Mode Coupling Strength Research Articles

1:1 internal mode coupling strength in GaAs doubly-clamped MEMS beam resonators with linear and nonlinear oscillations

We have investigated the 1:1 internal mode coupling strength in a GaAs doubly-clamped MEMS beam resonator by thermally tuning the frequencies of two neighbored resonant modes. The anti-crossing of the resonant frequencies indicates that the two modes couple with each other, and the mode coupling strength was estimated by the splitting of the anti-crossed frequencies of the two modes. When the oscillation amplitudes were tuned from the weakly-driven linear regime to the strongly-driven nonlinear regime, we observed slightly increased frequency splitting, indicating that the nonlinear oscillation only makes a very small contribution to the mode coupling strength.

Existence conditions for phononic frequency combs

The mechanical analog of optical frequency combs, phononic frequency combs, has recently been demonstrated in mechanical resonators and has been attributed to coupling between multiple phonon modes. This paper investigates the influence of the mode structure on comb generation using a model of two nonlinearly coupled phonon modes. The model predicts that there is only one region within the amplitude-frequency space where combs exist, and this region is a subset of the Arnold tongue that describes a 2:1 autoparametric resonance between the two modes. In addition, the location and shape of the comb region are analytically defined by the resonance frequencies, quality factors, mode coupling strength, and detuning of the driving force frequency from the mechanical resonances, providing clear conditions for comb generation. These results enable comb structure engineering for applications in areas as broad as sensing, communications, quantum information science, materials science, and molecular science.

Anisotropy and shaping effects on the stability boundaries of infernal ideal MHD modes in tokamak hybrid plasmas

Anisotropy and some limiting toroidal flow effects on the stability of nearly resonant ideal magnetohydrodynamic modes in hybrid shaped tokamak plasmas are investigated within the ideal MHD infernal mode framework. Such effects are found to alter the plasma magnetic well/hill, which can be interpreted as imparing the average curvature, and the strength of mode coupling. In line with previous results, it is found that better stability properties are achieved through deepening the magnetic well by special cases of uniform toroidal flow and parallel plasma anisotropy. Plasma shaping provides additional modifications to the magnetic well depth, whose global stabilising or destabilising effect depends on the mutual interplay of elongation, triangularity and toroidicity. Further stabilisation is achieved by weakening the mode drive in vertically elongated plasmas.

Decoupling from a Thermal Bath via Molecular Polariton Formation.

The coupling between an electronic system and an environmental bath plays a decisive role in the excited state dynamics of artificial/natural molecular condensed phases. Although it is generally difficult to control the coupling between the system and the thermal bath in condensed matter, a strong light-matter coupling can control system-bath coupling properties using the polaron decoupling effect, in which a coherent interaction between excitons and photons reduces the reorganization energy. Here we demonstrate that this polaron decoupling strongly reduces the fluctuations in electronic energy in tetraphenyldibenzoperiflanthene thin films embedded in an optical microcavity. Using two-dimensional electronic spectroscopy, the frequency-fluctuation correlation function of the lower polariton state was revealed, showing that the dynamic inhomogeneity due to bath coupling inside the microcavity almost vanishes completely. This was attributed to a significant delocalization of the lower polariton state over 105 molecules in the cavity, reducing the effective coupling strength of the bath modes.

Single-Photon Emission Mediated by Single-Electron Tunneling in Plasmonic Nanojunctions.

Recent scanning tunneling microscopy (STM) experiments reported single-molecule fluorescence induced by tunneling currents in the nanoplasmonic cavity formed by the STM tip and the substrate. The electric field of the cavity mode couples with the current-induced charge fluctuations of the molecule, allowing the excitation of photons. We investigate theoretically this system for the experimentally relevant limit of large damping rate κ for the cavity mode and arbitrary coupling strength to a single-electronic level. We find that for bias voltages close to the first inelastic threshold of photon emission, the emitted light displays antibunching behavior with vanishing second-order photon correlation function. At the same time, the current and the intensity of emitted light display Franck-Condon steps at multiples of the cavity frequency ω_{c} with a width controlled by κ rather than the temperature T. For large bias voltages, we predict strong photon bunching of the order of κ/Γ where Γ is the electronic tunneling rate. Our theory thus predicts that strong coupling to a single level allows current-driven nonclassical light emission.

