Research Projects

Below is a summary of our core research themes. For more information about the research please contact either Dr Stephen Morris (stephen.morris@eng.ox.ac.uk) or Professor Steve Elston () directly.

1. Aberration-corrected Direct Laser Writing in Liquid Crystals & Polymers

We are working closely with the Dynamic Optics and Photonics group of Professor Martin Booth and Dr Patrick Salter to develop new electro-optic effects and photonics technologies by writing micron-sized polymer features into polymerizable liquid crystal materials and devices. Two-photon polymerization direct laser writing (TPP-DLW) is a technique for creating micro- and nanoscale polymer objects and is a key advanced additive manufacturing or 3D printing technique. The capabilities of TPP-DLW go well beyond that offered by traditional lithographic processes, as it provides true 3D structuring rather than 2D layer-by-layer fabrication. It allows rapid prototyping and flexible, direct conversion from 3D design to 3D microstructure as well as the remarkable ability to create sub-diffraction limited polymer features, making it a true nanoprinting technique. By translating a photopolymerizable sample (a resin) in a controlled manner relative to the laser focus, 3D structures can be built-up from polymerized voxels (the 3D analogy of pixels) formed via free-radical polymerization. 

Figure 1. Direct laser writing of polymer pillars in liquid crystals.

Figure 2. Laser-written polymer bicycle in a liquid crystal device.

 Figure 3. Laser written QR code.

 A key challenge for emerging nanoprinting techniques is to increase the functionality and capabilities of the resin materials. We are directly addressing this by developing tunable resins using stimuli-responsive liquid crystal materials that change their optical and physical properties in response to external fields. Developing such bespoke resins that incorporate polymerizable liquid crystal molecules has allowed us to lock-in different alignments of the liquid crystal director (the average pointing direction of the molecules). Thus from a single resin, we are able to engineer polymer structures with different material properties in a single-step fabrication process, leading to new electro-optic capabilities and practical applications.

 Research Highlights:

 

Relevant publications

 

2. Characterising Laser Speckle and Liquid Crystal Speckle Reducers

 

The properties of laser light, such as the high spectral purity and collimation, make them ideal for display projection, microscopy and medical imaging applications. When multiple laser sources of different wavelengths are combined it allows for a much larger range of colours to be produced than with other light sources such as LEDs. Unfortunately, the coherence of a laser results in speckle which degrades the quality of an image. Existing technologies, such as rotating diffusers, can mitigate this problem but at the cost of increased size, power consumption and mechanical vibrations, which makes them unsuitable for many applications.

We have developed a compact technology that can reduce laser speckle by as much as 90%, making it virtually impossible to see with the naked eye. The basic operating principle of the device is that it consists of an electro-active thin film of liquid crystal that generates a continuous series of independent speckle patterns with the application of an external voltage. These statistically independent patterns are generated because the thin-film imparts a spatially random phase modulation to light that is transmitted through the device. When observed within a finite integration time, this results in a reduction in the observed speckle pattern (Figure 4).

 

Figure 4. Laser projection of a checkerboard with speckle reducer off (left) and on (right).

Our speckle reducers allow for analogue control of the speckle contrast that is hysteresis-free and stable over time. We have shown that the speckle reduction is consistent for at least 30 hours. The device can be operated at temperatures reaching 120 degrees and shows no damage when exposed to optical powers of at least 2W. Our device has no moving parts and so is vibration free and does not have the same lifetime issues as the alternative technologies. We have developed a range of integrated prototype devices for testing on optical benches and for integrating into existing optical systems.

 Figure 5. Automated experiment for laser speckle contrast measurements.

The team has also established a semi-automated characterisation system for quantifying the amount of speckle contrast present in an image (Figure 5), which has been set-up to emulate the response of the human eye in terms of how it perceives speckle. Moreover, the operating parameters of this characterisation system are in accordance with the recommended practice for the measurement of speckle in laboratory conditions as defined by the Laser Illuminated Projector Association. Our system (Figure 5) allows for a real-time measurement of the Speckle Contrast, C, and enables a full sweep of the electric field parameters of the optoelectronic device under test.

Relevant Publications

3. Inkjet Printing of Liquid Crystals & Polymers

Our research, which is in collaboration with the Fluid Dynamics group led by Professor Alfonso Castrejon-Pita, revolves around the printing of liquid crystals (LC) and polymer composites for the fabrication of next-generation thin-film optical elements and devices. Drop-on-Demand (DoD) inkjet printing is a non-contact technique that allows for the precise deposition of pico-litre volume droplets of ink to create micron-sized objects. The capabilities of DoD printing go well beyond the contact-based printing methods, such as gravure offset and screen printing, as it provides greater flexibility in terms of the variety of materials that can be deposited using a layer-by-layer approach on a range of substrates and surfaces including flexible substrates.

 

 Figure 6. An array of printed LC microlenses.

 Figure 7. Printed nematic LC droplet.

LCs are a class of functional fluid that possess unique electro-optical properties, providing enormous opportunities for the development of active and passive optical components. Combining LCs and DoD inkjet printing opens up new pathways in the fabrication of functional thin-film optical elements and devices (Figure 6). Chemical control of the printing substrate as well as the precise deposition of the LC based inks can lead to the engineering of structures and configurations that exhibit special optical properties (Figure 7). Our work includes a broad and in-depth investigation of the printing conditions that are required to deposit a range of LC ink formulations in a controlled and precise manner without the formation of satellite droplets or jet break-up. We are also study the effect of substrates on the printed drop formation as well as possible thin-film optical elements that can be fabricated using this additive manufacturing technique. 

