Application of Padé Approximation in Simulating Photonic Crystals

To save finite-difference time-domain（FDTD） computing time, several methods are proposed to convert the time domain FDTD output into frequency domain. The Pad6 approximation with Baker＇s algorithm and the program are introduced to simulate photonic crystal structures. For a simple pole system with frequency 160THz and quality factor of 5000, the intensity spectrum obtained by the Padé approximation from a 2^8-item sequence output is more exact than that obtained by fast Fourier transformation from a 2^20-item sequence output. The mode frequencies and quality factors are calculated at different wave vectors for the photonic crystal slab from a much shorter FDTD output than that required by the FFT method, and then the band diagrams are obatined. In addition, mode frequencies and Q-factors are calculated for photonic crystal microcavity.

Ultra-Wideband Electromagnetic Radiation from GaAs Photoconductive Switches

The experiment results of ultrawide band electromagnetic radiation with DC biased GaAs photoconductive semiconductor switch combining double ridge horn antenna triggered by high repeat frequency femto-second laser pulse are reported.The GaAs switches are insulated by solid multi-layer transparent dielectrics and the distance of two electrodes is 3mm.The electrode material of the switch is ohmic contact through alloy technics with definite proportion of Au/Ge/Ni.This switch and double ridge horn antenna are integrated and the receive antenna is connected with the test instrument.From receiving antenna,ultra fast electrical pulse of 200ps rise time and 500ps pulse width is obtained,the repetition rate of the pulse is about 82MHz and the frequency spectrum is in the range of 4.7MHz～14GHz.The radiation characteristic of the ultrafast electrical pulse is analyzed.

Study and Fabrication of a Au/n-ZnO/p-Si Structure UV-Enhanced Phototransistor

The fabrication and characterization of a Schottky-emitter heterojunction-collector UV-enhanced bipolar phototransistor （SHBT） are presented. The luminescence peak of the ZnO film is observed at 371nm in the PL spectrum. The sensitivity of the ultraviolet response from 200 to 400nm is enhanced noticeably, and the spectrum response at wavelengths longer than 400nm is also retained, The experiments show that the Au/n-ZnO/p-Si SHBT UV enhanced phototransistor enhances the sensitivity of the ultraviolet response noticeably. The UV response sensitivity at 370nm of the phototransistor is 5-10 times that of a ZnO/Si heterojunction UV enhanced photodiode.

时代气息的演绎——漫步者新音频主义E3100

当今的电脑音频,已经不再是那种喇叭简单发声了,面对音效震撼的DVD影片和令人身临其境的3D游戏,更多的用户要求享受“更高级”的声音。另一方面,除了音质的改善外,对音箱造型的要求也更高了。自漫步者推出彰显贵族气派的R331T后.全新的新音频主义E3100近日又隆重登场。这是漫步者最新的E系列产品之一,E系列是漫步者继经典R系列和高端S系列之后,融合了漫步者音频艺术(Thc Audio Artist)理念,改进了电子、电声、工艺等方面技术,并且在材料、造型设计下了不少功夫研发的新一代系列产品。至于这个E系列产品的第一炮产品有什么表现呢?让我们来看看吧。

Calculation of Valence Subband Structures for Strained Quantum-Wells by Plane Wave Expansion Method Within 6×6 Luttinger-Kohn Model

The valence subband energies and wave functions of a tensile strained quantum well are calculated by the plane wave expansion method within the 6×6 Luttinger Kohn model.The effect of the number and period of plane waves used for expansion on the stability of energy eigenvalues is examined.For practical calculation,it should choose the period large sufficiently to ensure the envelope functions vanish at the boundary and the number of plane waves large enough to ensure the energy eigenvalues keep unchanged within a prescribed range.

Nanoheterostructured photocatalysts for improving photocatalytic hydrogen production

Rapid industrialization has accordingly increased the demand for energy.This has resulted in the increasingly severe energy and environmental crises.Hydrogen production,based on the photocatalytic water splitting driven by sunlight,is able to directly convert solar energy into a usable or storable energy resource,which is considered to be an ideal alternative energy source to assist in solving the energy crisis and environmental pollution.Unfortunately,the hydrogen production efficiency of single phase photocatalysts is too low to meet the practical requirements.The construction of heterostructured photocatalyst systems,which are comprised of multiple components or multiple phases,is an efficient method to facilitate the separation of electron‐hole pairs to minimize the energy‐waste,provide more electrons,enhance their redox ability,and hence improve the photocatalytic activity.We summarize the recent progress in the rational design and fabrication of nanoheterostructured photocatalysts.The heterojunction photocatalytic hydrogen generation systems can be divided into type‐I,type‐II,pn‐junction and Z‐scheme junction,according to the differences in the transfer of the photogenerated electrons and holes.Finally,a summary and some of the challenges and prospects for the future development of heterojunction photocatalytic systems are discussed.

