Making quantitative predictions for quantum light-matter interactions in ultra thin solids involves several fundamental challenges. Any realistic theory must simultaneously account for the highly confined light modes and their quantization in the presence of optical losses, while simultaneously describing the electronic states from first principles. In this paper we develop a framework that combines density functional theory (DFT) with macroscopic quantum electrodynamics (MQED). We exemplify our methods by presenting ab-initio predictions for the Purcell effect in atomically thin van der Waals heterostructures. Our ab-initio predictions for the spontaneous emission rate enhancement of intersubband transitions in few-layer transition metal dichalcogenides (TMDs) sandwiched between graphene and a perfect conductor can approach $10^7$. Such enhancement corresponds to a lifetime reduction from microseconds to picoseconds due to the the extreme confinement of light modes associated with the graphene plasmons. The general validity of our methodology allows us to test the limitations of widely used approximations, including the dipole-approximation, 1D quantum well wave functions, and Fermi's Golden rule for spontaneous emission. Our approach lays the foundation for ab-initio modeling of light matter interactions in structures where the quantum nature of light influences the electron system.

A quantitative and predictive theory of quantum light-matter interactions in ultra thin materials involves several fundamental challenges. Any realistic model must simultaneously account for the ultra-confined plasmonic modes and their quantization in the presence of losses, while describing the electronic states from first principles. Herein we develop such a framework by combining density functional theory (DFT) with macroscopic quantum electrodynamics, which we use to show Purcell enhancements reaching 10^7 for intersubband transitions in few-layer transition metal dichalcogenides sandwiched between graphene and a perfect conductor. The general validity of our methodology allows us to put several common approximation paradigms to quantitative test, namely the dipole-approximation, the use of 1D quantum well model wave functions, and the Fermi's Golden rule. The analysis shows that the choice of wave functions is of particular importance. Our work lays the foundation for practical ab initio-based quantum treatments of light matter interactions in realistic nanostructured materials.

Due to their unique 2D nature, charge carriers in semiconducting transition metal dichalcogenides (TMDs) exhibit strong unscreened Coulomb interactions and sensitivity to defects and impurities. The versatility of van der Waals layer stacking allows spatially separating electrons and holes between different TMD layers with staggered band structure, yielding interlayer few-body excitonic complexes whose nature is still debated. Here we combine quantum Monte Carlo calculations with spectrally and temporally resolved photoluminescence (PL) measurements on a top-and bottom-gated MoSe2/WSe2 heterostructure, and identify the emitters as impurity-bound interlayer excitonic complexes. Using independent electrostatic control of doping and out-of-plane electric field, we demonstrate control of the relative populations of neutral and charged complexes, their emission energies on a scale larger than their linewidth, and an increase of their lifetime into the microsecond regime. This work unveils new physics of confined carriers and is key to the development of novel optoelectronics applications.

We find what mechanisms limit fundamental light-matter interactions of plasmons confined to the atomic scale, when interfacing two-dimensional semiconductor emitters. We show how nonlocality governs the dynamics, limiting the Purcell factor yet reaching ultra-strong coupling. © 2019 The Author(s)

Two-dimensional (2D) semiconducting materials are promising building blocks for optoelectronic applications, many of which require efficient dissociation of excitons into free electrons and holes. However, the strongly bound excitons arising from the enhanced Coulomb interaction in these monolayers suppresses the creation of free carriers. Here, we identify the main exciton dissociation mechanism through time and spectrally resolved photocurrent measurements in a monolayer WSe 2 p – n junction. We find that under static in-plane electric field, excitons dissociate at a rate corresponding to the one predicted for tunnel ionization of 2D Wannier–Mott excitons. This study is essential for understanding the photoresponse of 2D semiconductors and offers design rules for the realization of efficient photodetectors, valley dependent optoelectronics, and novel quantum coherent phases.

Phase transitions of solids and structural transformations of molecules are canonical examples of important photoinduced processes whose underlying mechanisms largely elude our comprehension due to our inability to correlate electronic excitation with atomic position in real time. Here, we present a decisive step towards such new methodology based on water-window covering (284-543 eV) attosecond soft x-ray pulses that can simultaneously access electronic and lattice parameters via dispersive x-ray absorption fine-structure (XAFS) spectroscopy. We validate this approach with an identification of the sigma* and pi* orbital contributions to the density of states in graphite simultaneously with its lattice's four characteristic bonding distances. This work demonstrates the potential of dispersive XAFS, in combination with attosecond pulses, as a powerful investigative tool that is equally applicable to gas, liquid, and condensed phase. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

