The fundamental goal of this research project is to develop a route to form high performance photonic devices within the volume of a silicon wafer rather than on its surface, as is generally the case. Photonic devices are integral to many important applications in telecommunications, computing, data storage and transfer, and consumer electronics. Silicon photonics has enabled circuits that are miniaturized and integrated. These technologies will dramatically improve in speed, density, and energy efficiency if they can be successfully integrated at a high enough density as a multi-plane photonic integrated circuit (PIC). Two key projected societal impacts are (1) ultra-high speed data transfer within next generation computers using channels that route light in 3D within a chip or between chips and (2) advanced all-optical signal processing using a PIC that has numerous planes for performing operations. The project also includes the training of undergraduate and graduate researchers in photonic device/circuit theory, microfabrication, metrology, and numerical simulation. Research and teaching will be closely integrated through three of the PI's courses: Materials in Nanotechnology, Integrated Optoelectronics, and Principles of Experimental Research. Participation of students from underrepresented groups will be broadened through undergrad research and by leveraging large existing K-12 outreach initiatives that will impact greater than a thousand students. The PIs propose a proof-of-concept project to realize multiple planes of interconnected micro-optic elements, waveguides, and passive photonic devices within the volume of a silicon wafer. Subsurface gradient refractive index (GRIN) devices will be fabricated within porous silicon (PSi) or silica (PSiO2 = oxidized PSi) via two-photon lithography to selectively polymerize photoresist within the porous host. This direct laser writing (DLW) approach enables index control (nPSi = 1.4-1.9, nPSiO2 = 1.15-1.3) at each voxel. Complex GRIN elements (e.g., compound apochromatic lenses, photonic nanojet emitters, diffractive and photonic bandgap structures, spiral phase plates, and index/mode matching bamboo-shaped tapers) plus conventional silicon photonic elements (e.g., interferometers, gratings, and microring multiplexers and filters) can all be fabricated in a self-aligned manner with < 1 micron critical dimensions across a 4" wafer. The research seeks to advance knowledge in the fields of subsurface fabrication and photonic integration. The interdisciplinary team of 2 PIs, 2 grad students, and 2 undergrad students will leverage their experience and training in GRIN PSi and PSiO2 optics, DLW, optical device/circuit theory and design, computational electromagnetics, imaging, metrology, and nanomanufacturing to answer fundamental scientific questions about the fabrication of complex optics by addressing five major goals: 1. Investigate and apply polymerization-based index control and optical characteristics in mesoporous scaffold-enabled two-photon lithography. 2. Measure the spectral dependence of the refractive index and absorption coefficient (400 - 1700 nm) as well as the 3D point spread function of the writing process. 3. Develop software tools to: (a) optimize the 3D index profile to achieve a given optical functionality and (b) determine the necessary two-photon lithography exposure conditions to realize said profile. 4. Design, fabricate, characterize, and model novel GRIN elements and subsurface 3D waveguides. 5. Demonstrate the aforementioned monolithically integrated 8-plane PIC and optical interposer. Demonstration of previously unachievable photonic geometries and functionalities will not only create a new paradigm for future PIC architectures but will also lead to new scientific applications such as lab-in-chip, interferometry, metrology, imaging, and quantum optics.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
