We theoretically discuss the possibility of having materials with zero effective permittivity that would create band gaps in a wide range of frequencies up to the visible. The physical realization of these materials is also discussed in terms of embedding metallic nanoparticles and nanowires in a dielectric medium. In the limit of long wavelengths, these composites will behave like a homogeneous medium with zero permittivity that will completely reflect electromagnetic waves. We present transmittivity calculations by using finite-difference time domain for periodic structures that proves the concept and shows the validity of the long wavelength approximation. The striking result is that the cutoff frequency ωc is determined by the lattice parameter of the composite. By properly choosing the lattice constant of the composite and permittivity of metal and dielectric constituents, we can have full band gaps at any frequency range but especially in the visible.

In this paper, high-speed surface-illuminated Ge-on-Si pin photodiodes with improved efficiency are demonstrated. With photon-trapping microhole features, the external quantum efficiency (EQE) of the Ge-on-Si pin diode is $>80\%$ at 1300 nm and 73% at 1550 nm with an intrinsic Ge layer of only 2 μm thickness, showing much improvement compared to one without microholes. More than threefold EQE improvement is also observed at longer wavelengths beyond 1550 nm. These results make the microhole-enabled Ge-on-Si photodiodes promising to cover both the existing C and L bands, as well as a new data transmission window (1620–1700 nm), which can be used to enhance the capacity of conventional standard single-mode fiber cables. These photodiodes have potential for many applications, such as inter-/intra-datacenters, passive optical networks, metro and long-haul dense wavelength division multiplexing systems, eye-safe lidar systems, and quantum communications. The CMOS and BiCMOS monolithic integration compatibility of this work is also attractive for Ge CMOS, near-infrared sensing, and communication integration.

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