Dielectric-loaded Surface Plasmon Polariton Waveguides Research Articles

Advanced mid-infrared plasmonic waveguides for on-chip integrated photonics

Long-wave infrared (LWIR, 8–14 µm) photonics is a rapidly growing research field within the mid-IR with applications in molecular spectroscopy and optical free-space communication. LWIR applications are often addressed using rather bulky tabletop-sized free-space optical systems, preventing advanced photonic applications, such as rapid-time-scale experiments. Here, device miniaturization into photonic integrated circuits (PICs) with maintained optical capabilities is key to revolutionize mid-IR photonics. Subwavelength mode confinement in plasmonic structures enabled such miniaturization approaches in the visible-to-near-IR spectral range. However, adopting plasmonics for the LWIR needs suitable low-loss and -dispersion materials with compatible integration strategies to existing mid-IR technology. In this paper, we further unlock the field of LWIR/mid-IR PICs by combining photolithographic patterning of organic polymers with dielectric-loaded surface plasmon polariton (DLSPP) waveguides. In particular, polyethylene shows favorable optical properties, including low refractive index and broad transparency between ∼2 μm and 200 µm. We investigate the whole value chain, including design, fabrication, and characterization of polyethylene-based DLSPP waveguides and demonstrate their first-time plasmonic operation and mode guiding capabilities along S-bend structures. Low bending losses of ∼1.3 dB and straight-section propagation lengths of ∼1 mm, pave the way for unprecedented complex on-chip mid-IR photonic devices. Moreover, DLSPPs allow full control of the mode parameters (propagation length and guiding capabilities) for precisely addressing advanced sensing and telecommunication applications with chip-scale devices.

Implementing commercial inverse design tools for compact, phase-encoded, plasmonic digital logic devices

Numerical simulations have become an essential design tool in the field of photonics, especially for nanophotonics. In particular, 3D finite-difference-time-domain (FDTD) simulations are popular for their powerful design capabilities. Increasingly, researchers are developing or using inverse design tools to improve device footprints and performance. These tools often make use of 3D FDTD simulations and the adjoint optimization method. In this work, we implement a commercial inverse design tool with these features for several plasmonic devices that push the boundaries of the tool. We design a logic gate with complex design requirements, as well as a y-splitter and waveguide crossing. With minimal code changes, we implement a design that incorporates phase encoded inputs in a dielectric-loaded surface plasmon polariton waveguide. The complexity of the requirements in conjunction with limitations in the inverse design tool force us to make concessions regarding the density of encoding and to use on-off keying to encode the outputs. We compare the performance of the inverse-designed devices to conventionally-designed devices with the same operational behavior. Finally, we discuss the limitations of the inverse design tools for realizing complex device designs and comment on what is possible at present and where improvements can be made.

Radiation Leakage Images For Two Different Dielectric-Loaded Surface Plasmon Polariton Waveguides

We have studied about leakage radiation images obtained for two different dielectric-loaded surface plasmon polariton waveguides based on routing structures; linear couplers and bent waveguides. By simultaneously imaging the conjugated aperture and field planes of the microscope, we unambiguously quantify the degeneracy lift occurring for strongly interacting dielectric loaded surface plasmon polariton waveguides and visualize the symmetry of the coupled modes. We have obtained the wave vector distribution and showed its evolution with the bend radius. We have developed a numerical and an analytical analysis for momentum distribution. We have found that for large radii i.e. for vanishing bending loss, we can link the plasmon in Fourier space with the geometrical and modal properties of the bend structure. It was also found that the radial dependence of the wave vector distribution is governed by the phase difference. The obtained results were found in good agreement with previously obtained results.

