‘Plasmofluidic’ Lens is Tunable, Reconfigurable

Laser-induced bubbles on a metal film are the first demonstration of a plasmonic lens in a microfluidic environment, report engineers at Pennsylvania State University. The unique integration of plasmonics and microfluidics could help in developing multifunctional plasmonic elements, highly sensitive biomedical detection systems, and on-chip, all-optical information processing.

A nanoscale light beam modulated by short electromagnetic waves, known as surface plasmon polaritons (labelled as SPP beam) enters the bubble lens, officially known as a reconfigurable plasmofluidic lens. The bubble controls the light waves, while the grating provides further focus. Images courtesy of Tony Jun Huang, Penn State.

Because it provides a way to manipulate light beyond the diffraction limit, plasmonics — the study of the interaction between the electromagnetic field and free electrons in a metal — is promising for the development of ultrasmall, ultrafast and power-efficient optical devices. Nanoplasmonics is used to combine the speed of optical communication with the portability of electronic circuitry in situations where conventional optics do not work; however, aiming and focusing this modulated light beam at desired targets is difficult.

The majority of plasmonic devices created to date are solid-state and have limited tunability or configurability, and solid-state plasmonic devices lack the ability to deliver multiple functions, the researchers said.

"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, Penn State associate professor of engineering science and mechanics. "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, all of which affect 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.

Huang's team created separate simulations of the light beams and bubble lens to predict their behaviors and optimize conditions before combining the two in the laboratory. To form their plasmofluidic device, the researchers used a low-intensity laser to heat water on a gold surface. The nanobubble's optical behavior remained consistent as long as the laser's power and the environmental temperature stayed constant.

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.

Laboratory images of a light beam without a bubble lens, followed by three examples of different bubble lenses altering the light.

"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, he said. Fine control over these light beams will enable 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.

Chenglong Zhao, postdoctoral fellow in engineering science and mechanics at Penn State, designed and conducted the experiment; Yongmin Liu, assistant professor of mechanical and industrial engineering, and electrical and computer engineering at Northeastern University, worked with Nicholas Fang, associate professor of mechanical engineering, MIT, to analyze the results and develop simulations; and Yanhui Zhao, graduate student in engineering science and mechanics at Penn State, fabricated the materials.

The work, which was funded by the National Institutes of Health, the National Science Foundation and the Penn State Center for Nanoscale Science, was published in Nature Communications (doi:10.1038/ncomms3305).

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