Graphene-based optical modulator realized in metamaterial split-ring resonators operating in the THz frequency range

The integration of quantum cascade lasers with devices capable of efficiently manipulating terahertz light, represents a fundamental step for many different applications. Split-ring resonators, sub-wavelength metamaterial elements exhibiting broad resonances that are easily tuned lithographically, represent the ideal route to achieve such optical control of the incident light. We have realized a design based on the interplay between metallic split rings and the electronic properties of a graphene monolayer integrated into a single device. By acting on the doping level of graphene, an active modulation of the optical intensity was achieved in the frequency range between 2.2 THz and 3.1 THz, with a maximum modulation depth of 18%.

Quantum tunneling through graphene nanorings

We investigate theoretically quantum transport through graphene nanorings in the presence of a perpendicular magnetic field. Our theoretical results demonstrate that the graphene nanorings behave like a resonant tunneling device, contrary to the Aharonov-Bohm oscillations found in conventional semiconductor rings. The resonant tunneling can be tuned by the Fermi energy, the size of the central part of the graphene nanorings and the external magnetic field.

Study of the wetting layer of InAs/GaAs nanorings grown by droplet epitaxy

The properties of the wetting layer (WL) of InAs nanorings grown by droplet epitaxy have been studied. The heavy-hole (HH) and light-hole (LH) related transitions of the In(Ga)As WL were observed by reflectance difference spectroscopy. From the temperature dependent photoluminescence behavior of InAs rings, the channel for carriers to redistribute was found to be the compressed GaAs instead of the In(Ga)As layer, which strongly indicated that the wetting layer was depleted around the rings. Futhermore, a complex evolution of the WL with In deposition amount has been observed. (c) 2008 American Institute of Physics.

Electronic structure and optical property of semiconductor nanocrystallites

Semiconductor nanostructures show many special physical properties associated with quantum confinement effects, and have many applications in the opto-electronic and microelectronic fields. However, it is difficult to calculate their electronic states by the ordinary plane wave or linear combination of atomic orbital methods. In this paper, we review some of our works in this field, including semiconductor clusters, self-assembled quantum dots, and diluted magnetic semiconductor quantum dots. In semiconductor clusters we introduce energy bands and effective-mass Hamiltonian of wurtzite structure semiconductors, electronic structures and optical properties of spherical clusters, ellipsoidal clusters, and nanowires. In self-assembled quantum dots we introduce electronic structures and transport properties of quantum rings and quantum dots, and resonant tunneling of 3-dimensional quantum dots. In diluted magnetic semiconductor quantum dots we introduce magnetic-optical properties, and magnetic field tuning of the effective g factor in a diluted magnetic semiconductor quantum dot. (C) 2004 Elsevier B.V. All rights reserved.

Formation of GaAs/AlGaAs and InGaAs/GaAs nanorings by droplet molecular-beam epitaxy

GaAs/AlGaAs lattice-matched nanorings are formed on GaAs (100) substrates by droplet epitaxy. The crucial step in the formation of nanorings is annealing Ga droplets under As flux for proper time. The observed morphologic evolution of Ga droplets during annealing does not support the hypothesis that As atoms preferentially react with Ga around the periphery of the droplets, but somehow relates to a dewetting process similar to that of unstable films. Photoluminescene (PL) test results confirm the quantum-confinement effect of these GaAs nanorings. Using similar methods, we have fabricated InGaAs/GaAs lattice-mismatched rings. (c) 2005 American Institute of Physics.

Evolution of InAs nanostructures grown by droplet epitaxy

The authors report the growth evolution of InAs dot and ring nanostructures with the indium deposition amount on GaAs (001) by droplet molecular beam epitaxy. There is a critical flux for the indium to form InAs dots even when there is no droplet. When the flux exceeds a critical value, In droplets form, which act as nucleation centers for the formation of InAs rings. (C) 2007 American Institute of Physics.

Electronic structures of GaAs/AlxGa1-xAs quantum double

In the framework of effective mass envelope function theory, the electronic structures of GaAs/AlxGa1-xAs quantum double rings(QDRs) are studied. Our model can be used to calculate the electronic structures of quantum wells, wires, dots, and the single ring. In calculations, the effects due to the different effective masses of electrons and holes in GaAs and AlxGa1-xAs and the valence band mixing are considered. The energy levels of electrons and holes are calculated for different shapes of QDRs. The calculated results are useful in designing and fabricating the interrelated photoelectric devices. The single electron states presented here are useful for the study of the electron correlations and the effects of magnetic fields in QDRs.

Complex quantum ring structures formed by droplet epitaxy

Well-defined complex quantum ring structures formed by droplet epitaxy are demonstrated. By varying the temperature of the crystallizing Ga droplets and changing the As flux, GaAs/AlGaAs quantum single rings and concentric quantum double rings are fabricated, and double-ring complexes are observed. The growth mechanism of these quantum ring complexes is addressed. (c) 2006 American Institute of Physics.

Resonant tunneling theory of planar quantum dot structures

The ballistic transport in the semiconductor, planar, circular quantum dot structures is studied theoretically. The transmission probabilities show apparent resonant tunneling peaks, which correspond to energies of bound states in the dot. By use of structures with different angles between the inject and exit channels, the resonant peaks can be identified very effectively. The perpendicular magnetic field has obvious effect on the energies of bound states in the quantum dot, and thus the resonant peaks. The treatment of the boundary conditions simplifies the problem to the solution of a set of linear algebraic equations. The theoretical results in this paper can be used to design planar resonant tunneling devices, whose resonant peaks are adjustable by the angle between the inject and exit channels and the applied magnetic field. The resonant tunneling in the circular dot structures can also be used to study the bound states in the absence and presence of magnetic field.

