Systematic characterization of gallium nitride photonic crystal surface-emitting lasers

In recent decades, photonic crystals (i.e., two dielectric materials arranged in a long-range periodic structure) have been widely applied to the development of optoelectronic devices. Photonic crystal surface-emitting lasers (PCSELs) are relatively new devices that are thought to be better than traditional vertical cavity surface-emitting lasers for future light-emitting applications.¹ This is because PCSELs can lase with a small divergence angle in single-mode operation and potentially with high output powers.² We previously reported that the threshold power, emission angles, and divergence angles of gallium nitride (GaN)-based PCSELs show an obvious variation when their lasing action occurs in different band-edge modes.³ Furthermore, the specific photonic crystal lattice type that is selected can restrict the formation of photonic band structures and can play an important role in realizing photonic crystal lasers with low thresholds and favorable characteristics. In the past, however, there has been no systematic analysis of threshold conditions and lasing characteristics for PCSELs with different lattices.

The advances in the application of photonic crystals have largely been possible because of the particulars of photonic band structures, which include the ability to tune light propagation and to inhibit spontaneous emission.⁴ Light at the photonic band edge satisfies specific Bragg diffraction conditions and can therefore strongly interact with the gain medium.⁵ Further generation of multidirectionally distributed feedback in the in-plane thus results, and stimulated resonant and vertical emission can be achieved simultaneously. This is therefore the origin of the term ‘photonic crystal surface-emitting lasers.’

In this work, we have fabricated and characterized GaN-based PCSELs that have three different common lattice structures (honeycomb, hexagonal, square). A schematic drawing for one of our GaN-based PCSELs is shown in Figure 1(a). Our laser devices consist of 25 pairs of aluminum nitride/GaN distributed Bragg reflectors and an active p-i-n junction region that has 10 pairs of indium gallium nitride multiple quantum wells. The p-i-n junction region has a thickness equal to five lasing wavelengths and is surrounded by a 320nm-thick n-type GaN layer, as well as 100nm-thick p-type GaN layers.

Figure 1. (a) Schematic diagram of a gallium nitride (GaN)-based photonic crystal surface-emitting laser (PCSEL). Scanning electron microscope images of (b) honeycomb, (c) hexagonal, and (d) square lattice structures on the GaN surface are also shown. In addition, the corresponding field patterns for the (e) honeycomb, (f) hexagonal, and (g) square lattices are shown on the right. PC: Photonic crystal. DBR: Distributed Bragg reflector. AlN: Aluminum nitride. InGaN: Indium gallium nitride. MQW: Multiple quantum well.

To investigate the threshold conditions of our PCSELs with honeycomb, hexagonal, and square lattices, we designed each specific lattice constant so that we could obtain the approximate lasing wavelength for each device. Our calculated lattice constants were 100, 176, and 150nm for the honeycomb, hexagonal, and square lattices, respectively. The corresponding scanning electron microscope images for each of the lattice structures are shown in Figure 1(b–d). The simulated field patterns are also shown in Figure 1(e–g).

We have been able to successfully characterize the optical properties of our fabricated devices when they were operated above the threshold condition. The measured output power intensities for our PCSELs are shown in Figure 2(a) as a function of pumping power. We measured a relatively low threshold power density of about 1.6mJ/cm² for the honeycomb lattice at room temperature. This low measurement is due to the flat photonic band causing the larger photonic density of states to enhance the strength of the light—matter interaction. We also observed a lasing wavelength of 393.8nm for this device. The threshold conditions for the other two lattice types were 2.3mJ/cm² (hexagonal) and 3.8mJ/cm² (square). In addition, we calculated—with the multiple scattering method—the threshold gains for our three different PCSEL device types.⁶ We find that our modeled threshold gains are consistent with our experimental results.

Figure 2. The output intensity of the PCSELs with honeycomb, hexagonal, and square lattice structures, as a function of (a) pumping power and (b) far-field divergence angle. a.u.: Arbitrary units.

In our work we have also analyzed additional characteristics of the lasers in our devices. For instance, we have characterized the divergence angle—see Figure 2(b)—and the degree of polarization (DOP) of the lasers in each of the different lattice devices. The laser device with the honeycomb lattice exhibits a lower threshold, higher DOP (86%), and lower divergence angle (1.3°) than the other two lattice devices. As such, we have clearly demonstrated that the application of a honeycomb lattice as the photonic crystal pattern is the best option.

We have fabricated and systematically characterized GaN-based PCSELs with different lattice structures. In addition, we have explicitly demonstrated that GaN-based PCSELs with a honeycomb lattice exhibit the lowest threshold at room temperature conditions. We have also compared the divergence angle and degree of polarization between our devices that have different lattices. Moreover, we used the multiple scattering method as an alternative (i.e., modeling) approach, and we were able to predict our experimental results.⁶ We are now planning to reduce the threshold condition of our devices further. We will achieve this by using novel photonic quasicrystals with spatial arrangements that are between conventional photonic crystals and random scattering patterns. With such arrangements used as cavities, we could potentially develop additional ways to exhibit micro- and nanoresonators, with even higher quality factors.⁷

The authors acknowledge the helpful contributions of S. W. Chen, T. T. Wu, and S. C. Wang to this study. This work was supported by the National Science Council of Taiwan under contract NSC99-2622-E009-009-CC3.