Figure 1: The experimental set-up for measuring the coherence between pulses in an attosecond laser pulse sequence. The higher harmonics of a light pulse (blue) are spatially separated from one another in the harmonic separator. Each harmonic is then split in two parts, which travel down the arm of the instrument, before being recombined and focused onto a CCD camera.
Reproduced, with permission, from Ref. 1 © (2009) by the American Physical Society
Scientists at RIKEN have developed a way to measure the wavelike properties of ultrafast (attosecond) light pulses—an important step toward being able to probe the dynamics of electrons, atoms and molecules.
Quantum mechanics theory can completely describe the structure of atoms and molecules. But directly observing electronic motion in an atom requires a technique that can take snapshots of the electron on time scales of less than a femtosecond (10-15 s). To this end, scientists are working to generate ultraviolet light pulses that are only 10–100 attoseconds (10-18 s) long.
Electrons, like light, have wavelike properties. Thus, when a fast optical pulse—or sequence of pulses—interacts with the electrons in an atom, it creates an interference pattern that can effectively image the electron over time.
The challenge is to create a sequence, or ‘train’, of pulses, each with the same, well-defined wavelike properties. For this reason, the technique developed by Yasuo Nabekawa and colleagues at the RIKEN Advanced Science Institute in Wako allows them to compare consecutive pulses in an attosecond light pulse series1.
“Ultimately, the goal of our research is to control atoms and molecules with the attosecond pulse train,” says Nabekawa.
To produce the attosecond pulses, the team started with a series of intense laser-generated ultraviolet light pulses, each approximately 40 femtoseconds in duration. When the laser pulses interacted with a gas of xenon atoms, they generated pulses of light with odd integer (1, 3, 5, etc…) multiples of the frequency of the original laser pulse. These higher frequency pulses—or, ‘harmonics’—reached into the attosecond range.
Detecting ultrafast motion in atoms and molecules requires that the pulses in the train are ‘coherent’ with each other, meaning they are in phase, similar to soldiers marching in lock-step. The team therefore designed its experiment specifically to determine the coherence between the pulses in each of the higher harmonics.
Spatially separating the harmonics allowed the team to measure the coherence between pulses of each harmonic individually. Each harmonic was then split into two beams that traveled down a long arm, before being recombined (Fig. 1). A CCD camera measured the interference pattern between the recombined beams, which provides a measure of the coherence between pulses.
While the current measurements relate to characterizing the optical pulse itself, the RIKEN team plans to build upon these experiments to study ionization and dissociation of electrons from atoms and molecules.