## 2 Experimental Setup and Methods  
Our optical system, not commercially existing and schematically shown in Figure 3, is obtained by integrating a femtosecond-SRS spectroscopy setup [20; 21; 22] with a C2 con  
Figure 2: Coherent Raman excitation with (**a**) FTL laser pulses, (**b**) laser pulses with the same chirp, (**c**) laser pulses with different chirps. \(\Omega\) is the vibration frequency given by \(\Omega=\omega_{\mathrm{P}}-\omega_{\mathrm{s}}\). focal Nikon microscope, which consists of an inverted Nikon Ti-eclipse microscope and a scan head. Experimental details of our setup have already been reported in our previous papers [22; 23; 24; 25]. Two femtosecond laser sources are involved in this setup: a Ti:Sapphire (Ti:Sa--Chameleon Ultra II--pulse duration = 140 fs, repetition rate = 80 MHz, emission wavelengths range = 680-1080 nm) and a femtosecond synchronized optical parametric oscillator (OPO--Chameleon Compact OPO--pulse duration = 200 fs, repetition rate = 80 MHz, emission wavelengths range = 1000-1600 nm). Typically, the temporal characterization of a signal is performed by measuring the correlation between the signal and its duplication. In particular, optical autocorrelation of the field intensity may be used to measure the second-order coherence degree and to assess the duration of short pulses. With this aim, an additional flip-flop mirror has been mounted in-between the mirror M4 and the input of the scan head in our optical architecture to deflect the laser pulse beams directly toward an autocorrelator (pulseCheck 50-A.P.E., Berlin) (see Figure 3); therefore, auto- and cross-correlation of the pulse beam were measured in the optical path just before the laser beam reaches the microscope. In an autocorrelator, the input pulsed beam is divided into two arms of a Michelson interferometer and a delay can be induced in the pulses in one arm with respect to the other one. Both pulses are then overlapped in a non-linear crystal and the generated signal is detected. The pulse duration can be evaluated from the measurement of the time delay and the intensity of the generated signal [26]. The detectable pulse width range is fixed by the delay range, whereas the measurable wavelength range is determined by the detector and the non-linear material. Here, the nonlinear process is the two photons absorption (TPA) and it is measured as a function of the time delay giving the beam autocorrelation function. Auto-correlators based on TPA have some important advantages: (i) a higher sensitivity can be obtained with respect to second harmonic generation (SHG) due to the involvement of fields only at the original frequency, \(\omega\), owing to the TPA resonant second-order transition nature; (ii) extremely short pulses can be characterized since TPA can operate in a wide wavelength range not restricted by the narrow phase matching bandwidth; (iii) in TPA, non-linear signal multiplication and detection are combined into one process, leading  
Figure 3: Experimental setup for stimulated Raman scattering imaging. (Ti:Sa): Titanium-Sapphire pulsed laser source; OPO: optical parameter oscillator; M: mirror; EOM: electro-optical modulator; GM: galvo-mirror; DM: dichroic mirror; OBJ: objective lens; PD: photo-detector; FFFC: flip flop fiber coupler; S: multimode optical fiber; OSA: optical spectrum analyzer. to a simplification and a higher efficiency with respect to a two-step process of optical non-linearity followed by linear detection. The used pulseCheck can be used in two measurement modes: collinear and non-collinear. Collinear, also known as interferometric or fringe-resolved mode, provides further qualitative information on the chirp and central wavelength of the pulse. Regarding non-collinear mode, it allows high dynamic range measurements that are background free [27].