Topographic control of open-access microcavities at the nanometer scale

Abstract

Open-access optical microcavities 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. In this article, we present recent developments in the fabrication of such devices demonstrating topographic control of the micromirrors at the nanometer scale as well as a high degree of reproducibility. Our method takes into account the template shape as well as the effect of the dielectric mirror growth. In addition, we present the optical characterization of these microcavities with effective radii of curvature down to 4.3 µm and mode volume of 16×(λ/2)(3). This work opens the possibility to fully engineer the photonic potential depending on the required properties.

Fabrication of mirror templates in silica with micron-sized radii of curvature

“…Only a few fabrication methods are known to produce concave microoptics with radii of curvature R 5 lm, e.g., focussed ion beam (FIB) milling, 8,9 femtosecond laser wet etching 10 and proximity-effect-assisted reflow techniques. 11 For an optical cavity with total round-trip losses L tot and ultra-high finesse F % p=L tot % 10 5 , acceptable round-trip scattering losses are below S % ð4pr=kÞ 2 ¼ 30 ppm, setting the upper limit for the surface roughness to a demanding r ¼ 2…6 Å in the VIS-NIR spectral range.…”

“…For FIB milling, the microoptics fabrication benefits from full shape control and low surface roughness and indeed depressions with curvatures as low as R ¼ 1.5 lm have been reported recently. 8 Yet, FIB represents a production method at significant investment costs.…”

Spectral engineering of coupled open‐access microcavities

“…Open cavities bring the additional advantage over micropillar structures that confinement of the optical mode can be achieved on sub-micron scales without either disrupting the integrity of the electronic system or causing scattering losses that inevitably occur with conventional etching and patterning techniques. Adding to that, a high degree of control over the topography [28], low mode volumes down to λ 3 and high finesses up to 2 × 10 5 were achieved with the current fabrication techniques [29][30][31]. Recent demonstrations of polariton formation with quantum wells [32] and Purcell enhancements with 2D materials [33] have evidenced the maturation of the open-access microcavity design.…”

Tunable polaritonic molecules in an open microcavity system

“…The individual hemispherical cavities of the molecules laterally confine the photonic field with a confining potential that is nearly harmonic giving rise to zero-dimensional cavity eigenstates which can be described by Hermite or Laguerre Gaussian profiles depending on the exact symmetry of the confining potential. 14,18 The RoC of the single cavity concave feature of 6 lm gives an expected Gaussian beam waist of the fundamental mode at the QW position of around 0.89 lm. coupled cavity system studied here, the longitudinal and higher order transverse Gaussian modes in adjacent cavities couple, giving rise to hybridisation of these eigenmodes into various bonding and antibonding eigenstates.…”

“…In addition, the small concave mirror radius of curvature (RoC) of 6 lm gives rise to micron sized polariton confinement in the hybridised modes, which combined with the narrow polariton linewidths at resonance of 150 leV, may allow the blockade regime for coupled cavities to be reached. 10,11 The shape of the coupled cavities is given by the surface formed from the isophase profile of two Gaussian modes calculated for a cavity length of 3 lm and RoC of 6 lm, separated by a distance d. 13 A 13 Â 13 array of coupled cavity features was fabricated using focused ion beam (FIB) milling, 13,14 where d was incrementally reduced from 10 lm to 7.5 lm as shown in the microscope image in Fig. 1(a).…”