“…The apparently abrupt cut-off of the SHG spectrum from of the even parity states has important implications for the utility of SHG and the even parity states in quantum optics applications. The factors governing the visibility of high-n Rydberg excitons have been extensively studied in absorption experiments on the nP series [18,20,32,58], and include Rydberg blockade [1], thermally or optically excited free carriers [20,59], charged defects [32,60,61], and local strain and disorder [18,20]. Therefore to gain further insight we carried out further experiments on the power dependence and spatial variation of the SHG spectrum.…”