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Record pulse provides new tool for observing quantum mechanics

Photonics Spectra
Nov 2012

The young field of attosecond science marked a significant milestone recently with the generation of the shortest laser pulse to date – 67 attoseconds.

The record, achieved by Dr. Zenghu Chang and colleagues at the University of Central Florida, marks the most significant breakthrough in laser pulses in four years and gives scientists a new tool for watching quantum mechanics in action.

With the development of attosecond light sources a decade ago, ultrafast laser science became a viable research field. It began in 2001, when groups in Saclay, France, and Vienna proved the production of attosecond pulse trains and isolated attosecond pulses through high-harmonic-generation processes, respectively.

Photonics Spectra caught up with Chang shortly after his return from The Workshop on Super Intense Laser-Atom Physics, held in China in late September.

“People were very excited” about his group’s achievement, said Chang, a professor in the physics department at UCF and CREOL, the College of Optics and Photonics.

Zenghu Chang and colleagues at the University of Central Florida have generated the shortest laser pulse to date – 67 attoseconds. The work gives scientists a new tool for observing quantum mechanics.

Chang’s team achieved the 67-as (1 attosecond is a quintillionth of a second) pulse using extreme-ultraviolet (EUV) light generated not by a mile-long particle accelerator or a Superdome-size synchrotron, but by using a technique the group created called double optical gating, or DOG (techniques in the field tend to have animal-themed acronyms, such as FROG-CRAB, or frequency-resolved optical gating for complete reconstruction of attosecond bursts).

The technique allows EUV light to be cut off in a way that concentrates the maximum amount of energy in the shortest possible pulse of light. The shortest possible light burst is a powerful tool for freezing electron motion in atoms, molecules and condensed matter.

In their paper, which appeared in Optics Letters, Chang and his team used two algorithms, FROG-CRAB and PROOF (phase retrieval by omega oscillation filtering) to characterize the 67-as pulse generated using DOG, and both yielded nearly identical results. The durations of the DOG-generated pulses were measured with a streak camera.

The pulse beat the previous record of 80 attoseconds that was set by the Max Planck Institute in Garching, Germany, in 2008; this is the first time an American-led team has set the record.

“With not a significant change [to the current setup], we think we can get to 50 attoseconds,” Chang told Photonics Spectra.

In principle, the DOG technique can achieve 25 as, which is near the atomic unit of time and fast enough to allow the observation of electron-electron interactions. But before that can happen, he said, a fundamental issue that must be addressed is chirp compensation.

“Chirp compensation doesn’t work well with DOG. We need to find new ways to compress the pulse,” he said, once the pulse duration is below 40 as.

“Dr. Chang’s success in making ever-shorter light pulses helps open a new door to a previously hidden world, where we can watch electrons move in atoms and molecules, and follow chemical reactions as they take place,” said UCF physicist and College of Sciences Dean Michael Johnson in a university release. “It is astounding to imagine that we may now be able to watch quantum mechanics in process.”

“We’ll see practical applications in five years or so,” Chang said, noting that “The Department of Defense is interested in attosecond science, and DARPA is interested.”

DARPA’s basic research program PULSE (Program in Ultrafast Laser Science and Engineering) aims to enhance the capabilities of tabletop high-peak-power pulsed-laser-driven x-ray generation techniques to produce high-flux coherent x-rays with wavelengths from 2.3 to 4.4 µm for bioimaging applications.

Next for Chang and his team are increasing the flux of attosecond pulses to strengthen the pulse and developing an attosecond pump probe.

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