Optical properties of nanowire heterojunctions in core‐shell geometries differ significantly from their bulk, thin‐film, counterparts1‐4. Figure 1a shows core‐shell nanowires (CSNW) of~ 100nm diameter core of GaAs, and~ 40nm thick AlGaAs grown on a silicon substrate6, with the inset showing a magnified top view image. Figure 1b (red) is the reflectivity spectrum of these CSs showing 2‐4% of light is reflected in the 700‐1200nm range, while they only occupy~ 15% of the volume compared to thin films of the same height. The figure also shows 3‐7% of light is reflected from GaAs substrate on which CSNWs are grown. Both cased have a signature sharp increase of reflectivity at GaAs bandgap, proving that the reflection is due to CSNWs, not the substrate. By contrast the same heterojunction in thin film has a reflectivity ranging from 30% to 55% to a normally incident light. Thus, normalized to volume, these wires absorb more than two orders of magnitude more light than their thin‐film counterparts. Figure 1c compares room tempera‐ture micro photoluminescence (PL) spectrum of bulk GaAs to CSNWs grown on GaAs, and two cuts of Si6. The ratio of Peak Luminescence of a) CS on GaAs, b) CS on Si [111], and c) Si (miscut) substrates to bulk GaAs are, respectively, 923, 311, and 10. Considering the beamwidth of~ 1μm, 5‐10 NW were excited, yet emitted over three order of magnitude more light compared to bulk. We describe the reasons for such large enhancement from the basic principle of Fermi Golden Rule for calculation of upward (absorption) and downward (emission) transition rates based. We posit that reduced dimensionality effects …