Experimental identification of vacuum heating at femtosecond-laser-irradiated metal surfaces

Summary form only given. The existence of a vacuum heating (VH) contribution to absorption of high-intensity ultrafast laser pulses at solid targets has been repeatedly predicted, but experimental confirmation has been lacking. The absorption occurs when electrons are drawn from a metallic surface by an intense, obliquely incident p-polarized laser field with L/spl lambda//sup 2//spl ges/10/sup 14/ W/spl mu/m/sup 2//cm/sup 2/, and driven back to the surface with approximately the quiver velocity. Simulations predict that this is an important absorption mechanism for both discrete surfaces and density gradients of scale length L/spl les/ x/sub asc/, where x/sub asc/=eE/mw/sup 2/ is the electron quiver amplitude. For longer gradients with L>x/sub asc/ RA becomes important instead. The anomalous skin effect and relativistic phenomena only occur for intensities above 10/sup 17/ W/cm/sup 2/, and are thus easily avoided. In order to distinguish VH from RA, the gradient scale length L resulting from excitation with a 120 fs, /spl lambda/=620 nm pulse at L = 3/spl times/10/sup 14/W/cm/sup 2/ was measured by means of noninvasive twin-pulse interferometry and time-resolved reflectivity measurements.