Fig. 3.

Photoluminescence (PL) spectra with 1-photon (1P) and 2-photon (2P) excitation. (top) Normalized PL spectra for 2P excitation (λ _exc = 800 nm) where the focal depth is varied from −3 (above the surface) to 25.5 µm. (bottom) Normalized PL spectra for 1P excitation (λ _exc = 488 nm) where the focal depth is varied from −3 (above the surface) to 10.5 µm