Tunable Photonic Crystal Based on SOI

Self-collimation in photonic crystals (PhCs) has been demonstrated providing a very promising light-guiding mechanism. The fact that self-collimation allows light-guiding without any physical boundary is beneficial in high-density photonic integrated circuits (PICs) in terms of efficient coupling and arbitrary beam routing with no crosstalk. In this paper, we demonstrate a tunable photonic crystal device by combining the self-collimation lattice and band-gap lattice. DOI: 10.2529/PIERS060907215746 It has been well known that photonic crystals (PhCs) may exhibit very different dispersion properties with that of either host material or guest material [1–3]. For example, while the equalfrequency contours (EFCs) of an unpatterned Si slab are circular, EFCs of a Si slab with different periodic patterns may exhibit different shapes [4]. A EFC is a cross section of dispersion surfaces and a dispersion surface is a surface which characterizes the relationship between all allowed wave vectors in the structure and their corresponding frequencies. Recently, there has been a growing interest in engineering the dispersion property of photonic crystals for possible applications. One of the very interesting phenomenons found in planar photonic crystals (PhCs) during these explorations is the self-collimation phenomenon [4, 5]. This behavior in which incident waves with a certain angular range are naturally collimated along only certain directions has been utilized to efficiently guide electromagnetic waves within a planar PhC without the use of channel defects or structural waveguides. Compared to its alternatives, namely dielectric waveguides and PhC line defect waveguides, this type of waveguides offers many advantages since it does not require physical boundaries to confine light. For instance, it releases the strict alignment requirements imposed by the coupling efficiency in the case of dielectric or PhC line defect waveguides. As such, it enables high efficient in-plane coupling. On the other hand, due to lack of structural interaction, light path can arbitrarily cross each other without any crosstalk, which is very important for high density PICs to achieve arbitrary routing. To allow more functionalities integrated into the selfcollimation lattice and thus a self-collimation based PICs, in this paper we demonstrated tunability of self-collimation photonic crystals by free carrier injection [6, 7]. The studied device is illustrated in Fig. 1. This device consists of the self-guiding region and the tunable region. The self-guiding region is the square lattice of air holes patterned on the silicon slab. The radius of air holes is 0.3a, where a is the lattice constant. The dispersion surface and the dispersion contour of this lattice are obtained with plane wave method (PWM). An effective index of 3 is used for Silicon to take the finite thickness of the Silicon slab into consideration. The self-guiding lattice has an approximately square EFC at the frequency of a/λ = 0.3, where a is the lattice constant and λ is the guiding wavelength. Since a = 0.45μm, electromagnetic wave with wavelength of 1.5μm can thus be self-guided in the lattice. The tunable region is a square lattice of air holes with the same lattice constant in order to align with the self-guiding lattice to minimize the leakage along the boundary between the two lattices. The tunable lattice is designed such that it has a band gap; the self-guiding frequency is located at its band edge of the band gap without injected free carriers. Under this condition, the self-guiding beam completely passes through this region. On the other hand, when injecting free carriers into this region to slightly reduce the effective index of Silicon slab in this region, the band is pulled up and the self-guiding frequency is thus shifted into the band gap. Depending on how deep the self-guiding frequency is pulled into the band gap, a various percentage of the self-guiding beam passes through this region to port A, the rest of it is reflected to port B. Fig. 2 shows the band diagrams of the tunable lattice with different effective indices. To validate the overall device design, we simulate the device shown in Fig. 1 using the finitedifference time-domain (FDTD) method. Fig. 4 shows the simulation results. To demonstrate the performance of the device experimentally, it was fabricated on a Siliconon-insulator (SOI) wafer. The Silicon device layer has a thickness of 260 nm. The thickness of PIERS ONLINE, VOL. 3, NO. 5, 2007 575 Tuable R eion Self-G uiding Region