Explore the vast range of titles available.

Interlayer excitons (ILXs) — electron–hole pairs bound across two atomically thin layered semiconductors — have emerged as attractive platforms to study exciton condensation 1 – 4 , single-photon emission and other quantum information applications 5 – 7 . Yet, despite extensive optical spectroscopic investigations 8 – 12 , critical information about their size, valley configuration and the influence of the moiré potential remains unknown. Here, in a WSe 2 /MoS 2 heterostructure, we captured images of the time-resolved and momentum-resolved distribution of both of the particles that bind to form the ILX: the electron and the hole. We thereby obtain a direct measurement of both the ILX diameter of around 5.2 nm, comparable with the moiré-unit-cell length of 6.1 nm, and the localization of its centre of mass. Surprisingly, this large ILX is found pinned to a region of only 1.8 nm diameter within the moiré cell, smaller than the size of the exciton itself. This high degree of localization of the ILX is backed by Bethe–Salpeter equation calculations and demonstrates that the ILX can be localized within small moiré unit cells. Unlike large moiré cells, these are uniform over large regions, allowing the formation of extended arrays of localized excitations for quantum technology. Imaging the electron and hole that bind to form interlayer excitons in a 2D moiré material enables direct measurement of its diameter and indicates the localization of its centre of mass.

With their long lifetime and protection against decoherence, dark excitons in monolayer semiconductors offer a promising route for quantum technologies. Optical techniques have previously observed dark excitons with a long-lived valley polarization. However, several aspects remain unknown, such as the populations and time evolution of the different valley-polarized dark excitons and the role of excitation conditions. Here, using time- and angle-resolved photoemission spectroscopy, we obtain a holistic view of the dynamics after valley-selective photoexcitation. By varying experimental conditions, we reconcile between the rapid valley depolarization previously reported in TR-ARPES, and the observation of long-lived valley polarized dark excitons in optical studies. For the latter, we find that momentum-dark excitons largely dominate at early times sustaining a 40% degree of valley polarization, while valley-polarized spin-dark states dominate at longer times. Our measurements provide the timescales and how the different dark excitons contribute to the previously observed long-lived valley polarization in optics. The authors showcase the capabilities of time-resolved momentum microscopy to image spin- and valley-resolved excitons in monolayer WS₂ with high energy resolution, revealing distinct long-lived, valley-polarized dark excitons that dominate at different timescales.

The transient terahertz (THz) pulse with high peak field has become an important tool for matter manipulation, enabling many applications such as nonlinear spectroscopy, particle acceleration, and high harmonic generation. Among the widely used THz generation techniques, optical rectification in lithium niobate (LN) has emerged as a powerful method to achieve high fields at low THz frequencies, suitable to exploring novel nonlinear phenomena in condensed matter systems. In this review, we focus on introducing single- to few-cycle THz generation in LN, including the basic principles, techniques, latest developments, and current limitations. We will first discuss the phase matching requirements of LN, which leads to Cherenkov-like radiation, and the tilted pulse front (TPF) technique. Emphasis will be put on the TPF technique, which has been shown to improve THz generation efficiency, but still has many limitations. Different geometries used to produce continuous and discrete TPF will be systematically discussed. We summarize the advantages and limitations of current techniques and future trends.

With their long lifetime and protection against decoherence, dark excitons in monolayer semiconductors offer a promising route for quantum technologies. Optical techniques have previously observed dark excitons with a long-lived valley polarization. However, several aspects remain unknown, such as the populations and time evolution of the different valley-polarized dark excitons and the role of excitation conditions. Here, using time- and angle-resolved photoemission spectroscopy, we obtain a holistic view of the dynamics after valley-selective photoexcitation. By varying experimental conditions, we reconcile between the rapid valley depolarization previously reported in TR-ARPES, and the observation of long-lived valley polarized dark excitons in optical studies. For the latter, we find that momentum-dark excitons largely dominate at early times sustaining a 40% degree of valley polarization, while valley-polarized spin-dark states dominate at longer times. Our measurements provide the timescales and how the different dark excitons contribute to the previously observed long-lived valley polarization in optics.

