Polarization rotation and coupling between silicon waveguide and hybrid plasmonic waveguide

Mode-evolution-based ultra-broadband polarization beam splitter using adiabatically tapered extreme skin-depth waveguide

We present an ultra-broadband silicon photonic polarization beam splitter (PBS) using adiabatically tapered extreme skin-depth (eskid) waveguides. Highly anisotropic metamaterial claddings of the eskid waveguides suppress the crosstalk of transverse-electric (TE) mode, while the large birefringence of the eskid waveguide efficiently cross-couples the transverse-magnetic (TM) mode. Two eskid waveguides are adiabatically tapered to smoothly translate TM mode to the coupled port via mode evolution while keeping the TE mode in the through port. The tapered cross-section of the eskid PBS was designed numerically, achieving a large bandwidth at 1400-1650 nm with extinction ratios >20dB. We experimentally demonstrated the tapered-eskid PBS and confirmed its broad bandwidth at 1490-1640 nm, limited by laser bandwidth. With its mode evolution, the tapered-eskid PBS is tolerant to fabrication imperfections and should be crucial for controlling polarizations in photonic circuits.

Modular nonlinear hybrid plasmonic circuit

Photonic integrated circuits (PICs) are revolutionizing nanotechnology, with far-reaching applications in telecommunications, molecular sensing, and quantum information. PIC designs rely on mature nanofabrication processes and readily available and optimised photonic components (gratings, splitters, couplers). Hybrid plasmonic elements can enhance PIC functionality (e.g., wavelength-scale polarization rotation, nanoscale optical volumes, and enhanced nonlinearities), but most PIC-compatible designs use single plasmonic elements, with more complex circuits typically requiring ab initio designs. Here we demonstrate a modular approach to post-processes off-the-shelf silicon-on-insulator (SOI) waveguides into hybrid plasmonic integrated circuits. These consist of a plasmonic rotator and a nanofocusser, which generate the second harmonic frequency of the incoming light. We characterize each component's performance on the SOI waveguide, experimentally demonstrating intensity enhancements of more than 200 in an inferred mode area of 100 nm², at a pump wavelength of 1320 nm. This modular approach to plasmonic circuitry makes the applications of this technology more practical.

Towards Deep Integration of Electronics and Photonics

A combination of computational power provided by modern MOSFET-based devices with light assisted wideband communication at the nanoscale can bring electronic technologies to the next level. Obvious obstacles include a size mismatch between electronic and photonic components as well as a weak light–matter interaction typical for existing devices. Polariton modes can be used to overcome these difficulties at the fundamental level. Here, we review applications of such modes, related to the design and fabrication of electro–optical circuits. The emphasis is made on surface plasmon-polaritons which have already demonstrated their value in many fields of technology. Other possible quasiparticles as well as their hybridization with plasmons are discussed. A quasiparticle-based paradigm in electronics, developed at the microscopic level, can be used in future molecular electronics and quantum computing.

Plasmonic polarization beam splitting based on single silver nanowire

Plasmonic waveguides have been indispensable "building-blocks" to construct functional elements for future integrated nano-photonic devices and circuits. In this paper, we demonstrate that a thick silver nanowire with well-defined end facets can provide multiple outcoupling channels, and the controllable beam splitting is realized. The propagating surface plasmons emission at nanowire end are split into two parts: I₁ and I₂, with the polarizations nearly perpendicular to the respective emitting facets. By changing incident polarization, the splitting ratio (I₁/I₂) can be tuned in the range of 1.52~0.36. Electromagnetic simulations indicate that polarization beam splitting mechanisms in this single thick nanowire are the interference of propagating surface plasmon modes and the superposition of excited dipoles at the nanowire end. These findings would deepen the understanding of manipulation of surface plasmons propagation/emission, and advance the development of plasmonic waveguide-based nano-photonic devices.

Controlling evanescent waves using silicon photonic all-dielectric metamaterials for dense integration

Ultra-compact, densely integrated optical components manufactured on a CMOS-foundry platform are highly desirable for optical information processing and electronic-photonic co-integration. However, the large spatial extent of evanescent waves arising from nanoscale confinement, ubiquitous in silicon photonic devices, causes significant cross-talk and scattering loss. Here, we demonstrate that anisotropic all-dielectric metamaterials open a new degree of freedom in total internal reflection to shorten the decay length of evanescent waves. We experimentally show the reduction of cross-talk by greater than 30 times and the bending loss by greater than 3 times in densely integrated, ultra-compact photonic circuit blocks. Our prototype all-dielectric metamaterial-waveguide achieves a low propagation loss of approximately 3.7±1.0 dB/cm, comparable to those of silicon strip waveguides. Our approach marks a departure from interference-based confinement as in photonic crystals or slot waveguides, which utilize nanoscale field enhancement. Its ability to suppress evanescent waves without substantially increasing the propagation loss shall pave the way for all-dielectric metamaterial-based dense integration.

Miniaturization of optical components could give way to dense photonic-integrated circuits. Here, the authors demonstrate the control of evanescent waves using all-dielectric metamaterials and show that they can reduce cross-talk and bending loss, which limit the integration density in photonic circuits.

High-speed plasmonic modulator in a single metal layer

Plasmonics provides a possible route to overcome both the speed limitations of electronics and the critical dimensions of photonics. We present an all-plasmonic 116-gigabits per second electro-optical modulator in which all the elements-the vertical grating couplers, splitters, polarization rotators, and active section with phase shifters-are included in a single metal layer. The device can be realized on any smooth substrate surface and operates with low energy consumption. Our results show that plasmonics is indeed a viable path to an ultracompact, highest-speed, and low-cost technology that might find many applications in a wide range of fields of sensing and communications because it is compatible with and can be placed on a wide variety of materials.

