Figure 1.

(a) Optical transmittance (T) of the [L1/S2], [L1/S4] and [L1/S6] superlattices (60 nm in total thickness) on STO substrates as a function of photon energy (ω). Optical spectra for an undoped bare STO substrate and an oxygen-vacancy-doped STO film (100 nm thick) on STO are also shown for comparison. Superlattice samples clearly show the Drude absorption and no noticeable absorption due to in-gap states. Inset: Photographs of a bare STO substrate, [L1/S4], and oxygen-vacancy-doped STO film. The oxygen-vacancy-doped STO absorbs a significant portion of visible light with clear evidence of in-gap defect states, as marked with arrows. (b) Extinction coefficient (k) and refractive index (n), and (c) optical conductivity (σ₁) of the [L1/S2], [L1/S4] and [L1/S6] superlattices obtained from a Drude fit to the experimental transmittance data. The net carrier density and mobility for each sample are listed for convenience. For all three samples the carrier density is within experimental error bars of 0.5 e⁻/interface¹³,¹⁴.