The role of quantum memory in ultrafast, nonperturbative lightwave electronics is discussed with several examples involving one- and two-dimensional quantum materials. Since interactions are strong in these materials, scattering emerges often on sub-100fs timescales. Nevertheless, quantum memory has extremely important implications for lightwave electronics – several examples will be given to demonstrate how omitting quantum memory eliminates many key effects of measured in lightwave electronics. These insights have direct implications to all aspects of lightwave electronics, including multi-photon absorption, optical switching, and high-harmonic generation.
As conventional electronics approaches its ultimate limits, novel concepts of fast quantum control have been sought after. Lightwave electronics – the foundation of attosecond science – has opened a new arena by utilizing the oscillating carrier wave of intense light pulses to control electrons faster than a cycle of light. We employ atomically strong terahertz electromagnetic pulses to accelerate electrons through the entire Brillouin zone of solids, drive quasiparticle collisions, and generate high-harmonic radiation as well as high-order sidebands. The unique band structures of topological insulators allow for all-ballistic and quasi-relativistic acceleration of Dirac quasiparticles over distances as large as 0.5 μm. In monolayers of transition metal dichalcogenides, we switch the electrons’ valley pseudospin, opening the door to subcycle valleytronics. Finally, we show that lightwave electronics can be combined with ultimate atomic spatial resolution in state-selective ultrafast scanning tunneling microscopy.
A microscopic theory is presented to describe high-harmonic generation in solids with the semiconductor-Bloch equations. The approach includes the relevant interband polarizations and intraband currents. The appearance of even harmonic orders is shown to require at least three electronic bands and a mutual interband coupling between them. In experimental and theoretical time-resolved studies, this also manifests as a unipolar emission signature of the high-harmonic radiation.
Quantum computing and ultrafast quantum electronics constitute pivotal technologies of the 21st century and revolutionize the way we process information. Successful implementations require controlling superpositions of states and coherence in matter, and exploit nonlinear effects for elementary logic operations. In the THz frequency range between optics and electronics, solid state systems offer a rich spectrum of collective excitations such as excitons, phonons, magnons, or Landau electrons. Here, single-cycle THz transients of 8.7 kV/cm amplitude centered at 1 THz strongly excite inter-Landau-level transitions of magnetically biased GaAs quantum wells, facilitating coherent Landau ladder climbing by more than six rungs, population inversion, and coherent polarization control. Strong, highly nonlinear pump-probe and four- and six-wave mixing signals, entirely unexpected for this paragon of the harmonic oscillator, are revealed through two-time THz spectroscopy. In this scenario of nonperturbative polarization dynamics, our microscopic theory shows how the protective limits of Kohn’s theorem are ultimately surpassed by dynamically enhanced Coulomb interactions, opening the door to exploiting many-body dynamics for nonlinear quantum control.
Dropletons are new highly correlated quasiparticles recently discovered in GaAs quantum wells. The dropleton discovery is verified with a new measurement set and the full identification cycle is presented. The analysis confirms that a dropleton contains four or more electron–hole pairs within a tiny correlation bubble and that dropleton’s electron–hole pairs are in a liquid-like state that is quantized due to quantum confinement.
Ultrafast transport of electrons in semiconductors lies at the heart of high-speed electronics, electro-optics and
fundamental solid-state physics. Intense phase-locked terahertz (THz) pulses at photon energies far below electronic
interband resonances may serve as a precisely adjustable alternating bias, strongly exceeding d.c. breakdown voltages.
Here, we exploit the near-field enhancement in gold metamaterial structures on undoped bulk GaAs, driven by few-cycle
THz transients centered at 1 THz, to bias the semiconductor substrate with field amplitudes exceeding 12 MV/cm. Such
fields correspond to a potential drop of the bandgap energy over a distance of only two unit cells. In this extremely
off-resonant scenario characterized by a Keldysh parameter of γK ≈ 0.02, massive interband Zener tunneling injects a
sizeable carrier density exceeding 1019 cm-3, and strong photoluminescence results. At a center frequency of 30 THz,
THz transients with peak fields of 72 MV/cm analogously excite carriers in a bulk, semiconducting GaSe crystal,
without metamaterial. Here, in contrast, we are able to drive coherent interband polarization and furthermore dynamical
Bloch oscillations of electrons in the conduction band, on femtosecond time scales. The dynamics entail the generation
of absolutely phase-stable high-harmonic transients containing spectral components up to the 22nd order of the
fundamental frequency, spanning 12.7 optical octaves throughout the entire terahertz-to-visible domain between 0.1 and
675 THz. Our experiments establish a new field of light-wave electronics exploring coherent charge transport at optical
clock rates and bring picosecond-scale electric circuitry at the interface of THz optics and electronics into reach.