Wide-Band Polarization-Independent Semiconductor Optical Amplifier Gate with Tensile-Strained Quasi-Bulk InGaAs

A semiconductor optical amplifier gate based on tensile strained quasi bulk InGaAs is developed.At injection current of 80mA,a 3dB optical bandwidth of more than 85nm is achieved due to dominant band filling effect.Moreover,the most important is that very low polarization dependence of gain (<0 7dB),fiber to fiber lossless operation current (70～90mA) and a high extinction ratio (>50dB) are simultaneously obtained over this wide 3dB optical bandwidth (1520～1609nm) which nearly covers the spectral region of the whole C band (1525～1565nm) and the whole L band (1570～1610nm).The gating time is also improved by decreasing carrier lifetime.The wide band polarization insensitive SOA gate is promising for use in future dense wavelength division multiplexing (DWDM) communication systems.

Ultraslow Optical Solitons in a Coupled Double Quantum-Well Nanostructure

We demonstrate the formation of ultraslow dark semiconductor double quantum well （SDQW） structure based optical solitons with a four-level scheme in an asymmetric on intersubband transitions by using only a low-intensity pulsed laser radiation. With appropriate conditions we show numerically that the dark optical soliton can travel with a ultraslow group velocity Vg/c - -10^-3. Such a semiconductor system is much more practical than its atomic counterpart because of its flexible design and the controllable interference strength. This nonlinear optical process in the SDQW solid-state material may be used for the control technology of optical delay lines and optical buffers.

Tradeoffbetween Narrowing Optical Band Gap and Enhancing Electrical Conductivity of the Metal Nanoparticles-Modified Titanium Oxide Films

The n-type semiconducting titanium oxide thin films are well-known as electron transporting interlayer in photovoltaic cells. The favorable characteristics of interlayers in photovoltaics are high optical transmittance （T%）, wide band gap energy （Eg） and high electrical conductivity （σ）. Modifying titanium oxide films with metal nanoparticles would increase electrical conductivity but reduce optical band gap energy. We developed the sol-gel derived titanium suboxide （TiOx） films modified with silver （Ag） or gold （Au） or copper （Cu） nanoparticles （NPs）. This study explores a tradeoff between narrowing optical band gap and enhancing electrical conductivity of nanostructured TiOx films by controlling the Au- or Ag- or Cu-NPs loading concentrations （mol%） in titania. The Au- and Cu-NPs loading concentration of 4 mol% should meet a tradeoff which yields the higher T%, wider Eg and higher compared to those of pure TiOx films. In addition, since the pure Cu is not thermodynamically stable in ambience as compared to Au and Ag, the stability of as-obtained colloidal CuNPs is also examined. A careful examination of the time evolution of surface plasmon resonance （SPR） bands of CuNPs indicates that their stability is only up to 4 h.

An Efficient Approach to Narrow the Emission Band of Pyrene-Based Emitters

The construction of high color purity and high resolution organic light-emitting diodes(OLEDs)is facilitated by the development of highly-efficient organic luminescent materials with narrow-band emission.Herein,in order to address the problem of broad emission spectra of organic luminescent materials,an effective molecular design strategy is presented to reduce the full width at half maximum(FWHM)of emission by integrating the steric hindrance effect in the pyrene system.As the bulky group was introduced into the 2-position,compounds 5 not only show a relative high quantum yield(>0.31)in the solid state,but also can suppress the molecular rotation of triphenylamine(TPA)at the 3-position to narrow the FWHM in the solid state compared to that in solution.Compound 5c containing biphenyl units exhibits a maximum emission peak at 484 nm with a quantum yield of 0.38 and FWHM value of 49 nm in the solid state.

