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Light Alters Interaction of Organic Molecules

CAMBRIDGE, Mass., Aug. 7, 2013 — A new platform enables emissions from organic molecules to be manipulated when the molecules are suspended on top of a photonic crystal surface. The finding could have implications for bio-imaging and biomolecular detection, OLEDs, and investigations involving Raman scattering or quantum dots.

Organic molecules (shown as yellow spheres) are suspended on a photonic crystal slab (shown as a gray substrate) supporting macroscopic resonances. Scientists at MIT discovered that when molecules are within 100 nm of the slab surface, they send light of the same wavelengths into specific directions (depicted by the light cones). Courtesy of Yan Liang and Bo Zhen

The same team from MIT published work last month demonstrating that a photonic crystal slab with a periodic array of holes allows light to be confined within a planar slab (See: A New Way to Trap Light). Suspended on the surface, however, molecules in solution no longer behave in their usual fashion. Instead of sending light isotropically in all directions, the path is specific.

"Most fluorescing molecules are like faint lightbulbs uniformly emitting light into all directions," said researcher and physics professor Marin Soljacic. "Now we can turn molecules from being simple lightbulbs to powerful flashlights that are thousands of times stronger and can all be aligned towards the same direction."

By introducing a microfluidic channel on top of the photonic crystal surface, organic molecules in solution are delivered to the active region, where interaction with light is enhanced. Each molecule then absorbs and emits significantly more energy with a designed directional pattern.

This method could be used in a number of practical applications, also. As researcher Dr. Ofer Shapira points out, it may assist with one’s yearly checkup.

"During normal blood tests, for example," said Shapira, "cells and proteins are labeled with antibodies and fluorescing molecules that allow their recognition and detection. Their detection limit could be significantly improved using such a system due to the enhanced directional emission from the molecules."

The researchers demonstrated also that the directional emission can be converted into organic lasers with lasing thresholds at least an order of magnitude lower than were reported previously using identical molecules.

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