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Laser Tweezers Take a Step Forward

Photonics Spectra
Aug 1998
Robert C. Pini

Imagine the neuroscientist who, surrounded by banks of equipment, is able to take a pair of tweezers and reach inside a cell to manipulate a tiny organelle or extract a virus. Such is the vision of Japanese researcher Katsuhiro Ajito, who recently demonstrated a laser system that one day may allow researchers to reach routinely inside cells to identify sample molecules.

Discerns molecular structure
The tweezers in this application are a trapping technique that employs gradient radiation pressure from a tightly focused laser beam to immobilize a microdroplet. The technique was reported first in 1970, and it earned a Nobel Prize in physics in 1997. However, until now it has been impossible to identify the trapped sample. Using the NTT Basic Research Laboratories, Ajito has combined the laser trapping technique with a Raman microprobe using near-infrared light to reach the next stage: identification of the sample's molecular structure.
As reported in Applied Spectroscopy (52:3:339-342), the setup for the experiment comprises a modified Renishaw Raman microprobe spectrometer for the near-IR light, a Schwartz Electro-Optics Ti:sapphire laser pumped by a Spectra-Physics solid-state visible continuous-wave laser, and a charge-coupled device camera from Wright Instruments Ltd. For his demonstration, Ajito trapped a single toluene microdroplet in a drop of water.
The new technique makes two improvements to the previous method. "NIR laser light reduces photolytic or thermal damage to an organic or biological microscale sample and reduces fluorescence interference in its Raman spectra," Ajito said. Also, the reduction of the focused spot of the laser microprobe to 1 µm in diameter makes it possible to combine the spectroscopic identification with the laser trapping technique.
Ajito foresees even higher levels of resolution. "Currently, I have demonstrated the manipulation and Raman spectroscopic measurement of microscale samples. I believe that it will be possible to apply the method to nanoscale samples or single molecules in the future."

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