Aperture-coupled MIM plasmonic ring resonators with sub-diffraction modal volumes

Propagation-Invariant Space-Time Plasmonic Pulse in Subwavelength MIM Waveguide

The metal-insulator-metal (MIM) plasmonic waveguide has been highly anticipated for confining and guiding surface plasmon polaritons (SPPs) on the subwavelength scale. However, perennial drawbacks such as a short propagation length and an unbounded transverse field have set limits on the use of the MIM waveguide in various applications. Herein, diffraction- and dispersion-free MIM modes are synthesized by using space-time wave packets (STWPs) and are therefore referred to as space-time MIM (ST-MIM) waveguide modes. Compared to a Gaussian pulse of the same duration and spectral bandwidth, the ST-MIM demonstrates enhanced propagation lengths of about 2.4 times for the symmetric mode and about 6.3 times for the antisymmetric mode. In the simulations, the ST-MIMs are confined in all transverse dimensions, thereby overriding the diffraction limits. In addition, the group velocities of the ST-MIMs can be arbitrarily designed, which makes it possible to synchronize the pulse propagation speeds of the symmetric and antisymmetric MIM modes.

The Generalized Analytical Expression for the Resonance Frequencies of Plasmonic Nanoresonators Composed of Folded Rectangular Geometries

A robust generalized analytical expression for resonance frequencies of plasmonic nanoresonators, which consists of folded rectangular structures, is proposed based on a circuit route. The formulation is rigorously derived from the lumped circuit analogue of the plasmon resonance in a rectangular metallic nanorod. Induced by the nonhomogeneous charge distributions in the plasmonic resonators of rectangular end-caps, the electromagnetic forces drive the harmonic oscillations of free electrons in the plasmonic nanoresonators, generating intrinsically nonlinear shape-dependent LC resonance responses. Even for the plasmonic nanoresonators with much larger structure sizes than the skin depths, the significant frequency deviations due to the phase-retardation behavior can still be adequately described by the generalized expression. Moreover, for a large range of plasmonic nanoresonators with various folded rectangular geometries, sizes and materials, the generalized analytical expression gives the underlining physics and provides accurate predictions, which are perfectly verified by a series of numerical simulations. Our studies not only offer quantitative insights of nearly any plasmonic nanoresonators based on folded rectangular geometries, but also reveal potential applications to design complex plasmonic systems, such as periodic arrays with embedded rectangular nanoresonators.

Tunable Nanosensor Based on Fano Resonances Created by Changing the Deviation Angle of the Metal Core in a Plasmonic Cavity

In this paper, a type of tunable plasmonic refractive index nanosensor based on Fano resonance is proposed and investigated. The sensor comprises a metal-insulator-metal (MIM) nanocavity with a center-deviated metal core and two side-coupled waveguides. By carefully adjusting the deviation angle and distance of the metal core in the cavity, Fano resonances can be obtained and modulated. The Fano resonances can be considered as results induced by the symmetry-breaking or geometric effect that affects the field distribution intensity at the coupling region between the right waveguide and the cavity. Such a field-distribution pattern change can be regarded as being caused by the interference between the waveguide modes and the cavity modes. The investigations demonstrate that the spectral positions and modulation depths of Fano resonances are highly sensitive to the deviation parameters. Furthermore, the figure of merit (FOM) value is calculated for different deviation angle. The result shows that this kind of tunable sensor has compact structure, high transmission, sharp Fano lineshape, and high sensitivity to the change in background refractive index. This work provides an effective method for flexibly tuning Fano resonance, which has wide applications in designing on-chip plasmonic nanosensors or other relevant devices, such as information modulators, optical filters, and ultra-fast switches.

Plasmofluidic Disk Resonators

Waveguide-coupled silicon ring or disk resonators have been used for optical signal processing and sensing. Large-scale integration of optical devices demands continuous reduction in their footprints, and ultimately they need to be replaced by silicon-based plasmonic resonators. However, few waveguide-coupled silicon-based plasmonic resonators have been realized until now. Moreover, fluid cannot interact effectively with them since their resonance modes are strongly confined in solid regions. To solve this problem, this paper reports realized plasmofluidic disk resonators (PDRs). The PDR consists of a submicrometer radius silicon disk and metal laterally surrounding the disk with a 30-nm-wide channel in between. The channel is filled with fluid, and the resonance mode of the PDR is strongly confined in the fluid. The PDR coupled to a metal-insulator-silicon-insulator-metal waveguide is implemented by using standard complementary metal oxide semiconductor technology. If the refractive index of the fluid increases by 0.141, the transmission spectrum of the waveguide coupled to the PDR of radius 0.9 μm red-shifts by 30 nm. The PDR can be used as a refractive index sensor requiring a very small amount of analyte. Plus, the PDR filled with liquid crystal may be an ultracompact intensity modulator which is effectively controlled by small driving voltage.

