Low-loss light transport at the subwavelength scale in silicon nano-slot based symmetric hybrid plasmonic waveguiding schemes

Silicon-Based Optoelectronics Enhanced by Hybrid Plasmon Polaritons: Bridging Dielectric Photonics and Nanoplasmonics

Silicon-based optoelectronics large-scale integrated circuits have been of interest to the world in recent decades due to the need for higher complexity, larger link capacity, and lower cost. Surface plasmons are electromagnetic waves that propagate along the interface between a conductor and a dielectric, which can be confined several orders smaller than the wavelength in a vacuum and offers the potential for minimizing photonic circuits to the nanoscale. However, plasmonic waveguides are usually accompanied by substantial propagation loss because metals always exhibit significant resistive heating losses when interacting with light. Therefore, it is better to couple silicon-based optoelectronics and plasmonics and bridge the gap between micro-photonics and nanodevices, especially some nano-electronic devices. In this review, we discuss methods to enhance silicon-based optoelectronics by hybrid plasmon polaritons and summarize some recently reported designs. It is believed that by utilizing the strong light confinement of plasmonics, we can overcome the conventional diffraction limit of light and further improve the integration of optoelectronic circuits.

LiNbO3 waveguide with embedded Ag nanowire and 3L MoS2 for strong light confinement and ultra-long propagation length in the visible spectral range

A vertical slot LiNbO₃ waveguide with an Ag nanowire and 3L MoS₂ embedded in the low-refractive index slot region is proposed for the purpose of improving light confinement. We find that the proposed waveguide has a novel dielectric based plasmonic mode, where local light field is enhanced by the Ag nanowire. The mode exhibits an extremely large figure of merit (FoM) of 6.5×10⁶, one order of magnitude larger than that the largest FoM of any plasmonic waveguide reported in the literature to date. The waveguide also has an extremely long propagation length of 84 cm in the visible wavelength at 680 nm. Furthermore, the waveguide has a low sub-micro bending loss and can be directly connected to all-dielectric waveguides with an extremely low coupling loss. The proposed vertical slot LiNbO₃ waveguide is a promising candidate for the realization of ultrahigh integration density tunable circuits in the visible spectral range.

Deep subwavelength confinement and threshold engineering in a coupled nanorods based spaser

In recent years, extensive efforts have been made for design and fabrication of low threshold spasers or plasmonic nanolasers at a deep subwavelength scale. Plasmonic nanolasers with coupled-nanorods structure can realize this purpose due to energy concentration in nano size volumes and effective amplification mechanisms. In this study, a group of structures based on metallic and CdS coupled nanorods are designed and analyzed using the finite element method (FEM). By changing the lateral adjacent surfaces of the metal and semiconductor nanorods through utilizing regular polygons as the cross sections of the nanorods, different characteristics of the plasmonic nanolaser are investigated. Simulation results show that the mode area normalized by the diffraction limit area is as low as 0.0062 in the structures based on hexagonal metallic core with circular semiconductor nanorods while structures based on circular Ag core with hexagonal CdS nanorods can provide a low threshold gain as 1.310 μm^-1. Also, it is shown that if ZnO be used as the semiconductor gain material instead of CdS, a normalized mode area of almost one tenth can be attained in a structure with dodecagonal metallic core and circular ZnO nanorods.

Ultra-high light confinement and ultra-long propagation distance design for integratable optical chips based on plasmonic technology