Ultrafast Plasmonic Graphene Photodetector Based on the Channel Photothermoelectric Effect

We propose an on-chip CMOS compatible graphene plasmonic photodetector based on the photo-thermoelectric effect (PTE) that occurs across an entire homogeneous graphene channel. The proposed photodetector incorporates the long-range dielectric-loaded surface plasmon polariton (LR-DLSPP) waveguide with a metal stripe serving simultaneously as a plasmon supporting metallic material and one of the metal electrodes. Large in-plane component of the transverse magnetic (TM) plasmonic mode can couple efficiently to the graphene causing large temperature increases across an entire graphene channel with a maximum located at the metal stripe edge. As a result, the electronic temperatures exceeding 12000K at input power of only a few tens of {\mu}W can be obtained at the telecom wavelength of 1550nm. Even with limitations such as the melting temperature of graphene (T= 4510 K), a responsivity exceeding at least 200 A/W is achievable at telecom wavelength of 1550 nm. It is also shown that under certain operation conditions, the PTE channel photocurrent can be isolated from photovoltaic and p-n junction PTE contributions providing an efficient way for optimizing the overall photodetector performance.

High-bandwidth and high-responsivity waveguide-integrated plasmonic germanium photodetector

Here we propose a waveguide-integrated germanium plasmonic photodetector that is based on a long-range dielectric-loaded surface plasmon polariton waveguide configuration. As this configuration ensures a long propagation distance, i.e., small absorption into metal, and a good mode field confinement, i.e., high interaction of the electric field with a germanium material, it is perfect for a realization of plasmonic germanium photodetectors. Such a photodetector even without optimization provides a responsivity exceeding 1 A/W for both wavelengths of 1310 nm and 1550 nm. To achieve such a responsivity, only a 5 {\mu}m-long waveguide is required for 1310 nm and 30 {\mu}m-long for 1550 nm. With optimization this value can be highly improved. In the proposed arrangement a metal stripe simultaneously supports a propagating mode and serves as one of the electrodes, while the second electrode is located a short distance from the waveguide. As a propagating mode is tightly confined to the germanium ridge, the external electrode can be placed very close to the waveguide without disturbing it. As such, the distance between electrodes can be smaller than 350 nm which allows one to achieve a bandwidth exceeding 100 GHz. However, as most of the carriers are generated inside a distance of 100 nm from a stripe, a bandwidth exceeding 150 GHz can be achieved for a bias voltage of -4 V.

Plasmonic Schottky photodetector with metal stripe embedded into semiconductor and with a CMOS-compatible titanium nitride

Here we propose an original waveguide-integrated plasmonic Schottky photodetector that takes full advantage of a thin metal stripe embedded entirely into a semiconductor. The photodetector is based on the long-range dielectric-loaded surface plasmon polariton waveguide with a metal stripe deposited on top of a semiconductor rib and covered by another semiconductor. As the metal stripe is entirely surrounded by semiconductor, all hot electrons with appropriate k-vectors can participate in transitions that highly enhances the electron transfer, and consequently the internal quantum efficiency. In addition, a high coupling efficiency from the photonic waveguide to the photodetector is simulated exceeding 90 % which enhances the external quantum efficiency. Calculations show that a responsivity exceeding 0.5 A/W can be achieved at telecom wavelength of 1550 nm and the bandwidth can exceed 100 GHz. Furthermore, it is shown that titanium nitride is a perfect material for the photodetector as it provides a low Fermi energy and long electron mean free path that enhance the hot electron transfer to the semiconductor. In addition, it shows reasonable metallic behavior and CMOS compatibility. Measurements showed that the Schottky barrier height between titanium nitride and p-doped silicon reaches 0.69–0.70 eV that matches the optimum signal-to-noise ratio operation calculated at 0.697 eV.

On-chip excitation of single germanium vacancies in nanodiamonds embedded in plasmonic waveguides

Monolithic integration of quantum emitters in nanoscale plasmonic circuitry requires low-loss plasmonic configurations capable of confining light well below the diffraction limit. We demonstrated on-chip remote excitation of nanodiamond-embedded single quantum emitters by plasmonic modes of dielectric ridges atop colloidal silver crystals. The nanodiamonds were produced to incorporate single germanium-vacancy (GeV) centres, providing bright, spectrally narrow and stable single-photon sources suitable for highly integrated circuits. Using electron-beam lithography with hydrogen silsesquioxane (HSQ) resist, dielectric-loaded surface plasmon polariton waveguides (DLSPPWs) were fabricated on single crystalline silver plates to contain those of deposited nanodiamonds that are found to feature appropriate single GeV centres. The low-loss plasmonic configuration enabled the 532-nm pump laser light to propagate on-chip in the DLSPPW and reach to an embedded nanodiamond where a single GeV centre was incorporated. The remote GeV emitter was thereby excited and coupled to spatially confined DLSPPW modes with an outstanding figure-of-merit of 180 due to a ~six-fold Purcell enhancement, ~56% coupling efficiency and ~33 μm transmission length, thereby opening new avenues for the implementation of nanoscale functional quantum devices.

