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This work focuses on the formation mechanism of a superconducting Fe1+xTe/Bi2Te3 bilayer fabricated through chemical reactions between a Bi2Te3 flux and an annealed Fe layer grown on a ZnSe(111)B buffer via the molecular beam epitaxy technique. The studies using energy-dispersive X-ray spectrometry and high-resolution transmission electron microscopy performed on a number of samples fabricated with different desired schemes provide evidence that several interfacial chemical reactions taking place at the Fe/ZnSe interface through the edges of voids of the annealed Fe layer contributed to the formation of the superconducting bilayer. We have also revealed that the bonding strength of the involved Fe compounds at the bottom interface of the annealed Fe layer seems to play an essential role.

Nanostructures of II–VI semiconductor materials could potentially offer novel and superior physical (in particular, optoelectronic) properties with respect to their bulk counterparts. Herein, we present our most recent research on several II–VI and related nanostructures grown by molecular beam epitaxy (MBE) technique. These include a ZnSe nanograting. This nanograting structure was realized at the surface of Fe/ZnSe bilayers grown on GaAs(001) substrates by thermal annealing. A model based on an Ewald construction is presented to explain its unusual reflection high-energy electron diffraction (RHEED) patterns. The formation mechanism of this one-dimensional (1D) nanostructure is possibly related to surface energy minimization, together with an Fe–Se exchange interaction and Fe-induced decomposition of several top ZnSe atomic layers during thermal annealing. Another nanostructure investigated was the ZnS Schottky barrier embedded with Fe quantum dots (QDs). Here, a Au/ZnS/Fe-QDs/ZnS/n +-GaAs(100) Schottky barrier structure containing five layers of spherical Fe quantum dots with a diameter of ~3 nm was fabricated. Its I–V characteristic measured from 5 K to 295 K displays negative differential resistance (NDR) for temperature ≤50 K. Staircase-like I–V characteristics were also observed at low temperature in some devices fabricated from this structure. Possible mechanisms that can account for the observed unusual I-V characteristic in this structure are presented. Finally, laterally grown Fe nanowires (NWs) on a ZnS surface were prepared. Under high growth/annealing temperature, two types of Fe NWs with specific orientations can be grown on the ZnS(100) surface. We propose a mean-field model that the torque exerted by type A Fe NWs could effectively turn the two components of type B Fe NWs slightly toward the ZnS [110] direction, leading to the observed misalignment of type B Fe NWs.

A Au/ZnS/Fe-quantum dots (QDs)/ZnS/n+-GaAs(1 0 0) Schottky-barrier structure containing five layers of spherical Fe QDs with diameter around 3 nm was fabricated by the molecular beam epitaxy technique. Its current–voltage (I–V) characteristics measured from 5 to 295 K display negative differential resistance (NDR) for temperature ⩽50 K, which is caused by the presence of Fe QDs. The highest peak-to-valley current ratio obtained at 5 K is as high as 15:1. Staircase-like I–V characteristic was also observed at low temperature in some devices fabricated from this structure. Possible mechanisms that can account for the observed unusual I–V characteristics in this structure were discussed.

Two types of self-assembled Fe nanowires (NWs) were grown by molecular beam epitaxy on ZnS(1 0 0) surface under high growth/annealing temperature. The type-A Fe NWs orient along the ZnS [1 1 0] direction with irregular shape, while the type-B Fe NWs orient along either the ZnS [1 8 0] or the [8 1 0] direction with seemingly straight shape. Detailed high-resolution transmission electron microscopy and selected area electron diffraction studies reveal that both types were single-crystalline with their elongated axis along the 〈1 0 0〉 direction family possibly due to the fact that the easy axis of Fe is along this direction. We have proposed a mean-field model to explain the slight misalignment of the type-B Fe NWs. The I–V characteristic of a single type-B Fe NW measured at room temperature displays a straight line nature corresponding to a resistivity of about 2.3×10−7 Ω m.

