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A new light wave

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Posted August 13, 2013

Yongmin Liu

Yongmin Liu, an assistant professor with joint appointments in the Department of Mechanical and Industrial Engineering and the Department of Electrical and Computer Engineering, researches nano-optics and metamaterials. Photo by Brooks Canaday.

Hold a mag­ni­fying glass over the dri­veway on a sunny day and it will focus sun­light into a single beam. Hold a prism in front of the window and the light will spread out into a per­fect rainbow. Lenses like these have been used for thou­sands of years, including, most recently, for sophis­ti­cated optical devices.

Until now, all lenses have shared one big lim­i­ta­tion: It’s impos­sible to focus light into a beam that’s smaller than half of the light’s wave­length, said assis­tant pro­fessor Yongmin Liu. It would be like trying to com­press golf balls into a cylinder whose diam­eter was smaller than the balls themselves.

This so-​​called “dif­frac­tion limit” has thus far pre­vented tech­nolo­gies based on pho­tons (instead of elec­trons) from com­peting with the small sizes achieved in elec­tronic devices—like the tiny chips pow­ering our palm-​​sized cell phones.

But an emerging field called plas­monics has turned this age-​​old truth on its head and could rev­o­lu­tionize high-​​tech devices from super-​​resolution optical micro­scopes, to ultra-​​bright LED dis­plays, to high-​​speed com­puters, said Liu. Researchers are now able to focus light into a beam as small as 10 nanome­ters in diam­eter, a small frac­tion of the shortest wave­length in the vis­ible spectrum.

The trick is to get the pho­tons and elec­trons to behave as one. When you do this, the resulting par­ticle emerges with the fea­tures and advan­tages of each. The unique com­bi­na­tion could lead to the cre­ation of an “ultra-​​small, ultra-​​fast, and energy effi­cient device,” said Liu, who holds joint appoint­ments in the Depart­ment of Mechan­ical and Indus­trial Engi­neering and the Depart­ment of Elec­trical and Com­puter Engi­neering.

Yet, despite this exciting advance­ment, Liu wasn’t sat­is­fied. The cur­rent devices in this field are all based on solid mate­rials. In a liquid envi­ron­ment, Liu con­jec­tured, he would have more con­trol on the devices’ prop­er­ties. Per­haps he could develop devices that are not only small and fast, but also recon­fig­urable and multifunctional.

Liu teamed up with Tony Jun Huang, an engi­neering pro­fessor at Penn­syl­vania State Uni­ver­sity who studies optoflu­idics, Penn State post­doc­toral fellow Chen­g­long Zhao and grad­uate stu­dent Yanhui Zhao, and Nicholas Fang, an asso­ciate pro­fessor of mechan­ical engi­neering at M.I.T. Com­bining their exper­tise, the team cre­ated the world’s first “plas­moflu­idic lens,” which uses water bub­bles to manip­u­late light instead of glass or poly­mers. This work was pub­lished Friday in the journal Nature Com­mu­ni­ca­tions.

Just as with glass blowing, the bub­bles are cre­ated from heat—only instead of coming from a burning metal rod six feet long, this heat comes from a tiny laser beam just a few microm­e­ters wide. And just like soap bub­bles in a bathtub, the micro­scopic water bub­bles are free to move around a sur­face, such as a piece of gold film.

The researchers can use the laser beam to change the size, shape, and loca­tion of the bub­bles, allowing them to con­trol exactly how and where the light is directed. With one lens, they can simul­ta­ne­ously focus, scatter, and align the pho­tons from a single beam of light.

Com­bining the size capa­bil­i­ties of elec­tronics with the speed capa­bil­i­ties of optics, and the ver­sa­tility of the fluid envi­ron­ment, said Liu, the approach pro­vides fer­tile ground for a host of new tech­nolo­gies we haven’t yet imag­ined. In par­tic­ular, it could open up an entirely new class of bio­med­ical diag­nostic tools.

Source: Northeastern University

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