Lasing from dye-doped liquid crystal systems has already been demonstrated in cholesteric and ferroelectric liquid crystals; as well as holographic polymer dispersed liquid crystals¹ We will present on lasing from a nematic liquid crystal whose photonic band gap at visible wavelengths is solely induced by the alignment layer. By exposing a linear polarization polymer (LPP) surface alignment layer to a holographic interference pattern generated by the intersection of a pair of right and left handed circularly polarized beams, a periodic rotating alignment layer can be generated.^2,3 The polarization grating formed by the interference pattern consists of a constant intensity, rotating linear polarization along one direction; this periodic, rotating pattern is 'written' into the surface alignment layers on both the top and bottom LPP coated substrates (Figure 1(a)). The orientation of the liquid crystal material will tend to follow this rotating alignment, creating a grating structure of periodic effective refractive index. By tailoring the pitch of the grating structure a photonic band gap in the visible regime can be formed. Lasing is achieved with the use of a pulsed Nd:YAG laser, operating at λ = 532 nm, with energies in the micro-Joule regime as the pump source. When doped with a laser dye, low threshold lasing occurs in these liquid crystal structures along the direction of the grating vector (Figure 1(b) and (c)); early results suggest this threshold is approximately 60 µJ (inset of Figure 1(c)). Significant line narrowing is observed when the pulse energy is greater than the threshold for lasing. With the application of an electric field across the dye-doped liquid crystal material, the grating structure will change and the lasing wavelength will shift, effectively creating a tunable laser cavity. We will present on these structures using a variety of pitches, laser dyes, and liquid crystal hosts. Investigations will focus on the threshold of lasing in these systems as well as the effect of different dyes and liquid crystals with different birefringences. A determination of the effect of an applied electric field on lasing within these systems will also be investigated. This will be the first study of lasing from a nematic liquid crystal system whose grating structure is dictated solely by an alignment layer.

1. V. I. Kopp, Z. Q. Zhang, and A. Z. Genack, "Lasing in chiral photonic structures," Progress in Quantum Electronics 27, 369-416 (2003).

2. G. P. Crawford, J. N. Eakin, M. D. Radcliffe, A. Callan-Jones, and R. A. Pelcovits, "Liquid-crystal diffraction gratings using polarization holography alignment techniques," Journal of Applied Physics 98 (2005).

3. J. N. Eakin, R. A. Pelcovits, G. P. Crawford, and M. D. Radcliffe, "Polarization holographic patterned alignment of nematic liquid crystals," Molecular Crystals and Liquid Crystals 438, 1749-1757 (2005).