First coherent Raman measurements

First coherent Raman measurements

The integration of Olympus FV3000 confocal laser scanning microscope with the Newport SF-TRU Timing and Recombination Unit in Helsinki is almost complete and the setup is ready for some test measurements plus performance optimization. With the customized non-descanned multiphoton detectors, the microscope has now the capability to measure CARS and SRS signals in both forward and backward scattered direction. The title picture shows a diffraction limited image of 200 nm polysterene beads imaged with SRS at 3054 cm-1, which is a good test for image resolution.

With the fast spectral focusing capability of SF-TRU, the instrument can record at maximum about three spectral points per second in imaging mode. Therefore, the spectral measurement of cyclohexane in the figure below, took about one minute to complete with 1.6 cm-1 step and 128×128 frame size. The inset shows an example frame. The figure also tells the maximum spectral range (about 200 cm-1) which can be measured without changing the laser wavelength.

SRS and CARS spectra of cyclohexane in the C-H stretching region measured with spectral focusing technique. Optical delay is the position of the linear stage, which controls the pump-Stokes pulse overlap for spectral focusing. The position of the optical delay line transfers directly to Raman shift and is, therefore, a subject of calibration.

Spectral focusing is a technique, which allows narrowband Raman excitation with short (fs) broadband pump and Stokes laser pulses. Conventionally, a selective Raman excitation requires long (ps) narrowband laser pulses as the spectral width of transform limited pulses directly defines the Raman excitation bandwidth as exemplified in the figure below for CARS (left part of the figure). Then, to access a different Raman frequency the laser wavelength needs to be changed. In spectral focusing, the broadband pulses are temporarily chirped in such a way that their instantaneous optical frequency changes linearly over time. With equally chirped pump and Stokes pulses, the instantaneous pump-Stokes frequency difference (IFD) is narrowband, and can selectively excite a Raman frequency as represented on the right side of the figure below. Furthermore, the IFD depends on the pump-Stokes pulse overlap, which can be rapidly changed with an optical delay line without changing the laser wavelength.

Concept of spectral focusing in the case of CARS. On the left side of the figure, transform limited pump-Stokes pulses excite Raman frequencies (Anti-Stokes signal), which depend directly on the spectral bandwidth of the pulses. In the case of broadband pulses, the Raman resolution is low. On the right side, linearly chirped pulses can selectively excite a Raman frequency Ω1, which can be rapidly tuned to another frequency Ω2 by adjusting the pulse overlap with an optical delay line (change of Δt).