Single pulse simplifies microscopy

02 Aug 2002

For the first time, scientists use a single pulse to both excite and probe samples in a Raman microscope.

A team of scientists in Israel has developed a method that could greatly simplify nonlinear Raman microscopy.

Yaron Silberberg and colleagues at the Weizmann Institute of Science used a single pulse from a coherently-controlled femtosecond laser to both excite and probe their samples (Nature 418 512).

The technique could potentially be applied to gather three-dimensional information on the molecular structure of live biological specimens.

At present, all nonlinear spectroscopy schemes require two or more laser beams. For example, in coherent anti-Stokes Raman (CARS) spectroscopy, several Raman levels in a molecule are simultaneously populated by one or two broadband excitation pulses, before being probed by a delayed pulse.

Silberberg and colleagues built a CARS microscope that operates with just one laser source - a Ti:sapphire laser emitting 20 fs pulses. All three photons that are needed for CARS are supplied by a single pulse.

The key to the new technique is coherent control. This uses a spatial light modulator (SLM) to shape the ultrashort pulse into the required phase pattern. By changing this pattern, the populations of different vibrational energy levels in a molecule can be controlled.

Pulse shaping has an added benefit - it suppresses nonresonant background light, helping to improve signal-to-noise.

At the moment, the system can probe Raman shifts of between 400 and 800 cm^-1, suitable for molecules containing carbon-halogen bonds, such as chloroform. The team demostrated this by producing CARS images of a glass capillary plate with holes containing dibromomethane (CH₂Br₂).

However, to become a useful tool for spectroscopists, the technique must be extended to the "fingerprint" region of the infrared spectrum (corresponding to Raman shifts of 1000-1500 cm^-1). Then it could be used to identify specific molecules.

According to the team, this can easily be achieved by using commercial lasers with slightly shorter pulses, and therefore with a wider spectral range.

"This concept will have a significant impact on nonlinear spectroscopy and microscopy," conclude the researchers in their paper.

Author
Michael Hatcher is technology editor of Opto and Laser Europe magazine.