A hyperbranched poly(aroylarylene) containing triphenylamine (TPA) moiety (hb-P1) is synthesized by metal-free polycyclotrimerization of N,N-bis(4-ethynylcarbonylphenyl)aniline (1) in a high yield (86%) with a perfect 1,3,5-regioregularity. The polymer is soluble in common organic solvents such as THF and chloroform and is thermally stable, losing 5% of its weight when heated to a temperature as high as ∼450 °C. The polymer is fluorescent and solvatochromic, whose emission color changes from ∼455 nm (blue) in toluene to 549 nm (yellow) in DMF. The polymer exhibits a large two-photon absorption cross-section due to the intramolecular charge transfer between its electron-donating TPA and electron-withdrawing 1,3,5-tricarbonylbenzene groups.

Molecular beam epitaxy (MBE) via the vapor-liquid-solid (VLS) reaction was used to grow ZnSe nanowires (NWs) on (111), (100), and (110) oriented GaAs substrates. Through detailed transmission electron microscopy (TEM) studies, it was found that 〈111〉 orientation is the growth direction for NWs with size ≥30 nm, while NWs with size around 10 nm prefer to grow along the 〈110〉 direction, with a small portion along the 〈112〉 direction. These observations have led to the realization of vertical ZnSe NWs with size around 10 nm grown on a GaAs (110) substrate. An ordered ZnSe NW array fabricated on a GaAs (111) substrate with a novel prepatterning method associated with plasma etching shows a high degree of ordering and a good size uniformity of the as-grown NWs. The diameter of the NWs in the array is around 80 nm and most of them are found to orient vertically, but some tilt to one of the six possible directions of the 〈111〉 family.

The 1/f noise and shot noise studies were carried out on molecular beam epitaxy (MBE)-grown ZnS0.85Se0.15 and Zn0.43Mg0.57S ultraviolet (UV) detectors packaged in surface mount lead frames. Three ZnS0.85Se0.15 detectors with different thicknesses of the active layer and the top electrode pad were used. The highest onset of reverse bias for the appearance of 1/f noise is −27.5 V, and the highest dark resistances at zero bias is R0=3.7×1013 Θ. The observed difference in their noise performance implies that the increase of the thicknesses of both the active layer and the top electrode pad can significantly lower the noise levels and in turn lead to higher detectivity. The best detectivity achieved is 8.75×1013 cm Hz1/2 W−1 in a ZnS0.85Se0.15 detector with an active layer of 5000 A and a top electrode pad of Cr (50 A)/Au (8000 A). The typical detectivity achieved on Zn0.43Mg0.57S devices that have an active layer thickness just exceeds the critical thickness of phase transition is 1.54×1012 cm Hz1/2 W−1.

We report on observation of simultaneous strong enhancements of second-harmonic generation (SHG) and third-harmonic generation (THG) in visible spectra region in a one-dimensional ZnSe-ZnMgS semiconductor photonic crystal. These enhancements come from phase matching and a high density of modes near the photonic band edge. Our result shows that the THG is due to direct /spl chi//sup (3)/ process instead of the cascaded two-step /spl chi//sup (2)/ process. Measured forward SH conversion efficiency close to 1% for only a few microns thick film demonstrates potential device applications. Theoretical calculations are in good agreement with the experimental measurements.

Au:TiO 2 multilayer composite films were prepared by the method of alternating multitarget magnetron sputtering technique. Samples were annealed afterwards. The surface plasmon resonances of the annealed films are around 700nm. TEM pictures showed the remained layered structure with grown gold particles between the layers. The values of χ(3) measured by degenerated four wave mixing (DFWM) are in the order of 10-7 esu which are one order larger than the previous work. The figure of merit (FOM), χ(3)/α, is in the order of 10-12 esu cm. The great improvement of χ(3) is assumed to be the result from the enhancement of field localization caused by the metallic/dielectric host multilayer structure.

Treatment of 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (2,4,6-(4-BrC6H4)3-1,3,5-C3N3)with HC⋮CSiMe3in the presence of Pd(PPh3)4/CuI and NEt3 produces 2,4,6-[4-(Me3SiC⋮C)C6H4]3-1,3,5-C3N3, which reacts with Bu4NF to yield 2,4,6-[4-(HC⋮C)C6H4]3-1,3,5-C3N3. Reaction of 2,4,6-[4-(HC⋮C)C6H4]3-1,3,5-C3N3 with AuCl(PPh3) in the presence of NEt3 produces the gold acetylide complex 2,4,6-[4-((PPh3)AuC⋮C)C6H4]3-1,3,5-C3N3. Treatment of 2,4,6-[4-(HC⋮C)C6H4]3-1,3,5-C3N3 with cis-RuCl2(dppe)2/NaPF6 followed by NEt3 gives 2,4,6-[4-(Cl(dppe)2RuC⋮C)C6H4]3-1,3,5-C3N3, which reacts with p-HC⋮CC6H4R (R = OCH3, NO2) to give 2,4,6-[4-((4-RC6H4C⋮C)(dppe)2RuC⋮C)C6H4]3-1,3,5-C3N3. The electrochemical and photophysical properties of the new complexes have been investigated.

