M. Kira

Nonlinear emission dynamics from semiconductor microcavities in the nonperturbative regime

Time-resolved normal-mode-coupling (NMC) oscillations in the nonlinear regime of a semiconductor microcavity with a large splitting to linewidth ratio are studied experimentally using upconversion. A reduction of the modulation depth of the NMC oscillations and reflection dips without a change in the NMC splitting and oscillation period is observed. Microscopic calculations attribute the observed features to excitonic broadening due to dephasing induced by carrier-carrier and polarization scattering processes.

Theory of the optical properties of semiconductor nanostructures

Abstract A microscopic many-body theory describing the optical and electronic properties of semiconductors and semiconductor nanostructures is briefly reviewed. At the semiclassical level, the optical response is computed using Maxwells equations together with the semiconductor Bloch equations which describe the dynamics of the diagonal and the off-diagonal terms of the reduced single-particle density matrix. These equations include the coupling between the semiconductor and the optical field as well as Coulomb many-body interactions among the optically excited carriers. Under quasi-equilibrium conditions, luminescence spectra can be obtained from absorption spectra on the basis of the Kubo–Martin–Schwinger relation for conditions usually limited to the regime of optical gain (lasers). More generally, light emission has to be computed at a fully quantum mechanical level leading to semiconductor luminescence equations.

Quantum correlations in semiconductor microcavities

The quantum mechanical nature of the light field in semiconductor microcavities leads to non-classical coupling effects between photons and electron–hole excitations. It is shown that these quantum correlations give rise to characteristic corrections of the semiclassical light-matter coupling dynamics. Examples of quantum correlation signatures include entanglement effects in the probe reflection of a microcavity system and squeezing in the incoherent emission.

Ultrashort pulse propagation effects in semiconductor microcavities

A microscopic theory is used to study femtosecond pulse propagation through a semiconductor microcavity containing one or more quantum wells. The ultrafast cavity mode build-up is investigated and the dynamical interplay of the field with the quantum-well electron-hole excitations is analyzed. Normal-mode coupling as well as radiative coupling between the quantum wells is shown to depend strongly on the placement of the quantum wells inside the microcavity.

High-harmonic generation in solids

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.

Thermal wakefield oscillations of laser-induced plasma channels and their spectral signatures in luminescence

Starting from a general microscopic model for an interacting two-component plasma including the interaction with a quantized light field, the equations of motion in the Wigner representation are derived. In contrast to the case of strongly focused laser beams which are known to leave behind so called wakefield oscillations of the electron plasma, thermal wakefield oscillations dominate the dynamics of a femtosecond laser generated plasma rod. It is shown that the photoluminescence from the resulting electron-ion plasma bears spectral features related to the plasma frequency due to these thermal radial wakefield oscillations.

Nonperturbative THz nonlinearities for many-body quantum control in semiconductors

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.

Sub-cycle control of multi-THz high-harmonic generation and all-coherent charge transport in bulk semiconductors

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.