In van der Waals (vdW) heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moiré superlattice. While it is widely recognized that a moiré superlattice can modulate the electronic band structure and lead to novel transport properties including unconventional superconductivity and insulating behavior driven by correlations, its influence on optical properties has not been investigated experimentally. We present spectroscopic evidence that interlayer excitons are confined by the moiré potential in a high-quality MoSe2/WSe2 heterobilayer with small rotational twist. A series of interlayer exciton resonances with either positive or negative circularly polarized emission is observed in photoluminescence, consistent with multiple exciton states confined within the moiré potential. The recombination dynamics and temperature dependence of these interlayer exciton resonances are consistent with this interpretation. These results demonstrate the feasibility of engineering artificial excitonic crystals using vdW heterostructures for nanophotonics and quantum information applications.

Improving the temporal resolution of single photon detectors has an impact on many applications 1 , such as increased data rates and transmission distances for both classical 2 and quantum 3-5 optical communication systems, higher spatial resolution in laser ranging and observation of shorter-lived fluorophores in biomedical imaging 6 . In recent years, superconducting nanowire single-photon detectors 7,8 (SNSPDs) have emerged as the highest efficiency time-resolving single-photon counting detectors available in the near infrared 9 . As the detection mechanism in SNSPDs occurs on picosecond time scales 10 , SNSPDs have been demonstrated with exquisite temporal resolution below 15 ps [11][12][13][14][15] . We reduce this value to 2.7±0.2 ps at 400 nm and 4.6±0.2 ps at 1550 nm, using a specialized niobium nitride (NbN) SNSPD. The observed photon-energy dependence of the temporal resolution and detection latency suggests that intrinsic effects make a significant contribution.Temporal resolution in SNSPDs, commonly referred to as jitter, is characterized by the width of the temporal distribution of signal outputs with respect to the photon arrival times. This statistical distribution is known as the instrument response function (IRF), and its width is commonly evaluated as

Multidimensional Coherent Optical Photocurrent Spectroscopy (MD-COPS) is implemented using unstabilized interferometers. Photocurrent from a semiconductor sample is generated using a sequence of four excitation pulses in a collinear geometry. Each pulse is frequency shifted by a unique radio frequency through acousto-optical modulation; the Four-Wave Mixing (FWM) signal is then selected in the frequency domain. The interference of an auxiliary continuous wave laser, which is sent through the same interferometers as the excitation pulses, is used to synthesize reference frequencies for lock-in detection of the photocurrent FWM signal. This scheme enables the partial compensation of mechanical fluctuations in the setup, achieving sufficient phase stability without the need for active stabilization. The method intrinsically provides both the real and imaginary parts of the FWM signal as a function of inter-pulse delays. This signal is subsequently Fourier transformed to create a multi-dimensional spectrum. Measurements made on the excitonic resonance in a double InGaAs quantum well embedded in a p-i-n diode demonstrate the technique.

In atomically thin two-dimensional semiconductors such as transition metal dichalcogenides (TMDs), controlling the density and type of defects promises to be an effective approach for engineering light-matter interactions. We demonstrate that electron-beam irradiation is a simple tool for selectively introducing defect-bound exciton states associated with chalcogen vacancies in TMDs. Our first-principles calculations and time-resolved spectroscopy measurements of monolayer WSe_{2} reveal that these defect-bound excitons exhibit exceptional optical properties including a recombination lifetime approaching 200 ns and a valley lifetime longer than 1  μs. The ability to engineer the crystal lattice through electron irradiation provides a new approach for tailoring the optical response of TMDs for photonics, quantum optics, and valleytronics applications.

We study an asymmetric double InGaAs quantum well using optical two-dimensional coherent spectroscopy. The collection of zero-quantum, one-quantum and two-quantum two-dimensional spectra provides a unique and comprehensive picture of the double well coherent optical response. Coherent and incoherent contributions to the coupling between the two quantum well excitons are clearly separated. An excellent agreement with density matrix calculations reveals that coherent inter-well coupling originates from many-body interactions.Coupled quantum wells (QWs) are one of the most fundamental topics of quantum mechanics. They can be realized in epitaxially-grown semiconductor materials, where the coupling can be exploited in optoelectronic devices such as quantum cascade lasers [1]. Furthermore, since QW and barrier sizes can be tailored, coupled semiconductor QWs can serve as a model for other systems. For example, the absence of vibrational coupling in semiconductor QWs allows isolation of electronic coupling; this distinctive feature may help understanding extremely efficient energy transfer in light harvesting complexes, where the roles played by electronic and vibrational coupling are under debate [2][3][4][5]. Semiconductor double QWs (DQWs) have attracted theoretical and experimental attention for more than twenty years. The roles of resonant transfer and wavefunction hybridization [6][7][8], phonon-assisted tunneling [9,10], dipole-dipole coupling [11], percolation of carriers through imperfect barriers [12], and thermally activated charge transfer [13] have been studied and discussed, as well as the formation of indirect excitons [14,15]. However, the role played by many-body effects-which have been shown to dominate the coherent response of semiconductor excitons [16,17]-in the coupling mechanism has been neglected so far.We use optical two-dimensional coherent spectroscopy (2DCS) to characterize coupling between the QW excitons, which are electron-hole pairs bound together by their Coulomb attraction. 2DCS is an extension of transient four-wave-mixing (FWM) spectroscopy, with the addition of interferometric stabilization of inter-pulse delays and measurement of the signal field. It is an ideal technique to study coupling between resonances, since unfolding one-dimensional spectra onto a second dimension distinguishes quantum beats from polarization interferences [18]. Additionally, 2DCS has been demonstrated as a powerful tool for revealing many-body effects in semiconductor nanostructures [16,17]. Several types of 2D spectra-isolating zero-, one-, and two-quantum coherences-have been shown in previous work to reveal information that one-dimensional techniques cannot access [19][20][21][22][23][24][25][26], but these different types of spectra have never been recorded and analyzed together for a single system so far. Previously, multidimensional spectroscopy showed electronic coherences between excitonic transitions of a GaAs/AlGaAs DQW [27][28][29]. However, the presence of heavy and light holes in each GaAs/Al...