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Engineering sub-wavelength silicon waveguides for sensing applications in the near-infrared and mid-infrared band (Conference Presentation)
No full textSilicon photonics is one of the most promising candidates to achieve lab-on-a-chip systems. Making use of the evanescent-field sensing principle, it is possible to determine the presence and concentration of substances by simply measuring the variation produced by the light-matter interaction in the real part of the mode effective index (in the near-infrared band), or in its imaginary part in a specific range of wavelengths (in the mid-infrared band). Regardless of which is the operating wavelength range, it is essential to select the proper sensing waveguide in order to maximize the device sensitivity. In this work we will review the potential of diffractionless subwavelength grating waveguides (SWG) for sensing applications by demonstrating their powerful capability to engineer the spatial distribution of the mode profile, and thereby to maximize the light-matter interaction. Among other things, we will demonstrate that the SWG waveguide dimensions used until now in the near-infrared are not optimal for sensing applications. In the mid-infrared band, due to the unacceptable losses of silicon dioxide for wavelengths longer than 4 \u3bcm, an additional effort is required to provide a more convenient platform for the development of future applications. In this regard, we will also show our recent progress in the development of a new platform, the suspended silicon waveguide with subwavelength metamaterial cladding. A complete set of elemental building blocks capable of covering the full transparency window of silicon (\u3bb < 3c8.5 \u3bcm) will be discussed.Peer reviewed: YesNRC publication: Ye
High-performance silicon Bragg filters exploiting sub-wavelength and symmetry engineering (Conference Presentation)
No full textBragg filters stand as a key building blocks of the silicon-on-insulator (SOI) photonics platform, allowing the implementation of advanced on-chip signal manipulation. However, achieving narrowband Bragg filters with large rejection levels is often hindered by fabrication constraints and imperfections. Here, we present a new generation of high-performance Bragg filters that exploit subwavelength and corrugation symmetry engineering to overcome bandwidth-rejection trade-off in state-of-the-art implementations. We experimentally show flexible control over the width and depth of the Bragg resonance, unlocking new tools for the implementation of notch filters with arbitrary bandwidth and rejection level. These results pave the way for the implementation of high-performance on-chip wavelength filters with a great potential for nonlinear-based applications, e.g. next generation Si-based photon-pair sources for quantum photonic circuits.Peer reviewed: YesNRC publication: Ye
Effect of low hole mobility on the efficiency droop of AlGaN nanowire deep ultraviolet light emitting diodes
No full textCompared to the extensive studies on the efficiency droop of InGaN visible light emitting diodes (LEDs), the efficiency droop of AlGaN deep ultraviolet (UV) LEDs is much less studied. In this context, we discuss the efficiency droop of AlGaN ternary nanowire deep UV LEDs. The device active region consisted of AlGaN double heterojunctions, which were grown by molecular beam epitaxy on silicon substrates. Through detailed analysis of the device optical characteristics under both continuous-wave and pulsed operations, as well as of the electrical characteristics from 293\u2009K to 77\u2009K, it is suggested that the efficiency droop is largely rooted in the low hole mobility, due to the dominant Mg impurity band conduction at room temperature in highly p-doped AlGaN alloys.Peer reviewed: YesNRC publication: Ye
Diffraction-less propagation beyond the sub-wavelength regime: a new type of nanophotonic waveguide
