Compact violet lasers by second-harmonic generation in KTP waveguides

Periodically segmented waveguides were fabricated in flux-grown KTP for quasi-phasematched second-harmonic generation (SHG) of a light beam with a wavelength of 425 nm. Diode-pumped violet laser sources are proposed on the basis of these waveguides. We shall show that a pulsed operation of the pump diode laser at a 940 MHz repetition rate enables the construction of sources with a very compact geometry, which are insensitive to temperature fluctuations. These sources may still be considered as quasi-continuous wave (cw) for applications in high-density optical recording.The most compact type of violet laser source has a size of 1 × 1 × 2 cm³. It contains only the diode pump laser, the KTP waveguide and a miniature lens to couple the pump beam to the waveguide. Time-averaged violet output powers up to 85 μW have been generated for many hours at room temperature without requiring an active temperature control. This output power may be sufficient for reading an optical disc.By optical feedback of a portion of the transmitted pump beam via an external grating it is possible to generate higher violet powers. In this way, the pump laser is forced to operate in a single spectral mode, the wavelength of which can be tuned to coincide with the phase-matching wavelength of the waveguide. This grating-controlled laser system is shown to generate a 425 nm beam with powers up to 0.5 mW. The total length of the device is about 7 cm.     

硅材料外加直流电场诱导的线性电光效应

在直流电场作用下,硅单晶的反演对称性被破坏,从而诱导产生二阶非线性光学效应.主要研究了硅材料在外加直流电场作用下诱导产生的场致线性电光效应.以沿(111)面切割的近本征硅材料为样品,采用平行板电容器结构,通过横向电光调制实验,观测到在不同直流偏压下,电光信号随调制电压而线性增加,且直流偏压越大,电光信号增加的斜率越大....     

Efficient second-harmonic generation in a double-quantum-well structure

Second-harmonic generation in a double-quantum-well nanostructure is analyzed. Optimization of the structure design from the viewpoint of attaining the highest intensity of the second-harmonic wave at the output is considered. Relationships between the quantum-well parameters in a structure satisfying the double-resonance condition are established. The dependence of the optical characteristics of the system on the quantum-well parameters is studied and the problem of optimization of the second-harmonic intensity is solved. It is demonstrated that the highest conversion efficiency is attained under the conditions where the second-harmonic generation coefficient is maximum.     

High efficiency quasi-phasematched frequency doubling waveguides inKTiOPO4 fabricated by electric field poling

Waveguides for quasi-phasematched frequency doubling were fabricated using an applied electric field to produce periodic poling and ion-exchange to produce channel waveguides. A blue output power of 12 mW was achieved with 146 mW of infrared light in a 3.6 mm-long waveguide     

Propagation in a crossed field periodic structure

A field theory of a crossed field periodic structure is given. Taking into account the effect of space charge, an integral equation method is employed to obtain a general dispersion equation for a structure supporting any space harmonic. The case of pure scalloping is treated in detail and the results are applied to forward and backward-wave amplifiers and backward-wave oscillators. Criteria for gain and oscillations are derived and corresponding expressions for the gain and start oscillation current are obtained. These results include the effect of all space harmonics.     

Resonant second harmonic generation by a semiconductor quantum wellin electric field

Both single- and double-resonance operating regimes are analyzed, and the latter is found to enable very high conversion efficiencies of pump to harmonic if accompanied by quasiphase matching by periodic alternation of the field direction. The self-consistent procedure of level energies and wave function calculation is also described     

垂直电偶极子在负折射媒质涂覆基底上产生的场

研究了垂直电偶极子在负折射媒质涂覆的理想导电基底上产生的电磁场,获得了场分量的解析表达式.分析和计算结果表明,电偶极子激发的电磁场可分解为直达波、反射波、侧面波和表面波.其中表面波在负折射媒质表面传播的波数介于空气层和介质层中传播波数之间,其幅度在距离向按波的几何扩散规律衰减,在垂直于介质层方向则以正弦或余弦规律变化.由于存在三种传播波数的波,它们共同作用形成的总场将发生复杂的干涉现象.     

