Absorption spectroscopy of the continuous transition from low to high electron density in a single modulation-doped InGaAs quantum well

We perform differential absorption spectroscopy of a single modulation-doped InGaAs quantum well as its electron density is varied continuously from full (N≊8×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$) to empty (N≊0) by a gate electrode, allowing direct measurement of the density-dependent changes of transition energies and strengths. We observe a progressive quenching of the optical transition at the first subband and red shifts and broadening at higher subbands. We show that the dominant mechanisms are phase-space filling and the quantum-confined Stark effect. This technique also allows direct in situ measurements of the density and temperature of the 2D electron gas in such field-effect–transistor structures.