No full textSub-wavelength grating (SWG) metamaterials have garnered a great interest for their singular capability to shape the propagation of light. However, practical SWG implementations are limited by fabrication constraints, such as minimum feature size. Here, we present a new nanophotonic waveguide grating concept that exploits phase-matching engineering to suppress diffraction effects for a period three times larger than those with SWG approaches. This long-period grating not only facilitates fabrication, but also enables a new diffraction-less regime with additional degrees of freedom to control light propagation. More specifically, the proposed phase-matching engineering enables selective diffraction suppression, providing new tools to shape propagation in the grating. We harness this flexible diffraction control to yield single-mode propagation in, otherwise, highly multimode waveguides, and to implement Bragg filters that combine highly-diffractive and diffraction-less regions to dramatically increase light rejection. Capitalizing on this new concept, we experimentally demonstrate a Si membrane Bragg filter with record rejection value exceeding 60\u2009dB. These results demonstrate the potential of the proposed long-period grating for the engineering of diffraction in nanophotonic waveguides and pave the way for the development of a new generation of high-performance Si photonics devices.Peer reviewed: YesNRC publication: Ye
Diffractive sidewall grating coupler: towards 2D free-space optics on chip
No full textSilicon photonics has been the subject of intense research efforts. In order to implement complex integrated silicon photonic devices and systems, a wide range of robust building blocks is needed. Waveguide couplers are fundamental devices in integrated optics, enabling different functionalities such as power dividers, spot-size converters, coherent hybrids and fiber-chip coupling interfaces, to name a few. In this work we propose a new type of nanophotonic coupler based on sidewall grating (SIGRA) concept. SIGRAs have been used in the Bragg regime, for filtering applications, as well as in the sub-wavelength regime in multimode interference (MMI) couplers. However, the use of SIGRAs in the radiation regime has been very limited. Specifically, a coarse wavelength division multiplexer was proposed and experimentally validated. In this work we study the use of SIGRAs in the diffractive regime as a mean to couple the light between a silicon wire waveguide mode and a continuum of slab waveguide modes. We also propose an original technique for designing SIGRA based couplers, enabling the synthesis of arbitrary radiation field profile by Floquet- Bloch analysis of individual diffracting elements while substantially alleviating computational load. Results are further validated by 3D FDTD simulations which confirm that the radiated field profile closely matches the target design field.Peer reviewed: YesNRC publication: Ye
Enhanced performance of integrated silicon nanophotonic devices engineered by sub-wavelength grating structures
No full textSub-wavelength gratings, segmented resonant-less structures with geometries featuring scales considerably smaller than the wavelength of light, have enabled an attractive technological concept to locally control light guiding properties in planar silicon chip architectures. This concept has allowed for additional degrees of freedom to tailor effective mode index, modal confinement, waveguide dispersion, as well as anisotropy, thereby providing a vital route towards high performing devices with engineered optical properties. Sub-wavelength integrated nanophotonics has opened up new horizons for realization of key building components that afford outstanding device performances, typically beyond those achieved by conventional design strategies, yet favorably benefiting from the sub-100-nm pattern resolution of established semiconductor manufacturing tools in nanophotonic foundries. The distinctive features of sub-wavelength grating structures are considered essential for future generation of chip-scale applications in optical communications and interconnects, biomedicine, as well as quantum-based technologies. In this work, we report recent advances in the development of high-performance on-chip nanophotonic waveguides and devices engineered with the sub-wavelength grating metamaterial structures. In particular, we discuss recent achievements of low-loss waveguides with controlled chromatic dispersion, high-efficiency fiber-to-chip surface grating couplers, micro-ring resonators, and grating-assisted waveguide filters, implemented on the mature silicon-on-insulator technology.Peer reviewed: YesNRC publication: Ye
The malate-activated ALMT12 anion channel in the grass Brachypodium distachyon is co-activated by Ca2+/calmodulin
