Low-threshold lasing is observed at the edge of the stop band of a one-dimensional structure-a dye-doped cholesteric liquid-crystal film. The mode closest to the edge has the lowest lasing threshold. The rates of spontaneous and stimulated emission are suppressed within the stop band and enhanced at the band edge. The ratio of right to left circularly polarized spontaneous emission is in good agreement with calculated density of photon states.

We demonstrate that twisting one part of a chiral photonic structure about its helical axis produces a single circularly polarized localized mode that gives rise to an anomalous crossover in propagation. Up to a crossover thickness, this defect results in a peak in transmission and exponential scaling of the linewidth for a circularly polarized wave with the same handedness as structure. Above the crossover, however, the linewidth saturates and the defect mode can be excited only by the oppositely polarized wave, resulting in a peak in reflection instead of transmission.PACS numbers: 42.70. Qs, 42.70.Df Localized modes produced by regular defects in periodic structures can serve as low threshold lasers [1], low loss waveguides, and narrow band filters in an expanding array of photonic devices [2,3]. When defects are randomly positioned, localized states are produced within the photonic band gap (PBG) by multiple scattering [4,5]. The photon dwell time at resonance with a defect state generally increases exponentially with the structure thickness L. This suppresses the lasing threshold in an active structure without the use of external feedback. In 3D PBGs, thresholdless lasing has been predicted when the band gap exceeds the emission bandwidth since emission can then couple only to the single defect mode [1]. Exceptionally low lasing thresholds have been achieved in vertical cavity surface emitting lasers (VCSELs), in which an amplifying defect layer is sandwiched between stacks of layers with periodically alternating refractive index [6]. A rich variety of physical effects have been observed when point, line and planar defects are introduced into periodic dielectric structures by the modifying refractive index or thickness of isotropic material. We consider here the unusual photonic properties of a defect created by twisting one part of an anisotropic structure relative to the rest of the sample. This "chiral twist" provides an additional degree of freedom for the design of photonic structures.Periodic chiral media appear in nature and can be synthesized as self-organized cholesteric liquid crystals (CLCs) [7] or fabricated by a variety of techniques, for example, by glancing-angle deposition on a rotating substrate [8]. In CLCs, the director, which is the average orientation of molecules in a plane, rotates with pitch P as shown in Fig. 1a. A reflection band forms for circularly polarized light with the same handedness as the structure itself. Light with opposite circular polarization is transmitted through the structure without attenuation. The edges of the band are defined by two sharp optical modes peaked at a wavelength inside the medium equal to P. These correspond to circularly polarized standing waves, in which the electric field in the bulk of the sample is aligned along either the ordinary or the extraordinary molecular axes [9]. It was in dyedoped CLCs that suppression of the density of states in a stop band and lasing in long-lived modes at the band edge were first demonstrated [10]. In analogy w...

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