Victor I. Kopp

Lasing in chiral photonic structures

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.

Twist defect in chiral photonic structures.

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.

Multichannel High-Bandwidth Coupling of Ultradense Silicon Photonic Waveguide Array to Standard-Pitch Fiber Array

A multichannel tapered coupler interfacing standard 250-μm-pitch low-numerical-aperture (NA) polarization-maintaining fiber arrays with ultradense 20- μm-pitch high-NA silicon waveguides is designed and fabricated. The coupler is based on an array of 12 dual-core glass waveguides on 250-μ m pitch that are tapered to a 20- μm pitch, simultaneously providing both pitch and spot-size conversion. At the wide end, the inner core matches the NA and mode profile of standard single-mode fiber. When drawn and tapered, the inner core “vanishes” and the outer core, surrounded by the clad, matches the NA and mode profile of the on-chip photonic waveguide. Ultradense high-efficiency coupling to an array of Si photonic waveguides is demonstrated using a 12-channel polarization-maintaining-fiber pigtailed tapered coupler. Coupling to Si waveguides is facilitated using SiON spot-size converters integrated into the Si photonic IC to provide 2-3-μm mode field diameters compatible with the tapered coupler. The tapered coupler achieves <; 1 dB coupling losses to photonic waveguides. Furthermore, eight-channel coupling is shown with less than -35 dB crosstalk between channels. Finally, a 640-Gb/s wavelength-division-multiplexing signal is coupled into four waveguides occupying 80 μm of chip edge, providing 160-Gb/s per-channel bandwidths.

Lasing from chiral photonic band gap materials based on cholesteric glasses

Glass-forming cholesteric liquid crystals were studied as promising dye-doped lasing materials at different pumping energies and temperatures. Cholesteric samples doped with laser dye pyrromethane 597 were pumped by Nd:YAG laser. Lasing was found to depend strongly on the vitrification rate of cholesteric samples, their temperature and multidomain structure. The lasing threshold and intensity as a function of thickness of cholesteric resonator are determined by two competing factors: narrowing of the band edge modes and increasing disorder.

Polarization properties of chiral fiber gratings

Recent experiments (Kopp et al 2007 J. Opt. Soc. Am. B 24 A48) have demonstrated that the polarization sensitivity of chiral fiber gratings depends strongly on the grating symmetry: double-helix fibers are polarization sensitive while single-helix fibers are not. A coupled-mode perturbation theory is developed and used to explain the polarization properties of chiral fiber gratings. Features of the transmission spectrum such as multiple dips in the spectrum and circular dichroism are also derived and attributed to chiral Bragg scattering of the core modes into the cladding modes of the fiber.

Large Coherence Area Thin-Film Photonic Stop-Band Lasers

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.

Single- and double-helix chiral fiber sensors

Copropagating core and cladding modes in optical fibers can be coupled by a grating with a period greatly exceeding the wavelength, since their propagation constants are similar. In contrast to conventional long-period gratings, in which the modulation is imposed by exposing a photosensitive core to ultraviolet light, we have created chiral long-period gratings with single- or double-helix symmetry by twisting optical fibers with nonconcentric or noncircular cores, respectively, as they pass through a short heat zone. The difference in symmetry between single- and double-helix gratings is manifested in their polarization properties. The use of these gratings as sensors of liquid level and temperature is demonstrated.

Chiral fiber gratings

Examples of adiabatic modification of optical fiber parameters while maintaining single-mode propagation are discussed in the paper. It is found that adiabatic variation of polarization of fiber mode enables efficient coupling of a PM fiber supporting linearly polarized modes and a chiral fiber supporting circularly or elliptically polarized modes

Chiral diffraction gratings in twisted microstructured fibers.

We observed dips in transmission spectra of uniformly twisted pure-silica microstructured fibers. The spectral positions of the dips and their insensitivity to the surrounding medium are consistent with Bragg diffraction from the helical structure. The reproducibility of the variation of the dip wavelength with temperature up to 1000 degrees C makes the chiral diffraction grating suitable for high-temperature sensing.

Double-helix chiral fibers

A polarization-selective photonic stop band is demonstrated in a new chiral fiber structure with double-helix symmetry. The stop band exists for only circularly polarized radiation with the same handedness as the structure and is centered at a wavelength in the fiber equal to the fiber pitch. When one part of the chiral fiber is twisted about its axis, a localized mode is produced, which can be tuned across the gap by changing the twist angle. Observations in single-mode fibers are in good agreement with one-dimensional simulations of a dispersive cholesteric material. At higher frequencies, however, we find a sharp onset of a broad polarization-selective scattering band, which is not present in one-dimensional simulations.