Abstract-Nonlinear spin control by terahertz-driven anisotropy fields

• S. Baierl,
• M. Hohenleutner,
• T. Kampfrath,
• A. K. Zvezdin,
• A. V. Kimel,
• R. Huber
• & R. V. Mikhaylovskiy

http://www.nature.com/nphoton/journal/v10/n11/full/nphoton.2016.181.html

Future information technologies, such as ultrafast data recording, quantum computation or spintronics, call for ever faster spin control by light^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}. Intense terahertz pulses can couple to spins on the intrinsic energy scale of magnetic excitations^5, 11. Here, we explore a novel electric dipole-mediated mechanism of nonlinear terahertz-spin coupling that is much stronger than linear Zeeman coupling to the terahertz magnetic field^5, 10. Using the prototypical antiferromagnet thulium orthoferrite (TmFeO₃), we demonstrate that resonant terahertz pumping of electronic orbital transitions modifies the magnetic anisotropy for ordered Fe³⁺ spins and triggers large-amplitude coherent spin oscillations. This mechanism is inherently nonlinear, it can be tailored by spectral shaping of the terahertz waveforms and its efficiency outperforms the Zeeman torque by an order of magnitude. Because orbital states govern the magnetic anisotropy in all transition-metal oxides, the demonstrated control scheme is expected to be applicable to many magnetic materials.

Abstract-Sub-cycle control of terahertz high-harmonic generation by dynamical Bloch oscillations

O. Schubert, M. Hohenleutner, F. Langer, B. Urbanek, C. Lange, U. Huttner, D. Golde, T. Meier, M. Kira, S. W. Koch, R. Huber

(Submitted on 13 Apr 2016)

http://arxiv.org/abs/1604.03732

Ultrafast charge transport in strongly biased semiconductors is at the heart of highspeed electronics, electro-optics, and fundamental solid-state physics. Intense light pulses in the terahertz (THz) spectral range have opened fascinating vistas: Since THz photon energies are far below typical electronic interband resonances, a stable electromagnetic waveform may serve as a precisely adjustable bias. Novel quantum phenomena have been anticipated for THz amplitudes reaching atomic field strengths. We exploit controlled THz waveforms with peak fields of 72 MV/cm to drive coherent interband polarization combined with dynamical Bloch oscillations in semiconducting gallium selenide. These dynamics entail the emission of phase-stable high-harmonic transients, covering the entire THz-to-visible spectral domain between 0.1 and 675 THz. Quantum interference of different ionization paths of accelerated charge carriers is controlled via the waveform of the driving field and explained by a quantum theory of inter- and intraband dynamics. Our results pave the way towards all-coherent THz-rate electronics.

Abstract-Coherent cyclotron motion beyond Kohn’s theorem

• T. Maag,
• A. Bayer,
• S. Baierl,
• M. Hohenleutner,
• T. Korn,
• C. Schüller,
• D. Schuh,
• D. Bougeard,
• C. Lange,
• R. Huber,
• M. Mootz,
• J. E. Sipe,
• S. W. Koch
• & M. Kira

http://www.nature.com/nphys/journal/v12/n2/full/nphys3559.html

In solids, the high density of charged particles makes many-body interactions a pervasive principle governing optics and electronics. However, Walter Kohn found in 1961 that the cyclotron resonance of Landau-quantized electrons is independent of the seemingly inescapable Coulomb interaction between electrons. Although this surprising theorem has been exploited in sophisticated quantum phenomena¹, such as ultrastrong light–matter coupling, superradiance and coherent control, the complete absence of nonlinearities excludes many intriguing possibilities, such as quantum-logic protocols. Here, we use intense terahertz pulses to drive the cyclotron response of a two-dimensional electron gas beyond the protective limits of Kohn’s theorem. Anharmonic Landau ladder climbing and distinct terahertz four- and six-wave mixing signatures occur, which our theory links to dynamic Coulomb effects between electrons and the positively charged ion background. This new context for Kohn’s theorem unveils previously inaccessible internal degrees of freedom of Landau electrons, opening up new realms of ultrafast quantum control for electrons.

