Direct measurement of light waves

Although the wave nature of light has long been known, it has not been possible to measure directly the oscillating field of light. Radiation in the visible and higher frequency spectral ranges can so far only be characterized in terms of physical quantities averaged over the wave period. Nonlinear optical techniques now allow measurement of εL(t), the amplitude envelope, and ωL(t), the carrier frequency, as a function of time t, for light pulses with durations that approach the wave cycle (1, 2). The carrier-envelope phase φ, which determines the timing between εL(t) and ωL(t), can also be measured (3). These measurements rely on carrier-envelope decomposition, which is physically meaningful only as long as the frequency spectrum of the wave is confined to less than one octave (4). If the radiation is composed of frequencies spanning a broader range (5-17), direct access to the field is required. Attosecond pulses of extreme ultraviolet (XUV) light were predicted to suit for this purpose (18, 19). We report the direct measurement of the buildup and disappearance of the electric field of a light pulse through the use of an attosecond probe.