Nanophotonic resonator

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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. [1] Nanophotonic resonators can be made with photonic crystals, silicon, diamond, or metals such as gold.

Contents

For a laser in a nanocavity, spontaneous emission (SE) from the gain medium is enhanced by the Purcell effect, [2] [3] equal to the quality factor or -factor of the cavity divided by the effective mode field volume, . Therefore, reducing the volume of an optical cavity can dramatically increase this factor, which can have the effect of decreasing the input power threshold for lasing. [4] [5] This also means that the response time of spontaneous emission from a gain medium in a nanocavity also decreases, the result being that the laser may reach lasing steady state picoseconds after it starts being pumped. A laser formed in a nanocavity therefore may be modulated via its pump source at very high speeds. Spontaneous emission rate increases of over 70 times modern semiconductor laser devices have been demonstrated, with theoretical laser modulation speeds exceeding 100 GHz, an order of magnitude higher than modern semiconductor lasers, and higher than most digital oscilloscopes. [2] Nanophotonic resonators have also been applied to create nanoscale filters [6] [7] and photonic chips [6]

Differences from classical cavities

For cavities much larger than the wavelength of the light they contain, cavities with very high Q factors have already been realized (~125,000,000). [8] However, high cavities on the order of the same size as the optical wavelength have been difficult to produce due to the inverse relationship between radiation losses and cavity size. [1] When dealing with a cavity much larger than the optical wavelength, it is simple to design interfaces such that light ray paths fulfill total internal reflection conditions or Bragg reflection conditions. For light confined within much smaller cavities near the size of the optical wavelength, deviations from ray optics approximations become severe and it becomes infeasible, if not impossible to design a cavity which fulfills optimum reflection conditions for all three spatial components of the propagating light wave vectors. [1] [9]

In a laser, the gain medium emits light randomly in all directions. With a classical cavity, the number of photons which are coupled into a single cavity mode relative to the total number of spontaneously emitted photons is relatively low because of the geometric inefficiency of the cavity, described by the Purcell factor . [10] The rate at which lasing in such a cavity can be modulated depends on the relaxation frequency of the resonator described by equation 1.

Where is the intrinsic carrier radiative lifetime of the bulk material, is the differential gain, is the group velocity, is the photon lifetime, is the lasing frequency, is the spontaneous emission coupling factor which is enhanced by the Purcell effect, and where is the non-radiative lifetime. In the case of minimal Purcell effect in a classical cavity with small , only the first term of equation 1 is considered, and the only way to increase modulation frequency is to increase photon density by increasing the pumping power. However, thermal effects practically limit the modulation frequency to around 20 GHz, making this approach is inefficient. [2] [11]

In nanoscale photonic resonators with high , the effective mode volume is inherently very small resulting in high and , and terms 2 and 3 in equation 1 are no longer negligible. Consequently, nanocavities are fundamentally better suited to efficiently produce spontaneous emission and amplified spontaneous emission light modulated at frequencies much higher than 20 GHz without negative thermal effects. [2] [12]

Materials and designs

A nanocavity can be created by introducing a defect in a photonic crystal lattice structure Photonic Crystal Nanocavity.png
A nanocavity can be created by introducing a defect in a photonic crystal lattice structure

Nanocavities made from photonic crystals are typically implemented in a photonic crystal slab structure. Such a slab will generally have a periodic lattice structure of physical holes in the material. For light propagating within the slab, a reflective interface is formed at these holes due to the periodic differences in refractive index in the structure.

