J. Eur. Opt. Society-Rapid Publ. 22, 1( 2026) 7
Figure 10. Schemes in direct and reciprocal( Ewald sphere’ s representation) spaces associated to angular positions of the Figure 3 and electric field E y amplitude:( a) First resonance at h s = 2.8 °( point A),( b) Second resonance at h s = 32.7 °( point B),( c) First crossing point h s = 46.8 °( point C) and( d) Second crossing point h s = 69.5 °( point E).
the grating for an eventual probing of the surrounding medium refractive index change.
4.2 Angular measurements 4.2.1 Optical setup
The angular response corresponding to the measurement of the propagative reflected orders intensities( 0th and �1st), looking like an eye shape in the Littrow angle vicinity and called“ switch eye”, is measured by an optical bench schematized in Figure 11a. The collimated beam emitted at wavelength 980 nm is provided by a laser diode and passes through a diaphragm for spatial filtering and through a polarizer to ensure TE-polarization according to the grating incidence plane. The laser diode, diaphragm and polarizer are set on a same rotative support and the photodiode is mounted on another motorized rotation stage. According to the incident angle on the grating, both 0th and �1st orders are diffracted and their efficiencies are measured sequentially by the photodiode, converting intensity into a current that is sent to the analog input of