The authors gratefully acknowledge the financial support of the EPSRC

1. The laser with 8 spacer units A slope efficiency of ~ 20 % FFO8OCB + PM597 E7 laser FFO8OCB + DCM High slope efficiency liquid crystal lasers designed through material parameter optimisation A. D. Ford*, S. M. Morris, M. N. Pivnenko, C. Gillespie and H. J. Coles** Centre of Molecular Materials for Photonics and Electronics Electrical Engineering Division, Cambridge University Engineering Department, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, UK * adf27@cam.ac.uk ** hjc37@cam.ac.uk LC MATERIALS INTRODUCTION EXPERIMENTAL SET-UP The original concept of lasing from liquid crystals (LCs) was put forward by Goldberg and Schnur in the form of a patent in 1973 [1]. Despite some experimental work (i.e. Il’chisin et al [2] demonstrated the modification of fluorescent emission in the presence of a reflection band) there was, at that time, no unequivocal demonstration of lasing from a LC medium. In recent years lasing in LCs has been revisited as a result of the pioneering work of Yablonovitch [3] and independently John [4] on photonic band gaps. In 1998, Kopp et al [5] experimentally demonstrated lasing from LCs for the first time. A homologous series of non-symmetric bimesogens was used as the LC hosts. For comparison, E7 (commercially available from Merck) was also used. LC HOST + CHIRAL DOPANT + DYE = LC LASER The even spaced molecule. The odd spaced molecule. The director within a chiral nematic LC, rotates about a single axis where one full rotation describes the pitch (P). Light incident in a direction parallel to the helix axis with the same handedness as the helix and with wavelength of the order of P will be forbidden to propagate through the structure and thus is reflected. Consequently chiral nematic LCs are self-organising, one dimensional photonic structures. With the inclusion of a laser dye and under optical excitation, low threshold lasing occurs at the band edge provided that the band edge coincides with the spontaneous emission spectrum of the dye. This series exhibits an odd-even effect (observed in the physical parameters) which is a result of the molecular packing variation. The elongated shape of the even-spaced molecules, in the all-trans configuration, increases the packing density compared to the bent-core shape of the odd-spaced molecules. DCM laser dye was used. In this paper we use a homologous series of non-symmetric bimesogen LCs as the hosts where the only variation between molecules is the length and parity of the spacer chain [6]. We examine the emission properties from these bimesogenic lasers and compare them to the emission properties of a laser using E7 as the host (a commonly used host for LC lasers [7]). By optimising the material parameters we show a slope efficiency of ~ 20 % . The Isotropic – Nematic phase transition temperature as a function of the number of methylene units in the spacer chain. The birefringence as a function of the number of methylene units in the spacer chain. For each bimesogenic LC laser, the emission energy as a function of excitation energy was measured at a shifted temperature of 20 ºC. LC HOST OPTIMISATION The odd spaced lasers The even spaced lasers The threshold energy (Eth) can be written as [9, 10]: The equation DOES accurately accounts for the experimental data! Where Dn is the birefringence The slope efficiency (hs) can be written as: The equation DOES NOT fully account for the experimental data! To fully account for the slope efficiencies, addition factors must be considered. • A correlation was observed between the elastic moduli and the slope efficiency. • The percentage of chiral dopant required to position the long wavelength band edge to 610 nm is a measure of the twist elastic constant. • The elastic moduli are inversely related to director fluctuations and consequently scattering losses. • This is speculation and ongoing research is being conducted to ascertain the validity of this relationship. All even spaced lasers exhibit: • Higher emission energies • the laser with 8 spacer units exhibits the highest Lower threshold energies • Higher slope efficiencies • the laser with 8 spacer units exhibits the highest Enhanced LC laser emission energies and slope efficiency Large elastic moduli give rise to reduced director fluctuations. Reduced scattering losses within the LC laser system. The odd spaced lasers exhibit slope efficiencies 3 times larger than the E7 laser [6, 7, 8]. CONCLUSIONS LASER DYE OPTIMISATION • A LC host with high birefringence gives rise to a laser with low threshold energy. • Birefringence is not the only factor to influence the slope efficiency [11]. • A correlation between the host elastic moduli and the slope efficiency was noted. • An increase in the elastic moduli reduces the scattering losses within the system. • By optimising all material parameters, a slope efficiency of ~ 20 % was achieved. • To optimise the LC laser, the bimesogen with 8 spacer units (exhibiting the lowest threshold energy and the highest slope efficiency) is used as the host. • An alternative laser dye is used, PM597, for which the absorption peak is optimised to the pump wavelength, 532 nm. The emission energy as a function of excitation energy was measured at an shifted temperature of 20 ºC, and compared to the emission energy from the equivalent DCM laser. References [1] Goldberg L. S. and Schnur J. M., Nov. 6, 1973, United States Patent, 3771065 [2] Il’chishin et al, JETP Lett. (1980) 32, 24 [3] Yablonovitch, E., Phys. Rev. Lett. (1987) 58, 2059 [4] John, S., Phys. Rev. Lett., (1987) 58, 2486 [5] Kopp V. I. et al, Opt. Lett., (1998) 23 (21) [6] Ford A. D. et al, Proc. SPIE, (2005) 5741, 217 [7] B. Taheri,et al, Mol. Cryst. Liq. Cryst. (2001)358, 73 [8] Morris S. M. et al, Proc. SPIE, (2005) 5741, 118 [9] Cao W. et al, Mol. Cryst. Liq. Cryst, (2005), 429, 101 [10] Morris S. M. et al, J. SID, (2006) 14 (6) [11] Morris S. M. et alJ. Appl. Phys, (2005)97, 023103 The absorption spectra of DCM (solid black curve) and PM597 (dashed curve). The vertical dashed line indicates the pump wavelength. The authors gratefully acknowledge the financial support of the EPSRC