Photonic crystal based sensing using band edge modulation
Çakmak, Atilla Özgür (2005) Photonic crystal based sensing using band edge modulation. [Thesis]
A photonic crystal based structure is proposed for sensing exceptionally small refractive index changes of a background medium. In a typical photonic crystal, we show that the band edges and the defect band(s) (if present) are very sensitive to the dielectric contrast. Hence, a propagating electromagnetic wave at a particular frequency gains significant phase shift due to the index changes. When the phase shift is measured interferometrically, it is possible to infer refractive index changes as small as 10⁻¹¹ per lattice distance. We have mainly concentrated on the band edge shifts of the two lowest bands of a square lattice of dielectric rods. Calculations reveal that an optimized design of 100x100 rods could be utilized to sense an index modulation of ten to the power minus thirteen per square roots Hertz with a modulation bandwidth of 1 Hz, which is approximately 10⁵ times better than the resolution of novel waveguide based sensor configurations. We also report that defect modes in photonic crystals are even better candidates to be adapted for sensing applications. A further resolution enhancement on the order of 10 could be achived due to the high sensitivity of the defect bands. We have performed proof of principle experiments in the microwave regime with a photonic crystal wave of 7x7 alumina rods placed in a gas chamber, whose inner pressure can be adjusted with a nitrogen tank. Obtaining the index modulations by changing the inner pressure, we witnessed that the experimental results are in very good argeement with the theory at the band adges.
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