Spoof localized surface plasmons on ultrathin textured MIM ring resonator with enhanced resonances

We numerically demonstrate that spoof localized surface plasmons (LSPs) resonant modes can be enhanced based on ultrathin corrugated metal-insulator-metal (MIM) ring resonator. Further enhancement of the LSPs modes has been achieved by incorporating an efficient and ease-of-integration exciting method. Quality factors of resonance peaks have become much larger and multipolar resonances modes can be easily observed on the textured MIM ring resonator excited by a microstrip line. Experimental results validate the high-efficiency excitation and resonance enhancements of spoof LSPs modes on the MIM ring resonator in the microwave frequencies. We have shown that the fabricated resonator is sensitive to the variation of both the dielectric constant and the thickness of surrounding materials under test. The spoof plasmonic resonator can be used as key elements to provide many important device functionalities such as optical communications, signal processing, and spectral engineering in the plasmonic integration platform.

Analogue of electromagnetically induced transparency in integrated plasmonics with radiative and subradiant resonators

We propose the use of radiative and subradiant resonators coupled to a metal-insulator-metal waveguide to represent the three-level energy diagram in conventional atomic systems and demonstrate a new realization of on-chip plasmonic analogue of electromagnetically-induced transparency (EIT) in integrated plasmonics. The radiative resonator is achieved with the help of aperture-coupling while evanescent coupling is relied for the subradiant resonator. Numerical simulation results demonstrate well-pronounced intermediate transmission peak through the bus waveguide and also show that the EIT effect can be easily controlled by the relative position of the two Fabry-Perot resonators.

Plasmon-induced transparency in metal-insulator-metal waveguide side-coupled with multiple cavities

We have demonstrated the analogue of electromagnetically induced transparency (EIT) in the metal-insulator-metal plasmonic waveguide, which consists of a bus waveguide side-coupled with a series of slot cavities. By finite-difference time-domain simulations, it is found that the resonance wavelength of the slot cavity can be controlled by adjusting the length of the cavity. Moreover, the EIT-like response is strongly dependent on the coupling separation between the corresponding adjacent cavities. Multiple-peak plasmon-induced transparency can be realized by cascading multiple cavities with different lengths and suitable cavity-cavity separations. This ultracompact plasmonic waveguide system may find important applications for multichannel plasmonic filter, nanoscale optical switching, and slow-light devices in highly integrated optical circuits and networks.

Silicon nitride based plasmonic components for CMOS back-end-of-line integration

Silicon nitride waveguides provide low propagation loss but weak mode confinement due to the relatively small refractive index contrast between the Si₃N₄ core and the SiO2 cladding. On the other hand, metal-insulator-metal (MIM) plasmonic waveguides offer strong mode confinement but large propagation loss. In this work, MIM-like plasmonic waveguides and passive devices based on horizontal Cu-Si₃N₄-Cu or Cu-SiO₂-Si₃N₄-SiO₂-Cu structures are integrated in the conventional Si₃N₄ waveguide circuits using standard CMOS backend processes, and are characterized around 1550-nm telecom wavelengths using the conventional fiber-waveguide-fiber method. The Cu-Si₃N₄(~100 nm)-Cu devices exhibit ~0.78-dB/μm propagation loss for straight waveguides, ~38% coupling efficiency with the conventional 1-μm-wide Si₃N₄ waveguide through a 2-μm-long taper coupler, ~0.2-dB bending loss for sharp 90° bends, and ~0.1-dB excess loss for ultracompact 1 × 2 and 1 × 4 power splitters. Inserting a ~10-nm SiO₂ layer between the Si3N4 core and the Cu cover (i.e., the Cu-SiO2(~10 nm)-Si₃N₄(~100 nm)-SiO2(~10 nm)-Cu devices), the propagation loss and the coupling efficiency are improved to ~0.37 dB/μm and ~52% while the bending loss and the excess loss are degraded to ~3.2 dB and ~2.1 dB, respectively. These experimental results are roughly consistent with the numerical simulation results after taking the influence of possible imperfect fabrication into account. Ultracompact plasmonic ring resonators with 1-μm radius are demonstrated with an extinction ratio of ~18 dB and a quality factor of ~84, close to the theoretical prediction.

