Abstract
Laser spectroscopy and chromoscopy permit precision measurement of quantum transitions and captures atomic-scale dynamics, respectively. Frequency- and time-domain metrology ranks among the supreme laser disciplines in fundamental science. For decades, these fields evolved independently, without interaction and synergy between them. This has changed profoundly with controlling the position of the equidistant frequency spikes of a mode-locked laser oscillator. By the self-referencing technique invented by Theodor Hänsch, the comb can be coherently linked to microwaves and used for precision measurements of energy differences between quantum states. The resultant optical frequency synthesis has revolutionized precision spectroscopy. Locking the comb lines to the resonator round-trip frequency by the same approach has given rise to laser pulses with controlled field oscillations. This article reviews, from a personal perspective, how the bridge between frequency- and time-resolved metrology emerged on the turn of the millennium and how synthesized several-cycle laser fields have been instrumental in establishing the basic tools and techniques for attosecond science.
Highlights
This article is part of the topical collection “Enlightening the World with the Laser” - Honoring T
The frequency of microwave to optical radiation absorbed or emitted upon electronic transitions between quantum states of atoms has been the most accurately measured physical quantity to date, allowing to put the laws of quantum physics to a test, determine fundamental constants, and define the standards for time and length [1]. Precision spectroscopy underlying these applications relies on excited quantum states the energy of which is precisely defined
In terms of which quantum mechanics describes molecular dynamics of any kind, occur on a timescale of several to several hundred femtoseconds (1 fs = 10−15 s), see Ref. [2], whereas electronic wavepacket dynamics underlying any dynamic change of electronic structure unfolds over tens to thousands of attoseconds (1 as = 10−18 s), see Ref. [3]
Summary
Krausz radically on the turn of millennium, thanks to gaining full control over the series of equidistant eigenfrequencies (frequency comb) of a mode-locked laser by Theodor Hänsch and his coworkers, Ronald Holzwarth and Thomas Udem In doing so, they have created a clockwork able to count optical field oscillations of more than 1015 cycles per second. The resultant optical frequency synthesis led to the first fully controlled femtosecond pulses from a modelocked laser, controlled in terms of their pulse envelope, and in terms of their field oscillations These advances have been instrumental in creating the ability to observe and control atomic-scale electronic motions in real time [3].
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