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THz Chip, Imager Research Presented at CLEO/QELS
May 2006
LONG BEACH, Calif., May 24, 2006 -- A terahertz (THz) biochip that can instantly identify illicit drugs, a THz imaging system, lightweight laser instruments for measuring concentrations of atmospheric gases, a tabletop optical imaging system that can reveal details smaller than 38 nanometers, and a femtosecond laser pulse that rips electrons from the periphery of molecules: These are among breakthroughs in the science and engineering of photons and light waves that are being presented by researchers from around the world at 2006 CLEO/QELS this week in Long Beach, Calif. The meeting is co-sponsored by the Optical Society of America (OSA), the American Physical Society Division of Laser Science (APS-DLS), and the IEEE Lasers & Electro-Optics Society (IEEE/LEOS).

Researchers include Chi-Kuang Sun, of National Taiwan University, whose team has built a tiny biochip that can instantly identify illicit drugs such as cocaine and amphetamines in their natural powdered form. In the new approach, researchers simply deposit powder in its natural form into a small, rectangular glass-and-plastic biochip containing some electronic components. The deposited powder settles into channels just 20 µm deep, the thickness of just a few red blood cells. Inside the biochip, a small transmitter beams electromagnetic radiation in the terahertz range -- between the microwave and infrared -- to which biomolecules are very sensitive.

By recording how much radiation the powder absorbs over a range of THz frequencies, the researchers obtain distinctive chemical fingerprints of the biomolecules that make up the powder. Using this method, they could distinguish powders of cocaine, which absorbs a maximum of radiation at 0.8 THz, and amphetamine (1.03 THz) from powders of potato starch, flour and lactose (0.53 THz) in only 2 to 5 seconds per scan.

The drugs' distinctive THz signatures makes them possible to detect even if they are mixed with an additional ingredient, such as flour. Present forensics techniques such as gas chromatography, in addition to being potentially bulky, all require tampering with the sample -- for example by vaporizing it or attaching a fluorescent molecule to it. The terahertz technique could be a more efficient alternative. The biochip could also identify specific molecules dissolved in water, which tends to absorb terahertz radiation strongly and obscure the signals from other molecules, for potential applications such as DNA identification in saliva and for molecular biology applications such as studying the folding patterns of proteins, which would be helpful for designing new drugs.

High-speed THz Imager

David Zimdars of Michigan-based Picometrix presented a fast and practical real-world system for terahertz (THz) imaging, which uses a band of electromagnetic radiation between the microwave and infrared spectrum to penetrate objects and look inside them. The new system can inspect a 1-square-meter area with 1.5-mm resolution in less than an hour, while smaller areas take just a few minutes.

NASA engineers have already used the Picometrix design to peer through the layer of spray-on foam insulation on the external fuel tanks of the space shuttle Discovery and inspect it for defects. It's also fast enough to monitor certain high-speed industrial processes. For example, the system can now be used to inspect paper products moving on an assembly line at 4 m/s with 1 mm spacing. The new system works in the "time domain"; it obtains information at different points in time to build up an image.

In the Picometrix system, laser pulses lasting just femtoseconds travel through an optical fiber to deposit energy onto a semiconductor material, which then generates THz radiation. The semiconductor material then aims THz rays at different areas of the object. The researchers expect it to be possible to develop much faster versions of this system for homeland security applications, such as airline screening of passengers and luggage.

Probing the Origin of Cirrus Clouds

Christopher Webster of the Jet Propulsion Laboratory and his colleagues have designed new lightweight laser instruments and made the first-ever in situ measurements of different water isotopes in and out of the clouds from the troposphere to stratospher. Measuring these gases more widely and frequently will give atmospheric researchers insights in studying weather, climate change and other phenomena on Earth and other planets and moons.

The tunable mid-infrared (IR) laser spectrometers produce light in the mid-infrared region, a part of the spectrum to which all atmospheric gases respond in a distinctive fashion. Tuning the lasers to produce light in a particular window of the mid-IR spectrum and recording the colors (wavelengths) that the atmosphere absorbs makes it very easy to obtain direct information on the concentration of gases that are present -- a boon to climate researchers who are accustomed to making limited or painstaking measurements of the upper atmosphere's crucial chemistry.

With the use of a laser spectrometer on NASA's high-altitude WB-57 spacecraft, this information is providing a wealth of data on the origin of cirrus clouds, the wispy masses that play a major role in warming the Earth. Webster has also designed a tunable laser spectrometer for the upcoming Mars Science Laboratory mission set for launch in 2009.

Tabletop microscope

Using state-of-the-art extreme ultraviolet laser technology, Courtney Brewer of Colorado State University and her colleagues have built a tabletop optical imaging system that can reveal details smaller than 38 nanometers (billionths of a meter), a world record for a compact light-based optical microscope. The work was done at Colorado State University in collaboration with the University of California Berkeley as part of the National Science Foundation Engineering Research Center for Extreme Ultraviolet Science and Technology.

The microscope can keenly inspect nanometer-scale devices designed for electronics and other applications. It will also be capable of catching subtle manufacturing defects in ultraminiaturized computer circuits, where defects just 50 nm in size that were once too small to cause trouble could wreak havoc in the nanometer scales of today's computer chips.

Except for some high-tech details, the microscope works very similarly to a conventional optical microscope. Light shines through the sample of interest, and the transmitted light is collected by an "objective zone plate," which forms an image on a CCD detector -- the same kind of device that records images in a digital camera. However, the sub-38-nm microscope uses a laser that produces light in the extreme-ultraviolet (EUV) spectrum, which has a very small wavelength that makes it possible to see tiny details.

The EUV light is created by ablating (boiling away) the surface of a silver or cadmium target material so the vaporized material forms a plasma -- a collection of charged particles -- that radiates laser light. To focus this light, the researchers said they avoid standard lenses, because they strongly absorb EUV radiation. Instead, the microscope uses "diffractive zone plates," structures containing nanometer-spaced concentric rings that focus the light in the desired fashion.

Other state-of-the-art optical microscopes have achieved resolutions as low as 15 nm, but they required the use of large particle accelerators called synchrotrons. This more compact and less expensive system has the potential to become more widely available to researchers and industry. In addition, since the extreme ultraviolet laser produces light pulses with very short duration (4 picoseconds, or trillionths of a second), the researchers believe it may be possible to create picosecond-scale snapshots of important processes in other applications.

Fast Lasers Set Sights on Electrons

David Villeneuve of the National Research Council of Canada and his colleagues have helped pioneer a method in which a femtosecond laser pulse rips electrons from the periphery of molecules. These electrons, feeling the electric field of the pulsed light, are first repelled, then very quickly recalled to their home molecule by the strong fields of the same pulse which, in its quick cycling, reverses direction. The electrons can then recombine into the parent molecule, and in the process emit extreme ultraviolet light of their own. This light can be used to perform a type of "tomographic" imaging of the molecule, or more particularly its orbitals. Thus, the electron is used to image its own domain. Villeneuve is reporting on his latest efforts to map orbitals and progress toward making movies of molecular dynamics.

Meeting topics at CLEO/QELS also include biologically inspired optical polymers, construction of the highest-powered laser in the world, a 3D-2D-3D photonic crystal, using optics to improve accuracy in determining blood flow rate, innovations in solid-state lighting, airborne lidar and satellite comparison of atmospheric aerosols, the light-emitting field-effect transistor, optical magnetic mirror, nanowell structures for sensing tiny particles, an optical lattice clock, entangled photons on demand from a single quantum dot, matter wave optics on an atom chip , solid-state single photon detectors, and nanoplasmonics.

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