Consider a stack of three wafers at room temperature. The top one is GaAs(E
gl=1.42eV), the middle one is Si(E g 2=1.12eV), and the bottom one is Ge(E g 3=0.66eV), as sketched below. To this stack, we shine three lasers with wavelengths λ 1 =0.85μm,λ 2=1.3μm, and λ 3 =1.55μm, once from above and a second time from below. For each illuminating condition, in which wafer is each laser beam absorbed? Explain.