Semiconductor Nanorings Research Articles

Broadband visible light absorber based on ultrathin semiconductor nanostructures**Project supported by the Natural Science Foundation of Guangdong Province, China (Grant Nos. 2018A030313854 and 2016A030313851).

It is desirable to have electromagnetic wave absorbers with ultrathin structural thickness and broader spectral absorption bandwidth with numerous applications in optoelectronics. In this paper, we theoretically propose and numerically demonstrate a novel ultrathin nanostructure absorber composed of semiconductor nanoring array and a uniform gold substrate. The results show that the absorption covers the entire visible light region, achieving an average absorption rate more than 90% in a wavelength range from 300 nm to 740 nm and a nearly perfect absorption from 450 nm to 500 nm, and the polarization insensitivity performance is particularly great. The absorption performance is mainly caused by the electrical resonance and magnetic resonance of semiconductor nanoring array as well as the field coupling effects. Our designed broadband visible light absorber has wide application prospects in the fields of thermal photovoltaics and photodetectors.

Ultrahigh Purcell Factor, Improved Sensitivity, and Enhanced Optical Force in Dielectric Bowtie Whispering-Gallery-Mode Resonators

We propose and theoretically investigate an all-dielectric bowtie whispering-gallery-mode (WGM) resonator which consists of two tip-to-tip coupled semiconductor nanorings with triangular cross section separated by low refractive index material gap. Mode splitting of symmetric WGM and antisymmetric WGM is observed and analyzed. Due to extremely field enhancements by the “slot” and “tip” effects, strong localization of light in the gap of symmetric WGMs leads to ultrasmall modal volume of $0.3\,\mu {\rm{m}}^{3}$ . This value is two orders of magnitude smaller than that in toroidal microresonator which is the typical WGM microcavity with tight mode confinement. Importantly, large amount of light confined in the gap in bowtie WGM resonator not only suppresses the radiation loss (radiation-loss-related quality factor is above $10^{8}$ ), but also makes it as an ideal platform for Purcell effect enhancement, ultrasensitive sensing, and optical trapping of nanoparticles. Calculation results show that Purcell factor can reach to $10^{8}$ at room temperature when assuming quantum dots or atoms placed in the gap. Refractive index sensitivity is improved to 700 nm/RIU as compared with conventional slot waveguide with rectangular cross section. The optical gradient force is greatly enhanced and allows efficient trapping of single nanoscale particle with diameter of 5 nm even at a relatively large gap of 100 nm.

Fabrication and room temperature operation of semiconductor nano-ring lasers using a general applicable membrane transfer method

Semiconductor nanolasers are potentially important for many applications. Their design and fabrication are still in the early stage of research and face many challenges. In this paper, we demonstrate a generally applicable membrane transfer method to release and transfer a strain-balanced InGaAs quantum-well nanomembrane of 260 nm in thickness onto various substrates with a high yield. As an initial device demonstration, nano-ring lasers of 1.5 μm in outer diameter and 500 nm in radial thickness are fabricated on MgF2 substrates. Room temperature single mode operation is achieved under optical pumping with a cavity volume of only 0.43λ03 (λ0 in vacuum). Our nano-membrane based approach represents an advantageous alternative to other design and fabrication approaches and could lead to integration of nanolasers on silicon substrates or with metallic cavity.

Енергетичний спектр електрона та сили осциляторів внутрішньозонних квантових переходів у подвійних напівпровідникових нанокільцях у магнітному полі

Енергетичний спектр електрона та сили осциляторів внутрішньозонних квантових переходів у подвійних напівпровідникових нанокільцях у магнітному полі

Energy levels and far-infrared optical absorption of impurity doped semiconductor nanorings: Intense laser and electric fields effects

The effects of electron-impurity interaction on energy levels and far-infrared absorption in semiconductor nanoring under the action of intense laser and lateral electric fields have been investigated. Numerical calculations are performed using exact diagonalization technique. It is found that the electron-impurity interaction and external fields change the energy spectrum dramatically, and also have significant influence on the absorption spectrum. Strong dependence on laser field intensity and electric field of lowest energy levels, also supported by the Coulomb interaction with impurity, is clearly revealed.

