Visible light communication (VLC), which utilizes LEDs, promises superior privacy and security and reduced impact on surrounding electronics compared to traditional Wi-Fi. This paper explores the potential of InGaN-based micro-light-emitting diodes (micro-LEDs) in high-speed VLC applications, focusing on yellow-green micro-LEDs with nanoporous distributed Bragg reflector (NP-DBR) and red InGaN micro-LEDs. Yellow-green micro-LEDs achieved a maximum external quantum efficiency (EQE) of 8.7%, bandwidth of 442 MHz, and data rate of 800 Mbit/s, while red micro-LEDs demonstrated an EQE of 5.95%, maximum bandwidth of 424 MHz, and data rate of 800 Mbit/s. The application of four core technologies, including circular devices and electrodes, reduced contact electrode area, atomic layer deposition (ALD) for passivation protection, and multi-chip parallel arrays, enhanced optoelectronic characteristics. This paper also highlights the superior performance of InGaN-based red micro-LEDs with a single quantum well (SQW) structure over double quantum wells (DQWs) for VLC applications. The SQW structure yielded higher maximum EQE, modulation bandwidth, and faster transmission rates, paving the way for the potential of full-color micro-display and high-speed VLC applications.
We demonstrate circularly-polarized-like modes with maximum chirality at the exceptional point (EP) of a photonic crystal (PhC) defect cavity. Reaching the EP requires a fine balance between the loss/gain and dielectric perturbation and a high consistency between fabrications and designs. In this work, we use the tunneling loss that can be controlled by the number of removed air holes in the PhC to induce the EP. By removing and repositioning the hole blocks, we make the EP insensitive to uncertainties in fabrications and improve the chirality of radiation field. Our results promote coherent chiral light sources at the chip level without auxiliary non-Hermitian and chiral structures.
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.
KEYWORDS: Nanostructures, Plasmonics, Near field scanning optical microscopy, Near field, Metals, Near field optics, Solids, Gold, Finite element methods, Glasses
We develop a method based on the reciprocity and Green function to efficiently obtain the far-field pattern of dipole emitters around plasmonic nanostructures. Applying this method to air hole arrays fabricated on metal films, we reveal their plasmonic characteristics in the near-field scanning optical microscopy. Modeling scanning-probe tips as surface plasmon launchers, we clarify the orientation effect of their equivalent dipoles and also how these effective dipoles contribute to the excitation of different plasmonic modes, resulting in distinguishable characteristics in the far-field imaging. The outcomes of our calculations are validated with the experimental data from a high-resolution raster scanning nano-focusing plasmonic tip. Satisfactory agreements between the model and measurements are demonstrated.
Optical isolators are important devices in photonic circuits. To reduce the unwanted reflection in a robust manner, several setups have been realized using nonreciprocal schemes. In this study, we show that the propagating modes in a strongly-guided chiral photonic crystal (no breaking of the reciprocity) are not backscattering-immune even though they are indeed insensitive to many types of scatters. Without the protection from the nonreciprocity, the backscattering occurs under certain circumstances. We present a perturbative method to calculate the backscattering of chiral photonic crystals in the presence of chiral/achiral scatters. The model is, essentially, a simplified analogy to the first–order Born approximation. Under reasonable assumptions based on the behaviors of chiral photonic modes, we obtained the expression of reflection coefficients which provides criteria for the prominent backscattering in such chiral structures. Numerical examinations using the finite-element method were also performed and the results agree well with the theoretical prediction. From both our theory and numerical calculations, we find that the amount of backscattering critically depends on the symmetry of scatter cross sections. Strong reflection takes place when the azimuthal Fourier components of scatter cross sections have an order l of 2. Chiral scatters without these Fourier components would not efficiently reflect the chiral photonic modes. In addition, for these chiral propagating modes, disturbances at the most significant parts of field profiles do not necessarily result in the most effective backscattering. The observation also reveals what types of scatters or defects should be avoided in one-way applications of chiral structures in order to minimize the backscattering.
