All-optical subcycle microscopy on atomic length scales.

Bringing optical microscopy to the shortest possible length and time scales has been a long-sought goal, connecting nanoscopic elementary dynamics with the macroscopic functionalities of condensed matter. Super-resolution microscopy has circumvented the far-field diffraction limit by harnessing optical nonlinearities1. By exploiting linear interaction with tip-confined evanescent light fields2, near-field microscopy3,4 has reached even higher resolution, prompting a vibrant research field by exploring the nanocosm in motion5-19. Yet the finite radius of the nanometre-sized tip apex has prevented access to atomic resolution20. Here we leverage extreme atomic nonlinearities within tip-confined evanescent fields to push all-optical microscopy to picometric spatial and femtosecond temporal resolution. On these scales, we discover an unprecedented and efficient non-classical near-field response, in phase with the vector potential of light and strictly confined to atomic dimensions. This ultrafast signal is characterized by an optical phase delay of approximately π/2 and facilitates direct monitoring of tunnelling dynamics. We showcase the power of our optical concept by imaging nanometre-sized defects hidden to atomic force microscopy and by subcycle sampling of current transients on a semiconducting van der Waals material. Our results facilitate access to quantum light-matter interaction and electronic dynamics at ultimately short spatio-temporal scales in both conductive and insulating quantum materials.

Build-up and dephasing of Floquet-Bloch bands on subcycle timescales.

Strong light fields have created opportunities to tailor novel functionalities of solids1-5. Floquet-Bloch states can form under periodic driving of electrons and enable exotic quantum phases6-15. On subcycle timescales, lightwaves can simultaneously drive intraband currents16-29 and interband transitions18,19,30,31, which enable high-harmonic generation16,18,19,21,22,25,28-30 and pave the way towards ultrafast electronics. Yet, the interplay of intraband and interband excitations and their relation to Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this link by visualizing the ultrafast build-up of Floquet-Bloch bands with time-resolved and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator32,33 with mid-infrared fields-strong enough for high-harmonic generation-and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our joint experiment and theory study provides a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.

Attosecond clocking of correlations between Bloch electrons.

Delocalized Bloch electrons and the low-energy correlations between them determine key optical1, electronic2 and entanglement3 functionalities of solids, all the way through to phase transitions4,5. To directly capture how many-body correlations affect the actual motion of Bloch electrons, subfemtosecond (1 fs = 10-15 s) temporal precision6-15 is desirable. Yet, probing with attosecond (1 as = 10-18 s) high-energy photons has not been energy-selective enough to resolve the relevant millielectronvolt-scale interactions of electrons1-5,16,17 near the Fermi energy. Here, we use multi-terahertz light fields to force electron-hole pairs in crystalline semiconductors onto closed trajectories, and clock the delay between separation and recollision with 300 as precision, corresponding to 0.7% of the driving field's oscillation period. We detect that strong Coulomb correlations emergent in atomically thin WSe2 shift the optimal timing of recollisions by up to 1.2 ± 0.3 fs compared to the bulk material. A quantitative analysis with quantum-dynamic many-body computations in a Wigner-function representation yields a direct and intuitive view on how the Coulomb interaction, non-classical aspects, the strength of the driving field and the valley polarization influence the dynamics. The resulting attosecond chronoscopy of delocalized electrons could revolutionize the understanding of unexpected phase transitions and emergent quantum-dynamic phenomena for future electronic, optoelectronic and quantum-information technologies.

Tunable non-integer high-harmonic generation in a topological insulator.

When intense lightwaves accelerate electrons through a solid, the emerging high-order harmonic (HH) radiation offers key insights into the material1-11. Sub-optical-cycle dynamics-such as dynamical Bloch oscillations2-5, quasiparticle collisions6,12, valley pseudospin switching13 and heating of Dirac gases10-leave fingerprints in the HH spectra of conventional solids. Topologically non-trivial matter14,15 with invariants that are robust against imperfections has been predicted to support unconventional HH generation16-20. Here we experimentally demonstrate HH generation in a three-dimensional topological insulator-bismuth telluride. The frequency of the terahertz driving field sharply discriminates between HH generation from the bulk and from the topological surface, where the unique combination of long scattering times owing to spin-momentum locking17 and the quasi-relativistic dispersion enables unusually efficient HH generation. Intriguingly, all observed orders can be continuously shifted to arbitrary non-integer multiples of the driving frequency by varying the carrier-envelope phase of the driving field-in line with quantum theory. The anomalous Berry curvature warranted by the non-trivial topology enforces meandering ballistic trajectories of the Dirac fermions, causing a hallmark polarization pattern of the HH emission. Our study provides a platform to explore topology and relativistic quantum physics in strong-field control, and could lead to non-dissipative topological electronics at infrared frequencies.

