carried out using a reference laser beam reflected on a metal film deposited on the sample surface. Knowing the reflectance of the metal, it is possible to estimate the absolute reflected flux of photons. The ratio between the photon flux collected by the microscope objective in front of the sample and the CCD count rate gives the required calibration coefficient.

The number of photons generated via the recombination of IXs was determined from the measured CCD count rate by taking into account the collection angle (which depends on the refractive index of the sample layers and on the numerical aperture of the objective lens) and the transmission of the semi-transparent metal electrodes. If the PL signal comes from the recombination of one type of carriers transported by the SAW with carriers of opposite polarity electrically injected from the electric contacts, we expect the contact current to be given by $I_c=e N_\mathrm{ph}$, where $N_\mathrm{ph}$ is the detected photon flux and $e$ the electron charge. If the measured current is much less than $I_c$, then at most of the electrons or holes forming the IXs must have been transported along the DQW structure by the SAW. For the experimental conditions of Fig. 