1 What is a liquid crystal 2 1.1 Nematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Cholesterics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Smectics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.4 Columnar phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.5 Lyotropic liquid crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Introduction to Liquid Crystals

We review the recently emerged class of hybrid metal-dielectric colloidal photonic crystals. The hybrid approach is understood as the combination of a dielectric photonic crystal with a continuous metal film. It allows to achieve a strong modification of the optical properties of photonic crystals by involving the light scattering at electronic excitations in the metal component into moulding of the light flow in series to the diffraction resonances occurring in the body of the photonic crystal. We consider different realizations of hybrid plasmonic-photonic crystals based on two- and three-dimensional colloidal photonic crystals in association with flat and corrugated metal films. In agreement with model calculations, different resonance phenomena determine the optical response of hybrid crystals leading to a broadly tuneable functionality of these crystals.

Hybrid Colloidal Plasmonic‐Photonic Crystals

We demonstrate how tuneable Distributed Bragg Reflectors (DBRs) and resonant micro-cavities can be built by a scalable layer assembly of the transparent utility rubbers polydimethylsiloxane and polystyrene-polyisoprene. Stretching the devices by more than 60% leads to an affine contraction of the layer thicknesses thereby tuning both DBR and cavity modes across the entire visible spectrum. Such rapidly- and reversibly- stretch-tuneable cavities can be used in tuneable micro-lasers and for quantitative optical strain sensing applications.

Stretch-tuneable dielectric mirrors and optical microcavities

https://onlinelibrary.wiley.com/doi/pdf/10.1002/qute.201900060

Tunable Open-Access Microcavities for Solid-State Quantum Photonics and Polaritonics

Arrays of half-symmetric Fabry-Perot micro-cavities were fabricated by controlled formation of circular delamination buckles within a-Si/SiO2 multilayers. Cavity height scales approximately linearly with diameter, in reasonable agreement with predictions based on elastic buckling theory. The measured finesse (F > 103) and quality factors (Q > 104 in the 1550 nm range) are close to reflectance limited predictions, indicating that the cavities have low roughness and few defects. Degenerate Hermite-Gaussian and Laguerre-Gaussian modes were observed, suggesting a high degree of cylindrical symmetry. Given their silicon-based fabrication, these cavities hold promise as building blocks for integrated optical sensing systems.

High-finesse cavities fabricated by buckling self-assembly of a-Si/SiO_2 multilayers

We measure experimentally the spatial intensity profiles and resonant frequencies of the transverse modes of nearly hemispherical microcavities for a range of cavity lengths and mirror curvatures. The experimental mode profiles are radially symmetric Gauss-Laguerre modes, but do not posses the radial frequency degeneracies typical of Gauss-Laguerre modes in large-scale optical cavities. We use a paraxial model of cavity propagation to interpret the experimental results. In particular, we show that the lifting of the radial frequency degeneracy may be due to the strength of spherical aberration in wavelength-scale microcavities.

Spectral properties and modes of surface microcavities

We measure experimentally the spatial intensity profiles and resonant frequencies of the transverse modes of nearly hemispherical microcavities with cavity length and mirror curvatures below 10 microm. These resonators possess axially symmetric Gauss-Laguerre-like modes, but do not display the frequency degeneracies typical of large-scale optical cavities. It is possible to interpret these results using a paraxial model of cavity propagation that includes nonparabolic optical elements.

Tracking spatial modes in nearly hemispherical microcavities.

We measure experimentally the spatial intensity profiles and resonant frequencies of the transverse modes of nearly hemi-spherical microcavities with cavity length and mirror curvatures below 10 mum. These resonators possess axially-symmetric Gauss-Laguerre-like modes, but do not display the frequency degeneracies typical of large-scale optical cavities. It is possible to interpret these results using a paraxial model of cavity propagation that includes non-parabolic optical elements.

Tracking spatial modes in nearly hemispherical microcavities

The authors have studied the mode structure of planar-spherical cavities. They have developed the possibility of templating a gold coated surface with a regular array of spherical dimples. A planar gold plated glass sheet a few microns is placed above the surface, thus forming a regular array of planar-spherical cavities. These cavities, are reproducible and with a well defined and adjustable geometry: both the cavity length and the depth of the spherical mirror can be changed. Moreover, they can be filled with a variety of materials, either linear or nonlinear and may therefore be an excellent building block of novel macroscopic optical structures. In order to model light-matter interaction in these cavities it is essential to have a good understanding of the mode structure of a single empty cavity. This paper reports the experimental and theoretical analysis of the cavity modes.

Transverse mode structure of hemispherical microcavities

Mapping the spatial and spectral resonant modes of wavelength-scale planar-hemispherical microcavities demonstrates that they have Laguerre-Gauss symmetry. However the spectral ordering of transverse modes confounds expectations from the paraxial approximation.

R.C. Pennington

Papers

Spectral properties and modes of surface microcavities

Tracking spatial modes in nearly hemispherical microcavities.

Tracking spatial modes in nearly hemispherical microcavities

Transverse mode structure of hemispherical microcavities

Tracking spatial/spectral mode structures in novel hemispherical microcavities