Stephen M. Morris

Blue-phase templated fabrication of three-dimensional nanostructures for photonic applications

A promising approach to the fabrication of materials with nanoscale features is the transfer of liquid-crystalline structure to polymers. However, this has not been achieved in systems with full three-dimensional periodicity. Here we demonstrate the fabrication of self-assembled three-dimensional nanostructures by polymer templating blue phase I, a chiral liquid crystal with cubic symmetry. Blue phase I was photopolymerized and the remaining liquid crystal removed to create a porous free-standing cast, which retains the chiral three-dimensional structure of the blue phase, yet contains no chiral additive molecules. The cast may in turn be used as a hard template for the fabrication of new materials. By refilling the cast with an achiral nematic liquid crystal, we created templated blue phases that have unprecedented thermal stability in the range -125 to 125 °C, and that act as both mirrorless lasers and switchable electro-optic devices. Blue-phase templated materials will facilitate advances in device architectures for photonics applications in particular.

Photonics and lasing in liquid crystals

Lasers were invented some 40 years ago and are now used in a plethora of applications. Stable liquid crystals were discovered at about the same time, and are now the basis of a large display industry. Both technologies involve photonics, the former in the creation and use of light and the latter in the control and manipulation of light. However, it is only recently that these two mature technologies have been combined to form liquid-crystal lasers, heralding a new era for these photonic materials and the potential for novel applications. We summarize the characteristics of liquid crystals that lead to laser devices, the wide diversity of possible laser systems, and the properties of the light produced.

Structured Organic-Inorganic Perovskite toward a Distributed Feedback Laser.

A general strategy for the in-plane structuring of organic-inorganic perovskite films is presented. The method is used to fabricate an industrially relevant distributed feedback (DFB) cavity, which is a critical step toward all-electrially pumped injection laser diodes. This approach opens the prospects of perovskite materials for much improved optical control in LEDs, solar cells, and also toward applications as optical devices.

Enhanced emission from liquid-crystal lasers

The performance of a photonic band-edge laser fabricated from a low molar mass dye-doped chiral nematic liquid crystal is found to have a strong thermal dependence. At each temperature the performance of the laser has been characterized by the slope efficiency which was calculated from a plot of the emission energy as a function of excitation energy. This slope efficiency was found to increase by 36% when the dye-doped chiral nematic liquid crystal was cooled from 53to43°C. The increase in slope efficiency is considered to be due to a change in the lasing conditions, in particular, changes in the emission efficiency of the dye and possibly the quality factor of the liquid-crystal resonator, which is dependent upon the linewidth of the resonant mode. The wavelength dependency of the spontaneous emission intensity and the quantum efficiency of the dye were not found to influence the lasing conditions in this case. The order parameters relating to the dye-doped chiral nematic liquid crystal were considered t...

Novel nonvolatile memory with multibit storage based on a ZnO nanowire transistor.

We demonstrate a room temperature processed ferroelectric (FE) nonvolatile memory based on a ZnO nanowire (NW) FET where the NW channel is coated with FE nanoparticles. A single device exhibits excellent memory characteristics with the large modulation in channel conductance between ON and OFF states exceeding 10(4), a long retention time of over 4 × 10(4) s, and multibit memory storage ability. Our findings provide a viable way to create new functional high-density nonvolatile memory devices compatible with simple processing techniques at low temperature for flexible devices made on plastic substrates.

Strong flexoelectric behavior in bimesogenic liquid crystals

In this paper, we demonstrate strong flexoelectric coupling in bimesogenic liquid crystals. This strong coupling is determined via the flexoelectro-optic effect in chiral nematic liquid crystals based on bimesogenic mixtures that are doped with low concentrations of high twisting power chiral additive. Two mixtures were examined: one had a pitch length of p∼300nm, the other had a pitch length of p∼600nm. These mixtures exhibit enantiotropic chiral nematic phases close to room temperature. We found that full-intensity modulation, that is, a rotation of the optic axis of 45° between crossed polarizers, could be achieved at significantly lower applied electric fields (E<5Vμm−1) than previously reported. In fact, for the condition of full-intensity modulation, the lowest electric-field strength recorded was E=2Vμm−1. As a result of a combination of the strong flexoelectric coupling and a divergence in the pitch, tilt angles of the optic axis up to 87°, i.e., a rotation of the optic axis through 174°, were obs...

Paintable band-edge liquid crystal lasers

In this paper we demonstrate photonic band-edge laser emission from emulsion-based polymer dispersed liquid crystals. The lasing medium consists of dye-doped chiral nematic droplets dispersed within a polymer matrix that spontaneously align as the film dries. Such lasers can be easily formed on single substrates with no alignment layers. The system combines the self-organizing periodic structure of chiral nematic liquid crystals with the simplicity of the emulsion procedure so as to produce a material that retains the emission characteristics of band-edge lasers yet can be readily coated. Sequential and stacked layers demonstrate the possibility of achieving simultaneous multi-wavelength laser output from glass, metallic, and flexible substrates.

Stretchable liquid-crystal blue-phase gels

Liquid-crystalline polymers are materials of considerable scientific interest and technological value. An important subset of these materials exhibit rubber-like elasticity, combining the optical properties of liquid crystals with the mechanical properties of rubber. Moreover, they exhibit behaviour not seen in either type of material independently, and many of their properties depend crucially on the particular mesophase employed. Such stretchable liquid-crystalline polymers have previously been demonstrated in the nematic, chiral-nematic, and smectic mesophases. Here, we report the fabrication of a stretchable gel of blue phase I, which forms a self-assembled, three-dimensional photonic crystal that remains electro-optically switchable under a moderate applied voltage, and whose optical properties can be manipulated by an applied strain. We also find that, unlike its undistorted counterpart, a mechanically deformed blue phase exhibits a Pockels electro-optic effect, which sets out new theoretical challenges and possibilities for low-voltage electro-optic devices.