Temporal and spectral fingerprints of ultrafast all-coherent spin switching.

Future information technology demands ever-faster, low-loss quantum control. Intense light fields have facilitated milestones along this way, including the induction of novel states of matter1-3, ballistic acceleration of electrons4-7 and coherent flipping of the valley pseudospin8. These dynamics leave unique 'fingerprints', such as characteristic bandgaps or high-order harmonic radiation. The fastest and least dissipative way of switching the technologically most important quantum attribute-the spin-between two states separated by a potential barrier is to trigger an all-coherent precession. Experimental and theoretical studies with picosecond electric and magnetic fields have suggested this possibility9-11, yet observing the actual spin dynamics has remained out of reach. Here we show that terahertz electromagnetic pulses allow coherent steering of spins over a potential barrier, and we report the corresponding temporal and spectral fingerprints. This goal is achieved by coupling spins in antiferromagnetic TmFeO3 (thulium orthoferrite) with the locally enhanced terahertz electric field of custom-tailored antennas. Within their duration of one picosecond, the intense terahertz pulses abruptly change the magnetic anisotropy and trigger a large-amplitude ballistic spin motion. A characteristic phase flip, an asymmetric splitting of the collective spin resonance and a long-lived offset of the Faraday signal are hallmarks of coherent spin switching into adjacent potential minima, in agreement with numerical simulations. The switchable states can be selected by an external magnetic bias. The low dissipation and the antenna's subwavelength spatial definition could facilitate scalable spin devices operating at terahertz rates.

Subcycle observation of lightwave-driven Dirac currents in a topological surface band.

Harnessing the carrier wave of light as an alternating-current bias may enable electronics at optical clock rates1. Lightwave-driven currents have been assumed to be essential for high-harmonic generation in solids2-6, charge transport in nanostructures7,8, attosecond-streaking experiments9-16 and atomic-resolution ultrafast microscopy17,18. However, in conventional semiconductors and dielectrics, the finite effective mass and ultrafast scattering of electrons limit their ballistic excursion and velocity. The Dirac-like, quasi-relativistic band structure of topological insulators19-29 may allow these constraints to be lifted and may thus open a new era of lightwave electronics. To understand the associated, complex motion of electrons, comprehensive experimental access to carrier-wave-driven currents is crucial. Here we report angle-resolved photoemission spectroscopy with subcycle time resolution that enables us to observe directly how the carrier wave of a terahertz light pulse accelerates Dirac fermions in the band structure of the topological surface state of Bi2Te3. While terahertz streaking of photoemitted electrons traces the electromagnetic field at the surface, the acceleration of Dirac states leads to a strong redistribution of electrons in momentum space. The inertia-free surface currents are protected by spin-momentum locking and reach peak densities as large as two amps per centimetre, with ballistic mean free paths of several hundreds of nanometres, opening up a realistic parameter space for all-coherent lightwave-driven electronic devices. Furthermore, our subcycle-resolution analysis of the band structure may greatly improve our understanding of electron dynamics and strong-field interaction in solids.

Lightwave valleytronics in a monolayer of tungsten diselenide.

As conventional electronics approaches its limits 1 , nanoscience has urgently sought methods of fast control of electrons at the fundamental quantum level 2 . Lightwave electronics 3 -the foundation of attosecond science 4 -uses the oscillating carrier wave of intense light pulses to control the translational motion of the electron's charge faster than a single cycle of light5-15. Despite being particularly promising information carriers, the internal quantum attributes of spin 16 and valley pseudospin17-21 have not been switchable on the subcycle scale. Here we demonstrate lightwave-driven changes of the valley pseudospin and introduce distinct signatures in the optical readout. Photogenerated electron-hole pairs in a monolayer of tungsten diselenide are accelerated and collided by a strong lightwave. The emergence of high-odd-order sidebands and anomalous changes in their polarization direction directly attest to the ultrafast pseudospin dynamics. Quantitative computations combining density functional theory with a non-perturbative quantum many-body approach assign the polarization of the sidebands to a lightwave-induced change of the valley pseudospin and confirm that the process is coherent and adiabatic. Our work opens the door to systematic valleytronic logic at optical clock rates.