This study focused on the investigation of the thermal stability of Fe on ZnSe and ZnS matrix using secondary ion mass spectroscopy (SIMS). Sandwiched three layer source structures of ZnSe/Fe/ZnSe and ZnS/Fe/ZnS were grown on GaAs and GaP substrates, respectively, by the molecular beam epitaxy technique. The bottom II–VI layers and the Fe- sandwiched layers in these structures are single crystalline while the top II–VI layers are polycrystalline. Thermal annealing was conducted in a range covering from 320 to 550 °C. The SIMS depth profiles of the as-grown and annealed structures reveal that (1 0 0) oriented single crystalline Fe/ZnSe interface is thermally stable at temperature as high as 450 °C while its polycrystalline counterpart suffers from fast diffusion even at the growth temperature. In contrast, (1 0 0) oriented polycrystalline Fe/ZnS interface is quite stable at least up to 200 °C. For both ZnSe/Fe/ZnSe and ZnS/Fe/ZnS systems, (1 1 1) oriented structures were found to have lower thermal stability than (1 0 0) oriented ones. These results provide important findings towards the optimization of Fe-based tunneling magneto-resistance structures using a II–VI semiconductor barrier.

Treatment of RuHCl(CO)(PPh3)3 with 1,3,5-triethynylbenzene produces the trimetallic complex 1,3,5-[Cl(CO)(PPh3)2RuCHCH]3C6H3, which reacts with PMe3 to give 1,3,5-[Cl(CO)(PMe3)3RuCHCH]3C6H3. 1,3,5-[Cl(CO)(PPh3)2RuCHCH]3C6H3 also reacts with pyridine (Py) and 4-phenylpyridine (PhPy) to give 1,3,5-[Cl(CO)(L)(PPh3)2RuCHCH]3C6H3 (L = Py, PhPy). Reactions of OsHCl(CO)(PPh3)3 with ArC⋮CH (Ar = Ph, p-tolyl) produce the six-coordinated insertion products OsCl(CHCHAr)(CO)(PPh3)3. Reaction of OsHCl(CO)(PPh3)3 with 1,3,5-triethynylbenzene produces the bimetallic complex 1-HC⋮C-3,5-[Cl(CO)(PPh3)3OsCHCH]2C6H3, which reacts with PMe3 to give 1-HC⋮C-3,5-[OsCl(CO)(PPh3)2(PMe3)OsCHCH]2C6H3. The electrochemical and second-order NLO properties of some of the new complexes have been investigated.

Quantum size effect in antimony thin films has been investigated. The Sb(1 1 1) thin films were grown on GaAs (0 0 1) substrates by molecular-beam epitaxy (MBE) at growth temperatures between 25 and 150 °C and with Sb film thicknesses between 5 and 200 nm. The film structures were analyzed by both reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM). The energy gap versus thickness for Sb films was tested. A semimetal-to-semiconductor transition in Sb films grown on GaAs substrates has been demonstrated. Narrow energy gap materials with tailorable gaps are of great interest as novel infrared detector materials. A design for Sb-GaAs quantum well infrared detector is presented.

A noise study on a set of Zns 0.58 Se 0.42 UV photodiodes with 1 μm thick Al bonding pads selected from a 2 wafer was carried out. Based on their 1/f noise behaviors, they were divided into three groups. The noise spectral density, impulse response time, bandwidth, responsivity, dark current at reverse bias up to 12 V and zero bias resistance were determined for three typical detectors, i.e. one from each of the three groups. The uneven performance of these detectors is mostly likely due to the different damage level induced in the wire-bonding process. With thicker Al bonding pads of 1.5 μm, a detectivity as high as 7.1 x 10 14 cm Hz 1/2 W -1 has been achieved.

