Zuyin Zhang

High-performance quantum-dot superluminescent diodes

By inclining the injection stripe of a multiple layer stacked self-assembled InAs quantum dot (SAQD) laser diode structure of 6/spl deg/ with respect to the facets, high-power and broad-band superluminescent diodes (SLDs) have been fabricated. It indicates that high-performance SLD could be easily realized by using SAQD as the active region.

Photoluminescence study of self-assembled InAs/GaAs quantum dots covered by an InAlAs and InGaAs combination layer

We have fabricated a quantum dot (QD) structure for long-wavelength temperature-insensitive semiconductor laser by introducing a combined InAlAs and InGaAs overgrowth layer on InAs/GaAs QDs. We found that QDs formed on GaAs (100) substrate by InAs deposition followed by the InAlAs and InGaAs combination layer demonstrate two effects: one is the photoluminescence peak redshift towards 1.35 mum at room temperature, the other is that the energy separation between the ground and first excited states can be up to 103 meV. These results are attributed to the fact that InAs/GaAs intermixing caused by In segregation at substrate temperature of 520 degreesC can be considerably suppressed by the thin InAlAs layer and the strain in the quantum dots can be reduced by the combined InAlAs and InGaAs layer

Ideal n=3 phase Ca4Mn3O10 of Ruddlesden–Popper series obtained using high pressure and high temperature

An ideal n=3 member of Ruddlesden–Popper (RP) phase Ca4Mn3O10 was obtained by solid-state reaction under high pressure. This phase has been studied by transmission electron microscopy, convergent beam electron microscopy, high-resolution transmission electron microscopy, and electron energy loss spectroscopy. The lattice parameters are derived as a=b=0.37u200anm and c=2.69u200anm with a space group of I4/mmm, which is the space group of ideal RP phase. The advantages of high pressure sintering are discussed.

Sensitive refractive index sensing with tunable sensing range and good operation angle-polarization-tolerance using graphene concentric ring arrays

A highly tunable refractive index sensor with excellent performance and good operation angle-polarization-tolerance is proposed and demonstrated numerically by means of the finite element method. The proposed sensor consists of a planar regular array of paired graphene concentric ring resonators sandwiched between a substrate and a sensing medium. Numerical calculation results show that a high sensitivity of 9.59 µm per refractive index unit and figure of merit of 5.82 can be reached for lower sensing medium refractive indices. The introduction of graphene in this sensor can enhance the absorption of biomolecules and make the sensing range actively tunable. Therefore, it can be conveniently used for possible detection of the refractive index variation of gases, liquids or mixed solutions. Also, we predict that a multi-channel sensor can be achieved by introducing several graphene concentric ring resonators into each unit cell of the array.

Dependence of optical properties on the structure of multi-layer self-organized InAs quantum dots emitting near 1.3 μm

Self-organized InAs quantum dots (QDs) have been fabricated by molecular beam epitaxy and characterized by photoluminescence (PL). For both single- and multi-layer QDs, PL intensity of the first excited state is larger than that of the ground state at 15 K. Conversely, at room temperature (RT), PL intensity of the first excited state is smaller than that of the ground state. This result is explained by the phonon bottleneck effect. To the ground state, the PL intensities of the multi-layer QDs are larger than that of the single-layer QDs at 15 K, while the intensities are smaller than that of the single-layer QDs at RT. This is due to the defects in the multi-layer QD samples acting as the nonradiative recombination centers. The inter-diffusion of Ga and In atoms in the growth process of multi-layer QDs results in the PL blueshift of the ground state and broadening of the full-width at half-maximum (FWHM), which can be avoided by decreasing the spacers growth temperature. At the spacers growth temperature of 520degreesC, we have prepared the 5-layer QDs which emit near 1.3 mum with a FWHM of 31.7 meV at RT, and 27.9 meV at 77 K

Two-band finite difference method for the bandstructure calculation with nonparabolicity effects in quantum cascade lasers

We present a two-band finite difference method for the bandstructure calculation of quantum cascade lasers (QCLs) based on the equivalent two-band model of the nonparabolic Schrodinger equation. Particular backward and forward difference forms are employed in the discretization procedure instead of the common central difference form. In comparison with the linearization approach of the nonparabolic Schrodinger equation, the method is as accurate and reliable as the linearization approach, while the velocity of the method is faster and the matrix elements are more concise, therefore making the method more practical for QCLs simulations.

TEM studies on La2−2xCa1+2xMn2O7 (x=0.5 and 0.25)

The nominal compositions and structures of the compounds La2-2xCa1+2xMn2O7 (x=0.5 and 0.25) prepared by solid-state reaction have been studied by transmission electron microscopy and high-resolution electron microscopy. Besides the observation of the major phase of a two-layer structure in the nominal composition La1.0Ca2.0Mn2O7, the embedded n = 3, n = 4 Ruddlesden-Popper phases and La rich phase in the two-layer structure were observed as defects. No layered structure was observed in the nominal composition La1.5Ca1.5Mn2O7. Instead, a major phase of orthorhombically distorted perovskite, a minor phase of an ideal cubic perovskite, and a small fraction of calcium-oxide were observed in the sample. The coexistence of the orthorhombic phase and ideal cubic perovskite phase, as well as the same orthorhombic phase with perpendicular directions, was observed in the nominal composition La1.5Ca1.5Mn2O7

The evolution of InAs/InAlAs/InGaAlAs quantum dots after rapid thermal annealing

We have studied how the optical properties of InAs self-assembled quantum dots (QDs) grown on GaAs substrate are affected when depositing an InAlAs/InGaAs combination overgrowth layer directly on it by rapid thermal annealing (RTA). The photoluminescence measurement demonstrated that the InAs QDs experiences an abnormal variation during the course of RTA. The model of transformation of InAs-InAlAs-InGaAlAs could be used to well explain the phenomena

A novel application to quantum dot materials to the active region of superluminescent diodes

We have proposed a new superluminescent diodes (SLD) aimed at wide spectrum-quantum dot superluminescent diodes (QD-SLD), which is characterized by the introduction of a self-assembled asymmetric quantum dot pairs active region into conventional SLID structure. We investigated the structure and optical properties of a bilayer sample with different InAs deposition amounts in the first and second layer. We find that the structure of a self-assembled asymmetric quantum dot pairs can operate up to a 150 nm spectral width. In addition, as the first QDs density can modulate the density of the QDs on the second layer, due to relatively high QDs density of the first layer, we can get the strong PL intensity from a broad range. We think that for the broad spectral width and the strong PL intensity, this structure can be a promising candidate for QW-SLD

A novel line-order of InAs quantum dots on GaAs

A novel line-order of InAs quantum dots (QDs) along the [1, 1, 0] direction on GaAs substrate has been prepared by self-organized growth. After 2.5 monolayer InAs deposition, QDs in the first layer of multi-layer samples started to gather in a line. Owing to the action of strong stress between layers, almost all the dots of the fourth layer gathered in lines. The dots lining up tightly are actually one-dimensional superlattice of QDs, of which the density of electronic states is different from that of isolated QDs or quantum wires. The photoluminescence spectra of our multi-layer QD sample exhibited a feature of very broad band so that it is suitable for the active medium of super luminescent diode. The reason of dots lining up is attributed to the hill-and-valley structure of the buffer, anisotropy and different diffusion rates in the different directions on the buffer and strong stress between QD layers