A NIR femtosecond pulse (pump pulse) induces electron oscillation, which is monitored by an extreme ultraviolet (EUV) IAP (probe pulse) based on transient absorption spectroscopy. (Credit: Nippon Telegraph and Telephone)

Attosecond light pulses have enabled Japanese researchers at Yokohama National University (YNU) and Nippon Telegraph and Telephone (NTT) to observe petahertz (10¹⁵ Hz) rate oscillations of electrons. The resulting periodic oscillations of 667-383 attoseconds is the fastest that has ever been measured using direct time-dependent spectroscopy in solid-state material.

Researchers generally use ultrafast light pulses to observe the ultrafast motion of an electron underlying a physical phenomenon. The shorter the pulse duration, the faster the electron oscillation that can be observed. The lightwave-field frequency of high-harmonic generation of light in the visible and UV region can reach the petahertz (PHz: 10¹⁵ Hz) region, which means that the oscillation periodicity can achieve attosecond (10^-18 of a second) duration.

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In previous studies, NTT researchers in the team generated an isolated attosecond pulse (IAP) [H. Mashiko et al., Nature Commun. 5, 5599 (2014)] and monitored the electron oscillation with 1.2-PHz frequency within a gallium nitride (GaN) semiconductor [H. Mashiko et al., Nature Phys. 5, 741 (2016)]. The next challenges will be to observe faster electron oscillation in a chromium doped sapphire (Cr:Al₂O₃) insulator and to characterize ultrafast electron dephasing.

The paper, published in the journal Nature communications reports a successful observation of near-infrared (NIR) pulse-induced multiple electronic dipole oscillations (periodicities of 667-383 as) in the Cr:Al₂O₃ solid-state material. The measurement is realized by the extreme short IAP (192 attosecond duration) and the use of stable pump (NIR pulse) and probe (IAP) system (timing jitter of ~23 attoseconds). The characterized electron oscillations are the fastest that have ever been measured using direct time-dependent spectroscopy.

The research results will aid in developing future electronic and photonic devices, says Hiroki Mashiko, a NTT scientist on the team.

(From: https://www.eurekalert.org/pub_releases/2018-04/ynu-ueo041818.php)