Negative Refraction in One-Dimensional Photonic Crystals with Tilted Interface

We investigate the wave propagation through the tilted interface of one-dimensional photonic crystals. Negative refraction can be realized by excitation of the Bloch states in the extended Brillouin zone with suppressed reflection. Equi-frequency surface analysis shows that the positive refraction, negative refraction or birefringence in this configuration can be achieved under a proper incident angle, which is confirmed by finite-difference timedomain simulations. The results may be useful in applications in the new devices based on one-dimensional photonic or optical waveguide arrays.     

Photoinduced linear and/or circular birefringences from light propagation through amorphous or smectic azopolymer films

The self-induced rotation of the azimuth of elliptically polarized light passing through birefringent azopolymer thin films is investigated. The experiments were carried out on thin films of the amorphous p(DR1M-co-MMA) and p(DR1M) azopolymer samples and of the p(6MAN) derivative in its glassy and liquid-crystalline phases. In fact, using various controlled input light ellipticities, linear birefringence (LB) and/or circular birefringence (CB) measurements were performed separately and in conjunction with polarization analyses of the transmitted pump beam. According to a general theoretical analysis based on Jones’ matrix formalism, it is thus shown that the induced rotation angle through the films depends mainly on the ellipticity of the input light, on the generated LB level and, to a lesser extent, on the CB photoinduced in the liquid-crystalline phase. In the latter case, it is concluded that irradiation with circularly polarized light does induce a chiral arrangement in the polymer film, although the photosensitive chromophores do not contain any optically active group. Received: 22 April 2002 / Revised version: 28 May 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-5/5684-8402; E-mail: csouri@morgane.lsmc.u-bordeaux.fr     

Temperature Dependence of Photoinduced Birefringence in an Azobenzene Polymer

The photoinduced birefringence in an azobenzene polymer is investigated at different temperatures between -20℃ to 50℃. It is found that there is a peak value of photoinduced birefringence in the temperature dependence of the photoinduced birefringence under a certain pumping intensity. With the pump light in 90mW/cm^2, the peak value of the photoinduced birefringence appeared at about 0℃ C. The effect of temperature on the photoinduced birefringence is discussed using the competition mechanism between the photoinduced reorientation and the thermal random motion.     

Photoinduced anisotropy in an azo-containing ionic liquid-crystalline polymer

Photoinduced anisotropy in an azobenzene ionic liquid-crystalline polymer was investigated through dichroism, birefringence and polarization holography. A dichroism degree of 1.58 and a birefringence value Δn ∼ 10⁻² were achieved in the polymer film at room temperature, and the polymer film was found to possess the characteristics of reversible and long-term optical storage. Particularly the stored birefringence could be enhanced to Δn ∼ 10⁻¹ by annealing the film, and it is attributed to the thermal self-organization of the molecules. Furthermore, linear- and circular-polarization holographic recordings were accomplished in the polymer film and pure polarization gratings were produced.     

Photoinduced macroscopic chiral structures in a series of azobenzene copolyesters

A study of the propagation of elliptically polarized light and the resulting formation of macroscopic chiral structures in a series of azobenzene side-chain copolyesters, in which the morphology is varied from liquid crystalline to amorphous, is reported. Real-time measurements are presented, showing the dynamic behavior of the photoinduced rotation of the polarization ellipse in the different samples. The relationship between the ellipticity of the recording light and the linear birefringence induced is studied. A numerical solution that takes into account the influence of the photoinduced linear dichroism on the light propagation through the samples is presented. Received: 10 June 2002 / Revised version: 26 July 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +45-4677/4588, E-mail: lian.nedelchev@risoe.dk RID="**" ID="**"Permanent address: Department of Chemistry and Physics, The Nottingham Trent Unversity, Clifton Lane, Nottingham NG11 8NS, England     

Bandwidth tuning of hybrid liquid-crystal Šolc filters based on an optical cancelling technique

We demonstrate that in addition to their role in tuning the wavelength of an N-stage hybrid liquid-crystal Šolc filter, liquid-crystal cells can also be used to vary the transmission bandwidth of such filter around any of the tuned wavelength. This bandwidth tuning is based on the variation of the number of stages by what we call here an “optical cancelling technique”. This is achieved by varying the birefringence of the liquid-crystal cells whose optical path difference switches between two particular values. We show that for a 10-stage filter and at λ_i = 1.532 μm, the calculated 3-dB bandwidth varies from 2.6 to 11.8 nm when the number of “optically-cancelled” hybrid plates increases from 0 to 8. During the tuning process, the contrast ratio remains equal to that of the equivalent classical Šolc filter.     

