Raman scattering from acoustic modes in Si/Ge superlattice waveguides

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Abstract

This paper discusses the design of acoustic vibrational modes in Si/Ge planar optical waveguides and its application in creating silicon-based Raman devices with a flexible spectrum. It addresses the deficiencies of the recently demonstrated Raman-based silicon lasers and amplifiers as they relate to spectral and low efficiency limitations of bulk silicon. The treatment is for in-plane scattering in a forward scattering configuration. In addition to calculating the spectrum and the efficiency for Raman active modes, it is shown that the negligible wave-vector of the phonons involved in this type of scattering allows for the use of the bandgap “pinching” effect to arrive at specific layer thicknesses for Si and Ge that optimize the scattering efficiency.

Section snippets

Motivation

The use of Raman nonlinear processes in silicon waveguides has emerged as a promising means to create optically active devices in silicon including amplifiers, lasers and wavelength converters [1], [2], [3], [4], [5]. Several areas of improvement remain, namely: waveguide side-wall smoothening [6], reduced waveguide dimensions [7], and low-loss fiber-to-waveguide coupling [8]. Other approaches to take advantage of the Raman effect have been suggested, such as the use of micro-ring resonators [9]

Theory and model

The geometry of interest in the case considered in this work will be linear scattering, forward and backward, along the direction of the plane of the superlattice, which will be assumed to grow along the (001) direction of the crystal. This configuration lends itself more easily for waveguide fabrication, in terms of layer growth, lithography and material deposition, and also from the optical confinement point of view.

This paper will not deal with the issues of optical mode-confinement and

Summary

In summary, we have shown that optical scattering from Raman-active acoustic phonon modes in SGSL offers an opportunity to modify the spectrum of Raman based silicon waveguide devices. The treatment of the Raman/Brillouin phenomenon for the in-plane forward scattering geometry, the most common geometry for waveguide devices, has been presented here. In addition to calculating the spectrum and the efficiency of Raman-active modes in these structures, it was found that the “bandgap pinching”

Acknowledgement

The authors wish to acknowledge the support of Dr. Jagdeep Shah of DARPA for the realization of this project.

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