It is widely believed that photonic crystals demonstrate transparency (the ability to transmit light without scattering or diffraction) only when the considered wavelength is much larger than the crystal’s lattice spacing. But researchers at Vrije Universiteit Brussel (Brussels, Belgium) have countered that finding by reporting on photonic crystals that are transparent (for TE polarized light), even when the photonic crystal’s lattice spacing is comparable to the free-space wavelength—that is, for normalized frequencies up to 0.9.
Together with colleagues from the University of Mons (Mons, Belgium) and the Leibnitz Institute of Photonic Technology (Jena, Germany), they used this phenomenon to design and fabricate a photonic-crystal fiber (PCF) with a holey cladding region that is transparent in the infrared (IR) wavelength region. The lack of diffraction and scattering in the microstructured cladding of the PCF allowed tight focusing of IR femtosecond laser pulses to the fiber’s core and, for the first time, the ability to inscribe fiber Bragg gratings through femtosecond laser point-by-point micromachining in a multi-ring hexagonal lattice PCF.
The discovery of anomalous transparency allows upscaling of photonic crystals by improving the manufacturing process and/or lowering their operational wavelength, which makes them amenable to applications in optical cloaking and graded-index guiding at shorter wavelengths than previously considered possible. Reference: T. Baghdasaryan et al., Nature Sci. Rep., 8, 5470 (Apr. 3, 2018).
