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J. Eur. Opt. Society-Rapid Publ. 22, 1( 2026)
as high as 3.44 can be obtained in the IR range while the material is transparent( absorption coefficient k = 0)[ 17 ]. These optical constants can be tuned by controlling the amorphous degree of Si and the concentration of Hydrogen [ 18 ]. Therefore, a-Si: H was chosen as the material that constitutes the resonant structure. The fabrication and the characterization of the layer are described in the following sections.
3.1.1 a-Si: H deposition
The deposition of hydrogenated amorphous silicon( a-Si: H) films is performed using magnetron sputtering in an industrial-scale TSD-550 machine from HEF Durferrit. The chamber is equipped with a magnetron sputtering cathode of Si( 99.99 %) operated in pulsed direct current( pDC) mode in the tens of kilohertz range, with argon and hydrogen gas lines respectively for sputtering gas and hydrogenation of the material. The H 2 / Ar gas ratio was fixed to 0.9 and the pDC power to 1 kW. The substrates were placed on a barrel-type substrate holder with a diameter of 550 mm and were heated at 573 ° K before and during deposition.
3.1.2 Ellipsometric measurements
Figure 4 shows the dispersion function of an a-Si: H layer on a BK7 substrate determined by spectroscopic ellipsometry. The refractive index values( blue curve) are around n wg = 3.5 and the absorption coefficient values( green curve) below 0.0038 above 800 nm wavelength, meaning that a-Si: H layer is almost transparent above 800 nm with high enough refractive index. Considering that the response of silicon-based photodetectors is maximum between 800 nm and 1 lm and that compact laser diodes emitting at 980 nm are available, the operating wavelength of the device is set to 980 nm. At k = 980 nm, the refractive index and the absorption coefficient values are n wg = 3.562 and k wg = 0.0036, respectively. The layer’ s thickness deduced from the ellipsometric measurements is equal to 336.65 ± 0.03 nm. These parameters are of great importance for prediction of the optical angular response to determine the best grating geometry leading to the optimized energy transfer as explained in Section 3.2.
3.2 Grating design 3.2.1 Resonant structure scheme: fixed parameters
Once the refractive index, the thickness of the a-Si: H layer and the operating wavelength are fixed, we used the“ MC Grating” software [ 22 ] based on the Rigorous Coupled Wave Analysis( RCWA) method to design the optimized structure, comprising the grating and the under layer, and to calculate its optical response.
Figure 5 shows the binary grating design with the structure parameters used for the simulation. The TE-polarized light at wavelength 980 nm is incident from the substrate before impinging into the a-Si: H. As recommended for probing from back-side( substrate), the transmission medium index( air, n super = 1) is lower than those of the incident medium corresponding here to the substrate
Figure 4. Ellipsometric measurements resulting from the hydrogenated amorphous silicon deposition( a-Si: H). The blue curve represents the refractive index and the green line the absorption depending on wavelength. At the operating wavelength k = 980 nm, the refractive index is n wg = 3.562 and the absorption coefficient k wg = 0.0036.
Figure
5. Structure used to model the interaction of a TEpolarized light beam emitting at 980 nm wavelength with a BK7 substrate above which a thin layer of a-Si: H is deposited and grooved.
( BK7, n s = 1.508) and the a-Si: H waveguide refractive index( n wg = 3.562) is larger than the one of the substrate.
Theperiodandthedutycyclearefixed to K = 390nm and f = 0.5, respectively. For this period, the Littrow angle h L = asin( k ⁄(( 2 n s K))) = 56.44 ° is above the critical angle h c = asin( n super ⁄ n s)= 42 °, which cancels the 0th transmitted order.
3.2.2 Parameters to optimize
Thicknesses Wg and d must be determined considering that the total thickness of a-Si: H layer( d + Wg = 337nm) is imposed by the layer deposition requirements( see §. 3.1). The 0th and �1st orders efficiencies versus these two parameters are mapped in Figure 6 at the Littrow angle( h s = h L = 56.44 ° in the BK7 substrate). The 0th and �1st orders efficiencies show minima( Fig. 6a) andmaxima