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1. Roughness Suppression in Electrochemical Nanoimprinting of Si for Applications in Silicon Photonics

   https://doi.org/10.1002/adma.202206608  

   Sharstniou, Aliaksandr; Niauzorau, Stanislau; Hardison, Anna L.; Puckett, Matthew; Krueger, Neil; Ryckman, Judson D.; Azeredo, Bruno (October 2022, Advanced Materials)

   Abstract Metal‐assisted electrochemical nanoimprinting (Mac‐Imprint) scales the fabrication of micro‐ and nanoscale 3D freeform geometries in silicon and holds the promise to enable novel chip‐scale optics operating at the near‐infrared spectrum. However, Mac‐Imprint of silicon concomitantly generates mesoscale roughness (e.g., protrusion size ≈45 nm) creating prohibitive levels of light scattering. This arises from the requirement to coat stamps with nanoporous gold catalyst that, while sustaining etchant diffusion, imprints its pores (e.g., average diameter ≈42 nm) onto silicon. In this work, roughness is reduced to sub‐10 nm levels, which is in par with plasma etching, by decreasing pore size of the catalyst via dealloying in far‐from equilibrium conditions. At this level, single‐digit nanometric details such as grain‐boundary grooves of the catalyst are imprinted and attributed to the resolution limit of Mac‐Imprint, which is argued to be twice the Debye length (i.e., 1.7 nm)—a finding that broadly applies to metal‐assisted chemical etching. Last, Mac‐Imprint is employed to produce single‐mode rib‐waveguides on pre‐patterned silicon‐on‐insulator wafers with root‐mean‐square line‐edge roughness less than 10 nm while providing depth uniformity (i.e., 42.9 ± 5.5 nm), and limited levels of silicon defect formation (e.g., Raman peak shift < 0.1 cm⁻¹) and sidewall scattering. 

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2. Highly efficient fiber to Si waveguide free-form coupler for foundry-scale silicon photonics

   https://doi.org/10.1364/PRJ.514999  

   Ranno, Luigi; Sia, Jia Xu Brian; Popescu, Cosmin; Weninger, Drew; Serna, Samuel; Yu, Shaoliang; Kimerling, Lionel C.; Agarwal, Anuradha; Gu, Tian; Hu, Juejun (May 2024, Photonics Research)

   As silicon photonics transitions from research to commercial deployment, packaging solutions that efficiently couple light into highly compact and functional sub-micrometer silicon waveguides are imperative but remain challenging. The 220 nm silicon-on-insulator (SOI) platform, poised to enable large-scale integration, is the most widely adopted by foundries, resulting in established fabrication processes and extensive photonic component libraries. The development of a highly efficient, scalable, and broadband coupling scheme for this platform is therefore of paramount importance. Leveraging two-photon polymerization (TPP) and a deterministic free-form micro-optics design methodology based on the Fermat’s principle, this work demonstrates an ultra-efficient and broadband 3-D coupler interface between standard SMF-28 single-mode fibers and silicon waveguides on the 220 nm SOI platform. The coupler achieves a low coupling loss of 0.8 dB for the fundamental TE mode, along with 1 dB bandwidth exceeding 180 nm. The broadband operation enables diverse bandwidth-driven applications ranging from communications to spectroscopy. Furthermore, the 3-D free-form coupler also enables large tolerance to fiber misalignments and manufacturing variability, thereby relaxing packaging requirements toward cost reduction capitalizing on standard electronic packaging process flows. 

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3. III/V silicon hybrid laser based on a resonant Bragg structure

   https://doi.org/10.1364/AO.390522  

   Davis, Jordan_A; Kim, Myun_Sik; El_Amili, Abdelkrim; Trotter, Douglas_C; Starbuck, Andrew_L; Dallo, Christina; Pomerene, Andrew_T; DeRose, Christopher_T; Lentine, Anthony_L; Fainman, Yeshaiahu (May 2020, Applied Optics)

   We demonstrate a laser tunable in intensity with gigahertz tuning speed based on a III/V reflective semiconductor optical amplifier (RSOA) coupled to a silicon photonic chip. The silicon chip contains a Bragg-based Fabry–Perot resonator to form a passive bandpass filter within its stopband to enable single-mode operation of the laser. We observe a side mode suppression ratio of 43 dB, linewidth of 790 kHz, and an optical output power of 1.65 mW around 1530 nm. We also investigate using a micro-ball lens as an alternative coupling method between the RSOA and the silicon chip. 

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4. On-chip spectrometers using stratified waveguide filters

   https://doi.org/10.1038/s41467-021-23001-6  

   Li, Ang; Fainman, Yeshaiahu (May 2021, Nature Communications)

   Abstract We present an ultra-compact single-shot spectrometer on silicon platform for sparse spectrum reconstruction. It consists of 32 stratified waveguide filters (SWFs) with diverse transmission spectra for sampling the unknown spectrum of the input signal and a specially designed ultra-compact structure for splitting the incident signal into those 32 filters with low power imbalance. Each SWF has a footprint less than 1 µm × 30 µm, while the 1 × 32 splitter and 32 filters in total occupy an area of about 35 µm × 260 µm, which to the best of our knowledge, is the smallest footprint spectrometer realized on silicon photonic platform. Experimental characteristics of the fabricated spectrometer demonstrate a broad operating bandwidth of 180 nm centered at 1550 nm and narrowband peaks with 0.45 nm Full-Width-Half-Maximum (FWHM) can be clearly resolved. This concept can also be implemented using other material platforms for operation in optical spectral bands of interest for various applications. 

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5. Selective phosphorus doping of polycrystalline silicon on glass using self-assembled monolayer doping (MLD) and flash anneal

   https://doi.org/https://doi.org/10.1016/j.matlet.2021.130780  

   Glenn Packard; Carolyn Spaulding; Alex Taylor; Karl Hirschman; Scott Williams, Santosh Kurinec (August 2021, Materials letters)

   Aldo R. Boccaccini (Ed.)

   We report doping of thin (~60 nm) amorphous silicon (a-Si) on glass substrate to form n + polycrystalline silicon on glass in selective regions using Monolayer doping (MLD) via Flash Lamp Annealing (FLA). The phosphorus monolayer was formed on the exposed regions of SiO2 patterned a-Si, through functionalization with chemically bound Diethyl vinylphosphonate (DVP) dopant molecules. The samples were capped with SiO2 and annealed using a single xenon flash pulse (5.0 J/cm2, 250 μs) to simultaneously crystallize a-Si, incorporate and activate phosphorus dopants. SIMS results show an average concentration of 8x1019 cm−3 in the 60 nm of thin silicon on glass. Electrical results show a resistivity of ~6.60x10−2 Ω.cm in doped regions. N-channel field effect transistor devices are successfully demonstrated using this MLD-FLA technique. 

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