In the Oct. 27 issue of Nature, scientists at Stanford University in Stanford, Calif., report the operation of a micron-scale optical modulator that is compatible with silicon CMOS fabrication techniques. The result is an important step in the effort to bring photonics functionality into the world of silicon electronics. Although silicon-based modulators have been demonstrated before, earlier devices were much larger -- millimeters in length -- or required precision-fabricated, high-Q ring resonators.

The new modulator is a simple system of multiple germanium quantum wells grown on a silicon substrate. A quantum well is a thin layer of semiconductor, in this case a 10-nm-thick slice of germanium, sandwiched between layers of barrier material. The potential energy of carriers in the well is lower than that of carriers in the barrier, and the boundary conditions at the well’s edge lead to distinct energy levels of the carriers in the well. These levels can be shifted by an electric field, in an effect known as the quantum-confined Stark effect.

The principle behind the modulator is to Stark-shift one of the levels to an energy exactly right to be excited by an incoming photon. The photon is absorbed, and an electron is boosted from the shifted energy level in the valence band to that in the conduction band.

The tricky part is that germanium’s bandgap is indirect, meaning that conservation of momentum prohibits a photon with the bandgap energy from boosting an electron from the valence band to the indirect conduction band. The scientists finessed this problem by using a high-energy, direct-bandgap transition to absorb the photon.