Polymer Arrayed Waveguide Grating with Box-Like Spectral Response

An efficient technique is used to flatten the spectral response of an arrayed waveguide grating （AWG） multiplexer. By subtracting an increment from the core width of odd arrayed waveguides and by adding the same increment to that of even arrayed waveguides,a box-like spectral response can be obtained. A 17 × 17 polymer AWG multiplexer with box- like spectral response has been made using FPE polymer materials. Measured result for the AWG shows that the box-like spectral response has a 3dB bandwidth of 0. 476nm, the crosstalk is about or less than - 21dB for every output channel,and the insertion loss is 13-15dB.

Composition-dependent ultra-high photoconductivity in ternary CdSxSe1-x nanobelts as measured by optical pump-terahertz probe spectroscopy

We employ optical pump-terahertz probe spectroscopy to investigate the composition-dependent photoconductivity in ternary CdSxSel_x nanobelts. The observed carrier dynamics of CdS nanobelts display much shorter lifetime than those of ternary CdSKSel_K nanobelts. This indicates the implementation of CdS nanobelts as ultrafast switching devices with a switching speed potentially up to 46.7 GHz. Surprisingly, ternary CdS,-Sel_x nanobelts are found to exhibit much higher photoconductivity than binary CdS and CdSe. This is attributed to the higher photocarrier densities in ternary compounds. In addition, the presence of Se in samples resulted in prominent CdSe-like transverse optical （TO） phonon modes due to electron-phonon interactions. The strength of this mode shows a large drop upon photoexcitation but recovers gradually with time. These results demonstrated that growth of ternary nanostructures can be utilized to alleviate the high surface defect density in nanostructures and improve their photoconductivity.

Linear and Nonlinear Optical Properties of Spherical Quantum Dots:Effects of Hydrogenic Impurity and Conduction Band Non-Parabolicity

Simultaneous effects of an on-center hydrogenic impurity and band edge non-parabolicity on intersubband optical absorption coefficients and refractive index changes of a typical GaAs/AlxGa1-xAs spherical quantum dot are theoretically investigated, using the Luttinger-Kohn effective mass equation. So, electronic structure and optical properties of the system are studied by means of the matrix diagonalization technique and compact density matrix approach, respectively. Finally, effects of an impurity, band edge non-parabolicity, incident light intensity and the dot size on the linear, the third-order nonlinear and the total optical absorption coemcients and refractive index changes are investigated. Our results indicate that, the magnitudes of these optical quantities increase and their peaks shift to higher energies as the influences of the impurity and the band edge non-parabolicity are considered. Moreover, incident light intensity and the dot size have considerable effects on the optical absorption coefficients and refractive index changes.

Simultaneous Effects of External Electric Field and Conduction Band Nonparabolicity on Optical Properties of a GaAs Quantum Dot Embedded at the Center of a GaAlAs Nano-Wire

An investigation of the optical properties of a GaAs spherical quantum dot which is located at the center of a Ga1-xAlx As cylindrical nano-wire has been performed in the presence of an external electric field. The band nonparabolieity effect is also considered using the energy dependent effective mass approximation. The energy eigenvalues and corresponding wave functions are calculated by finite difference approximation and the reliability of calculated wave functions is checked by computing orthogonality. Using computed energy eigenvalues and wave functions, the linear, third-order nonlinear and total optical absorption coefficients and refractive index changes are examined in detail. It is found that （i） Presence of electric field causes both blue and red shifts in absorption spectrum; （ii） The absorption coefficients shift toward lower energies by taking into account the conduction band nonparabolicity; （iii） For large values of electric field the effect of conduction band nonparabolieity is less dominant and parabolic band is estimated correctly; （iv） In the presence of electric field and conduction band nonparabolicity the nonlinear term of absorption coefficient rapidly increases by increasing incident optical intensity. In other words, the saturation in optical spectrum occurs at lower incident optical intensities.

Towards full-spectrum photocatalysis： Achieving a Z-scheme between Ag2S and TiO2 by engineering energy band alignment with interfacial Ag

A Z-scheme is a promising approach to achieve broad-spectrum photocatalysis. Integration of TiO2 with another semiconductor with a band gap of -1.0 eV would be ideal to harvest both ultraviolet and visible-near infrared light for photocatalysis; however, most narrow-bandgap semiconductors have straddling band structure alignments with TiO2, constituting an obstacle to forming the Z-scheme for photocatalytic hydrogen production. In this communication, we demonstrate Ag2S as a model system where the energy band upshift of the narrow-bandgap semiconductor that shares an interface with a metal can overcome this limitation. To fabricate the design, we developed a unique approach to synthesize Ag2S-Ag-TiO2 hybrid structures. The obtained ternary hybrid structures exhibited dramatically enhanced performance in photocatalytic hydrogen pro- duction under full-spectrum light illumination. The activities were significantly higher than the sum of those of Ag2S-Ag-TiO2 structures under λ〈 400 nm and λ 〉 400 nm irradiation as well as those of their counterparts under any light illumination conditions.

