Coupled-cavity Structures Research Articles

A Novel 2-D Slotted Structure Extended Interaction Oscillator

Numerous vacuum electronic devices (VEDs) have been widely studied, among which extended interaction oscillators (EIO) exhibit appealing characteristics, such as high frequency, high efficiency, lightweight, low working voltage, and stable working frequency. The slow wave structure typically adopts the coupled-cavity, folded-waveguide, ladder line, and other structures. The ladder line has a simple structure and is easy to process, with desirable heat dissipation and large power capacity. In this study, a novel slow wave structure being longitudinally slotted in the transverse grating of the original ladder line is proposed, and the 2-D slotted structure can be obtained. Encouragingly, the structure not only increases the cross-sectional area of the electron beam but also, more importantly, increases the effective interaction area to considerably improve the output power. The dispersion characteristics and field distribution are first studied through numerical and simulation calculations, and the optimal slotted structure parameters and working parameters are analyzed using PIC software. Next, a 94-GHz 2-D slotted structure EIO is designed. Finally, compared with the ladder line, the cavity characteristic impedance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{R}$</tex-math> </inline-formula> / <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{Q}$</tex-math> </inline-formula> is increased by 52.5%, and the output power is increased by 21.1%. This novel structure is expected to provide a new idea for the power enhancement of millimeter-wave and terahertz EIOs.

Improvement of the Beam-Wave Interaction Efficiency Based on the Coupling-Slot Configuration in an Extended Interaction Oscillator

A method aimed at improving the beam-wave interaction efficiency by changing the coupling slot configuration has been proposed in the study of extended interaction oscillators (EIOs). The dispersion characteristics, coupling coefficient and interaction impedance of the high-frequency structure based on different types of coupling slots have been investigated. Four types of coupled cavity structures with different layouts of the coupling slots have been compared to improve the beamwave interaction efficiency, so as to analyze the beam-wave interaction and practical applications. In order to determine the improvement of the coupling slot to a coupled cavity circuit in an EIO, we designed four nine-gap EIOs based on the coupled cavity structure with different coupling slot configurations. With different operating frequencies and voltages takes into consideration, beam voltages from 27 to 33 kV have been simulated to achieve the best beam-wave interaction efficiency so that the EIOs are able to work in the 2π mode. The influence of the Rb and the ds on the output power is also taken into consideration. The Rb is the radius of the electron beam, and the ds is the width of the coupling slot. The simulation results indicate that a single-slot-type EIO has the best beam-wave interaction efficiency. Its maximum output power is 2.8 kW and the efficiency is 18% when the operating voltage is 31 kV and electric current is 0.5 A. The output powers of these four EIOs that were designed for comparison are not less than 1.7 kW. The improved coupling-slot configurations enables the extended interaction oscillator to meet the different engineering requirements better.

Acoustic mode confinement using coupled cavity structures in UHF unreleased MEMS resonators

This papers investigates device approaches towards the confinement of acoustic modes in unreleased UHF MEMS resonators. Acoustic mode confinement is achieved using specially designed mechanically coupled acoustic cavities known as acoustic Bragg Grating Coupler structures to spatially localize the vibration energy within the resonators and thereby improve the motional impedance (R_x) and mechanical quality factor (Q). This enhancement in the mechanical response is demonstrated with numerical simulations using distinct unreleased resonator technologies involving dielectric transduction mechanisms. These initial investigations show improvements in the Q as well as enhanced vibrational amplitudes within the resonator domains (i.e. translating to improved R_x values) in the case of coupled cavities as opposed to single cavity designs. An initial approach to fabricate the devices in a CMOS compatible dual-trench technology are presented.

Control of coupling in 1D photonic crystal coupled-cavity nano-wire structures via hole diameter and position variation

We have successfully demonstrated close experimental control of the resonance splitting/free spectral range of a coupled micro-cavity one-dimensional photonic crystal/photonic wire device structure based on silicon-on-insulator. Clear splitting of the resonances, with FSR values ranging from 8 nm to 48 nm, was obtained through the use of different hole arrangements within the middle section of the device structures, between the coupled cavities. The results show good agreement with calculations obtained using a finite-difference time-domain simulation approach.

Spectrally resolved resonant propulsion of dielectric microspheres

Use of resonant light forces opens up a unique approach to high-volume sorting of microspherical resonators with much higher uniformity of resonances compared to that in coupled-cavity structures obtained by the best semiconductor technologies. In this work, the spectral response of the propulsion forces exerted on polystyrene microspheres near tapered microfibers is directly observed. The measurements are based on the control of the detuning between the tunable laser and internal resonances in each sphere with accuracy higher than the width of the resonances. The measured spectral shape of the propulsion forces correlates well with the whispering-gallery mode resonances in the microspheres. The existence of a stable radial trap for the microspheres propelled along the taper is demonstrated. The giant force peaks observed for 20-μm spheres are found to be in a good agreement with a model calculation demonstrating an efficient use of the light momentum for propelling the microspheres.

