Refractive index engineering is critical for fabrication of refractive optical elements directly inside transparent materials. Such three-dimensional optical engineering is only emerging for the technologically important material, silicon. Here we show the first analysis of refractive index and birefringence observations written with a beam other than Gaussian inside Si. Exploiting a Bessel-type beam, we created laser-written structures with average refractive index as high as 6 ✕ 10-^3 and retardance on the order of 20nm. These properties are studied as function of laser modulation and other relevant parameters, including writing geometry, and compared with results of Gaussian beam written structures.
The first successful nano-photonics element deep inside the silicon is created. This was achieved by creating nanoscale and high-aspect-ratio laser-written modifications inside Si, without altering the wafer surfaces. We exploit Bessel beams, in order to create 700-nm thick subsurface planes of 250 µm axial length, arrayed and layered to increase device efficiency. The length of modifications is controlled by precise axial stitching of individual subsurface lithographic layers. The maximum efficiency at the incident angle, satisfying the Bragg condition, is measured 85% for the two-layer grating, and its angular sensitivity is recorded, with a strong agreement between experiment and theory.
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