Optical properties of a designed passive unidirectional reflectionless PT metamaterial on an SOI platform.
(A) Schematic of the passive PT metamaterial on an SOI platform. Periodically arranged PT optical potentials with modulated dielectric permittivity of Δε=cos(qz)−iδsin(qz) (4nπ/q+π/q ≤z≤4nπ/q+2π/q) are introduced into an 800-nm-wide and 220-nm-thick Si waveguide embedded in SiO2. The period is 4π/q=575.5 nm. (B and C) Dependences of reflectance of the passive PT metamaterial on the modulation length with different values of δ at the wavelength of 1550 nm in forward (B) and backward (C) directions, respectively. In contrast to the monotonic decrease in the forward direction, reflection in the backward direction first reaches a minimum value of Rb=0 at δ=1 and then increases as δ becomes larger. (D) Periodically arranged 760-nm-wide sinusoidal-shaped combo structures are applied on top of an 800-nm-wide Si waveguide embedded inside SiO2 to mimic PT optical potentials, in which imaginary part modulation is implemented with 14 nm Ge/24 nm Cr structures, and 51 nm Si layers are for real part modulation. The designed PT metamaterial consists of 25 sets of top-modulated combo structures with a period of 575.5 nm and a width of 143.9 nm for each sinusoidal-shaped combo. (E) SEM picture of the device, where the boxed area indicates a unit cell. (F) Measured reflection spectra of the device through the waveguide coupler for both directions over a broad band of telecom wavelengths from 1520 to 1580 nm. Red and blue curves are Gaussian fits of raw data in forward (black) and backward (green) directions, respectively .