We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

In this article we review the experimental and theoretical investigations of the linear and nonlinear optical properties of semiconductor quantum well structures, including the effects of electrostatic fields, extrinsic carriers and real or virtual photocarriers.

Linear and nonlinear optical properties of semiconductor quantum wells

Hybrid perovskites are currently one of the most active fields of research owing to their enormous potential for photovoltaics. The performance of 3D hybrid organic–inorganic perovskite solar cells has increased at an incredible rate, reaching power conversion efficiencies comparable to those of many established technologies. However, the commercial application of 3D hybrid perovskites is inhibited by their poor stability. Relative to 3D hybrid perovskites, low-dimensional — that is, 2D — hybrid perovskites have demonstrated higher moisture stability, offering new approaches to stabilizing perovskite-based photovoltaic devices. Furthermore, 2D hybrid perovskites have versatile structures, enabling the fine-tuning of their optoelectronic properties through compositional engineering. In this Review, we discuss the state of the art in 2D perovskites, providing an overview of structural and materials engineering aspects and optical and photophysical properties. Moreover, we discuss recent developments along with the main limitations of 3D perovskites and assess the advantages of 2D perovskites over their 3D parent structures in terms of stability. Finally, we review recent achievements in combining 3D and 2D perovskites as an approach to simultaneously boost device efficiency and stability, paving the way for mixed-dimensional perovskite solar cells for commercial applications. Combining low-dimensional and 3D perovskites is a promising approach to achieve stable and efficient solar cells. In this Review, we discuss the structural, optical and photophysical properties of low-dimensional perovskites, compare the stability and efficiency of 2D and 3D perovskite devices, and consider 2D/3D composites as a strategy to increase the stability of perovskite solar cells.

Dimensional tailoring of hybrid perovskites for photovoltaics

Electron theory of the optical properties of laser-excited semiconductors

Breaking space-time symmetries in two-dimensional crystals (2D) can dramatically influence their macroscopic electronic properties. Monolayer transition metal dichalcogenides (TMDs) are prime examples where the intrinsically broken crystal inversion symmetry permits the generation of valley-selective electron populations [1–4], even though the two valleys are energetically degenerate, locked by time-reversal symmetry. Lifting the valley degeneracy in these materials is of great interest because it would allow for valley-specific band engineering and offer additional control in valleytronic applications. While applying a magnetic field should in principle accomplish this task, experiments to date have observed no valley-selective energy level shifts in fields accessible in the laboratory. Here we show the first direct evidence of lifted valley degeneracy in the monolayer TMD WS2 [5]. By applying intense circularly polarized light, which breaks time-reversal symmetry, we demonstrate that the exciton level in each valley can be selectively tuned by as much as 18 meV via the optical Stark effect. These results offer a novel way to control valley degree of freedom and may provide a means to realize new valley-selective Floquet topological phases [6–8] in 2D TMDs.

http://absimage.aps.org/image/MAR15/MWS_MAR15-2014-002163.pdf

Valley selective optical Stark effect in monolayer WS2

Multiple-quantum-well structures (MQWS) are attracting much interest for their potential use as efficient active devices in optoelectronics and nonlinear optical signal processing. Based on their strong excitonic optical nonlinearity, operation of MQWS as optical bistable elements and logical gates has been demonstrated both at low and room temperatures. To obtain information about the intrinsic response time of such devices, we have investigated the dynamics of exciton screening of a MQWS with subpicosecond resolution, at T = 15 K and T = 220 K. We have observed an unexpected shift of the exciton resonance to higher energy as a result of exciton-exciton interactions.

Blue Shift of the Exciton Resonance due to Exciton-Exciton Interactions in a Multiple-Quantum-Well Structure

A free-exciton gas is studied as a function of generated particle density in ${\mathrm{Cu}}_{2}$O at 1.5 K. A line-shape analysis of the subsequent recombination shows a gradual evolution, from a classical regime at low density, to a highly quantum-statistical one with chemical potential $\ensuremath{\mu}\ensuremath{\simeq}0$.

Evidence for Bose-Einstein Statistics in an Exciton Gas

We have studied the high-energy shift of excitons, observed in GaAs multiple-quantum-well structures at high exciton densities, as a function of well and barrier thickness. The magnitude of the blue shift was found to depend predominantly upon the GaAs layer thickness, increasing at smaller well sizes. This effect is discussed in terms of a quenching of long-ranged many-body Coulomb effects.

Well-size dependence of exciton blue shift in GaAs multiple-quantum-well structures

Measurements of the total energy reflected from crystalline and amorphous silicon illuminated by a femtosecond laser pulse are reported, for wavelengths of 0.62 and 0.31 \ensuremath{\mu}m. The results are interpreted by use of a highly nonlinear regime of light propagation in the dense plasma of electrons and holes ($e\ensuremath{-}h$). Densities higher than ${10}^{22}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ are reached. We demonstrate that free-carrier absorption is dominated by $e\ensuremath{-}h$ collisions.

D. Hulin

Papers

Blue Shift of the Exciton Resonance due to Exciton-Exciton Interactions in a Multiple-Quantum-Well Structure

Evidence for Bose-Einstein Statistics in an Exciton Gas

Well-size dependence of exciton blue shift in GaAs multiple-quantum-well structures

Energy Transfer during Silicon Irradiation by Femtosecond Laser Pulse

Long Exciton Lifetime in Cu 2 O