Wang He-Zhou

Broad Omnidirectional Reflection Band Forming using the Combination of Fibonacci Quasi-Periodic and Periodic One-Dimensional Photonic Crystals

By combining two Fibonacci quasi-periodic structures and a periodic structure to form a heterostructure, a broad omnidirectional reflection band is obtained. From the numerical results performed by the transfer matrix method, it is found that the reflection bands of the two Fibonacci quasi-periodic sub-structures and the periodic sub-structure can be compensated for each other. This method is expected to be useful in constructing a one-dimensional quasi-periodic photonic crystal structure with a broad omnidirectional reflection band.

Effect of invariant transformation in one-dimensional randomly-perturbed photonic crystal

An effect of invariant transformation in one-dimensional randomly perturbed photonic crystals is presented analytically and numerically. According to this effect, localization length can be investigated in different identical intervals, and the relations among these zones are governed by a simple expression. A concept of effective randomness is introduced as a result, which denotes the disorder of phase shifts actually. The divergent behaviour of localization length in the limit of low frequency can be interpreted directly through this effect. Since the effect is obtained without any approximation, it is expected to be useful in understanding the general intrinsic nature of one-dimensional randomly perturbed photonic crystals.

Fabrication of Compound Lattice by Holographic Lithography

Some compound lattices in photonic crystals with large complete photonic band gaps, such as the diamond structure and the two-dimensional hexagonal compound structure, is desired for fabrication. A novel method for fabrication of compound lattices by holographic lithography is reported. The key point is phase modulation by photoelectrical control and multiple-exposure techniques. This technique introduces many advances: for instance, using the same coherent beams and without changing the position of the sample during multiple-exposure.

Surface defect gap solitons in one-dimensional dual-frequency lattices

We study the surface defect gap solitons in an interface between a defect of one-dimensional dual-frequency lattices and the uniform media. Some unique properties are revealed that such lattices can broaden the region of semi-finite gap, and the semi-finite gap exists not only in the positive and zero defects but also in the negative defect; unlike in the regular lattices, the semi-finite gap exists in the positive and zero defects but does not exist in the negative defect. In particular, stable solitons exist almost in the whole semi-finite gap for the positive and zero defects. These properties are different from other lattices with defects. In addition, it is found that the existence of surface dual-frequency lattice solitons does not need a threshold power.

EXCITON RECOMBINATION DYNAMICS IN CdTe/CdZnTe QUANTUM WELLS

Quantum wells of CdTe/CdZnTe were grown by molecular beam epitaxy. The highest order of satellite peak of the sample is 5 from XRD spectra and excition emission linewidth is about 4.8 nm at 77 K. It was shown that our samples are very good. The recombination dynamics of exciton in high-quality CdZnTe/CdTe multiquantum wells were investigated by means of time-resolved photoluminescence(PL) spectra with different excitation power at 77 K and photoluminescence spectra with different temperature. When weaker excitation was used, radiative recombination decay time of the exciton was reduced as the excitation intensity was decreased; the results indicate that the dominant mechanism may be the quenching of exciton emission by impurities and defects. The linewidth of the exciton emission becomes broader with increasing temperature, the linewidth at low temperature is only due to the well thickness, and the broadening linewidth at high temperature is contributed by the interactions among the exciton and LO and TO phonons and ionized donor impurities. The PL intensities are reduced with increasing temperature, which is mainly due to the thermal dissociation of excitons, i.e., the electrons or holes jump into the barriers from the wells by thermal excitations. The exciton emission from n=2 heavy hole exciton at 77 K has been observed, and the n=2 heavy hole exciton luminescence decay time is shorter than that from n=1. The investigation indicates that there is the exciton energy relaxation from the n=2 to n=1 heavy hole exciton state by phonons.

Nonlinear Optical Studies of [(C4H9)4N][Ni(dmit)2] by Z-Scan Technique

Nonlinear optical properties of an organo-metallic compound [(C4H9)4N][Ni(dmit)2] (dmit=2-thioxo-1,3-dithiole-4,5-dithiolate), abbreviated as BuNi, dissolved in acetone, are investigated using the Z-scan technique with a 40ps pulse width at 1064nm and a 1 ns and 15ns pulse width at 1053nm. Strong saturable absorption is found when the sample solution is irradiated by 40ps and 1 ns laser pulses. When irradiated with a 15ns laser pulse, a stronger reverse saturable absorption is found. The nonlinear optical absorption coefficients are −1.06 × 10−11, −2.02 × 10−11 and 2.73 × 10−10 m/W, respectively. All the results suggest that BuNi may be a promising candidate in the application of laser pulse compression in the near-infrared waveband.

Existing conditions of full bandgaps and absolute negative refraction in metallic-dielectric photonic crystal

This paper has theoretically studied the characteristic frequencies of band structures in two-dimensional metallic-dielectric photonic crystals. It is demonstrated that a large filling fraction benefits the existence of absolute photonic band gap, while a smaller filling fraction benefits an absolute negative refraction band. In addition, it also finds that the relation between the cut-off frequency of E-polarized wave and the filling fraction exceeding 10% is content with a linear increasing function, whose coefficients are exponential to the normalized lattice constant. These investigations have significant implications for tuning the operational frequencies to desired applications and manufacturing photonic crystals.

Mechanism of low threshold lasing of dye solution embedding in nanoparticle fractal aggregates

We study the single-photon and two-photon induced emission properties of dye solution embedded with nanoparticle fractal aggregates. The spectral and temporal experimental results show that the lasing threshold of single-photon absorption of dye solution embedded with TiO2 nanoparticle fractal aggregates is significantly reduced compared with that of a neat dye solution; while that of two-photon absorption does not. These experimental results imply that this low threshold single-photon absorption lasing mainly results from the mechanism of random walking, instead of Anderson localization, unless something about two-photon induced emission is unknown.

Transient photoluminescence spectra of GaAs/AlGaAs quantum wells, quantum well wires, and quantum well boxes

The interaction of the electrons with the phonons in GaAs/AlGaAs quantum wells, quantum well wires, and quantum well boxes has been studied by meansuring their time resolved photoluminescence (PL) spectra. In the quantum well (QW) system, relaxation time of the electron energy of the narrower well is shorter than that of the wider well. It is attributed to the stronger interaction of the electrons with the interface mode phonons. In the QW wires and QW boxes, the shortening of the relaxation time is obvious. It is due to the stronger electron-phonon interaction caused by the reduction of screening effect. Our experiments also demonstrated the incident laser intensity dependence of the relaxation time.

Four-Photon Process Induced Upconversion Lasing

It is measured that upconversion fluorescent intensity of a new molecule is proportional to the quartic of excitation density. This demonstrates that it is a four-photon absorption process. Furthermore, at high-density ultrashort pulse laser pumping, an emission appears to satisfy four criteria of lasing, including the highly directional emission, the short emission duration, the spectral narrowing, and changing to linear output/input relationship above a threshold. The experimental results demonstrate that the four-photon-process-induced amplified spontaneous emission occurs.