Research – Soft Photonic Systems Group

Hybrid Photonic Metamaterials Coupled with Liquid Crystals at Nanoscale

LCMM_mechanics — Nano-electro-mechanical system (NEMS) under study. (a) Artistic impression of a mechanically reconfigurable zigzag metasurface (cross-section) infiltrated with a liquid crystal. Red and blue arrows indicate the directions of electrically induced displacements for the nanobridges baring opposite potentials. (b) SEM image of the fabricated metasurface. Scale bar corresponds to 2 μm. (c and d) Close-up view of the areas marked in panel (b) with green and purple boxes, respectively. Images were taken with SEM at 52° to the normal and color-coded to enhance the contrast between gold (yellow) and silicon nitride (gray). Scale bar corresponds to 300 nm.

Photonic metamaterials (MMs) is a novel class of nano-structured artificial media with optical properties not found, or superior, to those exhibited by natural materials. Nowadays, the scientific efforts are focused on development of active and tunable MMs, a generation of artificial photonic media with dynamically, on demand controlled optical properties. We have created tunable MMs by functionalising their fabric with liquid crystals (LCs), where the MM properties were tuned by changing the LC optical anisotropy with external electric field. The spectral tuning of hybrid LC-MM systems of up to 10% in the optical part of the spectrum using electric field was achieved, which had been challenging thus far due to strong anchoring of LC molecules to the surface of nano-structures [1].
In order to increase the tunability of LC-MM systems even further we made MM framework mechanically re-configurable and, for the first time, employed elastic properties of LCs for controlling its movement at the nanoscale [2]. The resulting hybrid nano-electro-mechanical systems (NEMS) were free from stiction and the robust control of nano-scopic actuations in such systems was achieved for the entire range of structurally allowed displacements. We experimentally demonstrated the full potential of electrically controlled LCs for tuning photonic MMs, and also introduced a novel type of NEMS, which are elastically coupled to and controlled by LCs.

1. O. Buchnev , N. Podoliak , M. Kaczmarek , N. I. Zheludev, and V. A. Fedotov, Electrically controlled nanostructured metasurface loaded with liquid crystal: toward multifunctional photonic switch, Adv. Optical Mater. 3, 674–679 (2015)
https://doi.org/10.1002/adom.201400494

2. O. Buchnev, N. Podoliak, T. Frank, M. Kaczmarek, L. Jiang, and V. A. Fedotov, Controlling stiction in nano-electro-mechanical systems using liquid crystals, ACS Nano 10, 11519–11524 (2016)
https://doi.org/10.1021/acsnano.6b07495

Controlling light with light

PAAD3 — Twisted LC cell. Linear polarised light causes PAAD molecules to align perpendicular of the light’s polarisation.

We study the intrinsic and photoinduced properties of azobenzene complex dyes (PAAD) in the visible and THz range. Optically controlled, rewritable modulators and phase shifters are developed, based on a twisted nematic liquid crystal cells. The anisotropy of azobenzene layers in THz is also investigated with indirect measurements by a shift of plasmonic resonance in matamaterials designed for an efficient active control of THz radiation.

E. Mavrona, S. Mailis, N. Podoliak, G. D’Alessandro, N. Tabiryan, M. Trapatseli, J.-F. Blach, M. Kaczmarek, and V. Apostolopoulos, “Intrinsic and photo-induced properties of high refractive index azobenzene based thin films,” Opt. Mater. Express 8 (2), 420-430 (2018)
https://doi.org/10.1364/OME.8.000420
E. Perivolari, J.R. Gill, N. Podoliak, V. Apostolopoulos, T.J. Sluckin, G. D’Alessandro and M. Kaczmarek, Optically controlled bistable waveplates, J. Mol. Liq. (2017)
https://doi.org/10.1016/j.molliq.2017.12.119

Multiscale modelling of doped liquid crystal systems

MMLC — Schematic of the scale separation used in the multiscale method of homogenisation. The macroscopic domain consists of an open region D, with external boundary ∂D.

