Photonic crystals, also known as photonic bandgap materials, are the subject of numerous investigations because of their unique characteristics, with prospective applications ranging from gas sensing to optical filters, inkless printing, and reflective displays. Photonic crystals are highly ordered materials with a periodic dielectric constant, whose period is on the order of the wavelength (or wavelength range) of interest. The effect of confining and controlling the propagation of light stems from the photonic bandgap, a band of frequencies in which light propagation in the photonic crystal is forbidden. Through variations in the refractive index and periodicity, it is possible to design one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) photonic crystals.

In order to combat the issues with using flat emitters, researchers turn their attention to three-dimensional microLEDs. Such emitters have the obvious advantage of increased surface area. Further, the increased dimensionality allows for the formation of quantum wells on the sides of the emitter. The increased dimensionality also provides access to various crystal faces. In the case of GaN, it includes the m-plane, that benefits from a reduced native electric field near the surface reducing the influence of the quantum confined stark effect (QCSE), thereby increasing emitter efficiency.

Schematic of microLED GaN/InGaN core-shell pillar cross-section (left) and a top-down microLED pillar array secondary electron image (right).

WARCL pattern at 580 nm.

To determine the photonic behavior of such an array, we employ the wavelength and angle-resolved CL (WARCL) technique. The WARCL pattern shows distinct anisotropy at several wavelengths relevant to the pillar material, dimensions, and periodicity.

This shows the anisotropy in the light emission as a function of wavelength, which is directly useful in potential display technology. However, the wavelength-angle bases are not ideal for the analysis of photonic modes; for this one, they may transform from wavelength-angle to energy-momentum bases.

Energy-momentum dispersion along 110 degrees from the +x axis.

In this way, CL is useful to probe the local density of optical states (LDOS) in the photonic specimen, which is of paramount importance in the design of nanophotonic structures.

Determining photonic band structure by energy-momentum spectroscopy in an electron microscope