Internal Mode Coupling Research Articles

Highly efficient waveguide-coupled output in asymmetrically deformed square cavity microlasers.

We propose and demonstrate deformed square cavity microlasers for realizing highly efficient output from a connected waveguide. The square cavities are deformed asymmetrically by replacing two adjacent flat sides with circular arcs to manipulate the ray dynamics and couple the light to the connected waveguide. The numerical simulations show that the resonant light can efficiently couple to the fundamental mode of the multi-mode waveguide by carefully designing the deformation parameter utilizing global chaos ray dynamics and internal mode coupling. An enhancement of approximately six times in the output power is realized in the experiment compared to the non-deformed square cavity microlasers, while the lasing thresholds are reduced by about 20%. The measured far-field pattern shows highly unidirectional emission agreeing well with the simulation, which confirms the feasibility of the deformed square cavity microlasers for practical applications.

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.

Giant Enhancement in the Thermal Responsivity of Microelectromechanical Resonators by Internal Mode Coupling

We report on a giant enhancement in the thermal responsivity of the doubly-clamped GaAs microelectromechanical (MEMS) beam resonators by using the internal mode coupling effect. This is achieved by coupling the fundamental bending mode with the fundamental torsional mode of the MEMS beam resonators through the cubic Duffing nonlinearity. In the mode coupling regime, we have found that, when the input heat to the MEMS resonators is modulated at a particular frequency, the resonance frequency shift caused by heating can be enhanced by almost two orders of magnitude. The observed effect is promising for realizing high-sensitivity thermal sensing by using MEMS resonators, such as ultrasensitive terahertz detection at room temperature.

Low temperature Raman and DFT study of creatinine

The Raman spectra of creatinine powder in the wavenumber region of 500–3500 cm−1 have been recorded as a function of temperature ranging from 293 K to 83 K and different clusters of creatinine were optimized with DFT method to determine hydrogen bonding network in crystalline creatinine. The accurate vibrational assignment of all vibrational bands of creatinine has been done on the basis of the calculated vibrational spectra and the potential energy distribution. The calculated Raman spectra of hydrogen bonded tetramer cluster matches nicely with the experimentally observed spectra confirming four molecules in unit cell as reported by crystallographic data. In addition signature of dimer, which is just half of the tetramer, has also been observed. The bands associated with CH3, CH2, ν(CO), ν(CN) and ν(CH) have been given more focus to see the behavior of hydrogen bond network with temperature. The analysis of these bands reveals that a gradual change (increase/decrease) in linewidth and peak position occurs with temperature as a consequence of the anharmonic coupling of internal modes with phonon mode and intermolecular interaction. The behavior of hydrogen bond network on cooling has been explained in terms of contraction of unit cell.

Dynamics of a polymer in a Brownian ratchet

We have used Brownian dynamics simulations to study the dynamics of a bead-and-spring polymer subject to a flashing ratchet potential. To elucidate the role of hydrodynamic (HD) interactions, simulations were carried out for the cases where HD interactions are present and when they are absent. The average speed of the polymer and its conformational properties were examined upon variation in the polymer length, N, and the ratchet spatial period, L. Two distinct dynamical regimes were evident. In the low-N/high-L regime, the velocity decreases with increasing N, and center-of-mass diffusion is a key part of the motional mechanism. By contrast, in the high-N /low-L regime, the velocity is insensitive to variation in N, and motion is achieved via the coupling of internal modes to the cycling of the ratchet potential. The location of the regimes is correlated with the average conformational state of the polymer. Incorporating HD interactions increases the average polymer velocity for all polymer lengths and ratchet spatial periods considered. The dynamical behavior of polymers in the low-N/high-L regime can be understood using simple a theoretical model that yields quantitatively reasonable predictions of the polymer velocity.

An external–internal mode coupling for a 3D hydrodynamical model for applications at regional scale (MARS)

This paper presents a 3D model in sigma coordinates. Although the principles it is based on have been established for some time, some original aspects for this type of 3D mode splitting model are presented here. The model was designed to simulate flows in various coastal areas from the regional scale down to the inshore scale of small bays or estuaries where circulation is generally driven by a mix of processes. The processes to be modeled enable simplifications of the Navier–Stokes equations on the classic Boussinesq and hydrostatic hypotheses. These equations are transformed within a sigma framework to make free surface processing easier. The main point of our demonstration focuses on the original aspect of the coupling between barotropic and baroclinic modes especially designed for ADI. It explains how full consistency of the transport calculated within the 2D and 3D equation sets was obtained. Lastly, we describe the physical processes simulated on a realistic configuration at a regional scale in the Bay of Biscay.