The introduction of foreign particles to alter and tune the properties of liquid crystal systems is an area of great interest and research. The addition of these particles allows for greater control, or even possibly entirely new, behaviours of the otherwise unaltered systems. Mathematical models of such doped systems are essential for their understanding and improvement but are often very computationally expensive. This is due to the microscopic effects of individual particles and the overall macroscopic behaviour of the system existing at vastly differing length scales. By employing multiscale averaging methods, we reduce the system to one that exists purely at the macroscopic scale for a single “averaged” material and computational study times are vastly reduced.

T.P. Bennett, G. D’Alessandro and K.R. Daly, Multiscale models of colloidal dispersion of particles in nematic liquid crystals, Phys. Rev. E 90(6), 062505 (2014)
https://doi.org/10.1103/PhysRevE.90.062505

T.P. Bennet, G. D’Alessandro and K.R. Daly, Multiscale models of metallic nanoparticles in nematic liquid crystals, SIAM J. Appl. Math. 78(2), 1228-1255 (2018)
https://doi.org/10.1137/18M1163919

Optical characterisation of liquid crystal materials

We have developed a new technique, that uses a periodic modulation of the voltage applied to the cell, to measure two sets of liquid crystal viscosity parameters:

T.P. Bennett, M.B. Proctor, M. Kaczmarek and G. D’Alessandro, Lifting degeneracy in nematic liquid crystal viscosities with a single optical measurement, J. Colloid Interface Sci. 497, 201-206 (2017)
https://doi.org/10.1016/j.jcis.2017.03.020

bennett2017 — Graphical abstract of Bennett et al (2017), J. Colloid Interface Sci. – We measure the cross-polarised intensity through a nematic liquid crystal cell at different frequencies of the applied voltage (high, top right; low, bottom right; intermediate, bottom left). Statistical analysis of these curves give the parameters listed on the top left.

Also a methodology to extract wide area information about nematic liquid crystal cells, e.g. a map of the cell thickness or of the pretilt angle has been developed. These measurements coupled with a bootstrapping statistical analysis allows us to obtain accurate measurements of the liquid crystal properties, e.g. its elastic constants. These results were reported in T.P. Bennett, M.B. Proctor, J.J. Forster, E. Perivolari, N. Podoliak, M. Sugden, R. Kirke, T. Regrettier, T. Heiser, M. Kaczmarek and G. D’Alessandro, Wide area mapping of liquid crystal devices with passive and active command layer, Appl. Optics 56, 9050-9056 (2017)
https://doi.org/10.1364/AO.56.00905010.1016

bennett2017a — PI planar cell filled with E7: spatial map of the liquid crystal (a) thickness and (c) pretilt angle; (b) and (d) are the corresponding errors. Circles are the fitted values, and the background color map is a piece-wise cubic interpolation between them.

The end result of this research project is a new instrument, the Optical Multi-Parameter Analysis (OMPA)

Voltage transfer function: optical characterisation method of electrical properties of liquid crystal devices

VTF — Comparison of measured (left column) crosspolarized intensity as a function of voltage and frequency for the E7 cell with that (right column) given by a nonlinear filter model.

There is considerable interest in the development of new aligning materials for liquid crystal devices to make them easier to fabricate and introduce less impurities in the process. Thin layers of different types of polymers or surfactants deposited on the device substrates typically are used for that purpose and, in most cases, play a passive role when considering the optical or electrical response. However, frequently their interface with liquid crystals can influence the electric field profile extending into the devices and ionic effects can be present, especially if impurities are present or as a result of long term exposure to light and electric field.

We have developed a new technique, called voltage transfer function, a rapid and visually effective method to determine the electrical response of liquid crystal systems using optics. This method relies on cross polarized intensity measurements as a function of the frequency and amplitude of the voltage applied to the device. Coupled with a mathematical model of the device it can be used to determine the device’s time constants and electrical properties.

J. Bateman, M. Proctor, O. Buchnev, N. Podoliak, G. D’Alessandro and M. Kaczmarek, Voltage transfer function as an optical method to characterize electrical properties of liquid crystal devices, Opt. Lett. 39(14), 3756-3759 (2014)
https://doi.org/10.1364/OL.39.003756