Electrically driven single photon source at high temperature

We present a theoretical model for an electrically driven single photon source operating at high temperatures. We show that decoherence, which is usually the main obstacle for operating single photon sources at high temperatures, ensures an efficient operation of the presented electrically driven single photon source at high temperatures. The single-photon source is driven by a single electron source attached to a heterostructure semiconductor nanoring. The electron’s dynamics in the nanoring and the subsequent recombination with the hole is described by the generalized master equation with a Hamiltonian based on tight-binding model, taking into account the electron-LO phonon interaction. As a result of decoherence, an almost 100% single photon emission with a strong antibunching behavior i.e. at high temperature up to 300 K is achieved.

Lithographically Patterned Nanoscale Electrodeposition of Plasmonic, Bimetallic, Semiconductor, Magnetic, and Polymer Nanoring Arrays.

Large area arrays of magnetic, semiconducting, and insulating nanorings were created by coupling colloidal lithography with nanoscale electrodeposition. This versatile nanoscale fabrication process allows for the independent tuning of the spacing, diameter, and width of the nanorings with typical values of 1.0 μm, 750 nm, and 100 nm, respectively, and was used to form nanorings from a host of materials: Ni, Co, bimetallic Ni/Au, CdSe, and polydopamine. These nanoring arrays have potential applications in memory storage, optical materials, and biosensing. A modified version of this nanoscale electrodeposition process was also used to create arrays of split gold nanorings. The size of the split nanoring opening was controlled by the angle of photoresist exposure during the fabrication process and could be varied from 50% down to 10% of the ring circumference. The large area (cm2 scale) gold split nanoring array surfaces exhibited strong polarization-dependent plasmonic absorption bands for wavelengths from 1 to 5 μm. Plasmonic nanoscale split ring arrays are potentially useful as tunable dichroic materials throughout the infrared and near-infrared spectral regions.

Decoherence and quantum interference assisted electron trapping in a quantum dot

We present a theoretical model for the dynamics of an electron that gets trapped by means of decoherence and quantum interference in the central quantum dot (QD) of a semiconductor nanoring (NR) made of five QDs, between 100 and 300 K. The electron's dynamics is described by a master equation with a Hamiltonian based on the tight-binding model, taking into account electron–LO phonon interaction. Based on this configuration, the probability to trap an electron with no decoherence is almost 27%. In contrast, the probability to trap an electron with decoherence is 70% at 100 K, 63% at 200 K and 58% at 300 K. Our model provides a novel method of trapping an electron at room temperature.

Effect of bipartition on spectral properties of nanorings

The effects of a special topological transformation on the energy levels, far-infrared spectra and magnetization of one electron inside a semiconductor nanoring under magnetic fields are studied. We have called this transformation as bipartition, and it is defined by a change in ring topological structure, when its shape is changed successively from a single oval-shaped loop up to quasi-two disconnected loops. Our results show that in the course of such transformation the low-lying energies dependencies with multiple crossovers between them, typical for the free electron rotation along an almost circular loop, are converted into a set of non-crossing double plaits related to a sub-barrier electron tunneling along a strongly non-circular loop in a quasi-local state. Also, we find that during the restructuring of the oval-shaped nanostructures the selection rules for the dipole transitions are changed allowing the appearance of additional peaks in the FIR spectrum.

