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×{}10¹¹ cm^-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.