Silicon Photonics: Fundamentals and Devices

7.3 Electrorefraction in Silicon

Having discussed in general terms some of the optical processes in semiconductors and how they affect the absorption and refraction in these materials, we now focus our attention to material silicon. We consider first the effects produced by an electric field (discussed in Section 2) and examine the importance of each of them in changing the RI of silicon. In this connection, change in absorption, or electroabsorption, in silicon will also be examined. The thermo-optic effect will be discussed in Section 4.

The EO effects in silicon were first examined critically by Soref and Bennett 9, and the following discussion is based mostly on their findings.

7.3.1 Electro-Optic Effects

Since silicon is a centro-symmetric crystal, the linear EO coefficient of it is zero. Its optical absorption spectrum is modified by the Franz–Keldysh (FK) effect. The electroabsorption spectra at the indirect edge as measured by different authors are utilized by Soref and Bennett, who performed a numerical Kramers–Kronig analysis of the spectra to obtain the values of electrorefraction at different wavelengths.

Soref and Bennett gave a plot of the RI change Δn as a function of wavelength ranging from 1.00 to 1.60 µm. Starting at 1.3 µm, as the wavelength decreases, the value of Δn rises and attains a maximum at 1.07 µm, a wavelength slightly below the gap wavelength. It then decreases, crosses zero at 1.05 µm, and becomes negative. The value of Δn = 1.3 × 10⁻⁵ is obtained ...