Photosensitive post tuning of chalcogenide Te20As30Se50 narrow bandpass filters

We present an experimental study on the photosensitive properties of narrow bandpass filters based on a chalcogenide Te₂₀As₃₀Se₅₀ (TAS) spacer. The transmittance curve of single TAS layer was shifted towards long wavelength direction after 2 h exposure by Xenon arc lamp. The refractive index and extinction coefficient were both increased together with a red shift of optical gap. A maximum 1.7% photo-induced effect was observed. Narrow band filters constituted by TAS and cryolite were manufactured by electron beam deposition. The transmittance spectrum of the filter during the exposure by a wide band source was in situ measured and the resonant wavelength was observed to turn longer gradually till saturation. A spatially localized central wavelength change up to 5.7 nm was finally obtained. The stability of the photo-induced effect was studied and some comments were given at the end of this paper.     

Giant Birefringence of Lithium Niobate Crystals in the Terahertz Region

Terahertz time-domaln spectroscopy （THz-TDS） is used to study the spectral response of lithium niobate crystals （LiNbO3） in the far infrared region. The optical constants are derived from the measured complex refractive index. A giant birefringence is observed in this material, and the average refractive-index difference between the ordinary wave and the extraordinary wave, no - ne, can reach up to about 1.6. Such a large birefrlngence is attributed to the different p honon modes orAl（z） and E （ x , y ）. This unusual property makes LiNbO3 a promising material to be used as a functional material in the terahertz region, e.g. employed as wave-plates and polarization separators.     

Rotations of eigenvibration direction in anisotropic crystals

The electric field of incident light induces dipoles in anisotropic media, vibrating in two perpendicular directions of the principal axes. Because of the tensor property of the dielectric constant, an induced dipole is subject to a torque, tending to rotate it about the axis parallel to the propagation direction. The directions of eigenvibration of the ordinary (o-ray) and extraordinary (e-ray) waves are no longer perpendicular in this sense. We propose here the relationships to describe the rotation of the induced dipole in the perpendicular electric fields. The rotation angles are found to increase with increasing dielectric constants and electric field strength of the incident light, exhibiting large values near the resonance frequencies in the infrared range at the azimuth angle /4 of the polarized incident light. Piezoelectric and ferroelectric crystals have a large value of the dielectric constant in the infrared frequency range. Rotations of the vibration direction of the o-ray and the e-ray waves are shown in the infrared transmission spectra recorded by incidence of the polarized light and transmission through piezoelectric and ferroelectric crystals (-quartz, LiNbO₃, and LiTaO₃). Interference of the o-ray and the e-ray waves transmitted through the crystals confirms the rotations of the vibration directions, a self-modulation effect of light in piezoelectric and ferroelectric crystals induced by the electric field of the propagating light.     

Polarization-selective grating mirrors used in the generation of radial polarization

Two novel methods to control the polarization of laser radiation are presented. The discrimination between different polarization distributions isperformed with a corrugation grating in the top high-index layer of a multilayer mirror, which couples the undesired polarization into a lossy waveguidemode of the multilayer. The generation of radially polarized radiation in a laser resonator is presented as a practical verification of the principle.This revised version was published online in May 2005. The Article Category was removed.This revised version was published online in August 2005 with a corrected cover date.     

Photonic Crystal Waveguide Intersection Based on Self-Imaging of Multi-Mode Interference

A new mechanism of intersection formed by two line defect photonic crystal （PC） waveguides are numerically investigated using the finite-diFference time-domain method. The results show that the normalized crosstalk is smaller than 10^-4; the reflection is smaller than 10^-3, and the transmission is larger than 0.999. The authors analyse the physical origins and find that a modified self-imaging process in the intersected multi-mode region is the main reason of the excellent performance. This kind of multi-mode interference based intersection may find potential applications in PC optical circuits.     

Photonic band gap properties of GaP opals with a new topology

In this paper, we propose that the “anomalous” optical response exhibited by GaP and InP infiltrated opals is due to the peculiar morphology shown by these materials when grown within the pores. In order to account for their optical response, we propose a new structural model consisting of a network of high dielectric spheres located in the pores of the bare opal, interconnected by cylinders of the same material. A fair agreement between the theoretical predictions using this model and the experimental measurements has been found. We also show that the inverse structure presents very interesting optical properties.     

Biphotonic effect of azo-dye-doped liquid crystals using the sequential Z-scan technique

This study investigates the biphotonic effect of azo-dye-doped liquid crystals (ADDLCs) using the sequential Z-scan technique. A spot on the sample is illuminated by a green light for 6 s, and then the same spot is illuminated simultaneously with a red light and a green light for 6 s. Measurements are made by scanning the sample near the beam waist of the green laser. The results show that the biphotonic effect is important to the nonlinear coefficient of the sample. The variations of the optical Kerr constant with intensity of red light are measured. Measurement results demonstrate that the molecular reorientation of liquid crystals induced by the photoisomerization of the azo dyes dominates at low red-light intensity, but the thermal effect compensates for the molecular reorientational nonlinearity of the sample at high red-light intensity.     

