Tunable photonic bandgaps
For more information about the fundamentals of what is a photonic band-gap, please click here.
Figure 1. The change in reflected colour of a polymer stabilized chiral nematic liquid crystal .
In recent years our research has focused on finding new ways to tune the photonic band gap of chiral nematic liquid crystals using electric fields. Methodologies include using polymer stabilization to form a structure whereby the helix axis lies in the plane of the device, which is formed by cooling the presence of a low frequency electric field. After polymerization, an electric field applied across the cell (normal to the substrates) results in a redshift in the reflected wavelengths as the helical structure is unwound. An example of the change in the reflection of a liquid crystal cell using this technique is shown in Figure 1 and the corresponding optical texture of the polymerized structure is shown in Figure 2.
Figure 2. The optical texture viewed between crossed polarizers on a microscope .
Another approach that has been explored is to use electrically commanded surfaces to tune the wavelength of the photonic band gap of a chiral nematic liquid crystal indirectly. A 100 nm layer of ferroelectric liquid crystal is coated onto each substrate before cell assembly. After cell fabrication, the application of an electric field results in the director in the FLC layer rotating in the plane of the device. This rotation creates a torque on the neighbouring chiral nematic liquid crystal molecules resulting in a macroscopic contraction of the pitch of the helix.
Figure 3. Schematic of the chiral nematic liquid crystal/ ferroelectric liquid crystal command surface cell configuration and proposed wavelength tuning mechanism .
A simple schematic of the basic principle of the tuning mechanism is shown in Figure 3 whilst an example of the results of the photonic band gap recorded on a spectrometer for different electric field strengths are presented in Figure 4.
Figure 4. Wavelength tuning of the photonic band gap using dual electrically commanded FLC surfaces. (a) Shift of the PBG with amplitude of electric field at a constant frequency of 1 kHz, and (b) PBG shift with frequency at constant amplitude of 20.8 V/micron. All data was recorded at a temperature of 25° C .
Doping the ferroelectric liquid crystal into the bulk of the chiral nematic liquid crystal has also been shown to result in wavelength tuning of the photonic band gap and is currently a subject that is under investigation at CMMPE. Examples of tuning using this approach is shown in Figures 5 and 6.
5. Broadband wavelength tuning of the photonic band gap.
Figure 6. The change in the reflected color from the cell as the electric field strength was increased, at a frequency of 1 kHz.
Electrically tuneable liquid crystal photonic bandgaps
The switching properties of chiral nematic liquid crystals
using electrically commanded surfaces
tuning the photonic band gap in chiral nematic liquid crystals using
electrically commanded surfaces
color switching from blue to red in a polymer stabilized chiral nematic
Liquid Crystal lasers
Recently, a new breed of laser, the organic laser, has attracted a great deal of attention from the science community. Liquid crystal lasers, in particular, have been the focus of intense research at CMMPE over the past few years. These organic lasers are of significant interest for a number of reasons:
1. A chiral nematic liquid crystal laser (red),
At present, our research activity is based on developing the understanding of the lasing process in these materials and the properties of the molecules that are important for achieving a low threshold, high output power laser. We are also examining a number of different structures ranging from one-dimensional to three-dimensional photonic band structures. Furthermore, investigations into the use of more practical pumping solutions (eg: inchoerent optical pumping, or electrical pumping) are also under way, with the intention of developing the technology into a commercially more useful device.
Figure 2. Section of the optical appratus required for LC lasing.
