Spatial Patterns of Dissipative Polariton Solitons in Semiconductor Microcavities.

We report propagating bound microcavity polariton soliton arrays consisting of multipeak structures either along (x) or perpendicular (y) to the direction of propagation. Soliton arrays of up to five solitons are observed, with the number of solitons controlled by the size and power of the triggering laser pulse. The breakup along the x direction occurs when the effective area of the trigger pulse exceeds the characteristic soliton size determined by polariton-polariton interactions. Narrowing of soliton emission in energy-momentum space indicates phase locking between adjacent solitons, consistent with numerical modeling which predicts stable multihump soliton solutions. In the y direction, the breakup originates from inhomogeneity across the wave front in the transverse direction which develops into a stable array only in the solitonic regime via phase-dependent interactions of propagating fronts.

Effects of spin-dependent interactions on polarization of bright polariton solitons.

We report on the spin properties of bright polariton solitons supported by an external pump to compensate losses. We observe robust circularly polarized solitons when a circularly polarized pump is applied, a result attributed to phase synchronization between nondegenerate TE and TM polarized polariton modes at high momenta. For the case of a linearly polarized pump, either σ+ or σ- circularly polarized bright solitons can be switched on in a controlled way by a σ+ or σ- writing beam, respectively. This feature arises directly from the widely differing interaction strengths between co- and cross-circularly polarized polaritons. In the case of orthogonally linearly polarized pump and writing beams, the soliton emission on average is found to be unpolarized, suggesting strong spatial evolution of the soliton polarization. The observed results are in agreement with theory, which predicts stable circularly polarized solitons and unstable linearly polarized solitons.

Synchronized photonic modulators driven by surface acoustic waves.

Photonic modulators are one of the most important elements of integrated photonics. We have designed, fabricated, and characterized a tunable photonic modulator consisting of two 180°-dephased output waveguide channels, driven by a surface acoustic wave in the GHz frequency range built on (Al,Ga)As. Odd multiples of the fundamental driven frequency are enabled by adjusting the applied acoustic power. A good agreement between theory and experimental results is achieved. The device can be used as a building block for more complex integrated functionalities and can be implemented in several material platforms.

Frequency modulation spectroscopy with a THz quantum-cascade laser.

We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser. High-frequency modulation (up to 50 MHz) of the laser driving current produces a simultaneous modulation of the frequency and amplitude of the laser output. The modulation generates sidebands, which are symmetrically positioned with respect to the laser carrier frequency. The molecular transition is probed by scanning the sidebands across it. In this way, the absorption and the dispersion caused by the molecular transition are measured. The signals are modeled by taking into account the simultaneous modulation of the frequency and amplitude of the laser emission. This allows for the determination of the strength of the frequency as well as amplitude modulation of the laser and of molecular parameters such as pressure broadening.

Noise-induced current switching in semiconductor superlattices: observation of nonexponential kinetics in a high-dimensional system.

We report on measurements of first-passage-time distributions associated with current switching in weakly coupled GaAs/AlAs superlattices driven by shot noise, a system that is both far from equilibrium and high dimensional. Static current-voltage (I-V) characteristics exhibit multiple current branches and bistability; precision, high-bandwidth current switching data are collected in response to steps in the applied voltage to final voltages V1 near the end of a current branch. For a range of V1 values, the measured switching times vary stochastically. At short times (≲10 µs), the switching time distributions decay exponentially, while at longer times the distributions develop nonexponential tails that follow an approximate power law over several decades. The power law decay behavior is attributed to the presence of multiple switching pathways, which may arise from small spatial variations in the superlattice growth parameters.

Lateral distributed-feedback gratings for single-mode, high-power terahertz quantum-cascade lasers.

We report on terahertz quantum-cascade lasers (THz QCLs) based on first-order lateral distributed-feedback (lDFB) gratings, which exhibit continuous-wave operation, high output powers (>8 mW), and single-mode emission at 3.3-3.4 THz. A general method is presented to determine the coupling coefficients of lateral gratings in terms of the coupled-mode theory, which demonstrates that large coupling strengths are obtained in the presence of corrugated metal layers. The experimental spectra are in agreement with simulations of the lDFB cavities, which take into account the reflective end facets.

Electrical suppression of spin relaxation in GaAs(111)B quantum wells.

