Fig. 1: A cryogenic, fiber-based technique for strain-dependent spectroscopy.

a A schematic of the fiber interfacing with an hBN/WSe₂/hBN heterostructure. The fiber is mounted on a piezoelectric nanopositioner (not pictured). Inset: Darkfield optical micrograph of a 240 nm radius tapered fiber. b We simulate excitation and collection profiles (for both circularly polarized (σ⁺/ σ⁻) and z-polarized (π) light, for z axis normal to the WSe₂) for a 240 nm radius fiber tip and 700 nm light. c White light transmitted through the device in a cryogenic environment (T = 5 K) and collected by the fiber (depicted schematically in inset) shows four transmittance dips. They correspond to the A and B excitons (X_A:1s, X_B:1s) and their first excited Rydberg states (X_A:2s, X_B:2s). d Fiber-collected PL at 5 K shows four pronounced features originating from (in decreasing energy) the neutral exciton (X⁰), the charged exciton (X⁻), the dark exciton (D⁰), and the dark exciton’s phonon replica (D^R). e As we push the device with the fiber by increasing the piezo-positioner voltage (V_p), we observe a decrease in the energy of the X_A:1s transmittance dip with increased fiber displacement. f Likewise, for the same fiber displacement as in e, we observe decreases in the energy of the X⁰ and X⁻ in PL. Data in c, d come from device D1, while data in e, f come from D2. D1 and D2 vary only by few-nm differences in hBN thicknesses.