No full textIn plants, strict regulation of stomatal pores is critical for modulation of CO\u2082 fixation and transpiration. Under certain abiotic and biotic stressors, pore closure is initiated through anionic flux, with calcium (Ca\ub2\u207a) playing a central role. The aluminum-activated malate transporter 12 (ALMT12) is a malate-activated, voltage-dependent member of the aluminum-activated malate transporter family that has been implicated in anionic flux from guard cells controlling the stomatal aperture. Herein, we report the characterization of the regulatory mechanisms mediating channel activities of an ALMT from the grass Brachypodium distachyon (BdALMT12) that has the highest sequence identity to Arabidopsis thaliana ALMT12. Electrophysiological studies in a heterologous cell system confirmed that this channel is malate- and voltage-dependent. However, this was shown to be true only in the presence of Ca\ub2\u207a. Although a general kinase inhibitor increased the current density of BdALMT12, a calmodulin (CaM) inhibitor reduced the Ca\ub2\u207a-dependent channel activation. We investigated the physiological relevance of the CaM-based regulation in planta, where stomatal closure, induced by exogenous Ca\ub2\u207a ionophore and malate, was shown to be inhibited by exogenous application of a CaM inhibitor. Subsequent analyses revealed that the double substitutions R335A/R338A and R335A/K342A, within a predicted BdALMT12 CaM-binding domain (CBD), also decreased the channels' ability to activate. Using isothermal titration calorimetry and CBD-mimetic peptides, as well as CaM-agarose affinity pulldown of full-length recombinant BdALMT12, we confirmed the physical interaction between the CBD and CaM. Together, these findings support a co-regulatory mechanism of BdALMT12 activation by malate, and Ca\ub2\u207a/CaM, emphasizing that a complex regulatory network modulates BdALMT12 activity.Peer reviewed: YesNRC publication: Ye
Characterization and reduction of in-use CH\u2084 emissions from a dual fuel marine engine using wavelength modulation spectroscopy
No full textIn-use exhaust stream CH\u2084 emissions from two dual fuel marine engines were characterized and strategies for CH\u2084 reduction were identified and evaluated. For this, a low-cost, portable, wavelength modulation spectroscopy (WMS) system was developed. The performance of the developed WMS sensor was assessed using gas standards and demonstrated on a heavy-duty, diesel pilot ignited, direct-injection natural gas research engine through comparison to a flame ionization detector. The WMS sensor was subsequently used to measure the exhaust-stream CH\u2084 concentration from two diesel pilot-ignited, port-injected natural gas engines on a coastal vessel while under normal operation. Using cylinder deactivation to reduce the excess air ratio, \u3bb, and vessel operation changes to minimize operation at lower loads, the total CH\u2084 emission were reduced by up to 33%. The measured, load specific CH\u2084 emissions were subsequently used to identify an improved vessel operation strategy, with an estimated 56\u201360% reduction in CH\u2084 emissions. These results demonstrate the importance of considering the real-world engine operation profile for accurate estimates of the global warming potential, as well as the utility of a WMS sensor for characterizing and mitigating in-use CH\u2084 emissions.Peer reviewed: YesNRC publication: Ye
Aerodynamic performance of a grooved cylinder in flow conditions encountered by bridge stay cables in service
No full textVibrations of dry bridge stay cables at relatively high wind velocities are of concern for a cable-stayed bridge. One approach to mitigate the vibrations is to modify the cross-sectional geometry of the cable and thereby its aerodynamic characteristics. Recent biological studies reveal that the cactus with a grooved cross-section can survive in high winds thanks to a less abrupt drag coefficient reduction with Reynolds number and a smaller fluctuating side force compared to a basic circular shape. This has inspired the attempt to take advantage of this natural evolution in the aerodynamic design of the protective sheath for bridge stay cables. This paper describes a series of wind tunnel tests on a cactus-shaped rigid cylinder consisting of eight ridges, eight troughs and with the groove depth-to-diameter ratio of 4%. It evaluates the static global aerodynamic forces under different yaw angles, angles of attack and segment rotations. The aerodynamic performance of the cactus cylinder is compared to a round cylinder from previous wind tunnel tests. The study aims to deepen the understanding of the particular groove effects in alleviating the wind excitation, and aid in the development of an alternative, aerodynamically efficient design of the protective sheath for stay cables.Peer reviewed: YesNRC publication: Ye
Flow visualization of a submarine model in a 9m wind tunnel
No full textPeer reviewed: NoNRC publication: Ye