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‘Plasmofluidic’ lens is tunable, reconfigurable

Photonics Spectra
Nov 2013
UNIVERSITY PARK, Pa. – Laser-induced bubbles on a metal film are the first demonstration of a plasmonic lens in a microfluidic environment. Integrating plasmonics and microfluidics could help in developing multifunctional plasmonic elements, highly sensitive biomedical detection systems and on-chip, all-optical information processing.

Plasmonics is promising for these applications because it enables light manipulation beyond the diffraction limit. Nanoplasmonics combines the speed of optical communication with the portability of electronic circuitry in situations where conventional optics do not work, although aiming and focusing are difficult.


A nanoscale light beam modulated by surface plasmon polaritons enters the bubble lens, officially known as a reconfigurable plasmofluidic lens. The bubble controls the lightwaves, while the grating provides further focus.


But the majority of plasmonic devices created to date are solid-state and lack the ability to deliver multiple functions.

“There are different solid-state devices to control [light beams], to switch them or modulate them, but the tenability and reconfigurability are very limited,” said Tony Jun Huang, associate professor of engineering science and mechanics at Pennsylvania State University. “Using a bubble has a lot of advantages.”

The main advantage of a “bubble” lens is just how quickly and easily its location, size and shape can be reconfigured, affecting the direction and focus of any light beam passing through it. The team’s “plasmofluidic lens” also doesn’t require sophisticated nanofabrication and uses only a single low-cost diode laser; the bubbles themselves are easy to dissolve, replace and move.


Laboratory images of a light beam without a bubble lens, followed by three examples of different bubble lenses altering the light. Photo courtesy of Tony Jun Huang, Penn State.


Simply moving the laser or adjusting its power can change how the bubble will deflect a light beam, either as a concentrated beam at a specific target or as a dispersed wave. Changing the liquid also affects how a light beam will refract.

To form the plasmofluidic device, Huang’s team used a low-intensity laser to heat water on a gold surface. The nano- bubble’s optical behavior remained consistent as long as the laser’s power and the environmental temperature stayed constant.

“In addition to its unprecedented reconfigurability and tenability, our bubble lens has at least one more advantage over its solid-state counterparts: its natural smoothness,” Huang said. “The smoother the lens is, the better quality of the light that pass through it.”

The next step is to find out how the bubble’s shape influences the direction of the light beam and the location of its focal point. Fine control over these light beams will lead to improvements for on-chip biomedical devices and superresolution imaging. “For all these applications, you really need to precisely control light in nanoscale, and that’s where this work can be a very important component,” Huang said.

In addition to researchers from Penn State, the work involved collaboration with Northeastern University and MIT.

The results were published in Nature Communications (doi: 10.1038/ncomms3305).


GLOSSARY
diffraction
As a wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefront as well as with each other to form various diffraction patterns.  
optical
Pertaining to optics and the phenomena of light.
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