AbstractSecond‐harmonic generation and two‐photon induced photoluminescence from arrays of ZnO nanowires and single nanowires under excitation with femtosecond pulses is experimentally studied. The ratio of relevant components of the nonlinear χ(2) tensor is obtained. The excitation of the nanowire array with femtosecond laser pulses induces significant heating of the free‐standing nanowires. The heat distribution is analyzed in phenomenological finite‐element studies. Two‐photon excitation of a broad internal photoluminescence in single ZnO nanowires is used to perform a single‐nanowire transmission experiment. From the transmitted intensity, the band‐gap shift in the excited nanowire is obtained. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)