Proceedings - Viện Vật lý

Advances _________________________________________________________________________________in Optics, Photonics, Spectroscopy and Applications. Aug. 2006, Cantho, Vietnam120 Intensity1008060200150100Intensity375380394385,4405,45399,3419,6426,4434,154020501801601401201008060400360 370 380 390 400 410 420 430 440Intensity380,3375,3Wavelength( nm)0360 370 380 390 400 410 420 430 440Wavelength( nm)(a)(b)Fig. 4: The spectra of discrete lasing modes from the ZnO sample.a- Excitation density of 900 KW/cm 2 and 1400KW/cm 2 .b- Excitation density of 900 KW/cm 2 and 1800KW/cm 2 .388,76406,2399,6394,2414,4409,7419,8426,9434,420015010050Intensity375380399,3394405,45419,6426,4434,1520360 370 380 390 400 410 420 430 440Wavelength (nm)(a)(b)Fig.5: Laser emission spectra with different observation angles. Angle between theobservation direction and the normal of sample surface is 0 o (a) and 45 o (b).The incident angle of the pumped beam is 45 .At excitation densities above the threshold (900KW/cm 2 ), many narrow peakswere observed in the emission spectrum. The line width of these speaks is about1nm. The wavelength spacing between two peaks estimates about 6-7nm.According to laser resonator theory, mode spacing for a cavity of length d is givenby: 20dnd(n )d2 00360 370 380 390 400 410 420 430 440Wavelength (nm)where n is the index of refraction at 0 and dn/d is index dispersion. Using thisequation we could calculate the cavity length (d~6m). This evidence oflongitudinal mode shows the lasing action in our experiments.In order to evaluate the average particle size we took the scanning electronmicroscope (SEM) image of our ZnO sanples. The average size of ZnO particle isabout 300nm - 400nm inside these ZnO pellets. With the average particle size, the301