Ultraslow waves on the nanoscale

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Title: Ultraslow waves on the nanoscale
Authors: Tsakmakidis, KL
Hess, O
Boyd, RW
Zhang, X
Item Type: Journal Article
Abstract: BACKGROUND The past 10 to 15 years have witnessed substantial advances in our ability to controllably adjust, slow down, or accelerate the speed of light signals propagating through dispersive optical media. In the fields of optoelectronics and photonics, we have for decades been accustomed to using propagating or guided light waves with speeds only slightly less than the speed of light in a vacuum, c—usually by a factor of 2 to 4. We now know that the group velocity of light waves entering a highly dispersive medium can be reduced by a factor of millions, down to the “human” scale, by exploiting judicious interference effects at the atomic scale of the material. Light decelerations by a factor of a few hundred, with low losses, can be attained in periodic dielectric structures, such as photonic crystal and coupled-resonator optical waveguides. Enabled applications include all-optical tunable delays for routers and data synchronization, optical buffers, enhanced light-matter interaction and nonlinear effects, and miniaturized photonic devices, such as modulators and interferometers. However, such atomic- or dielectric-media–based “slow light” is still fundamentally limited by the wavelength of light, λ, to spatial dimensions larger than ~λ/2 (~300 nm for visible light)—i.e., it is still diffraction limited and cannot reach true nanoscopic dimensions (e.g., below ~30 nm). It is at this point that media featuring negative electromagnetic parameters, such as negative-permittivity plasmonic media or negative–refractive index metamaterials, come to the rescue. ADVANCES Light deceleration in these media arises from the presence of a negative (real part of) electric permittivity, ε, or refractive index, n, leading to antiparallel power flows in the negative-parameter medium and the surrounding dielectric host—thereby giving rise to energy and group velocities that can be reduced even down to zero for a suitable choice of optogeometric parameters. This mechanism for slowing down light is thus nonresonant, and as such it can be broadband, with typical bandwidths being on the order of ~1 to 10 THz. Most notably, these structures support ultraslow surface-plasmon or surface-phonon polaritons that can be concentrated tightly into the nanoscale, at nanovolumes (at least) thousands of times smaller than λ3, upon suitable adiabatic tapering. This leads to large local field enhancements, of the order of 102 to 103, over small nanovolumes that can be exploited for a host of useful applications, including nanoimaging, biosensing and nanoscale chemical mapping, high-density magnetic data-storage recording, light harvesting, nanolasing, and nanoscale quantum and nonlinear optics. One must be mindful, though, of targeting applications (such as the ones mentioned above) for which dissipative losses, which are normally higher in these media compared to their dielectric counterparts, are not a prohibitive factor—i.e., applications for which energy efficiency is not the key figure-of-merit. OUTLOOK Thus far, this method of nanoscale, broadband slow light has relied on the use of uniform or nanostructured metals (plasmonic media), but continued advances in materials design and synthesis have recently enabled a transition to alternative media, such as doped semiconductors, graphene, hexagonal boron nitride, transition-metal dichalcogenides (such as TaS2), van der Waals crystals, and heterostructures. There are important ongoing efforts to push the response of these alternative media, which is currently mainly in the mid- and near-infrared regimes, into the visible region as they allow for considerably lower losses and enhanced controllability (e.g., simply by application of a gate voltage). With these new material platforms, opportunities for new applications emerge, too, particularly those requiring nanoscale (and even atomic) confinements and ultrahigh field enhancements, such as low-threshold single-photon nonlinearities and applications relying on the generation and manipulation of nonclassical light, at ambient conditions.
Issue Date: 20-Oct-2017
Date of Acceptance: 1-Oct-2017
ISSN: 0036-8075
Journal / Book Title: Science
Volume: 358
Issue: 6361
Copyright Statement: Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works This is an article distributed under the terms of the Science Journals Default License.
Sponsor/Funder: Engineering & Physical Science Research Council (E
Engineering & Physical Science Research Council (EPSRC)
Funder's Grant Number: RG72590
Keywords: Science & Technology
Multidisciplinary Sciences
Science & Technology - Other Topics
MD Multidisciplinary
General Science & Technology
Publication Status: Published
Article Number: eaan5196
Appears in Collections:Condensed Matter Theory
Faculty of Natural Sciences

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