Excitonic Aharonov-Bohm effect: Unstrained versus strained type-I semiconductor nanorings

We study how mechanical strain affects the magnetic field dependence of the exciton states in type-I semiconductor nanorings. Strain spatially separates the electron and hole in (In,Ga)As/GaAs nanorings which is beneficial for the occurrence of the excitonic Aharonov-Bohm (AB) effect. In narrow strained (In,Ga)As/GaAs nanorings the AB oscillations in the exciton ground-state energy are due to anticrossings with the first excited state. No such AB oscillations are found in unstrained GaAs/(Al,Ga)As nanorings irrespective of the ring width. Our results are obtained within an exact numerical diagonalization scheme and are shown to be accurately described by a two-level model with off-diagonal coupling $t$. The later transfer integral expresses the Coulomb coupling between states of electron-hole pairs. We also found that the oscillator strength for exciton recombination in (In,Ga)As/GaAs nanorings exhibits AB oscillations, which are superimposed on a linear increase with magnetic field. Our results agree qualitatively with recent experiments on the excitonic Aharonov-Bohm effect in type-I (In,Ga)As/GaAs nanorings.

The metal-clad semiconductor nanoring laser and its scaling properties

We proposed a metal-clad semiconductor nanoring laser structure that exhibited a superior scaling properties for D/λ(0) > 0.5 where D is the device diameter. We theoretically analyzed the metal-cald nanoring laser and compared its scaling properties with two other similar nanolaser structures. We found that the two design parameters, namely the ring width and the ring diameter, enable independent emission wavelength control from device dimension. This property in combination with other desirable features including in-plane out-coupling and monolithic integration make the metal-clad nanoring laser highly attractive for photonic integration.

Room‐Temperature Ferromagnetic Silver Vanadium Oxide (Ag1.2V3O8): A Magnetic Semiconductor Nanoring Structure

AbstractMagnetic nanoring structures are attractive for spintronic devices due to their unique attributes of well‐defined and reproducible magnetic states originating from their characteristic geometry. Almost all previous magnetic nanorings have been exclusively limited to traditional ferromagnetic materials, and a magnetic semiconductor (MSC) nanoring structure has been reported rarely during the past decades. Here, it is demonstrated that room‐temperature ferromagnetic Ag1.2V3O8 nanobelts and nanorings may be achieved by controlled oxidation of the V4+ precursors in an Ag+‐containing aqueous solution. The polarization‐induced self‐coiling of in situ formed Ag1.2V3O8 nanobelts is responsible for the formation of the perfectly circular nanoring geometry. The NEXAFS spectra and the density functional calculations clearly reveal that the electron transfer originates from the hybridization of the doped Ag+ and V4+ atoms, causing ordering of the magnetic moments that give rise to the intrinsic ferromagnetism of the Ag1.2V3O8 structure.

Gate controlled Aharonov-Bohm-type oscillations from single neutral excitons in quantum rings

We report on a magnetophotoluminescence study of single self-assembled semiconductor nanorings which are fabricated by molecular-beam epitaxy combined with AsBr3 in situ etching. Oscillations in the neutral exciton radiative recombination energy and in the emission intensity are observed under an applied magnetic field. Further, we control the period of the oscillations with a gate potential that modifies the exciton confinement. We infer from the experimental results, combined with calculations, that the exciton Aharonov-Bohm effect may account for the observed effects.

Multi-level multi-thermal-electron FDTD simulation of plasmonic interaction with semiconducting gain media: applications to plasmonic amplifiers and nano-lasers

Interactions between a semiconducting gain medium and confined plasmon-polaritons are studied using a multilevel multi-thermal-electron finite-difference time-domain (MLMTE-FDTD) simulator. We investigated the amplification of wave propagating in a plasmonic metal-semiconductor-metal (MSM) waveguide filled with semiconductor gain medium and obtained the conditions required to achieve net optical gain. The MSM gain waveguide is used to form a plasmonic semiconductor nano-ring laser(PSNRL) with an effective mode volume of 0.0071 microm3, which is about an order of magnitude smaller than the smallest demonstrated integrated photonic crystal based laser cavities. The simulation shows a lasing threshold current density of 1kA/cm2 for a 300 nm outer diameter ring cavity with 80 nm-wide ring. This current density can be realistically achieved in typical III-V semiconductor, which shows the experimental feasibility of the proposed PSNRL structure.

