Energy and Climate
Enabling Energy Efficiency

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Speakers: George Wang (EFRC Thrust Leader) and Tania Henry (Researcher)

Date: March 25, 2011

Event: Life at the Frontiers of Energy Research Contest

Sandia's Energy Frontier Research Center for Solid-State Lighting Science

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Speaker: Jeff Tsao, Distinguished Member of Technical Staff at Sandia National Laboratories and Chief Scientist of the Energy Frontier Research Center for Solid-State Lighting Science

Date: April 13, 2011

Event: Physical, Chemical, and Nano Sciences Colloquium

Abstract: Solid-state lighting is on the verge of replacing every lamp on earth, from the smallest signal lamp to the largest stadium-scale flood lamp. But many challenges remain, if solid-state lighting is to fulfill its ultimate promise. In this talk, I will give an overview of Sandia’ s Energy Frontier Research Center (EFRC) for Solid-State Lighting Science, one of the 46 EFRCs supported by DOE’s Office of Science to “establish the scientific foundation for a fundamentally new U.S. energy economy.” Our EFRC aims to deepen the foundational science of energy conversion and manipulation in tailored photonic structures, while informing, and being informed by, solid-state lighting technology.  Its overarching scientific thrusts are: 1) Competing Radiative and Non-radiative Processes (aimed at developing a microscopic understanding of the competition between radiative and non-radiative e-h recombination); 2) Beyond Free0Sapce Spontaneous Emission (aimed at exploring energy conversion routes that short-circuit conventional free-space spontaneous emission but end in free-space photons; and 3) Beyond-2D (aimed at exploring the use of non-planar nanoscale structures to modify energy conversion routes). 

III-Nitride Nanowires: Novel Emitters for Lighting

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Speaker: George Wang, EFRC Thrust Leader

Date: September 14, 2011

Event: Solid-State Lighting Science Workshop in Novel Emitters and Nanostructured Materials

Abstract: 1D nanostructures, such as nanowires and nanorods, based on the III nitride (AlGaInN) materials system have attracted attention as potential nanoscale building blocks in LEDs, lasers, sensors, photovoltaics, and high power and high speed electronics. Compared to planar films, III-nitride semiconductor nanowires have several potential advantages including higher crystalline quality and reduced strain, which enables growth on arbitrary substrates as well as allowing for a greater range of alloy compositions and hence energies to be achieved. However, before their promise can be fully realized, several challenges must be addressed in the areas of 1) controlled nanowire synthesis; 2) understanding and controlling the nanowire structural, electrical, thermal, and optical properties; and 3) nanowire device integration. Our work seeks to address these areas to lay the scientific and technological foundation for nanowire-based lighting and other energy-related applications. III-nitride nanowires can be fabricated by a variety of techniques, including “bottom-up” approaches and “top-down” lithographic approaches. Bottom-up techniques have been the dominant method and typically involve a nanoscale metal catalyst particle to direct the 1D growth or anisotropic growth conditions. Advantages of using this approach include nanowires free of detrimental crystal defects known as dislocations, and the ability to grow on inexpensive, lattice mismatched substrates, including glass and metal foil, which we have demonstrated in our lab. I will discuss recent results involving the aligned, bottom-up growth of Ni-catalyzed GaN and III-nitride core-shell nanowires, along with extensive results providing insights into the nanowire properties obtained using cutting-edge structural, electrical, and optical nanocharacterization techniques. Some topics I will cover include: in-situ TEM studies of nanowire electrical breakdown and nanomechanics, spatially-resolved cathodoluminescence studies of band-edge and defect luminescence in NWs; and strain-related spatial variation of In incorporation in InGaN shells. I will also discuss the development of an inexpensive, lithography-free technique employing nanowire templates for the growth of high-quality GaN, which could enable more efficient and longer-lifetime visible LEDs. Bottom-up nanowire growth methods do however have the disadvantage of requiring highly specific growth conditions to increase the on-axis growth rate while minimizing lateral growth, which can lead to non-optimal material quality and less flexibility in material design, such as doping and heterostructures. I will describe a new “top-down” approach for fabricating ordered arrays of high quality GaN-based nanorods with controllable height, pitch and diameter. This top-down method allows fabrication of nanorods from high quality, arbitrarily doped films grown by metal-organic chemical vapor deposition using standard, optimized conditions. The fabrication, structure, optical properties, lasing characteristics, and device performance of the nanorods and nanorod LEDs will be discussed.

