Polarization Stop Bands in Chiral Polymeric Three‐Dimensional Photonic Crystals

Abstract

Chiral 3D photonic crystals are an interesting subclass of 3D photonic crystals. For example, large complete 3D photonic bandgaps have been predicted for high-index-contrast silicon square-spiral structures; [1,2] corresponding experiments using glancing-incidence deposition, [3,4] interference lithography, [5] or direct laser writing [6,7] have been published. In addition to complete gaps or stop bands, theory [8] also predicts polarization stop bands, i.e., stop bands for just one of the two circular polarizations. Such polarization stop bands can give rise to strong circular dichroism, [4,9,10] which can potentially be used for constructing compact "thin-film" optical diodes. [11] In this report, we fabricate high-quality polymeric 3D spiral photonic crystals via direct laser writing. [12][13][14] The measured transmittance spectra of these low-index-contrast structures reveal spectral regions where the transmittance is below 5 % for one circular incident polarization and above 95 % for the other-for just eight lattice constants along the propagation direction. The experimental data are compared with scattering-matrix calculations [15,16] for the actual finite structures, leading to good agreement.For what conditions do we expect strong circular dichroism? For circular polarization of light, the tip of the electricfield vector simply follows a spiral. The pitch of this spiral is just the material wavelength k. Thus, intuitively, we expect a chiral resonance from spiral photonic crystals if the pitch of circularly polarized light matches the pitch of the dielectric spirals, i.e., the lattice constant a z . This condition, k/a z = 1, corresponds to the edge of the second Brillouin zone, i.e., to a wave number k z = 2p/k = 2p/a z . Recall that the edge of the first Brillouin zone is at k z = p/a z . Thus, one does not anticipate a strong chiral response around and below the fundamental stop band (or bandgap), but rather at higher frequencies. Theory [8] for high-index silicon-based structures confirms this intuitive reasoning. We have repeated similar calculations for low-index-contrast polymeric structures, revealing essentially the same trends. The parameters of the 3D spiral photonic crystals to be discussed below are the result of an optimization with respect to circular dichroism.The samples in our experiments are made by direct laser writing, which essentially allows for the fabrication of almost arbitrarily shaped 3D photoresist structures. [12][13][14] Details of our process based on the commercial thick-film resist SU-8 can be found in the Experimental section and in earlier work. [13] Our structures are mechanically supported by a 2D network of bars at, or close to, the top of the 3D crystal. As the spirals are not at all mechanically connected to their neighbors, very unstable low-quality structures would result without this grid. Furthermore, all the structures for optical experiments are surrounded by a thick massive wall (see Fig. 1a), which aims at reducing the effects of strain on the 2D grid cau...

Metamaterials: Optical Activity without Chirality

“…The effect has enormous importance for analytical chemistry, crystallography, molecular biology, and the food industry and is also a signature effect used to detect life forms in space missions. The recognition of chirality as a source negative refraction of light [1][2][3][4][5][6] needed for the creation of a perfect lens [7] inspired intense work in developing microwave and optical artificial chiral metamaterials [8][9][10][11][12] and yielded a demonstration of negative index due to chirality [8,13]. In this paper we present a somewhat surprising result that very strong optical activity may be seen in a metamaterial system consisting of meta-molecules that itself are not chiral.…”

“…differential absorbtion for left and right circular polarizations. The recent effort in creating artificial optically active metamaterials that aimed to achieve strong optical activity was focused on different types of arrays of 3D-chiral meta-molecules [8][9][10][11][12][14][15][16][17]. It is significantly less acknowledged that optical activity can also be seen when oriented non-chiral molecules make a chiral triad with the wave vector of light.…”

Optical activity in extrinsically chiral metamaterial

“…[3][4][5][6][7][8][9] Although not widely known, optical activity in the form of circular birefringence and dichroism can also be observed in extrinsically chiral systems. In such a system a non-chiral structure together with the incident wave forms a geometrical arrangement that cannot be superimposed with its mirror image and thus the whole arrangement is chiral.…”

Materials by design: Using architecture in material design to reach new property spaces

“…51 Wegener and co-workers pioneered the use of DLW TPL and double inversion to generate "woodpile" PhC architectures that contain defect cavities and act as resonators. 58 Direct laser writing also enables the fabrication of arbitrarily complex micro-and nanostructures and has been exploited to generate chiral PhCs with polarization stopbands 59 and 3D photonic quasicrystals 60 that exhibit novel photonic properties not available in strictly periodic structures. Fabrication and characterization of tunable 3D PhCs is an area of increasing research interest.…”