CLEO Technology Sparks Controversy

(Left: Data points from Osram Opto Semiconductor of Germany. Lower black square at 142 lm/W demonstrating record white-light LED efficiency; second black square at 160 lm/W shows projected efficiency after further optimization.)

You may not think that LED lighting would be a controversial topic, but as the New York Times reports, it may be indirectly responsible for new anger among conservative lawmakers in the United States. A U.S. federal law passed in 2007 by the Bush administration will, among other energy-saving measures, make the sale of the incandescent 100-watt light bulbs illegal in 2012. Republican representatives Joe Barton of Texas and Michelle Bachmann of Minnesota, and Republican Senator Rand Paul of Kentucky have recently become vocal about the rights of Americans to purchase lighting of their choice (energy-efficient or not).

The alternatives to the more-than-a-century-old technology spurning this debate are a new make-over of halogens (not much more efficient than incandescent bulbs at about 20 lumens/W), compact fluorescent bulbs (about70 lumens/W), and LED lighting (Osram Opto Semiconductors of Germany just recently claimed to have set a record for warm-white LED chips at 142 lumens/W). CLEO attendees beware: might tea-party members be planning protests of sessions within CLEO Science and Innovation 15: LEDs, Photovoltaics and Energy-efficient ("green") Photonics?

The "controversial" talks related to LED lighting can be found specifically in sessions "Nano-structured LEDs" on Monday, May 2, 1:30-3:15 pm and "Toward More Efficient Visible LEDs" on Wednesday, May 4, 1:30-3:10 pm. Many of these talks will address specific problems in the overarching goal of fabricating highly-efficient LEDs that can simultaneously mimic the white-light spectrum of an incandescent light bulb. White-light LEDs could reach luminous efficiencies of greater than 300 lumens/W once certain device and fabrication challenges are overcome.¹

One of the greater challenges is overcoming the emission gap in the green-yellow region of the visible spectrum (515-600 nm).^1,2 Whether multiple LEDs of different color are combined to produce white-light, or one or two different colored LEDs are used to pump phosphors to produce white light, current designs lack efficient production of yellow-green photons for true white-light color (a stinging irony for a technology slated as "green" photonics). Invited talk, CMU5, "Nitride-based Nano-columns and Applications" and contributed paper CMU6, "Diffraction-Coupled Plasmon-Enhanced Light Emission from InGaN/GaN Quantum Wells" in session "Nano-structured LEDs"will show different approaches to generating green light using nano-structures on InGaN. Lowering the dimensionality by using nano-structures allows one to play with defect, strain, and polarization properties of the material and hence light generating capability and extraction.²

Another challenge being addressed in session "Toward More Efficient Visible LEDs" is "efficiency droop" particular to InGaN. InGaN LEDs show great promise at low currents, but suffer an enormous efficiency reduction (the droop) at high-current injection, a problem for potential use in high-power applications. The jury is still out on the cause for the droop.² Contributed paper CWF3, "On the symmetry of efficiency-versus-carrier-concentration curves in GaInN/GaN light-emitting diodes and relation to droop-causing mechanisms" will be presenting evidence in support of carrier-leakage theory for droop. Contributed paper CWF4 "Efficiency Droop Reduction in InGaN/GaN Light-emitting Diodes by Graded-thickness Multiple Quantum Wells" will show work demonstrating droop reduction by inserting clever nano-structures into the design.

Whether or not you personally believe you should have the right to buy old-tech, 100-Watt incandescent light bulbs, one thing that is not a controversy is that LED technology is hot. It will likely be the future of lighting for illumination and displays. Don't miss out on these talks.

References

1. E Schubert and J.K. Kim, "Solid-State Light Sources Getting Smart," Science, 308, 1274-1278, (2005).

2. M. Crawford, "LEDs for Solid-State Lighting: Performance Challenges and Recent Advances," J. Sel. Topics in Quant. Electron., 15, 1028-1040, (2009).

Thoughts go out to our Japanese Colleagues

I know I am not alone when I write that my thoughts go out to our Japanese colleagues, collaborators, and friends as they cope with the aftermath of the March 11, earthquake and tsunami. The field of optics has a rich history of Japanese innovation. CLEO habitually hosts a significant number of contributed papers, tutorials, short courses, and plenary talks from Japanese scientists or those with ties to Japanese universities or companies. I hope for the safety and health of the survivors of this horrific disaster, and for what it may be worth, offer condolences to those who lost friends, family and loved-ones.

I am optimistic that the recent crises involving nuclear reactors will be solved without further disaster. My experience with Japanese technology and expertise is that it is thoughtful, long-term, and state-of-the-art. If anyone can come up with the right solution, it is them.

James Bond meets CLEO

(Above: From LLNL, Artist's concept of the MEGa-ray system)

The conference program for CLEO 2011 was just released on Wednesday, March 2. Among many other cutting-edge and ground-breaking contributed papers are those from the new conference to debut this May, CLEO: Applications and Technology. Browsing the topic subcategory, Lasers for Government Science and Security Applications, I came across titles that seemed to be the stuff out of a James Bond movie, ATuF2, "Mono-Energetic Gamma-rays (MEGa-rays) and the Dawn of Nuclear Photonics" and ATuF4, "2D+3D Face Imaging for Stand-off Biometric Identification." Can't you just picture the mad, evil-scientist-villain (disfigured in some way, stroking his cat) plotting to steal a MEGa-Ray device in one scene, and Q 3D-scanning 007's face in order for him to gain access into MI6 in another? Maybe I need to do more optics research and watch less movies, however, there's no question about impact of these papers.

Last February, David Gibson, Christopher Barty and colleagues at the National Ignition Facility and Photon Science Division at Lawrence Livermore National Laboratory (LLNL) published their initial results on a MEGa-Ray source they constructed as groundwork for a beefier machine (2 MeV) in the future. At 2 MeV, such a narrow-bandwidth, high-energy, x-ray source could provide brightness 15 orders of magnitude greater than those produced by synchotrons. The artist rendition of the facility shown in the figure above demonstrates the compact size (a large room) compared to the kilometer-scale rings or linacs conventionally used for generating high energy x-ray beams. Additionally, x-rays of this energy and brightness will find use in nuclear physics and applications such as detection of concealed nuclear material or specific isotope detection and quantification.

So how do you make a MEGa-Ray? By scattering high-intensity laser photons off of a relativistic electron beam (Compton Scattering). In fact, the relativistic electrons are made with a laser as well. The group at LLNL uses matched fiber laser oscillators and fiber-based amplifiers to make the relativistic electrons and the high-intensity scattering light to produce the end product.

Brian Redman from Lockheed Martin and his collaborators, on the other hand, are using light in a very different way- to scan human faces for secure identification and threat-detection. Biometric identification refers to a technique in which a subject can be identified by a unique physical trait or something they physically produce. Some examples are fingerprinting, iris scans, facial scans, and analysis of gait. This topic is particular fascinating to me since human identification is something that the human brain does remarkably well, and for which computers often have trouble. We can identify another person with a great success rate from a far distance based on how they walk, their gait (for a fun gait simulator click here). We have an impeccable ability to identify faces, particularly when we are young- babies can recognize different monkey faces. Therefore one of the directions of research on biometric identification is to improve computational algorithms.

One of Lockheed's specific objectives in their partnership with the FBI is to build a database of facial scans analogous to their database of fingerprints, the Integrated Automated Fingerprint Identification System (IAFIS). I look forward to hearing how Dr. Redman's CLEO talk addresses the optics involved in the facial scans, the use of both 2D and 3D scan information, and the success of the algorithms employed.