The situation is markedly different for researchers working in other parts of the electromagnetic spectrum, e.g. terahertz radiation (~0.1-10 THz). This spectral region is interesting due to its use for spectroscopic analysis in e.g. materials research, as many systems of interest have characteristic lifetimes on the picosecond scale. Security systems are also often mentioned as a potential application by using THz-imaging to visualize metal objects through clothes, and THz spectroscopy to detect the presence of specific chemicals in a box, be it drugs or explosives, without even opening the package.
So far, researchers working with THz radiation do not yet have all the near-infrared equivalents of photonic components to simplify their development of complex setups. However, bendable “fibers” (some mm thickness) with relatively low loss have been recently demonstrated made of the polymer Topas (Nielsen et al., Opt. Express 17, p. 8592, 2009). The present work by Zhou et al. adds another essential component to the toolbox of THz-researchers: a fiber Bragg grating that can be used for selective frequency filtering or sensing.
The fiber is first fabricated by extruding the polymer Topas to a fiber of either 0.3 or 0.6 mm diameter. The grating is then written in the fiber by simply using a UV laser to ablate notches into the side of the fiber with a pitch of either 361 µm or 509 µm. The researchers demonstrate that one of their Bragg gratings has a 60 dB reflectivity at 0.265 THz.
This work allows THz-researchers to make selective frequency filtering in a relatively low loss waveguide which also tightly confines the radiation. The tight confinement allows strong interaction of the electromagnetic field with a small sample of e.g. a liquid in which the grating is immersed, thus enabling development of new exciting sensing applications in the THz-regime.
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