A common photonic crystal nanocavity design shown is essentially a photonic crystal with an intentional defect (holes missing). This structure having periodic changes in refractive index on the order of the length of the optical wavelength satisfies Bragg reflection conditions in the and directions for a particular wavelength range, and the slab boundaries in the direction create another reflective boundary due to oblique reflection at dielectric boundaries. This results in theoretically perfect wave confinement in the and directions along the axis of a lattice row, and good confinement along the direction. [6] [7] Since this confinement effect along the and directions (directions of the crystal lattice) is only for a range of frequencies, it has been referred to as a photonic bandgap, since there is a discrete set of photon energies which cannot propagate in the lattice directions in the material. [6] However, because of the diffraction of waves propagating inside this structure, radiation energy does escape the cavity within the photonic crystal slab plane. The lattice spacing can be tuned to produce optimal boundary conditions of the standing wave inside the cavity to produce minimal loss and highest . [1] Beside those conventional resonators, they are some examples of rewritable and/or movable cavities, which are accomplished by a micro infiltration system [13] and by a manipulation of single nanoparticles inside photonic crystals. [14] [15]

Metals can also be an effective way to confine light in structures equal to or smaller than the optical wavelength. This effect is emergent from the confined surface plasmon resonance induced by the resonating light, which, when confined to the surface of a nanostructure such as a gold channel or nanorod, induces electromagnetic resonance. [16] Surface plasmon effects are strong in the visible range because the permittivity of a metal is very large and negative at visible frequencies. [17] [18] At frequencies higher than the visible range, the permittivity of a metal is closer to zero, and the metal stops being useful for focussing electric and magnetic fields. [18] This effect was originally observed in radio and microwave engineering, where metal antennas and waveguides may be hundreds of times smaller than the free-space wavelength. In the same way, visible light can be constricted to the nano level with metal structures which form channels, tips, gaps, etc. Gold is also a convenient choice for nanofabrication because of its unreactivity and ease of use with chemical vapour deposition. [19]

A thin film on top of a reflective substrate traps light inside Planar Nanocavity.png
A thin film on top of a reflective substrate traps light inside

A planar nanocavity consists of an absorptive semiconductive film no more than a few nanometers thick over a metal film also a few nanometers thick. [7] Incident light is absorbed and reflected off of both layers, the absorbed light then resonates between the two interfaces, transmitting some light back at after each cycle. Germanium is commonly used for the absorptive layer, while gold, aluminum, and aluminum oxide are used as alternatives as well. [7] Planar nanocavities are commonly used for thin film interference, which occurs when incident light waves reflected by the upper and lower boundaries of a thin film interfere with one another forming a new wave. An example of this is the colorful patterns produced by thin layers of oil on a surface. The difference in colors is due to minute differences in the distance reflected light travels whether it reflects from the top or bottom boundary of the oil layer. This difference is called the optical path difference, the difference in distance between the top and bottom reflection paths, which can be calculated with equation 2:

Where is the refractive index of the absorptive material, is the thickness of the absorptive film, and is the angle of reflection. As expressed in the equation 3, the optical path length difference (OPD) can be related to wavelengths which constructively interfere in the thin film. As a result, light which enters the film at different angles interferes with itself varying amounts, produces an intensity gradient for narrowband light, and a spectrum gradient for white light.

Examples/applications

Nanophotonic circuit designs are similar in appearance to microwave and radio circuits, minimized by a factor of 100,000 or more. Researchers have made nano-optical antennas which emulate the design and functionality of radio antennas. [16] There are a number of important differences between nanophotonics and scaled down microwave circuits. At optical frequency, metals behave much less like ideal conductors, and also exhibit plasmon-related effects like kinetic inductance and surface plasmon resonance. [20] A nantenna is a nanoscopic rectifying antenna, a technology being developed to convert light into electric power. The concept is based on the rectenna which is used in wireless power transmission. A rectenna functions like a specialized radio antenna which is used to convert radio waves into direct current electricity. Light is composed of electromagnetic waves like radio waves, but of a much smaller wavelength. A nantenna, an application of a nanophotonic resonator, is a nanoscale rectenna on the order of the optical wavelength size, which acts as an "antenna" for light, converting light into electricity. Arrays of nantennas could be an efficient means of converting sunlight into electric power, producing solar energy more efficiently than semiconductor bandgap solar cells. [20]

