High-QOne-Dimensional Photonic Crystal Nanocavity in a Silicon/SOI Platform

Eiichi Kuramochi, Takasumi Tanabe, Laurent-Daniel Haret,

Hideaki Taniyama, and Masaya Notomi

Optical Science LaboratoryIt is well known that a suitably designed chain of submicron holes in a silicon submincron wire-like waveguide fabricated on a silicon-on-insulator (SOI) substrate acts as a nanocavity. It belongs to one-dimensional (1D) photonic crystal (PhC) nanocavities and has attracted attention because it has the simplest design and smallest footprint. However, compared to two-dimensional (2D) PhC nanocavities, which have been highly developed and widely used, the 1D nanocavity have a relatively poor quality factor (

Q). In this study, applying the mode-gap confinement approach, which had achieved great success in 2D nanocavities, we found solutions for realizing an ultrahighQvalue in a 1D nanocavity [1].

We studied 1D nanocavities consisting of rectangular (R) and circular holes (C). The buried oxide (BOX) layer underneath the nanocavity was removed in an air-bridge structure (AB) and preserved in a SOI structure. The size of the holes was continuously modulated to generate a mode-gap barrier. Electro-magnetic simulations using the 3D finite-domain time-difference (FDTD) method demonstrated that both structures held ultrahighQvalues exceeding 10^{8}and that the C nanocavities had a modal volume (V) smaller than 1(λ/n)^{3}(Fig. 1) [1, 2]. Note that even in a SOI structure, our design allows an ultrahighQof 〜10^{8}, which is a unique feature of the 1D nanocavity. These numerical predictions were experimentally demonstrated by measuredQof 3.6×10^{5}in the SOI C and 7.2×10^{5}in AB C nanocavities (Fig. 2) [2].

Since air acts as thermal insulator, a 1D AB nanocavity has a considerably large heat resistance, which was demonstrated by numerical simulation. In such a nanocavity, thermo-optic nonliniearity is enhanced greatly. We observed the lowest thermo-optic bistabilty onset power of 1.6 µW in a AB R nanocavity (Fig. 3) which had a high opticalQof 2.2×10^{5}[3]. Considering the great structural difference between the 1D and the 2D nanocavities, we can expect unique advantages and applications of the former.

This work was partly supported by CREST of Japan Science and Technology Agency.[1] M. Notomi et al., Opt. Express

1611095 (2008).

[2] E. Kuramochi et al., Opt. Express1815859 (2010).

[3] L. D. Haret et al., Opt. Express1721108 (2009).

Fig. 1. Calculated QandV.

Fig. 2. Spectra of high- Qmode in

the nanocavities.

Fig. 3. Thermo-optic bistability

onset of a nanocavity mode.

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