Optimization of multiple quantum well electroabsorption modulators

A novel and simple approach for optimization of a multiple quantum well electroabsorption waveguide modulator is presented. In our approach, all four of the modulator characteristic parameters (on-off ratio, insertion loss, bandwidth, and driving voltage) are included. Design examples for GaAs/AlGaAs multiple quantum well modulators are presented. The accuracy of our model is confirmed by comparison between calculated and published experimental data.     

Optimizing Design for the Traveling Wave Electrodes in Low-Drive High-Speed Electro-Optic Polymer Modulators

Electro-optic (EO) polymer modulators are very promising in the realization of cost-effective and high-performance optical transmissions. In this article, general strategies and specific designs of the traveling wave electrodes in EO polymer modulators were presented to reduce the modulator drive power while maintaining a broadband response. The optimum device parameters and corresponding conditions were estimated using finite element method based on electrode design. In calculating the results, the comprehensive characteristics of polymer modulator with 1.21 V half-wave voltage and 91 GHz bandwidth was demonstrated with electro-optic interaction length is 20 mm, electro-optic coefficient is 55 pm/V, and operation wavelength is 1.319 μm. These results agree with the 0.8 V half-wave voltage and 30 mm electro-optic interaction length reported in Science. In the five designs presented, a hybrid electrode structure combining CPW and microstrip lines were advanced. The characteristics of this structure are like that of microstrip lines with a single-arm electrode on one arm of the waveguide, but it solves the problem of microstrip to coaxial line transition and corona polarization.     

A review on quantum well structures in photonic devices for enhanced speed and span of the transmission network

Quantum well devices feature heterostructures of very thin epitaxial layers of group III-V and II-VI semiconductor materials. Quantum well devices are integrated monolithically with various optoelectronics devices to provide photonic integrated circuits. The representative structure could be realized with GaAs wells with GaAlAs barriers for wavelengths around 0.9 μm and InGaAsP are used for longer wavelengths. Together with quantum well, superlattice structure is another popular design for InGaAs Avalanche Photo Diode (APD). Quantum well structures find their applications in improved lasers, superlattice for photodiodes, modulators and switches. Consequences of quantum well theory are available today in terms of quantum wires and quantum dots. Upon the application of the normal electric field to quantum well structures, exciton pairs becomes more and more confined and the sharp exciton absorption peaks are observed. The effect is termed as “Quantum Confined Stark Effect”. The electro-absorption effect is approximately 50 times larger in multiple quantum well structures than it is in bulk semiconductors. Another electro-absorption effect known as “Franz Keldysh Effect” has been employed in monolithic waveguide detector. These effects lead to electro-absorption lasers (EAL) as well as electro-absorption laser modulators (EML).     

Novel integrated optic intensity modulator based on mode coupling

Abstract

The analysis, design, realization, and measurements of a novel intensity modulator are reported. The operating principle is based on mode coupling between a passive low-loss SiON waveguide and an electro-optic high-loss polymer waveguiding structure. Matching the waveguides is critical and results in severe demands for the technology. Extended simulations by the Coupled Mode Theory, the Leaky Wave Model, and Finite Difference Beam Propagation Method resulted in the design of several modulator structures. After realization, modulation could be demonstrated at 632 nm and at 1523 nm using lossy waveguiding modes and surface plasmon modes.     

