CC BY
22LD-I-5
Stimulated Raman spectroscopy with femtosecond laser and spectral focusing detection
M. Veres1, D. A. Czitovszky1, L. Himics1, R. Holomb1, A. Nagy1, I. Rigo1, T. Vaczi1
1Wigner Research Centre for Physics, Department of Applied and Non-linear Optics, Budapest,
Hungary
During the last decades Raman spectroscopy became a routinely used characterization technique for many fields and materials. It gives information on vibrational modes and so on the structure (bonds, composition, functional groups, etc.) of the investigated medium. Raman scattering is of relatively low probability, however, several approaches can be used to enhance its sensitivity, including resonant Raman scattering or surface enhanced Raman scattering. Non-linear Raman techniques, like stimulated Raman spectroscopy are also available and with state of the art instrumentation they can offer even for video rate imaging and real time monitoring of cellular activity, biochemical reactions etc.
Normal Raman scattering is an inelastic light scattering process, in which part of the energy of the monochromatic incident light goes for the excitation of fundamental vibrations of the medium. SRS utilizes two coherent, temporally and spatially synchronized light sources (pump and Stokes beams having higher and lower energies, respectively) and when the energy difference of the two beams is equal to the energy of a vibrational transition of the investigated medium, stimulated excitation of that vibration occurs. The intensity of the scattered light at the pump wavelength experiences a stimulated Raman loss, while that of the scattered light at the Stokes wavelength experiences a stimulated Raman gain. The cross-section of the SRS process is around six orders of magnitudes higher than conventional Raman scattering, and so it has much better sensitivity. SRS microscopy is capable of high-speed spectral imaging of samples ranging from single cells to human patient tissues. Here we report on the development of an imaging SRS system utilizing femtosecond lasers and spectral focusing detection.
A dual output Coherent Chameleon Discovery femtosecond laser was used as light source for the SRS system, which was coupled into a Femtonics FemtoSmart two-photon scanning microscope. Special optics have been incorporated between the laser and the microscope consisting of a motorized delay line in the Stokes beam for temporal synchronization, acousto-optic modulator for the pump beam (for lock-in detection) and dichroic mirror for collinear arrangement of the two beams before introducing them into the microscope. Both pump and Stokes beams were chirped to achieve dispersive pulse stretching. The amount of chirp was set so that equal chirp parameters were obtained for both beams. As a result the two pulses have equal energy difference allowing to excite the same vibrational transition for the whole duration of the pulse. The probed wavenumber can be tuned by changing the delay between the pulses. The SRS system was used to characterize different inorganic and biological samples.
This work was supported by the European Commission through the H2020 FET-OPEN project NEURAM (grant agreement 712821) and H2020 MSCA-RISE-2016 project VISGEN (grant agreement: 734862).
