Observation of extraordinary optical activity in planar chiral photonic crystals Kuniaki Konishi1, Benfeng Bai2, Xiangfeng Meng2,3, Petri Karvinen2, Jari Turunen2, Yuri. P. Svirko2, and Makoto Kuwata-Gonokami1* 1 Department of Applied Physics, University of Tokyo, and Core Research for Evolutional Science and Technology (CREST), JST, 113-8656, Tokyo, Japan 2 Department of Physics and Mathematics, University of Joensuu, P.O. Box 111, FI-80101 Joensuu, Finland 3 Department of Precision Instruments, Tsinghua University, Beijing 100084, China * Corresponding author:
[email protected]
Abstract: Control of light polarization is a key technology in modern photonics including application to optical manipulation of quantum information. The requisite is to obtain large rotation in isotropic media with small loss. We report on extraordinary optical activity in a planar dielectric on-waveguide photonic crystal structure, which has no in-plane birefringence and shows polarization rotation of more than 25 degrees for transmitted light. We demonstrate that in the planar chiral photonic crystal, the coupling of the normally incident light wave with low-loss waveguide and Fabry-Pérot resonance modes results in a dramatic enhancement of the optical activity. ©2008 Optical Society of America OCIS codes: (050.5298) Photonic crystals; (230.5440) Polarization-selective devices.
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1. Introduction The control of light with artificial structures is one of the key issues in modern photonics. Recent advances in nanotechnology have made such a control possible using periodic subwavelength dielectric structures, i.e. photonic crystals. In particular, various photonic devices with planar photonic crystals have been employed to control propagation and/or emission of light and have attracted a widespread attention within the last decade. Among them are ultrahigh quality factor optical micro-cavities and control of spontaneous emission [1,2], ultraslow light propagation wave guiding [3,4] and control of the beam pattern emitted by a semiconductor laser [5]. Conventionally, photonic crystals are composed of simple elements such as square- or cylindrical-shaped holes. Introduction of low-symmetry elements allow one to employ the concept of chirality in the photonic crystal design and to control the polarization state of light. In particular, three-dimensional (3D) chiral photonic crystals that possess a well defined sense of twist allow one to create a polarization-sensitive stop-band, i.e., materials that reflect a particular circularly-polarized component of the incident light wave. Despite a considerable complexity, such 3D chiral photonic crystals can be fabricated by using a layer-by-layer approach [6] or by a two-photon direct-laser-writing technique [7]. The control of light polarization using planar structures has been achieved with metal nanogratings [8-10] in which strong coupling of photons with surface plasmons enhance the three dimensional polarization effect [11,12] in quasi-two-dimensional structures. Surface plasmon resonances give rise to such strong optical effects as enhanced light transmission through sub-wavelength holes [13], suppression of light extinction [14] and other linear and nonlinear optical phenomena [15-17]. Unfortunately, despite a high optical rotation power, excessive losses do not allow chiral metal nanogratings to compete with 3D all-dielectric structures in the polarization control. For applications of quasi-two-dimensional structures for polarization control the beam that corresponds to the zeroth transmission order in the diffraction pattern is most important because it preserves direction of propagation of the incident light wave. However in recent experiments on the polarization-sensitive diffraction in dielectric chiral planar structures [18,19], no polarization effect has been observed in the zeroth diffraction order, neither in the reflected nor in the transmitted light. This experimental finding strongly contradicts to the rigorous diffraction theory, which predicts a strong polarization rotation for a zeroth diffraction order in transmission [12].
#93812 - $15.00 USD
(C) 2008 OSA
Received 13 Mar 2008; revised 17 Apr 2008; accepted 30 Apr 2008; published 2 May 2008
12 May 2008 / Vol. 16, No. 10 / OPTICS EXPRESS 7190
In this letter, we report on the observation of optical activity with extraordinary rotational power in planar chiral photonic crystals formed by a dielectric chiral sub-wavelength-period grating on a planar dielectric waveguide. From the dependence of the polarization effect on the angle of incidence, we found that Fabry-Pérot resonance and the coupling between dielectric waveguide and photonic crystals forms high-Q resonant modes i.e. photonic crystal waveguide modes, which lead drastic resonant enhancement of optical activity.
Fig. 1. (a). Structure of the on-waveguide planar chiral photonic crystal. The polarization ellipse of the zeroth transmitted light is schematically shown. ϕ is the angle between the polarization azimuth of the incident wave and Y-axis of the square lattice.
ψ
is the incident
angle for p-polarized light. Δ represents the polarization azimuth rotation angle. The thickness of the chiral pattern is t = 410 nm, the waveguide layer thickness is h = 820 nm, widths of the gammadion line and opening are w = 120 nm, l = 70 nm, and the period is 600 nm. The arrows besides ϕ and ψ indicate the direction of rotations. The layout of this figure describes the
case of ϕ = 0 and ψ = 0 . (b). SEM image of the on-waveguide planar chiral structure. The scale bar corresponds to 600 nm. (c). Polarization azimuth rotation Δ for normal incidence measured at 634nm. The measured polarization azimuth rotation Δ is fitted by an oscillating function of the incident polarization azimuth ϕ ; Δ = θ + δθ cos(ϕ + ϕ0 ) , where θ is chirality-induced rotation, square pattern,
δθ