The ever-increasing demand for faster speed, broader bandwidth, and lower energy consumption of on-chip processing has motivated the use of light instead of electrons in functional communication components. However, considerable scattering loss severely affects the performance of nanoscale photonic devices when their physical sizes are smaller than the wavelength of light. Due to the tight localization of electromagnetic energy, plasmonic waveguides that work at visible and infrared wavebands have provided a solution for the optical diffraction limit problem and thus enable downscaling of optical circuits and chips at the nanoscale. However, due to the fundamental trade-off between propagation distance and light confinement, plasmonic waveguides, including conventional hybrid plasmonic waveguides (HPWGs), cannot be used as high performance integratable optical devices all the time. To solve this problem, a novel hybrid plasmonic waveguide is proposed where a hybrid metal-ridge-slot structure based on a two-dimensional (2D) transition metal dichalcogenide is embedded into two identical cylindrical dielectric waveguides. Benefiting from both the loss-less slot region and the high-index difference between the ultra-thin 2D material and the slot region, a 10 times longer propagation length and 100 times smaller mode area than the traditional HPWG are achieved at the telecommunication band. By removing the monolayer transition metal dichalcogenide, our designed waveguide shows a higher propagation length that is at least two orders of magnitude larger than its traditional HPWG counterpart. Therefore, the proposed hybridization waveguiding approach paves the way toward truly high-performance and deep-subwavelength integratable optical circuits and chips in the future.

Hybrid low-permittivity slot-rib plasmonic waveguide based on monolayer two dimensional transition metal dichalcogenide with ultra-high energy confinement

A hybrid plasmonic waveguide design is proposed that incorporates a two-dimensional transition metal dichalcogenide monolayer covered slot-rib in between a cylindrical waveguide and a metal surface. A deep optical energy confinement (mode area ranging from λ²/1000000-λ²/100000) along with a reasonable propagation length (5μm-25μm) can be realized at the working wavelength of 1550 nm. In comparison with a traditional hybrid plasmonic waveguide, the proposed waveguide structure exhibits a smaller mode area as well as a higher figure of merit. Investigation on the influence of various two-dimensional materials on modal properties reveals that a larger permittivity provides a stronger field confinement. Owing to its excellent energy field confinement with low transmission loss, the proposed waveguide can be utilized in a variety of plasmonic devices such as compact plasmonic chips, high-integration plasmonic nano-lasers and high-sensitivity plasmonic detectors.

Design and fabrication of hybrid SPP waveguides for ultrahigh-bandwidth low-penalty terabit-scale data transmission

Here we design and fabricate a hybrid surface plasmon polarities (SPP) waveguide on the silicon-on-insulator (SOI) photonics platform. The designed hybrid SPP waveguide is composed of a metal ridge, an air gap, and a silicon ridge. We simulate the mode characteristics in the structure and design the waveguide with a wide air gap that can simplify the fabrication process and maintain the advantages of the hybrid SPP mode. The performance of ultrahigh-bandwidth data transmission through the proposed waveguide is then investigated using 161 wavelength-division multiplexing (WDM) channels, each carrying a 11.2-Gbit/s orthogonal frequency-division multiplexing (OFDM) 16-ary quadrature amplitude modulation (16-QAM) signal. The bit-error rates (BERs) of all 161 channels are less than 1e-3. The favorable results show the prospect of on-chip optical interconnection using the proposed hybrid SPP waveguide.

Local field enhancement on demand based on hybrid plasmonic-dielectric directional coupler

The concept of local field enhancement using conductor-gap-dielectric-substrate (CGDS) waveguide structure is proposed. The dispersion equation is derived analytically and solved numerically. The solution of the dispersion equation reveals the anti-crossing behavior of coupled modes. the optimal gap layer thickness and the coupling length of the guided modes are obtained. The mechanism of the CGDS works as follows: Light waves are guided by conventional low-loss dielectric waveguides and, upon demand, they are transformed into highly confined plasmonic modes with strong local field enhancement, and get transformed back into low-loss dielectric modes. As an example, in a representative CGDS structure, the optimal plasmonic gap size is 17 nm, the local light intensity is found to be more than one order of magnitude stronger than the intensity of the dielectric mode at the film surface. The coupling length is only 2.1 μm at a wavelength of 632.8 nm. Such a local field confinement on demand is expected to facilitate efficient light-matter interaction in integrated photonic devices while minimizing losses typical for plasmonic structures.