Molecular beam epitaxial growth of Zn 1 x Mg x S alloy thin films on GaP (100) substrates is reported. In situ reflection high energy electron diffraction (RHEED) studies show that the alloys can be grown with stable zinc-blende structure up to x around 30%. For x > 30%, a phase transition will occur at a critical thickness which is sensitively dependent on the composition x. Several Schottky barrier photodetectors using Zn 1 x Mg x S layer, with thickness less than the critical thickness, as active layer were fabricated. High ultra-violet responsivity and excellent visible rejection are achieved. The response curve of the Zn 0.43 Mg 0.57 S device offers a long wavelength cut-off at 295 nm and closely matches the erythemal action spectrum that describes human skin sensitivity to UV radiation.

We demonstrated for the first time an autocorrelator based on two-photon absorption of ZnSSe photodetectors which is capable of measuring femtosecond laser pulses from 800 to 400 nm. This autocorrelator has a very broad bandwidth, high sensitivity and is capable of measuring laser pulses with high repetition rate. These results could lead to cost effective ultrafast autocorrelators with a single detector head to cover the whole of visible to near UV spectral region. Using a ZnMgS photodetector with a larger bandgap enables the autocorrelator to be more sensitive at 400 nm and extends the responsivity further into the ultraviolet wavelength of 350 nm.

Zn1−xMgxS alloy thin films covering Mg composition, x, from 0 to 1 were grown on GaP (100) substrates using molecular beamepitaxy. As observed by reflection high energy electron diffraction patterns, the alloys can be grown with stable zinc-blende structures up to x around 30%. For x>30%, a structural transition will occur at a critical thickness which is sensitively dependent on the composition x. Several Schottky barrier photodetectors using a Zn1−xMgxS layer with a thickness less than the critical thickness as the active layer were fabricated. The shortest cutoff wavelength achieved so far is 270 nm. Temporal response of the device shows that the photocurrent decays with a time constant as fast as 13 ns for a mesa area, of 1.5 mm2.

The time-resolved differential absorption of the ZnSe/ZnSTe superlattice is studied using femtosecond pump-probe measurements. Transient spectral hole burning due to the initial nonthermal carrier distribution is observed at zero time delay and the carriers are thermalized within 0.5 ps. The high-energy tail of the differential absorption spectra was used to deduce the effective temperature of the thermalized carriers. Rapid hot-carrier cooling from a temperature of 763 to 450 K within the first 4 ps is observed, with carrier cooling slowing down hence. This initial fast hot-carrier cooling is consistent with the strong carrier–phonon interaction in large gap II–VI semiconductors.

Continuous epitaxial films of cuprous oxide (Cu2O) have been formed by the thermal oxidation of 1.5-μm-thick Cu metal films deposited on MgO(110) substrates. These films melted at 1118 °C in air, in agreement with equilibrium phase diagrams. Upon cooling from the liquid, a highly crystalline, epitaxial, 2.5-μm-thick Cu2O film was formed. X-ray diffraction spectroscopy revealed that the Cu2O film crystal structure was orthorhombically distorted from the bulk cubic crystal structure. High-resolution transmission electron microscopy showed that the film is coherent, and energy dispersive x-ray spectroscopy showed that interdiffusion is limited to the interface. These results suggest that a new epitaxially stabilized phase of Cu2O has been formed.

This work focused on the studies of the high-temperature dependence of the responsivity of ZnS, ZnSTe and ZnSSe photodiodes. It was found that in general the responsivity at higher temperatures will shift to longer wavelengths because of band gap narrowing. A remarkable observation is that the near-band-edge responsivities of these diodes increase at higher temperature. We believe that this observation is attributed to the change of the density of state distribution due to lattice expansion at high temperature and a simplified model is used to illustrate this hypothesis.

The aim of this study is to reveal the underlying cause of the gradual turn-on characteristic of low Te containing ZnSTe Schottky barrier photodiodes. The results of photoresponse studies on ZnS, ZnSSe and ZnSTe diodes indicate that the Te isoelectronic trapping effect is responsible for the gradual turn-on characteristic of low Te containing ZnSTe Schottky barrier photodiodes. The results also reveal that the ZnSSe diode, having the advantage of being free of isoelectronic centers, is a more suitable choice for applications requiring high visible rejection power. It is demonstrated that highly UV sensitive responsivity with an abrupt long wavelength cutoff tailored to lie between 340–400 nm can be achieved in the ZnSSe diode system.

