Squeezing Light into Nanoscale Devices

As electronic devices and circuits shrink into the nano­scale, the ability to transfer data on a chip, at low power with little energy loss, is becoming a critical challenge. Over the past decade, squeezing light into tiny devices and circuits has been a major goal of nano­photonics researchers. Surface plasmon polari­tons have become an intense area of focus. Plasmons are hybrids of photons and electrons in a metal. If researchers can harness this nano­light, they will be able to improve sensing, subwave­length wave­guiding, and optical trans­mission of signals.

Pictorial representation of a surface plasmon polariton is in terms of a ripple of electron density on the surface of graphene sample. (Source: D. Basov, Columbia Univ.)

Columbia inves­tigators have made a major break­through in this research, with their invention of a novel cryogenic near-field optical micro­scope that has enabled them to directly image, for the first time, the propa­gation and dynamics of graphene plasmons at variable tempera­tures down to negative 250 degrees Celsius. “Our tempera­ture-dependent study now gives us direct physical insight into the funda­mental physics of plasmon propa­gation in graphene,” says Dimitri N. Basov, professor of physics at Columbia Univer­sity, who led the study together with col­leagues Cory Dean and James Hone.

“This insight was impos­sible to attain in previous nano­imaging studies done at room tempera­ture. We were parti­cularly surprised at disco­vering, after many years of failed attempts to get anywhere close, that compact nano­light can travel along the surface of graphene for distances of many tens of microns without unwanted scat­tering. The physics limiting the travel range of nanolight is a funda­mental finding of our study and may lead to new appli­cations in sensors, imaging, and signal proces­sing“, Basov said.

Basov, Dean, and Hone bring together years of experience in working with graphene, the one-atom-thick material that is one of the most promising candi­dates for novel photonic materials. Graphene’s optical proper­ties are readily tunable and can be altered at ultra­fast time scales. However, imple­menting nanolight without intro­ducing unwanted dissi­pation in graphene has been very difficult to achieve. The researchers developed a practical approach to confining light to the nano­scale. They knew they could form plasmon-polari­tons, or resonant modes, in the graphene that propa­gate through the material as hybrid excitations of light and mobile electrons. These plasmon-polariton modes can confine the energy of electro­magnetic radiation, or light, down to the nano­scale. The challenge was how to visua­lize these waves with ultra-high spatial reso­lution, so that they could study the perfor­mance of plasmonic modes at varying tempera­tures.

Alexander S. McLeod, a post­doctoral research scientist in the Basov Nano-optics Labora­tory, built a unique micro­scope that enabled the team to explore the plasmon-polariton waves at high resolution while they cooled the graphene to cryogenic tempera­tures. Lowering the tempera­tures allowed them to “turn off” various scat­tering, or dissi­pation, mechanisms, one after another, as they cooled down their samples and learned which mecha­nisms were relevant.

“Now that our novel nano­imaging capabi­lities are deployed to low tempera­tures, we can see directly the unmiti­gated wave propa­gation of collective light-and-charge exci­tations within graphene,” says McLeod. “Often times in physics, as in life, seeing truly is believing! The record-breaking travel range of these waves shows they’re destined to take on a life of their own, funneling signals and infor­mation back and forth inside next-gene­ration optical devices.”

The study is the first to demon­strate the funda­mental limita­tions for the propa­gation of plasmon polari­ton waves in graphene. The team found that graphene plasmons propa­gate ballis­tically, across tens of micro­meters, throughout the tiny device. These plasmon modes are confined within a volume of space hundreds, if not thousands, of times smaller than that occupied by freely propa­gating light. Plasmons in graphene can be tuned and controlled via an external electric field, which gives graphene a big advantage over conven­tional plasmonic media such as metal surfaces, which are inherently non-tunable. Moreover, the lifetimes of plasmon waves in graphene are now found to exceed those in metals by a factor of 10 to a 100, while propa­gating over comparably longer distances. These features offer enormous advan­tages for graphene as a plasmonic medium in next-generation opto­electronic circuits.

“Our results establish that graphene ranks among the best candidate materials for infrared plasmonics, with appli­cations in imaging, sensing, and nanoscale mani­pulation of light,” says Hone. “Further­more, our findings reveal the funda­mental physics of processes that limit propa­gation of plasmon waves in graphene. This monu­mental insight will guide future efforts in nano­structure engi­neering, which may be able to remove the remaining road­blocks for long-range travel of versatile nano­confined light within future optical devices.”

The current study is the beginning of a series of low-tempera­ture inves­tigations focused on control­ling and mani­pulating confined plasmons in nano­scale opto­electronic graphene devices. The team is now using low-temperature nanoimaging to explore novel plasmonics effects such as elec­trically-induced plasmonic reflection and modu­lation, topo­logical chiral plasmons, and also super­conducting plasmonics in the very recently dis­covered “magic angle” system of twisted bilayer graphene. (Source: CUS)

Reference: G. X. Ni et al.: Fundamental limits to graphene plasmonics, Nature 557, 530 (2018); DOI: 10.1038/s41586-018-0136-9

Link: Basov Infrared Laboratory, Dept. of Physics, Columbia University, New York, USA

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