Microspherical photonics: Sorting resonant photonic atoms by using light

A method of sorting microspheres by resonant light forces in vacuum, air, or liquid is proposed. Based on a two-dimensional model, it is shown that the sorting can be realized by allowing spherical particles to traverse a focused beam. Under resonance with the whispering gallery modes, the particles acquire significant velocity along the beam direction. This opens a unique way of large-volume sorting of nearly identical photonic atoms with 1/Q accuracy, where Q is the resonance quality factor. This is an enabling technology for developing super-low-loss coupled-cavity structures and devices.

Selective thermal emission from a patterned metalized plastic

A high-throughput hot-embossing and metal-deposition fabrication process based on compact-disc manufacturing has been employed to produce Coupled Resonant Cavity (CRC) structures for directionally and spectrally selective thermal emission at infrared wavelengths. The CRC structures were fabricated by hot embossing poly(methyl methacrylate) (PMMA) substrates, over which a layer of aluminum (Al) was deposited to support surface plasmons. The thermal emission behavior of the structures was predicted by Rigorous Coupled Wave Analysis (RCWA) of deposited metal profiles determined by the Blech model. It is shown that CRC structures which are initially under-imprinted may be brought to a resonant condition by increasing the metal deposition thickness, which is desirable for improving the thermal and mechanical properties of the structures. The primary source of deviation between the modeled and measured results was associated with surface curvature induced in the substrates during the fabrication process.

Microspherical Photonics: Ultra-High Resonant Propulsion Forces

In 1977, Arthur Ashkin and Joseph M. Dziedzic demonstrated that optical forces possess narrow spectral peaks determined by the internal resonances in microdroplets. These resonant optical forces open up a unique way of high-volume sorting of nearly identical “photonic atoms,” which may be used as building blocks for coupled-cavity structures and devices. However, to date, only relatively subtle resonant effects have been observed.

Terahertz Waveforms Generated by Second-Order Nonlinear Polarization in GaAs/AlAs Coupled Multilayer Cavities Using Ultrashort Laser Pulses

Temporal terahertz waveforms generated from GaAs/AlAs coupled multilayer cavity structures were simulated and compared with experimental results. Femtosecond laser pulses covering two cavity-mode frequencies were used for the difference frequency generation (DFG) in the terahertz region. The Fourier components dependent on the frequency and spatial position were determined for the second-order nonlinear polarization induced by a 100-fs Gaussian pulse injection. When the temporal waveform was simulated using the Fourier components, the oscillating behavior due to the efficient DFG of the two cavity modes was clearly observed after the initial ultrafast response near the incident surface. Assuming the exponential decay of signal sensitivity in the high-frequency region, the simulated results were consistent with the experimentally measured ones for coupled cavity structures grown on (113)B GaAs substrates.

All-Optical Plasmonic Switches Based on Coupled Nano-disk Cavity Structures Containing Nonlinear Material

All-optical plasmonic switches based on a novel coupled nano-disk cavity configuration containing nonlinear material are proposed and numerically investigated. The finite difference time domain simulation results reveal that the single-disk plasmonic structure can operate as an “on–off” switch with the presence/absence of pumping light. We also demonstrate that the proposed T-shaped plasmonic structure with two disk cavities can switch signal light from one port to another under an optical pumping light, functioning as a bidirectional switch. The proposed nano-disk cavity plasmonic switches have many advantages such as compact size, requirement of low pumping light intensity, and ultra-fast switching time at a femto-second scale, which are promising for future integrated plasmonic devices for applications such as communications, signal processing, and sensing.

Observation of coupled-cavity structures in metamaterials

In this letter, we investigated the transmission properties of metamaterial based coupled-cavity structures. We first calculated the effective parameters of a split-ring resonator (SRR) and composite metamaterial (CMM) structures. Subsequently, we introduced coupled-cavity structures and presented the transmission spectrum of SRR and CMM based coupled-cavity structures. The splitting of eigenmodes was observed due to the interaction between the localized electromagnetic cavity modes. Finally, the dispersion relation and normalized group velocity of the coupled-cavity structures were calculated. The maximum group velocity was found to be 100 times smaller than the speed of light in vacuum.

利用二维光子晶体耦合腔结构实现高效滤波

Transmission spectra of coupled cavity structures (CCSs) in two-dimensional (2D) photonic crystals (PCs) are investigated using a coupled mode theory, and an optical filter based on CCS is proposed. The performance of the filter is investigated using finite-difference time-domain (FDTD) method, and the results show that within a very short coupling distance of about 3λ, where λ is the wavelength of signal in vacuum, the incident signals with different frequencies are separated into different channels with a contrast ratio of 20 dB. The advantages of this kind of filter are small size and easily tunable operation frequencies.

Optical bistability and multistability in one-dimensional photonic band gap structures

We study optical bistability and multistability in multilayer coupled-cavity thin film structures comprised of high-index and low-index materials. The reduction of the bistable threshold and necessary index change are studied as functions of structural parameters by explicitly mapping the evolution of transmission and phase. We find that the coupled-cavity structure has no clear advantage over single-cavity structures for bistable operation. However, there is an advantage in using coupled-cavity structures for multistability in that the threshold for multistability can be reduced.