On-chip silicon dual-mode multiplexer via a subwavelength grating-based directional coupler and a mode blocker.

An on-chip silicon dual-mode multiplexer (MUX) is proposed via a subwavelength grating (SWG)-based directional coupler and a mode blocker. The proposed mode MUX is optimized by utilizing the three-dimensional full-vectorial finite-difference time-domain-based band structure and propagation calculations. The simulated results indicate that an ultralow mode cross talk of $ - {40}\,\,{\rm dB}$-40dB at the operating wavelength of 1550 nm can be achieved by introducing a ${{\rm TE}_0}$TE0 mode blocker. Benefiting from the triple SWG coupler, the mode-coupling strength can be significantly enhanced inside the proposed mode MUX, which can achieve a compact coupling length of 8.75 µm, a low insertion loss of 0.39 dB at 1550 nm, an ultrabroad bandwidth of 310 nm for the mode CT $ \lt - {15.0}\,\,{\rm dB}$<-15.0dB, and insensitivity to fabrication errors.

Dynamic modulation of modal coupling in microelectromechanical gyroscopic ring resonators

Understanding and controlling modal coupling in micro/nanomechanical devices is integral to the design of high-accuracy timing references and inertial sensors. However, insight into specific physical mechanisms underlying modal coupling, and the ability to tune such interactions is limited. Here, we demonstrate that tuneable mode coupling can be achieved in capacitive microelectromechanical devices with dynamic electrostatic fields enabling strong coupling between otherwise uncoupled modes. A vacuum-sealed microelectromechanical silicon ring resonator is employed in this work, with relevance to the gyroscopic lateral modes of vibration. It is shown that a parametric pumping scheme can be implemented through capacitive electrodes surrounding the device that allows for the mode coupling strength to be dynamically tuned, as well as allowing greater flexibility in the control of the coupling stiffness. Electrostatic pump based sideband coupling is demonstrated, and compared to conventional strain-mediated sideband operations. Electrostatic coupling is shown to be very efficient, enabling strong, tunable dynamical coupling.

Strong optomechanical coupling of light and highly confined acoustic phonons in slot dual-beam phoxonic crystal cavities

In this study, strong optomechanical (OM) coupling is proposed through the introduction of a slot in a dual-beam phoxonic crystal cavity structure. The structure can support a confined optical slot mode and localized phononic cavity modes to interact effectively through the slot surfaces. In addition to using conventional OM coupling rates for evaluating the coupling strength of photonic and phononic modes, we consider the optical forces induced by the optical slot mode to discuss the coupling between different mode pairs and the underlying mechanisms of the strong OM coupling. The optical field can induce radiation pressure and electrostrictive forces in the structure. We demonstrate that the radiation pressure dominates the coupling enhancement of photonic and phononic modes because of the slot, whereas the electrostrictive surface pressure plays a minor role and the electrostrictive body force has a negligible contribution. On the basis of the optical forces, we can then calculate the acoustic phonon spectrum through optical excitation. The spectrum indicates the appearance of strong OM coupling in the additional phononic cavity modes. The results suggest that the slot dual-beam cavity structure can be a promising choice for tailoring effective optical forces in micro- and nano-optomechanical systems for enhancing OM coupling.

Fundamental and Third Harmonic Mode Coupling Induced Single Soliton Generation in Kerr Microresonators

The single soliton propagating in a microresonator (MRR) is characterized by a smooth spectrum. However, the number of solitons that propagate in an MRR in the anomalous dispersion regime is usually stochastic and gives rise to a complex spectrum. In this paper, we introduce a new passive mechanism based on third-harmonic-induced nonlinear coupling for the formation of a single deterministic soliton in an MRR. We show that with appropriate fundamental and third harmonic mode coupling strength and a slight phase mismatch, deterministic single soliton generation can be realized by simply scanning the detuning of a continuous wave serving as a pump. Under strong mode coupling strength, a relatively smaller wave vector mismatch within suitable range can be used for single soliton generation. Our work provides a potential approach for on-chip generation of coherent optical Kerr combs and efficient THG conversion.

