In-chip devices fabricated with nonlinear laser lithography deep inside silicon
Ilday, Fatih Ömer
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The integration of photonic elements with electronic elements on the same chip is highly desirable, since it may lead to new generation of devices. One constraint in this direction is the limited space available on the wafer surface. Currently, conventional fabrication methods use only the top thin layer of the silicon platforms for device fabrication. Therefore, new architectural designs are necessary. Creating functional elements deep inside silicon without damaging the surfaces is a promising approach to overcome space bottleneck in electronicphotonic integration, since the bulk of the wafer can be utilized with this method. Laser-written devices have been demonstrated in various transparent materials, such as glasses and polymers. When focused, high-energy laser pulses can induce nonlinear breakdown and change the morphology of the interaction region enclosed by the material. This process enables the fabrication of a diverse set of devices, including interconnects, optical waveguides and quantum photonic devices. However, so far, similar approaches did not succeed in silicon. We demonstrated a similar enabling method inside silicon, where nonlinear e ects were exploited to generate highly controllable modi cations deep inside silicon. We used these modi cations as building blocks to create in-chip elements. We developed a simple, intuitive model to understand the structure formation in more detail, which indicated that nonlinear interaction between counterpropagating beams causes the self-focusing of the beam, resulting in disruption in crystal structure. Propagation of the pulses are recon gured by the previously modi ed region. The focal point of the pulse shifts, elongating the structure. These elongated structures can provide the necessary phase shift to build di ractive optical elements embedded in Si, among other optical elements. We demonstrated this concept by fabricating binary and grayscale Fourier holograms and a binary Fresnel hologram projecting four layers forming a 3D image. In an extension of this work, the algorithm is developed for greyscale Fresnel holograms and increased the possible numbers of projections layers three orders of magnitude. Moreover, we used in-chip modi cations for creating optical waveguides inside silicon with the lowest losses reported so far. By selectively etching the modi- cations, we showed a second set of applications. We sculpted the silicon with this method to fabricate micropillars, through-Si vias and micro uidic channels. Further, we extended the method to other semiconductors and nanostructured the bulk GaAs. We also investigated the possibility of new processing regimes by using Bessel beams and 2 m laser pulses.
Computer generated hologram