Super-resolution lightwave tomography of electronic bands in quantum materials.

Searching for quantum functionalities requires access to the electronic structure, constituting the foundation of exquisite spin-valley-electronic, topological, and many-body effects. All-optical band-structure reconstruction could directly connect electronic structure with the coveted quantum phenomena if strong lightwaves transported localized electrons within preselected bands. Here, we demonstrate that harmonic sideband (HSB) generation in monolayer tungsten diselenide creates distinct electronic interference combs in momentum space. Locating these momentum combs in spectroscopy enables super-resolution tomography of key band-structure details in situ. We experimentally tuned the optical-driver frequency by a full octave and show that the predicted super-resolution manifests in a critical intensity and frequency dependence of HSBs. Our concept offers a practical, all-optical, fully three-dimensional tomography of electronic structure even in microscopically small quantum materials, band by band.

Microscopic Coulomb interaction in transition-metal dichalcogenides.

The quasi-two dimensional Coulomb interaction potential in transition metal dichalcogenides is determined using the Kohn-Sham wave functions obtained fromab initiocalculations. An effective form factor is derived that accounts for the finite extension of the wave functions in the direction perpendicular to the material layer. The resulting Coulomb matrix elements are used in microscopic calculations based on the Dirac Bloch equations yielding an efficient method to calculate the band gap and the opto-electronic material properties in different environments and under various excitation conditions.

Control of the nonlinear response of bulk GaAs induced by long-wavelength infrared pulses.

The nonlinear optical response of GaAs is studied using extremely nonresonant 10 µm laser pulses with peak intensities greater than 2 GW/cm 2. We observe over an order of magnitude enhancement in the four-wave mixing efficiency by decreasing the CO 2 laser beat-wave frequency. This enhancement is attributed to currents of photoexcited unbound carriers modulated at the beat frequency, confirmed by measurements of nonlinear absorption at this long wavelength as well as a fully microscopic analysis of the excitation dynamics. Modeling of such nonperturbative semiconductor-laser interactions predicts that further decreasing the beat frequency can increase the nonlinear response and allow for its control over two orders of magnitude.

Memory effects in the long-wave infrared avalanche ionization of gases: a review of recent progress.

There are currently intense efforts being directed towards extending the range and energy of long distance nonlinear pulse propagation in the atmosphere by moving to longer infrared wavelengths, with the purpose of mitigating the effects of turbulence. In addition, picosecond and longer pulse durations are being used to increase the pulse energy. While both of these tacks promise improvements in applications, such as remote sensing and directed energy, they open up fundamental issues regarding the standard model used to calculate the nonlinear optical properties of dilute gases. Amongst these issues is that for longer wavelengths and longer pulse durations, exponential growth of the laser-generated electron density, the so-called avalanche ionization, can limit the propagation range via nonlinear absorption and plasma defocusing. It is therefore important for the continued development of the field to assess the theory and role of avalanche ionization in gases for longer wavelengths. Here, after an overview of the standard model, we present a microscopically motivated approach for the analysis of avalanche ionization in gases that extends beyond the standard model and we contend is key for deepening our understanding of long distance propagation at long infrared wavelengths. Our new approach involves the mean electron kinetic energy, the plasma temperature, and the free electron density as dynamic variables. The rate of avalanche ionization is shown to depend on the full time history of the pulsed excitation, as opposed to the standard model in which the rate is proportional to the instantaneous intensity. Furthermore, the new approach has the added benefit that it is no more computationally intensive than the standard one. The resulting memory effects and some of their measurable physical consequences are demonstrated for the example of long-wavelength infrared avalanche ionization and long distance high-intensity pulse propagation in air. Our hope is that this report in progress will stimulate further discussion that will elucidate the physics and simulation of avalanche ionization at long infrared wavelengths and advance the field.

Non-equilibrium dynamics in the dual-wavelength operation of vertical external-cavity surface-emitting lasers.

Microscopic many-body theory coupled with Maxwell's equation is used to study dual-wavelength operation in vertical external-cavity surface-emitting lasers. The intrinsically dynamic nature of coexisting emission wavelengths in semiconductor lasers is associated with characteristic non-equilibrium carrier dynamics, which causes significant deformations of the quasi-equilibrium gain and carrier inversion. Extended numerical simulations are employed to efficiently investigate the parameter space to identify the regime for dual-wavelength operation. Using a frequency selective intracavity etalon, two families of modes are stabilized with dynamical interchange of the strongest emission peaks. For this operation mode, anti-correlated intensity noise is observed in agreement with the experiment. A method using effective frequency selective filtering is suggested for stabilization of genuine dual-wavelength output.

Interlayer excitons in transition-metal dichalcogenide heterostructures with type-II band alignment.

Combining ab initio density functional theory with the Dirac-Bloch and gap equations, excitonic properties of transition-metal dichalcogenide hetero-bilayers with type-II band alignment are computed. The existence of interlayer excitons is predicted, whose binding energies are as large as 350 meV, only roughly 100 meV less than those of the coexisting intralayer excitons. The oscillator strength of the interlayer excitons reaches a few percent of the intralayer exciton resonances and their radiative lifetime is two orders of magnitude larger than that of the intralayer excitons.