Intersubband Polariton-Polariton Scattering in a Dispersive Microcavity.

The ultrafast scattering dynamics of intersubband polaritons in dispersive cavities embedding GaAs/AlGaAs quantum wells are studied directly within their band structure using a noncollinear pump-probe geometry with phase-stable midinfrared pulses. Selective excitation of the lower polariton at a frequency of ∼25 THz and at a finite in-plane momentum k_{‖} leads to the emergence of a narrowband maximum in the probe reflectivity at k_{‖}=0. A quantum mechanical model identifies the underlying microscopic process as stimulated coherent polariton-polariton scattering. These results mark an important milestone toward quantum control and bosonic lasing in custom-tailored polaritonic systems in the mid and far infrared.

Reprint of: Comparison of the Chemical Properties of Selenocysteine and Selenocystine with Their Sulfur Analogs.

Some chemical properties of cystine and cysteine have been compared with those of their selenium-containing analogs. Major differences were noted between their titration curves, pK values of 2.01, 5.24, and 9.96 were observed for the ionizations of the carboxyl, selenohydryl, and amino groups, respectively, of selenocysteine. These values are compared with a pK of 2.3 for the carboxyl group of cysteine and values in the range of 8-10 for the ionization of the sulfhydryl and amino groups. Selenocysteine is much more reactive with halo acid derivatives than is cysteine, and reacts readily with iodoacetate even at pH values much below the pK of the selenohydryl group. Selenocysteine has an apparent half-wave potential of -0.212 V compared with 0.021 V for cysteine. It is unstable to acid hydrolysis, being completely decomposed by heating at 110° in 6 n HCl. It is also more soluble in water than is cysteine.

Lightwave-driven quasiparticle collisions on a subcycle timescale.

Ever since Ernest Rutherford scattered α-particles from gold foils, collision experiments have revealed insights into atoms, nuclei and elementary particles. In solids, many-body correlations lead to characteristic resonances--called quasiparticles--such as excitons, dropletons, polarons and Cooper pairs. The structure and dynamics of quasiparticles are important because they define macroscopic phenomena such as Mott insulating states, spontaneous spin- and charge-order, and high-temperature superconductivity. However, the extremely short lifetimes of these entities make practical implementations of a suitable collider challenging. Here we exploit lightwave-driven charge transport, the foundation of attosecond science, to explore ultrafast quasiparticle collisions directly in the time domain: a femtosecond optical pulse creates excitonic electron-hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands of the optical excitation. A full quantum theory explains our observations microscopically. This approach enables collision experiments with various complex quasiparticles and suggests a promising new way of generating sub-femtosecond pulses.

Tailored Subcycle Nonlinearities of Ultrastrong Light-Matter Coupling.

We explore the nonlinear response of tailor-cut light-matter hybrid states in a novel regime, where both the Rabi frequency induced by a coherent driving field and the vacuum Rabi frequency set by a cavity field are comparable to the carrier frequency of light. In this previously unexplored strong-field limit of ultrastrong coupling, subcycle pump-probe and multiwave mixing nonlinearities between different polariton states violate the normal-mode approximation while ultrastrong coupling remains intact, as confirmed by our mean-field model. We expect such custom-cut nonlinearities of hybridized elementary excitations to facilitate nonclassical light sources, quantum phase transitions, or cavity chemistry with virtual photons.

Real-time observation of interfering crystal electrons in high-harmonic generation.