Valence band structures of the InAs/GaAs quantum ring

In the framework of effective-mass envelope function theory, the valence energy subbands and optical transitions of the InAs/GaAs quantum ring are calculated by using a four-band valence band model. Our model can be used to calculate the hole states of quantum wells, quantum wires, and quantum dots. The effect of finite offset and valence band mixing are taken into account. The energy levels of the hole are calculated in the different shapes of rings. Our calculations show that the effect of the difference between effective masses of holes in different materials on the valence subband structures is significant. Our theoretical results are consistent with the conclusion of the recent experimental measurements and should be useful for researching and making low-dimensional semiconductor optoelectronic devices. (C) 2002 American Institute of Physics.

Electronic states of InAs/GaAs quantum ring

In the framework of effective mass envelope function theory, the electronic states of the InAs/GaAs quantum ring are studied. Our model can be used to calculate the electronic states of quantum wells, quantum wires, and quantum dots. In calculations, the effects due to the different effective masses of electrons in rings and out rings are included. The energy levels of the electron are calculated in the different shapes of rings. The results indicate that the inner radius of rings sensitively changes the electronic states. The energy levels of the electron are not sensitively dependent on the outer radius for large rings. If decreasing the inner and outer radii simultaneously, one may increase the energy spacing between energy levels and keep the ground state energy level unchanged. If changing one of two radii (inner or outer radius), the ground state energy level and the energy spacing will change simultaneously. These results are useful for designing and fabricating the double colors detector by intraband and interband translations. The single electron states are useful for studying the electron correlations and the effects of magnetic fields in quantum rings. Our calculated results are consistent with the recent experimental data of nanoscopic semiconductor rings. (C) 2001 American Institute of Physics.

Design, simulation and testing of large area silicon drift detectors and detector array for X-ray spectroscopy

Large area (25 mm(2)) silicon drift detectors and detector arrays (5x5) have been designed, simulated, and fabricated for X-ray spectroscopy. On the anode side, the hexagonal drift detector was designed with self-biasing spiral cathode rings (p(+)) of fixed resistance between rings and with a grounded guard anode to separate surface current from the anode current. Two designs have been used for the P-side: symmetric self-biasing spiral cathode rings (p(+)) and a uniform backside p(+) implant. Only 3 to 5 electrodes are needed to bias the detector plus an anode for signal collection. With graded electrical potential, a sub-nanoamper anode current, and a very small anode capacitance, an initial FWHM of 1.3 keV, without optimization of all parameters, has been obtained for 5.9 keV Fe-55 X-ray at RT using a uniform backside detector.

Energy spectrum and persistent currents in finite-width mesoscopic ring with radial potential barrier threading a magnetic flux through its hole

The energy spectrum and the persistent currents are calculated for a finite-width mesoscopic annulus with radial potential barrier, threading a magnetic flux through the hole of the ring. Owing to the presence of tunneling barrier, the coupling effect leads to the splitting of each radial energy subband of individual concentrical rings into two one. Thus, total currents and currents carried by single high-lying eigenstate as a function of magnetic flux exhibit complicated patterns. However, periodicity and antisymmetry of current curves in the flux still preserve.

Morphology and wetting layer properties of InAs/GaAs nanostructures

In this work, we present the growth of InAs rings by droplet epitaxy. A complete process from the rings formation to their density saturation has been demonstrated: A morphological evolution with the varying of the indium deposition amount has been, clearly observed. Our results indicate that there, is a critical deposition amount (similar to 1.1 ML) for the indium to form InAs dots before droplets form; there is also a critical deposition amount (similar to 1.4 ML) to form InAs ring, but it is caused by the formation of droplets as the deposition amount increases. The density of the rings saturates when the deposition amount exceeds similar to 3.3 ML; because the adsorbed indium atoms block sites for further adsorption and the following supplied In only contributes to the size increase of In droplets. Still, as the In deposition amount increases, we can find coupled quantum rings. Moreover, the wetting layer properties of these structures are studied by reflectance difference spectroscopy, which shows a complicated evolution with the In amount. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Novel compact SOI-based reconfigurable optical add/drop multiplexer using microring resonators - art. no. 60190L

Novel compact design for 4-channel SOI-based reconfigurable optical add/drop multiplexer using microring resonators is presented and analyzed. Microring resonators have two important attributes as a key new technology for future optical communications, namely functionality and compactness. Functionality refers to the fact that a wide range of desirable filter characteristics can be synthesized by coupling multiple rings. Compactness refers the fact that ring resonators with radii about 30 mu m can lead to large scale integration of devices with densities on the order of 10(4) similar to 10(5) devices per square centimeter. A 4-channel reconfigurable optical add/drop multiplexer comprises a grid-like array of ridge waveguides which perpendicularly cross through each other. SOI-based resonators consisted of multiple rings at each of the cross-grid nodes serve as the wavelength selective switch, and they can switch an optical signal between two ports by means of tuning refractive index of one of the rings. The thermo-optic coefficient of silicon is 1.86x 10(-4) /K. Thus a temperature rise of 27K will increase the refractive index by 5 x 10(-3), which is enough to cause the switching of our designed microring resonators. The thermo-optic effect is used to suppress the resonator power transfer, rather than to promote loss. Thus, the input signal only suffers small attenuation and simultaneously low crosstalk can be achieved by using multiple rings.