Spin dephasing via the spin-orbit interaction (SOI) is a major mechanism limiting the electron spin lifetime in III-V zincblende quantum wells (QWs). The dephasing can be suppressed in GaAs(111) quantum wells by applying an electric field. The suppression has been attributed to the compensation of the intrinsic SOI associated with the bulk inversion asymmetry of the GaAs lattice by a structural induced asymmetry SOI term induced by an electric field. We provide direct experimental evidence for this mechanism by demonstrating the transition between the bulk inversion asymmetry-dominated to a structural induced asymmetry-dominated regime via photoluminescence measurements carried out over a wide range of applied fields. Spin lifetimes exceeding 100 ns are obtained near the compensating electric field, thus making GaAs(111) QWs excellent candidates for the electrical storage and manipulation of spins.

Strong correlation of electronic and lattice excitations in GaAs/AlGaAs semiconductor quantum wells revealed by two-dimensional terahertz spectroscopy.

Coulomb-mediated interactions between intersubband excitations of electrons in GaAs/AlGaAs double quantum wells and longitudinal optical phonons are studied by two-dimensional spectroscopy in the terahertz frequency range. The multitude of diagonal and off-diagonal peaks in the 2D spectrum gives evidence of strong polaronic signatures in the nonlinear response. A quantitative theoretical analysis reveals a dipole coupling of electrons to the polar lattice that is much stronger than in bulk GaAs, due to a dynamic localization of the electron wave function by scattering processes.

Two-dimensional terahertz correlation spectra of electronic excitations in semiconductor quantum wells.

We discuss a novel approach for nonlinear two-dimensional (2D) spectroscopy in the terahertz (THz) frequency range which is based on a collinear interaction geometry of a sequence of THz pulses with the sample. The nonlinear polarization is determined by a phase-resolved measurement of the electric field transmitted through the sample as a function of the delay τ between two phase-locked pulses and the "real" time t. The information provided by a single 2D scan along the τ and t axes is equivalent to that from a noncollinear photon-echo setup equipped with four local oscillators, each interacting with a different diffracted order. We address basic concepts of collinear 2D THz spectroscopy, in particular data analysis and phasing issues. Different rephasing and nonrephasing contributions to the third-order response are separated and 2D correlation spectra derived. As a prototype application, 2D correlation spectra of intersubband excitations of electrons in semiconductor quantum wells are presented.

High-field transport in an electron-hole plasma: transition from ballistic to drift motion.

The time evolution of high-field carrier transport in bulk GaAs is studied with intense femtosecond THz pulses. While ballistic transport of electrons occurs in an n-type sample, a transition from ballistic to driftlike motion is observed in an electron-hole plasma. This onset of friction is due to the holes, which are heated by THz absorption. Theoretical calculations, which reproduce the data quantitatively, show that both electron-hole scattering and local-field effects in the electron-hole plasma are essential for the time-dependent friction.

Polariton condensation in dynamic acoustic lattices.

We demonstrate that the tunable potential introduced by a surface acoustic wave on a homogeneous polariton condensate leads to fragmentation of the condensate into an array of wires which move with the acoustic velocity. Reduction of the spatial coherence of the condensate emission along the surface acoustic wave direction is attributed to the suppression of coupling between the spatially modulated condensates. Interparticle interactions observed at high polariton densities screen the acoustic potential, partially reversing its effect on spatial coherence.

A compact, continuous-wave terahertz source based on a quantum-cascade laser and a miniature cryocooler.

We report on the development of a compact, easy-to-use terahertz radiation source, which combines a quantum-cascade laser (QCL) operating at 3.1 THz with a compact, low-input-power Stirling cooler. The QCL, which is based on a two-miniband design, has been developed for high output and low electrical pump power. The amount of generated heat complies with the nominal cooling capacity of the Stirling cooler of 7 W at 65 K with 240 W of electrical input power. Special care has been taken to achieve a good thermal coupling between the QCL and the cold finger of the cooler. The whole system weighs less than 15 kg including the cooler and power supplies. The maximum output power is 8 mW at 3.1 THz. With an appropriate optical beam shaping, the emission profile of the laser is fundamental Gaussian. The applicability of the system is demonstrated by imaging and molecular-spectroscopy experiments.

Coherent ballistic motion of electrons in a periodic potential.

