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Octave-spanning frequency comb generation in the deep mid-infrared (>5.5 μm) typically requires a high pump power, which is challenging because of the limited power of narrow linewidth lasers at long wavelengths. We propose twofold dispersion engineering for a Ge-on-Si microcavity to enable both dispersion flattening and dispersion hybridization over a wide band from 3.5 to 10 μm. A two-octave mode-locked Kerr frequency comb can be generated from 2.3 to 10.2 μm, with a pump power as low as 180 mW. It has been shown that dispersion flattening greatly enhances the spectral broadening of the generated comb, whereas dispersion hybridization improves its spectral flatness.

We report that propagation loss of optical waveguides based on a silicon-on-insulator (SOI) material platform can be greatly reduced. Our simulations show that the loss, including SiO_2 absorption and substrate leakage, but no scattering loss, is 0.024 and 0.53 dB/cm in the deep mid-infrared at 4.8 and 7.1 μm wavelengths, where the material absorption in SiO_2 is 100 and 1000 dB/cm, respectively. The loss becomes negligible, compared to scattering loss in Si waveguides. This is enabled by using the TE_10 mode in a pedestal waveguide. We also show that the TE_10 mode can be excited in the proposed waveguide by the fundamental mode with a coupling efficiency of >94%. Low propagation loss, high coupling efficiency, and fabrication-friendly design would make it promising for practical use of SOI devices in the deep mid-infrared.

Research on two-dimensional designer optical structures, or metasurfaces, has mainly focused on controlling the wavefronts of light propagating in free space. Here, we show that gradient metasurface structures consisting of phased arrays of plasmonic or dielectric nanoantennas can be used to control guided waves via strong optical scattering at subwavelength intervals. Based on this design principle, we experimentally demonstrate waveguide mode converters, polarization rotators and waveguide devices supporting asymmetric optical power transmission. We also demonstrate all-dielectric on-chip polarization rotators based on phased arrays of Mie resonators with negligible insertion losses. Our gradient metasurfaces can enable small-footprint, broadband and low-loss photonic integrated devices.

We have fabricated gradient-grafted nanofoam films that are able to record the presence of volatile chemical compounds in an offline regime. In essence, the nanofoam film (100–300 nm thick) is anchored to a surface cross-linked polymer network in a metastable extended configuration that can relax back to a certain degree upon exposure to a chemical vapor. The level of the chain relaxation is associated with thermodynamic affinity between the polymer chains and the volatile compounds. In our design, the chemical composition of the nanofoam film is not uniform; therefore, the film possesses a gradually changing local affinity to a vapor along the surface. Upon vapor exposure, the nonuniform changes in local film morphology provide a permanent record or “fingerprint” for the chemical event of interest. This permanent modification in the film structure can be directly detected viachanges not only in the film surface profile but also in the film optical characteristics. To this end, we demonstrated that sensing/recording nanofoam films can be prepared and interrogated on the surfaces of optical waveguides, microring optical resonators. It is important that the initial surface profile and structure of the nanofoam film are encrypted by the distinctive conditions that were used to fabricate the film and practically impossible to replicate without prior knowledge.

This article reports on a small-scale empirical study of recasts—a form of corrective feedback. The subjects were eight adult L2 learners of English, four of whom were randomly assigned to a recast group and four to a nonrecast group. The study adopted a pretest, posttest, and delayed posttest design, with eight pedagogical recast sessions between the pretest and the posttest for the recast group paralleled by eight regular sessions for the nonrecast group. Data collected consisted of written and oral narratives primed by cartoon strips and produced by the subjects in both groups. Recasts appeared to be successful in this study in that they heightened the L2 learners' awareness and led to considerable improvement in their tense consistency during oral and written performance. Importantly, the study identified four conditions that may be necessary for recasts to facilitate learning: individualized attention, consistent focus, developmental readiness, and intensity. Further studies employing data from multiple sources are needed to validate these conditions and to identify more clearly aspects and stages of L2 acquisition that are susceptible to recasts.