Cavity-waveguide System Research Articles

Cavity-cavity coupling based on a terahertz rectangular subwavelength waveguide

The waveguide-cavity system has significant applications for sensing as well as for constructing highly integrated optical circuits. In this paper, we analyzed the mode coupling of resonant cavities integrated into a LiNbO3 rectangular subwavelength waveguide, which is able to serve as an on-chip integrated platform at terahertz frequencies. Using a time-resolved imaging method, we visualized terahertz field evolution in the waveguide-cavity structure and determined the resonant characteristics in transmission and reflection spectra. Moreover, a cavity-cavity coupling was realized, which was demonstrated by the transmission window in the transmission spectrum. We also showed that the localized fields in the cavity-cavity structure were enhanced at the transmission window. These findings would be helpful for the development of terahertz on-chip waveguide-based information processing and sensing systems.

Hybrid Toffoli gates with dipole-induced transparency effect in series and parallel cavity-waveguide systems

We propose two feasible schemes for realizing hybrid Toffoli gates with dipole-induced transparency (DIT) effect in series and parallel cavity-waveguide systems. The gate operations are accomplished by encoding information on flying photon and dipole emitter evanescently coupled to the microcavity, and the spatial as well as polarization states of photon will be flipped conditional on the quantum states of dipole emitters. The schemes could be physically realized in one step and straightforwardly extended to realize the N-qubit Toffoli gate, which greatly simplifies the experimental implementation of Toffoli gate and promises a much higher fidelity compared to those based on elementary gate decomposition. Benefiting from DIT effect, the schemes can be achievable without requirement of strong coupling condition of cavity-waveguide system, and they are insensitive to experimental noises, imperfections and the phase delay of adjacent microcavities, which may be feasible with present accessible technology.

S-parameters, non-Hermitian ports and the finite-element implementation in photonic devices with 𝒫𝒯-symmetry.

In Hermitian photonic devices, S-parameters, i.e., the elements of a scattering matrix based on integrated power flux and Hermitian modal orthogonality, are used to account for the transmission or reflection of light from one port to another. The definition of S-parameters in Hermitian settings becomes inappropriate in the non-Hermitian optical environment. Here we revisit the fundamental problems associated with extracting the S-parameters of light in photonic 𝒫𝒯-symmetric devices, i.e., waveguides or coupled waveguide-cavity systems, wherein the waveguide ports themselves may also be non-Hermitian. We first use the bi-orthogonal inner product that restores the modal orthogonality on the waveguide ports containing balanced gain and losses to quantify the modal overlapping instead of Hermitian inner product. Secondly, a finite element implementation is proposed and realized to extract the S-parameters on non-Hermitian ports. Lastly, we illustrate our approach of calculating the S-parameters on non-Hermitian ports via two waveguide-lattice structures. The numerical results of S-parameters are validated against the constraints imposed by reciprocity and 𝒫𝒯-symmetry.

Quantum Parity Gate with Dipole Induced Transparency Effect in the Drop-filter Cavity-waveguide System

We propose a feasible scheme for realizing quantum parity gate with dipole induced transparency (DIT) effect in the drop-filter cavity-waveguide system. The interference effect of dipole-microcavity systems plays the key role in the transmission spectrum, and quantum parity gate of dipole emitters can be realizable just by measuring the output photon in different waveguides. Benefiting from DIT effect, the scheme may work in the bad cavity regime and it is also insensitive to experimental noises and imperfections, which may be feasible with present accessible technology.

Hybrid transparency effect in the drop-filter cavity–waveguide system

We theoretically investigate hybrid transparency effect, which incorporates both dipole-induced transparency (DIT) and all-optical electromagnetically induced transparency (EIT)-like effects, in the drop-filter cavity–waveguide system. It is shown that the hybrid transparency effect originates from not only destructive interference of the cavity fields but also interference between DIT and all-optical EIT-like effects, thus the disappearance and revival phenomena of transparency windows occur. Especially, the transmission amplitude of hybrid transparency effect is even larger than that of all-optical EIT-like effect in the case of large intrinsic loss of microcavity. Benefiting from the hybrid transparency effect, photonic Stern–Gerlach-like and Faraday rotation effects can be realizable in multiple regimes, which may be useful for hybrid quantum information processing with photon and solid state qubit.