Dielectric-loaded plasmonic waveguide in the visible spectral range

Dielectric-loaded surface plasmon−polariton waveguides (DLSPPWs) are a practically valuable type of plasmonic waveguide. The properties of DLSPPWs at telecommunication wavelengths have been studied in detail. However, the efficient optical excitation of DLSPPWs in the visible spectral range has still not been realized. In this work, we present the results of our experimental investigations of DLSPPWs in the visible spectral range. In addition, a new configuration for the excitation and detection of the DLSPPW mode has been proposed and realized. The propagation of plasmon wave up to a distance of 45 µm in the DLSPPW has been demonstrated.

Single-Mode to Multi-Mode Crossover in Thin-Load Polymethyl Methacrylate Plasmonic Waveguides

Mode character and mode dispersion of sub-60-nm-thick polymethyl methacrylate dielectric-loaded surface plasmon-polariton waveguides (DLSPPWs) are investigated using photoemission electron microscopy and finite element method simulations. Experiment and simulation show excellent agreement and allow identifying a crossover from single-mode to multi-mode waveguiding as a function of excitation wavelength λ and DLSSPW cross section. Experiment and simulations yield, furthermore, indications for the formation of a surface plasmon-polariton cavity mode in the close vicinity of the waveguides.

Nanofabrication of Plasmonic Circuits Containing Single Photon Sources

Nanofabrication of photonic components based on dielectric-loaded surface plasmon-polariton waveguides (DLSPPWs) excited by single nitrogen vacancy (NV) centers in nanodiamonds is demonstrated. DLSPPW circuits are built around NV containing nanodiamonds, which are certified to be single-photon emitters, using electron-beam lithography of hydrogen silsesquioxane (HSQ) resist on silver-coated silicon substrates. A propagation length of ~20 {\mu}m for the NV single-photon emission is measured with DLSPPWs. A 5-fold enhancement in the total decay rate and up to 63% coupling efficiency to the DLSPPW mode is achieved, indicating significant mode confinement. Finally, we demonstrate routing of single plasmons with DLSPPW-based directional cou-plers, revealing the potential of our approach for on-chip realization of quantum-optical networks.

Spectral imaging of topological edge states in plasmonic waveguide arrays

We report on the observation of a topologically protected edge state at the interface between two topologically distinct domains of the Su-Schrieffer-Heeger model, which we implement in arrays of evanescently coupled dielectric-loaded surface plasmon polariton waveguides. Direct evidence of the topological character of the edge state is obtained through several independent experiments: Its spatial localization at the interface as well as the restriction to one sublattice is confirmed by real-space leakage radiation microscopy. The corresponding momentum-resolved spectrum obtained by Fourier imaging reveals the midgap position of the edge state as predicted by theory.

Nonreciprocal dielectric-loaded plasmonic waveguides using magneto-optical effect of Fe.

We have implemented the nonreciprocal propagation capabilities into plasmonic waveguides and have simulated the performances. We employed dielectric-loaded surface plasmon polariton waveguide (DLSPPW) and long-range DLSPPW (LR-DLSPPW) configurations, where ferromagnetic-metal Fe is used instead of noble metals in order to obtain nonreciprocal propagations by the transverse magneto-optical (MO) effect. The nonreciprocal performances were characterized by the finite-difference frequency-domain (FDFD) method in terms of the propagation losses in return for the nonreciprocal phase shift (NRPS) and nonreciprocal propagation loss (NRL). The NRPS and NRL of the DLSPPW configuration are larger than those of the previously reported semiconductor waveguide optical isolators owing to the large MO constant of Fe and the field confinement by surface plasmons although the propagation loss for NRL of 1 dB is at least 31 dB and the propagation length is limited to less than 10 μm. To reduce such a large propagation loss, we introduced the LR-DLSPPW configuration composed of Polymethyl methacrylate (PMMA) ridge and Benzocyclobutene (BCB) buffer layer. The Fe layer thickness and width are optimized to 50 nm and 500 nm, respectively, so that sizable MO effect and low propagation loss coexist. The propagation loss for NRL of 1 dB is suppressed to ~10 dB within a waveguide length of ~56 μm. Our comprehensive investigation offers fundamental information on practical magneto-plasmonic waveguides and how much nonreciprocal performances are expected, providing an insight into the integration of magneto-plasmonics with on-chip photonics and electronics.

