Research


  

 

Our research covers different aspects of materials research, nonlinear optics and optical characterisation technqiues, targeting optoelectronic applications of liquid crystals and their composites. The main experimental facilities include a set of specialised optical labs. In conjunction with the Nanomaterials Rapid Prototyping facility and Zepler Institute Clean Rooms at the University, we prepare and test all our samples in-house.


 

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, a goal that posed a significant challange before, 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 to control the MM 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 thin films of azobenzene complex dyes (PAAD) in the visible and in the THz range.

In the visible range we have successfully demonstrated strong, birefringent phase gratings light in thin 15-35 nm PAAD films. The gratings were found not to be associated with any periodic surface relief, a typical feature in thicker azobenzene layers.  We have also shown that the photo-induced refractive index change in PAADs is anisotropic with largest refractive index modulation in the direction parallel to the polarization of the writing beams [1].  The origin of such refractive index modulation cannot be fully attributed to a molecular reorientation effect. This intriguing mechanism of interaction of thin PAAD films with light will be studied further, in particular by considering the interaction with the substrate surface and its role in the formation of the gratings.

Azo PAAD films were also used as photoalignment layer in LC cells. Optically controlled, rewritable modulators and phase shifters are developed, based on a twisted nematic liquid crystal cells. The modulators are bistable, with switching between states controlled by one-step illumination with visible light. The photo-alignment properties of PAAD facilitated reversible switching between two perpendicular alignment states at the cell surface, resulting in controllable polarization manipulation. The resulting modulation in transmission for different visible and NIR wavelengths were demonnstrated with with an optical contrast of up to 90-100%. This result paves the way to LC modulators and phase shifters which can be controlled remotely using only external light sources [2]

We are also working on exploiting the anisotropy of azobenzene layers in THz region of spectrum and investigate, through indirect measurements, the shift of plasmonic resonance in matamaterials designed for an efficient, active control of THz radiation.

 

1. 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


2. 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 crystals

 

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

 


 

Light as a tool for characterisation of liquid crystals

 

 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 [1]

 

 

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 [2]. 

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 Analyser (OMPA) 

 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 devices time constants and electrical properties. 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. Our method has been successfully demonstrated in standard, LC cells, as well as in those with photo-active surface layers, including accumulated space charge layers [3]. 

 

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.

 

 1.  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

 

2. 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

 

3.  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