04 Jul 2007
US scientists have incorporated a nanowire laser into a prototype microscope that could enable biological samples to be imaged with sub-wavelength resolution.
The prototype instrument, developed by scientists at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, consists of a green-emitting nanowire laser that is optically trapped and pumped by a 1064 nm infrared laser (Nature 447 1098).
Peter Pauzauskie of UC Berkeley told optics.org that this is the first time a nanowire source has been able to generate a continuous beam of visible laser light at room temperature. The color of light emission can also be modified by changing the wavelength of the pump laser.
The nanowires, which are made from potassium niobate, are first dispersed into a sealed liquid chamber filled with water. The researchers then use a 50x objective lens to focus the infrared laser to a diffraction-limited spot about 1 µm in diameter.
At that spot the electric field gradient is strong enough to optically trap a single nanowire, while the high electric field and nonlinear polarization effects leads to second-harmonic generation – in other words, the incoming infrared light is frequency-doubled to create green light.
"Second harmonic photons are produced and waveguided along the length of the nanowire," said Pauzauskie. At the opposite side of the chamber, the light emitted from the nanowires is collected with another objective lens and sent to a camera. "Spectral analysis of this light showed that it was green, with a wavelength of about 532 nm."
The researchers tested their apparatus by using the optical tweezer to scan the nanowire over the surface of a thin sheet of glass that had been imprinted with 50 nm-thick lines of gold. As the nanowire passed over each gold line, the intensity of the frequency-doubled green light dropped, allowing them build up an image of the surface.
Such frequency doubling is useful because the green light would be able to effectively highlight parts of biological samples coated with fluorescent dye – a common "tagging" method used in biological imaging. And because the technique involves no wires, it can be operated in liquids safely, making it ideal for biological samples. "Unlike many other materials, such as cadmium selenide and indium antimonide, potassium niobate is not poisonous, lending itself for use in biomedical imaging," Pauzauskie added.
According to Deli Wang, a researcher working on nanowire LEDs at the University of California, San Diego, the development could open up a number of applications for nanowires in sub-wavelength imaging applications, such as fluorescence spectroscopy and near-field scanning microscopy. "The biggest advantage of using potassium niobate nanowires is the ease with which the emission wavelength can be tuned by altering the input source," he said.
The US group says that the next step is to refine the signal-processing techniques to make the imaging technique as practical as others, such as atomic force microscopy. But the work has also shown that the nanowire region can act as an effective photon-frequency mixer. In this project, the team has only demonstrated frequency generation using a single source, but the technique should lend itself to multiple light sources as well.