On Quantum Phase Transition. I. Spinless Electrons Strongly Correlated with Ions

We study a large class F of models of the quantum statistical mechanics dealing with two types of particles. First the spinless electrons are quantum particles obeying to the Fermi statistics, they can hop. Secondly the ions which cannot move, are classical particles. The Falicov–Kimball (FK) model⁽¹⁾ is a well known model belonging to F, for which the existence of an antiferomagnetic phase transition was proven in the seminal paper of Kennedy and Lieb.⁽²⁾ This result was extended by Lebowitz and Macris.⁽³⁾ A new approach to this problem based on quantum selection of the ground states was proposed in ref. 4. In this paper we extend this approach to show that, under the strong insulating condition, any hamiltonian of the class F admits, at every temperature, an effective hamiltonian, which governs the behaviour of the ions interacting through forces mediated by the electrons. The effective hamiltonians are long range many body Ising hamiltonians, which can be computed by a cluster expansion expressed in term of the quantum fluctuations. Our main result is that we can apply the powerfull results of the classical statistical mechanics to our quantum models. In particular we can use the classical Pirogov–Sinai theory to establish a hierarchy of phase diagrams, we can also study of the behaviour of the quantum inter- faces,⁽²⁹⁾ and so on...     

Harmonic and Intermodulation Performance of Fabry-Perot Intensity Modulator

Abstract

In this article, a mathematical model for the transfer function of the Fabry-Perot intensity modulator is presented. The model, basically a cosine-series function, can be used to obtain closed-form expressions for the amplitudes of the harmonic and intermodulation products of the Fabry-Perot intensity modulator driven by a multi-frequency radio frequency voltage. The special case of a Fabry-Perot intensity modulator driven by an equal-amplitude two-frequency radio frequency voltage is considered in detail, and the results are compared, whenever possible, with previously published experimental and numerically obtained results.     

Full separability criterion for tripartite quantum systems

In this paper, an intuitive approach is employed to generalize the full separability criterion of tripartite quantum states of qubits to the higher-dimensional systems [Phys. Rev. A 72, 022333 (2005)]. A distinct characteristic of the present generalization is that less restrictive conditions are needed to characterize the properties of full separability. Furthermore, the formulation for pure states can be conveniently extended to the case of mixed states by utilizing the Kronecker product approximate technique. As applications, we give the analytic approximation of the criterion for weakly mixed tripartite quantum states and investigate the full separability of some weakly mixed states.     

Electro-absorption and refraction in Fabry-Perot quantum well modulators: a general discussion

Fabry-Perot resonators have long been advocated to improve the limited contrast ratio of multiple quantum well optical modulators used in photonic switches based on self electrooptic effect devices (SEEDs) and in other array based optical interconnection schemes. Using data on field dependent GaAs/AlGaAs quantum well absorption and refraction, we have modelled the reflectivity, modulation depth and contrast ratio of resonant modulators. Our results are generally valid for any quantum well modulator and demonstrate 23the important role played by electro-refraction even in regions of strong absorption. Resonators give large contrast ratios but there are trade-offs in the maximum reflectivity change achievable with Fabry-Perot resonators compared to simple modulators. The model gives the optimum number of quantum wells and reflectivity values required to make a resonator at any wavelength for a given quantum well structure. Understanding the limits of Fabry-Perot quantum well modulator performance is important for their application in symmetric self electrooptic effectiveness for photonic switching where modulation and detection properties are both used and for optical interconnection systems.     

Generation of quantum‐entangled twin photons by waveguide nonlinear‐optic devices

This paper reviews the quasi‐phase‐matched (QPM) waveguide nonlinear‐optic device technologies for generation of quantum‐entangled twin photons indispensable for quantum‐information techniques. After a brief introduction to the concept of entanglement, quantum theory analysis of twin‐photon generation (TPG) is outlined to clarify the properties of twin photons. Then, methods for entangled‐photon generation are discussed. Practical design and theoretical performances of LiNbO₃ waveguide QPM TPG devices, as well as the fabrication techniques, are described. Finally, experimental demonstrations of polarization‐entangled twin‐photon generation by waveguide Type‐I and Type‐II QPM TPG devices are presented.     

Structural and response functions at equilibrium in path-integral quantum simple fluids

A discussion on the physical meaning of the r-space structures that can be defined in path-integral quantum simple fluids far from exchange is presented by making the connection with their associated experimentally measurable properties in k-space (response functions). The role played in this issue by weak external fields acting on the fluid is examined by considering both the standard quantum treatment of neutron scattering and the path-integral functional analysis approach. For the sake of completeness, the same discussion is presented for the approximate Gaussian Feynman-Hibbs effective potential picture that can be derived from the path-integral, and also the structural interrelations between both formalisms are stated. To illustrate the points addressed in this paper results for liquid helium-4 at 4.2 K (SVP), obtained with the use of the Aziz-Slaman and the ab initio SAPT2 pair potentials, are reported.     