(Lighting and) Solid-State Lighting: Science, Technology, Economic Perspectives

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View Slides: (Lighting and) Solid-State Lighting: Science, Technology, Economic Perspectives 

Speaker: Jeffrey Tsao,Distinguished Member of Technical Staff at Sandia National Laboratories and Chief Scientist of the Energy Frontier Research Center for Solid-State Lighting Science

Date: January 26, 2010

Event: SPIE Photonics West 2010 OPTO Symposium in San Francisco, California

Abstract: The lecture reviews two previous revolutions in which semiconductor-based technologies supplanted vacuum-tube technologies (electronics and displays) and then focuses on the third and upcoming revolution (illumination). Topics cover the basics of light-emitting diode (LED) operation; a 200-year history of lighting technology; the importance of white light and color vision to the evolutionary biology of trilobites, birds, mammals, and primates; ways in which white light can be created from colored light; ways in which LEDs are currently being used in colored-light applications; and speculations on possible future ways in which LEDs might be used in white-light applications.

Lighting Technologies, Costs, and Energy Demand: Global Developments to 2030

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View Slides: Lighting Technologies, Costs, and Energy Demand: Global Developments to 2030 

Speaker: Jeffrey Tsao, Distinguished Member of Technical Staff at Sandia National Laboratories and Chief Scientist of the Energy Frontier Research Center for Solid-State Lighting Science

Date: September 28, 2010

Event: Research Seminar at the World Bank, Washington DC.

Abstract: Artificial light has long been a significant factor contributing to the quality and productivity of human life; and increasingly so in lower-income countries. Humanity uses huge amounts of energy to produce it. The presenters review possible implications of an emerging technology, solid-state lighting, which promises performance features and efficiencies well beyond those of traditional artificial lighting, for human welfare and energy consumption. They discuss potential consequences of this technology with respect to (a) the global consumption of energy going into lighting up to 2030, and (b) the human productivity and welfare associated with that consumption. They first estimate baseline magnitudes using simple extrapolations of past behavior, and then discuss ways in which the future lighting demand could differ from this baseline. Even if solid-state lighting leads to substantial reductions in unit energy costs of lighting, global electricity consumption for lighting could still increase, due to great expected increases in lighting demand from lower-income countries.

Polarition Lasing by Intra-cavity Pumping in the Strong-Coupling Limit and Enhancement of Molecular Fluorescence in Critically Coupled Resonators

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Speaker: Vladimir Bulovic, Massachusetts Institute of Technology

Date: September 14, 2011

Event: Solid-State Lighting Science Workshop in Novel Emitters and Nanostructured Materials

Abstract:
The strongly coupled states of light and matter in microcavities, known as polaritons, can enable a radically new class of optoelectronic devices based on the macroscopic coherence of photons and excitons. In particular, strong optical absorption and efficient luminescence of molecular organic materials allow for strong coupling and polariton lasing to be achieved at room temperature and with substantially reduced requirements for cavity quality factor. The first part of the talk will present room temperature polariton lasing in a lambda-thick microcavity where a highly absorbing thin film of molecular J-aggregates serves as the strong-coupling material. We will show a new device excitation scheme of intra-cavity pumping which circumvents exciton-exciton annihilation losses inherent to organic materials at high optical excitation densities. Using this flexible cavity architecture, polariton lasing at room temperature is achieved.

The second part of the talk will show that one mirror and the J-aggregate active layer of the polariton laser microcavity can act as a critically coupled resonator, which can be used for scalable, tunable, and homogeneous enhancement of molecular fluorescence. The operation of this device, called a J-aggregate critically coupled resonator, is based on the excitonic energy transfer from a highly absorptive thin film of J-aggregating dye molecules positioned at the anti-node of an optical half-cavity to a overlaying film whose fluorescence is enhanced in this structure. A 20-fold enhancement in molecular fluorescence is demonstrated, with the role of optical interference, Förster energy transfer, and exciton diffusion quantitatively analyzed to optimize device geometry, which can be broadly useful in light emitting structures.

Quantum Optics with a Single Semiconductor Quantum Dot

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Speaker: Weng Chow, EFRC Scientist

Date: September 14, 2011

Event: Solid-State Lighting Science Workshop in Novel Emitters and Nanostructured Materials

Abstract: There is recent interest in strong light-matter interaction in semiconductors and its uses in optoelectronics. Exploring these possibilities requires extending the Jaynes-Cummings model to include many-body effects. The reason is that the matter, even for single quantum dot (QD), consists of multiple interacting charged particles.

This talk will show why the customary cluster expansion has a difficult time doing the job because of slow convergence. An alternate approach is proposed that allows numerical integration of coupled electron-photon-phonon equations of motion to arbitrary accuracy. Application of the approach is illustrated with examples involving quantum coherence, antibunching photon statistics and phonon-assisted polaritons.

Solid-State Lighting: A New Green Technology

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Speaker: Jerry Simmons (EFRC Director) and Mike Coltrin (EFRC Co-Director)

Date: December 13, 2010

Event: Inaugural Meeting of the New Mexico Energy Efficiency Working Group of the state’s Energy Conservation and Management Division.