Lightwave valleytronics in a monolayer of tungsten diselenide.

As conventional electronics approaches its limits 1 , nanoscience has urgently sought methods of fast control of electrons at the fundamental quantum level 2 . Lightwave electronics 3 -the foundation of attosecond science 4 -uses the oscillating carrier wave of intense light pulses to control the translational motion of the electron's charge faster than a single cycle of light5-15. Despite being particularly promising information carriers, the internal quantum attributes of spin 16 and valley pseudospin17-21 have not been switchable on the subcycle scale. Here we demonstrate lightwave-driven changes of the valley pseudospin and introduce distinct signatures in the optical readout. Photogenerated electron-hole pairs in a monolayer of tungsten diselenide are accelerated and collided by a strong lightwave. The emergence of high-odd-order sidebands and anomalous changes in their polarization direction directly attest to the ultrafast pseudospin dynamics. Quantitative computations combining density functional theory with a non-perturbative quantum many-body approach assign the polarization of the sidebands to a lightwave-induced change of the valley pseudospin and confirm that the process is coherent and adiabatic. Our work opens the door to systematic valleytronic logic at optical clock rates.

Author Correction: High-temperature operation of electrical injection type-II (GaIn)As/Ga(AsSb)/(GaIn)As "W"-quantum well lasers emitting at 1.3 µm.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

High-temperature operation of electrical injection type-II (GaIn)As/Ga(AsSb)/(GaIn)As "W"-quantum well lasers emitting at 1.3 µm.

Electrical injection lasers emitting in the 1.3 µm wavelength regime based on (GaIn)As/Ga(AsSb)/(GaIn)As type-II double "W"-quantum well heterostructures grown on GaAs substrate are demonstrated. The structure is designed by applying a fully microscopic theory and fabricated using metal organic vapor phase epitaxy. Temperature-dependent electroluminescence measurements as well as broad-area edge-emitting laser studies are carried out in order to characterize the resulting devices. Laser emission based on the fundamental type-II transition is demonstrated for a 975 µm long laser bar in the temperature range between 10 °C and 100 °C. The device exhibits a differential efficiency of 41 % and a threshold current density of 1.0 kA/cm2 at room temperature. Temperature-dependent laser studies reveal characteristic temperatures of T0 = (132 ± 3) K over the whole temperature range and T1 = (159 ± 13) K between 10 °C and 70 °C and T1 = (40 ± 1) K between 80 °C and 100 °C.

Atomic structure of 'W'-type quantum well heterostructures investigated by aberration-corrected STEM.

The atomic structure of (GaIn)As/Ga(AsSb)/(GaIn)As-'W'-type quantum well heterostructures ('W'-QWHs) is investigated by scanning transmission electron microscopy (STEM). These structures were grown by metal organic vapour phase epitaxy and are built for type-II laser systems in the infrared wavelength regime. For two samples grown at 525°C and 550°C, intensity profiles are extracted from the STEM images for each sublattice separately. These intensity profiles are compared to the one obtained from an image simulation of an ideal 'W'-QWH that is modelled in close agreement with the experiment. From the intensity profiles, the width of the different quantum wells (QWs) can be determined. Additionally, characteristics connected to the growth of the structures, such as segregation coefficients and material homogeneity, are calculated. Finally, composition profiles are derived from the STEM intensity profiles to a first approximation. For these composition profiles, the expected photoluminescence (PL) is computed based using the semiconductor luminescence equations. The PL spectra are then compared to experimental measurements for both samples.

Symmetry-controlled time structure of high-harmonic carrier fields from a solid.

High-harmonic (HH) generation in crystalline solids1-6 marks an exciting development, with potential applications in high-efficiency attosecond sources7, all-optical bandstructure reconstruction8,9, and quasiparticle collisions10,11. Although the spectral1-4 and temporal shape5 of the HH intensity has been described microscopically1-6,12, the properties of the underlying HH carrier wave have remained elusive. Here we analyse the train of HH waveforms generated in a crystalline solid by consecutive half cycles of the same driving pulse. Extending the concept of frequency combs13-15 to optical clock rates, we show how the polarization and carrier-envelope phase (CEP) of HH pulses can be controlled by crystal symmetry. For some crystal directions, we can separate two orthogonally polarized HH combs mutually offset by the driving frequency to form a comb of even and odd harmonic orders. The corresponding CEP of successive pulses is constant or offset by π, depending on the polarization. In the context of a quantum description of solids, we identify novel capabilities for polarization- and phase-shaping of HH waveforms that cannot be accessed with gaseous sources.

Self-Channeling of High-Power Long-Wave Infrared Pulses in Atomic Gases.

We simulate and elucidate the self-channeling of high-power 10 µm infrared pulses in atomic gases. The major new result is that the peak intensity can remain remarkably stable over many Rayleigh ranges. This arises from the balance between the self-focusing, diffraction, and defocusing caused by the excitation induced dephasing due to many-body Coulomb effects that enhance the low-intensity plasma densities. This new paradigm removes the Rayleigh range limit for sources in the 8-12 µm atmospheric transmission window and enables transport of individual multi-TW pulses over multiple kilometer ranges.

