Enlargement of omnidirectional photonic bandgap in porous silicon dielectric mirrors with a Gaussian profile refractive index

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Abstract
For enhancing the omnidirectional photonic bandgap (OPBG), we report the fabrication of two different configurations of one-dimensional, wavelength scalable dielectric multilayer structures of porous silicon, consisting of a unit cell formed by varying the refractive index of the multilayers according to the envelope of a Gaussian function. As compared to the already reported OPBG of 88 nm (in the complete angular range of 0° to 89°), an enhancement up to 204 nm (2.3 times) was observed on stacking, six different Gaussian structures (balanced mirror) with only 8 periods each. An unbalanced mirror structure, consisting of the six similar Gaussian structures as the balanced mirror, but having different sequence of periods, (configuration with 13, 6, 5, 5, 6, and 13 periods for each Gaussian, respectively) was seen to demonstrate the OPBG of 252 nm (enhanced by 2.86 times). The total optical thickness of both the structures was kept to be the same. The omnidirectional nature of the PBG was verified experimentally up to 68° and theoretically up to 89.9° angle of incidence.

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