Possessing both the high-speed characteristics of heterojunction bipolar transistors (HBTs) and enhanced radiative recombination of quantum wells (QWs), the light-emitting transistor (LET) which operates in the regime of spontaneous emissions has achieved up to 4.3 GHz modulation bandwidth. A 40 Gbit/s transmission rate can be even achieved using transistor laser (TL). The transistor laser provides not only the current modulation but also direct voltage-controlled modulation scheme of optical signals via Franz-Keldysh (FK) photon-assisted tunneling effect. In this work, the effect of FK absorption on the voltage modulation of TLs is investigated. In order to analyze the dynamics and optical responses of voltage modulation in TLs, the conventional rate equations relevant to diode lasers (DLs) are first modified to include the FK effect intuitively. The theoretical results of direct-current (DC) and small-signal alternating-current (AC) characteristics of optical responses are both investigated. While the DC characteristics look physical, the intrinsic optical response of TLs under the FK voltage modulation shows an AC enhancement with a 20 dB peak, which however is not observed in experiment. A complete model composed of the intrinsic optical transfer function and an electrical transfer function fed back by optical responses is proposed to explain the behaviors of voltage modulation in TLs. The abnormal AC peak disappears through this optoelectronic feedback. With the electrical response along with FK-included photon-carrier rate equations taken into account, the complete voltage-controlled optical modulation response of TLs is demonstrated.
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.
We have analyzed a hybrid photonic-plasmonic crystal nanocavity consisting of a silicon grating nanowire adjacent to a metal surface with a gain gap between. The hybrid plasmonic cavity modes are highly confined in the gap due to the coupling of photonic crystal cavity modes and surface plasmonic gap modes. Using the finite-element method, we numerically solve guided modes of the hybrid plasmonic waveguide at a wavelength of 1.55 μm. The modal characteristics such as waveguide confinement factors and modal losses of the fundamental hybrid plasmonic modes are explored as a function of the groove depth at various gap heights. After that, we show the band structure of the hybrid crystal modes, corresponding to a wide band gap of 17.8 THz. To effectively trap the optical modes, we introduce a single defect into the hybrid crystal. At a deep sub-wavelength defect length as small as 180 nm, the resonant mode exhibits a high quality factor of 566.5 and an ultrasmall mode volume of 0.00186 (λ/n) 3 at the resonance wavelength of 1.55 μm. In comparison to the conventional photonic crystal nanowire cavity in the absence of metal surface, the figure of merit Q/Vm is enormously enhanced around 15 times. The proposed nanocavities open up the opportunities for various applications with strong light-matter interaction such as nanolasers and biosensors.
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.
We analyze a plasmonic gap-mode Fabry-Perot nanocavity containing a metallic nanowire. The proper choice of the cladding layer brings about a decent confinement inside the active region for the fundamental and first-order plasmonic gap modes. We numerically extract the reflectivity of the fundamental and first-order mode and obtain the optical field inside the cavity. We also study the dependence of the reflectivity on the thickness of Ag reflectors and show that a decent reflectivity above 90 % is achievable. For such cavities with a cavity length approaching 1.5 μm, a quality factor near 150 and threshold gain lower than 1500 cm−1 are achievable.
There has been great interests on semiconductor quantum dot (QD) due to its novel physical properties and potential applications such as semiconductor lasers with high gain and narrow linewidth. The collection of carriers by the QDs is a critical issue for efficient gain of QD lasers. A tunneling injection quantum-dot laser has been researched recently. Direct, photon-, phonon-, and Auger-assisted tunneling are all possible mechanisms for carrier transfer from QW to QD. In this talk, we present a theoretical model for the phonon-assisted tunneling from a quantum well (QW) state to the QD ground state in the conduction band. We assume a quantum-disk model and use its analytical wave functions to calculate the tunneling rate based on Fermi's Golden rule. The single-LO-phonon-emission and absorption processes are modeled by Froelich Hamiltonian. The dependence of the tunneling rate on the QW carrier density, temperature, barrier width between QW and QD, and energy difference between the QW state and the QD state are studied. The tunneling time ranging from several to a few tens of picoseconds are possible depending on the thickness of the barrier and the energy spacing between the QW and QD states.
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