In this work, near-planar proton-implanted photonic crystal (PhC) VCSELs are fabricated and characterized. The photonic crystal design parameters are b (hole diameter) and a (hole pitch). A cross-sectional sketch and an optical microscope image of one completed device (without fan and pad metal) is shown in Figure 1 (a) and (b), respectively. With lithographically defined implant aperture, the freedom to design the current aperture size can be achieved. Furthermore, the requirement to etch trenches for the purpose of lateral oxidation as well as electrical isolation is eliminated, and this results in a more planar topographical feature as shown in Figure 1 (c). By etching PhC onto proton-implanted VCSELs, stronger index guiding is introduced and consequently, the variation in threshold current and efficiency from device to device and the discontinuity in LI is eliminated (see Figure 2). Note that for the proton-implanted VCSEL shown in Figure 2, the threshold current is reduced after PhC is etched due to the decrease in diffraction loss.
Oxide-confined PhC VCSELs fabricated for the purpose of endlessly single mode studies [1] are also investigated. Various mechanisms that affect the differential quantum efficiency and threshold current of photonic crystal VCSELs are studied. It is found that those mechanisms include spectral and spatial mode-gain overlap, optical loss, and thermal effects due to Joule heating. By etching PhC deeper, the VCSELs have lower optical loss [2] (see Figure 3) but more exacerbated heating as well as lower spatial mode-gain overlap. This is why the differential quantum efficiency does not increase monotonously with etch depth even though the optical loss decreases with etch depth, as evident in Figure 4.
There are three degrees of freedom in designing PhC VCSELs to maximize the laser performance in terms of efficiency and threshold current: epitaxial structure, relatively size of current aperture and transverse optical mode, and photonic crystal etch depth. The epitaxial structure determines the spectral mode-gain overlap, while the relative size of current aperture and optical mode dictates the spatial mode-gain overlap. The PhC etch depth has an impact on all the mechanisms mentioned above. To achieve high efficiency and low threshold current as well as to promote single mode lasing, it is necessary to have the gain peak comes into alignment with the resonance mode around lasing threshold. Depending on the application, the relative size of current aperture and optical mode can be designed to achieve either high efficiency or low series resistance by varying the current aperture diameter. Ultimately, optimized efficiency, threshold current and thermal as well as electrical resistance should lead to high-performance single mode PhC VCSELs.
Figure 1: (a) Cross-sectional sketch and (b) optical microscope image of a completed implant-confined PhC VCSELs (without fan and pad metal). (c) SEM image of an implant-confined PhC VCSEL, taken at 45 degrees from the wafer surface, showing planarity of such device.
Figure 2: LI of proton-implanted VCSELs before (blue) and after (red) PhC (b/a = 0.6 and a = 3.5) etch.
Figure 3: Fundamental mode optical loss of the oxide-confined PhC VCSELs as a function of etch depth normalized to the top DBR thickness, for b/a = 0.7, (a) a = 4.0 and (b) 4.5 um. For each PhC design, optical loss versus etch depth of two different oxide aperture diameters (13 and 18 um) are shown.
Figure 4: Differential quantum efficiency of the oxide-confined PhC VCSELs as a function of etch depth normalized to the top DBR thickness, for b/a = 0.7, (a) a = 4.0 and (b) 4.5 um. For each design, differential quantum efficiency versus etch depth of two different oxide aperture diameters (13 and 18 um) are shown.
References:
[1] A. M. Kasten, M. P. Tan, P. O. Leisher, and K. D. Choquette, “Endlessly single- mode photonic-crystal vertical-cavity surface-emitting lasers,” Proceedings of SPIE, vol. 6908, pp. 69080B1-8, 2008.
[2] D. F. Siriani, M. P. Tan, A. M. Kasten, A. C. Lehman, P. O. Leisher, J. D. Sulkin, J. J. Raftery, Jr., A. J. Danner, A. V. Giannopoulos, and K. D. Choquette, “Mode control in photonic crystal vertical-cavity surface-emitting lasers and coherent arrays (invited),” IEEE Journal of Selected Topics in Quantum Electronics, vol. 15, pp. 909-917, 2009.