Department of Biomedical Engineering and Computational Science


The calculated angle-resolved luminescence for a silver-grated GaN LED structure, for TM polarization. For more information, see J. Appl. Phys. 114, 223104 (2013).

The main goals of the nanoplasmonics project are four-fold: (i) understanding the main limitations and opportunities offered by nano-plasmonics, (ii) realizing significant luminescence enhancement in GaN LEDs by scattering optical and surface plasmon (SP) modes via metallic grating, (iii) developing electrical injection schemes to achieve the full potential of these plasmonic nanostructures and (iv) extending nano-plasmonic enhancement to solar cells and plasmon lasers. Our current activity involves studying the optical and absorption properties of these nanoplasmonic structures using advanced methods mainly Green’s function formalism. Theoretical work aims to explain experimental features, to predict emission enhancement, to understand the role of SPs in nano-photonic devices, and to propose optimal structures maximising light extraction.

Hot carrier transport and LED efficiency

The electron distribution function in the p-GaN layer as a function of energy and distance from QWs, for strong injection. For more information, see Appl. Phys. Lett. 105, 091106 (2014).

The origin of the efficiency droop, which seriously limits the performance of modern III-N light-emitting diodes (LEDs), has remained a topic of constant debate for over a decade. Auger recombination and electron leakage have been commonly suggested as the main causes of the droop, but a widely accepted evidence on its origin has been missing. The main goal of this work is to apply advanced microscopic Monte Carlo simulations of charge transport in III-N LEDs to clarify the relation between hot-electron transport and the droop.

The simulator developed in our group is able to account for electron transport and related scattering and recombination mechanisms under realistic conditions at the device level. Our recent work demonstrates in detail the relationship between Auger recombination and the hot electrons emitted at the LED surface, and shows that electron overflow from the quantum wells or other current leakage mechanisms cannot cause the droop. This strongly indicates that Auger recombination alone is responsible for the efficiency droop in III-N LED structures.

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Quantum Optics

Schematic illustration of the noiseless amplification of a weak coherent field. First, a single photon is subtracted from the field, then added back to the field, and finally again subtracted from the field. For more information, see Phys. Rev. A 86, 063804 (2012).

The goal of the research is to provide detailed understanding of optical energy transport in nanostructures and to use this understanding to propose new quantum optical experiments and to develop new applications. Reaching these goals requires better understanding of the physics of light-matter interaction, photon and phonon emission and absorption, interference of electromagnetic fields in lossy structures, electric charge transport, and near-field energy transfer. The research will focus on studying quantum optics of photon generation and absorption in nanostructures, amplification of weak optical signals, and propagation of photons in lossy optical cavities. The results are expected to give new insight in designing wide variety of photonic applications like new types of single photon sources and noiseless amplifiers.

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Electrical properties of high efficiency LEDs

Simulated band diagram of a multi-quantum well LED with an electron-blocking layer. Inset shows the quasi-Fermi level change leading to unequal emission from different quantum wells. For more information, see J. Appl. Phys. 111, 103120 (2012).

Improvements in the performance of inorganic light-emitting diodes (LEDs) have started a revolutionary change in the lighting industry. Despite the success, the technology still faces several challenges that stand in the way of further improvements. The goals of this research project are developing better theoretical and computational tools for studying the carrier dynamics in LEDs, understanding the operation of LEDs by analyzing data from diverse measurements, proposing new ways to significantly improve the performance of LEDs, and introducing new device applications. The scientific methods range from semiclassical transport equations to quantum-mechanical models, and we emphasize a very close collaboration with experimental partners. Our current research interests include white light generation by LEDs, thermophotonic cooling, LED efficiency droop, current injection to free-standing nanowire structures, and polarization doping.

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Light-matter interaction in LEDs and solar cells

External quantum efficiency of a thin-film GaN-InGaN LED as a function of the active layer thickness and its separation from the mirror. The dimensions are scaled by the emission wavelength in GaN, 184 nm, corresponding to 440 nm in air. For more information, see Appl. Phys. Lett. 102, 111111 (2013).

The research studies light-matter interaction in LEDs and solar cells by computational and theoretical methods and aims to provide insight to the optical processes behind their operation. Particular methods and focus areas are electromagnetic Green's functions, theory of spontaneous emission in excited and dissipative materials, interference effects in thin films and in stratified media, light scattering at rough surfaces and near-field energy transfer between semiconductors and metals. The theoretical framework and numerical simulations are used in the optimization of existing LED and solar cell technologies and in the development of new, more energy efficient device concepts.

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Nanoscale heat transfer

Kinetic temperature profiles in a two-dimensional nanoconstriction at (a) low temperature and (b) high temperature calculated using molecular dynamics. Atoms are set in a square lattice and neighboring atoms interact via Fermi-Pasta-Ulam interaction potential. For more information, see Phys. Rev. E 86, 031107 (2012).

The goal of the research topic is to explore various mechanisms of energy transfer in nanoscale, focusing especially on lattice vibrations, electromagnetic radiation and their interactions in semiconductors and biomolecules. Prevailing classical theories such as Fourier's law of thermal conduction and Planck's law of thermal radiation break down at nanoscale, and new effects such as the confinement, tunneling and interference of energy carriers open up possibilities for a myriad of applications in nanotechnology. These include more powerful thermal management in future electronics, design of efficient thermoelectric devices e.g. for waste to-electricity conversion, manipulation of biomolecules such as DNA and proteins by temperature and information processing by phonons. We develop and implement new computational methods to study how to exploit nanoscale effects in practical applications.

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Physics of strain induced quantum dots

(a) Electron and (b) hole probability densities of the four eigenstates, lowest in energy. The InP stressor island is shown in blue with respect to the ground-states. The position of the quantum well is shown in grey. For more information, see Rep. Prog. Phys. 70, 1425 (2007).

A quantum dot (QD) is a man-made solid-state structure which is able to confine one or several electrons and (or) holes to a nanometre-scale potential minimum. The size of a typical QD ranges from a few lattice constants to a few micrometres. In this project we have discussed theory and modelling of the electronic structure and carrier dynamics of strain induced quantum dots (SIQD) which confine electrons and holes to a lateral potential minimum within a near-surface quantum well (QW).

SIQD exhibit well-resolved and prominently atomic-like optical spectra, making them ideal for experimental and theoretical studies of mesoscopic phenomena in semiconductor nanocrystals. We analyzed the QD carrier dynamics using a master equation model, which accounts for the details of the electronic structure as well as the leading photon, phonon and Coulomb interaction processes. We also discussed the quantum confined Stark effect, the Zeeman splitting and the formation of Landau levels in external fields as well as the correlation effects in many electron many hole system.

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