August 9, 2013

By Angela Herring

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.