IMREF

For each photon energy hv, the PC response or gain [G.sub.B] accumulates effects due to all of the occupied SEs from the uppermost SE close to the Fermi level [E.sub.F] in Figure 1(a) (properly speaking, close to the surface imref [E.sub.Ps] of Figure 1(b)) to the lowest SE lying hv eV below the CB that photons are able to empty.

They a real so empty in the PC system because the weak light power handled in PC experiments is likely unable to induce a noticeable "pumping action" over the thermal activity, which makes the imref [E.sub.Ps] a kind of sharp borderline that separates the occupied SEs below [E.sub.Ps] from the empty SEs above [E.sub.Ps]; however, this will change under the strong illumination of the [P.sub.L] laser, as will be discussed.

What matters is to realise that we have Np and NH in close proximity at the surface, and the next point to consider is the effect of the optical emission of the [P.sub.L] laser emptying SEs lying below the imref [E.sub.Ps] in Figure 1(b).This has to do with the "pumping action" mentioned previously in this section.