A blue-violet photonic-crystal surface-emitting laser

Recently, there has been growing interest in photonic-crystal surface-emitting lasers (PC-SELs).^1,2,3,4 These lasers produce a high-quality beam over a large area (for example, devices >300μm in diameter),^1,2 and the polarization³ and shape of the beam⁴ can be controlled by appropriate design. Short-wavelength lasers have a wide range of potential applications. Super-high-resolution laser sources focused to a spot smaller than blue-violet wavelengths could be made available by the use of doughnut beams,^4,5 leading to optical tweezers or microscopes for ultrafine manipulation and observation. Furthermore, blue-violet PC-SELs would be suitable for very small scale biological or medical uses. However, it has proven difficult to develop a 2D surface-emitting laser with a sufficiently large band-edge effect to achieve such short wavelengths. Indeed, the shortest lasing wavelength achieved so far has been 980nm. We used a novel method to construct the 2D photonic crystal of a current-driven gallium nitride (GaN) PC-SEL to achieve lasing at blue-violet wavelengths (406.5nm).⁶

Figure 1 shows schematically the structure of this PC-SEL. Crucial to our success was fabricating a high-quality 2D GaN/air periodic structure with a lattice constant of 100–200nm, close to an active layer with optical gain. We achieved this by developing a new method that we call “air holes retained over growth’ (AROG), based on the particular characteristics of GaN growth, namely, that it proceeds much more rapidly in the lateral direction than it does vertically from the (0001) crystal plane.⁷

Figure 1. Schematic structure of the GaN PC-SEL created in this work. AlGaN: Aluminum gallium nitride. MQW: Multi-quantum well. PC: Photonic crystal.

Figure 2. (a) SEM image of the triangular lattice of air holes with a period of 186nm, a diameter of 85nm, and a depth of 100nm, formed in an epitaxial GaN/AlGaN layer above a GaN substrate. (b) Cross-sectional SEM image of the PC-SEL, showing a well-defined GaN/air periodic structure inside the GaN epitaxial layer. AROG: Air holes retained after growth. SiO₂: Silicon dioxide.

Establishing the AROG method allowed the formation of a 2D GaN/air PC structure near the active layer in the GaN system. Initially, a triangular lattice of air holes with a period of 186nm, a diameter of 85nm, and a depth of 100nm was formed in an epitaxial GaN/aluminum gallium nitride (AlGaN) layer situated above a GaN substrate as shown in Figure 2(a). A very thin (∼10nm) silicon dioxide (SiO₂) layer was then deposited at the bottom of each air hole. The GaN and successive layers, including indium gallium nitride (InGaN) multi-quantum wells (MQWs), were then overgrown by a metal-organic vapor-phase epitaxial (MOVPE) method. Figure 2(b) shows a cross-sectional scanning-electron microscope (SEM) image of the fabricated device. The periodic arrangement of air holes was well defined inside the GaN epitaxial layer. The GaN overgrowth appeared to proceed laterally, capping the top of the air holes, while the SiO₂ deposited at the bottom of the holes exhibited a growth-blocking action. The structures of the air holes were essentially uniform, and had not been degraded by the overgrowth process. Transmission-electron microscopy confirmed that no dislocations were generated at the overgrowth interface.

Figure 3. (a) Measured 2D photonic band structure of the GaN/air periodic structure constructed using the AROG method. The inset shows the spectrum observed at the Γ point, where four peaks (I–IV) are resolved. (b) Band structure calculated by the plane wave-expansion method.

To investigate whether the periodic structure thus constructed possessed the characteristics of a 2D PC, the photonic band structure of the device was measured. Figure 3(a) shows the results of the measurements, where Γ-X and Γ-J are two representative directions of the triangular 2D PC. Figure 3(b) shows the calculated band structure of the device, which was obtained by the plane wave-expansion method. The two figures seem to be in good agreement, especially for the central mode splitting (Γ point). As the in-plane k vector was increased, four peaks—denoted by I, II, III, and IV, respectively, in the inset of Figure 2(a)—could be distinguished for the normal direction (Γ point). Thus, four dispersion bands, including degenerate (or closely placed) bands, were constructed. These results indicate that the 2D GaN/air periodic structure formed in the device has the characteristics of a 2D PC.

We injected current into the device under pulsed conditions (a pulse width of 500ns and a repetition rate of 1kHz) and found that the device successfully oscillated at room temperature, where the lasing wavelength was 406.5nm and the threshold current was 6.6A. Figure 4(a) shows the top view of the device before current injection (left panel) and the near-field pattern after lasing oscillation (right panel), where the central square is the electrode (100×100μm). As can be seen in the figure, a large-area (>100×100μm) blue-violet lasing oscillation has been successfully achieved. Figure 4(b) shows the corresponding far-field pattern, where a very unique doughnut beam pattern is apparent and the divergence angle of the output beam is less than 1 degree. These results clearly reflect the unique nature of PC-SELs. The beam pattern can be controlled by engineering the photonic crystal structure⁴ as described above.

Figure 4. (a) Top view of the device before current injection (left panel) and the near-field pattern after lasing oscillation (right panel), where the central square is the electrode (100×100μm). (b) Corresponding far-field pattern, where a unique doughnut beam pattern has been observed.

Developing a current-driven GaN blue-violet PC-SEL is an important step toward realizing novel light sources for a variety of new scientific and engineering applications at blue-to-ultraviolet wavelengths. We hope to improve performance significantly in the future by modifying the growth conditions to improve the crystalline quality of the active layer; optimizing the distance between the active layer and the photonic crystal, which would increase the band-edge effect and reduce the threshold current; and using a transparent electrode to avoid blocking the surface emission.