Optical absorption is one of fundamental light-matter interactions. In most materials, optical absorption is a weak perturbation to the light. In this regime, absorption and emission are irreversible, incoherent processes due to strong damping. Excitons in monolayer transition metal dichalcogenides, however, interact strongly with light, leading to optical absorption in the non-perturbative regime where coherent re-emission of the light has to be considered. Between the incoherent and coherent limits, we show that a robust critical coupling condition exists, leading to perfect optical absorption. Up to 99.6% absorption is measured in a sub-nanometer thick MoSe2 monolayer placed in front of a mirror. The perfect absorption is controlled by tuning the exciton-phonon, exciton-exciton, and exciton-photon interactions by temperature, pulsed laser excitation, and a movable mirror, respectively. Our work suggests unprecedented opportunities for engineering exciton-light interactions using two-dimensional atomically thin crystals, enabling novel photonic applications including ultrafast light modulators and sensitive optical sensing.

We measure many-body interactions in isolated quantum dot states using double-quantum multidimensional coherent spectroscopy. Few states are probed in a diffraction limited spot, which is enabled by a novel collinear scheme in which radiated four-wave-mixing signals are measured with phase resolution. Many-body interactions are enhanced by an additional prepulse tuned to the delocalized quasi-continuum states. We propose this effect as a method for controlling coupling between quantum states. PACS numbers: 78.67.Hc, 78.47.nj Quantum dots (QDs) are often described as being noninteracting artificial atoms. Some optical spectroscopic experiments have been used to conclude that there are no measurable many-body interactions present for resonant excitation of interfacial ODs, which would support treating these QDs as non-interacting [1]. However, other optical techniques have yielded signatures of interactions between these QDs [2][3][4]. Outside of the spectroscopic differences, discrepancies exist regarding the presence of many-body effects in QD lasers [5]. The benefits of QD lasers arise from the discrete and narrow energy levels of QDs, but they are usually pumped by the injection of delocalized carriers [6]. Since many-body effects play a tremendous role in the theoretical treatment of semiconductors [7], it is important to understand the relevant interactions for calculating QD laser properties.Excitons and trions confined to QDs are potential candidates for qubits in quantum information [8][9][10]. The electronic states of a QD are accessible both optically and electronically. Also, the high oscillator strengths of electronic transitions in solid state systems facilitate their measurement and manipulation. Coherent control with ultrafast Rabi rotations has been demonstrated on both single and ensemble QD systems [11,12]. However, controlled qubit interaction remains one of the most challenging requirements for a functional quantum computer with few implementations for spin states in QDs [13,14] and none for the electronic states. The localization of excitons in QDs that gives them the benefit of being difficult to decohere also makes them difficult to entangle, or couple [15].Here we observe that the excitation of delocalized states not only enhances many-body effects, in agreement with theory [16], but can also turn them on. The physical mechanism responsible for enhancing many-body interactions in QDs may explain discrepancies in the literature. The mechanism may also be applied for turning on electronic coupling between isolated QD states.We use ultrafast coherent spectroscopy techniques to directly probe coupling and many-body interactions in a sub-micron-sized region containing a small number of distinct epitaxially-grown GaAs interfacial QDs at a temperature of 6 K. These interfacial QDs are exciton states bound by monolayer fluctuations in a narrow 4.2 nm GaAs quantum well with Al 0.3 Ga 0.7 As barriers [17]. The decreased transverse confinement binds the localized excitons by 10 meV, which energetic...

Transition metal dichalcogenides (TMDs) are regarded as a possible materials platform for quantum information science and related device applications. In TMD monolayers, the dephasing time and inhomogeneity are crucial parameters for any quantum information application. In TMD heterostructures, coupling strength and interlayer exciton lifetimes are also parameters of interest. However, many demonstrations in TMDs can only be realized at specific spots on the sample, presenting a challenge to the scalability of these applications. Here, using multi-dimensional coherent imaging spectroscopy (MDCIS), we shed light on the underlying physics - including dephasing, inhomogeneity, and strain - for a MoSe2 monolayer and identify both promising and unfavorable areas for quantum information applications. We furthermore apply the same technique to a MoSe2/WSe2 heterostructure. Despite the notable presence of strain and dielectric environment changes, coherent and incoherent coupling, as well as interlayer exciton lifetimes are mostly robust across the sample. This uniformity is despite a significantly inhomogeneous interlayer exciton photoluminescence distribution that suggests a bad sample for device applications. This robustness strengthens the case for TMDs as a next-generation materials platform in quantum information science and beyond.

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Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase TiO2 significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large (∼1 eV) band gap reduction of noncompensated codoped rutile TiO2. It is shown that Cr/N codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr 3d3 levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr 3d and delocalized N 2p states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications.