Dielectric-loaded Waveguide Research Articles

Analysis of Amplification Characteristics of Surface Plasmon Polaritons Based on the Rate-Equation Theory

A rate-equation theory model with the amplified spontaneous emission (ASE) effect considered for the surface plasmon polariton (SPP) waveguide amplifier is presented for analyzing the SPP amplifications with both side and SPP pumping techniques. By partitioning the cross section of the waveguide to suitable small units and introducing the overlapping integrated factor in each unit, the 3-D coupled equations of the model can be simplified to 1-D equations, which could be solved more conveniently. With our model, the gain and noise properties of a dielectric-loaded SPP waveguide amplifier are simulated. The results show that the ASE effect reduces the signal gain and deteriorates the signal-to-noise ratio of the SPP amplifier. For both the pumping schemes, the gains and noises for SPP multimode amplifications depend on the waveguide structure. By optimizing the waveguide structure, the gain-equalized amplification for different SPP modes may be achieved.

Experimental demonstration of CMOS-compatible long-range dielectric-loaded surface plasmon-polariton waveguides (LR-DLSPPWs).

We demonstrate the design, fabrication and experimental characterization of long-range dielectric-loaded surface plasmon-polariton waveguides (LR-DLSPPWs) that are compatible with complementary metal-oxide semiconductor (CMOS) technology. The demonstrated waveguide configuration represents a silicon nitride ridge atop a thin strip of metal, which is positioned on a partially oxidized layer of silicon supported by a silicon oxide layer. The demonstrated waveguides feature reasonable mode confinement (~0.5μm2) and show rather long propagation (~700 μm) at telecom wavelengths. Owing to the existence of a metal strip within the structure, one can envision the co-propagation of electrical and photonic signals within the structure, enabling thereby seamless integration of photonic and electronic circuits. Electrical signals in metal strips supporting plasmonic modes can be used for variety of applications, e.g. to control the propagation of radiation via the thermo-optic effect.

Spectral and mode properties of surface plasmon polariton waveguides studied by near-field excitation and leakage-mode radiation measurement

We present a method to couple surface plasmon polariton (SPP) guiding mode into dielectric-loaded SPP waveguide (DLSPPW) devices with spectral and mode selectivity. The method combined a transmission-mode near-field spectroscopy to excite the SPP mode and a leakage radiation optical microscope for direct visualization. By using a near-field fiber tip, incident photons with different wavelengths were converted into SPPs at the metal/dielectric interface. Real-time SPP radiation images were taken through leakage radiation images. The wavelength-dependent propagation lengths for silver- and gold-based DLSPPWs were measured and compared. It confirms that silver-based SPP has a propagation length longer than a gold-based one by 1.25, 1.38, and 1.52 times for red, green, and blue photons. The resonant coupling as a function of wavelength in dual DLSPPWs was measured. The coupling lengths measured from leakage radiation images were in good agreement with finite-difference time domain simulations. In addition, the propagation profile due to multi-SPP modes interference was studied by changing position of the fiber tip. In a multimode DLSPPW, SPP was split into two branches with a gap of 2.237 μm when the tip was at the center of the waveguide. It became a zigzag profile when the SPP was excited at the corner of the waveguide.

Plasma current start-up experiments using a dielectric-loaded waveguide array antenna in the TST-2 spherical tokamak

Plasma current start-up and ramp-up using the lower hybrid wave (LHW) were investigated on the TST-2 spherical tokamak. The LHW was launched by a dielectric-loaded waveguide array (grill) antenna. The antenna–plasma coupling of this antenna deteriorates as the input power exceeds several kW. This deterioration is believed to be caused by the density depletion due to the ponderomotive force. This conjecture was confirmed by the measurement of density reduction and the result of a non-linear full wave numerical calculation based on the finite element method (FEM). The plasma current was started and ramped up to 10 kA using this antenna. The ability of this grill antenna to excite the LHW with different n∥ = ck∥/ω was used to identify the most favourable n∥ spectrum for plasma current ramp-up. It was found that effective current drive can be achieved by the LHW with n∥ less than 6. However, even in this case, the energetic electrons which account for a large fraction of the driven current, are lost rapidly because the poloidal field generated by this level of plasma current is not sufficient to confine high energy electrons.

Mode division multiplexing in a polymer-loaded plasmonic planar waveguide.

A mode division multiplexer (MDM) based on in-plane diffractions is experimentally demonstrated in a polymer-loaded plasmonic planar waveguide. Three guided modes (TM₁, TE₁, and TM₂) were well demultiplexed by a focusing design with a focal length of about 40 μm, which are clearly distinguished by the polarization control. The experimental results well reproduced the theoretical design and calculation. Moreover, the demultiplexed focal spots directly reflect the different modes, by which a mode diagram of the dielectric-loaded planar waveguide was vividly mapped out by varying the polymer layer thickness. In this regard, the proposed device may not only serve as a MDM for the integrated optics but can also provide a new strategy in analyzing the guided modes.

脊位于窄边的介质加载双脊波导场结构的计算

脊位于窄边的介质加载双脊波导场结构的计算

2D theory of wakefield amplification by active medium

A train of microbunches generates in a passive dielectric-loaded waveguide an electromagnetic wake which propagates at the speed of the particles. This wake consists of propagating modes provided the electrons exceed the Cerenkov velocity. If the material is replaced with an active dielectric, identical to that of a laser, the wake is amplified. Another train of bunches, lagging many wavelengths behind, may be accelerated by this amplified wake. The gradient is limited by breakdown and saturation of the medium. Beam loading may be partially or even completely compensated by the gain along the trailing bunch. Preliminary results of a linear theory will be presented, assuming a 300MeV beam and high-pressure CO2 mixture as an active medium. In spite of many hundreds of modes excited by the front beam, the spectrum of the amplified field corresponds to a monochromatic wave determined primarily by the bandwidth of the medium. The analytic approach facilitates simple assessment of the effect of the various parameters on the accelerating gradient.

CMOS-Compatible Long-Range Dielectric-Loaded Plasmonic Waveguides

CMOS-compatible long-range dielectric-loaded plasmonic waveguides featuring long propagation lengths and strong mode confinements are designed and studied using the finite-element method. The waveguide composition includes a Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ridge on the top of a thin metal stripe deposited on a silicon-on-insulator (SOI) wafer with a thin top SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer produced by thermal oxidation. All the materials chosen in the design are compatible with complementary metal-oxide-semiconductor technology and can be integrated with plasmonic, electronic and photonic components. Influence of various waveguide parameters on the waveguide characteristics is analyzed, including the thicknesses of the Si layer and the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layers, the dimension of the ridge, as well as the parameters of the metal stripe. It is found that, at the telecom wavelength of 1.55 μm, the propagation length of CMOS-plasmonic waveguides can reach 1.07 mm with a lateral mode confinement of ~1.18 μm and 4.43 mm with a lateral mode width of ~2.32 μm. The bending loss and coupling length are both calculated to evaluate the applicability of the proposed device for high density integration and a figure of merit is proposed to show the tradeoff between the propagation length and the lateral mode width in the application of plasmonic devices.

Enhancing surface plasmon polariton propagation by two-layer dielectric-loaded waveguides on silver surface

We report a study of a two-layer dielectric-loaded surface plasmon polariton waveguide (TDLSPPW) which consisted of two dielectric layers (high-index/low-index) on a silver film. The discontinuity of the electric field at the interfaces resulted in a concentrated field in the low-index region. It efficiently reduced the propagation loss of the surface plasmon polariton mode. The mode fields and corresponding complex propagation constants were calculated by a vector finite-difference method. The propagation properties were measured by a modified near-field optical microscope. It is confirmed that the propagation length of the proposed TDLSPPW was about 1.6 times longer than conventional single-layer SPP waveguides. In addition, a 90∘ waveguide turn with 3 μm radius showed that the bending loss was smaller than 2 dB.

Gain-assisted light guiding at the subwavelength scale in a hybrid dielectric-loaded surface plasmon polariton waveguide based on a metal nanorod

Hybrid dielectric-loaded plasmonic waveguide consisting of an inverted U-shaped high-index ridge covering atop a low-index polymer coated metal nanorod is proposed and numerically investigated. Through incorporating a gain medium either in the ridge or the buffer layer, the loss of the supported plasmonic mode can be effectively compensated. For a typical configuration comprising a 450 nm × 160 nm InGaAsP ridge with a 5–40 nm thick PMMA layer and a 30 nm wide Ag nanorod, the necessary gain required in InGaAsP for lossless propagation of the quasi-TE type symmetric hybrid mode can be as low as 3.45–9.85 cm−1, along with a subwavelength mode area (λ2/60–λ2/148) attainable simultaneously. The configuration potentially offers a promising solution to enable lossless light transport at the sub-diffraction-limited scale with low compensation gain, thereby opening venues for ultra-compact active plasmonic devices and circuits.

Optimization Analysis on the Transmission Characteristics of Semi-Circle Long-Range Dielectric-Loaded Surface Plasmon-Polariton Waveguides

Semi-circle long-range dielectric-loaded surface plasmon-polariton waveguide (LR-DLSPPW) is proposed, in which a thin and narrow metal stripe (15-nm-thick and 500-nm-wide) is sandwiched between a semi-circle dielectric ridge(with the refractive index of 1.535 ,the radius of 777 nm) and a dielectric film (with the refractive index of 1.493) supported by low-index substrate (with the refractive index of 1.34). The mode effective index, propagation length, mode width and figures of merit of the fundamental mode supported using the finite-element method are calculated at the telecom wavelength λ =1.55μm for different dimensions (t) of the dielectric film. It was calculated that the optimized structure have the maximized parameters with figures of merit 3.75×10¬6and propagation length 3.7mm at t=570 nm. The semi-circle LR-DLSPPW structure is found to exhibit about 20% increase of the propagation length as compared to the conventional rectangular LR-DLSPPW while ridge thickness t≤600 nm. The proposed configuration allows for easy connection to electrodes enabling, and is technologically simple being compatible with planar fabrication using UV-lithography.

Transverse operator method for wakefields in a rectangular dielectric loaded accelerating structure

Cherenkov radiation generated by a relativistic electron bunch in a rectangular dielectric-loaded waveguide is analyzed under the assumption that the dielectric layers are inhomogeneous normal to the beam path. We propose a method that uses eigenfunctions of the transverse operator applied to develop a rigorous full solution for the wakefields that are generated. The dispersion equation for the structure is derived and the wakefield analysis is carried out. The formalism developed here allows the direct solution of the inhomogeneous system of Maxwell equations, an alternative analytic approach to the analysis of wakefields in contrast to the previously used impedance method for rectangular structure analysis. The formalism described here was successfully applied to the analysis of rectangular dielectric-lined structures that have been recently beam tested at the Argonne (ANL/AWA) and Brookhaven (BNL/ATF) accelerator facilities.

Momentum-space spectroscopy for advanced analysis of dielectric-loaded surface plasmon polariton coupled and bent waveguides

We perform advanced radiation leakage microscopy of routing dielectric-loaded plasmonic waveguiding structures. By direct-plane imaging and momentum-space spectroscopy, we analyze the energy transfer between coupled waveguides as a function of gap distance and reveal the momentum distribution of curved geometries. Specifically, we observed a clear degeneracy lift of the effective indices for strongly interacting waveguides in agreement with coupled-mode theory. We use momentum-space representations to discuss the effect of curvature on dielectric-loaded waveguides. The experimental images are successfully reproduced by a numerical and an analytical model of the mode propagating in a curved plasmonic waveguide.

Non-inductive plasma initiation and plasma current ramp-up on the TST-2 spherical tokamak

Plasma current (Ip) start-up in a spherical tokamak (ST) by waves in the lower-hybrid (LH) frequency range was investigated on TST-2. A low current (∼1 kA) ST configuration can be formed by waves over a broad frequency range (21 MHz–8.2 GHz in TST-2), but further Ip ramp-up (to ∼10 kA) is most efficient with waves in the LH frequency range. Ip ramp-up to 15 kA was achieved with 60 kW of net RF power PRF in the fast wave (FW) polarization at 200 MHz excited by the inductively coupled combline antenna. X-ray measurements showed that the photon flux and temperature are higher in the direction opposite to Ip, consistent with acceleration of electrons by a uni-directional RF wave. There is evidence that the LH wave is excited nonlinearly by the FW, based on the frequency spectra measured by magnetic probes. Similar efficiencies of Ip ramp-up were obtained with the inductive combline antenna and the dielectric-loaded waveguide array (‘grill’) antenna, and tendencies for the current drive efficiency to increase with plasma current and toroidal field were observed. During operation of the grill antenna, wavevector components were measured by an array of magnetic probes. Results were qualitatively consistent with expectations based on dispersion relations for the FW and the LH wave. A capacitively coupled combline antenna has been developed to improve coupling to the plasma and the wavenumber spectrum of the excited LH wave, and will be tested in 2013.

Compact electron acceleration and bunch compression in THz waveguides

We numerically investigate the acceleration and bunch compression capabilities of 20 mJ, 0.6 THz-centered coherent terahertz pulses in optimized metallic dielectric-loaded cylindrical waveguides. In particular, we theoretically demonstrate the acceleration of 1.6 pC and 16 pC electron bunches from 1 MeV to 10 MeV over an interaction distance of 20mm, the compression of a 1.6 pC 1 MeV bunch from 100 fs to 2 fs (50 times compression) over an interaction distance of about 18mm, and the compression of a 1.6 pC 10 MeV bunch from 100 fs to 1.61 fs (62 times) over an interaction distance of 42 cm. The obtained results show the promise of coherent THz pulses in realizing compact electron acceleration and bunch compression schemes.

