**6. Conclusion**

18 Photonic Crystals – Innovative Systems, Lasers and Waveguides

degree derived in the previous section. The reason that only one emission angle was obtained could be due to the fact that we only measured the AR μ-PL diagram along one Γ-

Fig. 21. The measured AR μ-PL diagram near the M3 mode ((a) pumped by YVO4 pulse laser; (b) pumped by He-Cd laser). The dash lines represent the calculated photonic band diagrams.

Each of PhC band-edge modes exhibited specific emission angle by different type of wave coupling mechanism in Fig. 19(a), Fig. 20(a), and Fig. 21(a). Finally, Fig. 22 shows the divergence angles of Γ1, K2, and M3 band-edge modes on the normal plane from the sample surface. The lasing emission angles are about 0, 29, and 59.5, and the divergence angles of laser beams are about 1.2, 2.5, and 2.2 for (Γ1, K2, and M3) band edge modes, respectively. Due to the alignment difficulties in the AR μ-PL system, the measured emission angles

might have some offset values of about 1 to 2.

the normal plane from the sample surface.

**1 K2 M3**

Fig. 22. The emission angles and divergence angles of Γ1, K2, and M3 band-edge modes on

M direction.

In the chapter, the optical pumped of GaN-based 2-D photonic crystal surface emitting lasers (PCSELs) with AlN/GaN distributed Bragg reflectors (DBR) are fabricated and measured. The laser has a 29-pair bottom DBR which plays the role of a low refractive index layer to enhance the coupling between photonic crystal (PhC) nanostructure and electrical field in the whole cavity. Therefore, the lasing action can be achieved in the optical pumping system. Each of these laser devices emits only one dominant wavelength between 395 nm and 425 nm.That normalized frequency of PhC lasing wavelength can be well matched to these three band-edge frequencies (Γ1, K2, M3) indicated that the lasing action can only occur at specific band-edges. In the angular-resolved μ-PL (AR μ-PL) system, the diffracted lines in the AR μ-PL diagrams of PCSEL devices can be further matched to the calculated 2- D TE-like photonic band diagram calculated by PWEM. These three band-edge frequencies (Γ1, K2, M3) have different emission angles in the normal direction of about 0, 29, and 59.5 and are further confirmed by the Bragg theory. The divergence angles of the (Γ1, K2, M3) modes are about 1.2, 2.5, and 2.2. Moreover, according to multiple scattering method (MSM), the resonant mode frequencies will approach to band edge frequencies compared with plane wave expansion method (PWEM). In addition, the threshold gain of four resonant modes varies with the filling factor. This result shows that the proper hole filling factor can control the PhC mode selection. Finally, all of these calculation and experiment results indicate that GaNbased PCSELs could be a highly potential optoelectronic device for lasers in the next generation.
