SEMINAR: In-chip devices fabricated with nonlinear laser lithography deep inside silicon
Ph.D. Defence in Electrical and Electronics Engineering
Supervisor: ASSOC. PROF. DR. F. ÖMER İLDAY
The seminar will be on Thursday, May 16, 2019 at 13:30, @EE-314
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 didn’t succeed in silicon. We demonstrated a similar enabling method inside silicon, where nonlinear effects were exploited to generate highly controllable modifications deep inside silicon. We used these modifications as building blocks to create in-chip elements. We developed a toy model to understand the structure formation in more detail and found out that nonlinear interaction between counter-propagating beams causes the self-focusing of the beam, resulting in disruption in crystal structure. Propagation of the next pulses are reconfigured by the previously modified region. The focal point of the pulse shifts, elongating the structure further. These elongated structures can provide the necessary phase shift to build diffractive optical elements embedded in Si. We demonstrated this concept by fabricating binary and grayscale Fourier holograms and a binary Fresnel hologram projecting four layers forming a 3D image. We further developed that algorithm for greyscale Fresnel holograms and increased the possible numbers of projections layers three order of magnitude. Moreover, we used the in-chip modifications for creating optical waveguides inside silicon with the lowest losses reported so far. By selectively etching the modifications, we showed a second set of applications. We sculpted the silicon with this method to fabricate micropillars, through-Si vias and microfluidic 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 µm laser pulses.