Reduced propagation loss in silicon strip and slot waveguides coated by atomic layer deposition

T Alasaarela,S Honkanen,A Tervonen,R Palmer,W Freude,A Säynätjoki,L Alloatti,D Korn,J Leuthold

doi:10.1364/oe.19.011529

Abstract

When silicon strip and slot waveguides are coated with a 50 nm amorphous titanium dioxide (TiO2) film, measured losses at a wavelength of 1.55 μm can be as low as (2 ± 1)dB/cm and (7 ± 2)dB/cm, respectively. We use atomic layer deposition (ALD), estimate the effect of ALD growth on the surface roughness, and discuss the effect on the scattering losses. Because the gap between the rails of a slot waveguide narrows by the TiO2 deposition, the effective slot width can be back-end controlled. This is useful for precise adjustment if the slot is to be filled with, e. g., a nonlinear organic material or with a sensitizer for sensors applications.

Highlights

Silicon photonic waveguides have gained a lot of interest over the past years
We previously suggested using conformal atomic layer deposition (ALD) for coating slot waveguides with amorphous titanium dioxide (TiO2) [5]
We demonstrate to which extent the losses of strip and slot waveguides can be reduced when covering them with a thin TiO2 layer, and in particular we show the first experimental characterization of slot waveguides with ALD controlled slot dimensions

Summary

Introduction

Silicon photonic waveguides have gained a lot of interest over the past years. Mature processes are available for fabricating electronics using the silicon platform, and it is highly promising to exploit the same technology for photonics at wavelengths near 1.55 μm. Our strip and slot waveguides were fabricated on silicon-on-insulator (SOI) wafers, where the index contrast between silicon (Si) with a refractive index of nSi = 3.48 and silicon dioxide (SiO2) with nSiO2 = 1.46 amounts to Δn ≈ 2.0 This requires very tight tolerances for the fabrication, and puts stringent conditions for the sidewall roughness of the waveguides. The requirements for an accurate smooth slot are especially high for sensor applications, where the slots are usually filled with air or a watery solution [3, 4] To solve these problems, we previously suggested using conformal atomic layer deposition (ALD) for coating slot waveguides with amorphous titanium dioxide (TiO2) [5]. The reduced loss and the possibility to control the slot width are of special advantage for nonlinear silicon-organic hybrid (SOH) slot waveguides [7, 8]

Surface roughness and loss

Atomic layer deposition of TiO2 for reducing surface roughness

Measurements

Experimental results

Conclusions