Integrating Sphere Fourier Microscopy of Highly Directional Emission

Abstract

Accurately controlling light emission using nano-and microstructured lenses and antennas is an active field of research. Dielectrics are especially attractive lens materials due to their low optical losses over a broad bandwidth. In this work we measure highly directional light emission from patterned quantum dots (QDs) aligned underneath all-dielectric nanostructured microlenses. The lenses are designed with an evolutionary algorithm and have a theoretical directivity of 160. The fabricated structures demonstrate an experimental full directivity of 61 ± 3, three times higher than what has been estimated before, with a beaming half-angle of 2.6°. This high value compared to previous works is achieved via three mechanisms. First, direct electron beam patterning of QD emitters and alignment markers allowed for more localized emission and better emitter−lens alignment. Second, the lens fabrication was refined to minimize distortions between the designed shape and the final structure. Finally, a new measurement technique was developed that combines integrating sphere microscopy with Fourier microscopy. This enables complete directivity measurements, contrary to other reported values, which are typically only partial directivities or estimates of the full directivity that rely partly on simulations. The experimentally measured values of the complete directivity were higher than predicted by combining simulations with partial directivity measurements. High directivity was obtained from three different materials (cadmium-selenide-based QDs and two lead halide perovskite materials), emitting at 520, 620, and 700 nm, by scaling the lens size according to the emission wavelength.

Perovskite Plasticity: Exploiting Instability for Self‐Optimized Performance

“…Thanks to the self-optimizing processes, a well-trained system performs better than what could be achieved in our previous work, where we used nanolithography to spatially confine and align emitters with lenses. [33] Fabrication and alignment errors limited the directivity to 12.9, while the self-optimized system results in directivity as high as 16.4. We have demonstrated how we can train a system of mixed halide perovskite with a microlens to self-optimize and retain the optimized configuration for highly directional emission.…”

“…[35] The directivity measurements were done using a Fourier microscope as described before. [33] This setup has an objective with a relatively low NA of 0.42, which allows for sufficiently intense excitation with a small spot size, while limiting angular spread in the excitation beam to mimic a spatially confined plane-wave source. Figure 3a shows the typical dynamics of the emission intensity and directivity as a function of time, when a lens is illuminated for the first time.…”

“…The microlenses used in this study were reported to provide highly directional emission. [ 33 ] The size of these lenses can be optimized to obtain maximal directivity at a desired emission wavelength, which in this case is the wavelength of the iodide‐rich regions. This lens design is particularly suitable because it is optimized based on the same principle of reciprocity that we use here for self‐alignment.…”

“…Thanks to the self‐optimizing processes, a well‐trained system performs better than what could be achieved in our previous work, where we used nanolithography to spatially confine and align emitters with lenses. [ 33 ] Fabrication and alignment errors limited the directivity to 12.9, while the self‐optimized system results in directivity as high as 16.4.…”

“…Encapsulation with AlO x and SU‐8 photoresist and lens fabrication was done as described in. [ 33 ] A 1:5 diluted SU‐8/cyclopentanone was used to obtain a layer of 250 nm to avoid laser damage to perovskite in the lithography process The lenses are fabricated as described before, [ 34 ] with two‐photon lithography.…”