A novel approach to fabricating a highly aligned one-dimensional (1D) surface nanostructure on the surface of Fe/ZnSe bilayers grown on GaAs (0 0 1) substrates is reported. The resulting 1D nano-grooves were found to align along the [1 1 0] direction with a spacing of about 7–10 nm and a height of about 1–2 nm, resulting in a circular arc reflection high-energy electron diffraction (RHEED) pattern when the e-beam is aligned with them. We present a model based on Ewald construction to explain the unusual RHEED observation. The formation mechanism of the 1D nano-grooves is possibly related to an Fe–Se exchange interaction and Fe-induced decomposition of several top ZnSe atomic layers with regional selection during high-temperature annealing.

ZnSe self-organized dot structures on ZnS thin films were fabricated by the molecular beam epitaxy technique. Atomic force microscopy (AFM) images of exposed dots taken within the same day of growth reveal that the dot density increases with increasing ZnSe coverage. AFM images taken at later times (up to six months) shows ripening effect. Room temperature photoluminescence studies on buried-dot structures show promising optical characteristics that are sensitive to the growth temperature used for dot formation.

A multilayer magnetic quantum dot sample containing 5 layers of Fe quantum dots (QDs) embedded in six layers of ZnS spacer was grown by molecular beam epitaxy (MBE). High-resolution transmission electron microscopy (HRTEM) observations reveal that the Fe QDs are single crystalline with spherical shape of diameters around 3 to 4 nm and area density of 1.5times1012 cm-2. Its zero-field cooled (ZFC) and field cooled (FC) curves measured at low field (100 Oe) show the magnetic relaxation effect with a blocking temperature around 26 K. The hysteresis loop measured at 5 K shows a coercivity of 83 Oe, confirming the slow relaxation process and coercivity enhancement attributed to the nanoparticle nature of the sample

Dy@C 82 has been prepared by an efficient solvent extraction procedure and characterized by negative-ion desorption chemical ionization mass spectrometry and UV-VIS–near-IR absorption. Degenerate four-wave-mixing measurements on the CS 2 solution of Dy@C 82 were performed using 70 ps laser pulses at 0.532 μm. The magnitude of the second hyperpolarizability γ 1111 of the Dy@C 82 molecule was (3.0±0.5)×10 −30 esu at 0.532 μm. The resonant enhancement and the electron transfer from the trapped dysprosium to the C 82 cage are suggested to bring about its larger non-linear optical response relative to the empty fullerene. The experimental results are in good agreement with our calculations based on an extended Su–Schrieffer–Heeger model and the sum-over-state method.

We report the results of our degenerate four-wave-mixing measurements (DFWM) on the fullerene series from C60 to C96 dissolved in CS2. The DFWM measurements were carried out using 70 ps laser pulses at the wavelength of 0.532 μm under the optimized experimental conditions. The second hyperpolarizabilities γ1111 of all these fullerenes were determined to be on the order of 10-31−10-30 esu, and overall they increase smoothly with the carbon cage size except for C70 and C78, which show anomalously larger γ1111 values than those of their neighboring cage sizes we investigated. The number of π-conjugated electrons, geometrical structure, and resonance enhancement are discussed as possible factors responsible for the observed third-order optical nonlinearities of the fullerenes.

We have demonstrated blue-light emission from ZnSTe-based electroluminescence (EL) devices. Photoelectroluminescence technique is used to study field-quenching effects on Te isoelectronic centers in undoped ZnSTe alloy. Heavy doping by Al in ZnSTe alloy is found to help reduce field quenching action and gives rise to much higher EL emission in ZnSTe:Al-based Schottky-barriery type structures. I – V characteristics and emission spectra reveal that a tunneling dominated mechanism is responsible for generating the carriers for impact excitation of the localized states associated with Te isoelectronic centers in these structures.

