Femtosecond Techniques Herald Attophysics

Daniel S. Burgess

In pursuit of shorter and shorter sources of illumination to probe dynamic physical phenomena, researchers at Technische Universität Wien in Austria and at Max Planck Institut für Quantenoptik in Garching, Germany, have used femtosecond techniques to produce laser pulses that can control electronic processes on the timescale of attoseconds.

The work promises to enable the investigation of ultrafast atomic processes and to give reality to theoretical understandings of matter.

Andrius Baltu(breve)ska of Technische Universität Wien compared the development to the work of Ahmed H. Zewail of California Institute of Technology in Pasadena, who won the 1999 Nobel Prize in chemistry for his efforts in the development of femtosecond pulse-probe spectroscopy. "Zewail used to speak of the femtosecond process like flash photography," said Baltu(breve)ska, explaining that the pulses enabled the user to take extremely brief "snapshots" of a chemical reaction as it progressed. "This is flash photography of atomic transitions."

Combining the work of two influential research groups, a femtosecond laser system produces phase-stabilized pulses for attosecond physics. Images courtesy of Technische Universität Wien.

The demonstration integrated the work by Theodor Hänsch's team at the German institute into the development of an all-optical-frequency standard based on mode-locked femtosecond lasers and on the efforts by Ferenc Krausz's group in Austria into the high-harmonic generation of ultrashort pulses of soft x-rays. By adding two feedback loops to stabilize the relationship of the position of the carrier wave and the pulse envelope of the laser pulses, the researchers produced a series of intense pulses that are identical on the level of 250 attoseconds.

In the experimental setup, a nonlinear interferometer based on a photonic crystal fiber measured the carrier-envelope phase drift in the pulses emerging from a Kerr-lens mode-locked Ti:sapphire seed oscillator. Varying the power of the CW Nd:YVO₄ laser that pumped the device compensated for this drift.

A second interferometer, incorporating a frequency-doubling crystal and a CCD camera, monitored the drift in the pulses that emerged from a multipass chirped-pulse Ti:sapphire amplifier seeded by the oscillator and pumped by a Q-switched frequency-doubled diode-pumped Nd:YAG laser. This offered an additional source of feedback with which the setup precompensated for drift by shifting the phase back at the oscillator.

Attosecond science

They next focused the 0.2-mJ, 5-fs phase-stabilized pulses from the laser system into a 2-mm-long jet of neon gas to produce pulses of soft x-rays. Using a spectrometer comprising a 10,000-line-per-millimeter transmission grating and a back-illuminated x-ray CCD camera to analyze the resulting spectrum at energies greater than 80 eV, they found that they could control the timing of the x-ray pulses on the order of less than 250 attoseconds by manipulating the phase of the laser pulses.Potential applications of the approach include attosecond spectroscopy and metrology. Theoreticians could test their predictions of the behavior of atoms, for example, in attosecond experiments. And just watching the way matter behaves at these timescales is likely to lead to a better understanding of the world. "That's the most straightforward way to study things," Baltu(breve)ska said. "Little advance knowledge is needed at the time to follow the dynamic changes in matter. You just observe the patterns. You look, attosecond by attosecond, at what is happening." Other applications include tabletop particle acceleration.

In a demonstration of the setup, the researchers produced ultrashort pulses of soft x-rays from a neon gas target.

The researchers plan to use the technique as they revisit an earlier investigation of Auger decay in photo-excited krypton that employed 900-attosecond x-ray pulses. They also would like to probe above-threshold ionization with the method.

More avenues of inquiry are bound to open. Baltu(breve)ska said that the apparatus is the first of its kind, but he again drew parallels with the emergence of femtochemistry. "In the years to come, you will see such setups built in labs around the world."