A highly strained ultrathin membrane of MoS2 could lead to the creation of a solar funnel, a new form of solar cell which absorbs a much broader range of the solar spectrum that a usual single junction device.

/pdf/strain-engineered-artificial-atom-as-a-broad-spectrum-solar-zr8r1jqfeg.pdf

Strain-engineered artificial atom as a broad-spectrum solar energy funnel

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

The authors review the physics of ultrafast dynamics in semiconductors and their heterostructures, including both the observed experimental phenomena and the theoretical description of the processes. These are probed by ultrafast optical excitation, generating nonequilibrium states that can be monitored by time-resolved spectroscopy. Light pulses create coherent superpositions of states, and the dynamics of the associated phase relationships can be directly investigated by means of many-pulse experiments. The commonly used experimental techniques are briefly reviewed. A variety of different phenomena can be described within a common theoretical framework based on the density-matrix formalism. The important interactions of the carriers included in the theoretical description are the phonon interactions, the interactions with classical and quantum light fields, and the Coulomb interaction among the carriers themselves. These interactions give rise to a strong interplay between phase coherence and relaxation, which strongly affects the non equilibrium dynamics. Based on the general theory, the authors review the physical phenomena in various semiconductor structures including superlattices, quantum wells, quantum wires, and bulk media. Particular results which have played a central role in understanding the microscopic origins of the relaxation processes are discussed in detail.

Theory of ultrafast phenomena in photoexcited semiconductors

The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton-exciton and exciton-phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors.

/pdf/intrinsic-homogeneous-linewidth-and-broadening-mechanisms-of-3ron9q6wji.pdf

Intrinsic homogeneous linewidth and broadening mechanisms of excitons in monolayer transition metal dichalcogenides

Radiative lifetime of free excitons in quantum wells

The ultrafast relaxation of excitons in GaAs, coherently excited by a short optical pulse and subjected to collisions with free carriers and noncoherent excitons which are independently created by a second synchronized light pulse, is studied directly in the time domain by a probing of the excitonic phase coherence. The relaxation rates reveal strong exciton-exciton scattering with a collision efficiency of 1.6 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{3}$ ${\mathrm{s}}^{\ensuremath{-}1}$ and even 10 times more efficient exciton-free-carrier scattering.

Ultrafast phase relaxation of excitons via exciton-exciton and exciton-electron collisions.

The phase relaxation of two-dimensional (2D) heavy-hole excitons in a 12-nm GaAs single quantum well subjected to collisions with either free carriers or incoherent heavy-hole excitons is investigated by time-resolved degenerate four-wave mixing. The homogeneous linewidth corresponding to the phase coherence time reveals a collisional broadening due to exciton-exciton scattering and an 8 times stronger exciton-free carrier scattering. Both scattering processes are enhanced for 2D excitons as compared to 3D excitons.

Collision broadening of two-dimensional excitons in a GaAs single quantum well

We report optical dephasing of two-dimensional excitons in $\mathrm{GaAs}\ensuremath{-}{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ single quantum wells by means of time-resolved degenerate-four-wave mixing and transmission experiments in the temperature range below 80 K. The observed absorption linewidths correspond directly to the excitonic phase coherence times, confirming the homogeneous broadening of the excitonic absorption. A linear temperature dependence of the homogeneous linewidth indicates acoustic-phonon scattering as the broadening mechanism.

Optical dephasing of homogeneously broadened two-dimensional exciton transitions in GaAs quantum wells.

The phase coherence and orientational relaxation of excitons resonantly excited in optically thin GaAs layers by a picosecond light pulse is explored by means of time-resolved degenerate-four-wave mixing. At low excitation intensities where contributions due to exciton-exciton scattering can be neglected, a rapid loss of the phase coherence as well as of the optical alignment of the excitonic dipoles within 7 ps is observed. This implies a weak coherent coupling between excitons and photons and raises the question about the range of validity of the commonly employed polariton concept for GaAs.

Picosecond phase coherence and orientational relaxation of excitons in GaAs.

Time‐resolved degenerate four‐wave mixing (DFWM) on two‐dimensional (2D) excitons in GaAs single quantum wells is studied. We observe backward coherent emission (DFWM signal) from the 2D excitons comparable in efficiency to that in forward direction. This reflection or backward DFWM configuration is extremely advantageous and useful in exploring optically thin semiconductor layers.

A. Honold

Papers

Ultrafast phase relaxation of excitons via exciton-exciton and exciton-electron collisions.

Collision broadening of two-dimensional excitons in a GaAs single quantum well

Optical dephasing of homogeneously broadened two-dimensional exciton transitions in GaAs quantum wells.

Picosecond phase coherence and orientational relaxation of excitons in GaAs.

Reflected degenerate four‐wave mixing on GaAs single quantum wells