Effects Of Mode Selection Research Articles

Excitation of torsional modes of proteins via collisional energy transfer: A quantum dynamical approach

Quantum dynamical calculations have been carried out on the excitation of the torsional vibrations of a protein by collision with a solvent molecule. This energy transfer process represents the first step in the unfolding of the protein. The method developed for this purpose is the torsional close coupling, infinite order sudden approximation. Both time-independent and time dependent methods are used to solve the scattering problem and individual excitation of all the torsional modes of the protein is treated. The method is applied to the excitation of the HIV protein gp41 colliding with a water molecule. This protein has 1101 atoms, 56 amino acids, and 452 torsional modes. A major mode-selective effect is found in the computations: it is much easier to excite backbone torsions than sidechain torsions in the protein. In addition, resonances arise in the collisional process and these complexes involve temporary trapping of the water molecule inside the pockets of the protein.

Microlensed vertical-cavity surface-emitting laser for stable single fundamental mode operation

We propose and demonstrate a vertical-cavity surface-emitting laser structure that operates predominantly in the single fundamental transverse mode over a wide operation current range. In this laser structure, a microlens is integrated on top of an otherwise ordinary surface-emitting laser so that a small portion of laser output is fed back into the cavity, forcing the lasing to occur in the fundamental mode. Model calculations reproduce the observed mode selection effect.

Effects of Mode Selection, Scaling, and Orthogonalization on the Dynamic Analysis of Flexible Multibody Systems∗

ABSTRACT This paper analyzes the effects of mode selection, scaling, and orthogonalization on dynamic analysis of flexible mechanical systems. The effects of mode selection are investigated by combining attachment modes and constraint modes with vibration normal modes to represent elastic deformations of flexible components. Four sets of input data that depend on the options for mode scaling and orthogonalization are generated. The dynamic response and computer simulation times for each set are compared. Two examples are chosen: (1) a cantilever beam and (2) a vehicle with a flexible chassis. With vibration normal modes and attachment modes, it was found that mode scaling was helpful for efficient simulation, while the effect of mode orthogonalization was problem-dependent. With vibration modes and constraint modes, however, mode scaling had no effect or detracted from numerical efficiency.

ChemInform Abstract: Comparison of Overtone‐Induced and Electronic Photochemistry of Liquid tert‐Butyl Hydroperoxide: Supporting Evidence for Vibrational Mode Selectivity

The spectroscopy and unimolecular decomposition kinetics of tert-butyl hydroperoxide have been studied both in neat liquid and diluted in carbon tetrachloride. The data show that the tail of a transition to a repulsive electronic state extends to 540 nm but it is insignificant at longer wavelengths. The peak cross section of the 6-0 O-H overtone of tert-butyl hydroperoxide at 535 nm is estimated to be 5-24 times more intense than the electronic tail. tert-Butyl alcohol can be observed when photolyzing on the electronic tail, but no vibration-induced effect can be observed when photolyzing on the 5-0 O-H overtone. This study supports the contention that a small mode-selective effect is present in the gas-phase unimolecular decomposition of tert-butyl hydroperoxide when the 5-0 O-H overtone is excited by excluding the possibility of the electronic tail contributing to the nonstatistical component. Furthermore, the lifetime of this nonstatistical component is estimated to be longer than 0.1 ps.

Dynamics of Flexible Mechanical Systems Using Vibration and Static Correction Modes

A finite-element-based method is developed and applied for geometrically nonlinear dynamic analysis of spatial mechanical systems. Vibration and static correction modes are used to account for linear elastic deformation of components. Boundary conditions for vibration and static correction mode analysis are defined by kinematic constraints between components of a system. Constraint equations between flexible bodies are derived and a Lagrange multiplier formulation is used to generate the coupled large displacement-small deformation equations of motion. A standard, lumped mass finite-element structural analysis code is used to generate deformation modes and deformable body mass and stiffness information. An intermediate-processor is used to calculate time-independent terms in the equations of motion and to generate input data for a large-scale dynamic analysis code that includes coupled effects of geometric nonlinearity and elastic deformation. Two examples are presented and the effects of deformation mode selection on dynamic prediction are analyzed.

Dynamics of Articulated Structures. Part II. Computer Implementation and Applications

ABSTRACT A computer program based on Part I of this paper has been developed for dynamic analysis of articulated structures that contain elastic bodies. The organization of an intermediate-processor that generates data for the dynamic analysis code, using modal mass and stiffness data from a finite element program, is presented. The spatial DADS (Dynamic Analysis and Design System) dynamic analysis code is used to assemble and solve the equations of motion of the system. Examples are presented and the effects of deformation mode selection on the accuracy and computational cost of the analysis are evaluated.