Highly Sensitive Biochemical Sensor Based on Two-Layer Dielectric Loaded Plasmonic Microring Resonator

In this paper, we propose and design a highly sensitive optical biochemical sensor based on two-layer dielectric loaded surface plasmon polariton waveguide (TDLSPPW)-based microring resonator (MRR). By optimizing the structure parameters, the propagation length of the proposed waveguide is ~126 μm, which is about 3 times of that of the polymer dielectric loaded surface plasmon polariton waveguide (DLSPPW) reported. It is demonstrated that the TDLSPPW-based MRR is operated at the under-coupling state, along with the quality factor (Q) of 541.2 and extinction ratio (ER) of 12.2 dB. Moreover, the Q and ER are much more sensitive to the structure parameters of the waveguide, including the waveguide width w, total thickness t, and coupling gap Wgap, compared to the low refractive index (RI) layer thickness t2. The simulation results on the biochemical RI sensing show that the sensitivities of 408.7 and 276.4 nm/RIU for glucose concentration in urine and chemical gases can be achieved, respectively. It is believed that the proposed sensor has potential applications in photonic-integrated biochemical sensing.

Efficient modal analysis of surface plasmon polariton waveguides using approximate semi-analytical method

A semi-analytical approximate method, called the semi-vector optimal variational method, is applied to dielectric loaded surface plasmon polariton (DLSPP) waveguides. The method provides a simple, fast and efficient model for modal analysis of SPP waveguides. The modal characteristics obtained by the present method show good agreement with the rigorous analysis of finite element method. The method provides a good alternate for effective index methods which have ambiguity in its application and do not provide satisfactory results for the DLSPP waveguides.

Fast response and low power consumption 1×2 thermo-optic switch based on dielectric-loaded surface plasmon polariton waveguides

In this paper, we present a 1 × 2 thermo-optic (TO) switch based on the integration of the dielectric-loaded surface plasmon polariton (SPP) waveguides with the silicon nanowires. Liquid-curable fluorinated resin (LFR) made of perfluorinated polymer was adopted as the ridge, which has a TO coefficient twice more than that of polymethyl methacrylate, leading to a significant decrease in the power consumption. It was shown that the response time of the dielectric-loaded SPP waveguide could be improved through optimizing the dimensions of the LFR polymer ridge without loss of relative high figure of merit and large confinement factor. Performance characteristics of such a 1 × 2 TO switch operating at a telecom wavelength of 1550 nm was investigated theoretically from the analysis of both heat and optical fields. The results reveal that a switching power as low as 7 mW and an extremely short switching time (with rise time of 3 μs and fall time of 6.7 μs) could be achieved with the proposed dielectric-loaded SPP-based 1 × 2 TO switch. In addition, the crosstalk could be enhanced to at least 40 dB with the applied power of 7 mW at the wavelength of 1550 nm, and it could be retained to be above 20 dB in the wavelength spectrum of 1500–1600 nm during the on/off state.

Waveguide Mode Splitter Based on Multi-mode Dielectric-Loaded Surface Plasmon Polariton Waveguide**Supported by the National Basic Research Program of China under Grant Nos 2011CBA00200 and 2011CB921200, the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant No XDB01030200, the National Natural Science Foundation of China under Grant No 11374289, the Fundamental Research Funds for the Central Universities

In photonic integrated circuits, information is usually encoded in the optical path. In this work, based on the multi-mode dielectric-loaded surface plasmon polariton waveguide, we numerically design a directional coupler, which can divide the different waveguide eigenmodes into different optical paths. The results show a possibility to encode information onto different waveguide modes. We also experimentally demonstrate that the splitting ratio of this directional coupler structure can be tuned without changing its size.

