Anomalous transparency in photonic crystals and its application to point-by-point grating inscription in photonic crystal fibers

Tigran Baghdasaryan,Kay Schuster,Jens Kobelke,Thomas Geernaert,Karima Chah,Christophe Caucheteur,Hugo Thienpont,Francis Berghmans

doi:10.1038/s41598-018-23867-5

Tigran Baghdasaryan, Kay Schuster + Show 6 more

Open Access

https://doi.org/10.1038/s41598-018-23867-5

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Abstract

It is common belief that photonic crystals behave similarly to isotropic and transparent media only when their feature sizes are much smaller than the wavelength of light. Here, we counter that belief and we report on photonic crystals that are transparent for anomalously high normalized frequencies up to 0.9, where the crystal’s feature sizes are comparable with the free space wavelength. Using traditional photonic band theory, we demonstrate that the isofrequency curves can be circular in the region above the first stop band for triangular lattice photonic crystals. In addition, by simulating how efficiently a tightly focused Gaussian beam propagates through the photonic crystal slab, we judge on the photonic crystal’s transparency rather than on isotropy only. Using this approach, we identified a wide range of photonic crystal parameters that provide anomalous transparency. Our findings indicate the possibility to scale up the features of photonic crystals and to extend their operational wavelength range for applications including optical cloaking and graded index guiding. We applied our result in the domain of femtosecond laser micromachining, by demonstrating what we believe to be the first point-by-point grating inscribed in a multi-ring photonic crystal fiber.

Highlights

Photonic crystals (PhCs) are man-made wavelength scale optical media enabling unprecedented control over the propagation of light[1]
We first demonstrate that a PhC with specific feature sizes can behave in an isotropic manner for a range of normalized optical frequencies taken above the first directional stop band, using regular photonic band theory
After unfolding the irreducible Brillouin zone, one can use the symmetry properties of the lattice to reconstruct the mode map for the complete Brillouin zones[36,37]. This essential procedure is well-known in the field of solid state physics[34], it is often omitted in the photonics domain, which essentially complicates the interpretation of the dispersion properties using folded diagrams

Summary

Introduction

Photonic crystals (PhCs) are man-made wavelength scale optical media enabling unprecedented control over the propagation of light[1]. For particular applications such as transformation optics[12,13,14,15] and metamaterials[15,16,17,18,19], one prefers working with PhCs that behave to a homogeneous medium in terms of transparency and isotropy Those typically work in the so-called ‘low frequency’ region, where the period and feature sizes of the PhC are much smaller than the wavelength of light, so that light propagating through the PhC experiences a spatially varying effective refractive index, as it essentially sees the spatial average of the structured medium instead of the individual domains of the lattice[16,17,20]. Using our findings we were able to design and to manufacture a PCF with a cladding that is transparent for the grating writing beam, and we inscribed what we believe to be the first infrared femtosecond laser pulse point-by-point Bragg grating in a multi-ring PCF32

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