Compact All Pass Transmission Filter using Photonic Crystal Slabs
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L2.3.1
Compact All Pass Transmission Filter using Photonic Crystal Slabs
Wonjoo Suh and Shanhui Fan Department of Electrical Engineering, Stanford University, Stanford, CA 94305
Abstract We show that both the coupled photonic crystal slab and the single photonic crystal slab structure can function as an optical all-pass transmission filter for normally incident light. The filter function is synthesized by designing the spectral properties of guided resonance in the slab. We expect this compact device to be useful for optical communication systems. Introduction Compact optical filter structures are of great interest for optical communication applications. In particular, optical all-pass transmission filters, which generate significant delay at resonance, while maintaining 100% transmission both on and off resonance have been useful for applications such as optical delay or dispersion compensation[1][2] and the demand for making compact optical filters with these characteristics is currently increasing. In all pass reflection filters such as Gires-Tournois interferometers, proper signal processing is needed in signal extraction. Also, cascading multiple devices to obtain a high capacity delay line remains a challenge in this reflection mode. Here, we introduce a new type of optical all-pass transmission filter based upon guided resonance in photonic crystal slabs, which consist of a periodic lattice of air holes introduced into the dielectric slab.
Operating Mechanism
Guided resonances in photonic crystal slabs [3]-[10] provide a very compact way to generate useful spectral functions for externally incident light.
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Figure 1. (a) Schematic of a photonic crystal filter consisting of a single photonic crystal slab. The arrow represents the direction of the incident light. (b) Schematic of a theoretical model for a resonator system that supports two resonant states with opposite symmetry with respect to the mirror plane perpendicular to the incident light.
An example of a photonic crystal slab consists of a periodic array of air holes introduced into a high index dielectric slab, as shown in Figure 1(a). Wang and Magnusson showed that a slab can function as a notch filter with a Lorentzian reflection line shape, when the slab thickness is appropriately chosen and a single resonance is placed within the vicinity of the signal frequency[11]. However, the spectral response has a strong variation in the intensity and therefore the filter is in the reflection mode. Therefore, in order to obtain 100% transmission while maintaining the group delay on resonance, we need two resonances that are degenerate. With two degenerate resonances with opposite symmetry, we are able to cancel out the effect of the strong variation in the transmission intensity. This is schematically shown in Figure 1(b), and this is referred to as accidental degeneracy.
Coupled Slab All Pass Filter
First we demonstrate a coupled photonic crystal slab configuration as a physical realization of creating an even and odd mode to generate a large
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