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Ultrafast quantum physics on the sub-cycle time scale

Final 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 [Nature Communications 5, 4648 (2014), Nature Materials 13, 857 (2014), Nature Materials 14, 889 (2015)] whereas custom-tailored metallic nanostructures offer versatile means to chisel THz waves with subcycle 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:

1. Charge control: The electric carrier field component of intense THz pulses has been exploited as a transient biasing field to drive charge currents through three- and two-dimensional solids. This idea, which is often referred to as “lightwave electronics”, has been used to drive Bloch oscillations in bulk solids [Nature Photonics 8, 119 (2014)], to resolve the subcycle time structure of the concomitant high-harmonic radiation, for the first time [Nature 523, 572 (2015)], and to shape the waveforms of high-harmonic pulses by exploiting crystal symmetry [Nature Photonics 11, 227 (2017)]. Furthermore, the intense lightwaves have been used to demonstrate a novel quasiparticle collider [Nature 533, 225 (2016)]. Beside the translational motion, we have also managed to control internal degrees of freedom of electrons, such as the valley pseudospin. In atomically thin layers of WSe2, intense multi-THz pulses can switch the valley pseudospin on a subcycle scale, opening the door to future room-temperature quantum information processing based on lightwave valleytronics [Langer et al., Nature 557, 76 (2018)]. The high sensitivity of shot-noise reduced electro-optic sampling [Opt. Lett. 39, 2435 (2014)] has also allowed us to bring subcycle resolution to the nanometer length scale in ultrafast near-field microscopy of semiconductor nanowires [Nature Photonics 8, 841 (2014)] and photo-switchable surface plasmons on black phosphorus [Nature Nanotechnology 12, 207 (2017)]. By combining lightwave-electronics with atomic resolution scanning tunneling microscopy, we were the first to take femtosecond snapshots of individual molecular orbitals of single molecules and to record the first-ever femtosecond movie of a single vibrating molecule [Nature 539, 263 (2016)].

2. Control of electron spins: Utilizing low-frequency intense THz pulses, we have successfully entered the realm of a field-induced nonlinear spin response optionally mediated by the magnetic [Baierl et al., Physical Review Letters 117, 197201 (2016)] or the electric field [Baierl et al., Nature Photonics 10, 715 (2016)]. By enhancing the local electric field in the vicinity of a metallic THz nanoantenna, we have observed characteristic fingerprints of spin switching.

3. Control of vacuum photons: Even without the presence of any light, electromagnetic fields exhibit quantum fluctuations. This vacuum field can be harnessed to explore a new realm of quantum electrodynamics. We have exploited the field-enhancement in the vicinity of custom-tailored metallic nanoantennas to couple such a vacuum field to cyclotron resonances of two-dimensional electron gases in semiconductor quantum wells. By optimizing the system, we have reached a regime in which the rate of photon exchange between the cyclotron and the metamaterial resonances exceeds the resonance frequency of the metamaterial itself [Nano Letters 17, 6340 (2017)]. Under these conditions, ultrafast changes of the coupling strength have been theoretically predicted to lead to the generation of real photons out of the quantum vacuum. We have demonstrated that non-adiabatic changes of the cyclotron [Nature Physics 12, 119 (2016)] and the metamaterial [Physical Review Letters 113, 227401 (2014)] resonances my indeed be driven by strong THz fields. With our improved subcycle quantum sensitivity, we think that the stage is now set to detect nonadiabatic quantum vacuum radiation.