 

 Relevant Publications


4. Liquid Crystal Phase Modulators

 


5. Thin-Film Lasers


Our research on organic laser devices involves using a combination of liquid crystals and polymer materials to create hybrid lasers that are wavelength tuneable (Figure 4). We are interested in developing new laser sources based upon these organic materials in an attempt to create low-threshold lasers that are potentially compatible with low-cost manufacturing techniques. The research includes both fundamental studies of the emission characteristics such as coherence properties as well as the design and fabrication of new device architectures.

We are working in collaboration with the research groups of Professors Henry Snaith FRS and Moritz Reade in the Department of Physics (University of Oxford) to develop new thin-film lasers that combine the absorption and emission properties of perovskite structures with the feedback and reflection properties of chiral liquid crystalline materials. The aim of this work is to develop new, low threshold laser sources that are photostable and can be tuned using external stimuli. We have recently developed an amplified spontaneous emission source based upon a thin layer of perovskite material sandwiched between a chiral nematic (cholesteric) liquid crystal reflector and a gold layer (see publication below). Further work on combining the perovskites with tuneable liquid crystalline polymer reflectors for laser devices is currently in progress. This work was also carried out in collaboration with Professor Albert Schenning's group at the Eindhoven University of Technology and the Optoelectronics Research Group in the Cavendish Laboratory at the University of Cambridge.

Figure 6. ASE from a perovskite structure using a chiral nematic (cholesteric) reflector and a gold reflector.

Emission on flexible chiral nematic liquid crystal substrates.

 

LC Perovskite 

Figure 7. ASE from a perovskite structure using a flexible chiral nematic (cholesteric) reflector.

 

Distributed feedback structures for laser emission.

 

 

Figure 8. Illustration of a distributed feedback perovskite laser.

Clustomesogens

Figure 9. Clustomesogen structures synthesised by the team of Dr. Yann Molard (University of Rennes 1)

 

Clustomesogens 2

Figure 10. Emission characteristics of Molybdenum-based clustomesogens in chiral nematic liquid crystals.


Relevant publications:


  • Wavelength-tuneable laser emission from stretchable chiral nematic liquid crystal gels via in situ photopolymerization  RSC Advances6, 31919-31924 (2016)
  • Enhanced Amplified Spontaneous Emission in Perovskites using a Flexible Cholesteric Liquid Crystal Reflector  Nano Letters 15, 4935-4941 (2015)
  • Structured Organic–Inorganic Perovskite toward a Distributed Feedback Laser Advanced Materials, 28, 923-929 (2016)
  • Polarized Phosphorescence of Isotropic and Metal-Based Clustomesogens Dispersed into Chiral Nematic Liquid Crystalline Films Advanced Optical Materials, 3, 1368-1372, (2015)


6. Smart Materials


 a. Chiral Polymer Scaffolds 

Our research in this area involves the fabrication and characterisation on new photonic and meta-materials based upon chiral nematic and blue phase liquid crystals that have been ‘templated’ using a polymer scaffold that has been created by a cross-linked polymer network. The aim of this work is to harness the self-organising feature of liquid crystalline phases that naturally assemble to form a structure with a periodic modulation of the refractive index that is on the order of the wavelength of visible light. Furthermore, depending upon the liquid crystal phase, this periodicity can exist in either 1-dimension (chiral nematic) or 3-dimensions (blue phase). By forming a template of these structures with a polymer network it is then possible to impart this macroscopic structure onto other materials. An example of the 'templating' process of a blue phase is shown in Figure 1. This work is carried out in collaboration with Dr Flynn Castles in the Department of Materials at the University of Oxford and also previously with the Centre of Molecular Materials for Photonics and Electronics at the University of Cambridge.

 

Blue phase template

Figure 1. A Blue phase template (a self-assembled 3D photonic crystal). 

b. Stretchable Photonic Crystal-like Gels

In addition, we are also studying the mechanochromic and electro-optic properties of chiral nematic blue phase liquid crystalline gels, which have recently been demonstrated as a result of research carried out at the Departments of Engineering at both Oxford and Cambridge. Examples of free-standing blue phase gels from a recent publication in Nature Materials are shown in Figure 2.

Stretchable liquid crystalline blue phase gels

Figure 2. Stretchable liquid crystalline blue phase gels.

We are also working on stretchable laser gels based upon chiral nematic and blue phase liquid crystals. An example of wavelength tuning of the laser emission from a liquid crystal laser gel is shown in Figure 3. 

 

Figure 3. Stretchable cholesteric liquid crystalline gels

Figure 4. Stretchable cholesteric liquid crystalline gels 

Relevant publications:

  • Wavelength-tuneable laser emission from stretchable chiral nematic liquid crystal gels via in situ photopolymerization  RSC Advances, 6, 31919-31924 (2016)
  • Stretchable liquid-crystal blue-phase gels Nature Materials, 13, 817-821(2014)
  • Blue-phase templated fabrication of three-dimensional nanostructures for photonic applications Nature Materials 11, 599 (2012)
  • Simultaneous red-green-blue reflection and wavelength tuning from an achiral liquid crystal and a polymer template Adv. Mater., 22, 53-56, (2010)