An alternative source of isolated attosecond light pulses

Electronic motion inside atoms and molecules happens on time scales of a few tens to a few thousand attoseconds (1as ≡ 10⁻¹⁸s). Probing these dynamics with attosecond light pulses makes it possible to capture electrons in the act of rearranging themselves during the making or breaking of chemical bonds.^1,2

The standard route to creating isolated attosecond pulses starts with an intense, few-cycle IR laser pulse generated via self-phase modulation (which leads to spectral broadening) in a hollow fiber.³ Focusing this pulse into a gas of atoms, a highly nonlinear interaction process (high harmonic generation, HHG), yields coherent, extreme UV (XUV) radiation only near the peak of the laser pulse.¹ We have recently shown, through large-scale calculations, that few-cycle laser pulses that have self-compressed through filamentation present an interesting alternative to the actively-compressed hollow-fiber pulses as a source of isolated attosecond XUV pulses.⁴

Filamentation-driven self-compression in noble gases yields intense, few-cycle laser pulses.⁵ In our calculations, we used a self-compressed 4fs laser pulse to produce single-attosecond XUV pulses via the process of HHG. We found that the time structure of the XUV pulses depends sensitively on both the amplitude and phase modulation that are induced in the driving pulse during the self-compression process.⁴

Figure 1.(a) Intensity profile of a filamented laser pulse in neon at the point of maximum temporal compression. (b) Isolated attosecond extreme UV pulse with a photon energy around 40eV, produced by the laser pulse in (a) via harmonic generation in a separate argon-gas cell.

Our calculations proceed in two steps.⁴ In the first step, we describe the propagation, filamentation, and self-compression of an 800nm, 30fs laser pulse down to a few optical cycles in a long neon-gas cell at atmospheric pressure, via a numerical solution of the nonlinear envelope equation. The laser pulse is extracted from the filament at the point of maximum compression, and is used in the second step to drive HHG in a separate, short argon gas cell at much lower pressure. We simultaneously solve the time-dependent Schrödinger equation for the nonlinear laser-atom interaction and the wave equation for the propagation of the laser and XUV fields through the argon gas.

Figure 1(a) shows the spatiotemporal profile of the maximally-compressed laser pulse, which has a duration of 4fs and a peak intensity of 3×10¹⁴W/cm². Figure 1(b) shows the spatiotemporal profile of the highest-energy XUV radiation (around 40eV) produced by the pulse in Figure 1(a) via HHG in argon. The XUV pulse has a duration of 560as and an energy of a few picojoules.

The spatial and temporal characteristics of the driving pulse have important consequences for the generated XUV radiation. The self-compressed pulse is naturally collimated by the filamentation process.

This improves the phase-matching conditions for the HHG process and leads to well-collimated XUV harmonics: see Figure 1(b). As is also typical of filamentation-driven self-compression, the pulse shown in Figure 1(a) is not transform-limited. Self-phase modulation, plasma defocusing, and self-steepening lead to an intensity envelope that is asymmetric in time, and to a rapid blueshift of the instantaneous frequency over the peak of the pulse. Consequently, whereas the rising edge of the high-energy XUV radiation generated by this pulse is shaped by the increase in the laser intensity, the end of the XUV emission happens when the laser frequency becomes too high (in harmonic generation, a low-frequency field will produce higher XUV frequencies than a high-frequency field with the same intensity).⁶ It is this combined intensity-frequency effect, resulting from the self-compression process, that allows for the synthesis of the isolated 560as pulse shown in Figure 1(b).

Isolated attosecond XUV pulses can be produced by filamentation-driven self-compressed few-cycle laser pulses. The short duration of the self-compressed pulse, in combination with the rapid dynamical blueshift of its instantaneous frequency, present a steep time-frequency gate for the emission of XUV radiation with the highest frequencies. These results constitute a promising starting point for ultra-fast nonlinear optics experiments using self-compressed pulses.