Lighting the way nm by nm

Researchers at the US Department of Energy’s (DOE) Berkeley laboratory have created a nano-sized light source capable of emitting light across the visible spectrum.

The Lawrence Berkeley National Laboratory scientists, in conjunction with those from the University of California at Berkeley, said that the nano-sized light source could be developed to be used in single cell endoscopy and other forms of subwavelength bio-imaging, integrated circuitry for nanophotonic technology, and cyber cryptography.

‘This nanowire light source is like having a tiny flashlight that we can potentially scan across a living cell, visualising the cell while mechanically interacting with it,’ said Jan Liphardt, a biophysicist working on the project who holds a joint appointment with Berkeley Lab’s Physical Biosciences Division and the university’s physics department.

Chemist Peidong Yang, one of the principal investigators on the project who also holds a joint appointment with the Berkeley Lab and the university’s chemistry department, described how the technology would be applied.

‘Working with individual nanowires, we’ve developed the first electrode-free, continuously tuneable coherent visible light source that’s compatible with physiological environments,’ he said.

‘We’ve also demonstrated that it is possible to trap and manipulate single nanowires with optical tweezers, a critical capability not only for bio-imaging but also for wiring together nanophotonic circuitry.’

To create the light source, nanowires of potassium niobate were synthesised in a hot water solution and separated using ultrasound. The wires created were several microns long, but only 50 nanometres in diameter.

A beam from an infrared laser was used to create an optical trap that grabbed and manipulated individual nanowires. The same beam of light was also used as an optical pump to cause the nanowires to emit visible light, the colour of which could then be selected.

‘Our potassium niobate nanowires have diameters that are substantially below the wavelengths of visible light,’ said Yang.

‘They also have excellent electronic and optical properties, and low toxicity, plus they are chemically stable at room temperatures. This makes them ideal for subwavelength laser and imaging technology.’

‘In microscopy, the general rule has always been that you can look at an object or you can touch it,” said Liphardt. ‘With our nanowire light source technology, we combine both these capabilities in a single device. This opens up the possibility of being able to manipulate a specimen as you visualize it.’

The non-linear optical properties of potassium niobate enabled the frequencies of the incoming infrared light to be mixed or doubled, through second harmonic generation (SHG) or sum frequency generation (SFG), before being emitted as visible and tuneable light.

The scientists carried out a demonstration of the technology, using the nanowire light sources to generate fluorescence from specially treated beads.

When a nanowire light source hit a fluorescent bead, the bead emitted a distinct orange fluorescence at the contact point. When the nanowire was removed, the orange fluorescence was immediately reduced 80-fold, which confirmed the scientists’ theory that the fluorescence was caused by the nanowire.

According to Yang, the researchers will next explore the technology’s use in single cell endoscopy.

‘The next direction we would like to push is single cell endoscopy, in which we use these nanoscale light source and subwavelength waveguides to do high resolution imaging inside the individual cell.’