An international team of researchers has developed designs for electro-optical (EO) free-space and waveguided 2 × 2 switches that operate at the 1550 nm wavelength and are based on germanium selenide (GeSe), a IV-VI semiconductor phase-change material that is transparent at 1550 nm and has two stable, self-sustaining states: amorphous, and crystalline. The reversible phase transition can be made to occur within 100 ns via an applied voltage of about 80 V/μm. The researchers hail from the University of Massachusetts at Boston, the Air Force Research Laboratory (Wright-Patterson AFB, OH), McMaster University (Hamilton, ON, Canada), the University of Washington (Seattle), Massachusetts Institute of Technology (MIT; Cambridge), and Shandong University (China).
The device’s operation is based on the change in refractive index n between the crystalline phase (n = 2.97) and the amorphous phase (n = 2.4) of GeSe. In one configuration, a 10 to 100 nm film of GeSe is sandwiched between two prisms made of crystalline gallium phosphide (GaP), which has a refractive index of 3.06 at 1550 nm. In another configuration (see figure), one of the prisms is severely truncated and used in total internal reflection to form a 1 × 4 EO switch that switches both the s and p polarizations. In a third configuration, silicon (Si) waveguides are used instead, and a GeSe ribbon is embedded in a length of locally doped Si; this can be used, for example, to form a very compact directional-coupler switch or Mach-Zehnder device. Simulations using COMSOL (Burlington, MA) Multiphysics software show a background loss of about 0.004 dB per micron of active length for the waveguide version. Reference: Richard Soref et al., Opt. Express (2015); http://dx.doi.org/10.1364/OE.23.001536.