Inducing novel quantum phases and topologies in materials using intense light fields is a key objective of modern condensed matter physics, but nonetheless faces significant experimental challenges. Alternately, theory predicts that in the dense limit, excitons - collective excitations composed of Coulomb-bound electron-hole pairs - could also drive exotic quantum phenomena. However, the direct observation of these phenomena requires the resolution of electronic structure in momentum space in the presence of excitons, which became possible only recently. Here, using time- and angle-resolved photoemission spectroscopy of an atomically thin semiconductor in the presence of a high-density of resonantly and coherently photoexcited excitons, we observe the Bardeen-Cooper-Schrieffer (BCS) excitonic state - analogous to the Cooper pairs of superconductivity. We see the valence band transform from a conventional paraboloid into a Mexican-hat like Bogoliubov dispersion - a hallmark of the excitonic insulator phase; and we observe the recently predicted giant exciton-driven Floquet effects. Our work realizes the promise that intense bosonic fields, other than photons, can also drive novel quantum phenomena and phases in materials.

Interlayer excitons (ILXs) - electron-hole pairs bound across two atomically thin layered semiconductors - have emerged as attractive platforms to study exciton condensation, single-photon emission and other quantum-information applications. Yet, despite extensive optical spectroscopic investigations, critical information about their size, valley configuration and the influence of the moiré potential remains unknown. Here, we captured images of the time- and momentum-resolved distribution of both the electron and the hole that bind to form the ILX in a WSe2/MoS2 heterostructure. We thereby obtain a direct measurement of the interlayer exciton diameter of ~5.4 nm, comparable to the moiré unit-cell length of 6.1 nm. Surprisingly, this large ILX is well localized within the moiré cell to a region of only 1.8 nm - smaller than the size of the exciton itself. This high degree of localization of the interlayer exciton is backed by Bethe-Salpeter equation calculations and demonstrates that the ILX can be localized within small moiré unit cells. Unlike large moiré cells, these are uniform over large regions, thus allowing the formation of extended arrays of localized excitations for quantum technology.

An exciton, a two-body composite quasiparticle formed of an electron and hole, is a fundamental optical excitation in condensed-matter systems. Since its discovery nearly a century ago, a measurement of the excitonic wavefunction has remained beyond experimental reach. Here, we directly image the excitonic wavefunction in reciprocal space by measuring the momentum distribution of electrons photoemitted from excitons in monolayer WSe2. By transforming to real space, we obtain a visual of the distribution of the electron around the hole in an exciton. Further, by also resolving the energy coordinate, we confirm the elusive theoretical prediction that the photoemitted electron exhibits an inverted energy-momentum dispersion relationship reflecting the valence band where the partner hole remains, rather than that of conduction-band states of the electron.

The nonlinear propagation of an initially sinusoidal acoustic wave is described by modelling the field with a Gaussian beam. The theoretical model takes account of the effects of nonlinear distortion, absorption, dispersion and diffraction, although the application is mainly limited to the field on the acoustic axis of a transducer. The field of a circular disc radiator is explicitly considered, for both the unfocused and the focused case, and the numerical solution of the basic equations is described. The application of previously derived, approximate solutions is demonstrated and these solutions are assessed for their accuracy, particularly when used to predict the amplitudes of the harmonic components. Experimental verification of the theoretical predictions has been obtained for propagation in water in the frequency range 1 to 100 MHz and also for propagation in a tissue-mimmicking gel. A particular feature of the measurement system, which uses a broad-band hydrophone, is the ability to record many harmonic components. It is demonstrated that nonlinear distortion occurs in media with acoustic properties similar to those of tissue and that the characteristic behaviour of the field is significantly different to the behaviour of the corresponding field in water. Consequently it is difficult to predict the behaviour of the field in tissue from measurements made in water, but nevertheless a procedure has been developed to make such predictions. As an application of the work described here, a new method for the absolute calibration of hydrophones over a wide range of frequencies has been developed as well as a technique for intercomparing the sensitivities of hydrophones at a number of frequencies simultaneously.

When all else is equal, how do business-to-business (B2B) customers choose between competing suppliers? As we have shown in our previous book, Winning Behavior, when competing suppliers offer essentially the same technical skills, experience, solutions, and price, behavior becomes the most important factor in the customer's selection decision - and this is happening with greater frequency today as competing companies become more and more alike. Because competing suppliers look very much alike and offer fundamentally the same products, services, and advantages at much the same price, the customer's final selection decision will not be driven by the question 'Can you do the work?' That question has already been answered. The customer's key question now is, 'Do we want to work with you?' and the answer, positive or negative, will reside in what the customer has observed in each finalist's behaviors.