Less Complex Approach to Laser Pulse Measurement Provides Precise Information

Until now, a complex experimental setup was required to measure the shape of a laser lightwave with a high degree of accuracy. This measurement can now be done using a small crystal with a diameter of less than 1 mm. The new method, developed by scientists at the Max Planck Institute (MPI) for Quantum Optics, Ludwig Maximilian University (LMU), and the Vienna University of Technology (TU Wien), uses extremely short pulses, with a duration in the order of femtoseconds.

“In order to create an image of such lightwaves, they must be made to interact with electrons,” professor Joachim Burgdörfer said. “The reaction of the electrons to the electric field of the laser gives us very precise information about the shape of the light pulse.”

The traditional way to measure an infrared (IR) laser pulse has been to add a much shorter laser pulse with a wavelength in the x-ray range and send both pulses through a gas medium. The x-ray pulse ionizes individual atoms. Electrons are released, which are then accelerated by the electric field of the IR laser pulse. The motion of the electrons is recorded, and if the experiment is carried out many times with different time shifts between the two pulses, the shape of the IR laser pulse can eventually be reconstructed. “The experimental effort required for this method is very high,” professor Christoph Lemell said.

The researchers took a simpler approach — to begin, they decided to measure light pulses not in a gas but in a solid. “In a gas you have to ionize atoms first to get free electrons,” researcher Isabella Floss said. “In a solid it is sufficient to give the electrons enough energy so that they can move through the solid, driven by the laser field.” As electrons move through the solid, an electric current is generated that can be directly measured.

The researchers fired two different laser pulses at silicon oxide crystals that were just a few hundred μm in diameter. The wavelength of the pulse to be measured could range from ultraviolet (UV) to visible (VIS) to longwave infrared (LWIR). The second pulse was a strong pulse in the IR range.

Two light pulses are hitting the silicon dioxide crystal. Courtesy of TU Wien.

While the pulse being measured penetrated the crystal, the second pulse was fired at the target. “This second pulse is so strong that nonlinear effects in the material can change the energy state of the electrons so that they become mobile. This happens at a very specific point in time, which can be tuned and controlled very precisely,” Burgdörfer said.

The electrons were accelerated by the electric field of the first beam as soon as they moved through the crystal, producing an electric current that could be measured directly at the crystal and that contained precise information about the shape of the light pulse.

Researchers at TU Wien studied the method theoretically and analyzed it in computer simulations, and researchers at the MPI for Quantum Optics performed the experiment. “Thanks to the close cooperation between theory and experiment, we have been able to show that the new method works very well, over a large frequency range, from ultraviolet to infrared,” Lemell said. “The waveform of light pulses can now be measured much more easily than before, with the help of a much simpler and more compact setup.”

It could be possible to use this method to precisely characterize new materials, to answer fundamental physical questions about the interaction of light and matter, and even to analyze complex molecules, for example, to detect diseases by examining tiny blood samples.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-019-14268-x).