From the combined pioneering theoretical work of Yablonovitch [Phys.Rev.Lett. 58, 2059 (1987)] and John [Phys.Rev.Lett. 58, 2486 (1987)] on photonic band structures it was realised that spontaneous emission could be inhibited within the photonic band gap and photons localised at the band-edge. In addition to photonic crystals, there are also a variety of liquid crystal phases, primarily those that contain chiral components, which exhibit a periodic structure and consequently give rise to a photonic band gap. One advantage of using liquid crystals as photonic band structures is that the periodic structure spontaneously forms naturally without the need for complex fabrication procedures. The first unequivocal observations of band-edge lasing from liquid crystals were by Kopp et al. [Opt.Lett. 23(21), 1707 (1998)] and independently by Taheri et al. [ALCOM Symp. Chiral Materials & Applications, (1999)] in dye-doped chiral nematic liquid crystals. Liquid crystals are a promising candidate as low-threshold micro-lasers as they spontaneously form a periodic structure and are wavelength tunable by virtue of the sensitivity of the structure to electric fields.
Figure 3. Laser emission at the long-wavelength band-edge.
A typical laser emission spectrum at the edge of a chiral nematic PBG when photopumped with 5ns pulses from a Q-switched Nd:YAG laser of wavelength 532nm is shown in Figure 3. Emission is bidirectional along the helix axis. In this case, the sample was a standard commercial liquid crystal mixture doped with chiral additive and DCM dye contained in a 7.5 micron-thick planar aligned glass cell. It was operated at a temperature of 30degC. To show the emission relative to the band-gap, the transmission spectrum for white light is plotted on the primary y-axis whereas the laser emission spectrum is presented on the secondary axis. Here, the spectrometer has captured the long wavelength part of the PBG, the flat region from ~590-603nm, as well as the maxima and minima of four subsidiary oscillations. The emission intensity of the laser line in this case is found to peak at 604.65nm. The peak of the laser line (lr=604.65nm) is located at a slightly longer wavelength than the transmission maximum (le=604.36nm). This offset is to be expected since in finite samples the laser mode is not positioned at the band-edge. It is also shown in this example that the linewidth of the laser line is 0.13nm. This yields a Q-factor of the laser mode (Q = lr/Dl) of ~4600. From this we can make a rough estimate of the coherence length, x, using the linewidth (x=lr2/Dlr) which we find to be ~5mm.
Figure 4. Far-field interference fringes from coherent LC laser emission.
For band-edge laser emission to occur in liquid crystal media two primary factors are of importance. Firstly, a light harvester of some form must be present in the interior of the liquid crystal medium. This can be either a laser dye such as DCM or a rare earth element. Moreover, the dye can either be dispersed into the liquid crystal matrix or it can be grafted onto the liquid crystal molecule directly. There have been a number of studies reporting the synthesis of novel liquid crystal/dye hybrids. Something that we have been working on here at Cambridge is the synthesis of so-called bimesogenic dye structures whereby a light emitter of some form is attached to a liquid crystal mesogenic unit. The term ‘mesogenic’ refers to an element or sub-unit that forms a mesophase (‘meso’ been the Greek word for middle), ie: a phase inbetween crystalline solid and liquid. To date, we have attached high quantum efficency perylene dyes onto liquid crystals with surprising results including extraordinarily high dye concentrations and concentration-controlled colour shifts. Other reports have shown that even lanthanide ions can be attached to liquid crystal molecules without disrupting or destroying the liquid crystallinity.
The second factor is the position, in terms of the electromagnetic spectrum, of the photonic band gap relative to the emission spectrum of the light emitter. In order to achieve laser action one edge of the photonic band gap must be overlapped with the emission spectrum. To maximise coupling efficiency and achieve the lowest possible threshold, the band-gap must be matched in k-space to the gain maximum. For liquid crystals the band-edges are not equivalent, this is due to the alignment of the electric field vector of the propagating mode at the band-edge relative to the alignment of the transition dipole moment of the dye. For the majority of the studies carried out so far the long wavelength band-edge has been the lowest threshold mode because the transition dipole moment of the dye and the electric field vector are more collinear than they are perpendicular to one another. In a study by Schmitdke and Stille on dye-doped systems, the importance of this orientation has been considered both theoretically and experimentally.
Figure 5. Output beam profiles for an LC laser.
Parallel and complimentary work in this area has also been carried out by the CAPE-funded project, Chiralase.