Highly tunable nanoscale metal-insulator-metal split ring core ring resonators (SRCRRs)

A class of nano-scale wavelength-selective optical filters is proposed where the core of a metal-insulator-metal square ring is replaced with a split-ring core (SRC). The proposed resonator supports split-ring-resonator-like (SRR-like) resonant modes that are characteristics of the structure. These resonant modes are highly adjustable, via the gap size of the split-ring core, over a range of hundreds of nanometers. The proposed resonator can also incorporate tunable materials localized in the gap of the SRC or placed throughout the resonating path. By varying the refractive index (1 to 2) of the material in the gap of the SRC, first and second SRR-like modes can be tuned over ~200 and 300 nm, respectively. A circuit model based on transmission-line theory is proposed for the structure and used to derive the resonance conditions of the split-ring-resonator-like modes; the model compares favorably to the numerical results. The proposed resonator has the potential to be utilized effectively in integrated nano-scale optical switches and tunable filters.

Radiation guiding with surface plasmon polaritons

Surface plasmon polaritons (SPPs) are electromagnetic (EM) modes propagating along metal-dielectric interfaces, in which surface collective excitations of free electrons in the metal are coupled to evanescent EM fields in the dielectric. Various SPP modes can be supported by flat and curved, single and multiple surfaces, exhibiting remarkable properties, including the possibility of concentrating EM fields beyond the diffraction limit, i.e. on the nanoscale, while enhancing local field strengths by several orders of magnitude. This unique feature of SPP modes, along with the ever-increasing demands for miniaturization of photonic components and circuits, generates an exponentially growing interest in SPP-mediated radiation guiding and SPP-based waveguide components. Here we review the current status of this rapidly developing field, starting with a brief presentation of the main planar SPP modes along with the techniques employed for their excitation and manipulation by sets of nanoparticles. We then describe in detail various SPP-based waveguide configurations that ensure two-dimensional mode confinement in the plane perpendicular to the propagation direction and compare their characteristics. Excitation of SPP waveguide modes and recent progress in the development of SPP-based waveguide components are also discussed, concluding with our outlook on challenges and possible future developments in this field.

Selective-mode optical nanofilters based on plasmonic complementary split-ring resonators

A nanoplasmonic optical filtering technique based on a complementary split-ring resonator structure is proposed. The basic and modal properties of the square-nanoring are studied using the group theory. Degeneracy and non-degeneracy of the possible TM odd- and even-modes are characterized based on the symmetry elements of the ring structure. Distinctively, the proposed technique allows selecting and exciting the proper plasmonic modes of the nanoring in the side-coupled arrangement. It is found that the non-integer modes can be excited due to the presence of a metallic nano-wall. These modes are highly sensitive to the nano-wall dimensions, in contrast to the regular integer modes. Moreover, the transmission-line theory is used to derive the resonance condition of the modes. The results show the optical transmission spectrum of the investigated filter can be efficiently modified and tuned either by manipulation of the position or by variation of the width of the employed nano-wall inside the ring. The numerical results support the theoretical analysis.

Optical bistability effect in plasmonic racetrack resonator with high extinction ratio

In this paper, optical bistability effect in an ultracompact plasmonic racetrack resonator with nonlinear optical Kerr medium is investigated both analytically and numerically. The properties of optical bistability and pump threshold are studied at 1.55 µm with various detuning parameters by an analytical model. The transmission switch from the upper branch to the lower branch with a pulse is also demonstrated by a finite-difference time-domain method. An extinction ratio of 97.8% and a switching time of 0.38 ps can be achieved with proper detuning parameter. Such a plasmonic resonator design provides a promising realization for highly effective optical modulators and switch.

Plasmon-induced transparency with detuned ultracompact Fabry-Perot resonators in integrated plasmonic devices

We demonstrate the realization of on-chip plasmonic analogue of electromagnetically induced transparency (EIT) in integrated plasmonic devices using detuned Fabry-Perot resonators aperture-side-coupled to a metal-insulator-metal (MIM) waveguide, with the transmission peak occurring at the intermediate wavelength. Strong MIM mode confinement along with localized side-coupling allows one to realize subwavelength photonic components with EIT-like transmission. Numerical results show that MIM components exhibiting pronounced EIT-like spectra in near infrared with the footprint of < 0.15 μm2 and group index of ~26 can be designed.

Numerical analysis of deep sub-wavelength integrated plasmonic devices based on Semiconductor-Insulator-Metal strip waveguides

We report the first study of nanoscale integrated photonic devices constructed with semiconductor-insulator-metal strip (SIMS) waveguides for use at telecom wavelengths. These waveguides support hybrid plasmonic modes transmitting through a 5-nm thick insulating region with a normalized intensity of 200-300 μm(-2). Their fundamental mode, unique transmission and dispersion properties are consistent with photonic devices for guiding and routing of signals in communication applications. It has been demonstrated using Finite Element Methods (FEM) that the high performance SIMS waveguide can be used to fabricate deep sub-wavelength integrated plasmonic devices such as directional couplers with the ultra short coupling lengths, sharply bent waveguides, and ring resonators having a functional size of ≈1 µm and with low insertion losses and nearly zero radiation losses.