Mid-infrared nonlinear optical response of Si-Ge waveguides with ultra-short optical pulses.

L Carletti,P Ma,J M Hartmann,S Madden,C Monat,P Labeye,Y Yu,C Grillet,M Sinobad,R Orobtchouk,D Allioux,M Brun,D J Moss,S Nicoletti,B Luther-Davies,S Ortiz

doi:10.1364/oe.23.032202

Abstract

We characterize the nonlinear optical response of low loss Si(0.6)Ge(0.4) / Si waveguides in the mid-infrared between 3.3 μm and 4 μm using femtosecond optical pulses. We estimate the three and four-photon absorption coefficients as well as the Kerr nonlinear refractive index from the experimental measurements. The effect of multiphoton absorption on the optical nonlinear Kerr response is evaluated and the nonlinear figure of merit estimated providing some guidelines for designing nonlinear optical devices in the mid-IR. Finally, we compare the impact of free-carrier absorption at mid-infrared wavelengths versus near-infrared wavelengths for these ultra-short pulses.

Highlights

The mid-infrared region of the spectrum, at wavelengths between 3 μm and 20 μm, is currently gaining formidable momentum since it is rich in applications that can potentially impact many important aspects of our everyday life such as chemical and biological sensing, active imaging, tissue ablation, secure free-space communication, multiwavelength light sources, and many others [1,2]
We report experimental measurements of the nonlinear optical response of step index Si-Ge/Si waveguides in the mid-IR obtained by measuring self-phase modulation (SPM) and the nonlinear transmission of these waveguides at high optical intensities using femtosecond optical pulses at wavelengths between 3.3 μm and 4 μm
We observe that the impact of free-carriers is significant even though the pulse duration is only a few hundreds of femtoseconds, in contrast with what is usually observed at near-IR wavelengths, and highlights the need to accurately account for free-carrier effects even when using ultra-short optical pulses in the mid-IR

Summary

Introduction

The mid-infrared (mid-IR) region of the spectrum, at wavelengths between 3 μm and 20 μm, is currently gaining formidable momentum since it is rich in applications that can potentially impact many important aspects of our everyday life such as chemical and biological sensing, active imaging, tissue ablation, secure free-space communication, multiwavelength light sources, and many others [1,2]. Despite the tremendous recent progress in miniaturisation, reduced power consumption, and increased tunability of detection and spectroscopy platforms developed far for the mid-IR [6,7], current technologies are still primarily based on stand-alone single frequency laser sources that operate in relatively narrow spectral regions. This limits the number of different molecules that can be detected with a single system. Apart from one report of nonlinear optics out to 6 μm in the silicon on sapphire (SOS) platform [9], Si has hardly been investigated beyond the short-wavelength infrared (SWIR), limited to 3 μm, due to both absorption in the cladding material (i.e. silica in the case of the silicon on insulator (SOI)) and the impact of higher order (3 and 4) photon absorption and the ensuing free carrier absorption [18,19]

Results

Discussion

Conclusion