8
J. Eur. Opt. Society-Rapid Publ. 22, 1( 2026)
Figure 11.( a) Angular characterization bench,( b) Side view of the sample and half sphere holder,( c) Photograph of the holder designed to maintain the half sphere and the sample.
an acquisition card connected to a computer, which stores the intensities for each incident angle. The grating is probed by the laser diode from the back side as shown in Figure 1.
Direct measurement of the angular response on the whole range of 0 °– 90 ° is impossible due to the high refractive index of the resonant waveguide layer that leads to total internal reflection at BK7 substrate and a-Si: H interface.
To ensure the angular scanning by the incident beam within the resonant structure, a half sphere with the same refractive index as the substrate is added with an indexmatching fluid on the back side of the substrate to couple the light from the air to the a-Si: H grating with a grazing angle between 40 ° and 80 °. To support the grating and the half sphere, a holder was specially designed as shown in Figures 11b and 11c. The sphere must be properly positioned on the characterization bench so that the incident beam is centered with no tilt in normal incidence. The grating is also rotated in its support to have the diffracted orders in the horizontal plane.
4.2.2 Experimental results
After optical alignments, measurements were carried out to verify the ability of the sample to exhibit an energy transfer between the 0th and �1st diffracted orders. For each incident angle h s from 0 ° to 70 ° with a step of 1 °, controlled by the laser diode rotation stage, the 0th order and the �1st order diffraction intensities are measured by the photodiode. Experimental values( points in black and in red for the 0th and �1st orders respectively) are plotted in Figure 12 and superimposed to the theoretical curves( thin lines) simulated with the actual grating parameters values.