Topographically induced mode-coupling: Beyond FIB-milled single open microcavities

Open-access optical microcavities, depicted in figure 1 a), are emerging as an original tool for light-matter studies thanks to their intrinsic tunability and the direct access to the maximum of the electric field along with... [ view full abstract ]

Open-access optical microcavities, depicted in figure 1 a), are emerging as an original tool for light-matter studies thanks to their intrinsic tunability and the direct access to the maximum of the electric field along with their small mode volume. They have been attracting a lot of attention recently, especially in Europe with a total of 10 groups now working with open microcavities configuration producing high-profile Science including QED experiments, strong coupling with quantum wells and 2D materials, micro-lasing, nano-sensing and nano-tweezing.

The fabrication challenge of these devices lies in making the concave mirror substrate as small and smooth as possible while avoiding cracks in the Dielectric Bragg Reflector (DBR). Both, CO2 laser ablation and Focused Ion Beam (FIB) milling have produced low mode volume (≈ λ^3) with high Finesse (> 10^4) single microcavity. However, the FIB milling strategy is the only one providing nanometric topographic control [1] of the photonic potential with a high degree of reproducibility (see figure 1 b) ). This feature opens the possibility to investigate high quality extended structures (> 100 µm x 100 µm) beyond the single microcavity.

I will firstly discuss our optical study of two coupled microcavities for which the coupling strength depends on the barrier height in between the two microcavities [2]. As a second example, I will show that the mirror topography can induce an anti-crossing and an hybridization in between the microcavity localized modes and the surrounding planar modes. Then, I will show our latest effort in fabrication and surface characterization to extend these devices towards super-structures such as photonic squared and hexagonal crystals and photonic circuitry as shown in figure 1 c).

The FIB strategy offer the unique possibility to manipulate the flow of light on-a-chip while maintaining high finesse and low mode volume and therefore, brings open-microcavities into the realm of photonic circuits and superstructures with a wide range of applications in nanophotonics.

[1] A. A. P. Trichet, et al., Optics Express, 23, 13, 17205-17216 (2015)
[2] L. C. Flatten, et al., Laser & Photonics Reviews, 10, n°2, 257-263 (2016)