High efficiency second harmonic generation in one-dimensional photonic crystal coupled cavity structures

Second harmonic generation of coupled cavity structures (CCSs) fabricated in one-dimensional photonic crystals, which are composed of dispersive materials, is investigated. The fundamental wave is a localized mode, and the second harmonic wave is a traveling mode of the CCS. Using transfer matrix and effective refractive index methods, we analyze the localization and phase matching of the two modes. Using a nonlinear finite-difference time-domain method, we simulate the nonlinear process with a Gaussian pumping pulse, and both transmitted and reflected second harmonic signals are obtained. We investigate the effect of cavity number on the conversion efficiency and find the material dispersion can be compensated by the structural dispersion. For this reason, the conversion efficiency in a CCS is three orders larger than that in a bulk material of the coherence length.

Coupling of whispering-gallery modes in size-mismatched microdisk photonic molecules

Mechanisms of whispering-gallery (WG)-mode coupling in microdisk photonic molecules (PMs) with slight and significant size mismatches are numerically investigated. The results reveal two different scenarios of modes interaction depending on the degree of this mismatch and offer new insight into how PM parameters can be tuned to control and modify WG-mode wavelengths and Q factors. From a practical point of view, these findings offer a way to fabricate PM microlaser structures that exhibit low thresholds and directional emission and at the same time are more tolerant to fabrication errors than previously explored coupled-cavity structures composed of identical microresonators.

Minimizing finite-size effects in artificial resonance tunneling structures

We consider finite-size effects in coupled cavity structures. Starting with microring resonator structures well described by transfer matrices, we obtain conditions that lead to the minimization of finite-size effects. Our approach does not require numerical optimization and requires only slight modification of design parameters guided by closed-form analytical expressions. Using a Breit-Wigner scattering formalism, we demonstrate that the scheme can be used to minimize finite-size effects in a general class of coupled cavity structures. The strength of the present technique lies in its simplicity and its applicability to a wide variety of structures described by tight-binding formalisms.

Highly dispersive photonic crystal-based coupled-cavity structures

We measured the wavelength dependence of the group velocity dispersion (GVD) for different photonic-crystal coupled-cavity structures through a phase analysis of the transmitted modulated signal. GVD values as large as 106–107 times the dispersion of a standard single mode fiber are obtained when operating close to the band edge of the miniband, in agreement with the calculated group index. The GVD is found to be smaller for the structure based on more open cavities.

Comparative Analysis of the Curnow and Malykhin–Konnov–Komarov (MKK) Circuits as Representations of Coupled-Cavity Slow-Wave Structures

Two lumped element models of coupled-cavity slow-wave structures, one due to Curnow (1965) and another due to Malykhin, Konnov, and Komarov (MKK) (2003), are compared. The basis of comparison is the level of accuracy with which the models reproduce the cold circuit phase velocity and characteristic impedance of the structures as functions of frequency, as obtained from numerical solutions of Maxwell's field equations. Two distinct types of coupled-cavity structure are analyzed using these two models. These are a C-band double staggered ladder circuit and an L-band slot coupled Hughes-type structure. We find for the C-band case, both the Curnow and MKK models give excellent representations of both the cavity and slot bands, but for the L-band case the Curnow model has no solution when an attempt is made to fit both the cavity and slot bands, while the MKK model accurately represents both bands in this case.

A Study of Infrared Photonic Crystal Devices Using New ADI-FDTD Algorithm

To investigate the infrared photonic crystal devices numerically, the traditional finite-difference time-domain (FDTD) method has been modified by combining with a new alternating direction implicit (ADI) algorithm. An improvement of two-five in speed over previous FDTD methods can be obtained by calculating the envelope rather than the fast-varying field, and the numerical errors are minimized. Consider the isolated localized coupled-cavity modes, the phenomenon of eigenmode splitting has been observed when the coupled-cavity structures in two dimension triangular dielectric photonic crystals are simulated. The results are in good agreement with experiments.

Multilayer thin-film structures with high spatial dispersion.

We demonstrate how to design thin-film multilayer structures that separate multiple wavelength channels with a single stack by spatial dispersion, thus allowing compact manufacturable wavelength multiplexers and demultiplexers and possibly beam-steering or dispersion-control devices. We discuss four types of structure--periodic one-dimensional photonic crystal superprism structures, double-chirped structures exploiting wavelength-dependent penetration depth, coupled-cavity structures with dispersion that is due to stored energy, and numerically optimized nonperiodic structures utilizing a mixture of the other dispersion effects. We experimentally test the spatial dispersion of a 200-layer periodic structure and a 66-layer nonperiodic structure. Probably because of its greater design freedom, the nonperiodic structure can give both a linear shift with wavelength and a larger usable shift than the thicker periodic structure gives.