Browsing by Author "Hsieh, Chi-Ti"

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Absorption enhancement in type-II coupled quantum rings due to existence of quasi-bound states
(2018-01-01)
Hsieh, Chi-Ti
;
Lin, Shih-Yen
;
Chang, Shu-Wei
;
光電工程學系
;
Department of Photonics
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-to continuum 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.
Bound-to-continuum absorption with tunneling in type-II nanostructures: a multiband source-radiation approach
(2013-12-16)
Hsieh, Chi-Ti
;
Chang, Shu-Wei
;
光電工程學系
;
Department of Photonics
We convert calculations of the bound-to-continuum absorption in type-II semiconductor quantum wells into an equivalent source-radiation problem under the effective-mass approximation with band mixing. Perfectly matched layers corresponding to the eight-band Luttinger-Kohn Hamiltonian are introduced to incorporate the effect of quasi-bound states in open regions. In this way, the interplay between quantum tunneling and optical transitions is fully taken into account. From resulted lineshapes of the Fano resonance, we can evaluate tunneling rates of these metastable states and related absorption strengths relative to those of the continuum. The approach here is useful in estimations of carrier extraction rates from type-II nanostructures for photovoltaic applications. (C) 2013 Optical Society of America
Carrier lifetime of heavily p-doped base in light-emitting transistors and transistor lasers
(2019-01-01)
Hsieh, Chi-Ti
;
Cheng, Pi-Ju
;
Lin, Chih-Hsien
;
Chang, Shu-Wei
;
光電工程學系
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 10(19)cm(-3) and might be even shorter in the case of heavier doping.
Circularly-polarized lasing of ultraviolet plasmonic gammadion nanocavity
(2019-01-01)
Cheng, Pi-Ju
;
Lai, Ming-Sheng
;
Hsiao, Yu-Hao
;
Yu, Cheng-Li
;
Chang, Chiao-Yun
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.
Comparison of wavefront coding optical system using two conjugate phase masks among cubic, sinusoidal, and tangent phase masks
(2017-01-01)
Chen, Shao-Pai
;
Cheng, Pi-Ju
;
Hsieh, Chi-Ti
;
Chang, Shu-Wei
;
光電工程學系
Wavefront coding is an effective technique for extending the depth of field. We verified the usefulness of the method of generating symmetrical point spread function by using two conjugate phase masks. According to the results of this study, this method is effective at a small defocus.
Effect of Heavily P-Doped Base on Radiative Recombination of Transistor Laser
(2019-11-01)
Hsieh, Chi-Ti
;
Chang, Shu-Wei
;
光電工程學系
;
Department of Photonics
The p-type doping in base regions of transistor lasers (TLs) plays an essential role in both the electrical and optical functionalities. The holes in heavily p-doped bases lay the ground for not only the control of current signals but also stimulated emission of lasing modes. In this paper, however, we show that an exceedingly high doping level does not further enhance the radiative recombination in TLs. With heavy doping, the distorted band profile induced by the slight charge separation of holes and ionized acceptors results in no bound valence subbands in the quantum well (QW). This effect stops holes from entering the QW, and most radiative transitions take place inefficiently between bound electrons in the QW and quasi-free holes in the base. As a result, the radiative lifetime is only shortened to the (sub-) nanosecond range, suggesting that the previously reported picosecond carrier lifetime of TLs might originate from other recombination processes.
Enhanced Absorption Due to Formation of Quasi-Bound States in Type-II Coupled Quantum Rings
(2018-03-01)
Hsieh, Chi-Ti
;
Lin, Shih-Yen
;
Chang, Shu-Wei
;
光電工程學系
;
Department of Photonics
The absorption of type-II nanostructures is often weaker than type-I counterpart due to spatially indirect optical transitions. This phenomenon limits the application of type-II nanostructures in photon detections. In this paper, we show that with proper arrangements of conduction barriers, the formation of quasi-bound states can significantly boost up the absorption of type-II coupled quantum rings. Meanwhile, the rapid tunneling of electrons in these leaky states should make the corresponding external quantum efficiency comparable to that of single (uncoupled) rings. These featuresmay improve the performance of photon detectors based on type-II semiconductor nanostructures.
Enhancing accuracy with subpixel smoothing for multiband effective-mass Hamiltonians of semiconductor nanostructures
(2016-01-01)
Hsieh, Chi-Ti
;
Hsieh, Tung-Han
;
Chang, Shu-Wei
;
光電工程學系
;
Department of Photonics
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 eigenenergies 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.
Improving accuracy using subpixel smoothing for multiband effective-mass Hamiltonians of semiconductor nanostructures
(2016-04)
Hsieh, Chi-Ti
;
Hsieh, Tung-Han
;
Chang, Shu-Wei
;
光電工程學系
;
Department of Photonics
We develop schemes of subpixel smoothing for the multiband Luttinger-Kohn and Burt-Foreman Hamiltonians of semiconductor nanostructures. With proper procedures of parameter averages at abrupt interfaces, computational errors of envelope functions due to the discontinuity of heterostructures are significantly reduced. Two smoothing approaches are presented. One is based on eliminations of the first order perturbation in energy, and the other is an application of the Hellmann-Feynman theorem. Using the finite-difference method, we find that while the procedure of perturbation theory seems to be more robust than that of Hellmann-Feynman theorem, the errors of both schemes are (considerably) lower than that without smoothing or with direct but unjustified averages of untransformed parameters. The proposed approaches may enhance numerical accuracies and reduce computational cost for the modeling of nanostructures. (C) 2015 Elsevier B.V. All rights reserved.