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.).

By Patricia Daukantas

I thought I’d give my readers a quick summary of some of the interesting sessions I’ve been attending at CLEO/QELS and CLEO:Applications.

As you might expect, both terahertz technology and metamaterials are really hot topics right now. Put them together and you get a well-attended session on terahertz metamaterials. Hui Tao, Richard D. Averitt and colleagues at Boston University (U.S.A.) built structurally reconfigurable metamaterials using the techniques of micro-electro-mechanical-systems (MEMS) fabrication.

Basically, the BU team built an array of tiny split-ring resonators (SRRs), 72 x 72 m m in size, with an in-plane periodicity of 100 m m. These things are made of bianisotropic gold-silicon nitride and they tilt up and down like an array of little flaps, thus controlling the beam of terahertz radiation passing through the metamaterial. (Note: Tao recently finished his Ph.D. and is a postdoctoral fellow at Tufts University, also in Massachusetts, U.S.A.)

Also on the topic of metamaterials, a German group led by Benjamin Reinhard of the University of Kaiserslautern studied surface terahertz surface waves on an array of copper SRRs. Their experiment’s unit size was 41 x 41 m m, or about 1/7 of a wavelength at 1 THz, and every sixth row of the array was removed. They modeled the metamaterial as a thin-slab waveguide and found that their experimental (See the May 1 issue of Optics Letters -- vol. 35, p. 1320 -- for more details on their work.)

I always enjoy the Market Focus sessions (PhAST in past years, CLEO:Applications in 2010), and this year’s have been no exception.

At an energy-related session, Corey Dunsky, president of Aeos Consulting Inc. (U.S.A.), made some powerful arguments that lasers can help lower manufacturing costs and improve the conversion efficiency of photovoltaic panels. In one example he offered, single-step laser doping is well-positioned to be the best way of fabricating selective emitters on the surfaces of crystalline silicon solar panels. Andrew Masters, a vice president of Veeco Instruments (U.S.A.), described how his company’s atomic force microscopes can quantify solar cells’ efficiency to within plus-or-minus 0.5 percent.

Another session of Market Focus examined various optical methods for detecting chemical threats and explosives.

Price Kagey of Surface Optics Corp. (U.S.A.) reviewed a whole list of technologies -- laser-induced breakdown spectroscopy, Raman spectroscopy, quantum-cascade-laser-based heating and thermography, broadband heating and emission spectroscopy, and hyperspectral imaging -- and found that all of them came up short in one way or another (cost, eye safety, ease of remote deployment, etc.).

Dan Strellis of Rapiscan Systems Ltd. (United Kingdom) said his company is working with both terahertz and X-ray technologies to develop a scanner specifically for shoes, so that airline passengers wouldn’t have to take off their shoes at security checkpoints. According to Adam Bingham of ICx Technologies (U.S.A.), workable systems for detecting explosives at checkpoints will probably require pairs of systems, one for wide-area scanning and a different kind of detector for zooming in to provide “high-confidence confirmation.”

Today (Thursday in California) is the final day that CLEO’s exhibit hall is open, so I’ll be making a final swing through there -- and getting one last glimpse of the laser history exhibit (more on that shortly). Tonight features the three concurrent CLEO/QELS postdeadline paper sessions.

One thing I’ve forgotten to mention in previous posts is a special LaserFest presentation: reprints of a collection of early quantum-electronics papers from the Soviet Union. It’s titled “Beginning of the Laser Era in the USSR,” and some of the articles have been translated into English for the first time. You can find it on the LaserFest website, Optics InfoBase or

By Patricia Daukantas

Most OSA members, I’m sure, are aware that negative-refractive-index metamaterials have been a hot topic of research for the past few years. Although scientists have found success making materials “invisible” to microwaves, several teams have been racing to extend these materials into the optical range – a development that would have much more interesting applications.

This week, two new papers from a California-based team hit the news wires. The researchers, based at the University of California at Berkeley and Lawrence Berkeley National Laboratory, made a prism out of optically negative-index metamaterial that closely resembles layers of tiny fishnets, according to the letter that appeared in Nature.

Xiang Zhang’s group made their metamaterial out of a stack of alternating layers of silver and magnesium fluoride; the size of the “fishnet” cells was on the order of a few hundred nanometers. The scientists tested the prism’s refractive index at near-infrared wavelengths from 1,200 to 1,800 nm and found that the index went to zero at around 1,475 nm.

In the brief Science paper, Zhang and colleagues reported on a different experiment that refracted red light via an aluminum-oxide array of nanometer-sized holes filled with silver. Negative refraction, however, happened only for transverse magnetic polarized light, not for transverse electric polarized light.

Journalistic interest, of course, is driven by the age-old human dream of invisibility, never mind the cloak and cloaking devices in the Harry Potter and “Star Trek” tales. Fortunately, some of the news coverage took pains to explain that practical devices made from negative-index metamaterials are still many years in the future.

By Patricia Daukantas

Despite all the history-making world and national news going on, optics made the front page of this morning’s Washington Post. An article by staff writer Rick Weiss highlights both the black-material research going on at Rensselaer Polytechnic Institute (RPI) and the “invisibility-cloak” metamaterials studies by several other teams of scientists.

Shawn-Yu Lin, a physics professor at RPI, and his colleagues recently created a thin material that absorbs more than 99.9 percent of incident light. Their research was published last month in the journal Nano Letters.

The RPI technique is different from the femtosecond-laser-ablation technique developed by Chunlei Guo and Anatoliy Vorobyev at the University of Rochester (see last week's blog post). Lin’s team used low-density arrays of vertically aligned carbon nanotubes to create the light-absorbing material.

I was amused to read Weiss’s metaphorical description of the blackest-black material as “a roach motel for photons—light checks in, but it never checks out.” The Post article goes on to describe recent research in the invisibility properties of metamaterials and quotes both OSA Fellow Vladimir Shalaev and 2007 CLEO/QELS plenary speaker Sir John Pendry.

By Patricia Daukantas

Metals are going multicolored, thanks to the femtosecond-laser-ablation technique developed by Chunlei Guo at the University of Rochester. Without paint, aluminum and platinum can look just like gold—or like a blue sky.

Guo, an assistant professor of optics, and postdoctoral fellow Anatoliy Vorobyev have been studying the roughness of metallic surfaces at the nanoscale level. Their research on precision control of the surface structures at the nanometer scale led to the new colorizing technique.

The February 2007 issue of OPN described the previous work of Guo and Vorobyev in blackening gold surfaces. However, the multicolor technique is a significant advance over the research that preceded it.

The Rochester team’s original “black metal” surfaces absorbed all wavelengths of light. However, “the key to the coloring technique is that we now can control the spectral responsivity of metals,” Guo said.

In other words, the researchers change the size of the tiny, polarization-dependent structures etched onto the metallic surfaces in order to change the surface colors.

“The most significant step forward is that we now have a precise control in the range of nanostructure sizes formed, and these nanostructures can affect the light response of the metal surface and give us the appearance of a color or range of colors,” Guo said.

The researchers control the size of the region that gets colored by varying the width of the focused laser beam, which is typically 100 microns to 1 mm wide. A narrowly focused beam produces a tinier area of color change, opening the potential for using the technique to etch multicolored patterns on metallic surfaces.

Guo and Vorobyev have produced solid gold, blue and gray colors on the surfaces of aluminum, platinum, titanium, tungsten and silver. In addition, the researchers have produced iridescent surfaces, which make a metal appear variously colored at different viewing angles.

The research was recently published in Applied Physics Letters and also garnered attention with an article in the New York Times.