## 2 Materials and Methods
For ISRS measurements, the pump delay was periodically modulated at 10 Hz by the oscillation of the actuator placed in the pump arm, and the signals produced by the detection photodiodes were digitized as a function of time. Using the scan averaging method with 200 averages and about 20 s acquisition time, we obtained the data shown in Figure 2a,b for the spectral and refractive detection channels respectively; from the Raman spectra shown in Figure 2c we calculated a signal-to-background ratio equal to 460 and 195 by dividing the Raman peak intensity values at 173 cm\({}^{-1}\) by the average values of the background in the 200-500 cm\({}^{-1}\) region, for the two detection channels respectively. The absolute pump-probe delay due to the actuator movement, i.e., the conversion of the apparent time scale (see Figure 2a,b), was calibrated using the ISRS signal produced by a routinely used reference sample with a known low-frequency Raman resonance, i.e., a CH\({}_{2}\)Br\({}_{2}\) sample in the present case (see [5] for details). For the Z-scan measurements, the sample position was made to oscillate around the beam focus using the actuator which holds the cuvette, and the signal produced by the probe beam on the closed-aperture photodiode (i.e., the Kerr lens detection assembly) was registered as a function of time. To obtain the Z-scan trace as a function of the  
Figure 2: ISRS signals measured on CH\({}_{2}\)Br\({}_{2}\) at the sample positions corresponding to the maximum Raman signal intensities using: (**a**) spectral shift detection; (**b**) the Kerr lens detection system. (**c**) Fast Fourier transform of the oscillating regions of the signals, showing the Raman mode at 173 cm\({}^{-1}\) of the sample. sample position, the actuator movement was calibrated by matching the data with those measured in the usual way, i.e., moving the sample with a linear stage. This arrangement (MZ-scan) allowed highly sensitive and noiseless measurements of the Z-scan traces [24], as is necessary in the present work to observe the very small effects of the Raman-induced Kerr lens. The two arms of the interferometer can be used to perform both the usual single-beam measurements using the probe pulse alone, or pump-probe measurements at various delays, by moving the linear stage placed in the pump arm, with the delay actuator at rest. Demonstration of the Z-scan operation mode is shown in Figure 3, where a single-beam measurement on a carbon disulfide (CS\({}_{2}\)) standard reference sample, obtained by closing the pump beam, is reported. As evidenced by an attempt to fit the data with the commonly used simple theoretical model [20], assuming a TEM\({}_{00}\) Gaussian beam, thin sample and small nonlinearities, the experimental traces deviated substantially from the model. Actually, we verified that in our experimental setup, the excitation beam profile could not be described as a fundamental Gaussian beam; moreover, the thin sample approximation was not fully verified. Therefore, we used the known value of the CS\({}_{2}\) sample nonlinear refractive index for hundred-femtosecond pulses, i.e., \(\gamma_{ref}\) = 2 \(\times\) 10\({}^{-6}\) cm\({}^{2}\)/GW [23; 25; 26; 27], as a reference value to calibrate our system and to obtain the Kerr nonlinearity of other samples, using the method described in [28], essentially exploiting the linear dependence of the Z-scan peak-valley amplitude \(\Delta T_{pv}\) on the nonlinear refractive index for small nonlinearities. Here, the peak-valley amplitude of the CS\({}_{2}\) reference sample was \(\Delta T_{pv}^{ref}\) = 13%, and thus, given the measured value \(\Delta T_{pv}^{sample}\) of an unknown sample in the same excitation conditions, the associated refractive index is given by  
\[\gamma_{sample}=\gamma_{ref}\Delta T_{pv}^{sample}/\Delta T_{pv}^{ref} \tag{1}\]  
It is worth underlining that while the absolute value of the nonlinearity can be of importance, the results reported below can actually be discussed in relative terms without affecting their validity.