Electro-optic tuning of vacuum Rabi coupling is demonstrated on a very
high quality semiconductor quantum microcavity structure with reflect
ivity linewidths at resonance of 0.9 and 1.1 meV. The variation in ene
rgy of the reflectivity peaks as a function of electric field is fitte
d very well by a transfer matrix reflectivity (TMR) model, using an ex
perimental determination of exciton oscillator strength, and hence exc
iton-photon coupling strength, with electric field. In high sensitivit
y photo-current experiments we obtain clear evidence for coupling to d
iscrete excited and continuum exciton states.