An on-chip hybrid plasmonic light steering concentrator with ∼96% coupling efficiency

We, for the first time, propose and theoretically study a plasmonic light steering concentrator (PLSC) that is based on a hybrid photonic-plasmonic sandwich structure. In this device, a transverse electric (TE) polarization guided mode supported by a silicon-on-insulator (SOI) waveguide is vertically coupled to a metal-dielectric-metal sandwich structure, while the structure steers the light to a perpendicular metal taper and focuses the light on the apex of the taper with a small radius of 15 nm. Based on the coupled-mode theory, the two supermodes (quasi-TM modes) are clarified to illustrate the coupling mechanism of the device. We numerically obtain over 96% coupling efficiency at the 1500 nm telecommunication wavelength, and the mode width supported by the apex is limited laterally within the range of ∼110 nm, where the field enhancement calculated is found to be more than 10⁷ compared to that of light in the silicon waveguide.

Integrated InP polarization rotator using the plasmonic effect

we report on an integrated InP based polarization rotator scheme using the plasmonic effect. It operates as a half-wave retarder in ridge waveguide structure. The rotation angle of the eigenmode axes of the half-wave retarder waveguide is determined by the position off a bottom corner of a metal layer placed above the waveguide core in the upper cladding region. The simple rotator structure enables an easy and tolerant fabrication process. The length of the fabricated device is less than 50 μm, and a polarization extinction ratio (PER) of 20 dB has been achieved.

Hybrid plasmonic mode converter: theoretical formulation and design with a graphical approach

The theoretical formulation based on rigorous transmission-line networks is developed for a general mode conversion problem and lays the groundwork for a simpler yet more efficient graphical design approach for hybrid plasmonic mode converters (HPMCs). The concurrence of co- and cross-polarization conversion to and among higher-order photonic and HP modes followed by subsequent power redistributions and losses over the course of the HPMC can lead to performance degradation and largely determines the silicon core thickness. Using gradient ascent of the TM polarization fraction incorporated with modal index contours sets critical perturbation parameters for required transverse structural asymmetry. Polarization reversal estimates are shown to be practically applicable for about 60% of the total device length. The mode conversion efficiency (MCE), insertion loss (IL), and the polarization conversion efficiency of the proposed HPMC (<7×0.4  μm²) at λ₀=1550  nm are 90.04%, 0.4691 dB, and 99.96%, respectively. The 85%-bandwidth of the MCE is 135 nm, while the IL stays below 0.5 dB over a 68-nm spectral range.

8.13 μm in length and CMOS compatible polarization beam splitter based on an asymmetrical directional coupler

A compact silicon polarization beam splitter (PBS) is proposed based on an asymmetrical directional coupler consisting of a wide waveguide and a dielectric loaded narrow waveguide. Given that TE and TM polarizations are the dominant mode in the wide and narrow waveguides, respectively, a perfect phase-matching condition in the TM mode but a huge phase mismatching in the TE mode can be achieved. Therefore, the TE mode is almost uncoupled regardless of device length but the TM mode can only completely couple to the cross port at an appropriate coupling length. An ultrashort (∼8.13  μm long) PBS is designed based on a silicon-on-insulator nanowire with a loading refractive index of 2.0 and a gap width of 200 nm. Numerical simulations show that the proposed PBS has a broad bandwidth (∼100  nm) with large extinction ratio (>10  dB) and low insertion loss (<0.61  dB). The fabrication-error tolerance of the PBS is also discussed.

Ultracompact polarization rotator in an asymmetric single dielectric loaded rib waveguide

A compact polarization rotator (PR) with an asymmetric single dielectric loaded rib waveguide is proposed. The core of the waveguide is designed to have a specific rectangular configuration. The waveguide requires only a single asymmetrical dielectric loading on the core to complete the polarization conversion. The optical field is confined to the vicinity of the core center, which matches the optical field of the input/output waveguides. The transition loss of the PR is as low as 0.03-0.21 dB/facet without the taper or offset schemes. Such results can facilitate the fabrication of a PR with an operating length of 10 μm. In a comprehensively designed PR with a length of 7.92 μm, a -1  dB bandwidth for polarization conversion efficiency (PCE) is greater than 100 nm at the communicating wavelength of 1550 nm. The loading width and thickness with ±20  nm tolerance exhibit -0.87 and -0.49  dB changes in PCE, respectively.

Mode-evolution-based polarization rotation and coupling between silicon and hybrid plasmonic waveguides

Hybrid plasmonic (HP) modes allow strong optical field confinement and simultaneously low propagation loss, offering a potentially compact and efficient platform for on-chip photonic applications. However, their implementation is hampered by the low coupling efficiency between dielectric guided modes and HP modes, caused by mode mismatch and polarization difference. In this work, we present a mode-evolution-based polarization rotation and coupling structure that adiabatically rotates the TE mode in a silicon waveguide and couples it to the HP mode in a strip silicon-dielectric-metal waveguide. Simulation shows that high coupling factors of 92%, 78%, 75%, and 73% are achievable using Ag, Au, Al, and Cu as the metal cap, respectively, at a conversion length of about 5 μm. For an extremely broad wavelength range of 1300–1800 nm, the coupling factor is >64% with a Ag metal cap, and the total back-reflection power, including all the mode reflections and backscattering, is below −40 dB, due to the adiabatic mode transition. Our device does not require high-resolution lithography and is tolerant to fabrication variations and imperfections. These attributes together make our device suitable for optical transport systems spanning all telecommunication bands.