Electron-related optical responses in triple δ-doped quantum wells

A detailed theoretical study on the electron-related optical responses in triple δ-doped GaAs quantum wells in the presence of non-resonant, monochromatic intense laser field is presented. For this purpose, we first obtained the bound subband energy levels and their corresponding envelope wave functions of the structure for different central doping concentrations within the effective-mass approximation. Then, we calculate the effect of the non-resonant intense laser field on the optical properties of this structure using the compact-density-matrix approach via the iterative method. We found that the optical absorption coefficients and refractive index changes in the triple δ-doped GaAs quantum well can be modulated by changing the central doping concentration and the intensity of the non-resonant, monochromatic laser field. In addition, it is shown that a sufficiently intense laser field suppresses the multiple quantum well configuration towards a single potential well one and the optical response becomes practically independent of the δ-doping concentration.     

Numerical on-the-fly implementation of the action of the kinetic energy operator on a vibrational wave function: application to methanol

ABSTRACT

In quantum dynamics, physically well-adapted curvilinear coordinates are coordinates that lead to a Hamiltonian operator as separable as possible, in order to simplify the resolution of the corresponding time-independent or time-dependent Schrödinger equations. Various equivalent curvilinear expressions of the kinetic energy operator (KEO) are well known. They can be used in either an analytical or a numerical approach. The latter has the feature of allowing to straightforwardly compute the KEO in terms of sophisticated (yet easy to define) physically well-adapted curvilinear coordinates. Nevertheless, the number of terms to be computed on a full grid, scales as n²/2 (n being the number of degrees of freedom), so that, for systems with n?>?10, the memory storage of the KEO's becomes extremely demanding and therefore often unrealistic. We show here that it is possible, starting from the basic quantum expression of the KEO as a curvilinear Laplacian operator, to reduce the memory storage bottleneck by numerically computing the KEO on-the-fly, i.e. each time it is required, without computing the extrapotential term. This new approach opens the way to rigorous quantum studies of systems with many degrees of freedom. The comparison of torsional levels of methanol obtained by the present on-the-fly method with our previous results shows excellent agreement.     

Optimization of an Externally Modulated RF Photonic Link

Abstract

The noise figure and sub-octave spurious-free dynamic range of a carrier-suppressed analog RF photonic link are examined. Expressions for noise figure and dynamic range are derived as a function of the modulator bias angle. A closed form expression for the bias angle that optimizes link noise figure and spurious-free dynamic range is also derived. Experimental results are presented and shown to agree with calculations. These experimental results include some of the lowest noise figure and highest dynamic range results published to date for an RF photonic link incorporating a standard Mach-Zehnder modulator: 6 dB and 122 dB·Hz^2/3, respectively.     

Intensity-dependent nonlinear optical properties in a modulation-doped single quantum well

In the present work, the changes in the intersubband optical absorption coefficients and the refractive index in a modulation-doped quantum well have been investigated theoretically. Within the envelope function approach and the effective mass approximation, the electronic structure of the quantum well is calculated from the self-consistent numerical solution of the coupled Schrödinger-Poisson equations. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. The numerical results GaAs/Al_xGa_1−xAs are presented for typical modulation-doped quantum well system. The linear, third-order nonlinear and total absorption and refractive index changes depending on the doping concentration are investigated as a function of the incident optical intensity and structure parameters, such as quantum well width and stoichiometric ratio. The results show that the doping concentration, the structure parameters and the incident optical intensity have a great effect on the optical characteristics of these structures.     