Strong longitudinal coupling of Tamm plasmon polaritons in graphene/DBR/Ag hybrid structure

In this paper, strong longitudinal coupling of the Tamm plasmon polaritons (TPPs) is investigated in a graphene/DBR/Ag slab hybrid system. It is found that TPPs can be excited at both the top graphene and the bottom silver slab interface, which can strongly interact with each other in this coupled structure. Numerical simulation results demonstrate that the vertical Tamm plasmon coupling can be either tuned by adjusting the geometric parameters or actively controlled by the Fermi energy in graphene sheet as well as the incident angle of light, allowing for strong light-matter interaction with a tunable dual-band perfect absorption. Moreover, the coupling strength of the hybrid modes exhibits a large tuning range, from a large Rabi splitting to an extremely narrow induced transparency in this coupled regime. Coupled mode theory has been employed to explain the strong coupling phenomenon. The controllable TPP coupling with an ultrahigh dual-band absorption capability offered by this simple layered structure opens new avenues for developing a broad range of graphene-based active optoelectronic and polaritonic devices.

Controlling the Performance of Polymer Lasers via the Cavity Coupling.

The polarization and threshold of distributed feedback (DFB) polymer lasers were controlled by adjusting the cavity coupling. The cavity of DFB polymer lasers consisted of two gratings, which was fabricated by a two-beam multi-exposure holographic technique. The coupling strength of the cavity modes was tuned by changing the angle between the two gratings. The threshold of the polymer lasers decreased with reducing the coupling strength of the cavity modes. A minimum threshold was observed at the lowest coupling strength. Moreover, the azimuthally polarized output of the polymer lasers was modified by changing the cavity coupling. These results may provide additional perspectives to improve the performance of DFB polymer lasers.

Localized Surface Magnetic Modes Propagating Along a Chain of Connected Subwavelength Metamaterial Resonators

Controlling the propagation of electromagnetic waves at the subwavelength scale is hampered by diffraction. This study uses a chain of connected subwavelength metamaterial resonators to solve the problem, by elegantly improving the coupling strength of localized surface magnetic modes in the planar system. The metamaterial elements are connected physically, so that conduction current can be used to transfer energy efficiently. Of further interest is that the surface wave can be switched between forward and backward modes. These results are expected to impact the design of integrated photonic devices.

Entangling Cavity Modes in a Double-Cavity Optomechanical System

We study entanglement of the cavity modes in a double-cavity optomechanical system in strong-coupling regime. The system consists of two optomechanical systems coupled by a single photon hopping between them. With the radiation pressure of the photon, entanglement of the cavity modes can be generated. The concurrence between the cavity modes is at least twice larger than that between the mechanical modes. Moreover, when we change the ratio between coupling strength and resonant frequency of mechanical modes, the entanglement in cavity and mechanical modes are influenced differently.

Analytical solution and applications of three qubits in three coupled modes without rotating wave approximation

We study the dynamics of the three-qubit system interacting with multi-mode without rotating wave approximation (RWA). A physical realization of the system without direct qubits interactions with dephasing bath is proposed. It is shown that non-Markovian characters of the purity of the three qubits and the coupling strength of modes are stronger enough the RWA is no longer valid. The influences of the dephasing of qubits and interactions of modes on the dynamics of genuine multipartite entanglement and bipartite correlations of qubits are investigated. The multipartite and bipartite quantum correlations could be generated faster if we increase the coupling strength of modes and the RWA is not valid when the coupling strength is strong enough. The unitary transformations approach adopted here can be extended to other systems such as circuit or cavity quantum electrodynamic systems in the strong coupling regime.

Analytical modeling of dual-frequency solid-state lasers including a buffer reservoir for noise cancellation.

A theoretical model describing the dynamical behavior of dual-frequency solid-state lasers including a buffer reservoir (BR) is presented. It relies on the introduction of two additional coupled rate equations describing the interaction of the two laser modes with the BR. The relative intensity noise is derived by taking into account the fluctuations of both pump intensity and intra-cavity photons. This modelling approach accurately predicts the experimental noise spectra obtained with an Er,Yb:glass dual-frequency laser implemented in different cavity architecture configurations. The mode coupling strength in the BR is shown to rule the reduction efficiency of the excess noise lying at the in-phase and anti-phase frequencies.

