Three-dimensional nanoporous silicon (PSi), with inherently large surface areas, tunable pore sizes, film thicknesses, and effective refractive indices, has been utilized as a platform for the detection of biomolecules and high-dose radiation. A brief overview of the fabrication and characterization of the nanoporous framework is presented for novel applications that benefit from such sponge-like, high surface area devices. For many of these applications, it is necessary to ensure that the PSi surfaces are well-passivated and stabilized for subsequent conjugation with linker molecules and for emitters to maintain their emissive properties post-integration with the porous matrix. We present a detailed analysis of the influence that varied levels of interfacial oxide (SiOx) growth has on the optical properties of quantum dots (QDs) immobilized within the PSi thin-films. Reflectance spectroscopy, continuous wave photoluminescence (CWPL) and time-resolved photoluminescence (TRPL) studies provide a comprehensive understanding of the complex QD exciton dynamics at the PSi/SiOx-QD interfaces. The gradual conversion of PSi thin-films into fully-oxidized porous silicon oxide (PSiO2) thin-films is shown to significantly suppress non-radiative recombination pathways of photogenerated QD excitons and achieve almost a five-fold increase in QD exciton lifetimes. This conversion of PSi into PSiO2, a wide bandgap nanoporous material, also circumvents loss of QD emission due to absorption by PSi based devices. Future avenues of research into PSi based devices will be presented based on analyzing the optical scattering response of nanoscale PSi annular rings fabricated over PSi Bragg mirrors via dark field microscopy.
We demonstrate enhanced detection sensitivity of a slow light Mach-Zehnder interferometer (MZI) sensor by incorporating multi-hole defects (MHDs). Slow light MZI biosensors with a one-dimensional photonic crystal in one arm have been previously shown to improve the performance of traditional MZI sensors based on the increased lightmatter interaction that takes place in the photonic crystal region of the structure. Introducing MHDs in the photonic crystal region increases the available surface area for molecular attachment and further increases the enhanced lightmatter interaction capability of slow light MZIs. The MHDs allow analyte to interact with a greater fraction of the guided wave in the MZI. For a slow light MHD MZI sensor with a 16 μm long sensing arm, a bulk sensitivity of 151,000 rad/RIU-cm is demonstrated experimentally, which is approximately two-fold higher than our previously reported slow light MZI sensors and thirteen-fold higher than traditional MZI biosensors with millimeter length sensing regions. For the label-free detection of nucleic acids, the slow light MZI with MHDs also exhibits a two-fold sensitivity improvement in experiment compared to the slow light MZI without MHDs. Because the detection sensitivity of slow light MHD MZIs scales with the length of the sensing arm, the tradeoff between detection limit and device size can be appropriately mitigated for different applications. All experimental results presented in this work are in good agreement with finite difference-time domain-calculations. Overall, the slow light MZI biosensors with MHDs are a promising platform for highly sensitive and multiplexed lab-on-chip systems.
Silicon micro-ring biosensors demonstrate great potential for high sensitivity and multiplexed lab-on-chip systems. In
this work, we characterize the sensing performance of suspended TM-mode silicon micro-ring resonators, 5 μm in
radius, and demonstrate an enhanced sensitivity to molecular binding on the ring after suspension. In the TM-mode, the
overall field intensity exists primarily outside of the waveguide core, with high electric field intensities present near the
top and bottom surfaces. In traditional micro-ring resonators, only the top surface of the ring is available for surface
analyte attachment, while the electric field intensity near the bottom surface dissipates by leaking into the underlying
silicon dioxide substrate. In our approach, we suspend the TM-micro ring resonators in order to increase the surface area
for binding events and increase the light-matter interaction with analytes. The suspended rings demonstrate excellent
mechanical stability to multiple rinsing, soaking and nitrogen drying steps during the sensing procedure. We show that
the resonance shift achieved by the suspended micro-rings after attachment of small chemical molecules and DNA is at
least twice that of micro-rings supported by the silicon dioxide substrate.