  To translate electrical into optical signals one uses the modulation of
either the refractive index or the absorbance of a material by an electric
field. Contemporary electroabsorption modulators (EAMs) employ the quantum
confined Stark effect (QCSE), the field-induced red-shift and broadening of the
strong excitonic absorption resonances characteristic of low-dimensional
semiconductor structures. Here we show an unprecedentedly strong transverse
electroabsorption (EA) signal in a monolayer of the two-dimensional
semiconductor MoS2. The EA spectrum is dominated by an apparent linewidth
broadening of around 15% at a modulated voltage of only Vpp = 0.5 V. Contrary
to the conventional QCSE, the signal increases linearly with the applied field
strength and arises from a linear variation of the distance between the
strongly overlapping exciton and trion resonances. The achievable modulation
depths exceeding 0.1 dBnm-1 bear the scope for extremely compact, ultrafast,
energy-efficient EAMs for integrated photonics, including on-chip optical
communication.
