Two‐Photon Photoconductivity Measurements in a Hexagonal ZnO Crystal

Two‐photon‐photoconductivity (TPPC) measurements in ZnO, using two different dye lasers as sources, are reported. Photocurrents produced by each laser beam separately were determined and subtracted. Linear dependencies of TPPC as a function of the intensity of one source while the other source intensity was kept constant, were found forvarious source frequencies. The spectral measurements of TPPC as a function of the frequency of one source while the other source frequency is constant show a square root dependent rise of TPPC at the crystal energy gap, at 3.38 eV, and a second very strong and approximately linear rise at 4.25 eV. This second rise is due to a forbidden one‐photonbut allowed two‐photon transition from a deeper valence band to the first conduction band in the ZnO crystal, and was also seen in ZnO crystals. A theoretical calculation, predicting qualitatively the behaviours of TPPC as a function of the sum of the energy ofthe two photons, is also given.