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Vanadium dioxide (VO2) is a phase change material (PCM) that exhibits a large change in complex refractive index on the order of unity upon switching from its dielectric to its metallic phase. Although this property is key for the design of ultracompact optical modulators of only a few microns in footprint, the high absorption of VO2 leads to appreciable insertion loss (IL) that limits the modulator performance. In this paper, through theory and numerical modeling, we report on a new paradigm, which demonstrates how the use of a hybrid plasmonic waveguide to construct a VO2 based modulator can improve the performance by minimizing its IL while achieving high extinction ratio (ER) in comparison to a purely dielectricwaveguide. The hybrid plasmonic waveguide that contains an additional metal layer with even higher loss than VO2 enables unique approaches to engineer the electric field (E-field) intensity distribution within the cross section of the modulator. The resulting figure-of-merit, FoM = ER/IL, is much higher than what is possible by simply incorporating VO2 into a silicon wire waveguide. A practical modulator design using this new approach, which also includes input and output couplers yields ER = 3.8 dB/µm and IL = 1.4 dB/µm (FoM= 2.7), with a 3-dB optical bandwidth>500 nm, in a device length= 2 µm, and crosssectional dimensions = 200 nm x 450 nm. To the best of our knowledge, this is one of the smallest modulator designs proposed to date that also exhibits amongst the highest ER, FoM, and optical bandwidth, in comparison to existing designs. In addition to VO2, we investigate two other PCMs incorporated within the waveguide structure. The improvements obtained for VO2 modulators do not extend to other PCMs. [ABSTRACT FROM AUTHOR]

An ultra-compact 90° bend between silicon (Si) wire and silicon slot waveguide (SSW) with a footprint of ~725 nm × 500 nm is experimentally demonstrated on silicon-on-insulator substrate. By achieving momentum matching of the waveguides through an orthogonal junction placement, and maximizing modal overlap through an angled facet, coupling efficiency of ~70% and 3-dB bandwidth of over 500 nm has been achieved. The nominal experimental transmission through cascaded input (Si wire to SSW) and output (SSW to Si wire) orthogonal junctions match closely those obtained from simulations, both in the range from 1270 to 1360 nm and from 1480 to 1590 nm. For slot widths ranging from 30 to 230 nm, our Si wire-SSW bend can achieve coupling efficiency comparable to that of a direct butt-coupler over a 400 nm bandwidth. This compact and wideband waveguide bend serves as an important component to enable dense integration between conventional Si wire and SSW. [ABSTRACT FROM PUBLISHER]

Momentum matching at hybrid junctions is examined for efficient broadband energy transfer between internal reflection guided waves and evanescence-based plasmonic-gap guided waves. We demonstrate a nanoscale orthogonal junction coupler between 50 nm air-filled plasmonic slot waveguides (PSWs) and 450 nm silicon rib waveguides. Non-resonant junction coupling efficiency of 50 ± 2 % between 1450 nm and 1650 nm is achieved experimentally and PSW propagation loss is directly measured to be only 2.5 dB/μm. This taperless hybrid junction reduces PSW-based device footprint and enhances device tolerance to temperature and fabrication process variations, serving as a potential platform for hybrid silicon-plasmonic interconnects. [ABSTRACT FROM AUTHOR]

We theoretically investigate the enhancement of surface enhanced Raman spectroscopy (SERS) using hyperbolic stratified nanostructures and compare to metal nanoresonators. The photon Green function of each nanostructure within its environment is first obtained from a semianalytical modal theory, which is used in a quantum optics formalism of the molecule-nanostructure interaction to model the SERS spectrum. An intuitive methodology is presented for calculating the single-molecule enhancement factor (SMEF), which is also able to predict known experimental SERS enhancement factors of a gold nanodimer. We elucidate the important figures-of-merit of the enhancement and explore these for different designs. We find that the use of hyperbolic stratified materials can enhance the photonic local density of states (LDOS) by close to two times in comparison to pure metal nanostructures, when both designed to work at the same operating wavelengths. However, the increased LDOS is accompanied by higher electric field concentration within the lossy hyperbolic material, which leads to increased quenching that serves to reduce the overall detected SERS enhancement in the far field. For nanoresonators with resonant localized surface plasmon wavelengths in the near-infrared, the SMEF for the hyperbolic stratified nanostructure is approximately one order of magnitude lower than the pure metal counterpart. Conversely, we show that by detecting the Raman signal using a near-field probe, hyperbolic materials can provide an improvement in SERS enhancement compared to using pure metal nanostructures when the probe is sufficiently close (<50nm) to the Raman active molecule at the plasmonic hotspot. [ABSTRACT FROM AUTHOR]

We study the optical properties of quantum dipole emitters coupled to hyperbolic metamaterial nanoresonators using a semianalytical quasinormal mode approach. We show that coupling to metamaterial nanoresonators can lead to significant Purcell enhancements that are nearly an order of magnitude larger than those of plasmonic resonators with comparable geometry. However, the associated single-photon output β-factors are extremely low (around 10%), far smaller than those of comparably sized metallic resonators (70%). Using a quasinormal mode expansion of the photon Green function, we describe how the low β-factors are due to increased Ohmic quenching arising from redshifted resonances, larger quality factors, and stronger confinement of light within the metal. These results explain why hyperbolic metamaterial nanostructures likely make poor choices for single-photon sources. [ABSTRACT FROM AUTHOR]