Multisubband electron mobility in a parabolic quantum well structure under the influence of an applied electric field

We study the multisubband electron mobility in a barrier delta doped AlχGal-χAs parabolic quantum well structure under the influence of an applied electric field perpendicular to the interface plane. We consider the alloy fraction χ = 0.3 for barriers and vary x from 0.0 to 0.1 for the parabolic well. Electrons diffuse into the well and confine within the triangular like potentials near the interfaces due to Coulomb interaction with ionized donors. The parabolic structure potential, being opposite in nature, partly compensates the Coulomb potential. The external electric field further amends the potential structure leading to an asymmetric potential profile. Accordingly the energy levels, wave functions and occupation of subbands change. We calculate low temperature electron mobility as a function of the electric field and show that when two subbands are occupied, the mobility is mostly dominated by ionised impurity scattering mediated by intersubband effects. As the field increases transition from double subband to single subband occupancy occurs. A sudden enhancement in mobility is obtained due to curtailment of intersubband effects. Thereafter the mobility is governed by both impurity and alloy disorder scatterings. Our analysis of mobility as a function of the electric field for different structural parameters shows interesting results.     

Composite material behavior under applied electric field: material switches to a high-resistive state

Current-voltage characteristics of the electrically conductive silver-filled epoxy Ablefilm ECF-563 preform switches to a high-resistive state under low bias voltage. The observed phenomena is argued to be an intrinsic property of electrically conducting composite materials caused by strong localized centers that introduce space charge.     

Formation of a periodic magnetic field by a sequence of rings with different magnetic and electric properties

The electrodynamic analysis of the topography of magnetic fields generated in a spatially periodic structure is performed. The structure is a solenoid containing periodically spaced rings made from materials with different electric and magnetic properties. The system of equations relating the amplitudes of spatial harmonics of the magnetic field is derived and numerically analyzed. Dependences of the amplitudes of longitudinal and transverse components of the modulated magnetic field on the ring dimensions, the current frequency, and the magnetic and electric properties of the ring materials are investigated. The field amplitudes obtained during the complete electrodynamic analysis are compared with the amplitudes of a periodic magnetic field obtained in the impedance approximation.     

The energy structure of quantum dots induced in quantum wells by a nonuniform electric field

The study is focused on a theoretical treatment of electrons in quantum dots produced by a nonuniform electric field in a heterostructure with a single quantum well. A nonuniform electric field can be formed with the use of a mosaic electrode with a regular system of nanometer-sized apertures. A structure involving a one-aperture electrode deposited onto the surface is considered. The potential in the entire region between the electrode plates is calculated numerically. The analytical expression for the depth of the potential well is derived, and the potential profile in the vicinity of the bottom of the well is determined. The energy level structure in the vicinity of the bottom of the potential well is calculated. The geometric characteristics of the mosaic electrode most favorable for conservation of electron spin in the quantum dots of this type are determined. It is shown that in the quantum dots of this type the spin lifetime can be expected to lie within the microsecond region at a liquid-helium temperature.     

Full-wave analysis of discontinuities in planar waveguides by themethod of lines using a source approach

An accurate full-wave analysis of discontinuities in shielded microwave structures by the method of lines is presented for use in microwave integrated circuit design. The input currents are derived from a sinusoidal source representation and modeled by inhomogeneous boundary conditions. Interacting discontinuities can be treated without segmentation. Inhomogeneous boundary conditions are introduced to account for the excitation. Analysis shows that in discretizing the differential operators additional source terms which result in inhomogeneous differential equations are obtained. A deterministic equation for the current distribution is derived. For higher flexibility, nonequidistant discretization is used. The scattering parameters are presented for the microstrip step discontinuity and for a single-filter structure with interacting discontinuities, namely a coupled line filter. The filter characteristics are compared with measurements. Measurements verify the filter calculations     