Lightwave-driven quasiparticle collisions on a subcycle timescale.

Ever since Ernest Rutherford scattered α-particles from gold foils, collision experiments have revealed insights into atoms, nuclei and elementary particles. In solids, many-body correlations lead to characteristic resonances--called quasiparticles--such as excitons, dropletons, polarons and Cooper pairs. The structure and dynamics of quasiparticles are important because they define macroscopic phenomena such as Mott insulating states, spontaneous spin- and charge-order, and high-temperature superconductivity. However, the extremely short lifetimes of these entities make practical implementations of a suitable collider challenging. Here we exploit lightwave-driven charge transport, the foundation of attosecond science, to explore ultrafast quasiparticle collisions directly in the time domain: a femtosecond optical pulse creates excitonic electron-hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands of the optical excitation. A full quantum theory explains our observations microscopically. This approach enables collision experiments with various complex quasiparticles and suggests a promising new way of generating sub-femtosecond pulses.

Interaction-induced nonlinear refractive-index reduction of gases in the midinfrared regime.

The nonlinear optical response of a dilute atomic gas to ultrashort high-intensity midinfrared pulse excitation is calculated fully microscopically. The optically induced polarization dynamics is evaluated for the interacting many-electron system in a gas of hydrogen atoms. It is shown that the many-body effects during the excitation distinctly influence not only the atomic ionization dynamics, but also the nonlinear polarization response in the midinfrared regime. The delicate balance between the Kerr focusing and the ionization-induced defocusing is dramatically modified and a significant decrease of the nonlinear refractive index is predicted for increasing wavelength of the exciting pulse.

Optically bright p-excitons indicating strong Coulomb coupling in transition-metal dichalcogenides.

It is shown that the strong Coulomb coupling in intrinsic suspended semiconducting transition metal dichalcogenides can exceed the critical value needed for an excitonic ground state. The dipole-allowed optical excitations then correspond to intra-excitonic transitions such that the optically bright excitonic transitions near the Dirac points have a p-like symmetry, whereas the s-like states are dipole forbidden. The large intrinsic coupling strength seems to be a generic property of the semiconducting transition metal dichalcogenides and strong Coulomb-coupling signatures in the form of the optical selection rules can be observed even in samples grown on typical substrates like SiO2. For the examples of WS2 and WSe2, excellent agreement of the computed excitonic resonance energies with recent experiments is demonstrated.

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

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. The recent discovery of high-harmonic generation in bulk solids combines the idea of ultrafast acceleration with complex condensed matter systems, and provides hope for compact solid-state attosecond sources and electronics at optical frequencies. 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, 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.

Novel type-II material system for laser applications in the near-infrared regime.

The design and experimental realization of a type-II "W"-multiple quantum well heterostructure for emission in the λ > 1.2 µm range is presented. The experimental photoluminescence spectra for different excitation intensities are analyzed using microscopic quantum theory. On the basis of the good theory-experiment agreement, the gain properties of the system are computed using the semiconductor Bloch equations. Gain values comparable to those of type-I systems are obtained.

Coherent terahertz control of vertical transport in semiconductor heterostructures.

Coherent-control protocols are introduced to selectively transport electrons, excitons, or pure two-particle correlations through semiconductor interfaces. The scheme is tested in a double-quantum-well structure where a sequence of terahertz pulses is applied to induce the vertical excitation transfer between the wells. Using a microscopic theory, it is shown that efficient and highly selective transfer can be realized even in the presence of the unavoidable scattering and dephasing processes.

Quantum-memory effects in the emission of quantum-dot microcavities.

The experimentally measured input-output characteristics of optically pumped semiconductor microcavities exhibits unexpected oscillations modifying the fundamentally linear slope in the excitation power regime below lasing. A systematic microscopic analysis reproduces these oscillations, identifying them as a genuine quantum-memory effect, i.e., a photon-density correlation accumulated during the excitation. With the use of projected quantum measurements, it is shown that the input-output oscillations can be controlled and enhanced by an order of magnitude when the quantum fluctuations of the pump are adjusted.

Influence of many-body interactions during the ionization of gases by short intense optical pulses.

The excitation of atomic gases by short high-intensity optical pulses leads to significant electron ionization. In dilute systems, the generated distribution of ionized electrons is highly anisotropic, reflecting the quantum mechanical properties of the atomic states involved in the many photon transitions. For higher atomic densities, the Coulomb interaction in the electron-ion system leads to the development of an isotropic electron plasma. To study the ionization process in the presence of the many-body interaction, a fully microscopic model is developed that combines a generalized version of the optical Bloch equations describing the optical excitation with a microscopic description of the many-body interactions. The numerical evaluation shows that the Coulomb interaction significantly modifies the distribution anisotropy already during the excitation process. Whereas a reduced anisotropy is still present after the pulse for low ionization degrees and pressures, it is completely absent for elevated gas densities. An ionization degree is predicted that is significantly enhanced by the many-body interactions.