Stephan W. Koch

We use microscopic many-body models based on first principle density functional theory to investigate the high harmonic generation (HHG) in monolayer transition metal dichalcogenides (ML-TMDCs) at the example of MoS₂. A two-dimensional bandstructure model is utilized that allows for the realistic inclusion of Coulomb correlations. It is shown that for off-resonant mid-IR excitation Coulomb correlations lead to a dramatic enhancement of HHG intensity by up to two orders of magnitude. For resonant excitation near the fundamental excitonic resonance the Coulomb interaction leads to dressed harmonics. These have a sub-floor of broad spectral contributions. The amplitude of these contributions is about four to six orders of magnitude below the peak. The width scales linearly with the exciting field and can reach hundreds of meV.

Type-II heterostructures as active layers for semiconductor laser devices combine the advantages of a spectrally broad, temperature stable, and efficient gain with the potential for electrical injection pumping. Their intrinsic charge carrier relaxation dynamics limit the maximum achievable repetition rates beyond any constraints of cavity design or heat dissipation. Of particular interest are the initial build up of gain after high-energy injection and the gain recovery dynamics following depletion through a stimulated emission process. The latter simulates the operation condition of a pulsed laser or semiconductor optical amplifier. An optical pump pulse injects hot charge carriers that eventually build up broad spectral gain in a model (Ga,In)As/GaAs/Ga(As,Sb) heterostructure. The surplus energies of the optical pump mimic the electron energies typical for electrical injection. Subsequently, a second laser pulse tuned to the broad spectral gain region depletes the population inversion through stimulated emission. The spectrally resolved nonlinear transmission dynamics reveal gain recovery times as fast as 5 ps. These data define the intrinsic limit for the highest laser repetition rate possible with this material system in the range of 100 GHz. The experimental results are analyzed using a microscopic many-body theory identifying the origins of the broad gain spectrum.

Fully microscopic many-body models based on inputs from first principles density functional theory are used to calculate the carrier losses due to radiative- and Auger-recombinations in bulk tellurium. It is shown that Auger processes dominate the losses for carrier densities in the range typical for applications as lasers. The Auger loss depends crucially on how far energetically lower hole bands are detuned from the valence band edge. Values for this detuning range throughout literature from being about equal to the bandgap to being on the order of 100 meV larger than the bandgap. We find that at cryogenic temperatures of 50 K (100 K), the Auger coefficient, C, is about six (three) orders of magnitude smaller if this detuning is as in our calculations at the low end of the published values rather than at the high end where it exceeds the bandgap. At room temperature, the sensitivity is reduced to about a factor of four with C values ranging between 0.4 and 1.6×10−27 cm6 s−1. Here, radiative losses dominate for carrier densities up to about 1016/ cm3 with a loss coefficient B≈10−11 cm3 s−1. The radiative losses are about two to three times lower than in typical bulk III–V materials for comparable wavelengths.

An ab initio based fully microscopic many-body approach is used to study the carrier relaxation dynamics in monolayer transition-metal dichalcogenides. Bandstructures and wavefunctions as well as phonon energies and coupling matrix elements are calculated using density functional theory. The resulting dipole and Coulomb matrix elements are implemented in the Dirac–Bloch equations to calculate carrier–carrier and carrier–phonon scatterings throughout the whole Brillouin zone (BZ). It is shown that carrier scatterings lead to a relaxation into hot quasi-Fermi distributions on a single femtosecond timescale. Carrier cool down and inter-valley transitions are mediated by phonon scatterings on a picosecond timescale. Strong, density-dependent energy renormalizations are shown to be valley-dependent. For MoTe₂, MoSe₂ and MoS₂ the change of energies with occupation is found to be about 50% stronger in the Σ and Λ side valleys than in the K and K′ valleys. However, for realistic carrier densities, the materials always maintain their direct bandgap at the K points of the BZ.

Probing quantum materials: Unraveling the functionalities of quantum materials such as spin-valley-electronic, topological, and many-body effects provides a route to exploiting these materials for applications. Borsch et al. introduce a spectroscopic technique based on the concept of crystal-momentum combs. By extending the ideas of frequency combs of metrology and superresolution imaging, they demonstrate the ability to directly map out the properties of quantum electronic structures under ambient conditions. Using this technique combined with accurate many-body computations, they were able to reveal tomographic images of two-dimensional quantum materials. Science , this issue p. 1204

The local density approximation-1/2 method is employed in density functional theory calculations for the electronic structure of III–V dilute bismide systems. For the representative example of Ga(SbBi) with Bi concentrations below 10%, it is shown that this method works very efficiently, especially due to its reasonably low demand on computer memory. The resulting band structure and wavefunctions are used to compute the interaction matrix elements that serve as input to the microscopic calculations of the optical properties and intrinsic losses relevant for the optoelectronic applications of dilute bismides.