Lasers Break an Optical Barrier -- Stimulated Emission Depletion (STED)

This year's 10th annual German Innovation Award has been awarded to Stefan W. Hell, the director of the Max Planck Institute for Biophysical Chemistry, for his project "Light Microscopy with Unprecedented Resolution".

More than 100 years ago, Ernst Abbe laid down the law that the wave nature of light limits the smallest resolvable spot size to one-third of a wavelength in diameter, or approximately 200 nm. He predicted that any features smaller than this must always be blurred by diffraction and hence be indistinguishable by focusing light microscopes. Professor Hell resolved a spot that was 30 percent smaller than Abbe's minimum using a focusing fluorescence microscope and a pair of laser beams operating at different wavelengths. One beam -- a frequency-doubled UV-pulse -- caused excited molecules to be distributed by the fluorophore, while the other, slightly red-shifted fundamental beam quenched them through stimulated emission.

The researchers generated the two pulses with a Mira 900 Ti:sapphire laser, pumped by an Innova 400 argon-ion laser, both from Coherent Inc. of Santa Clara, Calif. The Ti:sapphire operated at a wavelength of 766 nm and a 120-fs pulse length. The outgoing beam was divided into an excitation beam and a fundamental. While the pulses of the first beam were frequency-doubled with a nonlinear optical crystal made by Fuzhou, China-based Casix Inc., the pulses of the fundamental were stretched to 40 ps by a Coherent grating compressor/decompressor. The technique requires pulsed lasers because, to eliminate fluorescence efficiently, the fundamental pulse must do its job in a much shorter time -- a few picoseconds --than the fluorescence lifetime of the dye -- a few nanoseconds.

related link: STED microscopy

(from Photonics.com)