position in space. The spatial frequency is a measure of how often sinusoidal components (as determined by the Fourier tran ...
. The higher spatial frequencies correspond to the very fine features and sharp edges. In nanophotonics, strongly localized radiation sources (dipolar emitters such as
fluorescent Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with color ...
molecules) are often studied. These sources can be decomposed into a vast
spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...
of
plane wave In physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of ...
s with different
wavenumber In the physical sciences, the wavenumber (or wave number), also known as repetency, is the spatial frequency of a wave. Ordinary wavenumber is defined as the number of wave cycles divided by length; it is a physical quantity with dimension of ...
s, 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 The near field and far field are regions of the electromagnetic (EM) field around an object, such as a transmitting antenna, or the result of radiation scattering off an object. Non-radiative ''near-field'' behaviors dominate close to the an ...
. Thus,
subwavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same '' phase'' on ...
information from the emitter is blurred out; this results in the
diffraction limit In optics, any optical instrument or systema microscope, telescope, or camerahas a principal limit to its resolution due to the physics of diffraction. An optical instrument is said to be diffraction-limited if it has reached this limit of res ...
in the optical systems. Nanophotonics is primarily concerned with the near-field evanescent waves. For example, a
superlens 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 ...
(mentioned above) would prevent the decay of the evanescent wave, allowing higher-resolution imaging.


Metamaterials

Metamaterial A metamaterial (from the Greek word μετά ''meta'', meaning "beyond" or "after", and the Latin word ''materia'', meaning "matter" or "material") is a type of material engineered to have a property, typically rarely observed in naturally occu ...
s 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

*''
ACS Photonics ''ACS Photonics'' is a monthly, peer-reviewed, scientific journal, first published in January 2014 by the American Chemical Society. The current editor in chief is Romain Quidant (ETH Zurich). The interdisciplinary journal publishes original resear ...
'' *
Photonics Photonics is a branch of optics that involves the application of generation, detection, and manipulation of light in the form of photons through emission, transmission, modulation, signal processing, switching, amplification, and sensing. E ...
*''
Photonics Spectra Photonics Spectra is a monthly business-to-business (B2B) magazine published for the engineers, scientists, and end users who develop, commercialize and buy photonic products. It provides both technical and applications information for all aspe ...
'' journal * Ultraperformance Nanophotonic Intrachip Communications


References


External links


ePIXnet Nanostructuring Platform for Photonic IntegrationOptically induced mass transport in near fields"Photonics Breakthrough for Silicon Chips: Light can exert enough force to flip switches on a silicon chip," by Hong X. Tang, ''IEEE Spectrum,'' October 2009
*''Nanophotonics, nano-optics and nanospectroscopy'' A. J. Meixner (Ed.

in the
Open Access Open access (OA) is a set of principles and a range of practices through which nominally copyrightable publications are delivered to readers free of access charges or other barriers. With open access strictly defined (according to the 2001 de ...
Beilstein Journal of Nanotechnology {{Authority control Photonics Nanoelectronics