Microstructured void gratings for outcoupling deep-trap guided modes

Breaking the total internal reflection far above a critical angle (i.e., outcoupling deep-trap guided modes) can dramatically improve existing light-emitting devices. Here, we report a deep-trap guided modes outcoupler using densely arranged microstructured hollow cavities. Measurements of the leaky mode dispersions of hollow-cavity gratings accurately quantify the wavelength-dependent outcoupling strength above a critical angle, which is progressively improved over the full visible spectrum by increasing the packing density. Comparing hollow- and filled-cavity gratings, which have identical morphologies except for their inner materials (void vs. solid sapphire), reveals the effectiveness of using the hollow-cavity grating to outcouple deep-trap guided modes, which results from its enhanced transmittance at near-horizontal incidence. Scattering analysis shows that the outcoupling characteristics of a cavity array are dictated by the forward scattering characteristics of their individual cavities, suggesting the importance of a rationally designed single cavity. We believe that a hollow-cavity array tailored for different structures and spectra will lead to a technological breakthrough in any type of light-emitting device.