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61.  
Researchers Develop Simplified Approach for High-Power, Single-Mode Lasers
Researchers Develop Simplified Approach for High-Power, Single-Mode Lasers
McCormick News Article - August 23, 2012
When it comes to applications like standoff sensing—using lasers to detect gas, explosives, or other materials from a safe distance—the laser’s strength is of the utmost importance. A stronger and purer beam means devices can sense danger more accurately from a greater distance. Northwestern University researchers have developed a new resonator that that controls both wavelength and beam quality, creating the purest, brightest, and most powerful single-mode quantum cascade lasers yet at the 8-12 micron range. The feat is achieved through the use of a new type of “distributed feedback” mechanism called Β-DFB, a simple diffractive feedback in an angled laser cavity. A paper describing the findings, “Angled Cavity Broad Area Quantum Cascade Lasers,” was published August 21 in the journal Applied Physics Letters ( DOI:10.1063/1.4747447). ... [read more]
 
62.  
Improved LEDs and photovoltaics by hybridization and nanostructuring
Improved LEDs and photovoltaics by hybridization and nanostructuring
SPIE Newsroom - June 15, 2012
There has been rapid industrial development for optoelectronic devices based on III-Nitrides, which span a direct bandgap from deep UV to IR, and are currently widely used in commercial white, UV, blue, and green LEDs. This alloy system is now projected to provide a platform for the development of novel multi-junction photovoltaics (PVs) with an unprecedented fit to the solar spectrum. However, improving the efficiency of InGaN-based p-n junctions is a very complex and multifaceted task for a number of reasons.
Zinc oxide (ZnO) is a remarkable, multifunctional, and biocompatible direct-, wide-bandgap semiconductor, with a distinctive property set and a unique potential for nanostructuring. Recently, there has been a surge of activity surrounding ZnO to the point where the number of publications now rivals that for GaN. Due to their similar crystal structures and bandgaps, ZnO and GaN can be combined in new ways, which opens up the prospect of novel optoelectronic devices and has the potential to solve many of the issues with existing III-Nitride devices. ... [read more]

 
63.  
Superlattice sees colder objects in two colors and high resolution
Superlattice sees colder objects in two colors and high resolution
SPIE Newsroom - February 10, 2012
The capabilities of IR sensing, commonly used for night vision, have been extended to see colder objects at high speed and potentially made cheaper by using a semiconductor material called the type II superlattice. The wavelength tunability and material robustness of type III-V superlattice have generated much attention in recent years. This is especially true with respect to the performance of the material in narrow-energy-gap detectors, and specifically in a part of the electromagnetic spectrum between 8 and 12 μm, called the long-wavelength IR (LWIR). The benefits of this technology have enabled us to push type II superlattice to its logical next step: a camera capable of seeing distinct wavebands or colors in an all-in-one package, a feat not previously demonstrated by this material system in the LWIR. Our group engineered the detection energies on the cameras to be extremely narrow, with cutoff wavelengths at 9.5 and 13 μm, in the range of roughly 0.1eV in energy. Realizing the camera was a difficult task because the light-absorbing layers are prone to surface leakage effects due to the size of the pixels, which are 30μm wide. We first reported a dual-band LWIR 320256 pixel-sized type II superlattice in July 2011. More recently, we also demonstrated a large-format 640 x 512 pixel type II superlattice camera based on the same material design. The 4x increased resolution is necessary for many advanced applications. ... [read more]
 
64.  
Compact terahertz device could improve security screening
Compact terahertz device could improve security screening
Photonics Spectra magazine, Vol. 45, Issue 12, p. 19-20 - December 31, 2011
Using two mid-infrared laser beams, researchers have finally generated single-chip terahertz radiation at room temperature. The technology could speed up and improve a range of processes, including high-sensitivity biological and chemical analysis, astronomical study, security screening, border protection and agricultural inspection. The project got its start in an unscientific place: the airport security lineup. Like most travelers, Manijeh Razeghi, a professor at Northwestern University's McCormick School of Engineering and Applied Science, was concerned with both the delays in the process and its accuracy. The technology to safely and easily inspect items for hazardous substances is expensive and bulky, so much of it is underused, Razeghi said. The same concerns — time, reliability and cost — are found in medical diagnostics, tumor detection and package inspection. She wanted to come up with "something useful that can overcome these basic limitations and allow terahertz technology to truly become pervasive in order to make everyone's life a little safer and easier." Coherent terahertz radiation historically has been very difficult to generate, and the search for a compact easy-to-use source continues today. Existing terahertz sources are large multicomponent systems that may require complex vacuum electronics, external pump lasers and/or cryogenic cooling. A single-component device that does not have these limitations could enable next-generation terahertz systems. ... [read more]
 
65.  
Focal-Plane Arrays:  Northwestern develops solar-blind, deep-UV FPA
Focal-Plane Arrays: Northwestern develops solar-blind, deep-UV FPA
Laser Focus World magazine - November 2, 2011
In the solar-blind region, the ozone layer in our atmosphere absorbs nearly 100% of the Sun’s energy for wavelengths shorter than 285 nm. Within this region, 254 nm—the dominant ultraviolet (UV) emission line of low-pressure mercury lamps used in germicidal disinfection—can cause damage to the human cornea, making it an especially important wavelength to monitor and control in clinical environments. Current detection methods for this deep-UV spectral range include photocathode and microchannel-plate combinations or silicon-photodetector arrays with bandpass filters. However, these options are fragile (vacuum-tube based) and require high-voltage power supplies or are not intrinsically solar blind and become complex and inefficient due to filtering requirements, respectively. Progress in back-illuminated, aluminum-gallium-nitride (AlGaN)-based photodetectors has eliminated many of these drawbacks. Low quality of the AlGaN layers—hindered by the need for high Al content to make the detectors truly solar blind—has limited this progress. By refining the metal-organic chemical-vapor-deposition (MOCVD) growth process, researchers at Northwestern University (Evanston, IL) have improved the quality and increased the Al content of the AlGaN layers and successfully fabricated the first deep-UV focal-plane array (FPA). ... [read more]
 
