NIST Researchers Uncover Missing Light
John Lehman, Katherine Hurst, and Lara Roberson, along with collaborators Kathryn Nield and John Hamlin of New Zealand’s Measurement Standards Laboratory, have identified unique features in the reflectance spectra of single-wall carbon nanotubes (SWCNT) that were previously misidentified in the literature as absorbance. In the process, they have demonstrated a novel method for determining SNCWT absorbance that, unlike other published accounts, is independent of sample concentration and does not rely on data post-processing to remove the effects of contaminants. Prior to this work, other published results for SWCNT absorbance relied on transmittance measurements of SWCNTs in solution using spectrophotometers. The spectrophotometer method assumes that the reflectance of the sample is negligible and that the absorbance is complementary to the transmittance (A = 1 – T). The NIST results, which appeared in the August 7, 2008, issue of the Journal of Physical Chemistry C, demonstrate that reflectance is not negligible and in fact dominates absorbance near excitonic transitions. The significance of the measurement results is that current documentary standard efforts to assess SWCNT purity based on transmittance spectra may lead to misinterpretation of SWCNT optical properties; e.g., emissivity and dielectric function.

NIST Researchers Receive Award for Excellence in Technology Transfer
EEEL Researchers Chris Cromer, John Lehman, and Xiaoyu Li have received the 2008 Award for Excellence in Technology Transfer by the Federal Laboratory Consortium for Technology Transfer (FLC). The FLC award recognizes federal laboratory employees who have accomplished outstanding work in the process of transferring a technology to the commercial marketplace. Cromer, Lehman, and Li were recognized for their pioneering work in commercializing optical trap detectors for laser power measurements. These optical trap detectors represent the state-of-the-art in calibration transfer standards for laser power measurements and are critical to obtaining traceability through NIST calibration services. The novel trap design allows the measurement of a variety of optical beam geometries, thus supporting diverse optical sources such as laser beams, optical fibers, light emitting diodes (LEDs), and lamps, providing improved accuracy while increasing the tolerance to optical beam variations. The device has successfully decreased metrological uncertainty by National Metrology Institutes worldwide. Recently, the NIST-designed detector was successfully commercialized by Spectrum Detector, Inc., working in collaboration with the EEEL researchers. The commercial availability of a metrology-grade trap-detector transfer will allow companies that manufacture and use laser power and energy meters to obtain stable, high-accuracy transfer standards, at a reasonable price. Using these advanced detectors for NIST calibrations to obtain optical power traceability will enable a widespread improvement in laser power calibrations by decreasing the uncertainty associated with the detector.

Igor Vayshenker Receives Allen V. Astin Award
Igor Vayshenker, Calibration Leader of the NIST Optical Fiber Power Laboratory, received the Allen V. Astin Award, which is given annually to recognize outstanding achievement in the advancement of measurement science. Mr. Vayshenker was recognized for his leadership in developing and providing modern standards and measurement services for optical fiber power. Optical fiber power meters are one of the most common forms of test equipment for optical fiber communication networks. For nearly two decades, Mr. Vayshenker has provided critical support to the optoelectronics and telecommunications industries through his measurement services, directly impacting every American who makes a long distance phone call or uses the Internet. In establishing the Optical Fiber Power Laboratory, Mr. Vayshenker has created a traceability chain for optical fiber power meters. This chain includes absolute measurements of optical fiber power and detector linearity. Industry experts around the world consider Mr. Vayshenker’s work crucial for their companies. In addition to his research, Mr. Vayshenker has led several national and international comparisons of optical fiber power measurements, establishing a worldwide network for optical fiber power measurements.

New Measurement Technique for Carbon Nanotubes Properties
Katie Hurst, John Lehman and collaborators have developed a new technique for measuring the recombination lifetimes of carbon nanotubes (CNT) called the resonant-coupled photoconductive decay (RCPCD) method. The carrier recombination lifetime is a fundamental property of carbon nanotubes that is typically determined by contact-based techniques or spectroscopic methods. These methods do not readily allow for the characterization of bulk material properties. The new measurement technique is based on a pump-probe technique (1) in which an optical pump and a low frequency microwave probe are employed. RCPCD offers the first rapid, non-contact technique for routine nanometrology of carbon nanotube electronic properties.

The researchers conducted measurements of carrier lifetimes for multi-walled and single-walled CNT thin films. The thin films were made by depositing a ~30 micrometer thick CNT coating on a glass slide by an air-brush technique. The researchers also considered the influence of material purity on the measurement of lifetimes in these nano-scale systems. Raman spectroscopy and UV-VIS absorption measurements provide further identification and characterization of nanotube samples to enable correlation of nanotube properties with the efficiency of charge transport in these samples. RCPCD is shown to be a fast and effective method for measuring the lifetimes of bulk carbon nanotubes, thereby overcoming present issues of routine carbon nanotube electronic nanometrology. This work was presented at the recent 54th American Vacuum Society International Symposium and Exhibition in Seattle.
(1) R.K. Ahrenkiel, S.W. Johnston Mater. Sci. Eng. B 102 (2003) 161.

