Nanowire lasers get in tune

A wish list of features for any new biological imaging technique would almost certainly include the ability to produce images with subwavelength resolution, a light source that can be tuned over a wide range of frequencies or wavelengths, and compatibility with physiological conditions. By combining and improving a number of existing optical technologies, Peidong Yang, Jan Liphardt and colleagues at the University of California at Berkeley and the Lawrence Berkeley National Laboratory have developed a new optical probe that just might fit the bill (Nature 447, 1098–1101; 2007).

Researchers have been exploring the optical properties of nanowires made of semiconductors such as zinc oxide and gallium nitride for a number of years. When these nanowires are excited by an external light source, they can emit laser radiation — but only at certain frequencies. Being able to tune the output of nanowire lasers over a range of frequencies would significantly increase their usefulness, especially in subwavelength applications, which exploit the fact that nanowires have cross-sections that are much smaller than the wavelengths used in many imaging experiments.

The Berkeley team has now made a tunable nanowire laser from potassium niobate (KNbO₃) — a material that is well known for having nonlinear optical properties. When KNbO₃ is illuminated at two different input frequencies, it can produce light at four output frequencies, including the sum and the difference of the two input frequencies. Therefore, if a standard tunable laser provides one or both input signals, it will be possible to tune the output frequency of the nanowire laser.

Yang and co-workers perform a 'nanowire scanning microscopy' experiment to demonstrate the potential of the nanowire lasers (see image). Optical tweezers hold the nanowire (blue) in position (and also excite the lasing action in the KNbO₃) while a piezoelectric stage moves the sample so that it is scanned by the laser radiation (green) from the nanowire. The image is built up by measuring the amount of laser radiation that is transmitted by the sample as a function of position. The test sample (right) was a pattern of gold lines — 200 nm wide, 50 nm thick, with separation decreasing from left to right — and the line separations measured by the nanowire scanning microscope along the red line in the figure agreed with those recorded by an atomic force microscope to within ∼10%.

In addition to acting as sources, the nanowires could also be operated as subwavelength photon detectors, and as they can work in sealed containers, there may also be applications in microfluidics.