NANOIMAGING: Tunable nanoscopic lasers probe cellular domains

Researchers at Lawrence Berkeley National Laboratory (LBNL; Berkeley, CA) and the University of California at Berkeley have invented a bio-friendly nanosize electrode-free light source capable of emitting coherent light that can be continuously tuned across the visible spectrum. The device may eventually become a key enabler for subwavelength optics-a laser source for use in the physical, information, and biological sciences that is stable at room temperature and compatible with the fluid-filled environments of life at the cellular level.1


A laser-trapped and pumped potassium niobate nanowire was used to scan a thermally evaporated pattern of gold stripes on a glass coverslip, producing a result comparable to atomic-force microscopy. (Courtesy of LBNL)
Click here to enlarge image

The work combines previously reported developments in the same lab: a nanoscopic light source in air and a passive subwavelength waveguide, said Peidong Yang, one of two principal investigators on the team. The new and enabling piece in this work is the nonlinear optical conversion performed in potassium niobate nanowires. Potassium niobate offers low toxicity, chemical stability, large effective nonlinear optical coefficients at room temperature, large refractive indices, and transparency across a wide range of wavelengths, including the visible spectrum.

The nanowires (each measuring several microns in length and about 100 nm in diameter) were synthesized in a hot-water solution and separated using ultrasound. Infrared laser radiation (which is compatible with living tissue) was used to trap and manipulate individual nanowires and as an optical pump, causing the nanowires to emit visible light that is wavelength tunable via single-harmonic generation or wave mixing. The researchers used the nanoscopic light sources in solution to perform optical imaging and near-field scanning with resolution down to 200 nm.

Fluorescence

The nanowire light sources were also used to generate fluorescence when placed in contact with fluorescent beads. When a nanowire light source was touched to 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, confirming that the nanowire was the predominant source of fluorescent excitation.

“In microscopy, the general rule has always been that you can look at an object or you can touch it,” said Jan Liphardt, also a principal investigator on the project. “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 small size of the source also opens interesting possibilities. “The next direction we would like to push is single-cell endoscopy, in which we use these nanoscale light sources and subwavelength waveguides to do high-resolution imaging inside the individual cell,” Yang said. “The ability to monitor processes within living cells should greatly improve our fundamental understanding of cell functions, intracellular physiological processes, and cellular signal pathways.”

The ability to trap and manipulate single nanowires with optical tweezers turns out to be critical not only for bioimaging, but also potentially for wiring together nanophotonic circuitry. “Lasers, waveguides, nonlinear optical converters, and photodetectors are all important components for photonic technology,” said Yang. “A full-fledged nanophotonic technology will require these elements to create integrated nanophotonic circuitry. They are also quite important for other applications such as lab-on-a-chip technologies or quantum cryptography.”

Yang and Liphardt caution that the nanowire light-source technology is at a very early stage of development. Liphardt compared it to the level at which atomic-force microscopy was some ten years ago. He also said the technology is not intended to replace existing microscopy technologies, but will enable researchers to do things that cannot be done with current technology.

Hassaun A. Jones-Bey

REFERENCE

1. Y. Nakayama et al., Nature 447, 1098 (June 28, 2007).

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