Near-Field Birefringence Response of Liquid Crystal Molecules in Thickness Direction of Liquid Crystal Thin Film Orientated by Shear Force

Information of molecular orientation in nematic liquid crystal （LC） is attractive and important for applications in the field of display devices. We demonstrate a novel method using a birefringence scanning near-field optical microscope （Bi-SNOM） with a probe which is inserted into the LC thin film to detect the molecular orientation from its birefringence responses in the thickness direction of the LC thin film. The probe is laterally vibrated when going forward into the LC thin film, and the retardation and azimuth angle are recorded as the probe going down. Firstly, the thickness of the LC thin film is measured by the shear force detection. Since the shear force acts as a stimulation to reorientate the LC molecules above the substrate surface, we can detect the molecular orientation caused by a polyimide alignment substrate and the effect to molecular orientation caused by vibration of fibre probe. As a result, the orientation profiling of the LC film in depth direction is obtained in both the cases that the direction of probe vibrating is vertical/parallel to the rubbing direction of the alignment film. Furthermore, the thickness of completely orientated layers just above the substrate surface can also be obtained by either vibrating probe or no-vibrating probe. Ultimately, the LC thin film can be modelled in thickness direction from all the results using this method.     

Efficient transmission in cascaded photonic crystal waveguides via a strong coupling effect

A design of cascaded photonic crystal waveguide is proposed in this paper inspired by the work of Tang et al. [D. Tang, L. Chen, W. Ding, Appl. Phys. Lett. 89 (2006) 131120]. In contrast to a conventional waveguide source, a plane wave source is applied in the current design. We show that an efficient guide mode in the photonic band gap can be achieved. The same idea also works for a slight variation by defects introduction in the photonic crystal. Finally, the strong coupling effect present in the cascaded waveguides is demonstrated by an analogy with photonic quantum wells.     

Roughness losses and volume-current methods in photonic-crystal waveguides

We present predicted relative scattering losses from sidewall roughness in a strip waveguide compared to an identical waveguide surrounded by a photonic crystal with a complete or incomplete gap in both 2d and 3d. To do so, we develop a new semi-analytical extension of the classic “volume-current method” (Green’s functions with a Born approximation), correcting a longstanding limitation of such methods to low-index contrast systems (the classic method may be off by an order of magnitude in high-contrast systems). The resulting loss predictions show that even incomplete gap structures such as photonic-crystal slabs should, with proper design, be able to reduce losses by a factor of two compared to an identical strip waveguide; however, incautious design can lead to increased losses in the photonic-crystal system, a phenomena that we explain in terms of the band structure of the unperturbed crystal.     

Topological structure in polarization resolved conoscopic patterns for nematic liquid crystal cells

We investigate the polarization structure of coherent light, produced by a convergent light beam transmitted through nematic liquid crystal (NLC) cells with different director configurations. Employing solutions to the transmission problem for the case when plane wave propagates through an anisotropic layer, we analyze the arrangement of the topological elements, such as polarization singularities (C points with circular polarization and L lines with linear polarization), saddle points and extrema of polarization azimuth. We observe transformations of the topological structure under the variation of the incident light ellipticity and represent it by corresponding trajectories of topological elements in three-dimensional space. For the cells with uniform and non-uniform director configuration we describe the processes of creation/annihilation of C point pairs, which can be controlled precisely in the case of the cell with non-uniform director. Our experimental measurements for the homeotropically oriented NLC cells are in good agreement with the theoretical predictions.     

Effective dielectric constant of two-dimensional photonic crystals in optical bands

By using the Fourier expansion method, we have developed an approach to calculate the effective dielectric index of a two-dimensional photonic crystal. The approach is very general: it can take into account various Bravais lattice structure as well as arbitrary spatial variation of the dielectric index. It has been found that near a nondegenerate frequency ω_n(Γ) at Γ point, the transverse magnetic (TM) mode is ordinary, as it is independent of the propagation direction, whereas in general the transverse electric (TE) mode depends on the propagation direction, it is extraordinary. Therefore, a two-dimensional photonic crystal can always be described by an effective dielectric index for TM mode near the nondegenerate frequency ω_n(Γ). However, the TE mode is much more complicated unless the lattice structure is highly symmetric. Moreover, a two-dimensional square photonic crystal has been identified as an effective birefringent crystal having two negative refractive indexes from the perspective of Snell's law.     

Photonic band gap in the visible range in a three-dimensional solid state lattice

We report on the photonic band gap effect in the visible range in a three-dimensional dielectric lattice formed by closely packed silica spherical clusters and by interconnected cavities filled with various liquids. The spectral position and the spectral width of the optical "stop-band" depend on the lattice period and on the relative sphere/cavity refraction indexn. The stop band peak wavelength shows a linear dependence on n. Transmission characteristics of the lattice have been successfully simulated by numerical calculations within the framework of a quasicrystalline approximation.     

Modified Spontaneous Emission from Dye Molecules inside a Photonic Crystal Microcavity

We fabricate a photonic crystal microcavity containing Alq3 in a sandwiched structure by the self-assemble method. The angle-dependent photoluminescence （PL） spectra and the variation of the PL lifetime demonstrate the effect of the photonic band gap on the spontaneous emission of Alq3 in the photonic crystals.