Silicon-based integrated photonics holds the promise of revolutionizing key technologies, such as telecommunications, computing, and lab-on-chip systems. One can achieve diverse functionalities in two ways: on the wafer surface ("on-chip") or within its bulk ("in-chip"), the latter gaining recognition due to recent advancements in laser lithography. Until recently, 3D in-chip laser writing has only been utilized for single-level devices, leaving a vast potential for monolithic and multilevel functionality within silicon untapped. In our previous research, we successfully designed and fabricated multilevel, high-efficiency diffraction gratings in silicon using nanosecond laser pulses. Their high performance stemmed from effective field enhancement at Talbot self-imaging planes. Our current work takes a theoretical approach, investigating how varying the grating period affects the performance of in-chip multilevel gratings. We demonstrate that the previously achieved 95% diffraction efficiency at a 1550 nm wavelength is also attainable with a reduced period of 3 μm. This smaller period is predicted to allow for spectral filtering, nearly equivalent to commercially available filters in terms of Full Width at Half Maximum (FWHM). Our findings underscore the potential of volumetric Si photonics and mark a significant step towards realizing 3D-integrated monolithic chips.
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.
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.
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