Molecular plasmonics

While graphene plasmonics has been well-studied in the IR, shifting the plasmon resonance of graphene to the visible region of the spectrum would require extremely small graphene structures with dimensions smaller than can be fabricated by the best currently available top-down fabrication methods.

In a plenary talk at SPIE Optics + Photonics, SPIE Fellow Naomi Halas of Rice University discussed taking plasmons down to the sub-nanometer scale - to the scale of molecules.

Halas is the Stanley C. Moore Professor in Electrical Engineering at Rice, and Director of the Smalley-Curl Institute. Halas is a pioneering researcher in plasmonics, creating the concept of the "tunable plasmon". She pursues research in plasmonics and its applications in biomedicine, optoelectronics, chemical sensing, photocatalysis and sustainability. 

Halas opened with a short history of plasmons - how it all started with gold colloid, the origin of the deep red color seen in stained-glass windows.

"The property of gold colloid, this beautiful red color you see, is because very small gold particles absorb green light" Halas explained. "They have a well-defined resonance and this resonance is due to the collective oscillation of the conduction electrons at 2.3 electron volts."

After discussing plasmonic resonances in metallic nanoparticles, Halas talked about how graphene has become a popular substance in the field of plasmonics. Graphene can support surface plasmons and in its extended state can have plasmon waves.

"That's what's special about graphene," said Halas. "Because it's a pore metal, one can change the carrier density rather easily by adding or removing charge either chemically or by applying voltage, so one can modify the plasmon resonance of a material like graphene."

Halas also covered working with polycyclic aromatic hydrocarbon (PAH) molecules -- the picoscale versions of graphene. Charged PAH molecules can possess molecular plasmon resonances, where the addition of removal of one or more electrons leads to strong absorption features in the visible wavelength range. PAHs show outstanding potential as low-voltage electrochromic media for color-changing walls or windows.

Halas discussed molecular plasmon dynamics including the characterization of collective resonance in molecular-size systems, distinguishing plasmons from excitons in a few-electron system experimentally, and probing longer-lived molecular plasmon lifetimes using Transient Absorption Spectroscopy.

She concluded with possible applications of molecular plasmons including large-area, low-voltage optical devices such as windows, walls, vehicles, or wearables. Charge transfer could lead to ultra-fast changes in optical transparency, as in shutters for eye or sensor protection. The use of OLEDs offer the potential for higher quantum yields that neural fluorophores.

SPIE Optics + Photonics 2017, 6-10 August in San Diego, CA (USA), featured 3300 technical presentations on light-based technologies in 69 conferences. It was also the venue for a three-day industry exhibition with 180 companies; a two-day Career Center job fair; 34 courses and workshops; and several networking opportunities for professionals and students. Read more news from SPIE Optics + Photonics 2017.

SPIE Optics + Photonics 2018 will run 19-23 August in San Diego.

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