Symmetry-Breaking Effect on the Electromagnetic Properties of Plasmonic Trimers Composed of Graphene Nanodisks

Plasmonic trimers composed of equal-sized graphene nanodisks are proposed in this paper. The symmetry-breaking effect on the electromagnetic properties of the nanostructure is numerically investigated by studying plasmon energy diagrams and optical scattering spectra in mid-infrared range with a gradient vertex angle. The degenerate plasmonic modes are lifted and new modes appear with increased vertex angle. The energy diagrams are consistent with scattering extinction spectra, about which the dipole moment distribution of the proposed structure is discussed to demonstrate the coupling strength of the collective plasmonic modes of the trimer. More specifically, the frequency tunability of the plasmonic trimer is pointed out by modifying the chemical potential of the graphene nanodisks without varying the geometric configuration. The proposed structure might find applications such as light-matter interaction, single molecule detection, and high-sensitivity chemical sensing.

Impact of modulation bandwidth on multiplexing using principal modes in MMF links.

Multimode fibers (MMFs) are widely used for short fiber links. However, the data rates through MMFs is limited owing to modal dispersion. The so-called "principal modes" (PMs) permit transmission and multiplexing through the MMFs without modal dispersion for small modulation bandwidths. For larger modulation bandwidths, however, they lose their dispersion-free nature. In this paper, we model the impact of modulation bandwidth and mode coupling strength on the performance of PMs. We develop a simulator that characterizes the dispersion and cross-talk of the PMs of few-mode and large-core graded-index MMFs with mode-dependent losses (MDL). Simulations reveal that for fibers without MDL, for modulation frequencies beyond 10 GHz diminishes the PMs' frequency response by more than 1 dB for 100 m in large-core MMF links and 10 km few-mode fiber links. With MDL, simulations reveal that for modulation bandwidths beyond 2 GHz diminishes the frequency response by 3 dB for a 1 km few-mode fiber and by more than 4 dB for a 1 km large-core multimode fiber. While multiplexing using PMs in large-core MMFs with MDL, we find that for modulation bandwidths beyond 3 GHz, the cross-talk is 20 dB in 1 km large-core MMF links, thereby limiting system performance.

Ringing phenomenon based measurement of weak mode-coupling strength in an optical microresonator

There is always a coupling between the degenerate clockwise (CW) and counter-clockwise (CCW) modes in a whisperinggallery- mode (WGM) optical microresonator, since the surface of the microresonator can not be perfectly smooth. It is important to measure this coupling strength in many applications. When the coupling strength is strong, the conventional method by observing mode splitting in the stationary spectrum can be used to measure its value. However, when the coupling strength is weak, the conventional method will not work. We experimentally demonstrate that the ringing phenomenon can be used to measure weak coupling strength between the CW and CCW modes in a WGM optical microresonator.

Temperature Correction for Cylindrical Cavity Perturbation Measurements

The need for accurate material property measurements using microwave cavities requires a form of compensation to correct for changes in temperature and other external influences. This paper details a method for temperature correcting microwave cavity perturbation measurements by monitoring two modes; one which is perturbed by the sample and one which is not (referred to as a nodal mode). The nodal modes used (TM310 and TE311 for an axial sample in a cylindrical cavity) are subject only to sample-independent influences. To demonstrate this technique, the bulk permittivity of a PTFE rod has been measured under varying temperature conditions. The results show that without correction, the measured temperature-dependent dielectric constant has large variations associated with the stepped and linear temperature ramping procedures. The corrected response mitigates systematic errors in the real part. However, the correction of the imaginary part requires careful consideration of the mode coupling strength. This paper demonstrates the importance of temperature correction in dynamic cavity perturbation experiments.

Investigations on perturbations of microwave dielectric resonator thermometer

Investigations of antenna probe length, antenna-dielectric distance, cavity filling and humidity on microwave resonator thermometer with respect to Q, spurious mode depression, coupling strength, accuracy, shock resistance or sensitivity were carried out in order to improve the dielectric resonator thermometer performance. Significant improvement of Q and depression of spurious mode coupling were obtained when the antenna length was reduced. It also turns out that the Q and spurious mode coupling strength vary with the distance between dielectric and antenna pin, as well under appropriate antenna length. Filling the cavity with nitrogen increases coupling strength but decrease frequency-temperature sensitivity compared to a vacuum-pumped cavity. Besides, preliminary results on the microwave resonator sensitivity to air humidity were obtained.