Amplitude and Phase Control of Attosecond Light Pulses

We report the generation, compression, and delivery on target of ultrashort extreme-ultraviolet light pulses using external amplitude and phase control. Broadband harmonic radiation is first generated by focusing an infrared laser with a carefully chosen intensity into a gas cell containing argon atoms. The emitted light then goes through a hard aperture and a thin aluminum filter that selects a 30-eV bandwidth around a 30-eV photon energy and synchronizes all of the components, thereby enabling the formation of a train of almost Fourier-transform-limited single-cycle 170 attosecond pulses. Our experiment demonstrates a practical method for synthesizing and controlling attosecond waveforms.

Figure 1

Principle of the experiment. The generated XUV radiation is compressed in two steps: amplitude and phase control using a 600 nm-thick aluminum film; spatial filtering with a 1.6 mm iris located 35 cm after the harmonic generation cell. The spectral and temporal evolution of the XUV radiation, simulated by integrating the time-dependent Schrödinger equation and by solving the Maxwell wave equation for the generated fields, illustrates these effects for the three indicated regions (a)–(c). The generated attosecond pulse train is indicated at the top.

Figure 2

Attosecond pulse characterization. The measurement, known as RABITT, is based on the two-color two-photon ionization process pictured on the right. A typical acquisition with harmonic and sideband signals (in false colors) is shown as a function of the delay. The IR pulse is overlapped with the XUV pulse by means of a convex spherical mirror with a hole in the center. The curvature and position of this mirror is chosen so that its virtual focal point coincides with the point of harmonic generation, allowing us to match the divergence of the XUV and IR beams. Both beams are refocused by a toroidal platinum mirror into the sensitive region of a magnetic bottle electron time-of-flight spectrometer filled with argon or neon up to a static pressure of $\sim {10}^{-4}\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$.

Figure 3

Compression of XUV radiation via amplitude and phase control. The effect of the aluminum films on the intensities and the synchronization of the harmonics is presented in (a) and (b) for one (blue) and three (red) 200 nm-thick films. These results contain sufficient information to reconstruct the average attosecond pulse in the train (c). The green line shows an extrapolation to zero films assuming the same bandwidth as in the one-filter case. The aluminum improves the synchronization of the harmonics resulting in a compression of the pulse duration from 480 as (zero filter, green curve) down to 280 as (one filter, blue curve) and 170 as (three filters, red curve). The latter is very close to the transform limit of 150 as shown by the dashed line and corresponds to only 1.2 cycles of the carrier frequency (7 PHz). The inset shows the XUV electric field assuming a cosine carrier. The measured transmission (red symbols) and group delay (blue symbols) of our filter are compared to tabulated values (solid lines) in (d).