Acceleration and collision of particles has been a key strategy for exploring the texture of matter. Strong light waves can control and recollide electronic wavepackets, generating high-harmonic radiation that encodes the structure and dynamics of atoms and molecules and lays the foundations of attosecond science. The recent discovery of high-harmonic generation in bulk solids combines the idea of ultrafast acceleration with complex condensed matter systems, and provides hope for compact solid-state attosecond sources and electronics at optical frequencies. Yet the underlying quantum motion has not so far been observable in real time. Here we study high-harmonic generation in a bulk solid directly in the time domain, and reveal a new kind of strong-field excitation in the crystal. Unlike established atomic sources, our solid emits high-harmonic radiation as a sequence of subcycle bursts that coincide temporally with the field crests of one polarity of the driving terahertz waveform. We show that these features are characteristic of a non-perturbative quantum interference process that involves electrons from multiple valence bands. These results identify key mechanisms for future solid-state attosecond sources and next-generation light-wave electronics. The new quantum interference process justifies the hope for all-optical band-structure reconstruction and lays the foundation for possible quantum logic operations at optical clock rates.

Addressing COVID-19 challenges in a randomised controlled trial on exercise interventions in a high-risk population.

BACKGROUND: The coronavirus disease 2019 (COVID-19) pandemic is a threat to ongoing clinical trials necessitating regular face-to-face, in-person meetings, particularly in participants with a high risk of complications. Guidance on how to handle and safely continue such trials is lacking. Chronically ill elderly individuals require-in addition to protection from infection-regular physical exercise and social contact to remain healthy. Solutions on how to handle these conflicting necessities are needed. The ENTAIER-randomised controlled trial was investigating the influence of mindful movements on fall risk, fear of falling, mobility, balance, life quality, and other outcomes. The study population was planned to comprise of 550 chronically ill elderly individuals with a high risk of falling. The movements were regularly performed in coached groups over 6 months. After the trial began, COVID-19 lockdowns stopped all in-person meetings, and it was expected that the limitations of this pandemic would continue for a long term. Therefore, the exercise programme, which involved complex movements and was typically conducted face-to-face in groups, had to be substituted by a telemedicine programme within a short timeframe. The objectives, therefore, were to identify challenges and tasks that could to be resolved and steps that could to be taken to achieve high-quality, efficacy, safety, and enable human encounter and motivation. METHODS: We proceeded with four steps: 1) A literature review on the quality and feasibility issues of telemedicine in general, and specifically, in exercise training in elderly individuals. 2) Participation in two international telemedicine task forces on integrative medicine, particularly, mind-body medicine. 3) Interviews with study therapists, (for practical purposes, eurythmy therapists and Tai Chi teachers are summarized here as therapists) personnel, and international experts on providing mindful movement exercises and other physiotherapies via live telecommunication technology, and with scientists and patient representatives. 4) Final evaluation by the core trial team and subsequent planning and implementation of changes in the trial organisation. RESULTS: Various tasks and challenges were identified: for the technical equipment for therapists and patients; for the ability of therapists and trial participants to adequately manage the technology and telemedicine intervention; the reservations and concerns about the technology among therapists and participants; safety and data protection in using the technology; and study design. The two major options found on how to continue the trial in the COVID-19 situation were a complete switch to telemedicine and a partial switch in the form of risk management implemented into the former design. CONCLUSIONS: The management of an ongoing clinical trial in a national or international crisis with a minimum of available time and extra financial resources, alongside with two checklists on steps and procedures for trial continuation and telemedicine implementation, may be informative for other researchers or healthcare providers faced with similar challenges and making similar decisions in the current situation or similar future scenarios. TRAIL REGISTRATION: www.drks.de . DRKS00016609. Registered July 30, 2019.

Full exome sequencing of 11 families with Hidradenitis suppurativa.

BACKGROUND: Hidradenitis suppurativa (HS) is not a well-studied or easily treated disease. Genetic information is essential for advances in the understanding and treatment of HS. This study aims to examine mutations in the gamma-secretase complex, the Notch signalling pathway and to perform a Mendelian analysis of genetic variants that segregated with disease in a full exome sequencing of 11 families with HS. METHOD: Whole-exome sequencing and Mendelian analysis of 11 families with HS from Denmark. Patients with a clinical diagnosis of active HS and a positive family history of HS were recruited. Consenting family members were enrolled and examined for HS as well. We included 11 families, with a total of 51 participants, 24 with HS and 27 without. Whole-exome sequencing using HiSeq platform as paired-end 2 × 150 bases was used. RESULTS: We found mutations in the Notch pathway for all families. We found mutations in the PSENEN and APH1B of the gamma-secretase genes. We also report 161 variants of unknown significance that segregated with the disease within these families. CONCLUSIONS: We did not find causative mutation for each family in this study, supporting the theory that HS is rarely caused by single-gene mutations. We suggest that future genetic studies should be focused on genome-wide association with thousands of cases, as this technique is better suited for suspected polygenic diseases.