Strong exciton-photon coupling in a high-Q microcavity leads to the formation of two new eigenstates, called excitonpolaritons.
We present the quantum dynamics of exciton-polaritons driven by strong few-cycle THz pulses. Our study
focuses on an intriguing question of how THz radiation interacts with the strongly coupled light-matter system. We
performed THz-pump and optical-probe experiments to answer the question: we observed the time-resolved optical
reflectivity of the lower and higher exciton-polariton (LEP and HEP) modes in a QW microcavity in the presence of
strong few-cycle THz pulses. In a previous study with a bare QW, a strong THz field tuned to the 1s-to-2p intraexciton
transition induced an excitonic Rabi splitting. Since THz radiation interacts only with the excitonic components of
exciton-polaritons and has no impact on cavity modes, it is an interesting question how THz radiation drives the excitonpolariton
states to higher energy states in the microcavity system. Our study shows that THz radiation resonantly drives
the exciton-polariton polarizations giving rise to LEP-to-2p or HEP-to-2p transitions. LEP-to-HEP transition is
forbidden because they have the same symmetry. Our experimental and theoretical investigations demonstrate that LEP,
HEP, and 2p-exciton states form a three-level Λ system in an optically excited QW microcavity.
Phase-locked electromagnetic transients in the terahertz (THz) spectral domain have become a unique contact-free probe
of the femtosecond dynamics of low-energy excitations in semiconductors. Access to their nonlinear response, however,
has been limited by a shortage of sufficiently intense THz emitters. Here we introduce a novel high-field source for THz
transients featuring peak amplitudes of up to 108 MV/cm. This facility allows us to explore the non-perturbative
response of semiconductors to intense fields tailored with sub-cycle precision. In a first experiment intense transients
drive Rabi-oscillations between excitonic states in Cu2O, implying exciting perspectives for future THz quantum optics.
At electric fields beyond 10 MV/cm, we observe the breakdown of the power expansion of the nonlinear polarization in
bulk semiconductors. Furthermore, we employ the intense magnetic field components of our transients to coherently
control spin waves in antiferromagnetically ordered solids. Finally, intersubband cavity polaritons in semiconductor
microcavities are exploited to push light-matter coupling to an unprecedented ultrastrong and sub-cycle regime.
We perform ultrafast pump-probe experiments on a 50 period Ge/SiGe multiple-quantum-well structure held
at room temperature. Tunable 80 fs pulses emitted by an opto-parametric amplifier are used as a pump and a
white-light supercontinuum generated directly from a 1 kHz Ti:sapphire regenerative amplifier system is used as
a probe. The resulting spectro-temporal response shows three distinct temporal regimes. Coherent oscillations
dominate at negative times yielding a well-defined time zero across the whole detected spectral range. Dynamics
are observed within the direct conduction band valley during and shortly after the excitation while the electrons
are also scattered towards the indirect minima. After several hundreds of fs to a few ps almost all electrons
populate the L-valley states. These carriers decay out of the L-valleys on a timescale longer than several ns.
During the first ps, carrier inversion is obtained for strong enough pumping due to faster intra-valley than intervalley
scattering. The obtained gain values are similar in magnitude to those observed in typical III-V compound
The exciton binding energy in GaAs-based quantum-well (QW) structures is in the range of ~10 meV, which falls in the
THz regime. We have conducted a time-resolved study to observe the resonant interactions of strong narrowband THz
pulses with coherent excitons in QWs, where the THz radiation is tuned near the 1s-2p intraexciton transition and the
THz pulse duration (~3 ps) is comparable with the exciton dephasing time. The system of interest contains ten highquality
12-nm-wide GaAs QWs separated by 16-nm-wide Al 0.3Ga 0.7As barriers. The strong and narrowband THz pulses
were generated by two linearly-chirped and orthogonally-polarized optical pulses via type-II difference-frequency
generation in a 1-mm ZnTe crystal. The peak amplitude of the THz fields reached ~10 kV/cm. The strong THz fields
coupled the 1s and 2p exciton states, producing nonstationary dressed states. An ultrafast optical probe was employed to
observe the time-evolution of the dressed states of the 1s exciton level. The experimental observations show clear signs
of strong coupling between THz light and excitons and subsequent ultrafast dynamics of excitonic quantum coherence.
