Invited Article: Coherent Raman and mid-IR microscopy using shaped pulses in a single-beam setup

Niklas Müller,Lukas Brückner,Marcus Motzkus

doi:10.1063/1.5030062

Abstract

The possibility to exploit shaped ultrabroadband laser pulses for molecular coherent control offers a unique implementation of numerous nonlinear spectroscopic methods in one setup. Nonlinear processes, such as coherent Raman microscopy, second harmonic generation, or two-photon fluorescence, which have been applied in many different applications in both the material and life sciences, can be selectively addressed and optimized just by changing the phase imprinted by a programmable pulse shaper. Here, the experimental realization of this concept for multimodal nonlinear microscopy is discussed and the successful implementation of adaptive spectral focussing schemes not only for nonlinear Raman but also for difference frequency generation based mid-infrared (Mid-IR) spectroscopy using a single broadband pulse from a Ti:sapphire laser is shown. Flexible pulse shaping enables tuning of the resonance frequency and the spectral width of the excitation. By variation of the instantaneous frequency difference and the amount of chirp, the experiment can be optimized to achieve high resolution spectroscopy reaching up to 20 cm−1. Matching the resolution of the experiment with the linewidths of the sample on the other hand optimizes the contrast for imaging at high signal levels. The combination and flexible switching between Raman or mid-IR excitation for spectroscopy and microscopy is demonstrated on alkynes, polymer films, and skin tissue. The simple addition of this complementary modality to an existing nonlinear microscope is a further step toward an all-purpose laser excitation source for multimodal microscopy.

Highlights

Multimodal microscopy has seen tremendous development over the recent years
Further improvements in multimodal microscopy have been achieved by implementing techniques which are sensitive to the molecular specific vibrations
For coherent anti-Stokes Raman scattering (CARS) spectroscopy, the anti-Stokes signal was detected by the CCD spectrometer and integrated over the pump probed region to give the CARS intensity at a certain resonance, which corresponds to the applied instantaneous frequency difference (IFD)

Summary

Introduction

Multimodal microscopy has seen tremendous development over the recent years. Further improvements in multimodal microscopy have been achieved by implementing techniques which are sensitive to the molecular specific vibrations. Raman spectroscopy and infrared spectroscopy are the two most common methods providing contrast which is solely based on molecular vibrations. Besides differences in the sensitivity, Raman spectroscopy shows a higher spatial resolution in imaging due to the Abbe limit and far superior contrast in aqueous samples, which is due to the transparency in the visible region in comparison to the infrared region. On the other hand, offers higher coupling efficiencies to the vibrational modes. The higher coupling efficiencies in the infrared region are reasoned by the first order susceptibility χ1 in comparison to the third order one, χ3, for coherent anti-Stokes Raman scattering (CARS).

Methods

Results

Conclusion