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We present theoretical and experimental results on high-order harmonic generation in a low-density few-centimeter-long gas medium (${L}_{\mathrm{med}}\ensuremath{\leqslant}$ 10 cm). We study the dependence with focal length of harmonic efficiency. Theoretically, we consider in detail the generation of the $25\mathrm{th}$ harmonic of a short pulse Ti:sapphire laser in argon. Within the strong-field approximation for the atomic dipole, and a complete account of the macroscopic propagation, we compute the number of photons produced as a function of the medium parameters and the focusing conditions. The simulations show that, at constant intensity, the emission of the $25\mathrm{th}$ harmonic scales with the focal length as $\ensuremath{\sim}$$f$${}^{4}$ at low pressure ($P=2$ Torr) and as $\ensuremath{\sim}$$f$${}^{6}$ at higher pressure ($P=5$ Torr). At constant laser energy, we find that the harmonic signal scales approximately as $f$${}^{2}$ at low pressure and as $f$${}^{4}$ at higher pressure. Those numerical results are compared with experimental data.

By using a self-referencing technique, we have experimentally measured the influence of the carrier-envelope phase of femtosecond light pulses on the phase of the electric field of the radiation produced by high-order harmonic generation. We show that, in particular experimental conditions, the temporal evolution of the electric field of the attosecond pulses, is directly controlled by the carrier-envelope phase of the driving pulses.

A model is proposed to account for Kerr-like nonlinearity induced by femtosecond pulses via terahertz generation and electro-optical effect. This phenomenon, so far overlooked, is evidenced in a zinc blende single crystal with a heterodyne optical Kerr effect setup. The spectral evolution of this phenomenon as well as its noninstantaneous response character are reported. Its competition with a third-order optical Kerr effect is demonstrated.