We have prepared crystalline beta barium borate, β-BaB2O4 (the so-called BBO), thin films on several kinds of substrates using pulsed laser deposition (PLD) and opposed-targets magnetron sputtering (OTMS) techniques. From the measurement of X-ray diffraction, we have found that the crystallinity of the BBO film grown by both techniques is highly dependent on the deposition temperature and the choice of the substrates. There is a different phase transition temperature for the BBO films grown on different substrates and by different deposition techniques. The best films obtained were the ones grown on a sapphire (0 0 0 1) substrate at a deposition temperature of 800°C by PLD and 700°C by OTMS, which show a single X-ray diffraction (0 0 6) peak normal to the surface of the film. The second harmonic generation (SHG) signal of these films reveals a 6-fold symmetry with a large magnitude contrast between its maximum (1.2 pm/V) and minimum (0.2 pm/V).

The technology of a new type of optical waveguide, the silicon-on-insulator (SOI) movable integrated optical waveguide, is discussed. The fabrication method based on the surface-micromachining technique and wafer bonding is described. Applications in optical switching and optical scanning probing at the communication wavelength of 1300 nm are demonstrated.

The optical and light emitting properties of ZnSTe ZnSe strained superlattices are investigated. The hh1-e1 exciton absorption peak is observed both in absorption and photoluminescence excitation spectra at low temperatures. The Stokes shift between the photoluminescence and absorption peak is ∼ 6 meV indicating that the recombining excitons are only weakly localized and that the sample quality is high. Strong exciton-phonon coupling in the ZnSe wells is evidenced from the observation of oscillatory peaks with energy separation of 31 meV in the PLE spectra. These result from hot excitonic creation and subsequent relaxation via multiple longitudinal-optical phonon emission. Room-temperature stimulated emission by photopumping in the deep-blue spectral region is also observed. An upper limit of the threshold carrier density required to achieve lasing is estimated to be ∼ 5 × 1018/cm3 for the ZnSTe ZnSe superlattices.

The in-plane optical anisotropy of the ZnSe/GaAs interface is studied using reflectance difference (RD) spectroscopy in the 300-700 nm spectral range. Strong resonances near the E 0 bandgap energy and the E 0 + Δ gap energy of bulk ZnSe are observed in both the real and the imaginary parts of the RD spectra. The shape of these resonances changes as a function of ZnSe top layer thickness. Such change is well explained by a three-layer optic model. Possible causes of the resonances are explored, but none has been confirmed so far.

Polysulfones, such as poly(oxy-1,4-phenylenesulfonyl-1,4-phenyleneoxy-1,4-phenyleneisopropylid-ene-1,4-phenylene), poly(phenylene ether sulfone), poly(phenylene sulfone), and aliphatic polysulfone, poly(bicycloheptene sulfone), were employed as a host resin. N-ethyl-N-hydroxyethyl-4-(4'-nitrophenylazo)phenylamine (Disperse Red 1) acted as a guest molecule and was dispersed in those hosts with a concentration of 5 wt. % to 10 wt.%. Their second harmonic generation properties were characterized and compared. Xijk values obtained varied from 11 pm/V to 21 pm/V. The merits of polysulfones as a host material are good optical clarity, easy film forming, and fairly high glass transition Tg temperature (140 °C to 190 °C). Chemically, compared to polyimides they have good resistance to moisture, i.e., less vulnerable to hydrolysis. It is a good candidate for electro-optical materials.

Undoped epitaxial silicon cantilevers with micro-size cross section are fabricated on silicon-on-insulator (SOI) substrates using the surface-micromachining technique. It is demonstrated that these cantilevers function well as a movable optical waveguide at 1.3 μm, an important wavelength for telecommunication applications. The propagation loss of these waveguides is measured to be less than 1.8 dB cm −1 with a polarization dependence of not more than 0.5 dB. A deflection amplitude of 1400 μm is demonstrated for 3400 μm long cantilevers without losing the guided wave. Electrostatic force is used for the actuation.

ZnS 1− x Te x /ZnSe superlattices with x = 0.02 and 0.07 have been grown using the molecular beam epitaxy (MBE) technique. Room temperature photoluminescence from ZnS 0.98 Te 0.02 /ZnSe superlattices is dominanted by strong recombination radiation from ZnSe wells. Under high photoexcitation intensity, room temperature stimulated emission from this superlattice has been observed. In contrast, ZnS 0.93 Te 0.07 /ZnSe superlattices only exhibits a weak, broad luminescence band that is characteristic of isoelectronic centers. These observed results can be attributed to the difference in the energies of hole traps in the barrier layers of these superlattices.

