We demonstrate the first electrically injected GaN-based vertical-cavity surface-emitting lasers (VCSELs) with a TiO₂ high-index-contrast grating (HCG) as the top mirror. Replacing the top distributed Bragg reflector (DBR) with an HCG offers substantial thickness reduction, polarization-pinning, and setting of the resonance wavelength by the grating parameters. Conventional HCGs are usually suspended in the low refractive index material, such as air, in order to create the largest refractive index contrast. However, the mechanical stability of such structures can be questioned and creating free-hanging GaN-membrane on top of GaN is problematic. We have therefore fabricated TiO₂–HCGs resting directly on GaN without an air-gap. No DBR layers are used below the HCG to boost the reflectivity. A VCSEL with an aperture diameter of 10 μm shows a threshold current of 25 mA under pulsed operation at room temperature. The lasing modes locate around 400 nm and are transversely electrically -polarized with a line width of 0.5 nm. The full-width half-maximum beam divergence is 10°. This demonstration of a TiO₂–HCG VCSEL offers a new route to achieve polarization pinning and could also allow additional benefits such as postgrowth setting of the resonance wavelength.

中文翻译：

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带TiO ₂高折射率光栅反射器的电注入GaN基垂直腔表面发射激光器

我们展示了第一批以TiO ₂高折射率对比光栅（HCG）为顶部镜的电注入GaN基垂直腔面发射激光器（VCSEL）。用HCG代替顶部分布的布拉格反射器（DBR）可以大大降低厚度，极化钉扎以及通过光栅参数设置共振波长。常规的HCG通常悬浮在低折射率材料中，例如空气中，以产生最大的折射率对比。但是，这种结构的机械稳定性可能会受到质疑，在GaN顶部形成自由悬挂的GaN膜是有问题的。因此，我们制造了TiO ₂–HCG直接位于GaN上而没有气隙。HCG下方未使用任何DBR层来提高反射率。孔径为10μm的VCSEL在室温下以脉冲操作显示的阈值电流为25 mA。激光模式位于约400nm处，并且以0.5nm的线宽被横向电极化。全宽半最大光束发散角为10°。TiO ₂ -HCG VCSEL的演示提供了实现极化固定的新途径，并且还可以带来其他好处，例如共振波长的后生长设置。