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Transitions metal dichalcogenides (TMDs) are direct semiconductors in theatomic monolayer (ML) limit with fascinating optical and spin-valleyproperties. The strong optical absorption of up to 20 % for a single ML isgoverned by excitons, electron-hole pairs bound by Coulomb attraction. Excitedexciton states in MoSe$_2$ and MoTe$_2$ monolayers have so far been elusive dueto their low oscillator strength and strong inhomogeneous broadening. Here weshow that encapsulation in hexagonal boron nitride results in emission linewidth of the A:1$s$ exciton below 1.5 meV and 3 meV in our MoSe$_2$ andMoTe$_2$ monolayer samples, respectively. This allows us to investigate theexcited exciton states by photoluminescence upconversion spectroscopy for bothmonolayer materials. The excitation laser is tuned into resonance with theA:1$s$ transition and we observe emission of excited exciton states up to 200meV above the laser energy. We demonstrate bias control of the efficiency ofthis non-linear optical process. At the origin of upconversion our modelcalculations suggest an exciton-exciton (Auger) scattering mechanism specificto TMD MLs involving an excited conduction band thus generating high energyexcitons with small wave-vectors. The optical transitions are furtherinvestigated by white light reflectivity, photoluminescence excitation andresonant Raman scattering confirming their origin as excited excitonic statesin monolayer thin semiconductors.

Exciton states in monolayer MoSe2 and MoTe2 probed by upconversion spectroscopy