Aharonov-Bohm oscillations in a ring with a quantum well

Aharonov-Bohm oscillations in a ring with a quantum well are investigated in the ballistic regime. It is shown that when trajectories with multiple circuits around the ring are taken into account, the maxima in the conductivity correspond to resonance levels of an isolated ring. The results obtained are in qualitative agreement with the experiment performed by Yakoby, Heiblum, Mahalu, and Shtrikman [Phys. Rev. Lett. 74, 4047 (1995)]: Although the scattering phase of an electron scattered by a quantum well changes by π on passage through each resonance, the Aharonov-Bohm curves for the centers of neighboring resonances are identical. In the simplified interpretation employed by Yakoby et al. the latter result looks like an identical scattering phase in neighboring resonances. Pis’ma Zh. ωksp. Teor. Fiz. 66, No. 9, 605–610 (10 November 1997)     

非线性包层多量子阱波导的TE波

给出用以分析非线性包层多量子阱波导ＴＥ模光学非线性与双稳性的理论公式与计算方法。指出用均方根等效折射率法解本征方程是有效的简化方法。用本文方法分析了模折射率对波导总功率的依赖关系,芯区功率与总功率之间的双稳性以及模场分布与模折射率的关系。讨论了波导参数对光学非线性、双稳性及模场的分布的影响。     

用二维时域有限差分法分析条形多层波导

为精确分析条形多层波导和多量子阱波导，本文提出了一种以二维时 限差分法为基础的全波数值方法，给出了场分布的色散特性的数值结果，并与矢量有限元法已有的结果作了比较；文中还研究了以二维时域有限差分法为基础的标量近似技术，提出了它的有效性和精度的局限性。     

Selection Rules for Multiple Quantum NMR Excitation in Solids: Derivation from Time-Reversal Symmetry and Comparison with Simulations and C NMR Experiments

New derivations of selection rules for excitation and detection of multiple quantum coherences in coupled spin-1/2 systems are presented. The selection rules apply to experiments in which the effective coupling Hamiltonian used for multiple quantum excitation is both time-reversal invariant and time-reversible by a phase shift of the radiofrequency pulse sequence that generates the effective couplings. The selection rules are shown to be consequences of time-reversal invariance and time-reversibility and otherwise independent of the specific form of the effective coupling Hamiltonian. Numerical simulations of multiple quantum NMR signal amplitudes and experimental multiple quantum excitation spectra are presented for the case of a multiply ¹³C-labeled helical polypeptide. The simulations and experiments confirm the selection rules and demonstrate their impact on multiple quantum ¹³C NMR spectra in this biochemically relevant case.     

Design of micro resonator quantum well intensity modulator

A compact rectangular resonator quantum well intensity modulator for operation in the wavelength band around 1?μm is described. The modulator is realized using InGaAs/GaAs modulation-doped quantum wells and operates on the principles of index change caused by blue shifts of the absorption edge. High efficiency 90° bends are used to form the resonator and to provide optimal coupling to the external waveguide. The benefits are to reduce loss, to relax the lithography requirements and to provide more flexible contact designs to the modulator. The characteristics of the modulator are analyzed using MATLAB and FDTD simulation tools with refractive index profiles based on measured absorption parameters. A model including parasitics is developed for HSPICE transient simulations and is run in the AGILENT ADS environment. The performance parameters are determined to be an extinction ratio of 10.4 dB, a bandwidth of 33?GHz, and a dc power less than 1 mW for device dimensions of 16?×?6?μm².     

Single-particle distribution function of a quantum dot system at finite temperature

In the present paper, we shall rigorously re-establish the result of the single-particle function of a quantum dot system at finite temperature. Unlike the proof given in our previous work (Phys. Rev. B 74 195414 (2006)), we take a different approach, which does not exploit the explicit expression of the Gibbs distribution function. Instead, we only assume that the statistical distribution function of the quantum dot system is thermodynamically stable. As a result, we are able to show clearly that the electronic structure in the quantum dot system is completely determined by its thermodynamic stability. Furthermore, the weaker requirements on the statistical distribution function also make it possible to apply the same method to the quantum dot systems in non-equilibrium states.