The science and applications of electronics and optoelectronics have been driven for decades by progress in the growth of semiconducting heterostructures. Many applications in the infrared and terahertz frequency range exploit transitions between quantized states in semiconductor quantum wells (intersubband transitions). However, current quantum well devices are limited in functionality and versatility by diffusive interfaces and the requirement of lattice-matched growth conditions. Here, we introduce the concept of intersubband transitions in van der Waals quantum wells and report their first experimental observation. Van der Waals quantum wells are naturally formed by two-dimensional materials and hold unexplored potential to overcome the aforementioned limitations-they form atomically sharp interfaces and can easily be combined into heterostructures without lattice-matching restrictions. We employ near-field local probing to spectrally resolve intersubband transitions with a nanometre-scale spatial resolution and electrostatically control the absorption. This work enables the exploitation of intersubband transitions with unmatched design freedom and individual electronic and optical control suitable for photodetectors, light-emitting diodes and lasers.

Phase transitions of solids and structural transformations of molecules are canonical examples of important photo-induced processes, whose underlying mechanisms largely elude our comprehension due to our inability to correlate electronic excitation with atomic position in real time. Here, we present a decisive step towards such new methodology based on water-window-covering (284 eV to 543 eV) attosecond soft X-ray pulses that can simultaneously access electronic and lattice parameters via dispersive X-ray absorption fine-structure (XAFS) spectroscopy. We validate attoXAFS with an identification of the σ* and π* orbital contributions to the density of states in graphite simultaneously with its lattice's four characteristic bonding distances. This work demonstrates the concept of attoXAFS as a powerful real-time investigative tool which is equally applicable to gas-, liquid- and condensed phase.

Sumary form only given. A multitude of existing technologies are based on the ability of converting light into electrical signals. Graphene has demonstrated a number of optical and transport properties [1] which are promising for this type of optoelectronic applications and great efforts have been devoted to the development of graphene-based photodetectors. Being a gapless semiconductor, graphene enables light absorption over a wide energy spectrum, spanning from the ultraviolet to the far infrared. Moreover, its absorption is wavelength-independent and its optical properties are tunable via electrostatic doping. Finally, it displays low dissipation rates and high carrier mobility and it enables electromagnetic-energy confinement to extremely small volumes [2]. However, graphene devices on standard SiO 2 -substrates display properties that are far inferior to that of the suspended graphene. This motivates the research for dielectrics that allow substrate-supported geometry while retaining the intrinsic quality of graphene. The photodetection efficiency is ultimately defined by the magnitude and the speed of the photoresponse of the detecting-material. Here, we present an experimental study of the photoresponse of encapsulated graphene by high sensitivity ultrafast transient absorption experiments, in transmission and reflection geometry. We investigate the effect of the encapsulant material on the hot-electron cooling by comparing SiO 2 -supported graphene with graphene encapsulated into hBN and multilayer MoS 2 . In order to unveil the dominating cooling mechanism, we discuss the dependence of the relaxation dynamics on the lattice temperature and on the initial hot-electron temperature, which is tuned by changing the excitation power.

Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities and superior electrical and optoelectronic properties 1-7 . Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting 8-16 require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements, we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons 17-19 in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism using distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures.

Summary form only given. The discovery of intersubband transitions in III-V semiconductor heterostructures [I] had a huge impact on large parts of the condensed matter physics community and ultimately led to the development of quantum well infrared photodetectors [2] and quantum cascade lasers [3]. One of the main constraints, however, are the strict lattice matching conditions of the heterostructures - limiting the available materials to combine - and its expensive and complicated growth.Here, we present an innovative measurement technique that allows us to overcome these constraints We observe resonant absorption in the conduction band of MoS2 and valence band of WSe2 that we can attribute to intersubband transitions. By varying the doping of the TMD flakes we can compare the extracted values of the absorption to a simple model for an infinite square well potential and find a good quantitative agreement.

Finding alternative optoelectronic mechanisms that overcome the limitations of conventional semiconductor devices is paramount for detecting and harvesting low-energy photons. A highly promising approach is to drive a current from the thermal energy added to the free-electron bath as a result of light absorption. Successful implementation of this strategy requires a broadband absorber where carriers interact among themselves more strongly than with phonons, as well as energy-selective contacts to extract the excess electronic heat. Here we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath leading to a thermalized hot carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the photo-thermionic effect enables detection of sub-bandgap photons, while being size-scalable, electrically tunable, broadband and ultrafast.

Photodetectors based on graphene/WSe 2 /graphene heterostructures combine an ultrafast photoresponse with high quantum efficiency.