Ultrafast transition between exciton phases in van der Waals heterostructures.

Heterostructures of atomically thin van der Waals bonded monolayers have opened a unique platform to engineer Coulomb correlations, shaping excitonic1-3, Mott insulating4 or superconducting phases5,6. In transition metal dichalcogenide heterostructures7, electrons and holes residing in different monolayers can bind into spatially indirect excitons1,3,8-11 with a strong potential for optoelectronics11,12, valleytronics1,3,13, Bose condensation14, superfluidity14,15 and moiré-induced nanodot lattices16. Yet these ideas require a microscopic understanding of the formation, dissociation and thermalization dynamics of correlations including ultrafast phase transitions. Here we introduce a direct ultrafast access to Coulomb correlations between monolayers, where phase-locked mid-infrared pulses allow us to measure the binding energy of interlayer excitons in WSe2/WS2 hetero-bilayers by revealing a novel 1s-2p resonance, explained by a fully quantum mechanical model. Furthermore, we trace, with subcycle time resolution, the transformation of an exciton gas photogenerated in the WSe2 layer directly into interlayer excitons. Depending on the stacking angle, intra- and interlayer species coexist on picosecond scales and the 1s-2p resonance becomes renormalized. Our work provides a direct measurement of the binding energy of interlayer excitons and opens the possibility to trace and control correlations in novel artificial materials.

A multi-centre, parallel-group, randomised controlled trial to assess the efficacy and safety of eurythmy therapy and tai chi in comparison with standard care in chronically ill elderly patients with increased risk of falling (ENTAiER): a trial protocol.

BACKGROUND: In elderly poeple, multimorbidity and polypharmacy increase while sensory, motor and cognitive functions decrease. Falls occur in 30% of people aged 65 years and older at least once per year, with injuries at 10-20%. Reducing falls and enhancing physical, emotional and cognitive capacities are essential for healthy aging despite chronic disease. Eurythmy therapy (EYT) and Tai Chi train balance, mobility and concentrative and sensory capacities. METHODS: In eight trial sites (academic or community hospitals), 550 outpatients aged 65 years and older with chronic disease and increased risk of falling (history of imbalance, Berg Balance Scale (BBS) score ≤ 49) will be randomly assigned (1:1:1) to receive either EYT or Tai Chi (each provided in one-hour group sessions, twice, later once per week plus practice at home, for over 24 weeks) added to standard care or standard care alone. Standard care includes a detailed written recommendation on fall prevention and the visit of a primary care doctor. Seniors living a reclusive life or economically disadvantaged elderly will be particularly addressed. A motivation and communication concept supports the trial participants' compliance with trial procedures and practicing. Public and patient representatives are involved in the planning and conduction of the trial. Falls will be documented daily in a diary by the participants. These falls as well as injuries and complications will be ascertained during monthly phone visits. The falls efficacy scale, BBS, cognition (MoCA), Mood (GDS-15), quality of life (SF12), instrumental activities of daily living (IADL), use of medical and non-medical services (FIMA) and adherence will be assessed at months 3, 6, and 12 and inner correspondence with practices (ICPH) at month 6. The trial is funded by the Federal Ministry of Education and Research (BMBF 01GL1805). DISCUSSION: This study will determine whether EYT and Tai Chi reduce falls, injurious falls, fear of falling and healthcare utilisation and improve mobility, cognition, mood, quality of life and functional independence. A reduction of fall risk and fear of falling and an improvement of mobility, autonomy, quality of life, mood, and cognition are highly relevant for older people to cope with aging and diseases and to reduce healthcare costs. TRAIL REGISTRATION: www.drks.de. DRKS00016609. Registered 30th July 2019.

Immunotherapeutic maintenance treatment with toll-like receptor 9 agonist lefitolimod in patients with extensive-stage small-cell lung cancer: results from the exploratory, controlled, randomized, international phase II IMPULSE study.