Binding energy of magneto-biexcitons in semiconductor nano-rings

We report a simulation on the magneto-biexciton system of asymmetrical InAs/GaAs nano-rings, using a smooth three dimensional confinement potential which realistically describes electronic properties of the rings. In our calculation, we use the Hartree approximation to calculate the self-consistent energy for exciton and biexciton confined in the system. We simulate recombination energy, binding energy and their diamagnetic shifts and compare those results with experimental data obtained from the magneto-photoluminescence measurement. We found that using the realistic geometry and composition of the asymmetric nano-ring in our simulations we are able to reproduce experimental data with good accuracy.

Magnetoexcitons in Semiconductor Quantum Rings with Complicated (Kane's) Dispersion Law

The influence of the nonparabolicity of charge carriers dispersion law (Kane’s dispersion) on a magnetoexciton energy spectrum in InSb quantum rings is theoretically investigated. The analytical expression for the energy spectrum of exciton in a narrow-gap semiconductor nanoring in a magnetic field is obtained. The Aharonov‐Bohm oscillations in the energy of excited states are studied. PACS numbers: 73.21.ib, 71.35.Ji

Effect of eccentricity in the spin accumulation effect induced in semiconductors rings with Rashba spin orbit interaction

A spin accumulation effect (SAE) is induced in a semiconductor nanoring with Rashba spin orbit interaction and pierced by a magnetic flux. We show that when the sample is not perfectly symmetric, the profile of the SAE can be highly inhomogeneous along specific orientations. In particular, we analyze the anisotropy generated in the angular profile by a finite eccentricity. We discuss the feasibility of detecting the effect with usual magneto optical techniques for a number of electrons and values of magnetic fluxes experimentally accessible.

Few-electron artificial molecules formed by laterally coupled quantum rings

We study artificial molecular states formed in laterally coupled double semiconductor nanorings by one, two, and three electrons. An interplay of the interring tunneling and the electron-electron interaction is described, as well as its consequences for the magnetization and charging properties of the system. It is shown that both the magnetic dipole moment generated by the double-ring structure and the chemical potential of the system as function of the external magnetic field strongly depend on the number of electrons and the interring barrier thickness. Both the magnetization and chemical potentials exhibit cusps at the magnetic fields inducing ground-state parity and/or spin transformations. The symmetry transformations are discussed for various tunnel coupling strengths, from rings coupled only electrostatically to the limit of coalesced rings. We find that in the ground states for rings of different radii the magnetic field transfers the electron charge from one ring to the other. The calculations are performed with the configuration-interaction method based on an approach of Gaussian functions centered on a rectangular array of points covering the studied structure. Electron-electron correlation is also discussed.

Effects of lateral asymmetry on electronic structure of strained semiconductor nanorings ina magnetic field

The influence of lateral asymmetry on the electronic structure and optical transitions inelliptical strained InAs nanorings is analyzed in the presence of a perpendicular magneticfield. Two-dimensional rings are assumed to have elliptical inner and outer boundariesoriented in mutually orthogonal directions. The influence of the eccentricity of the ring onthe energy levels is analyzed. For large eccentricity of the ring, we do not find anyAharonov–Bohm effect, in contrast to circular rings. Rather, the single-particle states ofthe electrons and the holes are localized as in two laterally coupled quantum dotsformed in the lobes of the nanoring. Our work indicates that the control of shape isimportant for the existence of the Aharonov–Bohm effect in semiconductor nanorings.

Noncircular semiconductor nanorings of types I and II: Emission kinetics in the excitonic Aharonov-Bohm effect

Transition energies and oscillator strengths of excitons in dependence on magnetic field are investigated in types I and II semiconductor nanorings. A slight deviation from circular (concentric) shape of the type II nanoring gives a better observability of the Aharonov-Bohm oscillations since the ground state is always optically active. Kinetic equations for the exciton occupation are solved with acoustic phonon scattering as the major relaxation process, and absorption and luminescence spectra are calculated, showing deviations from equilibrium. The presence of a nonradiative exciton decay leads to a quenching of the integrated photoluminescence with magnetic field.