Proceedings - Viện Vật lý

_________________________________________________________________________________Những tiến bộ trong Quang học, Quang tử, Quang phổ và Ứng dụng. 8/2006, Cần Thơ, Việt NamFig.2: X-ray spectrum of the ZnO sample100000008000000Intensity515Intensity8007003876000000600500576,8400000040030050020000000375,52001000350 400 450 500 550 600 650Wavelength(nm)(a)350 400 450 500 550 600 650Wavelength( nm)Fig.3: The fluorescence spectra of ZnO sample excited by lamp (a) and by N 2 laser (b)The much stronger 375-430nm red-broaden emission band excited by N 2 lasersuggests a stimulated emission with excitonic origin occurred .The SE peak wasattributed to exciton-exciton scattering and lies below the free-exciton energy by anexciton binding energy plus the mean kinetic energy 3/2.k B T where k B T is thethermal energy [6]. The observed red -broaden band (375-430nmnm) may beexplained by the fact that the inelastic exciton-exciton scattering leaves in anexcited state which in turn reduces the emission energy of the recombining exciton[1,12]In order to separate spontaneous emission from stimulated one at higherexcitation intensities we observed the stimulated emission at the moment delayedabout 250ns from the first fluorescence emission. The spectra of discrete lasingmodes were obtained above a determined threshold (Fig 4).(b)300