Zhao-Qing Zhang

Publications (2)0 Total impact

• Source

  Available from: arxiv.org

  Article: Large Coherence Area Thin-Film Photonic Stop-Band Lasers

  Victor I Kopp, Zhao-Qing Zhang, Azriel Z Genack
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  ABSTRACT: We demonstrate that the shift of the stop band position with increasing oblique angle in periodic structures results in a wide transverse exponential field distribution corresponding to strong angular confinement of the radiation. The beam expansion follows an effective diffusive equation depending only upon the spectral mode width. In the presence of gain, the beam cross section is limited only by the size of the gain area. As an example of an active periodic photonic medium, we calculate and measure laser emission from a dye-doped cholesteric liquid crystal film.
  09/2000;
• Source

  Available from: physics.qc.cuny.edu

  Article: Lasing in chiral photonic structures

  Victor I. Kopp, Zhao-Qing Zhang, Azriel Z. Genack
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  ABSTRACT: This article presents a review of the lasing and photonic properties of periodic one-dimensional anisotropic structures with the symmetry of a double helix. Examples are self-organized cholesteric liquid crystals (CLCs) and sculptured thin films created by vapor deposition. A reflection band with sharp, closely spaced transmission peaks at its edges occurs for circularly polarized light with the same handedness as the helical structure. Within the reflection band, this wave is evanescent, corresponding to a vanishing density of states (DOS). Oppositely polarized light is uniformly transmitted. Since optical emission is proportional to the DOS, it is suppressed within the reflection band. However, it is enhanced at the band edge, where a series of narrow long-lived transmission modes are found. For this reason lasing in dye-doped CLCs occurs at the edge of the stop band rather than at its center, where reflection is highest. Introducing an additional rotation or an isotropic layer within a chiral structure creates a single circularly polarized localized mode with the same handedness as the structure. A resonance appears in the transmission of light of this polarization in thin samples. In thicker samples, the resonance appears instead in the reflection for oppositely polarized light. In contrast to strong modulation of the intensity within the sample on a wavelength scale, a characteristic of layered dielectric medium, the intensity within a chiral medium varies slowly when the sample is excited either at the band edge or at a localized mode. A transverse coherence is created in emission over a length scale proportional to the square root of the photon dwell time at resonance with long-lived modes. This makes possible spatially coherent lasing over a large area in thin films. The photonic properties of chiral thin films make them promising candidates for a variety of filter and laser applications.
  Progress in Quantum Electronics.