By Patricia Daukantas

To wind up OPN’s coverage of CLEO/QELS 2010, I would like to spotlight some of the interesting things that I didn’t get a chance to write about during the conference.

Weather Guy to Lidar Specialists: Please Help

A meteorologist at California State University at Chico (U.S.A.) presented a list of opportunities for lidar researchers to help improve his group’s technology for studying atmospheric aerosols.

Shane D. Mayor uses a direct-detection infrared lidar instrument dubbed REAL (for Raman-shifted Eye-safe Aerosol Lidar) to study how particulate matter moves with air currents. He has participated in several simulations of bacterial agent plumes – this knowledge could be important in the case of a biological weapons attack.

REAL operates at 1,543 nm, which Mayor said is in the “sweet spot” between shorter-wavelength retinal hazards and insufficient detector performance above 2 µm. The 1.5-µm zone also offers such desirable qualities as low molecular scattering, low background radiation from the sky, and compatibility with telecom components. Mayor and a colleague designed a Raman shifter for converting the REAL Nd:YAG laser output from 1,064 nm to 1,543 nm (Appl. Opt. 46, 2990).

Unfortunately, the flashlamps on REAL’s pump laser need replacement every 20 million shots or 23 days – at $200 each, that amounts to $6,350 per year, Mayor noted. Also, the CSU-Chico lidar setup requires a tractor-trailer for transportation, but Mayor’s goal is to shrink that down to fit in a more mobile van.

Mayor listed a number of ways that optical scientists could help improve REAL. They include:

• Reduce or eliminate flashlamp replacement.
• Increase efficiency by reducing power consumption and waste heat.
• Further reduce the mass of the beam steering unit mirrors, which are now 14 kg each.
• Develop a high-precision pulse energy monitor that can measure shots to ± 1 percent.
• Figure out how to monitor beam divergence continuously on the fly.

What Makes Counterfeit Money Funny?

Can measuring the intrinsic fluorescence lifetime of U.S. paper money distinguish between phony bills and the real thing? Researchers from Yale University (U.S.A.) believe that this technique could be used for forensic identification of counterfeit money.

The key, according to biomedical engineers Michael J. Levene and Thomas Chia, is that the paper for all U.S. currency comes from a single source. Although the exact “recipe” for that paper is secret, it has a very consistent fluorescence lifetime “signature” that differs from that of other types of papers made from wood, cotton and linen pulp. U.S. currency ink is essentially non-fluorescent, although the serial numbers on bills scatter light. (Inks on some non-U.S. currency, like the Mexican 100-peso note, do fluoresce, so that could be important in detecting foreign fakes.)

The researchers used a custom-built two-photon microscope, with an excitation wavelength of 735 nm, to study U.S. currency – mostly $100 banknotes, since they are the highest-valued bills targeted by counterfeiters. (They also tested some lower denominations as a control group.) They also tested three kinds of counterfeit bills provided to them by investigators: digital scans onto printer paper, counterfeit bills printed on cotton-linen-blend paper, and so-called “bleached” low-denomination bills that were illicitly reprinted with a higher denomination.

Levene and Chia didn’t know the exact provenance of the counterfeit money. “They [investigators] don’t like us to hold onto the bills for more than a few hours and won’t tell us much about where they came from, and we’re not going to make our own,” Levene said.

All the genuine currency notes had consistent short- and long-lifetime components to their fluorescence. The printer-paper fakes had only the longer-lifetime component. Other counterfeit bills had noticeably shorter long-lifetime components.

The testing group included bills dating back to the 1970s, and because the United States has been using the same paper supplier for so many decades, the two-component intrinsic fluorescence lifetime signature is “remarkably consistent” over the years, Levene said.

Small and Big Lasers

Qi Qin of the Massachusetts Institute of Technology (U.S.A.) and colleagues at two other labs built a tunable terahertz “wire laser” whose cavity is much narrower than its operating wavelength. The researchers tuned the laser by moving either a metal or dielectric “plunger” outside the laser cavity. (The gold plunger shifted the wavelength shorter and the silicon plunger made the operating wavelength longer.)

The group’s first design, as reported in the original CLEO proceedings, achieved 137 GHz of tuning centered on 3.8 THz. To get rid of the static friction that made the plunger stick and jump, they designed a MEMS-type plunger made up of layers of gold, silicon and silicon dioxide. The revised laser, only 10.5 µm wide, registered a total shift of 330 GHz between 3.85 and 4.2 THz, or about 8.5 percent. Such lasers could be used to detect explosives, which have spectroscopic “fingerprints” in the terahertz range, according to Qin.

On the opposite end of the laser size spectrum, Textron Defense Systems (U.S.A.) is building a 100-kW laser as part of the Pentagon’s Joint High Power Solid State Laser Program. Invited speaker Alex Mandl traced the history of Textron’s efforts from its initial “membership in the kilowatt club” (1.2 kW achieved in February 2004) to its laser’s performance of more than 100 kW in final government tests (exactly how much more, he couldn’t divulge).

Textron calls its technology ThinZag because the beam path inside the laser zigzags through a comparatively thin slab of ceramic (not crystalline) Nd:YAG material. The final laser configuration consists of six ThinZag 15-kW-class lasers in series (yes, 15 × 6 = 90, but again, there may have been other technological tweaks to get it over the 100-kW mark).

Social Media and Postdeadline Papers

I would like to tip my hat to the four CLEO/QELS bloggers – Jim van Howe, Ksenia Dolgaleva, Xiaoyu Miao and David Nugent – who have been contributing to the conference’s social media hub. If you haven’t done so already, please check out their coverage of CLEO/QELS.

Van Howe, a professor at Augustana College in Rock Island, Ill. (U.S.A.), blogged about a couple of postdeadline papers I missed because the room was full and the entryway was clogged. (The paper numbers, though, were QPDA5 and QPDA6.) Since those papers seemed to generate a lot of buzz, I’ll summarize them here.

The group that presented QPDA5, from Yale University (U.S.A.), said that an arbitrary body can be made perfectly absorbing at discrete frequencies, thanks to the interaction of optical absorption and wave interference. “It is thus the time-reversed process of lasing at threshold,” A. Douglas Stone and colleagues wrote.

In QPDA6, Evgenii Narimanov of Purdue University and two colleagues from Norfolk State University (all U.S.A.) found a new approach to the “blacker than black” phenomenon of radiation absorption: something called hyperbolic metamaterials. Hyperbolic dispersion means that a metamaterial has negative electric permittivity in the direction perpendicular to its surface and positive electric permittivity parallel to its surface. The researchers tested their ideas by building an experimental array of silver nanowires.

In the postdeadline session where I did find a space to put myself, Aleksandr Biberman of Columbia University (U.S.A.) described his group’s demonstration of a 40-Gbps electro-optic switch for photonic networks-on-chip (paper CPDA11). Such CMOS-compatible switches will be needed as more photonic networks are built inside the computer as well as between computers. Biberman worked with researchers from both Columbia and Cornell University (U.S.A.).

2010-05 May, CLEO/QELS, Lasers cleo, cleo10, lasers, lidar, metamaterials, social media, optics, atmospheric optics, terahertz technology, biomedical optics, laserfest