In this work, we present the model of plasmonic chrial nanolasers composed of aluminum-coated gallium-nitride (GaN) gammadions, which may lase with a high degree of circular polarization at room temperatures. Using the finite-element method, we examine resonant modes of the four-fold rotationally symmetric cavities of gammadions whose resonant frequencies lie in the gain spectrum of GaN. We find a degenerate doublet of resonant modes which can couple to plane waves in the far-field zone above gammadions. Their near-field profiles exhibit localized distribution in the arms of gammadions and a Fabry-Perot standing-wave pattern along the post. In practice, fabrication imperfections would inevitably spoil the four-fold rotation symmetry of gammadions. Typical perturbation could lift the degeneracy of doublet and leads to mixing of the two degenerate modes which may still output signals with observable handedness above gammadions. Considering a gammadion cavity with a single elongated arm, we show that the magnitude of dissymmetry factor of its resonant mode can be larger than unity. Our calculations are consistent with the experimental results, indicating that the right-handed gammadion cavities lase with a magnitude of dissymmetry factors near 1 at a wavelength of 364 nm. The dimensionless effective mode volume scaled by the cube of effective wavelength is 2.62, reflecting a modal distribution remarkably confined in the plasmonic structures and the capability of enhancing the spontaneous-emission rate noticeably. These chiral nanolasers with an ultrasmall footprint could be potentially utilized as future circularly-polarized photon source at the chip level.
Light-emitting transistor (LET) and transistor laser (TL) can provide the high-speed electrical and optical modulations simultaneously, advancing light-emitting diodes and diode lasers. Still, between experimental data and rate-equation modeling, there are two-order-of-magnitude uncertainties on the carrier lifetimes of quantum wells (QWs) inserted in heavily p-doped bases of these devices. In view of the importance of this timescale on the modulation speed, we provide a comprehensive approach to calculate carrier lifetimes under such circumstances. We model the Hartree potential energy with self-consistent solutions of the Schrodinger’s and Poisson’s equations. The hole distribution is obtained from real-space density of states through multiband retarded Green functions, taking the outgoing-wave features of hole quasi-bound states into account. We then estimate the carrier lifetimes based on a multiband source-radiation approach including both bound-to-bound and bound-to-continuum components of spontaneous (SP) emissions. Under low surface carrier injections, a large Hartree potential is formed, and the valence band around the QW is strongly tilted. Both bound and quasi-bound valence states are present, and quasi-bound holes may tunnel out of QW and reemerge in the base. The SP spectrum from the QW in the heavily doped base is significantly larger than that from an undoped one due to preexisting holes. At the high injection level, the screening effect significantly reduces the Hartree potential and band bending. We also include the nonradiative Auger recombination to evaluate the total carrier lifetime. Overall carrier lifetimes and small-signal ones are estimated as hundred picoseconds at a doping density of 1019 cm−3 and might be even shorter in the case of heavier doping.
The absorption of type-II nanostructures is often weaker than type-I counterpart due to spatially separated electrons and holes. We model the bound-to-continuum absorption of type-II quantum rings (QRs) using a multiband source-radiation approach using the retarded Green function in the cylindrical coordinate system. The selection rules due to the circular symmetry for allowed transitions of absorption are utilized. The bound-tocontinuum absorptions of type-II GaSb coupled and uncoupled QRs embedded in GaAs matrix are compared here. The GaSb QRs act as energy barriers for electrons but potential wells for holes. For the coupled QR structure, the region sandwiched between two QRs forms a potential reservoir of quasi-bound electrons. Electrons in these states, though look like bound ones, would ultimately tunnel out of the reservoir through barriers. Multiband perfectly-matched layers are introduced to model the tunneling of quasi-bound states into open space. Resonance peaks are observed on the absorption spectra of type-II coupled QRs due to the formation of quasi-bound states in conduction bands, but no resonance exist in the uncoupled QR. The tunneling time of these metastable states can be extracted from the resonance and is in the order of ten femtoseconds. Absorption of coupled QRs is significantly enhanced as compared to that of uncoupled ones in certain spectral windows of interest. These features may improve the performance of photon detectors and photovoltaic devices based on type-II semiconductor nanostructures.