The pulsed laser deposition technique was used to grow silicon oxynitride (SiOxNy) thin films at low temperatures (25–300 °C). The thin films were found to be smooth in surface morphology, extremely inert in HF H 2 O solution, and highly transparent in the optical range from 0.3 μm to 5 μm. The refractive index is tunable between 1.4 and 2.1 by the addition of oxygen in substitution for nitrogen in the thin films.

The temporal decay study of second harmonic generation from poled films of two new NPP and PNP-substituted acrylates and (meth)acrylates shows that these films retain well over 80% of their initial second order nonlinearity over a period of three months. This degree of temporal stability is much higher than those observed in similar copolymers. FT-IR studies indicate that the enhanced stabilization of these films may be attributable to the existence of strong interaction between chromophores and (meth)acrylate spacers.

We have used high-resolution X-ray diffraction (HRXRD) to study the strain relaxation in ZnSe GaAs (001) heterostructures grown by molecular beam epitaxy (MBE). For layers thinner than a critical thickness, X-ray interference fringes are observed. Using a simulation program based on the dynamical theory, we are able to determine with high degree of precision the layer thickness and the (001) Poisson ratio of ZnSe. A transition from partially strained to fully relaxed layer is observed as a function of increasing film thickness for films thicker than a critical thickness, which is determined to be between 1600 to 1850 A. The (001) axis of the epilayer is observed to tilt away from the (001) axis of the substrate. The inclination angle for pseudomorphic layers is fairly constant. For films thicker than the critical thickness, there is no clear relationship between the tilt angle and the film thickness.

We report an experimental demonstration of optimal storage and retrieval of heralded single-photon wave packets using electromagnetically induced transparency (EIT) in cold atoms at a high optical depth. We obtain an optimal storage efficiency of (49+/-3)% for single-photon waveforms with a temporal likeness of 96%. Our result brings the EIT quantum light-matter interface close to practical quantum information applications., Comment: 5 pages, 4 figures

Various microstructural and chemical analysis techniques were applied to study two types (type-A and B) of self-assembled laterally aligned Fe nanowires (NWs) fabricated by molecular beam epitaxy on a ZnS buffer layer. The formation of the three-dimensional shapes of these NWs was found to be driven by the principle of surface energy minimization. We have provided phenomenological models to address the factors affecting the observed topological shape of these NWs, including the role of the lattice relationship between the Fe NWs and the underlying buffer layer, growth temperature, Fe nominal coverage and substrate orientation. Magnetic hysteresis measurements were performed at different temperature, demonstrating the Fe NWs possess a coercivity about 30 times larger than that of a Fe thin film. The observed gradual magnetization reversal indicates the magnetization process is accomplished by the rotation of magnetic moments within a single domain.

We generate narrow-band frequency-tunable entangled photon pairs from spontaneous four-wave mixing in three-level cold atoms and study their two-photon quantum interference after a beam splitter. We find that the path-exchange symmetry plays a more important role in the Hong-Ou-Mandel interference than the temporal or frequency indistinguishability, and observe coalescence interference for both degenerate and nondegenerate photons. We also observe a quantum beat in the same experimental setup using either slow or fast detectors.

We report the direct observation of optical precursors of heralded single photons with step- and square-modulated wave packets passing through cold atoms. Using electromagnetically induced transparency and the slow-light effect, we separate the single-photon precursor, which always travels at the speed of light in vacuum, from its delayed main wave packet. In the two-level superluminal medium, our result suggests that the causality holds for a single photon.

We observe optical precursors generated from slow and fast light cold atomic media. Using constructive interference between sequenced precursors, we produce optical transient pulses with peak powers of about 9 times the input power.