Effects of resonant mode coupling on optical Characteristics of InGaN-GaN-AlGaN lasers

The phenomenon of resonant mode coupling is modeled and numerically investigated in InGaN-based multiple-quantum-well laser structures. It is shown that under resonant conditions, the internal mode coupling can lead to severe distortions of both near- and far-field characteristics. Taking into account the carrier-induced change of the modal effective index for the lasing mode, we show that the resonant internal mode coupling can result in a strongly nonlinear dependence of the modal gain on injection current, with possible region of negative differential modal gain.

Electronically induced dynamics in surface layers

Electronic excitations induced in well-defined surface layers (adsorbates as well as condensates) by electron impact or photon absorption evolve in time in terms of both internal and external coordinates of the electrons as well as of the nuclei. A survey is given to show how the combination of electron spectroscopies with the analysis of desorbing particles (fragments and molecules) liberated by such excitations can be used to investigate the interplay of these evolutions with each other. Of particular interest is the coupling of internal modes of the surface species to the substrate or the surroundings in terms of energy and charge exchange. First the main principles governing these processes that have become understood over the years are briefly summarized. On this basis some recent, partly quite unexpected results are discussed. The first utilizes laser spectroscopy for a detailed analysis of the energy distribution over the internal modes of molecules desorbed by electron impact; the other two combine electron and ion yields under highly resolved photon irradiation of condensates. They illustrate the potential of such investigations for the understanding of the mechanisms of electronically induced desorption, of the analysis of changes induced in the molecular properties by the situation at the surface or in the condensate including symmetry breaking in the surface field, and for the improved assignment and selective detection of complex excitations in solids, surfaces, and molecules.

Infrared spectroscopic studies of hydrogen bonding in triethylammonium salts. Part 3.—Strong hydrogen bonding

Infrared absorption spectra of solutions of a number of strongly hydrogen-bonded triethyl-ammonium salts Et3N+H⋯A– have been obtained. Special attention has been paid to changes in the band parameters of the νN+H stretching mode. A comparison is made between Fermi resonance of internal modes of the triethylammonium ion and a new type of strong resonance coupling of internal modes in which the intermolecular bending mode of the hydrogen-bonded complex Et3N+H⋯A– participates. The zero, first and second moments of the νN+H stretching-mode bands have been measured. The inter-relation between these values has been checked over the permissible range of interionic hydrogen-bonding strengths in the ion pairs Et3N+H⋯A–. Hydrogen-bond energies of the triethylammonium salts studied have been estimated by a spectral method.

Dynamic light‐scattering studies of DNA. I. The coupling of internal modes with anisotropic translational diffusion in congested solutions

Dynamic light‐scattering studies of DNA. I. The coupling of internal modes with anisotropic translational diffusion in congested solutions

Dynamic light-scattering studies of DNA. I. The coupling of internal modes with anisotropic translational diffusion in congested solutions.

AbstractA model for the coupling between internal modes, or molecular rotation, and anisotropic translational diffusion in congested solutions is proposed to account for the anomalously slow component that has appeared ubiquitously in reported autocorrelation functions of Rayleigh scattered light from solutions of DNA's with molecular weights greater than about 107. The predicted existence of an anomalously slow mode in addition to a faster “normal” mode, as well as the predicted relative amplitudes of both fast and slow components, are qualitatively in agreement with the observations. For sufficiently long‐wavelength fluctuations all of the amplitude appears in the slower mode, which then exhibits an appropriately averaged translational diffusion coefficient.In support of the model it is shown in the Appendix that nonideal central interactions between macromolecules are by themselves insufficient to generate isolated internal mode relaxation terms in the autocorrelation function, unless translational ordering of the macromolecules extends over the illuminated observation region.

Experimental determination of the radiation pattern of optical fibres

The analysis of the output radiation pattern of clad optical fibres has shown the existence of strong mode coupling effects on two particular samples. Further, it is shown on a low-loss sample that the amount of mode conversion can be externally controlled by the application of a stress. Measurements performed on a 1 km waveguide show how internal mode coupling leads to a steady-state radiation pattern.