This feature issue compiles nineteen papers addressing both theoretical and experimental studies of fundamental optical properties in semiconductors (FOPS) and in solids in general. The papers include a review, invited papers, and contributed papers. Many of the papers in this issue proceed from the Fundamental Optical Processes in Semiconductors meeting held August 2015 in Breckenridge, Colorado; however, submissions were solicited regardless of attendance at that meeting, and some of the papers in the issue were not the results of conference presentations.

Recently, experimental activity in the field has been driven by studies of new materials and development of new light sources and spectroscopy, while theoretical efforts have been motivated by both the experimental activity and improvements in the theoretical approaches to describing the many-body physics underlying the optical processes. All of these aspects of the fields are reflected in this feature issue.

Semiconductor quantum dots with an electron wave function confined in all three dimensions have been studied for many years using various types of optical spectroscopy. Such studies are now focused on exploring their quantum properties on the level of individual quantum dots for quantum technologies such as quantum information. Individual self-assembled quantum dots may act as single photon source with high purity, photon indistinguishability, and brightness as discussed in the invited article by Gazzano and Solomon [1]. One of the greatest advantages of quantum dot based single photon sources is the possibility of integrating them in all solid-state photonic circuits. The electron-lattice coupling may inevitably lead to performance degradation of the single photon source. On the other hand, the dynamic vibronic coupling may provide a different approach for enabling population inversion using properly detuned excitation lasers as discussed by Brash and coworkers in an invited paper [2]. A related, but distinct, solid-state quantum optical phenomenon, Dicke superradiance, is discussed by Cong and coworkers in an invited review article in this special issue [3]. In contrast to atomic and molecular gas, both radiative coupling and Coulomb interaction may lead to collective photon emission in solids. The authors suggest that such cooperative phenomena are not small corrections only observable under special conditions. Instead, such phenomena can play a dominant role in non-equilibrium dynamics and light emission processes of interacting electrons.

Excitonic physics has been an intense subject of study in the FOPS community throughout its entire history. Advanced and recently developed spectroscopic methods, such as optical two-dimensional coherent spectroscopy, continue to provide new insight on exciton physics in GaAs quantum wells. Using two-dimensional double-quantum spectroscopy, Tollerud and Davis discovered new effects of carrier scattering at low-excitation density regime previously unexplored [4]. Using similar techniques, Singh and coworkers provide quantitative studies of spectral diffusion of excitons in quantum wells by directly measuring the frequency–frequency correlation function and its temperature dependence [5].

Exciton physics is particularly relevant in an emerging class of atomically thin semiconductors known as the transition metal dichalcogenides (TMDs) because the large exciton binding energy (200–500 meV) makes excitons stable even at room temperature. An invited article by Moody et al. provides a brief overview of this new and rapidly evolving subfield [6]. Excitons residing at the corner of the Brillouin zone in TMDs inherit the valley degree of freedom, which is locked with electron spins. Existing studies of exciton dynamics and valley depolarization dynamics are not conclusive partially due to disorder from defects and impurities in these materials. A contribution from Kolmakov and coworkers proposes the possibility of realizing room temperature superfluidity of exciton polarization based on these TMD monolayers embedded in a cavity [7].

Excitonic physics is also the subject of intense theoretical activity. An invited article by Kwong et al. [8] presents low-dimensional models that help understand the occurrence of instability-driven excitonic polariton-density patterns in semiconductor microcavities. Also working on microcavity polaritons, Ostatnicky [9] proposes a theoretical model consisting of excitonic cavity polaritons in a rectangular potential trap where particle–particle scattering induces two-particle tunneling. Pure excitons are studied by Vänskä et al. [10], who present a combined approach of first-principles density-functional approach and systematic cluster-expansion scheme. This work predicts that a rutile ${\mathrm{TiO}}_2$ has a dipole-forbidden but quadrupole-allowed 1s-exciton state.

Semiconductor lasers have also been a central topic of FOPS. Junges et al. [11] present a numerical study of two mutually delay-coupled semiconductor lasers. They predict that a complex distribution of periodic and chaotic laser oscillations appears as function of coupling and detuning. In a connected area, quantum-optical studies have been a consistent theme in the field of FOPS. In an invited paper, Kabuss et al. [12] discuss a theory for coherent quantum feedback and Pyragas control, and analyze the quantum kinetics of entanglement growth by computing multi-time correlations.

Terahertz (THz) frequency electromagnetic fields are resonant with many of the relevant quasiparticle states in solids, and the number of THz investigations has steadily increased. Springer et al. [13] propose a new numerical method to solve for excitonic states when electronic masses are anisotropic. The approach is applied to show that the mass anisotropy generates two spectrally distinct THz resonances in germanium.

High-harmonic generation (HHG) has been studied in atomic system for decades, but for only a few years in semiconductors. In an invited paper, Huttner et al. [14] overview a many-body theory that describes atomic as well as semiconductor HHG with one and the same theory. They demonstrate that band structure as well as delocalization of the initial electronic states can introduce significant differences between atomic and solid-state HHG. Multiphoton aspects behind the HHG are studied by Khurgin [15], who finds unambiguous connection between the nonlinear optical conductivity and the odd-order nonlinear optical susceptibility. The ability to generate attosecond pulses using high harmonic generation has been one of the drivers for its development. Attosecond pulses produced by high harmonic generation are used by Borja et al. [16] to probe transient absorption in solids, including germanium, on a femtosecond attosecond timescale.

Beyond semiconductors, studies of related materials are also interesting to those working on FOPS. In an invited article, Gehl et al. [17] present progress on hybrid superconductor/semiconductor optical sources. The dispersion of surface plasmon polaritons in metal films is directly imaged by Ives et al. [18]. In an invited article, the structure-property relations in individual carbon nanotubes are studied by Yao et al. [19].

This special issue is dedicated to the memory of Galina Khitrova, who was a key researcher driving the broader field of FOPS and specifically the series of FOPS meetings. She was one of the organizers of the first FOPS meeting in 2001, and stayed involved with the organization of the series of meetings. Her strong contributions, not only scientific but also in fostering an atmosphere of lively and stimulating discussions, will be sorely missed.