We demonstrate a new CARS microscopy method based on fast switching of effective vibrational excitation frequency from chirped femtosecond laser pulses. Broadband pump and Stokes pulses excite a single vibrational mode with a high spectral resolution when the two pulses are identically chirped and their pulse durations are approaching the dephasing time of the excited vibrational state. This "spectral focusing" mechanism is applied to CARS microscopy with a single broadband Ti:Sapphire laser. The vibrational excitation frequency is controlled simply by the time delay between the pump and Stokes pulses and fast switching of the excitation frequency (~100 kHz) is achieved with a Pockels cell and polarization optics. Lock-in detection of the difference between the two CARS signals at nearby vibrational frequencies not only eliminates the non-resonant background but also generates a spectral line shape similar to the spontaneous Raman scattering. We demonstrate both micro-spectroscopy and vibrational imaging with various samples.

Date Published: 26 February 2010
PDF: 7 pages
Proc. SPIE 7569, Multiphoton Microscopy in the Biomedical Sciences X, 756909 (26 February 2010); doi: 10.1117/12.841497

Published in SPIE Proceedings Vol. 7569:
Multiphoton Microscopy in the Biomedical Sciences X
Ammasi Periasamy; Peter T. C. So; Karsten König, Editor(s)