Nonlinear absorption and refraction in crystalline silicon in the mid‐infrared

Abstract

The wavelength dependence of the nonlinear absorption and the third order nonlinear refraction of crystalline silicon between 2.75 mu m 5.5 mu m as well as at 1.55 mu m have been measured. It was found that at all wavelengths multi-photon and free carrier absorption can be significant. In particular nonlinear absorption can affect silicon devices designed for the mid-infrared that require strong nonlinear response, such as for the generation of a supercontinuum

Nonlinear optical response of low loss silicon germanium waveguides in the mid-infrared

“…The asymmetry in the transmitted pulses is actually more pronounced than one would expect for only multi photon effects and the derived mean wavelength shift seems to increase with the wavelength. Although, at a fixed light intensity, one would expect that fewer free carriers are generated as the wavelength increases, the free carrier dispersion coefficient, k c , scales as λ 2 [12], therefore leading to a stronger effect at longer wavelengths and thus potentially explaining the stronger asymmetry and the larger mean wavelength shift observed.…”

“…λ th ≈2.45 μm [16]) the nonlinear transmission observed in Fig. 4 is necessarily due to higher order multiphoton absorption, such as three-and four-photon absorption, as well as the resulting freecarrier absorption [12]. …”

“…(2) [20]. As a starting point, we used the values of Si for the free-carrier absorption and dispersion parameters and we applied the scaling rules for the wavelength used in [12]. Thus we defined σ(λ) = 1. , where λ is the wavelength (in μm).…”

“…For nonlinear devices, the issue of two-photon absorption (TPA) that limits device performance in the near-infrared region, vanishes at longer wavelengths (>2.2 μm) [1,3], and this has motivated some impressive nonlinear optical demonstrations in silicon on insulator (SOI) waveguides near 2 μm [7,10], as well as in silicon on sapphire (SOS) [11]. However, it has recently been shown that, although TPA vanishes beyond 2 μm, higher order (3 and 4) photon absorption and the ensuing free carrier absorption (FCA) in Si remain far from negligible [12]. Therefore, a careful design is needed for applications requiring a strong nonlinear response, such as for supercontinuum generation, in this wavelength range.…”

Silicon-Based Photonic Integration Beyond the Telecommunication Wavelength Range

“…Silicon-on-insulator is a very attractive material platform for nonlinear optics, given the high refractive index contrast available and the high intrinsic Kerr nonlinearity of silicon. Especially in the short-wave infrared wavelength range silicon becomes an excellent material for nonlinear optics due to the absence of parasitic two-photon absorption [39]. Based on these properties several new device functionalities were demonstrated on the 220-nm silicon waveguide platform such as 1.5 to 2.5-μm supercontinuum generation using a 2120-nm wavelength picosecond pulsed laser source in crystalline silicon waveguides [40] or a similar bandwidth supercontinuum source using a 1950-nm picosecond thulium-doped fibe laser in hydrogenated amorphous silicon waveguides [41].…”