Directional coupling in long-range dielectric-loaded plasmonic waveguides

Directional couplers (DCs) based on long-range dielectric-loaded surface plasmon-polariton waveguides (LR-DLSPPWs) operating at telecom wavelengths are studied both numerically and experimentally. The investigated LR-DLSPPWs are formed by ~1.2-µm-high and 1-µm-wide polymer ridges fabricated atop of 15-nm-thick and 500-nm-wide gold stripes supported by a 288-nm-thick Ormoclear polymer deposited on a low-index (n(s) ≈1.34) layer of Cytop. DC structures consisting of sine-shaped S-bends (having an offset of ~10 µm over a distance of ~20 µm) and ~100-µm-long parallel LR-DLSPPWs with a center-to-center separation of 2 µm are characterized using scanning near-field microscopy. The experimentally obtained values of the propagation length (~400 µm), S-bend loss (~4 dB) and coupling length (~100 µm) are found in good agreement with the numerical simulations, indicating a significant potential of LR-DLSPPWs for the realization of various plasmonic components.

Detuned-resonator induced transparency in dielectric-loaded plasmonic waveguides

We report on the experimental demonstration of detuned-resonator induced transparency in the near-infrared (~800 nm) using two detuned racetrack resonators side-coupled to a bus waveguide. Both resonators and the bus waveguide are in the form of dielectric-loaded surface plasmon polariton waveguides. Leakage radiation microscopy imaging is employed to measure transmission spectra, featuring local maxima at intermediate wavelengths with asymmetrical profiles that are found in good agreement with full-wave numerical simulation results.

Power monitoring in dielectric-loaded plasmonic waveguides with internal Wheatstone bridges

We report on monitoring the mode power in dielectric-loaded surface plasmon polariton waveguides (DLSPPWs) by measuring the resistance of gold electrodes, supporting the DLSPPW mode propagation, with internal (on-chip) Wheatstone bridges. The investigated DLSPPW configuration consisted of 1-μm-thick and 10-μm-wide cycloaliphatic acrylate polymer ridges tapered laterally to a 1-μm-wide ridge placed on a 50-nm-thin and 4-um wide gold stripe, all supported by a ~1.7-µm-thick Cytop layer deposited on a Si wafer. The fabricated DLSPPW power monitors were characterized at telecom wavelengths, showing very high responsivities reaching up to ~6.4 μV/μW (for a bias voltage of 245 mV) and the operation bandwidth exceeding 40 kHz.

Analysis of hybrid dielectric-plasmonic slot waveguide structures with 3D Fourier Modal Methods

Recently, plasmonic waveguides have been intensively studied as promising basic building blocks for the construction of extremely compact photonic devices with subwavelength characteristic dimensions. A number of different types of plasmonic waveguide structures have been recently proposed, theoretically analyzed, and their properties experimentally verified. The fundamental trade-off in the design of plasmonic waveguides for potential application in information technologies lies in the contradiction between their mode field confinement and propagation loss: the higher confinement, the higher loss, and vice versa. Various definitions of figures of merit of plasmonic waveguides have been also introduced for the characterization of their properties with a single quantity. In this contribution, we theoretically analyze one specific type of a plasmonic waveguide – the hybrid dielectric-loaded plasmonic waveguide, or – as we call it in this paper – the hybrid dielectric-plasmonic slot waveguide, which exhibits very strong field confinement combined with acceptable losses allowing their application in some integrated plasmonic devices. In contrast to the structures analyzed previously, our structure makes use of a single low-index dielectric only. We first define the effective area of this waveguide type, and using waveguide parameters close to the optimum we analyze several waveguide devices as directional couplers, multimode interference couplers (MMI), and the Mach-Zehnder interferometer based on the MMI couplers. For the full-vector 3D analysis of these structures, we use modelling tools developed in-house on the basis of the Fourier Modal Method (FMM). Our results thus serve to a dual purpose: they confirm that (i) these structures represent promising building blocks of plasmonic devices, and (ii) our FMM codes are capable of efficient 3D vector modelling of plasmonic waveguide devices.

Submicrometer radius and highly confined plasmonic ring resonator filters based on hybrid metal-oxide-semiconductor waveguide

We numerically report the submicrometer radius (0.5 μm) and high confinement (mode area ~λ(2)/1200) plasmonic ring resonators for both all-pass and add-drop filters based on the hybrid metal-oxide-semiconductor (Ag-SiO(2)-Si) waveguide platform. The best tradeoff between the propagation length and the confinement of this hybrid plasmonic waveguide platform is also discussed and compared to the dielectric-loaded plasmonic waveguide counterpart. We show that the ring resonator all-pass filter features an extinction ratio as high as 23 dB with a transmission loss of 1.5 dB, and a wide free spectral range of 168 nm with a bandwidth of 14 nm. Moreover, the demonstrated add-drop filter achieves an extinction ratio larger than 12 dB with a channel isolation between the through and drop channels of 13.5 dB at the resonant wavelength. These demonstrated plasmonic devices reveal as potential building blocks for future nanoscale electronic-photonic integrated circuits.