New Model for Pulse Oximetry
Shao Yang along with collaborators Paul Batchelder and Dena Raley of Clinimark, a Boulder-based company that specializes in accurate pulse oximetry measurements, have developed a new model of pulse oximetry that addresses the disagreement between theoretical calibration curves based on Beer-Lambert’s Law and test results on human subjects. This work was published in the December 2007 issue of the Journal of Clinical Monitoring and Computing. Pulse oximeters provide a noninvasive measurement of the concentration of hemoglobin with bonded oxygen in arterial blood. Since their introduction, pulse oximeters have become an indispensible means of monitoring a patient’s oxygen levels in operating rooms, emergency rooms, and intensive care units. A pulse oximeter consists of two optical sources, operating at two different wavelengths, and an optical detector. Traditional calibration curves assume a one-dimensional optical path to describe how light is transmitted through tissue. The new model is a two-dimensional model that takes into account the effect of other tissue outside of the blood vessels on the transmitted light. The new model is in good agreement with results from human test subjects, while the old model underestimates the arterial oxygen saturation levels as measured using human test subjects. Currently, pulse oximeters are validated by means of measurements on human test subjects. In these measurements, human test subjects undergo a rigorously monitored process where the arterial oxygen saturation levels are gradually reduced. Because of the dangers related to hypoxia, it is not possible to record oxygen saturation level measurements in human subjects below 60 %.

Rapid, Inexpensive Identification of Bulk Carbon Nanotubes
Katie Hurst and John Lehman, along with collaborators Anne Dillon and Jeff Blackburn from the National Renewable Energy Laboratory, have estimated the metal-to-semiconductor ratio of bulk carbon nanotubes using an effective medium approximation and the measured spectral responsivity of a LiTaO3 pyroelectric detector as a fixed platform for single-wall carbon nanotubes (SWNTs). This is the first such demonstration of a simple quantitative method for determining the metal-to-semiconductor ratio for solid-state forms of SWNTs. Carbon nanotubes are revolutionary materials, exhibiting properties that are vastly different than any bulk form of carbon. They have valuable electrical, optical, mechanical, and thermal characteristics, due to their unique quasi-one-dimensional electron confinement. At least 50 different SWNT species have been identified, over half of which are semiconducting. Current manufacturing processes do not produce a single type of SWNT. Instead, these processes produce an unknown mixture of metallic and semiconducting SWNT species. As a result, technologists confront SWNT quality issues at every level, ranging from composite manufacturers integrating SWNTs into high strength structures, to scientists who dream of the next generation of optical sources, detectors, and displays. Advanced, cost effective analytical techniques are needed so that carbon nanotube manufacturers, product developers, and regulatory agencies can truly "see" what they have.

Despite the visibly black appearance of bulk SWNTs, their dielectric properties yield variations in the absorption coefficient. This is due to features such as chirality, diameter, purity, and bulk topology. By measuring the detector responsivity, the team takes advantage of a relatively large specular absorptance at normal incidence, which is easily modeled from simplified Fresnel equations for normal incidence. Further analysis reveals that a change in responsivity as small as 4 % is manifested by a change in ratio of metallic-to-semiconducting SWNTs of approximately 10 %. The relative expanded uncertainty of their measured responsivity is 1.24 %. The fixed platform spectral responsivity method has an advantage over other quantitative methods which require samples of SWNTs in solution. Arguably, evaluation of SWNTs is less repeatable over time if the SWNTs fall out of suspension. SWNTs on a fixed platform such as a pyroelectric detector are repeatable over time and are compatible with other qualitative measurement techniques such as Raman spectroscopy.

Excimer Laser Treatment Process for Carbon Nanotube Coatings
John Lehman, Katie Hurst, Darryl Keenan, Natalia Varaksa and Stephen Russek have demonstrated an excimer laser treatment process for carbon nanotube (CNT) coatings. The researchers documented their results through a series of scanning electron microscope (SEM) images and spectral responsivity measurements on both multi-wall and single-wall carbon nanotubes. The multi-wall carbon nanotubes were grown by chemical vapor deposition on substrates made of either copper or lithium niobate. The single-wall carbon nanotubes were deposited on silicon carbide, quartz, and lithium tantalate. These treatment studies have implications not only for enhancement and evaluation of thermal detector coatings, but for other CNT applications such as cleaning, purification, and production of CNT-based field emitters. This work is part of a larger program to develop the next generation of optical coatings based on CNT technology. The CNT coatings, which have been deposited on a variety of optical detector platforms, exhibit remarkable thermal and mechanical properties making them ideal coatings for absolute standards of laser power and energy measurements. These properties include resistance to damage and aging while maintaining desirable optical and thermal properties over the broad range of laser wavelengths (0.157 μm to 10.6 μm) served by the Optoelectronics Division's calibration services for laser power and energy.