Fig. 1

Band structure engineering by biaxial strain. a Schematic illustration of the experiment setup used for applying in-plane biaxial strain by heating or cooling the PP substrate. b A typical IR extinction spectrum (1 − T/T₀) for a 6L BP on PP substrate under zero strain, with the incident light polarized along the AC direction. Top inset is a schematic illustration for optical transitions between different subbands. Bottom inset is an optical image of this 6L BP sample. Scale bar: 20 μm. c IR extinction spectra (1 − T/T₀) for the 6L BP under varying biaxial tensile (>0) and compressive (<0) strains. For clarity, the spectra are vertically offset. Dashed lines are guides to the eye. d The E₁₁ and E₂₂ peak energies as a function of biaxial strain. The solid lines are linear fits to the data, which give shift rates for E₁₁ and E₂₂ of 222 and 167 meV/%, respectively. The inset shows (E₂₂ − E₁₁) as a function of biaxial strain and the solid line is a linear fit to the data