Generation & application of circularly-polarized high-order harmonics and attosecond pulses
High harmonic generation (HHG) is an extreme nonlinear optical process where many visible or infrared photons are converted into a single high-energy photon in the extreme UV and x-ray spectral regions. Light sources based on HHG exhibit unique and very useful features, including pulses with attosecond to femtosecond duration, and broadly tunable bandwidths (femto and atto stand for 10-15 and 10 -18). Indeed, over the past several years scientists around the world learn to employ HHG for exploring new frontiers in optical, atomic, molecular and solids state physics.
Peculiarly, one fundamental property of HHG has defied control: the polarization. Circularly polarized HHG radiation is promising for exploring the structural, electronic and magnetic properties and dynamics of atoms, molecules and materials. While changing the polarization of visible light is as simple as rotating a waveplate, it is more complicated in the XUV and x-ray spectral regions where waveplates are very lossy and spectrally limited. Hence, it is desirable to control the polarization of HHG-based sources in the generation step.
Science’s cover highlighting our demonstration of bright circularly-polarized high-harmonics in its special issue for 2015 international year of light. When an atom is shined by a strong rosette-shape laser pulse (purple), which corresponds to superposition of red and blue circularly-polarized lasers with opposite helicity, an electron (green) is ripped from, and recollides with the parent ion from three directions. As a result, circularly-polarized high-order harmonics are emitted. This long-sought-for bright and chiral probe at the extreme UV allows unraveling the physics of ultrafast chirality.
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