Coherent Anti-Stokes Raman Scattering System Uses Chirped Pulses
Daniel S. Burgess
Using a variant of coherent anti-Stokes Raman scattering that employs chirped femtosecond pulses, researchers at the University of California, Berkeley, hope to reveal new details regarding stellar and planetary formation. The spectromicroscopy technique, which they have demonstrated to be suitable with liquid and solid samples, will be used to examine interstellar dust particles that were returned to Earth on NASA’s Stardust space probe in January.
The coherent anti-Stokes Raman scattering system incorporates a “temporal gate” to enable the simultaneous excitation of several vibrational resonances using a single femtosecond laser with high spectral resolution. Courtesy of the University of California, Berkeley.
In the coherent anti-Stokes Raman scattering process, laser pulses that are tuned such that the difference in their energies coincides with a vibrational resonance of a molecular species of interest interact in a sample. The intensity of the response at the anti-Stokes wavelength reveals the presence and density of that molecule at the spot where the pulses overlap, enabling the method to generate chemical maps of the sample.
Because the excitation sources in this nonlinear process need not have the same pulse widths, it is possible to simultaneously excite several vibrational resonances. One approach to do so employs synchronized femtosecond and picosecond lasers. A less complex solution uses only a femtosecond laser, in which part of the output is extracted and converted to picosecond pulses with a series of prisms or gratings and a mechanical slit.
The method is suitable for the generation of chemical maps of liquid and solid samples — here, polystyrene beads. At top and bottom, the scale bars are 3 and 1 μm, respectively. Courtesy of the University of California, Berkeley. ©2006, American Chemical Society.
The new technique omits the slit, using only a pair of gratings from Edmund Optics Inc. of Barrington, N.J. The grating pair stretches some of the 90-fs-long, 800-nm pulses produced by a homebuilt Ti:sapphire laser and amplified in a Spitfire regenerative amplifier from Spectra-Physics of Mountain View, Calif., to 9.4 ps, with the blue light preceding the red — a “negative chirp.”
Other 90-fs pulses pump an optical parametric amplifier from Light Conversion Ltd. of Vilnius, Lithuania, the output of which is doubled in a BBO crystal to offer an 800- to 1175nm tunable Stokes beam. This beam traverses a delay stage so that it temporally overlaps the chirped pulses. Both beams are focused to a 1-µm spot on the sample to effect chemical imaging, and the resulting anti-Stokes signal is dispersed with a spectrometer and imaged with a CCD or a photomultiplier tube, enabling the acquisition of a 300-cm–1 spectrum with each laser pulse.
In proof-of-principle experiments reported in 2004, the scientists demonstrated that the approach is suitable for collecting coherent anti-Stokes Raman scattering spectra from liquid hydrocarbons that have spectral resolution of at least 10 cm–1. They have shown that it can be used to chemically image 3- and 1-µm-diameter polystyrene beads.
Richard J. Saykally, a professor of chemistry at the university and leader of the research group, said that the simplicity and generality of the method are its strengths. Scanning transmission x-ray microscopy is an alternative technology that produces chemical maps at a significantly higher spatial resolution — 30 to 40 nm — than coherent anti-Stokes Raman scattering (which offers a spatial resolution on the order of 300 nm), but it requires a synchrotron source and a high-vacuum sample environment. The less complicated setup, he said, will satisfy a demand for a means of characterizing complex materials such as polymer blends, semiconductor heterostructures and biological specimens.
A hitch with the technique stems from the use of a femtosecond source. Saykally noted that multiphoton processes in the sample can result in optical damage. He said that the team is making progress in its efforts to minimize the effect.
The investigators are expecting to receive samples of interstellar dust from the Stardust mission, which they will image in the aromatic and aliphatic carbon domains, with hopes of learning more about the local conditions at the time and how the dust formed. They also are focused on the characterization of segregation and domain formation in polymer blends.
Journal of the American Chemical Society B, March 30, 2006, pp. 5854-5864.
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