Chirp modulation stimulated Raman scattering microscopy

Coherent Raman Microscopy (CRM), a nonlinear optical version of Raman microscopy, offers rapid, chemical-specific, label-free imaging. Unfortunately, competing background optical processes limit its sensitivity and contrast in many materials. Existing modulation transfer schemes developed to reduce these are based on well-known concepts in linear signal processing, such as amplitude, polarization or frequency modulation. No existing schemes simultaneously remove all background types in CRM, resulting in sample-dependent sensitivity and contrast. Here we propose and demonstrate a novel CRM modulation scheme, based on rapid modulation of the higher order optical phases of the input beams, which removes all non-Raman background signals: Chirp-Modulation Stimulated Raman Scattering (CM-SRS). The modulation of higher order phases, unimportant in linear optics, is remarkably effective in nonlinear optical spectroscopy and microscopy. We exemplify this concept through a modulation of - exclusively - the relative sign of the quadratic phase (linear chirp) of the input lasers, keeping all other laser parameters fixed. We show that CM-SRS removes all non-Raman backgrounds, even in samples near electronic resonances which can stymie other modulations schemes. Importantly, this technique remains linear in both Raman oscillator strength and concentration, allowing for high sensitivity, quantitative studies. We present applications of CM-SRS to traditionally challenging samples such as plant materials. We also use the high sensitivity and background removal afforded by CM-SRS to demonstrate the monitoring of small molecule pharmacokinetics in single living cells.