**3.2 Threshold characteristics of fundamental mode of PhC lasers**

In the optical pumped experiments of PCSEL devices, the lasing action was clearly observed in several devices with different lasing wavelength ranging from 395 nm to 425 nm. Fig. 7 shows the output emission intensity versed the pumping energy density with the PhC lattice constant of about 254nm. In the figure, the clear threshold pumping energy shows at the threshold pumping energy density of 2.8 mJ/cm2, and a peak power density of 5.6 MW/cm2. When the laser pumping energy exceeds the threshold energy, the laser output intensity increases abruptly and linearly with the pumping energy. Fig. 8 shows the excitation energy dependent emission spectrums of 0.8 Eth, 1 Eth, 1.2 Eth, and 1.3 Eth. These spectrums clearly show the transition behavior from spontaneous emission to stimulated emission. Furthermore, above the threshold, only one dominant peak wavelength of 419.7 nm with a linewidth of 0.19 nm can be observed.
