Optical metamaterial waveguide for light spectral reshaping and filtering

An optical waveguide is defined as a structure that can confine and guide optical light to follow a predefined path from a transmitter to a receiver. The waveguide that we are discussing is made of silicon semiconductor material that is built over a bulk insulating substrate made of silicon dioxide material. This silicon-on-insulator structure is fabricated with a well-known, simple, and cheap method called CMOS technology. The novel metamaterial presented in this work comprises periodic nano-size air gap unit cells that are distributed along a planar lightwave waveguide length. That metamaterial structure can have useful applications in light spectral reshaping to implement various types of optical filters. A light spectrum defines the distribution of light energy over some specific wavelength range. Optical filters can allow light to pass through at a certain designed wavelength range, and to stop light at some other wavelength ranges.

The novel metamaterial here is named a partial-width entrenched-core (PWEC) waveguide. It has a series of periodic nano-wide rectangular air gaps embedded within the centre of a few-mode waveguide core. The gaps have a smaller width than that of the waveguide and thus partially fill the waveguide core. The core is the main part of a waveguide in which the light is mostly confined. An input infrared light to the core suffers from periodic perturbations during propagation along this metamaterial waveguide due to passing through nano-gaps. Thus, the input light can couple into two modes: Zero-order (called fundamental mode) mode and second-order mode. That results in two distinctive Bragg wavelengths that are closely separated, which form a double-hump (DH) spectrum. That is unlike the well-known standard Bragg gratings. The well-known standard Bragg gratings usually consist of distributed partial reflectors that cause constructive interference between different light waves at certain specific wavelengths called Bragg wavelengths.

The width and periodicity parameters of nano air gaps (also called trenches) are critical in the construction of the DH-spectrum shape. By engineering the trenches’ two parameters, the DH-spectrum can be reshaped to have different humps bandwidth and separation within the shortwave infrared wavelength range. For example, the double-hump reflection and transmission spectra can be reshaped to form different types of optical filters such as double stop-bands, wide pass-band of 200nm, narrow pass-band of 10nm, notch-bands of 10nm, or ultra-wideband of 530nm working as a fully reflective mirror.