Light spectral filter based on Spatial Adiabatic Passage

We report the first experimental realization of an analogous of the Stimulated Raman Adiabatic Passage (STIRAP) technique [1] working as a light spectral filter [2,3]. The system consists of three identical coupled total internal reflection silicon oxide waveguides designed such that there is a counterintuitive variation of the evanescent couplings between the waveguides along the propagation direction (see Fig.1(a)). If couplings are strong and wide enough along the propagation direction, which only occurs for long enough wavelengths, this geometry allows for an adiabatic following of a supermode that implies only the outermost waveguides. Thus, if light is injected into the right waveguide of our device, after propagating along the system, long wavelengths are transferred into the left output whereas short wavelengths end up into the central and right outputs. In this way, the system operates as a simultaneous high and low-pass spectral filter in the visible range. We refer to this device as Spatial Adiabatic Passage (SAP) filter. Regarding the filtering efficiency, the measured stopband reaches values up to -11dB for the left output, and around -20dB for the right plus central outputs. The passband values are close to 0dB for both cases. Fig. 1(b) shows the experimentally measured intensities at the output of the left waveguide and the sum of the outputs of the right and central waveguides as the transmittance relative to the total output intensity for one of the fabricated devices.