Abstract: In Simmon’s and Coltrin’s talk titled, “Solid-State Lighting: A New Green Technology,” they provide an introduction to various lighting technologies in use today, and the energy-saving features that are possible with the use of solid-state lighting. The working group’s purpose is to exchange information among practitioners about trends and activities, whether they are related to technology, policies or practices. Other speakers at the initial meeting discussed the adoption of an updated building code and the status of energy efficiency projects funded by the Recovery Act.

Solid State Lighting: The III-V Epi Killer App

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View Slides: Solid State Lighting: The III-V Epi “Killer App” 

Speaker: Jeffrey Tsao, DistinguishedMember of Technical Staff at Sandia National Laboratories and Chief Scientist of the Energy Frontier Research Center for Solid-State Lighting Science

Date: May 23-28, 2010

Event: 15th International Conference on Metal Organic Vapor Phase Epitaxy (ICMOVPE) in Lake Tahoe, Nevada

Abstract: Throughout its history, lighting technology has made tremendous progress: the efficiency with which power in converted into usable light has increased 2.8 orders of magnitude over three centuries. This progress has, in turn, fueled large increases in the consumption of light and productivity of human society. In this talk, we review an emerging new technology, solid-state lighting: its frontier performance potential; the underlying advances in physics and materials that might enable this performance potential; the resulting energy consumption and human productivity benefits; and the impact on worldwide III-V epi manufacture.

Spontaneous Coherence and Spin Texture in a Cold Exciton Gas

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Speaker: Leonid Butov, University of California, San Diego

Date: September 14, 2011

Event: Solid-State Lighting Science Workshop in Novel Emitters and Nanostructured Materials

Abstract: An indirect exciton is a bound pair of an electron and a hole confined in spatially separated layers. Indirect excitons have long lifetimes and long spin relaxation times, can cool down to low temperatures well below the temperature of quantum degeneracy, can travel over large distances before recombination, and can be in situ controlled by voltage. Due to these properties, indirect excitons form a model system both for the studies of basic properties of cold bosons and for the development of optoelectronic devices. In this contribution, we report on the observation of a pattern of spontaneous coherence, spin texture, and phase singularities in a cold gas of indirect excitons in a GaAs/AlGaAs coupled quantum well structure. The observed features of spin texture include a vortex of linear polarization with polarization perpendicular to the radial direction around an exciton source and a periodic spin texture around the macroscopically ordered exciton state formed in the exciton ring. Extended spontaneous coherence of excitons is observed in the region of the polarization vortices and in the region of the macroscopically ordered exciton state. The coherence length in these regions is much larger than in a classical gas, indicating a coherent exciton state with a much narrower than classical exciton distribution in momentum space, characteristic of a condensate. The observed phase singularities include phase domains and fork-like dislocations in the interference pattern. Extended spontaneous coherence, spin texture, and phase singularities are spatially correlated and emerge when the exciton gas is cooled below a few Kelvin.

The Next Semiconductor Revolution: This Time It's Lighting!

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View Slides:The Next Semiconductor Revolution: This Time It’s Lighting! 

Speaker: Jeffrey Tsao, Distinguished Member of Technical Staff at Sandia National Laboratories and Chief Scientist of the Energy Frontier Research Center for Solid-State Lighting Science

Date: March 31, 2010

Event: Lecture at Albuquerque Academy

Abstract: Throughout its history, lighting technology has made tremendous progress: the efficiency with which power is converted into usable light has increased 2.8 orders of magnitude over three centuries. This progress has, in turn, fueled large increases in the consumption of light and productivity of human society. In this talk, we review an emerging new technology, solid-state lighting: the underlying advances in physics and materials that have enabled its current performance, its frontier performance potential, the energy consumption and human productivity benefits associated with achieving this performance potential, and scientific challenges that lie en route.

Unconventional Lasing in Organic Semiconductors

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Speaker: Stephane Kena-Cohen, Imperial College, UK

Date: September 14, 2011

Event: Solid-State Lighting Science Workshop in Novel Emitters and Nanostructured Materials

Abstract: The high optical gain and versatility afforded by organic semiconductors make them an attractive medium for the realization of coherent light sources. It is clear, however, that lasing thresholds must be further reduced to achieve lasing via electrical injection. In this talk we present two novel approaches to organic lasing. In the first, a microcavity containing an anthracene single crystal in the strong exciton-photon coupling regime is realized. Coherent emission of polaritons, the resulting mixed light-matter states, is achieved, via a mechanism that does not require a population inversion. In the second approach, we demonstrate that low threshold random lasing can be obtained using as-grown doped organic films containing dicyanomethylene-based dyes without the need for optical feedback or infiltrated scatterers.

 

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