Generation of enhanced even and odd harmonics in a two-color laser field

We present results on an enhanced production of high harmonics (HHs) with a two-color laser field $(\omega_{0} +2\omega_{0} )$ in argon (Ar), nitrogen (N$_{\mathrm{2}})$ and carbon dioxide (CO$_{\mathrm{2}})$ gases having close ionization potentials, $I_{p} =15.6eV$ , $I_{p} =15.7eV$, and $I_{p} =13.77eV$ respectively. The fundamental radiation at $\omega_{0} $corresponds to 50fs pulses from a Ti:sapphire chirped-pulse amplifier laser system at 800nm. Odd $\left( {2n+1} \right)\omega_{0} \quad \left( {n=2-10} \right)$ and even $2\left( {2m+1} \right)\omega_{0} \quad \left( {m=1-3} \right)$ harmonics, were observed in the $\omega_{0} +2\omega_{0} $ driving field. High harmonics were measured for parallel and perpendicular relative polarizations orientations of the $\omega_{0} $and $2\omega_{0} $-fields, and intensity of $\omega_{0} $ beam was$\sim 1.5\times 10^{14}W/cm^{2}$, and $2\omega_{0} $ beam was in the range$\sim 3\times 10^{12}-3\times 10^{13}W/cm^{2}$. An increased conversion efficiency into HHs, notably of the $2\left( {2n+1} \right)$ orders was found for the latter case. With the combined $(\omega_{0} +2\omega_{0} )$field, which was obtained by inserting a doubling crystal into the fundamental $\left( {\omega_{0} } \right)$beam, more than an order of magnitude enhancement of the HHs signal was observed compared to the$\omega_{0} $-field or its second harmonic $\left( {2\omega_{0} } \right)$ alone. We also discuss similarities and differences of the HH generation in the three gas media with a single and two-color beams.