High-harmonic generation consists of a series of bursts - the blue emission spikes pictured at right - twice per optical cycle. As early as 1994, Paul Corkum and Misha Ivanov recognized that HHG might generate closely spaced trains of such attosecond pulses and perhaps even provide the means to obtain isolated pulses. Three years later, our group proposed the most straightforward way to generate an isolated attosecond pulse. As the figure illustrates, reducing the pulse width of the femtosecond laser also reduces the number of attosecond bursts, until - using a roughly 5 fs pulse, say - the high-energy harmonics are generated during only 3 half cycles of the laser pulse. To isolate a single subfemtosecond burst from that 5-fs pulse, a thin metal film can serve as a filter (the dashed lines) to pass only the highest cutoff energies. Such isolated attosecond pulses would be more useful in experiments designed to resolve dynamics that occur between 1 and 10 fs.

Experimental confirmation of the concept required measuring the structure of the short wavelength emission. Harm Muller (Institute for Atomic and Molecular Physics in Amsterdam) and Pierre Agostini (Atomic Energy Commission in Saclay, France) introduced techniques that rely on ionization of an atom by the harmonic light in the presence of a strong, visible light pulse. When such a pulse occurs, the electron is born in a rapidly oscillating potential; after the pulse leaves, the energy of the ejected electron is modulated by an amount that depends on exactly when within the optical cycle the electron was ionized. This modulation can manifest as either electron energy sidebands or as a continuous energy shift. In 2001, the Muller-Agostini team experimentally confirmed the attosecond pulse train, and 3 years later Ferentc Krausz (Max Planc Institute for Quantum Optics in Garching, Germany) and his group confirmed the production of a single attosecond burst (see Physics Today, October 2004, pp 21).