S. W. Koch

On the importance of radiative and Auger losses in GaN-based quantum wells

Fully microscopic many-body models are used to study the importance of radiative and Auger carrier losses in InGaN∕GaN quantum wells. Auger losses are found to be negligible in contrast to recent speculations on their importance for the experimentally observed efficiency droop. Good agreement with experimentally measured threshold losses is demonstrated. The results show no significant dependence on details of the well alloy profile.

Sub-cycle control of terahertz high-harmonic generation by dynamical Bloch oscillations

Terahertz waveforms with peak fields of 72 MV cm−1 and a central frequency of 30 THz drive interband polarization in bulk GaSe off-resonantly and accelerate excited electron–hole pairs, inducing dynamical Bloch oscillations. This results in the emission of phase-stable, high-harmonic transients over the whole frequency range of 0.1–675 THz.

Density-activated defect recombination as a possible explanation for the efficiency droop in GaN-based diodes

It is shown that a carrier loss process modeling density-activated defect recombination can reproduce the experimentally observed droop of the internal quantum efficiency in GaN-based laser diodes.

Quantum theory of spontaneous emission and coherent effects in semiconductor microstructures

Abstract A fully quantum-mechanical theory for the interaction of light and electron–hole excitations in semiconductor quantum-well systems is developed. The resulting many-body hierarchy for the correlation functions is truncated using a dynamical decoupling scheme leading to coupled semiconductor luminescence and Bloch equations. For incoherent excitation conditions, the theory is used to describe nonlinear excitonic emission properties of single-quantum wells, optically coupled multiple quantum-well systems, and quantum wells in a microcavity. Resonant coherent optical excitation leads to a direct coupling between the induced coherent polarization and photoluminescence. The resulting quantum corrections to the semiclassical semiconductor Bloch equations and the coherent contributions to the semiconductor luminescence equations are discussed. The secondary emission in directions deviating from the coherent excitation direction after femtosecond-pulse excitation is studied. Coherent control and quadrature squeezing for the light emission are analyzed.

Room temperature continuous wave milliwatt terahertz source

We present a continuous wave terahertz source based on intracavity difference frequency generation within a dual color vertical external cavity surface emitting laser. Using a nonlinear crystal with a surface emitting phase matching scheme allows for high conversion efficiencies. Due to the tunability of the dual mode spacing, the entire spectral range of the terahertz gap can be covered. The terahertz output scales quadratically with the intracavity intensity, potentially allowing for terahertz intensities in the range of 10s of milliwatts and beyond.

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.

Temperature-dependence of the internal efficiency droop in GaN-based diodes

The temperature dependence of the measured internal efficiencies of green and blue emitting InGaN-based diodes is analyzed. With increasing temperature, a strongly decreasing strength of the loss mechanism responsible for droop is found which is in contrast to the usually assumed behavior of Auger losses. However, the experimental observations can be well reproduced assuming density activated defect recombination with a temperature independent recombination time.

Semiconductor‐Laser Physics

1. Semiconductor Laser Diodes 2. Basic Concepts 3. Free-Carrier Theory 4. Coulomb Effects 5. Many-Body Gain 6. Band Mixing and Strain in Quantum Wells 7. Semiclassical laser Theory 8. Multimode Operation 9. Quantum Theory of the Laser 10. Propagation Effects 11. Beyond Quasiequilibrium Theory, Appendices A-e, Index

Classical theory for second-harmonic generation from metallic nanoparticles

In this paper, we develop a classical electrodynamic theory to study the optical nonlinearities of metallic nanoparticles. The quasi free electrons inside the metal are approximated as a classical Coulomb-interacting electron gas, and their motion under the excitation of an external electromagnetic field is described by the plasma equations. This theory is further tailored to study second-harmonic generation. Through detailed experiment-theory comparisons, we validate this classical theory as well as the associated numerical algorithm. It is demonstrated that our theory not only provides qualitative agreement with experiments but it also reproduces the overall strength of the experimentally observed second-harmonic signals.

Optical nonlinearities of glasses doped with semiconductor microcrystallites.

The origin of optical nonlinearities of glasses doped with CdS(x)Se(1-x) microcrystallites is studied both experimentally and theoretically. At room temperature a large shift ( congruent with50 meV) of the absorption spectrum to higher energies is observed in a CdS(x)Se(1-x) glass with increased light intensity. This blue shift is analyzed by using a semiconductor plasma theory. Experimental and theoretical results suggest that band filling is the dominant mechanism for the observed nonlinearity. An independent measurement for the intensity-dependent dispersive changes is presented.