Community Research and Development Information Service - CORDIS



Project reference: 305003
Funded under: FP7-IDEAS-ERC

Mid-Term Report Summary - QUANTUMSUBCYCLE (Ultrafast quantum physics on the sub-cycle time scale)

QUANTUMsubCYCLE utilizes precisely defined optical waveforms to drive quantum physics faster than a single cycle of light. Especially, intense terahertz (1 THz = 10^12 Hz) pulses provide direct access to key elementary dynamics in solids whereas custom-tailored metallic nanostructures offer versatile means to chisel THz waves with sub-cycle temporal and sub-wavelength spatial precision. This concept is ideally suited to observe and control charge, spin and photon dynamics via the electric and magnetic field of light. In the first project phase, tailored THz fields and ultrasensitive electro-optic sampling [Opt. Lett. 39, 2435 (2014)] have allowed us to achieve the following main results:

1. We use custom-tailored metallic THz nanoantennas to generate record large THz ponderomotive potentials >1 keV, driving massive interband tunnelling in a semiconductor. Electron-hole pair densities > 10^19 cm^-3 are created within one half cycle of the THz field, causing bright interband photoluminescence for future THz-controlled optoelectronics [Phys. Rev. Lett. 113, 227401 (2014)].
2. Intense THz pulses can also drive a two-dimensional electron gas beyond the limits of the famous Kohn theorem, giving rise to unexpected anharmonic population transfer and THz multi-wave mixing on sub-cycle scales. These results open up new realms of ultrafast quantum control of many-body systems [submitted].
3. Atomically strong multi-THz transients with peak fields of 72 MV/cm are shown to drive a counterintuitive quantum motion of electrons – called dynamical Bloch oscillations – in bulk semiconductors. Here the lightwave induces a coherent charge transport throughout the entire Brillouin zone and generates high-harmonic radiation covering the THz-to-visible domain between 0.1 and 675 THz [Nature Photonics 8, 119 (2014)]. With a new correlation scheme we clock the sub-cycle emission of the high-harmonic bursts, for the first time. The recorded temporal structure originates from a novel strong-field quantum interference between electrons on different quantum paths. The results open new vistas for coherent lightwave electronics and solid-state attosecond sources [Nature 523, 572 (2015)].
4. Investigating the sub-cycle charge dynamics of layered van-der-Waals crystals, we identify the microscopic origin of the famous charge-density-wave transition in TiSe_2 [Nature Materials 13, 857 (2014)] and trace the internal structure as well as the ultrafast radiative recombination of excitons – Coulomb-bound electron-hole pairs – in monolayers of WSe_2 [Nature Materials 14, 889 (2015)].
5. While custom-tailored metallic nanoantennas permit four-dimensional shaping of electromagnetic waveforms, these structures cannot be repositioned. We have, therefore, started to exploit the near-field of scanning probe microscopes. Using electro-optic characterization of THz pulses scattered from the near-field of a metallized atomic force microscope tip, we demonstrate scanning near-field optical microscopy with simultaneous sub-cycle (10 fs) time and sub-nanoparticle (10 nm) spatial resolution [Nature Photonics 8, 841 (2014)]. We also use an intense THz half-cycle as an AC bias of a scanning tunnelling microscope, combining sub-cycle with atomic resolution [in preparation].
6. When THz pulses with magnetic amplitudes of up to 0.4 Tesla drive collective magnon oscillations in NiO a narrowband Faraday signal at the second harmonic of the dominant 1-THz magnon mode is observed, manifesting the first magnetically induced THz spin nonlinearity. This observation foreshadows physics that may become essential in sub-cycle spin switching [submitted]. In a further study, we demonstrate a large nonlinearity occurring when the quasi-ferromagnetic magnon in TmFeO_3 is resonantly driven close to a reorientational phase transition. For strong THz fields the magnon amplitude increases by 500% with respect to its linear response, potentially heralding spin reorientation [in preparation].

The fundamental idea to control charge currents and spin dynamics directly by the electric and magnetic carrier waves of light has been dubbed ‘lightwave electronics’ and ‘lightwave magnonics’. Our results confirm the validity of these concepts. In the future, they may well be employed for the implementation of information technology at optical clock rates. In addition, sub-cycle manipulations of light-matter coupled systems are expected to become a rich playground for novel non-adiabatic quantum electrodynamics, analogous to the predicted Unruh-Hawking radiation of black holes.


KOEHLER, Matthias
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Record Number: 183583 / Last updated on: 2016-06-16