Dynamic Control of Double Plasmon-Induced Transparencies in Aperture-Coupled Waveguide-Cavity System

A theoretical model, for dynamic control of double plasmon-induced transparencies in aperture-coupled metal-dielectric-metal (MDM) waveguide-cavity system, is proposed. Based on the theoretical and numerical results, the formation and evolution mechanisms of double plasmon-induced transparencies are investigated more accurately compared to previous works. The symmetries, positions and measured full width at half maximum (FWHM) of the double transparent windows, and the transmission phase shift can be tuned freely by structural parameters. Moreover, the slot cavity location relative to aperture affects the phase difference between the left and right slot cavities to aperture center and determines the wavelength of resonance mode in aperture-side-coupled slot cavity, which plays an important role in the appearance or disappearance of transparent windows. In particular, supported by the consistency between the theoretical analysis and simulations, we obtain the threshold of coupling distance between two apertures, at which the abrupt phase shift happens. The detailed analysis may open up the possibility for precise control of light in highly integrated optical circuits.

Broadband near total light absorption in non-PT-symmetric waveguide-cavity systems.

We introduce broadband waveguide absorbers with near unity absorption. More specifically, we propose a compact non-parity-time-symmetric perfect absorber unit cell, consisting of two metal-dielectric-metal (MDM) stub resonators with unbalanced gain and loss side-coupled to a MDM waveguide, based on unidirectional reflectionlessness at exceptional points. With proper design, light can transport through the perfect absorber unit cell with reflection close to zero in a broad wavelength range. By cascading multiple unit cell structures, the overall absorption spectra are essentially the superposition of the absorption spectra of the individual perfect absorber unit cells, and absorption of ~ 100% is supported in a wide range of frequencies.

Quasinormal mode theory and design of on-chip single photon emitters in photonic crystal coupled-cavity waveguides.

Using a quasinormal mode (QNM) theory for open cavity systems, we present detailed calculations and designs of a photonic crystal nanocavity (PCN) side-coupled to a photonic crystal waveguide (PCW) for on-chip single photon source applications. We investigate various cavity-waveguide geometries using an L3 PCN coupled to a W1 PCW, obtaining the quality factors, effective mode volumes, and single photon Purcell factors of the complete loaded cavity-waveguide system as a function of spatial separation between the two. We also show that the quality factor does not monotonically increase with increasing separation between the PCN and PCW, and we identify a particular hole/defect which acts as the key structural parameter in the cavity-waveguide coupling.

An analytical approach for beam loading compensation and excitation of maximum cavity field gradient in a coupled cavity-waveguide system

The critical process of beam loading compensation in high intensity accelerators brings under control the undesired effect of the beam induced fields to the accelerating structures. A new analytical approach for optimizing standing wave accelerating structures is found which is hugely fast and agrees very well with simulations. A perturbative analysis of cavity and waveguide excitation based on the Bethe theorem and normal mode expansion is developed to compensate the beam loading effect and excite the maximum field gradient in the cavity. The method provides the optimum values for the coupling factor and the cavity detuning. While the approach is very accurate and agrees well with simulation software, it massively shortens the calculation time compared with the simulation software.

Unidirectional reflectionless propagation in plasmonic waveguide-cavity systems at exceptional points.

We design a non-parity-time-symmetric plasmonic waveguide-cavity system, consisting of two metal-dielectric-metal stub resonators side coupled to a metal-dielectric-metal waveguide, to form an exceptional point, and realize unidirectional reflectionless propagation at the optical communication wavelength. The contrast ratio between the forward and backward reflection almost reaches unity. We show that the presence of material loss in the metal is critical for the realization of the unidirectional reflectionlessness in this plasmonic system. We investigate the realized exceptional point, as well as the associated physical effects of level repulsion, crossing and phase transition. We also show that, by periodically cascading the unidirectional reflectionless plasmonic waveguide-cavity system, we can design a wavelength-scale unidirectional plasmonic waveguide perfect absorber. Our results could be potentially important for developing a new generation of highly compact unidirectional integrated nanoplasmonic devices.

Plasmonic coaxial waveguide-cavity devices.

We theoretically investigate three-dimensional plasmonic waveguide-cavity structures, built by side-coupling stub resonators that consist of plasmonic coaxial waveguides of finite length, to a plasmonic coaxial waveguide. The resonators are terminated either in a short or an open circuit. We show that the properties of these waveguide-cavity systems can be accurately described using a single-mode scattering matrix theory. We also show that, with proper choice of their design parameters, three-dimensional plasmonic coaxial waveguide-cavity devices and two-dimensional metal-dielectric-metal devices can have nearly identical transmission spectra. Thus, three-dimensional plasmonic coaxial waveguides offer a platform for practical implementation of two-dimensional metal-dielectric-metal device designs.

On-chip electrically controlled routing of photons from a single quantum dot

Electrical control of on-chip routing of photons emitted by a single InAs/GaAs self-assembled quantum dot (SAQD) is demonstrated in a photonic crystal cavity-waveguide system. The SAQD is located inside an H1 cavity, which is coupled to two photonic crystal waveguides. The SAQD emission wavelength is electrically tunable by the quantum-confined Stark effect. When the SAQD emission is brought into resonance with one of two H1 cavity modes, it is preferentially routed to the waveguide to which that mode is selectively coupled. This proof of concept provides the basis for scalable, low-power, high-speed operation of single-photon routers for use in integrated quantum photonic circuits.

Octagonal toroid microcavity for mechanically robust optical coupling

Light is usually coupled to a whispering gallery mode cavity using a tapered fiber. However, it is difficult to stabilize the optical coupling against mechanical vibration because it requires sub-μm control of the gap distance between the fiber and cavity. In this study, we experimentally demonstrate mechanically robust coupling that we realize by allowing the tapered fiber to touch the sidewall of the cavity. By using an octagonal toroid microcavity, we prevent the cavity-waveguide system from over coupling and achieve critical coupling even when the fiber is in contact with the surface of the cavity. We show by numerical analysis that such a deformed microcavity is required if we need to control the coupling, since a circular cavity usually overcouples when the fiber contacts the surface. The fabricated octagonal silica toroid microcavity exhibits a quality factor of 2.2 × 104 when the tapered fiber touches a cavity with a diameter of 80 μm.

Low Power and Large Modulation Depth Optical Bistability in an Si Photonic Crystal L3 Cavity

Optical bistability with low power and large modulation depth is observed in the silicon 2D photonic crystal 3 defect-long (L3) cavity coupling with a photonic crystal waveguide experimentally. The cavity-waveguide resonator system operates under the critical coupling condition. The triangular line shape of the cavity resonance and the hysteresis loop of the system is observed. The switching contrast is 7.7 dB and the modulation depth is 0.70 at the switching power of −12.1 dBm. A nonlinear coupled mode model is established to analyze the behavior of the cavity, and numerical simulation results are in good agreement with the experiment results.

High-Q silica zipper cavity for optical radiation pressure driven MOMS switch

We design a silica zipper cavity that has high optical and mechanical Q (quality factor) values and demonstrate numerically the feasibility of a radiation pressure driven micro opto-mechanical system (MOMS) directional switch. The silica zipper cavity has an optical Q of 4.0 × 104 and an effective mode volume Vmode of 0.67λ3 when the gap between two cavities is 34 nm. The mechanical Q (Qm) is determined by thermo-elastic damping and is 2.0 × 106 in a vacuum at room temperature. The opto-mechanical coupling rate gOM is as high as 100 GHz/nm, which allows us to move the directional cavity-waveguide system and switch 1550-nm light with 770-nm light by controlling the radiation pressure.