66.  
Superlattice cameras add color to night-vision imaging
Superlattice cameras add color to night-vision imaging
Laser Focus World magazine - November 2, 2011
In addition to their recent development of a narrowband terahertz source, Manijeh Razeghi’s group at Northwestern University’s Center for Quantum Devices have built an infrared camera that can see more than one optical waveband or “color” in the dark. The semiconducting material used in the camera--a type-II superlattice--can be tuned to absorb a wide range of infrared wavelengths, and now, a number of distinct infrared bands at the same time. The idea of capturing light simultaneously at different wavelengths is not new. Digital cameras in the visible spectrum are commonly equipped with detectors that sense red, green, and blue light to replicate a vast majority of colors perceived by the human eye. Multi-color detection in the infrared spectrum, however, offers unique functionalities beyond color representation. The resonant frequencies of compounds can often be found in this spectral range, which means that chemical spectroscopy can be relayed in images real-time. ... [read more]
 
67.  
Type II superlattice enables high operating temperature
Type II superlattice enables high operating temperature
SPIE Newsroom - October 25, 2011
Currently, commercial technologies for MWIR detection and imaging are based on indium antimonide (InSb), mercury cadmium telluride (HgCdTe or MCT), or quantum-well IR photodetector systems. However, these materials are fundamentally limited. We recently developed a novel variant of Type-II superlattices, the M-structure superlattice with large effective mass and tunability of band-edge energies. This M-structure has been incorporated in a novel photodiode architecture for operation at high temperatures. A focal plane array based on this new material and detector architecture with improved electrical performance provides mid-wavelength-IR imaging of a human being at 170K. ... [read more]
 
68.  
New Generation of Superlattice Camera Add More Color to Night Vision
New Generation of Superlattice Camera Add More Color to Night Vision
McCormick News Article - October 19, 2011
Recent breakthroughs have enabled scientists from the Northwestern University’s Center for Quantum Devices to build cameras that can see more than one optical waveband or “color” in the dark. The semiconducting material used in the cameras – called type-II superlattices – can be tuned to absorb a wide range of infrared wavelengths, and now, a number of distinct infrared bands at the same time. The idea of capturing light simultaneously at different wavelengths isn’t new. Digital cameras in the visible spectrum are commonly equipped with detectors that sense red, green, and blue light to replicate a vast majority of colors perceived by the human eye. Multi-color detection in the infrared spectrum, however, offers unique functionalities beyond color representation. The resonant frequencies of compounds can often be found in this spectral range, which means that chemical spectroscopy can be relayed in images real-time. Razeghi’s group engineered the detection energies on the cameras to be extremely narrow, close to one-tenth of an electron volt, in what is known as the long-wave infrared window. Creating the cameras was difficult, however, because the light-absorbing layers are prone to parasitic effects. Furthermore, the detectors were designed to be stacked one on top of another, which provided spatially coincident pixel registration but added significantly to the growth and fabrication challenges. Nevertheless, a dual-band long-wave infrared 320-by-256 sized type-II superlattice camera was demonstrated for the first time in the world, the results of which were published in the July 2011 issue of Optics Letters. ... [read more]
 
69.  
Razeghi and group develop small, narrowband room-temperature terahertz source
Razeghi and group develop small, narrowband room-temperature terahertz source
Laser Focus World magazine - October 2, 2011
A chip containing two mid-IR quantum-cascade (QC) lasers is at the heart of a small room-temperature terahertz-radiation source developed by researchers at Northwestern University.1 Using intracavity difference-frequency generation, the chip emits 4 THz radiation with a linewidth of only 6.6 GHz and an output power of up to 8.5 μW. The research was headed by a scientist well-known for her work on QC lasers -- Manijeh Razeghi, a professor at Northwestern University's McCormick School of Engineering and Applied Science. As is well-known to Laser Focus World readers, terahertz radiation can be used for security screening, terahertz-absorption spectroscopy for detecting biological and chemical compounds, and, through mixing, detection of weak terahertz signals from deep space. Coherent terahertz radiation has historically been very difficult to generate, and the search for an easy-to-use, compact source continues today. Existing terahertz sources are large, multi-component systems that may require complex vacuum electronics, external pump lasers, and/or cryogenic cooling. A single-component device without any of these limitations is highly desirable for next-generation terahertz systems. ... [read more]
 
70.  
Researchers Realize High-Power, Narrowband Terahertz Source at Room Temperature
Researchers Realize High-Power, Narrowband Terahertz Source at Room Temperature
McCormick News Article - September 27, 2011
Researchers at Northwestern University have developed a simpler way to generate single-chip terahertz radiation, a discovery that could soon allow for more rapid security screening, border protection, high sensitivity biological/chemical analysis, agricultural inspection, and astronomical applications. Coherent terahertz radiation has historically been very difficult to generate, and the search for an easy-to-use, compact source continues today. One possible avenue toward achieving a robust single component terahertz source is to create and mix two mid-infrared laser beams within a single semiconductor chip in the presence of a giant nonlinearity. The researchers at the CQD have incorporated a novel dual-wavelength diffraction grating within the laser cavity to create single mode (narrow spectrum) mid-infrared sources, which in turn has led to very narrow linewidth terahertz emission near 4 terahertz. The initial device yielding powers of 10 microwatts, but Razeghi said her group will continue in hopes of reaching higher power levels: “Theory says that it is possible, and we have all of the tools necessary to realize this potential.” ... [read more]
 

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