**3.1 Experimental**

**97** The experimental system utilised here is identical to the one used towards increasing the interaction length between the pump and the non-linear medium and the same would therefore, not be repeated here, and the reader may refer to **Figure 2** of Section 2.1 and its description therein instead. To be noted here that **Figure 2a** depicts the case of uniform illumination while **Figure 2b** represents the case of non-uniform illumination. The CO2 laser was tuned to the 10P(32) line giving rise to emission at 10.72 μm and consequently phase matching for frequency doubling was found to occur for an external angle of incidence of ~36<sup>ᵒ</sup> . The crosssection of the pump beam on the crystal entrance face was restricted to ~5.0 mm diameter that allowed its clear passage through the crystal. Although the crystal

was AR coated over broad range covering 5–10 micron on both input and exit faces for normal angle of incidence (AOI), the small Fresnel reflection from the entrance face of the crystal, that was inevitable at oblique AOI, was utilised to monitor both energy and temporal profile of the pump pulse. The energy and temporal profile of the SH beam were monitored after blocking the residual pump beam, that also emerged with it through the exit face of the crystal, by a sapphire plate. By virtue of its multi-atmosphere operation, the CO2 laser possessed intrinsically very high gain and thus delivered a pulse of relatively short duration (FWHM ~110 nsec).
