Unprecedentedly bright quantum light generates high harmonics
Quantum optics and high harmonic generation (HHG) have historically been extremely prolific research areas with little or no overlap. This is because, until very recently, quantum light was too weak to drive nonlinear effects beyond the perturbative regime. It could not enable HHG – a strong-field effect particularly important for the study of ultrafast phenomena in matter. Now, two groups at the Max Plank Institute for the Science of Light (MPL) and researchers from Canada and Israel have succeeded in generating unprecedentedly bright quantum light and using it for HHG – a groundbreaking result published in Nature Physics.
As in all optical experiments, the key point is alignment. © Tanya Chekhova
HHG is an extreme, nonlinear process in which intense ultrashort pulses of light drive electrons to emit high harmonics of the driving field frequency. The process lies at the foundation of attosecond science and creates unique sources of ultrashort pulses in extreme ultraviolet and X-ray spectral regions. Previously, HHG has been driven only by classical light. Now, the groups of Maria Chekhova and Francesco Tani at MPL have observed HHG from a bright squeezed vacuum – a quantum state of light – and shown it to be more efficient than using classical light. “We know that most interesting things happen at the boundaries of different fields, and when we reach extreme values of some parameter. Our work is at the boundary of quantum optics and strong-field physics, and the parameter that is very large is the number of photons in a squeezed vacuum state”, says Prof. Maria Chekhova.
The MPL group engineered a source of quantum light whose number of photons per pulse is as high as for very powerful lasers – up to 1013 – but whose photon statistics are very different from those of laser sources. This state of light, known as bright squeezed vacuum, has pronounced quantum features despite its extremely high number of photons. For instance, its electric field is in a quantum superposition of being ‘up’ and ‘down’. Using bright squeezed vacuum, researchers generated up to the seventh optical harmonic in solids, achieving enhanced efficiency and unusual photon statistics compared to the case of coherent light excitation. Moreover, excitation with bright squeezed vacuum has proven less invasive for the sample and to enable a broader range of electron excitation to be probed simultaneously.
This work opens a path for the study of quantum effects in strong-field physics. Further, scientists expect to see quantum features in the behavior of electrons excited by quantum light. This will enable coupling between quantum light and quantum matter, another boundary-breaking field in quantum physics.
Scientific contacts:
Prof. Maria Chekhova
Research Group Leader "Quantum Radiation"
Max Planck Institute for the Science of Light
maria.chekhova@mpl.mpg.de
Dr. Francesco Tani
Research Group Leader "Ultrafast & Twisted Photonics"
Max Planck Institute for the Science of Light
francesco.tani@mpl.mpg.de