Graphene-based long-range SPP hybrid waveguide with ultra-long propagation length in mid-infrared range

A graphene-based long-range surface plasmon polariton (LRSPP) hybrid waveguide, which is composed of two identical outer graphene nanoribbons and two identical inner silica layers symmetrically placed on both sides of a silicon layer, is investigated using the finite-difference time-domain method. By combining the simulated results with the coupled mode perturbation theory, we demonstrate that the LRSPP and short-range SPP (SRSPP) modes originate from the coupling of the same modes of the two graphene nanoribbons. For the LRSPP mode, an ultra-long propagation length (~10 μm) and an ultra-small mode area (~10^-7A₀, where A₀ is the diffraction-limited mode area) can be simultaneously achieved. This waveguide can be used for future photonic integrated circuits functional in the mid-infrared range.

Utilization of Field Enhancement in Plasmonic Waveguides for Subwavelength Light-Guiding, Polarization Handling, Heating, and Optical Sensing

Plasmonic nanostructures have attracted intensive attention for many applications in recent years because of the field enhancement at the metal/dielectric interface. First, this strong field enhancement makes it possible to break the diffraction limit and enable subwavelength optical waveguiding, which is desired for nanophotonic integrated circuits with ultra-high integration density. Second, the field enhancement in plasmonic nanostructures occurs only for the polarization mode whose electric field is perpendicular to the metal/dielectric interface, and thus the strong birefringence is beneficial for realizing ultra-small polarization-sensitive/selective devices, including polarization beam splitters, and polarizers. Third, plasmonic nanostructures provide an excellent platform of merging electronics and photonics for some applications, e.g., thermal tuning, photo-thermal detection, etc. Finally, the field enhancement at the metal/dielectric interface helps a lot to realize optical sensors with high sensitivity when introducing plasmonic nanostrutures. In this paper, we give a review for recent progresses on the utilization of field enhancement in plasmonic nanostructures for these applications, e.g., waveguiding, polarization handling, heating, as well as optical sensing.

Silicon-organic hybrid slot waveguide based three-input multicasted optical hexadecimal addition/subtraction

By exploiting multiple non-degenerate four-wave mixing in a silicon-organic hybrid slot waveguide and 16-ary phase-shift keying signals, we propose and simulate three-input (A, B, C) multicasted 40-Gbaud (160-Gbit/s) optical hexadecimal addition/subtraction (A + B - C, A + C - B, B + C - A, A + B + C, A - B - C, B - A - C). The error vector magnitude (EVM) and dynamic range of signal power are analyzed to evaluate the performance of optical hexadecimal addition/subtraction.

Long range mid-infrared propagation in Si and Ge hybrid plasmonic-photonic nano-ribbon waveguides

We have investigated a hybrid plasmonic-photonic mode in Si and Ge channel waveguides over the 1.55-8.0 μm wavelength range. A 10-nm Cu ribbon was buried midway within a Si₃N₄ "photonic slot" centered in the semiconductor strip. For the TMo mode, propagation lengths L of several millimeters are predicted for a waveguide cross-section of about 0.7λ/n x 0.7λ/n which offers optical confinement mainly within the ~λ²/400-area slot. The L increased strongly with λ. For 0.4λ/n x 0.4λ/n channels, we found multi-centimeter propagation, but there ~60% of the propagating energy had leaked out into the thick, all-around Si₃N₄ cladding.

Highly confined guiding of low-loss plasmon waves in hybrid metal-dielectric slot waveguides

We report the observation of strongly confined plasmon modes in hybridized metal-dielectric slot waveguides, which consist of semiconductor-insulator-semiconductor nanostructures embedded inside the low-index gaps of conventional hybrid plasmonic configurations. Owing to the combined effects induced by the high-refractive-index-contrast dielectric slot and semiconductor-insulator-metal configurations, tight field localization (Aeff ~ λ(2)/1250-λ(2)/55) in conjunction with large propagation distances (L ~ 70-180 μm) can be realized simultaneously at telecommunication wavelength. Through comprehensive numerical simulations, the characteristics of the fundamental hybrid modes are revealed in detail by optimizing key structural parameters of the waveguides. The advantages over their traditional hybrid waveguiding counterparts are unraveled. In addition, the possibilities of extending our current design into other metal/dielectric composites are also discussed. Our studies regarding hybrid metal-dielectric slot structures and their alternatives in this paper are expected to provide effective approaches for the enhancement of traditional hybrid modes' properties and open up new opportunities for the constructions of high-performance plasmon waveguides and devices.

