Fabrication and Physics of Low-Loss Photonic Crystal Slab Waveguides

Eiichi Kuramochi, Stephen Hughes, and Masaya Notomi
Optical Science Laboratory

 Recently, the two-dimensional photonic crystal slab (PCS) has attracted great interest because it has led to the development of high quality-factor photonic nanocavities and low-loss waveguides [1]. It has been revealed that a guided mode induced by a line defect in ideal PCS is theoretically lossless when it is designed to be inside the photonic band gap and under the light line. However, current nanolithography technology allows nanometer-order disorder and it is believed that such disorder causes a considerable loss increase due to out-of-plane scattering.
 One aim of our research is to demonstrate that waveguide loss can be reduced by reducing fabrication disorder. We patterned PCSs by using a precise electron beam lithography system with 1-nm position accuracy and 100-kV acceleration, where we paid great effort to minimizing the proximity effect. We observed the fabricated Si PCS with scanning electron microscopes and found that disorder (σ, RMS) was about 3 nm (Fig. 1). We measured the propagation loss of PCS waveguides by the cut-back method. The minimum loss measured was 5dB/cm (Fig. 2), which is record loss for photonic crystal waveguides [2].
 Another aim of our research is to reveal the physics of PCS waveguide loss. We applied the photon Green function tensor (GFT) formalism [3] to disorder-related scattering in the guided mode. We derived the loss formula as a simple product of the disorder and local density of states of light between the initial state and final state. We calculated the loss assuming σ of 3nm, and the result showed surprisingly good agreement with the experimental data (Fig. 2). This loss calculation by our GFT method is the most detailed ever achieved. We investigated the scattering-loss mechanism in detail by theory and experiment. We revealed for the first time that the loss characteristics of PCS waveguides are unique and complex because the loss mechanisms (out-of-plane scattering, intermode scattering, and backward scattering) are tightly bound with photonic band structure.
 In conclusion, by experiment and theory, we successfully revealed the detailed physics of scattering loss in PCS waveguides caused by fabrication disorder.
[1] M. Notomi et al., Optics Express 12, 1551 (2004).
[2] E. Kuramochi et al., LEOS2004, WF6 (2004).
[3] S. Hughes et al., Phys. Rev. Lett. 94, 033903 (2005).


Fig. 1. SEM images of Si PCSs fabricated in this study.
Fig. 2. Measured and calculated loss spectra of PCS waveguides.

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