Third-order harmonic generation by self-guided femtosecond pulses in air

Different-sized CuO nanocrystals were prepared by precipitation-pyrolysis method, with sizes of 7 nm, 10 nm, and 30 nm. The absorption spectra of CuO nanocrystals showed clear bandshift as an evidence of the quantum size effect. On a measurement device with a diamond anvil cell, the pressure-resistance relations of different-sized CuO nanocrystals under pressure up to 20 GPa at room temperature were obtained. The experimental results showed varying pressure-dependent resistance of CuO nanocrystals of different sizes. There was an interesting phenomenon occurring for the smallest CuO nanocrystal (7 nm), that is, a significant resistance fluctuation of large amplitude under pressure appears with a frequency-field dependence. This fluctuation clearly originates from surface charge oscillations on the surface of many nanocrystals with the incident field due to the size effects.