The spontaneous emission of an excited molecule can be tailored by its environment. Modifications of the spontaneous emission rate using plasmonic structures are widely investigated for applications ranging from the near-field optics, nanophotonics, to biomedical imaging. It is possible to track the spontaneous emission rate of a dipole emitter which responds to spatial changes of the environment and therefore reflect the morphology of surface of interest. In this work, we model the fluorescence lifetime imaging of gold nanorod dimers by utilizing a single dipole emitter as a sensitive probe scanning along one dimension above the metallic nanostructures. The fluorescence lifetime is spatially mapped out as an attempt to reconstruct the corresponding images. However, it is found that the lifetime imaging is not always consistent with the real morphology of nanostructure. Artifacts in lifetime imaging may arise due to the strong coupling fields in the resonance structures. The sharpness of nanorod dimers could make spontaneous emission rate of a dipole emitter change dramatically and play a key role in artifacts. The operation frequency of a dipole emitter can also influence the lifetime and contribute to artifacts. Here, we will investigate the relation between orientations of dipole emitters and spatial profile of the image. In addition, we will address strategies to distinguish these artifacts from the real morphology and present a theoretical model based on the waveguide geometry to examine possible origins of artifacts.
The spatial discontinuity of physical parameters at an abrupt interface may increase numerical errors when solving partial differential equations. Rather than generating boundary-adapted meshes for objects with complicated geometry in the finite-element method, the subpixel smoothing (SPS) replaces discontinuous parameters inside square elements that are bisected by interfaces in, for example, the finite-difference (FD) method, with homogeneous counterparts and matches physical boundary conditions therein. In this work, we apply the idea of SPS to the eight-band effective-mass Luttinger-Kohn (LK) and Burt-Foreman (BF) Hamiltonians of semiconductor nanostructures. Two smoothing approaches are proposed. One stems from eliminations of the first-order perturbation in energy, and the other is an application of the Hellmann-Feynman (HF) theorem. We employ the FD method to numerically solve the eigenvalue problem corresponding to the multiband Schrodinger’s equation for circular quantum wires (QWRs). The eigen-energies and envelope (wave) functions for valence and conduction states in III-V circular QWRs are examined. We find that while the procedure of perturbation theory seems to have the better accuracy than that of HF theorem, the errors of both schemes are considerably lower than that without smoothing or with direct but unjustified averages of parameters. On the other hand, even in the presence of SPS, the numerical results for the LK Hamiltonian of nanostructures could still contain nonphysical spurious solutions with extremely localized states near heterostructure interfaces. The proper operator ordering embedded in the BF Hamiltonian mitigates this problem. The proposed approaches may improve numerical accuracies and reduce computational cost for the modeling of nanostructures in optoelectronic devices.
Electrons and holes in type-II nanostructures are spatially separated. Therefore, both the radiative and nonradiative
recombination rates are reduced. Although the photon conversion efficiency is hence decreased, the lowered
nonradiative recombination such as Auger process benefits photovoltaic applications. Furthermore, if generated
carriers can be rapidly removed from nanostructures through quasi-bound states, the photon absorption may be
designed and enhanced regardless of the concern on nonradiative mechanisms. Here, we model the bound-tocontinuum
absorption of type-II nanostructures in the presence of tunneling using the density-matrix formalism
and convert it into a radiation problem in the multiband space with band mixing. An effective source is derived
from the eight-band momentum operator, and the corresponding field is expressed in terms of the source and
retarded Green’s function of the eight-band Luttinger-Kohn Hamiltonian. On the other hand, the response is
actually calculated without the Green’s function. Perfectly-matched layers in the multiband space are introduced
to model the effect of quasi-bound states in open regions. In this way, the interplay between photon absorption
and tunneling is fully taken into account. We present both the transverse-electric and transverse-magnetic
absorption spectra of type-II GaAs 0:65Sb0:35/GaAs coupled quantum wells. The corresponding lineshape broadening near the resonant energy can be divided into two parts. One comes from various incoherent relaxation
mechanisms, and another well-fitted by the Fano resonance originates from the coherent tunneling. For a 2-nm
potential barrier, the tunneling times of metastable states in nanostructures are around 20 fs, and their degrees
of mixing to the continuum are high.
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