Photonic Band Gap Materials: Light Control at Will




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Color Production in Natural Amorphous Photonic Structures



Jian Zi

Department of Physics, Fudan University, Shanghai 200433

Email: jzi@fudan.edu.cn


Abstract


Structural coloration is widespread in the biological world, produced by photonic structures via optical phenomena. Intriguing examples include periodic photonic structures (photonic crystals) which produce iridescent structural colors. Color production for these photonic-crystal structures is due to partial photonic band gaps. The directional dependence of the partial photonic band gaps is responsible for iridescence.

It has been found that some bird feathers contain amorphous photonic structures (APSs) and hence display non-iridescent structural coloration. The origin of such non-iridescent structural coloration has long been controversial. Mechanisms proposed include incoherent scattering such as Rayleigh and Mie scatterings and coherent scatterings. In this talk, we would like to present our experimental and theoretical studies on natural APSs and to show the ultimate mechanism of non-iridescent structural coloration by APSs.


Fabrication of photonic crystals by two-photon polymerization with femtosecond laser pulses



Yan Li, Haibo Cui, Zhaopei Liu, Hong Yang and Qihuang Gong

State Key Laboratory for Mesoscopic Physics & Department of Physics, Peking University,

Beijing 100871, China


Abstract


Femtosecond laser induced two-photon photopolymerization has been utilized to fabricate various 3D structures with sub-diffraction-limit feature-sizes. When the near-infrared pulses are tightly focused into a UV curable resin, a highly localized chemical reaction results in an organic crosslinking, i.e. the material is transformed from the liquid into the solid state. Since the process of multiphoton absorption depends nonlinearly on the light intensity, the interaction region is limited to the focal volume, while outside of the focus the material stays unchanged. Therefore, the feature size can be much smaller than the wavelength. In addition, by moving the focus in three directions, arbitrary 3D structures like photonic crystals with the sub-diffraction-limit resolution can be created.

To fabricate a perfect woodpile photonic crystal consisting of piles of nanorods, one challenge is to polymerize uniform nanorods with the reduction of the feature-sizes and the spacing for visible wavelengths. Recently, we have paid much attention to the suspended polymerized rods. Their feature sizes were reduced less than 20 nm by controlling the incident laser power and the laser focus scan speed. With the reduction of the feature size, distortions of the nanorods produce more pronounced detrimental effects on the performances of the structures. For example, the transmission spectra of polymerized photonic crystals indicate that the appearance of bandgaps, especially when using low refractive index materials, is very sensitive to all kinds of disorder such as roughness of the rods, aspect ratio, or rod thickness. To fabricate uniform structures, several techniques have been proposed to smooth down the surface, to control the aspect ratio, to compensate for the shrinkage , and to address the stiction problem.

We observed novel phenomena such as nonuniform shrinkage, stretching, tapering of nanorods. We found that the fabricated structures shrank after polymerization and rinsing as reported before, but the suspended nanorods were also stretched due to the shrinkage of the supports. The shrinkage and extension strains were measured by adding periodical points onto the nanolines. Suspended nanorods gradually thickened in the scanning direction when the scanning speed was faster than 60 m/s. A bidirectional scanning technique was proposed to effectively eliminate the size difference, resulting in uniform suspended nanorods.We investigated the stiction phenomenon in which suspended nanorods would stick to each other by the capillary force when the spacing was too close. We obtained a modified model to calculate the detachment lengths that was nicely confirmed by the experimental results.


Slow light propagation in semiconductor heterostructures


Guoxiang Huang

State Key Laboratory of Precision Spectroscopy,

and Department of Physics, East China Normal University, Shanghai

200062, China

Institute of Nonlinear Physics, and Department of

Physics, Zhejiang Normal University, Zhejiang 321004, China


Abstract


We investigate the formation and propagation of optical solitons in an asymmetric double quantum well structure. Using a standard method of multiple scales we derive a nonlinear Schroedinger (NLS) equation with some high-order correction terms that describe effects of linear and differential absorption, nonlinear dispersion, delay response of nonlinear refractive index, and third-order dispersion of a probe field. We show that in order to make slowly varying envelope approximation be valid an excitation scheme of interband transition should be adopted. We also show that for realistic quantum-well parameters the probe field with time length of picosecond or shorter must be used to make dispersion and nonlinear lengths of the system be smaller than absorption length, only by which a shape-preserving propagation of optical solitons is available. In addition, we clarify validity domains for the perturbed NLS equation as well as the high-order NLS equation and provide various optical soliton solutions in different regimes both analytically and numerically. We demonstrate that the solitons obtained have ultraslow propagating velocity and can be generated under very low input light intensity [1].


[1]. Chengjie Zhu and Guoxiang Huang, Phys. Rev. B, accepted for publication.


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