It has been suggested that nanophotonic resonators be used on multi core chips to both decrease size and boost efficiency. [21] This is done by creating arrays of nanophotonic optical ring resonators that can transmit specific wavelengths of light between each other. Another use of nanophotonic resonators in computers is in optical RAM (O-RAM). O-Ram uses photonic crystal slab structure with properties such as strong confinement of photons and carriers to replace the functions of electrical circuits. The use of optical signals versus electrical signals is a 66.7% decrease in power consumption. [22] Researchers have developed planar nanocavities that can reach 90% peak absorption using interference effects. This result is useful in that there are numerous applications that can benefit from these findings, specifically in energy conversion [7]

Related Research Articles

<span class="mw-page-title-main">Laser</span> Device which emits light via optical amplification

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 is an anacronym that originated as an acronym for light amplification by stimulated emission of radiation. The first laser was built in 1960 by Theodore Maiman at Hughes Research Laboratories, based on theoretical work by Charles H. Townes and Arthur Leonard Schawlow.

<span class="mw-page-title-main">Optical amplifier</span> Device that amplifies an optical signal

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 from the cavity is suppressed. Optical amplifiers are important in optical communication and laser physics. They are used as optical repeaters in the long distance fiber-optic cables which carry much of the world's telecommunication links.

<span class="mw-page-title-main">Laser diode</span> Semiconductor laser

A laser diode is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction.

Q-switching, sometimes known as giant pulse formation or Q-spoiling, is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high (gigawatt) peak power, much higher than would be produced by the same laser if it were operating in a continuous wave mode. Compared to modelocking, another technique for pulse generation with lasers, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations. The two techniques are sometimes applied together.

<span class="mw-page-title-main">Fabry–Pérot interferometer</span> Optical device with parallel mirrors

In optics, a Fabry–Pérot interferometer (FPI) or etalon is an optical cavity made from two parallel reflecting surfaces. Optical waves can pass through the optical cavity only when they are in resonance with it. It is named after Charles Fabry and Alfred Perot, who developed the instrument in 1899. Etalon is from the French étalon, meaning "measuring gauge" or "standard".

<span class="mw-page-title-main">Photonic crystal</span> Periodic optical nanostructure that affects the motion of photons

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 natural crystals gives rise to X-ray diffraction and that the atomic lattices of semiconductors affect their conductivity of electrons. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, as artificially produced, promise to be useful in a range of applications.

<span class="mw-page-title-main">Dye laser</span> Equipment using an organic dye to emit coherent light

A dye laser is a laser that uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths, often spanning 50 to 100 nanometers or more. The wide bandwidth makes them particularly suitable for tunable lasers and pulsed lasers. The dye rhodamine 6G, for example, can be tuned from 635 nm (orangish-red) to 560 nm (greenish-yellow), and produce pulses as short as 16 femtoseconds. Moreover, the dye can be replaced by another type in order to generate an even broader range of wavelengths with the same laser, from the near-infrared to the near-ultraviolet, although this usually requires replacing other optical components in the laser as well, such as dielectric mirrors or pump lasers.

<span class="mw-page-title-main">Optical cavity</span> Arrangement of mirrors forming a cavity resonator for light waves

An optical cavity, resonating cavity or optical resonator is an arrangement of mirrors or other optical elements that forms a cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric oscillators and some interferometers. Light confined in the cavity reflects multiple times, producing modes with certain resonance frequencies. Modes can be decomposed into longitudinal modes that differ only in frequency and transverse modes that have different intensity patterns across the cross section of the beam. Many types of optical cavity produce standing wave modes.

<span class="mw-page-title-main">Optical ring resonators</span>

An optical ring resonator is a set of waveguides in which at least one is a closed loop coupled to some sort of light input and output. The concepts behind optical ring resonators are the same as those behind whispering galleries except that they use light and obey the properties behind constructive interference and total internal reflection. When light of the resonant wavelength is passed through the loop from the input waveguide, the light builds up in intensity over multiple round-trips owing to constructive interference and is output to the output bus waveguide which serves as a detector waveguide. Because only a select few wavelengths will be at resonance within the loop, the optical ring resonator functions as a filter. Additionally, as implied earlier, two or more ring waveguides can be coupled to each other to form an add/drop optical filter.