As a consequence, we demonstrate frequency conversion between optical and THz pulses induced by nonlinear
interactions of the THz pulses with excitons in semiconductor QWs.
A microscopic theory for the terahertz response of a semiconductor quantum well under coherent conditions is
presented. It is shown that excitonic effects influence the intersubband absorption under certain conditions. For
high-quality samples, one should be able to resolve both band-to-band and excitonic intersubband transitions
in an terahertz absorption measurement. Due to the competition of intersubband transitions and classical field-induced
carrier accelerations, an unexpected Fano feature is observed in the terahertz spectra. This result is in
excellent agreement with recent measurements.
The terahertz response of a two-dimensional electron gas (2DEG) is investigated theoretically. The developed
microscopic model shows that the terahertz absorption sensitively detects Coulomb-induced many-body correlations
within the entire 2DEG system. In particular, the resulting response follows from a nontrivial competition
between the ponderomotive and the Coulomb-correlation contributions. The result is in good agreement with
recent experiments while the response cannot be explained with a simple Drude-model analysis.
The optical response of semiconductor quantum wells is investigated theoretically to explain nonlinear transients
generated via intense terahertz (THz) fields. A microscopic description of THz-induced interaction processes is
developed while several numerical examples are presented to illustrate properties in a typical THz-pump and
optical-probe configuration. The results identify signatures of the ac-Stark effect, ponderomotive contributions,
and extreme-nonlinear dynamics.
The interaction of semiconductors with terahertz radiation is discussed. The main ingredients of a consistent
microscopic description are presented. The theory is evaluated to analyze direct terahertz emission features of
Quantum-electrodynamic calculations predict that truly incoherent light can be used to efficiently generate quantum-degenerate exciton population states. Resonant incoherent excitation directly converts photons into excitons with vanishing center of mass momentum. The populated exciton state possesses long-range order, is very stable against perturbations, and should be observable via its unusual directional and density dependence in luminescence measurements.
A fully quantum mechanical theory for a system of photons and Coulomb interacting electron-hole pairs in semiconductors is investigated. The resulting semiconductor luminescence equations are discussed and evaluated for a variety of examples. For a quantum-well system, it is shown how luminescence at the exciton resonance can result from an incoherent electron-hole plasma. Also changes in carrier lifetimes due to radiative recombination are studied.
Coherent signatures in the semiconductor light emission are studied using a fully quantum mechanical theory for the system of photons and Coulomb interacting electron-hole pairs. The dominant light-matter correlations couple the semiconductor Bloch and luminescence equations yielding significant quantum corrections. A coherent excitation leads to squeezing of the emitted light as well as to entanglement between light and matter.
The normal modes in a nonperturbatively coupled quantum-well semiconductor microcavity are linear superpositions of the QW exciton and cavity mode when the QW exciton transition is resonant with the cavity mode. When the lower normal mode is excited by a phase-locked pair of optical pulses, the nonlinear response of a probe pulse tuned to the upper mode is controlled. Thus the normal modes are coupled in their nonlinear optical response due to the nonlinearity of the exciton underlying the two normal modes. The cavity enhancement of the excitonic nonlinearity gives rise to a large signal; modulating the relative phase of the excitation pulses produces a differential reflectivity of up to 10%. Besides the coherent control of normal modes which is explainable with in the frame of semiclassical models, we observe a purely quantum mechanical phenomenon in our system. The quantum correlations between the field and carrier density lead to intraband coherences which live much longer than the interband dephasing time.
A microscopic interpretation of spontaneous and stimulated emission in semiconductor microcavities is developed using Semiconductor Luminescence Equations obtained from a quantum theory for the interacting electron-hole system and microcavity photons. The properties of bare quantum well luminescence as well as nonlinear photoluminescence of microcavity systems showing a threshold-like transition are consistently explained.
The threshold-like growth of the higher energy photoluminescence from a normal-mode-coupling microcavity was previously attributed to exciton polariton lasing (boser) based on Bose condensation into the upper polariton branch. Experimental evidence is presented here showing that this boser crossover occurs just as normal-mode coupling collapses to the perturbative weak coupling, so that boser action is fermionic after all. I.e., it can be understood as electron-hole recombination within a microcavity with density-dependent emission properties.
The optical properties of semiconductor microcavity systems are studied theoretically on the level of a fully quantum mechanical nonequilibrium theory. The normal mode coupling of the exciton and cavity resonances is investigated for various excitation conditions. Transmission, reflection, photoluminescence, and lasing characteristics are analyzed using the full quantum electrodynamic theory.