Theoretical study of all-optical NRZ to RZ format conversion with tunable pulse width based on XPM in a silicon waveguide

An all-optical format conversion from non-return-to-zero （NRZ） to return-to-zero （RZ） is presented based on cross-phase modulation （XPM） in a silicon waveguide with a detuned optical bandpass filter （OBPF）. The simulation results show that the tunable bandwidth of the OBPF leads.to RZ signals with tunable pulse width. The conversion efficiency （CE） and the pattern effect of the RZ signal are attributed to the parameters of the pump pulse and the OBPE The converted RZ signal exhibits lower timing jitter than the NRZ signal.

Strain-induced direct-indirect bandgap transition and phonon modulation in monolayer WS2

In situ strain photoluminescence （PL） and Raman spectroscopy have been employed to exploit the evolutions of the electronic band structure and lattice vibrational responses of chemical vapor deposition （CVD）-grown monolayer tungsten disulphide （WS2） under uniaxial tensile strain. Observable broadening and appearance of an extra small feature at the longer-wavelength side shoulder of the PL peak occur under 2.5% strain, which could indicate the direct-indirect bandgap transition and is further confirmed by our density-functional-theory calculations. As the strain increases further, the spectral weight of the indirect transition gradually increases. Over the entire strain range, with the increase of the strain, the light emissions corresponding to each optical transition, such as the direct bandgap transition （K-K） and indirect bandgap transition （F-K, ≥2.5%）, exhibit a monotonous linear redshift. In addition, the binding energy of the indirect transition is found to be larger than that of the direct transition, and the slight lowering of the trion dissociation energy with increasing strain is observed. The strain was used to modulate not only the electronic band structure but also the lattice vibrations. The softening and splitting of the in-plane E＇ mode is observed under uniaxial tensile strain, and polarization-dependent Raman spectroscopy confirms the observed zigzag-oriented edge of WS2 grown by CVD in previous studies. These findings enrich our understanding of the strained states of monolayer transition-metal dichalcogenide （TMD） materials and lay a foundation for developing applications exploiting their strain-dependent optical properties, including the strain detection and light-emission modulation of such emerging two-dimensional TMDs.

Ag4Hg（SeO3）2（SeO4）: a novel SHG material created in mixed valent selenium oxides by in situ synthesis

Explorations of new second harmonic generation materials in Ag^+-Hg^2+/Bi^3+-selenites systems afforded three new silver selenium oxides, namely, Ag4 Hg(SeO3)2(SeO4)(1), Ag2Bi2(SeO3)3(SeO4)(2) and Ag5 Bi(SeO3)4(3). They exhibit flexible crystal chemistry. Compounds 1 and 2 are mixed valence selenium oxides containing Se(IV) and Se(VI) cations simultaneously. Compounds 1 and 3 exhibit a 3 D open framework with 4-, 6-and 8-member polyhedral ring tunnels along a, b and c axes. Compound 1 crystallized in a polar space group and could display a subtle frequency doubling efficiency about 35% of the commercial KH2PO4(KDP). UV-vis-NIR spectra reveal that compounds 1–3 are wide-band semiconductors with the optical bandgaps of 3.11, 3.65, 3.58 e V respectively. Theoretical calculations disclose that compounds2 and 3 are indirect band gap structures and their bandgaps are determined by Ag, Bi, Se and O atoms together.

Controllable tune of the cutoff frequencies in a photonic crystal waveguide with hexagonal lattice

In this paper,configuration parameters of the waveguide are altered independently or simultaneously to control the cutoff frequencies of the guided band.The independent control range of the upper and lower cutoff frequencies is 55.0% and 63.9% of the photonic band gap(PBG),respectively.The regulating range of the simultaneous tuning can be as large as 28.6% in terms of the PBG,or 240% in terms of the bandwidth.This tuning cutoff frequency method provides an efficient way to tailor the guided band and further tune the optical properties of PhCWs.