DOI: 10.1002/adma.200501849 Chiroptical Properties Induced in Chiral Photonic-Bandgap Liquid Crystals Leading to a Highly Efficient Laser-Feedback Effect** By Seiichi Furumi* and Yoshio Sakka Polarization control of the emission properties of organic and polymeric media has attracted considerable interest from a technological viewpoint because of its advantages in high- density photonic devices, such as light-emitting diodes and op- tical amplifiers, and optical information storage. [1] Much effort has been focused on the generation of circularly polarized (CP) emission, i.e., emissive light with different intensities for right-handed circularly polarized (R-CP) and left-handed cir- cularly polarized (L-CP) components, induced by the intrinsic structural asymmetry of chiral (optically active) compounds. Progress in this area has not been as rapid as that for linearly polarized emission due to the relative difficulties in preparing a helical molecular arrangement by conventional techniques such as mechanical stretching and buffing for uniaxial orienta- tion. It is well known that the introduction of enantiomerically pure chiral compounds or groups into a molecular system in- duces the chiroptical properties of optical rotatory dispersion (ORD) and circular dichroism (CD) in the ground state. In contrast, it is much more difficult to induce chiroptical proper- ties for CP emission in the excited state. Intriguing attempts have been made to increase the degree of CP emission by using p-conjugated polymers with chiral side chains. [2–6] How- ever, the degree of CP achieved is not sufficient for practical applications in dissymmetric light sources. In this context, cholesteric liquid crystals (CLCs) have the potential to form supramolecular helical assemblages because of their intrinsic twisting abilities. [7] The CLC mesophase emerges from intrinsically chiral compounds or mixtures in which at least one of the components is chiral. The CLC mesophase has an orientational order similar to nematic or smectic liquid crystals; however, the local director is con- strained in a layerlike structure and periodically rotates around a helical axis. When CLC materials are sandwiched between two substrates under homogeneous anchoring condi- tions, the helical axis is spontaneously aligned normal to the surface of the substrate. Because of its helical arrangement, the CLC cell shows negative birefringence along the helical axis. Selective reflection of light is the most important prop- erty exhibited by these helical CLC structures. For instance, when linearly polarized light propagates into a right-handed CLC cell along the helical axis, R-CP and L-CP light are re- flected and transmitted, respectively, under the following con- ditions: The maximum wavelength (k max ) and bandwidth (Dk) of the selective reflection are quantitatively determined by the physical parameters of the CLCs, such as the helical pitch length (p), average refractive index (n av ), and optical anisot- ropy (Dn), where k max = n av p and Dk = Dnp. When a fluores- cent dye is dissolved in the CLC media, in order to generate highly CP emission by utilizing selective reflection by the CLC, the CLC reflection band should be superimposed on the broad fluorescence band of the guest dye. A few researchers have reported broad CP emission with an opposite handed- ness to the molecular helical sense of the CLC host under illu- mination with nonpolarized light. [8–11] Recent advances in su- pramolecular helical CLC assemblies, in terms of self- organized 1D photonic bandgap (PBG) crystals, have led to new perspectives in the field of photonics. [12–24] Photoexcita- tion of dye-doped CLC cells with a pulsed laser beam causes stimulated laser emissions at the PBG edge(s) [12–24] and/or within the band [20,22] due to the internal laser-feedback effect. This mirrorless lasing from CLCs is currently a widely investi- gated research topic with the potential for a wide variety of technological applications. Thus far, more emphasis has been placed on the reduction of the lasing threshold excitation en- ergy (or peak power) by linearly polarized excitation for rea- lizing continuous wave (CW) excited laser devices. However, the chiroptical properties of the dye-doped CLCs still remain obscure from the viewpoint of molecular chirality. In this communication, we report the CP characteristics of the ground and excited states of an achiral (optically inactive) fluorescent dye embedded in CLC media with respect to the laser-feedback behavior arising from the chiral PBG effect of the cholesteric host. While carrying out detailed studies of the chiroptical properties, we have also established a simple strat- egy to enable mirrorless lasing action at an extremely low ex- citation peak power. In order to prepare the dye-doped CLC cells, we have used an achiral nematic liquid crystal, a chiral compound, and an COMMUNICATIONS Adv. Mater. 2006, 18, 775–780 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 775 – [*] Dr. S. Furumi,Dr. Y. Sakka National Institute for Materials Science (NIMS) 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan) E-mail: FURUMI.Seiichi@nims.go.jp [**] The authors express their sincere thanks to the reviewers for their kind attention and helpful comments. We acknowledge DuPont-Toray and Rodic for supplying polyimide films and liquid crystalline mate- rials, respectively, Dr. S. Yokoyama and Dr. A. Otomoof the Nation- al Institute of Information and Communications Technology (NICT) for valuable advice, and Dr. N. Tamaoki of the National Institute of Advanced Industrial Science and Technology (AIST) for measure- ment of the CD spectra. S. F. is extremely grateful for financial sup- port through an Academia Showcase Research Grant from the Japan Chemical Innovation Institute (JCII). Supporting Information is available online from Wiley InterScience or from the author.