A Raman laser is a specific type of laser in which the fundamental light-amplification mechanism is stimulated Raman scattering. In contrast, most "conventional" lasers rely on stimulated electronic transitions to amplify light.

Quantum-cascade lasers (QCLs) are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jérôme Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.

<span class="mw-page-title-main">Frequency comb</span> Laser source with equal intervals of spectral energies

A frequency comb or spectral comb is a spectrum made of discrete and regularly spaced spectral lines. In optics, a frequency comb can be generated by certain laser sources.

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 random laser (RL) is a laser in which optical feedback is provided by scattering particles. As in conventional lasers, a gain medium is required for optical amplification. However, in contrast to Fabry–Pérot cavities and distributed feedback lasers, neither reflective surfaces nor distributed periodic structures are used in RLs, as light is confined in an active region by diffusive elements that either may or may not be spatially distributed inside the gain medium.

An optical transistor, also known as an optical switch or a light valve, is a device that switches or amplifies optical signals. Light occurring on an optical transistor's input changes the intensity of light emitted from the transistor's output while output power is supplied by an additional optical source. Since the input signal intensity may be weaker than that of the source, an optical transistor amplifies the optical signal. The device is the optical analog of the electronic transistor that forms the basis of modern electronic devices. Optical transistors provide a means to control light using only light and has applications in optical computing and fiber-optic communication networks. Such technology has the potential to exceed the speed of electronics, while conserving more power. The fastest demonstrated all-optical switching signal is 900 attoseconds, which paves the way to develop ultrafast optical transistors.

The Purcell effect is the enhancement of a quantum system's spontaneous emission rate by its environment. In the 1940s Edward Mills Purcell discovered the enhancement of spontaneous emission rates of atoms when they are incorporated into a resonant cavity. In terms of quantum electrodynamics the Purcell effect is a consequence of enhancement of local density of photonic states at the emitter position. It can also be considered as an interference effect. The oscillator radiates the wave which is reflected from the environment. In turn the reflection excites the oscillator either out of phase resulting in higher damping rate accompanied with the radiation enhancement or in phase with the oscillator mode leading to the radiation suppression.

Laser linewidth is the spectral linewidth of a laser beam.

A liquid-crystal laser is a laser that uses a liquid crystal as the resonator cavity, allowing selection of emission wavelength and polarization from the active laser medium. The lasing medium is usually a dye doped into the liquid crystal. Liquid-crystal lasers are comparable in size to diode lasers, but provide the continuous wide spectrum tunability of dye lasers while maintaining a large coherence area. The tuning range is typically several tens of nanometers. Self-organization at micrometer scales reduces manufacturing complexity compared to using layered photonic metamaterials. Operation may be either in continuous wave mode or in pulsed mode.

A quantum dot single-photon source is based on a single quantum dot placed in an optical cavity. It is an on-demand single-photon source. A laser pulse can excite a pair of carriers known as an exciton in the quantum dot. The decay of a single exciton due to spontaneous emission leads to the emission of a single photon. Due to interactions between excitons, the emission when the quantum dot contains a single exciton is energetically distinct from that when the quantum dot contains more than one exciton. Therefore, a single exciton can be deterministically created by a laser pulse and the quantum dot becomes a nonclassical light source that emits photons one by one and thus shows photon antibunching. The emission of single photons can be proven by measuring the second order intensity correlation function. The spontaneous emission rate of the emitted photons can be enhanced by integrating the quantum dot in an optical cavity. Additionally, the cavity leads to emission in a well-defined optical mode increasing the efficiency of the photon source.

<span class="mw-page-title-main">Pr:YLF laser</span> Type of solid-state laser

A Pr:YLF laser (or Pr3+:LiYF4 laser) is a solid state laser that uses a praseodymium doped yttrium-lithium-fluoride crystal as its gain medium. The first Pr:YLF laser was built in 1977 and emitted pulses at 479 nm. Pr:YLF lasers can emit in many different wavelengths in the visible spectrum of light, making them potentially interesting for RGB applications and materials processing. Notable emission wavelengths are 479 nm, 523 nm, 607 nm and 640 nm.

References

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