Journal Article

Optical attosecond pulses and tracking the nonlinear response of bound electrons

Luu, T. T., Karpowicz, N., Zheltikov, A. M., Moulet, A., Pervak, V., Goulielmakis, E., Hassan, M. Th, Zhokhov, P., Raskazovskaya, O., Garg, M., Krausz, F.

2016

Overview

Intense light pulses in the visible and adjacent spectral ranges with their energy mostly confined to a half wave cycle—optical attosecond pulses—are synthesized and used to measure the time it takes electrons to respond to light. Sub-femtosecond control of bound electrons A fundamental speed limit for controlling matter through the electromagnetic force of light arises from the time it takes bound electrons to respond. Experiments have shown that this response is not instantaneous, but the lack of sufficiently fast probes has prevented direct measurements. Eleftherios Goulielmakis and colleagues have now produced intense light pulses in the visible and nearby spectral ranges and with energy largely confined to a half wave cycle, and show that these so-called optical attosecond pulses can control and measure the dynamics of bound electrons in krypton atoms. Proof-of-principle measurements establish the value of optical attosecond pulses for probing and manipulating bound electrons in atoms, molecules or solids, and suggest they may also find use in light-based nonlinear photonics operating on sub-femtosecond time scales and petahertz rates. The time it takes a bound electron to respond to the electromagnetic force of light sets a fundamental speed limit on the dynamic control of matter and electromagnetic signal processing. Time-integrated measurements of the nonlinear refractive index 1 of matter indicate that the nonlinear response of bound electrons to optical fields is not instantaneous; however, a complete spectral characterization of the nonlinear susceptibility tensors 2 —which is essential to deduce the temporal response of a medium to arbitrary driving forces using spectral measurements—has not yet been achieved. With the establishment of attosecond chronoscopy 3 , 4 , 5 , the impulsive response of positive-energy electrons to electromagnetic fields has been explored through ionization of atoms 6 and solids 7 by an extreme-ultraviolet attosecond pulse 8 or by strong near-infrared fields 9 , 10 , 11 . However, none of the attosecond studies carried out so far have provided direct access to the nonlinear response of bound electrons. Here we demonstrate that intense optical attosecond pulses synthesized in the visible and nearby spectral ranges allow sub-femtosecond control and metrology of bound-electron dynamics. Vacuum ultraviolet spectra emanating from krypton atoms, exposed to intense waveform-controlled optical attosecond pulses, reveal a finite nonlinear response time of bound electrons of up to 115 attoseconds, which is sensitive to and controllable by the super-octave optical field. Our study could enable new spectroscopies of bound electrons in atomic, molecular or lattice potentials of solids 12 , as well as light-based electronics operating on sub-femtosecond timescales and at petahertz rates 13 , 14 , 15 .

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/624/400/584 / 639/766/400/385 / Atomic physics / Driving ability / Electromagnetic fields / Electromagnetics / Electrons / Humanities and Social Sciences / Ionization / Krypton / Laser pulses, Ultrashort / letter / Light / multidisciplinary / Properties / Science / Signal transduction / Studies