Attosecond pulse generation and detectionThe conjecture of producing a train of attosecond pulses using high-order harmonic generation (HHG) has been put forward almost 10 years ago. More recently, detailed theoretical investigations have confirmed this possibility and shown that it is indeed conceivable to generate a train or even a single attosecond pulse. The method utilizes the coherent properties of the high harmonics produced in the interaction of laser light with atoms in a manner analogous to the short pulse production in mode-locked lasers. Nonetheless, it is only now that experimental evidence has started accumulating, indicating that the femtosecond barrier towards attosecond pulses might have fallen. Provided that they are well characterized, they may serve as the fastest camera, able to record frozen snapshots of ultra fast electron dynamics. Consequently, they are of great interest to a variety of scientific areas including atomic, molecular, solid state and plasma physics, as well as to material science. The challenging problem is to find a measuring technique that unequivocally verifies the existence of attosecond pulses and it provides a precise temporal characterization.
In principle, one should be able to apply the well known techniques from femtosecond pulse metrology like successive optical auto-correlations of increasing order to obtain the answer, i.e., characterize the attosecond pulse and ultimately measure its exact duration. The difficulty arises from the fact that, in contrast to femtosecond, the attosecond pulses are necessarily in the UV-XUV spectral range, they are orders of magnitude weaker and spectrally much broader. In case of femtosecond pulses with frequencies in the visible spectral region, an amplitude splitting interferometer (Michelson or Mach-Zehnder) in conjunction with a second harmonic crystal is commonly used. Both basic components, interferometer and detector, have to be appropriately modified or adapted for operation in the XUV spectral region. In this case, a beam splitting interferometer using a free-standing transmission grating, adapted for operation in the 10 - 100 nm spectral region is ideally suited for the temporal characterization of attosecond pulses. In conjunction with focusing mirrors, this interferometer can be made to exhibit dispersion characteristics that allow measurement of pulses with few as duration [(see: E. Goulielmakis et al., Appl. Phys. B 74, 197(2002)] .
The idea of utilizing a grating as a beam splitter is based on the natural splitting of an incident beam through dispersion into different orders. This is depicted in the figure above where a monochromatic beam incident on the grating is diffracted into one zeroth order and two first orders (for simplicity, only one is shown). Two mirrors reflect the zeroth order and one of the first orders straight back through the grating. In the second passage through the grating, the first order of the primary zeroth order automatically propagates in the same direction as the zeroth order of the primary first order. Because of the equal number of dispersions into the first and zeroth order involved, the splitting of the original beam in the two arms of the interferometer is exactly 50% independently of the efficiency of the grating. The transmission grating interferometer exhibits an additional feature, namely it spectrally analyzes the incident radiation. This property is especially desirable in case of gas harmonics produced by relatively long laser pulses where the spectrum is discrete. Then, the isolation of a single harmonic or a group of harmonics can be easily implemented by simple geometrical obstacles like apertures or knife-edges.
The first experimental utilization of this grating based interferometer was the characterization with respect to its duration and spatial coherence of the third harmonic of a 1kHz repetition rate Ti:sapphire laser system delivering ~50fs pulses . The harmonic was generated in an Ar gas cell and its temporal characterization was performed by measuring the second order interferometric as well as the intensity AC traces in a second cell containing gaseous Toluene (C7H8). The results are depicted in the figure below along with the Fourier spectrum of the autocorrelation trace.
The observation of a non-linear process, e.g. multiphoton ionization or inner-shell transition, induced by short-pulse coherent XUV radiation, such as the high-order harmonics of fs-laser radiation from rare gases, has been a challenging problem for a long time now. This is not only because of the interesting new physics inherent to the process, but also because it opens up the possibility of applying well established approaches in fs-laser pulse metrology to short pulses in the XUV wavelength region by providing an appropriate non-linear detector. Thus, second or higher order auto-correlation techniques can be appropriately modified for XUV radiation and utilized for the temporal characterization of individual higher-order harmonics or of a harmonic superposition.