Electroabsorption modulators work by changing the optical absorption in a semiconductor structure by applying voltage to it. There are two related mechanisms: the Franz–Keldysh effect, seen in bulk semiconductors, and the quantum-confined Stark effect (QCSE), seen in quantum-confined structures such as thin (e.g., 10 nm) quantum-well layers.
These effects are very closely related, with the Franz–Keldysh effect being the limit of the QCSE as the quantum-well layers are made thicker. The QCSE has more spectrally abrupt and somewhat stronger changes in absorption coefficient as a consequence of the discretization of the density of states and the stronger excitonic effects in quantum-confined structures.
Both effects require operating electric fields in the range of 1–10 V/μm.
Readily obtained by reverse biasing p-i-n diodes that contain the bulk semiconductor or quantum well materials in the intrinsic (i) region of the diodes.
Both effects are seen near the direct bandgap optical absorption edge and give rise to increases in the optical absorption below the bandgap photon. The Franz–Keldysh effect gives a long, smooth absorption tail, with typical induced absorption coefficient values in the range of a few hundred cm⁻¹.
The QCSE gives more abrupt steps in absorption that shift to lower energy with field, with absorption coefficient values that can be up to several thousand cm⁻¹. With their weaker absorption coefficients, Franz–Keldysh devices are used in waveguide structures that can have the necessary longer interaction lengths. QCSE devices are used in waveguide structures too, but, with their stronger absorption coefficient changes, they can be used for “surface normal” devices of micrometer vertical dimensions. QCSE modulators are widely used in telecommunications, especially in integrated laser-modulator structures.
Because the mechanisms are so strong, low operating energies are likely possible even without resonators. The core of the operating energy is the energy required to charge up the active volume of the device to the operating field. For a field of 5×10⁴ V/cm (5 V/μm), that energy is 2.5 fJ/μm³. Even without resonators, devices with energies of tens of femtojoules should be feasible.
Some level of temperature stabilization would be required for electroabsorption modulators because the bandgap energy of direct gap semiconductors shifts with temperature (typically 0.4 meV/K) and because any resonators used will also have some temperature dependence from the temperature dependence of the refractive index. Because the electroabsorptive effects can be so strong, however, high Q resonators are not required, and hence these devices are likely much less temperature sensitive than microring resonator refractive devices, for example.
Strong and clear QCSE was recently observed in Ge quantum wells on silicon, the first time the QCSE was clearly observed in any indirect gap or Group IV material. The first modulator devices have recently been demonstrated, including operation at ≤ 1 V swing. Liu et al. have demonstrated a waveguide Franz–Keldysh modulator in a CMOS compatible process, and with an estimated 50 fJ per bit of energy.
Though substantial work remains to be done on optimizing device structures and integration approaches, these Ge devices are very promising for high-speed low-energy optical output devices for optical interconnects to Si. The QCSE devices in particular are promising not only for waveguide devices but also for surface-normal modulators for free-space optical systems.