Second harmonic generation from a 1.064 μm fundamental was measured on BaTiO 3 thin films derived from metalorganic chemical vapor deposition. Significant enhancement of the second harmonic generation was observed upon poling of the film using corona discharge. The second harmonic intensity of the poled film was studied as a function of the incident fundamental polarization, showing that there was no in-plane symmetry in the poled films. The measured macroscopic second-order nonlinear optical coefficient of the poled film, d film 33 , is ∼ 7.5 pm/V. This value is comparable to that of the bulk BaTiO 3 along c -axis, d BaTiO 3 33 , suggesting that the majority of the crystalline c -axis of the poled film was aligned to the film normal.

Epitaxial growth of Zn1−xMgxS alloy thin films on GaP(100) substrates was carried out using the molecular-beam-epitaxy technique. In situ reflection high-energy electron diffraction studies show that the alloys can be grown with a stable zinc-blende structure up to x around 30%. For x>30%, a structural transition will occur at a critical thickness which is sensitively dependent on the x composition. A near-band-edge peak with a full width at half maximum of about 10 nm was observed in room-temperature photoluminescence measurements made on as-grown alloy thin films. Several Zn1−xMgxS-based Schottky barrier photodetectors were fabricated. Room-temperature photoresponse measurements were performed on these detectors and abrupt long-wavelength cutoffs covering 325, 305, 295, and 270 nm were achieved for devices with Mg composition of 16%, 44%, 57%, and 75%, respectively. The response curve of the Zn0.43Mg0.57S device offers a close match to the erythemal action spectrum that describes human skin sensitivity ...

We report the generation of nondegenerate narrow-bandwidth paired photons with time-frequency and polarization entanglements from laser cooled atoms. We observe the two-photon interference caused by Rabi splitting with a coherence time of about 30 ns and a visibility of 81.8% which verifies the time-frequency entanglement of the paired photons. The polarization entanglement is confirmed by polarization correlation measurements which exhibit a visibility of 89.5% and characterized by quantum-state tomography with a fidelity of 90.8%. Taking into account the transmission losses and duty cycle, we estimate that the system generates hyperentangled paired photons into opposing single-mode fibers at a rate of 320 pairs per second.

We study weak anti-localization (WAL) effect in topological insulator Bi2Te3 thin films at low temperatures. Two-dimensional WAL effect associated with surface carriers is revealed in the tilted magnetic field dependence of magneto-conductance. Our data demonstrates that the observed WAL is robust against deposition of non-magnetic Au impurities on the surface of the thin films. But it is quenched by deposition of magnetic Fe impurities which destroy the pi Berry's phase of the topological surface states. The magneto-conductance data of a 5 nm Bi2Te3 film suggests that a crossover from symplectic to unitary classes is observed with the deposition of Fe impurities., 4 pages, 3 figures. Corresponding author email address: phjwang@ust.hk

We report the observation of coherent two-photon free-induction decay (FID) in a three-level Lambda system excited by a low-intensity square-modulated laser pulse. Using electromagnetically induced transparency in an Rb85 cold atomic cloud, for what we believe to be the first time, we observe FID signals with coherence time exceeding the relevant atomic excited-state natural lifetime. Because of the on-resonance enhancement, the two-photon FID signal can be observed at the falling edge without using heterodyne means.

We experimentally demonstrate a technique for shaping the temporal quantum waveform of narrow-band biphotons generated in a cold atomic ensemble via four-wave mixing by periodically modulating the two input classical lasers. We show that it is possible to generate nonclassical paired photons with a predesigned shape of the correlation function.

We report theoretical and experimental studies of the propagation of a square-modulated laser pulse through a laser-cooled atomic ensemble of two-level absorbers, as well as through a three-level system with electromagnetically induced transparency. We find that the transmission characteristics over a wide range of optical depth can be fully accounted for as optical coherent transients excited by the wide spectral content of the square-modulated optical pulse. We show that both time-domain atom-field coupled equations and frequency-domain linear dispersion theory give precisely the same description of the optical transients at a weak power limit. At low optical depth with moderate absorption, resonant excitation dominates and free-induction decay (FID) contributes mostly to the transient field. At high optical depth when absorption and propagation effects become significant, lossless optical precursors start to dominate the transient response. By varying the optical depth from 0 to 45, we observe that optical transients evolve gradually from FID to optical precursors. We thus show that FID and optical precursors, which have been considered as two different optical transients for many decades, can be unified within a single theoretical frame and they are the manifestations of the same physical process in two different regimes.