Material reference

SiliconSi

The workhorse of integrated photonics. High index contrast on SOI enables dense circuits at telecom wavelengths, at the cost of two-photon absorption, no native gain, and no second-order nonlinearity.

Silicon dominates integrated photonics for one main reason: the silicon-on-insulator (SOI) platform rides on CMOS fabrication. The index contrast between silicon ( $n \approx 3.48$ ) and its silica cladding ( $n \approx 1.44$ ) confines light tightly, so waveguides bend in a few micrometers and full transceivers fit on millimeter-scale die.

Key properties

Property	Value	Conditions
Refractive index $n$	3.476	1550 nm, 300 K
Refractive index $n$	3.503	1310 nm, 300 K
Bandgap $E_g$	1.12 eV (indirect)	300 K
Transparency window	~1.1–8 μm	absorption edge to multiphonon onset
Thermo-optic coefficient $dn/dT$	$+1.8 \times 10^{-4}$ K⁻¹	1550 nm, 300 K
Kerr index $n_2$	$\approx 4.5 \times 10^{-18}$ m²/W	1550 nm
TPA coefficient $\beta$	~0.8 cm/GW	1550 nm
Thermal conductivity	149 W·m⁻¹·K⁻¹	300 K

What the numbers mean in practice

The indirect 1.12 eV bandgap sets the absorption edge near 1.1 μm, which is why silicon photonics lives in the O- and C-bands and why silicon makes an excellent detector for visible light but a useless one at 1550 nm (germanium is grown on top for that). The same indirect gap is why silicon does not lase: III-V gain material has to be attached by bonding, transfer printing, or hybrid assembly.

Because the bandgap exceeds one photon at 1550 nm but not two, two-photon absorption appears at high intensity, and the free carriers it generates absorb more light still. This caps the power handling of silicon waveguides and resonators well below what silicon nitride tolerates.

The large positive $dn/dT$ cuts both ways. It makes silicon rings and MZIs easy to tune with small heaters, and it makes every filter on the chip drift with ambient temperature unless actively stabilized or athermalized.

Typical propagation loss for a single-mode strip waveguide (220 nm SOI) is 1–3 dB/cm, dominated by sidewall-roughness scattering; wide rib geometries reach below 0.5 dB/cm.

References

H. H. Li, "Refractive index of silicon and germanium and its wavelength and temperature derivatives," J. Phys. Chem. Ref. Data 9, 561 (1980).
B. J. Frey, D. B. Leviton, T. J. Madison, "Temperature-dependent refractive index of silicon and germanium," Proc. SPIE 6273, 62732J (2006).
R. A. Soref, B. R. Bennett, "Electrooptical effects in silicon," IEEE J. Quantum Electron. 23, 123 (1987).
Y. A. Vlasov, S. J. McNab, "Losses in single-mode silicon-on-insulator strip waveguides and bends," Opt. Express 12, 1622 (2004).
E. D. Palik (ed.), Handbook of Optical Constants of Solids, Academic Press (1985).