Twisted-terraced-substrate GaAs/AlGaAs lasers

We present a new structure of AlGaAs/GaAs DH lasers consisting of two oppositely facing terraced-substrate cavities in series. Polarisation change at the junction results in internal reflection and produces a mode-selection effect. A stable single longitudinal mode is observed over a wide current range from 100 to 150 mA. The dominant lasing mode is locked over a 7° temperature variation. The lowest observed threshold current is 50 mA.

Real-time measurements of semiconductor laser spectra

A technique for measuring the instantaneous spectrum of a semiconductor laser is described. Spectral fluctuations of several different devices, including three different length channel-substrate buried-crescent lasers emitting at 1.52 μm were examined. The shortest (100-μm) device showed a trend towaxds single-mode operation, and it is believed that devices of up to 70-μm length should emit predominantly in one longitudinal mode. Mode selection effects, thought to be due to reflections from the monitor diode, were observed in a 1.3-μm packaged laster. The technique has also been used to measure correlations between successive pulses, and between spectral width and mean position.

Channeled substrate buried heterostructure GaAs- (GaAl)As injection lasers

A technique for fabricating stripe geometry GaAs/(GaAl)As injection lasers with optical and carrier confinement not only in the plane perpendicular but also parallel to the p-n junction is reported. This independently developed channeled substrate technique is a simple extension of conventional liquid phase epitaxy requiring no regrowth and relies on the strong tendency of epitaxial layers to smooth out as they grow. The active regions of the preliminary 10- and 20-μm-wide stripe lasers that have been fabricated to date are parabolic in cross section, tapering gradually to zero at the edges and completely embedded in GaAlAs. Unlike the ’’etched buried heterostructure lasers’’ recently reported by Burnham and Scifres, the n- (GaAl)As layer is continuous, thus preventing the direct injection of carriers into the substrate. Room-temperature pulsed threshold currents are in the range 150–350 mA with efficiencies up to 30%. This paper describes the LPE growth techniques that we used to produce the two preliminary wafers. A first-order theory is presented which describes the transverse mode structure of the unusually shaped waveguide which forms the active region of the lasers. The observed mode structure agrees closely with that calculated. A mode selection effect has been observed which favors low-order transverse mode operation close to threshold in the multimode waveguides which we produced. Finally, we report the cw operation of these lasers and discuss the more significant aspects of their operation.

Single-mode operation of diode lasers coupled to hemispherical resonators

Uncoated GaAs diode laser of different structures are coupled to external spherical mirrors. The emission characteristics of these systems are investigated. Mode selection effects are observed and single-mode operation may be achieved in the hemispherical resonator case if the length of the external cavity resonator is close to the optical diode length. Low coupling losses between the diode and the resonator are characteristic in this case and result in an increase of light output at coupling.

Spectral Characteristics and Inhomogeneities near Active Regions of Double-Heterostructure (GaAl)As–GaAs Lasers with Stripe Geometry Configuration

It is shown experimentally that several types of spectra of double heterostructure (GaAl)As-GaAs lasers are connected with corresponding inhomogeneities in and around their active regions. i) Families of axial modes belonging to different transverse modes along the junction plane often show different axial mode spacings, indicating filamentary nature along the axial direction. This type of oscillation may be called “quasi-filamentary”. ii) Families of axial modes in one transverse mode were also observed, which arise from the mode selection effect associated with sharp changes of the thickness of the active layer. iii) For some lasers, evidence is given that local boundaries rather than the heteroboundaries play an essential role. iv) The sustained pulsations having characteristics of the second order mode locking are connected with saturable absorption which arises from the existence of nonuniformity in the active region.

2A4 - Analysis of room temperature CW ruby lasers

Ruby lasers of between 10 mm and 75 mm length were operated continuously at room temperature in ellipsoidal pumping systems. Mode selection, relaxation oscillations, mode interaction, and thermal effects are described. CW operation was also studied with external mirrors, in 100 c/s giant pulse emission, and in single-mode emission. A ruby Pulslaser is described, that can be operated in 1 ms pulses at 50 to 120 c/s.

A Ruby Laser with External Mirrors of Large Spacing

Characteristics of a ruby laser are studied when the separation between mirrors of the resonator is as long as 10.5 m and 24.5 m. The experiments performed are: (1) the observation of beat signals between axial modes by an oscilloscope, and (2) Fabry-Perot analysis of the spectrum. Relation between the two results is discussed. Mode selection effect due to the characteristics of its double resonator is found.