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.

3Spontaneous symmetry breaking gives rise to a new quantum ground state featuring characteristic low-energy elementary excitations 3,11,14,[18][19][20][21][22] Ultrashort pulses in the terahertz (1 THz = 10 12 Hz) range have been used to trace electronic order via direct coupling to such excitations 22,23 . We demonstrate that THz pulses may simultaneously also track the crystalline order during an ultrafast phase transition.This idea is tested in a prominent reference system, 1T-TiSe 2 . Within the family of layered transition-metal dichalcogenides, this material has attracted special attention: Upon cooling below T c ≈ 200 K, it undergoes a transition into a commensurate CDW accompanied by the formation of a structural (2×2×2) superlattice 21 (Fig. 1a). In its high-temperature phase, TiSe 2 is a semimetal 20 with electron and hole pockets at the L and  points of the Brillouin zone, respectively 15,24 (Fig. 1b). The spatial reconstruction due to the CDW maps these two points on top of each other and leads to the partial opening of an electronic energy gap as well as a dramatic reduction of the density of free charge carriers 20 (Fig. 1b). Superconductivity emerges when the CDW is suppressed, e.g. by Cu intercalation 7 or pressure 25 . This discovery as well as novel chiral properties 26 have intensified the interest in the nature of the CDW in 1T-TiSe 2 . Yet, the microscopic mechanisms remain elusive 24,[27][28][29] . A first hypothesis assumes electron-phonon coupling based on a Jahn-Teller effect as the driving force 27 . A competing model suggests that the transition is purely electronically driven 24,28 . Coulomb attraction may render the system unstable against the formation of excitons between the electron-and hole-like Fermi pockets, leading to lattice deformation with the corresponding wave vector. Combinations of the two scenarios have also been proposed 29 . Time-resolved x-ray diffraction 16 and photoemission 10,15 experiments have separately tracked the dynamics of either structural or electronic orders. 4Evidence for both excitonic and phononic contributions was obtained in this way, leaving a controversial picture.Here we disentangle the two coupled components of the CDW order parameter by simultaneously tracing the ultrafast THz response of PLD-related phonons and electronic conductivity while a femtosecond pulse selectively melts the excitonic order. Our data reveal a transient phase in which the PLD persists in the absence of excitonic correlations. A quantummechanical theory 29 corroborates our conclusions.In TiSe 2 , the transition to the CDW ordered phase modifies the low-frequency optical response in three distinct ways: (i) The CDW-induced energy gap introduces a broad single- (Fig. 1d). Above T c , we observe a single TO phonon resonance at 17 meV. Below T c , back-folding of the uppermost acoustic branch from the L to the  point 21 yields an additional IR-active in-plane mode at 19 meV. The weaker peak at 22 meV likely originates from a folded optical branch at the M point 21 . 5W...

Condensation of bosons causes spectacular phenomena such as superfluidity or superconductivity. Understanding the nature of the condensed particles is crucial for active control of such quantum phases. Fascinating possibilities emerge from condensates of light–matter-coupled excitations, such as exciton–polaritons, photons hybridized with hydrogen-like bound electron–hole pairs. So far, only the photon component has been resolved, while even the mere existence of excitons in the condensed regime has been challenged. Here we trace the matter component of polariton condensates by monitoring intra-excitonic terahertz transitions. We study how a reservoir of optically dark excitons forms and feeds the degenerate state. Unlike atomic gases, the atom-like transition in excitons is dramatically renormalized on macroscopic ground state population. Our results establish fundamental differences between polariton condensation and photon lasing and open possibilities for coherent control of condensates.

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