Lensless mirror array focuses light with subwavelength resolution

Researchers at MIT put coherent superposition to the test and focus 15 laser beams into a 180 nm diameter spot without using a lens.

A team from MIT, US, has managed to focus 15 laser beams into a 180 nm spot without using a lens. Known as “coherent superposition” the technique could be ideal x-ray, ultraviolet and infrared regions of the spectrum, where focusing with a lens can be difficult or even impossible. (Appl. Phys. Lett. 88 261107)

Experimental apparatus

Superposition set-up: more than 60 flat mirrors convert an expanded beam of laser light into a converging cone of 15 beams. (Credit: MIT, US)

“The approach can be implemented entirely using reflective elements and does not require subwavelength accuracy in the construction of the apparatus,” said the authors in their paper.

Using a series of adjustable plane mirrors, the team varied the phase and amplitude of a pattern of interfering beams (See photo) to recreate the intensity distribution of a focused beam. The method follows from the principle that a focused beam by can be synthesized directly from its angular spectrum of plane waves.

In their proof-of-concept experiment, the scientists converted the expanded linearly polarized beam of an argon ion laser (488 nm) into a converging circular cone of 15 beams. Phase actuators within each beam path consisted of a flat mirror glued to a flexure mount and were driven by off-the-shelf piezo stacks.

To measure the intensity distribution, researchers projected the cone of beams through a glass cover slip on to the focal plane of a 160X / 1.4 NA oil-immersion microscope objective and recorded the magnified image on to a CCD.

“We have been considering using our technique for fluorescence microscopy, for which argon lasers are ideal light sources,” Stanley Hong of MIT’s Department of Electrical Engineering and Computer Science told optics.org. “Other applications include optical trapping, x-ray microscopy, maskless lithography and semiconductor metrology.”

Back in the lab, the group is now busy increasing the power density and mechanical stability of its apparatus.

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