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:
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.
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.
1. A chiral nematic liquid crystal laser (red),
The physics of LC lasers
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.
4. Far-field interference fringes
Funding and collaborators:
This work forms part of the EPSRC basic technology project, COSMOS (Coherent Optical Sources using Micromolecular Ordered Structures), which is in collaboration with Prof. Sir Richard Friend (Optoelectronics, Department of Physics), Prof. Eugene Terentjev (Biological Soft Sytems, Department of Physics), Prof. Willhelm Huck (Melville Lab, Department of Chemsitry) and Prof. Ian White (Centre for Photonic Systems, Department of Engineering).
Parallel and complimentary work in this area is also been carried out by the CAPE-funded project, Chiralase.