Biexcitons generation in a polymer by a femtosecond electric pump pulse

By applying a femtosecond electric pump pulse to a polymer, biexcitons are obtained and the relation between its yield and the photoexciting process is also presented. The simulations are performed within the framework of an extended version of one-dimensional Su–Schrieffer–Heeger tight-binding model combined with a nonadiabatic evolution method. In the discussions, effects of both the photoexciting energy and intensity are considered. The main result is that the yield ratio of biexcitons and excitons increases rapidly with the photoexciting intensity, and there exists a critical photoexciting intensity, beyond which the yield efficiency of biexcitons is even higher than that of excitons. The result theoretically verifies that, by increasing the photoexciting intensity, we can obtain biexcitons efficiently, which is consistent with the experimental speculation.     

Studies of the mobility of charge carriers in low-dimensional systems in a transverse DC electric field

The mobility of charge carriers μ in a parabolic quantum well in an electric field E directed along the size-confinement axis is calculated. With consideration for scattering of charge carriers at a rough surface, the mobility μ is shown to decrease with increasing E. A physical interpretation of this effect is proposed.     

Effect of a transverse applied electric field on electron drift velocity in a GaAs quantum wire: A Monte Carlo simulation

A Monte Carlo simulation is run to study the electron transport in a thin undoped GaAs quantum wire under the influence of a transverse applied electric field. Phonon and surface-roughness scattering are included. Electron drift velocity is investigated as a function of roughness amplitude at a temperature of 77 or 300 K and a longitudinal electric field of 10⁴ or 10⁵ V/m. A transverse applied field is shown to provide a means of controlling drift velocity, affecting the scattering rate.     

Probing carrier behavior in organic semiconductor device by electric field induced optical second harmonic generation measurement

Electric field induced optical second harmonic generation (EFISHG) measurement is capable of probing carrier motions in organic devices. By measuring nonlinear polarization induced in active layer of organic devices, the carrier motion is visualized. After summarizing the conventional SHG measurement as a tool for material characterization, we show that the EFISHG is a way for probing carrier motions, and thus extends the potentiality of the conventional SHG measurement. We employ the EFISHG measurement to probe carrier behaviors in double-layer metal-insulator–metal diodes, e.g., Au/TIPS-pentacene/PI/ITO diodes, and demonstrate how the time-resolved SHG measurement probes carrier motion in the diodes, in terms of the I–V and C–V characteristics. Results give an insightful picture on the carrier injection, and the succeeding carrier transport.     

Probing a two-step channel formation process in injection-type pentacene field-effect transistors by time-resolved electric-field-induced optical second-harmonic generation measurement

By using time-resolved electric-field-induced optical second-harmonic generation measurement, we studied carrier motion in pentacene field-effect transistors (FETs) with poly-4-vinylphenol (PVP) and with polyimide (PI) gate-insulator whose active layers were depleted by pre-biasing. Upon removal of the pre-biasing, channel formation proceeded as a two-step process in FETs with PVP gate-insulator and a conduction channel was formed eventually. On the other hand, no conduction channel was formed in FETs with PI gate-insulator but two-step carrier propagation was observed in a similar way. Results showed that a local electric field induced on the gate-insulator surface gives a significant effect on the carrier injection and the following carrier transport.     

Electron heating by a strong longitudinal electric field in quantum wells

The electron heating by a strong longitudinal electric field and the energy losses due to the scattering of nonequilibrium electrons by polar optical phonons in rectangular GaAs/AlGaAs quantum wells are studied. A simple model is suggested to calculate the rate of energy losses due to the scattering of electrons by nonequilibrium optical phonons. Some of the experimental results on the heating of charge carriers in quantum wells are discussed, and it is shown that taking nonequilibrium optical phonons into account significantly improves the agreement between the theoretical and experimental data.