Fluorinated photopolymer thermo-optic switch arrays with dielectric-loaded surface plasmon polariton waveguide structure

Novel polymer thermo-optic switch arrays were successfully designed and fabricated based on dielectric-loaded surface plasmon polariton waveguide (DLSPPW) structure. Highly fluorinated low-loss photopolymers and organic–inorganic grafting materials were used as the waveguide core and cladding, respectively. The low absorption loss characteristics and excellent thermal stabilities of the core and cladding materials were obtained. The proposed DLSPPW model was included of fluorinated polymer ridge with 4 × 4 μm2 size loaded on 60-nm thin gold stripe electrode heaters, organic-inorganic grafting material cladding and PMMA substrate. The operation of the device at signal wavelengths is controlled via the thermo-optic effect by electrically heating the gold stripes of dielectric-loaded surface plasmon polariton waveguides. The optimized structural properties of dielectric-loaded surface plasmon polariton waveguides were provided. The propagation loss of a 4-μm wide straight DLSPPW was measured as 0.55 dB∕cm at 1550 nm wavelength. The insertion loss of the device was measured to be about 4.5 dB. The switching rise and fall time of the device applied by 200 Hz square-wave voltage were obtained as 287.5 μs and 370.2 μs, respectively. The switching power was about 5.6 mW, and the extinction ratio was about 13.5 dB. The flexible low-loss multi-functional waveguide switch arrays are suitable for realizing large-scale optoelectronic integrated circuits.

Single-mode lasers and parity-time symmetry broken gratings based on active dielectric-loaded long-range surface plasmon polariton waveguides.

Single-mode distributed feedback laser structures and parity-time symmetry broken grating structures based on dielectric-loaded long-range surface plasmon polariton waveguides are proposed. The structures comprise a thin Ag stripe on an active polymer bottom cladding with an active polymer ridge. The active polymer assumed is PMMA doped with IR140 dye providing optical gain at near infrared wavelengths. Cutoff top ridge dimensions (thickness and width) are calculated using a finite element method and selected to guarantee single-mode operation of the laser. Several parameters such as the threshold number of periods and the lasing wavelength are determined using the transfer matrix method. A related structure based on two pairs of waveguides of two widths, which have the same imaginary part but different real part of effective index, arranged within one grating period, is proposed as an active grating operating at the threshold for parity-time symmetry breaking (i.e., operating at an exceptional point). Such "exceptional point" gratings produce ideal reflectance asymmetry as demonstrated via transfer matrix computations.

Low-loss dielectric-loaded graphene surface plasmon polariton waveguide based biochemical sensor

We have modeled and numerically simulated the performance of a dielectric-loaded graphene surface-plasmon-polariton (DL-GSPP) waveguide as a biochemical sensing device. In our device, the conventionally used gold layer is replaced with a graphene microribbon for the detection of biochemical molecules. The graphene layer is incorporated to minimize ohmic losses and to enhance the adsorption of biomolecules so that the sensor sensitivity is increased significantly. The sensor performance is quantified through numerical simulations carried out by varying device parameters such as waveguide length, effective mode index, dimension of the dielectric ridge, and the length and the number of graphene layers. One of the prominent features of our DL-GSPP waveguide sensor is that its length is in the millimeter range, an essential requirement for realistic plasmonic waveguide sensors. The average sensitivity of DL-GSPP structure is found to be in the range of 3–6 μRIU (refractive index units), which is comparable to the values obtained using surface-plasmon resonance (1–10 μRIU) and long-range waveguide sensors (0.1–5 μRIU).

Compact wavelength add-drop multiplexers using Bragg gratings in coupled dielectric-loaded plasmonic waveguides.

We report a novel design of a compact wavelength add-drop multiplexer utilizing dielectric-loaded surface plasmon-polariton waveguides (DLSPPWs). The DLSPPW-based configuration exploits routing properties of directional couplers and filtering abilities of Bragg gratings. We present practical realization of a 20-μm-long device operating at telecom wavelengths that can reroute optical signals separated by approximately 70 nm in the wavelength band. We characterize the performance of the fabricated structures using scanning near-field optical microscopy as well as leakage-radiation microscopy and support our findings with numerical simulations.