Electro-optical channel drop switching in a photonic crystal waveguide-cavity side-coupling system

The electro-optical channel drop switching in a photonic crystal waveguide-cavity side-coupling system is reported. The line waveguide is formed by removing a single row of dielectric cylinders. The twin optical microcavities side coupled between linear waveguides is studied by solving Maxwell's equations. We determine the general characteristics of the coupling element required to achieve channel drop tunneling. By modulating the conductance of the twin microcavities, the electrical tunability of the resonant modes is observed in the transmission spectrum. The spectral characteristics suggest a potential application for this switching device as an efficient multichannel optical switch in the photonic integrated circuits.

Efficient numerical method for analyzing optical bistability in photonic crystal microcavities

Nonlinear optical effects can be enhanced by photonic crystal microcavities and be used to develop practical ultra-compact optical devices with low power requirements. The finite-difference time-domain method is the standard numerical method for simulating nonlinear optical devices, but it has limitations in terms of accuracy and efficiency. In this paper, a rigorous and efficient frequency-domain numerical method is developed for analyzing nonlinear optical devices where the nonlinear effect is concentrated in the microcavities. The method replaces the linear problem outside the microcavities by a rigorous and numerically computed boundary condition, then solves the nonlinear problem iteratively in a small region around the microcavities. Convergence of the iterative method is much easier to achieve since the size of the problem is significantly reduced. The method is presented for a specific two-dimensional photonic crystal waveguide-cavity system with a Kerr nonlinearity, using numerical methods that can take advantage of the geometric features of the structure. The method is able to calculate multiple solutions exhibiting the optical bistability phenomenon in the strongly nonlinear regime.

Circuit model for efficient analysis and design of photonic crystal devices

We substitute different types of photonic crystal waveguide components by approximate transmission line circuits. The proposed distributed circuits exploit the analogy of wave propagation in photonic crystal waveguides and transmission lines. They are either cascaded to each other or inserted like stubs to imitate wave propagation within the photonic structure. Notable examples, e.g. coupled waveguide-cavity systems, sharp 90° bends, and T-junctions, are studied in detail. It is shown that analysis of the proposed circuits here can yield accurate enough results and thus substitute the brute-force numerical methods. The privilege of having analytical models is exploited to improve the performance of sharp 90° bends and T-junctions. The presented results are verified by using the standard finite-difference time domain (FDTD) method.

Nonlinear photon transport in a semiconductor waveguide-cavity system containing a single quantum dot: Anharmonic cavity-QED regime

We present a semiconductor master equation technique to study the input/output characteristics of coherent photon transport in a semiconductor waveguide-cavity system containing a single quantum dot. We use this approach to investigate the effects of photon propagation and anharmonic cavity-QED for various dot-cavity interaction strengths, including weakly-coupled, intermediately-coupled, and strongly-coupled regimes. We demonstrate that for mean photon numbers much less than 0.1, the commonly adopted weak excitation (single quantum) approximation breaks down, even in the weak coupling regime. As a measure of the anharmonic multiphoton-correlations, we compute the Fano factor and the correlation error associated with making a semiclassical approximation. We also explore the role of electron--acoustic-phonon scattering and find that phonon-mediated scattering plays a qualitatively important role on the light propagation characteristics. As an application of the theory, we simulate a conditional phase gate at a phonon bath temperature of $20 $K in the strong coupling regime.

High resonant transmission contrast filter based on the dual side-coupled cavities plasmonic structure

By using indirect coupling between two side-coupled cavities, we propose a method to enhance the resonant transmission contrast ratio in the lossy metal–dielectric–metal waveguide cavity system. We find that the dual side-coupled cavity structure has higher on-resonance transmission contrast ratio than the single side-coupled cavity structure for involving more coherent interfering paths. The filter operating around the wavelength of 1550 nm is analyzed based on coupled-mode theory, and the optimal on-resonance transmission contrast condition is obtained. Numerical experiments confirm the enhancement of the resonant contrast.