Ultraslow waves on the nanoscale

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Science  20 Oct 2017:
Vol. 358, Issue 6361, eaan5196
DOI: 10.1126/science.aan5196

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Slow light on the nanoscale

When light passes through an optical material, its speed is reduced by the refractive index of that material. Under exceptional circumstances, light can be slowed to a walking pace or even stopped momentarily. Exploring approaches for practical applications, Tsakmakidis et al. review how the speed of light can be controlled using designed materials and fabricated structures. They show how the combination of slow light and nanotechnology gives rise to a number of effects of interest in signal processing and optoelectronic communication.

Science, this issue p. eaan5196

Structured Abstract


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.


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.


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.

Broadband subdiffraction ultraslow and stopped light.

(Left) Light beams incident (red arrows) on a Si film bounded symmetrically by a negative-permittivity (plasmonic) medium (εr < 0) excite guided light pulses that can have zero group velocity, vg, decaying with time (dashed arrows) exactly at their injection points rather than propagating along the Si film. (Right) Surface plasmon polaritons propagating along an adiabatically tapered plasmonic nanoguide (white prism) are slowed down over a broad range of frequencies (“colors”), accumulating at a deep-subdiffraction spot (d << λ/2) at the tip of the guide and leading to large local field enhancements.


There has recently been a surge of interest in the physics and applications of broadband ultraslow waves in nanoscale structures operating below the diffraction limit. They range from light waves or surface plasmons in nanoplasmonic devices to sound waves in acoustic-metamaterial waveguides, as well as fermions and phonon polaritons in graphene and van der Waals crystals and heterostructures. We review the underlying physics of these structures, which upend traditional wave-slowing approaches based on resonances or on periodic configurations above the diffraction limit. Light can now be tightly focused on the nanoscale at intensities up to ~1000 times larger than the output of incumbent near-field scanning optical microscopes, while exhibiting greatly boosted density of states and strong wave-matter interactions. We elucidate the general methodology by which broadband and, simultaneously, large wave decelerations, well below the diffraction limit, can be obtained in the above interdisciplinary fields. We also highlight a range of applications for renewable energy, biosensing, quantum optics, high-density magnetic data storage, and nanoscale chemical mapping.

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