# Generation of Ultrafast Optical Pulses via Molecular Modulation in Ambient Air
## 2 Experimental Setup  
Figure 1 shows the experimental setup. We employed a flash-lamp pumped picosecond (ps) Nd:YAG laser (APL2100, EKSPLA) and a regenerative amplifier system (PL-2250, EKSPLA). The laser system produces up to 400 mJ 1064 nm Fourier transform limited pulses (\(\sim\)50 ps full width at half maximum (FWHM)) at 10 Hz repetition rate. The laser system has optional outputs with second (532 nm) and third (355 nm) harmonics of the fundamental radiation. For the following experiments we used the second harmonic (SH) pulse, with the average energy 20.6 mJ and less than 1% pulse-to-pulse energy fluctuations. The third harmonic (TH), in its turn, pumps the tunable optical parametric oscillator (OPG) system (PG 400, EKSPLA). The OPG produces 607.4 nm pulses with the average energy 2.6 mJ (6% fluctuations). We chose the operating wavelength of the OPG so that the frequency difference between the generated pulse (Stokes) and 532 nm (pump) pulse matches the fundamental vibrational frequency of nitrogen molecule (2331 cm\({}^{-1}\)). The pump pulse is appropriately time delayed for maximizing temporal overlap with the 607.4 nm pulse and then spatially combined with it by a dichroic mirror (DM). Afterwards, the collinearly propagating beams are focused in the air (FWM1, Figure 1) by the convex lens L1 (f = 150 mm) and then collimated by the lens L2 (f = 100 mm). The two laser pulses overlap temporally and spatially in the focal plane of the lens, where they generate multiple Raman sidebands. The energy level of this process is shown in Figure 1B. The frequency difference between the adjacent sidebands equals the fundamental vibrational frequency by nitrogen molecule. We note that chromatic aberrations of the focusing would have provided only a limited detrimental effect on the sidebands generation as all the pump, Stokes, and generated sidebands propagate collinearly, and the actual interaction region is on the order of the Rayleigh length in our experiment (0.5 mm) [19]. In the experiment, we confirmed the ideal spatial and temporal overlaps by maximizing the generated sidebands energies after FWM1 point. Before the beams interacted the second time (FWM2, Figure 1), we equalized the energies of the pump, Stokes, and the generated sidebands by properly attenuating the pump and Stokes pulses with the 532 nm high reflection mirror (HM, Figure 1). The HM mirror reflects more than 95% of the pump and 50% of Stokes radiation, so the intensity of the Stokes pulse becomes approximately the same as the intensity of the first generated anti-Stokes sideband. After the HM, the attenuated pump, Stokes pulses, and the sidebands pass through a pair of 12-mm thickness N-BK7 glass plates. These plates are installed in mirror mounts equipped with precise actuators (Newport, mirror mount: U200-A3K, actuator: AJS100-0.5K-NL). This allows us to accurately control the incident angle of the input beams, which results in change of the optical path through the glass (Figure S1 in the Supplementary Materials). The change of the optical path, in turns, lead to the modification of the relative phase among several pulses [20]. By using two glass plates and rotating them in opposite direction, we can change the optical paths of the pulses without shifting the directions of the beams. First, we set two mirror mounts with BK7 glasses at an angle of 27 degrees (written \(\alpha\) in Figure 1). After passing through these two BK7 plates, the multiple sidebands were focused in the air again (FWM2, Figure 1). The 3 rd anti-Stokes (AS3 at 387 nm) generated at FWM2 was separated from others with the prism, and its spectrum recorded with a fiber-coupled multichannel spectrometer (HR 4000, Ocean Optics, integration time: 150 msec, average: 100 times). We used this configuration to record the peak signal of AS3 sideband while changing the angle of the glass plates. We performed these measurements for the angle of the glass plates from 27.00 to 32.09 degrees. Figure 1: **(A)** Experimental setup for generating multiple Raman sidebands in air.