Background: The immune surveillance reactivator lefitolimod (MGN1703), a DNA-based TLR9 agonist, might foster innate and adaptive immune response and thus improve immune-mediated control of residual cancer disease. The IMPULSE phase II study evaluated the efficacy and safety of lefitolimod as maintenance treatment in extensive-stage small-cell lung cancer (ES-SCLC) after objective response to first-line chemotherapy, an indication with a high unmet medical need and stagnant treatment improvement in the last decades. Patients and methods: 103 patients with ES-SCLC and objective tumor response (as per RECIST 1.1) following four cycles of platinum-based first-line induction therapy were randomized to receive either lefitolimod maintenance therapy or local standard of care at a ratio of 3 : 2 until progression or unacceptable toxicity. Results: From 103 patients enrolled, 62 were randomized to lefitolimod, 41 to the control arm. Patient demographics and response patterns to first-line therapy were balanced. Lefitolimod exhibited a favorable safety profile and pharmacodynamic assessment confirmed the mode-of-action showing a clear activation of monocytes and production of interferon-gamma-induced protein 10 (IP-10). While in the intent-to-treat (ITT) population no relevant effect of lefitolimod on progression-free and overall survival (OS) could be observed, two predefined patient subgroups indicated promising results, favoring lefitolimod with respect to OS: in patients with a low frequency of activated CD86+ B cells (hazard ratio, HR 0.53, 95% CI: 0.26-1.08; n = 38 of 88 analyzed) and in patients with reported chronic obstructive pulmonary disease (COPD) (HR 0.48, 95% CI: 0.20-1.17, n = 25 of 103). Conclusions: The IMPULSE study showed no relevant effect of lefitolimod on the main efficacy end point OS in the ITT, but (1) the expected pharmacodynamic response to lefitolimod, (2) positive OS efficacy signals in two predefined subgroups and (3) a favorable safety profile. These data support further exploration of lefitolimod in SCLC.

Ultrafast Mid-Infrared Nanoscopy of Strained Vanadium Dioxide Nanobeams.

Long regarded as a model system for studying insulator-to-metal phase transitions, the correlated electron material vanadium dioxide (VO2) is now finding novel uses in device applications. Two of its most appealing aspects are its accessible transition temperature (∼341 K) and its rich phase diagram. Strain can be used to selectively stabilize different VO2 insulating phases by tuning the competition between electron and lattice degrees of freedom. It can even break the mesoscopic spatial symmetry of the transition, leading to a quasiperiodic ordering of insulating and metallic nanodomains. Nanostructuring of strained VO2 could potentially yield unique components for future devices. However, the most spectacular property of VO2--its ultrafast transition--has not yet been studied on the length scale of its phase heterogeneity. Here, we use ultrafast near-field microscopy in the mid-infrared to study individual, strained VO2 nanobeams on the 10 nm scale. We reveal a previously unseen correlation between the local steady-state switching susceptibility and the local ultrafast response to below-threshold photoexcitation. These results suggest that it may be possible to tailor the local photoresponse of VO2 using strain and thereby realize new types of ultrafast nano-optical devices.

Resonant internal quantum transitions and femtosecond radiative decay of excitons in monolayer WSe2.

Atomically thin two-dimensional crystals have revolutionized materials science. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, owing to their direct energy gaps in the optical range. Their electronic and optical properties are dominated by Coulomb-bound electron-hole pairs called excitons, whose unusual internal structure, symmetry, many-body effects and dynamics have been vividly discussed. Here we report the first direct experimental access to all 1s A excitons, regardless of momentum--inside and outside the radiative cone--in single-layer WSe2. Phase-locked mid-infrared pulses reveal the internal orbital 1s-2p resonance, which is highly sensitive to the shape of the excitonic envelope functions and provides accurate transition energies, oscillator strengths, densities and linewidths. Remarkably, the observed decay dynamics indicates an ultrafast radiative annihilation of small-momentum excitons within 150 fs, whereas Auger recombination prevails for optically dark states. The results provide a comprehensive view of excitons and introduce a new degree of freedom for quantum control, optoelectronics and valleytronics of dichalcogenide monolayers.