Optical design of InGaN/GaN nanoLED arrays on a chip: towards highly resolved illumination

The group at University of Rome Tor Vergata has studied in detail optical crosstalk between neighboring pixels of 3D structured nanoLED arrays and its dependence on LED size and pitch.

In order to quantify crosstalk, the light spot of the emission from a single pixel at the object plane has been looked at. It has been found that LED size and the pitch between individual pixels has strong and non-monotonous influence on crosstalk, and thus on the resolution capability of the nanoLED light source. Specifically, an LED size on the order of 100 nm and pitch around 300 nm appears to represent the minimum design parameters which allow for negligible optical crosstalk for the studied device geometry.

For smaller dimensions, light guiding is lost, and the light spot covers multiple pixels, as shown in the middle row of the figure. This has immediate consequences on the capability to resolve objects put in the object plane of the array. Here, we used two gold spheres at a distance of twice the pitch as a test setup.

In fact, the larger LED array with 300 nm pitch can easily resolve the two spheres, while the dense array leads to strong smearing of the signal detected from each individual LED. As a conclusion, very dense arrays would require a change in device structure in order to reduce optical crosstalk.

Reference: https://doi.org/10.1088/1361-6528/abcd60

Figure: Light spot of a single LED on the object plane (top row) of LED arrays with different pitch. The optical field when illuminating gold spheres distant twice the pitch has been calculated (middle row), switching on individual pixels. The light intensity  emitted towards the detector for each pixel is shown in the bottom row.