Abstract-Real-time observation of interfering crystal electrons in high-harmonic generation

• M. Hohenleutner,
• F. Langer,
• O. Schubert,
• M. Knorr,
• U. Huttner,
• S. W. Koch,
• M. Kira
• & R. Huber

http://www.nature.com/nature/journal/v523/n7562/full/nature14652.html

Acceleration and collision of particles has been a key strategy for exploring the texture of matter. Strong light waves can control and recollide electronic wavepackets, generating high-harmonic radiation that encodes the structure and dynamics of atoms and molecules and lays the foundations of attosecond science^1, 2, 3. The recent discovery of high-harmonic generation in bulk solids^4, 5, 6combines the idea of ultrafast acceleration with complex condensed matter systems, and provides hope for compact solid-state attosecond sources^6, 7, 8 and electronics at optical frequencies^3, 5, 9,10. Yet the underlying quantum motion has not so far been observable in real time. Here we study high-harmonic generation in a bulk solid directly in the time domain, and reveal a new kind of strong-field excitation in the crystal. Unlike established atomic sources^{1, 2, 3, 9, 11}, our solid emits high-harmonic radiation as a sequence of subcycle bursts that coincide temporally with the field crests of one polarity of the driving terahertz waveform. We show that these features are characteristic of a non-perturbative quantum interference process that involves electrons from multiple valence bands. These results identify key mechanisms for future solid-state attosecond sources and next-generation light-wave electronics. The new quantum interference process justifies the hope for all-optical band-structure reconstruction and lays the foundation for possible quantum logic operations at optical clock rates.

Abstract-Extremely Nonperturbative Nonlinearities in GaAs Driven by Atomically Strong Terahertz Fields in Gold Metamaterials

C. Lange, T. Maag, M. Hohenleutner, S. Baierl, O. Schubert, E. R. J. Edwards, D. Bougeard, G. Woltersdorf, and R. Huber

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.227401

Terahertz near fields of gold metamaterials resonant at a frequency of 0.88 THz allow us to enter an extreme limit of nonperturbative ultrafast terahertz electronics: Fields reaching a ponderomotive energy in the keV range are exploited to drive nondestructive, quasistatic interband tunneling and impact ionization in undoped bulk GaAs, injecting electron-hole plasmas with densities in excess of 1019  cm−3. This process causes bright luminescence at energies up to 0.5 eV above the band gap and induces a complete switch-off of the metamaterial resonance accompanied by self-amplitude-modulation of transmitted few-cycle terahertz transients. Our results pave the way towards highly nonlinear terahertz optics and optoelectronic nanocircuitry with subpicosecond switching times.

DOI: http://dx.doi.org/10.1103/PhysRevLett.113.227401

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• Published 26 November 2014
• Received 18 April 2014

© 2014 American Physical Society

Abstract-Sub-cycle control of terahertz high-harmonic generation by dynamical Bloch oscillations

• O. Schubert,
• M. Hohenleutner,
• F. Langer,
• B. Urbanek,
• C. Lange,
• U. Huttner,
• D. Golde,
• T. Meier,
• M. Kira,
• S. W. Koch
• & R. Huber
• • http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2013.349.html

Ultrafast charge transport in strongly biased semiconductors is at the heart of high-speed electronics, electro-optics and fundamental solid-state physics^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}. Intense light pulses in the terahertz spectral range have opened fascinating vistas^{14, 15, 16, 17, 18,19, 20, 21}. Because terahertz photon energies are far below typical electronic interband resonances, a stable electromagnetic waveform may serve as a precisely adjustable bias^{5, 11, 17, 19}. Novel quantum phenomena have been anticipated for terahertz amplitudes, reaching atomic field strengths^8, 9, 10. We exploit controlled (multi-)terahertz waveforms with peak fields of 72 MV cm⁻¹ to drive coherent interband polarization combined with dynamical Bloch oscillations in semiconducting gallium selenide. These dynamics entail the emission of phase-stable high-harmonic transients, covering the entire terahertz-to-visible spectral domain between 0.1 and 675 THz. Quantum interference of different ionization paths of accelerated charge carriers is controlled via the waveform of the driving field and explained by a quantum theory of inter- and intraband dynamics. Our results pave the way towards all-coherent terahertz-rate electronic