Abstract
The debate on tunnelling times have always been full of contradictions and the attoclock experiments that measure tunnelling delays in strong-field ionization are no exception. The current review presents the debate and discussions concerning the studies of tunnelling times based only on the attoclock technique. We review them with their implications and pitfalls identified due to lack of accurate strong field models that validate the observations in interpreting the measurements performed on noble gases. In order to provide a complete picture, the review begins with a background on some of the popular tunnelling time definitions, most of them conceived during the late 1980s debate, which are often cited in the attoclock literature. We then discuss various attoclock experiments on noble gas atoms and their interpretations in context of the tunneling time debate. The recently performed attoclock experiment and numerical modelling using atomic hydrogen are also presented as an attempt at resolving the controversy. We conclude with the current status of the debate.
Highlights
It is central to many chemical and biological processes [1], to electron transport in semiconductor diodes/ transistors [2, 3], molecular junctions [4], nano-structures [5, 6] etc quantum tunnelling is well understood and exploited in various applications, there is no consensus in the scientific community on ‘How long does it take for a particle to tunnel through the barrier?’ This question forms the very crux of the tunnelling time problem
In contrast to the above interpretation leading to finite tunnelling times, Pollak [158, 159] defines tunnelling flight time wherein the tunnelled wavepacket through a barrier is measured in a time-of-flight kind of measurement
The vast difference in the ionisation potential of these two species pose a challenge experimentally to exploit the right intensity regime to explore tunnelling delays. It is important for the tunnelling time community to understand that the attoclock debate has its origin primarily in the inability of strong-field models to predict accurately the ionization dynamics of complex systems like noble gases
Summary
Quantum tunnelling is one of the key features of quantum mechanics. It is central to many chemical and biological processes [1], to electron transport in semiconductor diodes/ transistors [2, 3], molecular junctions [4], nano-structures [5, 6] etc quantum tunnelling is well understood and exploited in various applications, there is no consensus in the scientific community on ‘How long does it take for a particle to tunnel through the barrier?’ This question forms the very crux of the tunnelling time problem. These debates of 1980-90s together with the experiments performed could not answer the question unambiguously keeping physicists in a limbo It took another 20 years for the precision metrology using ultrafast laser technology to offer a new experimental approach in the form of attosecond angular streaking (AAS) [12]. The current review is focused on the discussion of tunnelling times (or tunnelling delays as they are called in the strong-field community) from the perspective of previous attoclock experiments [12, 16–21] and the intricacies of their interpretations [21–32]. It presents the recent studies with atomic hydrogen (H) [33, 34] that were performed in an attempt to resolve the ongoing controversy on tunnelling delays. The current state of the debate is presented with concluding remarks
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