Excitons in atomically-thin semiconductors necessarily lie close to asurface, and therefore their properties are expected to be strongly influencedby the surrounding dielectric environment. However, systematic studiesexploring this role are challenging, in part because the most readilyaccessible exciton parameter -- the exciton's optical transition energy -- islargely \textit{un}affected by the surrounding medium. Here we show that therole of the dielectric environment is revealed through its systematic influenceon the \textit{size} of the exciton, which can be directly measured via thediamagnetic shift of the exciton transition in high magnetic fields. Usingexfoliated WSe$_2$ monolayers affixed to single-mode optical fibers, we tunethe surrounding dielectric environment by encapsulating the flakes withdifferent materials, and perform polarized low-temperature magneto-absorptionstudies to 65~T. The systematic increase of the exciton's size with dielectricscreening, and concurrent reduction in binding energy (also inferred from thesemeasurements), is quantitatively compared with leading theoretical models.These results demonstrate how exciton properties can be tuned in future 2Doptoelectronic devices.
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