## 2 Experimental Setup  
Figure 1c shows the experimental setup for our oxygen concentration measurements. As a laser source, we employ a Ti:sapphire regenerative amplifier (1 kHz repetition rate, 0.73 mJ/pulse, Legend, Coherent, Santa Clara, CA, USA) with two equally pumped optical parametric amplifiers (OPAs: OPerA-VIS/UV and OPerA-SFG/UV Coherent, Santa Clara, CA, USA). The outputs of the two OPAs are used as the pump and Stokes pulses (<130 fs). A small fraction of the amplifier output passes through a homemade pulse shaper with an adjustable slit and is used as a probe beam. The adjustable slit width allows us to select a narrow spectral band (\(\Delta\
u\approx\) 11 cm\({}^{-1}\)) at 806 nm, which corresponds to about 2 ps pulse duration. The pump and the probe beams pass through delay stages (DS1,2), so that the probe pulse is time delayed with respect to the pump and Stokes pulses. Choosing time delay for the probe pulse along with its pulse shaping, provide a clear CARS signal with suppressed non-resonant background from the four-wave mixing signal generated by the three beams.  
The collinear geometry configuration for all beams aids to simplify optical alignment and signal collection, with automatically satisfied phase-matching conditions in gases (Figure 1b). The wavelength (FWHM bandwidth) of the pump, Stokes pulses and probe beams are set to 556 nm (13.6 nm), 610 nm (14.1 nm) and 806 nm (0.7 nm), with 3.1 \(\upmu\)J, 2.6 \(\upmu\)J and 0.9 \(\upmu\)J pulse energies respectively. When the probe delay is varied, the full hybrid CARS spectrogram can be observed (for example, see the graphical abstract for this paper). In present experiments, the probe delay with respect to the pump/Stokes pulses is fixed at an optimum values of 2.1 ps. The beams are combined by two dichroic beamsplitters (BS1,2) and focused by a convex lens L1 (\(f\) = 100 mm) in the homemade gas cell (Borosilicate glass, 1 mm thickness, 25.2 cm\({}^{3}\), 8 cm long) filled with N\({}_{2}\) and air gas mixture. The three beams and the generated CARS signal are collimated by another lens (L2,\(f\) = 100 mm) and filtered by a set of neutral density (FW2 AND Thorlabs, Newton, NJ, USA) and bandpass filters (FF01-732/68 Semrock, Rochester, NY, USA) (ND+BFF). After passing through the filters, the beam is focused by the lens L3 (\(f\) = 100 mm) on the entrance slit of the spectrometer: a spectrograph (Chromex Spectrograph 250 is, Albuquerque, NM, USA), which has a liquid-nitrogen-cooled CCD camera (CCD: uncoated Spec-10:400B, Princeton Instruments, Trenton, NJ, USA) attached.  
The spectral resolution of the experimental setup is limited by two factors: resolution of the spectrometer and the probe pulse spectral width. The spectrometer resolution is determined by its slit size and was set to 0.12 cm\({}^{-1}\) for all measurements, thus the total spectral resolution during experiments was mainly limited by the width of the probe pulse (\(\approx\)11 cm\({}^{-1}\)). The probe pulse duration (\(\approx\)2 ps) constrains the temporal resolution. The spatial resolution in transverse to the beams propagation direction is mainly limited by a beam focal spot diameter (estimated at \(\approx\)20 \(\upmu\)m). However, the axial spatial resolution, i.e., in the direction of the beams propagation, can be potentially limited by the fact that the CARS signal is generated in the region up to 6 times larger than the Rayleigh range (\(\approx\)0.4 mm for our beams configuration) [8] in the collinear beams configuration. This fact does not limit the concentration dependence measurements since the gas cell length is significantly larger than the estimated length of this region (2.4 mm).