Silicon photonics
Which correctly pairs the photon energy of 1.55 µm light with its behavior in a silicon waveguide? (E [eV] ≈ 1.24 / λ [µm])
- AAbout 1.9 eV. It exceeds silicon's band gap, so it is strongly absorbed and converted into current
- BAbout 0.8 eV. It is smaller than silicon's band gap, so it excites electrons into the conduction band and is strongly absorbed
- CAbout 0.8 eV. It is smaller than silicon's band gap, so it propagates with almost no absorption
Show answer and explanation
Answer: C. About 0.8 eV. It is smaller than silicon's band gap, so it propagates with almost no absorption
1.24 ÷ 1.55 ≈ 0.80 eV, which is smaller than silicon's band gap of about 1.12 eV, so it cannot lift valence-band electrons into the conduction band and no band-to-band absorption occurs. Silicon is therefore nearly transparent at telecom wavelengths and can serve as a waveguide material. Conversely, silicon alone cannot make a photodetector for this wavelength. Silicon absorbs light with wavelengths shorter than about 1.1 µm (energies above 1.12 eV). About 1.9 eV is the result of multiplying 1.24 by 1.55, and corresponds to the energy of red light at a wavelength of about 650 nm.