Electrons in bulk n-doped GaAs at a lattice temperature of 300 K are driven by ultrashort high-field transients of up to 300 kV/cm in the terahertz frequency range. In the lowest conduction band the carriers show coherent ballistic motion, which is detected via the THz field emitted by them. This partial Bloch oscillation is reproduced by a quantum-kinetic theory of coherent transport on ultrafast time scales.

Effect of interactions on vortices in a nonequilibrium polariton condensate.

We demonstrate the creation of vortices in a macroscopically occupied polariton state formed in a semiconductor microcavity. A weak external laser beam carrying orbital angular momentum (OAM) is used to imprint a vortex on the condensate arising from the polariton optical parametric oscillator (OPO). The vortex core radius is found to decrease with increasing pump power, and is determined by polariton-polariton interactions. As a result of OAM conservation in the parametric scattering process, the excitation consists of a vortex in the signal and a corresponding antivortex in the idler of the OPO. The experimental results are in good agreement with a theoretical model of a vortex in the polariton OPO.

Internal motions of a quasiparticle governing its ultrafast nonlinear response.

A charged particle modifies the structure of the surrounding medium: examples include a proton in ice, an ion in a DNA molecule, an electron at an interface, or an electron in an organic or inorganic crystal. In turn, the medium acts back on the particle. In a polar or ionic solid, a free electron distorts the crystal lattice, displacing the atoms from their equilibrium positions. The electron, when considered together with its surrounding lattice distortion, is a single quasiparticle, known as the Fröhlich polaron. The basic properties of polarons and their drift motion in a weak electric field are well known. However, their nonlinear high-field properties--relevant for transport on nanometre length and ultrashort timescales--are not understood. Here we show that a high electric field in the terahertz range drives the polaron in a GaAs crystal into a highly nonlinear regime where, in addition to the drift motion, the electron is impulsively moved away from the centre of the surrounding lattice distortion. In this way, coherent lattice vibrations (phonons) and concomitant drift velocity oscillations are induced that persist for several hundred femtoseconds. They modulate the optical response at infrared frequencies between absorption and stimulated emission. Such quantum coherent processes directly affect high-frequency transport in nanostructures and may be exploited in novel terahertz-driven optical modulators and switches.

Anisotropic spin transport in (110) GaAs quantum wells.

Mobile piezoelectric potentials are used to coherently transport electron spins in GaAs (110) quantum wells (QW) over distances exceeding 60 microm. We demonstrate that the dynamics of mobile spins under external magnetic fields depends on the direction of motion in the QW plane. This transport anisotropy is an intrinsic property of moving spins associated with the bulk inversion asymmetry of the underlying GaAs lattice.

Nonlinear terahertz response of -type GaAs.

Excitation of an n-type GaAs layer by intense ultrashort terahertz pulses causes coherent emission at 2 THz. Phase-resolved nonlinear propagation experiments show a picosecond decay of the emitted field, despite the ultrafast carrier-carrier scattering at a sample temperature of 300 K. While the linear THz response is in agreement with the Drude response of free electrons, the nonlinear response is dominated by the super-radiant decay of optically inverted impurity transitions. A quantum mechanical discrete state model using the potential of the disordered impurities accounts for all experimental observations.

Phonon-induced optical superlattice.

We demonstrate the formation of a dynamic optical superlattice through the modulation of a semiconductor microcavity by stimulated acoustic phonons. The high coherent phonon population produces a folded optical dispersion relation with well-defined energy gaps and renormalized energy levels, which are accessed using reflection and diffraction experiments.

Quantized dispersion of two-dimensional magnetoplasmons detected by photoconductivity spectroscopy.

We find that the long-wavelength magnetoplasmon, resistively detected by photoconductivity spectroscopy in high-mobility two-dimensional electron systems, deviates from its well-known semiclassical nature as uncovered in conventional absorption experiments. A clear filling-factor dependent plateau-type dispersion is observed that reveals a so far unknown relation between the magnetoplasmon and the quantum Hall effect.

Fröhlich mass in GaAs-based structures.

The Fröhlich interaction is one of the main electron-phonon intrinsic interactions in polar materials originating from the coupling of one itinerant electron with the macroscopic electric field generated by any longitudinal optical (LO) phonon. Infrared magnetoabsorption measurements of doped GaAs quantum well structures have been carried out in order to test the concept of Fröhlich interaction and polaron mass in such systems. These new experimental results lead one to question the validity of this concept in a real system.