Abstract

Several groups have now reported the observation of high-order (>11th) harmonic generation resulting from the interaction of intense (>10¹⁴W/cm²) radiation with atomic gases. The phenomenon has been observed with a variety of sources including KrF,¹ dye lasers,² and Nd:glass.³ In all these experiments, short-wavelength radiation near 10 nm has been produced. As the laser intensity is increased, the probability for tunneling ionization increases and multiphoton processes such as harmonic generation may reach a limit. The Keldysh parameter, γ, is used as a figure of merit where γ << 1 defines the regime where tunneling ionization is dominant and γ >> 1 is the regime of multiphoton ionization. We present results of experiments performed in He and Ne that span a range from γ> 10 to γ= 10⁻².

© 1992 Optical Society of America