This paper discusses multilayer platforms using silicon nitride and silicon waveguides. This technology allows 3-D photonic circuits to be created.

We present passive, visible light silicon nitride waveguides fabricated on ≈ 100 µm thick 200 mm silicon wafers using deep ultraviolet lithography. The best-case propagation losses of single-mode waveguides were ≤ 2.8 dB/cm and ≤ 1.9 dB/cm over continuous wavelength ranges of 466-550 nm and 552-648 nm, respectively. In-plane waveguide crossings and multimode interference power splitters are also demonstrated. Using this platform, we realize a proof-ofconcept implantable neurophotonic probe for optogenetic stimulation of rodent brains. The probe has grating coupler emitters defined on a 4 mm long, 92 µm thick shank and operates over a wide wavelength range of 430-645 nm covering the excitation spectra of multiple opsins and fluorophores used for brain stimulation and imaging.

A novel hollow-core photonic crystal fiber platform was used for the first time to observe clear vibrational modes of the CdTe core, CdS(0.7)Te(0.3) interface, and carboxylate-metal complexes in dilute aqueous CdTe quantum dot (QD) solutions. These modes demonstrate the presence of crystalline cores, defects, and surface passivation responsible for photoluminescent efficiency and stability. In addition, 3-mercaptopropionic acid (MPA)-capped QDs show higher crystallinity and stability than those capped with thioglycolic acid (TGA) and 1-thioglycerol (TG). This detailed, nondestructive characterization was carried out using Raman spectroscopy for solutions with QD concentration of 2 mg/mL, which is similar to their concentration during synthesis process. This platform can be extended to the in situ studies of any colloidal nanoparticles and aqueous solutions of relevant biological samples using Raman spectroscopy.

Compact power splitters designed ab initio using binary particle swarm optimization in a 2D mesh for a standard foundry silicon photonic platform are studied. Designs with a 4.8 μm × 4.8 μm footprint composed of 200 nm × 200 nm and 100 nm × 100 nm cells are demonstrated.Despite not respecting design rules, the design with the smaller cells had lower insertion losses and broader bandwidth and showed consistent behavior across the wafer. Deviations between design and experiments point to the need for further investigations of the minimum feature dimensions. Foundry fabricated silicon (Si) photonics seek to implement highly sophisticated photonic integrated circuits (PICs) at low cost by using the mature manufacturing process of microelectronics [1][2][3]. The high refractive index contrast in Si photonic platforms not only allows for compact device footprints but also makes possible device concepts that can take advantage of the strong optical confinement and scattering (e.g., grating couplers, micro-resonators, photonic crystals) [4][5][6]. The growing availability of foundry Si photonics, in combination with expanded computation capabilities for detailed electromagnetic simulations, open the opportunity to explore device designs that cannot be implemented in traditional, lower index contrast PIC platforms such as silica and compound semiconductors and are yet volume manufacturable.Device design performed by topology optimization without any a priori assumptions on the geometry has recently generated much interest [7][8][9][10]. As opposed to conventional design methodologies in which a few critical geometric parameters are tuned on a fixed geometry, topology optimization can find unexpected solutions with good performance within demanding constraints by exploring much larger parameter spaces. An example is the polarization beam splitter of [9], which had a design footprint constraint of 2.4 μm × 2.4 μm. However, usual optimization approaches (e.g., in [8][9][10]) rely on highresolution rendering of intricate geometric features, such as through electron-beam lithography, which can result in designs that are incompatible with the design rules and minimum feature sizes in foundry processes, which use deep ultraviolet (DUV) photolithography.In this Letter, we investigate foundry fabrication of an optimization designed 2 × 2 3 dB power splitter. Power splitters are a common building block in PICs and, in Si photonic platforms, are typically implemented as multimode interference (MMI) couplers, directional couplers, and adiabatic couplers. Typical footprints in a standard Si photonic platform have footprints around 39 μm × 5.2 μm for a 3 dB directional coupler and 158 μm × 4.1 μm for an MMI coupler [11]. Because power splitters may be instantiated many times in a PIC, a size reduction of the 2 × 2 splitter can save substantial circuit area. Here, we explore the design and implementation of 2 × 2 power splitters with a footprint constraint of 4.8 μm × 4.8 μm designed through optimization that accounted for the minimum ...

Automatic resonance alignment tuning is performed in high-order series coupled microring filters using a feedback system. By inputting only a reference wavelength, a filter is tuned such that passband ripples are dramatically reduced compared to the initial detuned state and the passband becomes centered at the reference. The method is tested on 5th order microring filters fabricated in a standard silicon photonics foundry process. Repeatable tuning is demonstrated for filters on multiple dies from the wafer and for arbitrary reference wavelengths within the free spectral range of the microrings.Comment: 12 pages, 13 figures, Submitted to IEEE Journal of Quantum Electronic