Copper nanorod array assisted silicon waveguide polarization beam splitter

We present the design of a three-dimensional (3D) polarization beam splitter (PBS) with a copper nanorod array placed between two silicon waveguides. The localized surface plasmon resonance (LSPR) of a metal nanorod array selectively cross-couples transverse electric (TE) mode to the coupler waveguide, while transverse magnetic (TM) mode passes through the original input waveguide without coupling. An ultra-compact and broadband PBS compared to all-dielectric devices is achieved with the LSPR. The output ports of waveguides are designed to support either TM or TE mode only to enhance the extinction ratios. Compared to silver, copper is fully compatible with complementary metal-oxide-semiconductor (CMOS) technology.

Guiding properties of asymmetric hybrid plasmonic waveguides on dielectric substrates

We proposed an asymmetric hybrid plasmonic waveguide which is placed on a substrate for practical applications by introducing an asymmetry into a symmetric hybrid plasmonic waveguide. The guiding properties of the asymmetric hybrid plasmonic waveguide are investigated using finite element method. The results show that, with proper waveguide sizes, the proposed waveguide can eliminate the influence of the substrate on its guiding properties and restore its broken symmetric mode. We obtained the maximum propagation length of 2.49 × 103 μm. It is approximately equal to that of the symmetric hybrid plasmonic waveguide embedded in air cladding with comparable nanoscale confinement.

Asymmetric hybrid plasmonic waveguides with centimeter-scale propagation length under subwavelength confinement for photonic components

An asymmetric hybrid plasmonic metal-wire waveguide is proposed by combining the advantages of symmetric and hybrid plasmonic modes. The idea of asymmetric structure eliminates the adverse effect of a substrate and enhances the optical performance of the waveguide. The guiding properties of the proposed waveguide are intensively investigated using the finite elements method. The results exhibit a quite long propagation length of 2.69 cm with subwavelength confinement. More importantly, an extremely large figure of merit of 139037 is achieved. Furthermore, the proposed waveguides can be used as directional couplers. They can achieve a coupling length of only 1.01 μm at S = 0.1 μm with negligible loss. A strong dependence of coupling length on the operating wavelength makes the proposed waveguide promising for realizing wavelength-selective components at telecommunication wavelengths.

Ultra-long-range symmetric plasmonic waveguide for high-density and compact photonic devices

This study reports a symmetric hybrid plasmonic waveguide consisting of a cylindrical metal nanowire surrounded by low-index SiO₂ and high-index Si covered with SiO₂. The symmetric circumambience relative to the metal nanowire significantly facilitates the present design to minimize the energy attenuation resulting from Ohmic losses while retaining highly confined modes guided in the low-index nanoscale gaps between the metal nanowire and the high-index Si. The geometric dependence of the mode characteristics on the proposed structure is analyzed in detail, showing long propagation lengths beyond 10 mm with normalized mode areas on the order of 10⁻². In addition to enabling the building of long-range plasmonic circuit interconnects, the compactness and high-density integration of the proposed structure are examined by analyzing crosstalk in a directional coupler composed of two such waveguides and bending losses for a 90° bend. A relatively short coupling length of 1.16 μm is obtained at a center-to-center separation of 0.26 μm between adjacent waveguides. Increasing the separation to 1.65 μm could completely prevent coupling between waveguides. Power transmission exceeds 80% in the case of a 90° bend with small radius of curvature of 0.5 μm. Moreover, the dependence of spectral response on coupling length and the transmission of a 90° bend, ranging from telecom wavelengths of 1.40 to 1.65 μm, are investigated. Over a wide wavelength range, a strong coupling length dependence on wavelength and a high transmission for a 90° bend also make the proposed plasmonic waveguide promising for the realization of wavelength-selective components.