In recent years, experimental work has shown that significant luminescence enhancement can be obtained from quantum-well (QW) light-emitting diodes (LEDs) by using metallic grating, which diffracts efficiently optical modes and resonances trapped in these structures and converts surface plasmon (SP) modes into radiative modes. We employ a powerful simulation tool to provide a deep insight into the physics of plasmonic enhancement and present guidelines on how to optimize light-extraction in III-nitride LED structures incorporating an emitting InGaN QW located in the vicinity of a grated silver surface. The model uses first-principle theory, coupling the dyadic Green’s function formalism for solving Maxwell’s equations to fluctuational electrodynamics, and employs a recursive and transparent solution method allowing the fields to be written in a closed form. We demonstrate the significant effect of the type of the periodic grating and layer structure on light-extraction efficiency by simulating various structures with different grating shapes and dimensions. Careful optimization of the grating features shows that the maximum enhancement can reach a factor of around 8 as compared to the flat semiconductor structure and that the plasmonic losses can be significantly reduced.
Recent measurements have generated a need to better understand the physics of hot carriers in III-Nitride (III-N) lightemitting diodes (LEDs) and in particular their relation to the efficiency droop and current transport. In this article we present fully self-consistent bipolar Monte Carlo (MC) simulations of carrier transport for detailed modeling of charge transport in III-N LEDs. The simulations are performed for a prototype LED structure to study the effects of hot holes and to compare predictions given by the bipolar MC model, the previously introduced hybrid Monte Carlo–drift-diffusion (MCDD) model, and the conventional drift-diffusion (DD) model. The predictions given by the bipolar MC model and the MCDD model are observed to be almost equivalent for the studied LED. Therefore our simulations suggest that hot holes do not significantly contribute to the basic operation of multi-quantum well LEDs, at least within the presently simulated range of material parameters. With the added hole transport simulation capabilities and fully self-constistent simulations, the bipolar Monte Carlo model provides a state-of-the-art tool to study the fine details of electron and hole dynamics in realistic LED structures. Further analysis of the results for a variety of LED structures will therefore be very useful in studying and optimizing the efficiency and current transport in next-generation LEDs.
The unique properties of surface plasmons (SPs) are expected to provide a great improvement of light extraction in light-emitting diodes (LEDs). Surface plasmon modes are characterized by a high local density of states, and if scattered by gratings, significantly high emission enhancement is achievable. We investigate the physical role of SPs in improving light extraction from GaN quantum-well (QW) light-emitting structures incorporating metallic grating, by using first-principle theory based on Maxwell's equations and fluctuational electrodynamics. We demonstrate how careful nano-engineering, specifically by choosing the right nano-grating period, can reduce absorption losses and provide optimal enhancement; in the investigated test geometries, light extraction is increased by a factor of four, with the plasmonic losses being reduced from ~ 90% to below ~ 60% thanks to the metallic grating. While the results confirm a strong enhancement and reduction in the plasmonic losses, the overall losses still represent a significant obstacle for plasmonic-enhanced emission. With further optimization of the structure, the grating shapes and the materials, a much larger enhancement and lower losses are expected to be possible.
Performance of III-N based solid-state lighting is to a large extent limited by current transport effects that are also expected
to contribute to the efficiency droop in real devices. To enable studying the contributions of electron transport in drooping
more accurately, we develop and study a coupled Monte Carlo–drift-diffusion (MCDD) method to model the details of
electron current transport in III-N optoelectronic devices. In the MCDD method, electron and hole distributions are first
simulated by solving the standard drift-diffusion (DD) equations. The hole density and recombination rate density obtained
from solving the DD equations are used as inputs in the Monte Carlo (MC) simulation of the electron system. The MC
simulation involves solving the Boltzmann transport equation for the electron gas to accurately describe electron transport.
As a hybrid of the DD and MC methods, the MCDD represents a first-order correction for electron transport in III-N LEDs
as compared to DD, predicting a significant hot electron population in the simulated multi-quantum well (MQW) LED
device at strong injection.
It has recently been shown that multiphoton absorption in cavities containing an emitter and a nonlinear mirror or a two photon absorber can be used to create antibunched photons (i.e. nonclassical light). We investigate the generation of nonclassical photon states using nonlinear laser cavities where the excitation has been modified so that it consists of short current pulses. The light fields in the studied setups are ideally formed of superpositions of zero photon and one photon states. Our goal is to study and develop single photon sources which are needed e.g. in quantum information processing and quantum computing, and fundamental quantum optical experiments. We investigate the effect of exciting the photon emitter with time dependent current pulses to provide single-photon-on-demand sources. We maximize the probability of the single photon state by optimizing the strengths of linear losses, nonlinear absorption, photon emission, and the length of the current injection pulse into the amplifier. Furthermore, we analyze the output photon statistics and waiting times using Monte Carlo simulations. This type of a setup is technologically attractive since it potentially provides room temperature realization of photon antibunching with essentially standard optoelectronic materials and processing techniques.
Surface plasmons (SPs) have recently gained substantial attention due to their sub-wavelength localization and strong interactions in the near-field. Their unique properties are expected to be essential for the next-generation photonic nanodevices, for instance, to improve light extraction in light-emitting diodes (LEDs). We discuss and develop a rigorous and transparent method to model luminescence enhancement and absorption in grated multilayer structures. The method is based on Green's functions, obtained as a perturbative solution to Maxwell's equations, and the fluctuational electrodynamics description of the structures. The model provides an analytical alternative to numerical methods such as finite-element methods and gives insight beyond the numerical solutions, offering a direct means of studying emission and luminescence from the periodic structures. The model is applied to answer key fundamental questions regarding luminescence enhancement, absorption and reflection in realistic
plasmonic GaN light-emitting diode (LED) structures. Two aspects are considered in particular: (1) modeling the reflectometry measurements of grated LED structures to explain and map the interference patterns observed experimentally by our collaborators, and (2) modeling the enhancement in plasmonic structures where the emission takes place in quantum wells in the vicinity of the metallic grating. The results clearly reveal e.g. the SP-related luminescence enhancement in InGaN quantum well structures incorporating periodic silver grating.