Simultaneous red-green-blue reflection and wavelength tuning from an achiral liquid crystal and a polymer template

Adv. Mater. 2010, 22, 53–56 2010 WILEY-VCH Verlag Gmb T IO N It is well known that one-dimensional Bragg reflectors such as chiral nematic liquid crystals (N*LCs) spontaneously form a photonic band structure (PBS) and exhibit vivid Bragg reflections at visible wavelengths when the periodicity of the helix matches the wavelength of light. The self-organizing feature of the PBS has attracted considerable interest recently for low-threshold laser devices and display devices. However, direct electronic control of the PBS and, therefore, the wavelength of the reflected light using N*LCs is a nontrivial task. This is due to the fact that, under the influence of an electric field, the periodic structure becomes distorted in a non-uniform manner and a concomitant loss of the Bragg reflection is then observed. The main reasons for the disruption of the PBS, for the case of an electric field applied parallel to the helix axis, are due to out-of-plane rotations of the N*LCmolecules from the Grandjean (uniform standing helix) state to the disordered focal conic (i.e., scattering) phase and, at low frequencies, the presence of electro-hydrodynamic instabilities (EHDIs). Unwinding of the helix using an electric field perpendicular to the helix axis can lead to a change in the wavelength of the reflection band and thus some color tuning, but such an approach requires fringe-fields that can locally distort and disrupt the homogeneity of the structure. The Helfrich deformation, has also been shown to provide a means of changing the wavelength of the reflection. However, a marked deterioration in the quality of the macroscopic helix can result due to the presence of EHDIs. There have also been a number of reports on wavelength tuning using polymer-stabilized N*LCs; however, it should be noted that this too involved a Helfrich deformation and a deterioration of the structure was observed. Consequently, direct electrical control of the N*LC using such methods does not generally permit a smooth change of the reflected wavelength as the quality of the periodic structure is not maintained. To circumvent this problem, we recently demonstrated wavelength tuning of the photonic bandgap by employing a passive switching mechanism, using electrically commanded surfaces, which responded to electric fields of a frequency greater than 1 kHz. Broadband (>100 nm) wavelength tuning was observed without any deterioration of the optical quality of the bandgap. However, these passive switching methods which involved command surface switching using a ferroelectric liquid crystal (FLC) on the inner substrates of the cell or an FLC dispersed in to the bulk of the N*LC, suffered from being rather slow (>25 s) and are thus limited in terms of the scope of their potential applications. A further limitation of the previous approach is that the method was restricted to liquid crystalline compounds which possess a negative dielectric anisotropy. Typically, such compounds possess, intrinsically, a low birefringence, which is not ideal for applications such as liquid crystal lasers where a high birefringence is required for low thresholds and high slope efficiencies. In short, a photonic bandgap that could be fabricated from a conventional positive dielectric anisotropy LC that would respond rapidly to electrical addressing and could be wavelength tuned over a broad range, would be of significant interest. This motivated us to find an alternative method for creating a PBS from an achiral nematic LC and for using simple dielectric coupling in order to control the spectral properties of the bandgap. For this purpose, we employed the concept of so-called ‘‘molecular imprinting’’. According to de Gennes, nonchiral liquid crystalline molecules in the presence of a helical structure should adopt the helical configuration defined by the structure. A number of theoretical and experimental studies carried out by Warner and Terentjev, in particular, have considered molecular imprinting in detail using cholesteric elastomers. In addition, Jakli et al. have also verified experimentally that macroscopic chirality can be achieved from nonchiral LCs by transferring chirality from polymer networks to the ferroelectric LC phase of banana-shaped molecules. In this paper we demonstrate directly addressable wavelength tuning of the photonic bandgap from a hybrid structure consisting of an achiral nematic liquid crystal and a periodic polymer template. The wavelength tuning is found to be fast ( 43ms) and broadband (greater than 100 nm). For our study, a polymer template was prepared by photopolymerizing a monomer that was dispersed within a chiral nematic pre-mixture, which initially defined the periodicity or pitch, p, of the structure, but was then subsequently removed. A detailed schematic of the preparation and subsequent mechanism for tuning is described in Figure 1. The pre-mixture was composed of 40wt% reactive mesogen (UCL-011-K1, DIC), 28wt% of the chiral dopant R2011 (Merck KGaA) and 32wt% of the negative dielectric anisotropy liquid crystal MLC-7029 (Merck). Upon photo-polymerization, the remaining small molecule mixture was then washed out, using acetone, leaving behind the rigid chiral polymer template. An achiral nematic LC, BL006 (Merck KGaA), which has a higher birefringence (Dn1⁄4 0.286) than MLC-7029, and a positive

Comparison of the performance of photonic band-edge liquid crystal lasers using different dyes as the gain medium

The primary concern of this work is to study the emission characteristics of a series of chiral nematic liquid crystal lasers doped with different laser dyes (DCM, pyrromethene 580, and pyrromethene 597) at varying concentrations by weight (0.5–2 wt %) when optically pumped at 532 nm. Long-wavelength photonic band-edge laser emission is characterized in terms of threshold energy and slope efficiency. At every dye concentration investigated, the pyrromethene 597-doped lasers exhibit the highest slope efficiency (ranging from 15% to 32%) and the DCM-doped lasers the lowest (ranging from 5% to 13%). Similarly, the threshold was found to be, in general, higher for the DCM-doped laser samples in comparison to the pyrromethene-doped laser samples. These results are then compared with the spectral properties, quantum efficiencies and, where possible, fluorescence lifetimes of the dyes dispersed in a common nematic host. In accordance with the low thresholds and high slope efficiencies, the results show that the ...