to develop novel optical measurement methods for
solving problems in critical and emerging technology areas.
INTENDED OUTCOME AND BACKGROUND
The Division strives to improve the accuracy,
quality, and utility of optical measurements
in burgeoning technology areas, such as nanotechnology, biological and medical physics, climate change,
quantum information, and national and homeland security.
In the area of nanotechnology, metal and magnetic nanoparticles, quantum dots, and nanoshells are being
developed for use as quantitative probes to interrogate and manipulate chemical, physical, and biological
phenomena within complex biological systems. Raman spectroscopy is being applied to the determination of the
size homogeneity of carbon nanotubes separated using advanced chromographic methods. Such well characterized
pure samples have the potential to serve as standards for researchers exploring applications of
carbon nanotubes which have size-specific chemical and physical properties.
The Division has a strong biophysics program targeting the development of the critical measurement science
infrastructure for systems biology. Spectroscopy, microscopy, and other optical technologies are being developed
to characterize and control interactions between biomolecules.
Such technologies include single-molecule microscopy, fluorescence resonance energy transfer (FRET), optically
trapped hydrosomes containing single biological molecules, and THz spectroscopy.
The THz spectroscopic measurements provide benchmark quantitative measurements of the large-amplitude
vibrational modes in biomolecules important for folding and function. These benchmark measurements are
compared against state-of-the-art molecular models widely used within the chemical, biotechnology, and
pharmaceutical
industries. Complementing these molecular and cellular-scale technologies, hyperspectral imaging and optical
scatterometryare being developed for macroscopic
imaging of tissue to improve cancer diagnosis, guide surgical procedures, and improve surgical outcomes.
Climate-change research places some of the most stringent demands on optical radiation measurement due to the
need to quantify extremely small changes in the average incident solar radiation, reflected solar radiation,
and outgoing infrared radiation over a decadal time scale. In response to these measurement demands, the
Division has developed expertise in space sensor calibration and standards in support of the satellite programs
of NASA, NOAA, and USGS. The Division also works with land-and sea-based sensor programs to help ensure
measurement accuracy and quality.
The Division has a long history of supporting
our Nation’s national defense by working with the Calibration Coordination
Group of the Department of Defense to ensure that the standards needs of the military are met in the area of
optical radiation measurement. We have developed specialized calibration chambers to mimic the cold thermal
background of space to ensure the comparability and accuracy of the sensor measurements of the Missile Defense
Agency and its aerospace contractors. Correlated-photon sources are being developed to absolutely calibrate
photon-counting detectors with applications to quantum communication and quantum cryptography.
Such technology may eventually allow all of the Division’s fundamental radiation measurements to be tied to
quantum-based standards.
Accomplishments
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A Practical Method for Spectral and Spatial Stray-Light Correction in Optical
Measurement Systems
The Division has developed a simple and effective method to correct array spectroradiometers
for spectral stray-light errors. Such spectroradiometers are increasingly used in remote sensing, photometry,
colorimetry, and radiometry due to their low cost, compact size, and high measurement speed.
However, their measurement accuracy is often less than conventional monochromator-based systems due to the
presence of significant spectral stray light. Such stray light increases the detected light intensity at
wavelengths where the relative output radiation from the viewed source is low, leading to significant
measurement error.
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Figure 4.The stray-light distribution function (SDF)
of a typical CCD-based spectroradiometer. |
To implement the NIST stray-light correction
method, the spectroradiometer first measures several spectrally narrow laser lines
distributed over the wavelength range of the instrument. Due to stray-light effects, the spectroradiometer
will detect light not only at the wavelength corresponding to the laser wavelength, but also weakly in channels
corresponding to other wavelengths. Typical data are shown in Fig. 4.
From these data we derive a correction matrix,which is used to reduce measurement error.
This method has been shown to reduce the effect of stray light in commercial
array spectroradiometers by as much as two orders of magnitude, leading to a dramatic improvement in
measurement accuracy.
Follow-on Satellite Instrument Calibration Conference
NIST, through its U.S. Measurement System
Initiative, has cosponsored a follow-on conference to the highly successful “Satellite
Instrument Calibration for Measuring Global Climate Change” conference held at the University of Maryland in
November of 2002. The first conference discussed the requirements for achieving satellite measurements with
sufficient accuracy and long-term precision to monitor climate change. The report has been widely disseminated
as NISTIR 7047.
The follow-on conference, “Achieving Satellite
Instrument Calibration for Climate Change” or ASIC3 held in May 2006, was organized by the Space Dynamics
Laboratory of the Utah State University with additional cosponsorship by the National Oceanic and Atmospheric
Administration (NOAA) and the National Polar-orbiting Operational Environmental Satellite System
(NPOESS) office. The primary goal of the conference was to develop a calibration
science strategy to achieve the measurement
requirements determined in the first conference. Breakout groups addressed the following areas: Infrared Sensors; Ultraviolet, Visible, and Near Infrared Sensors;
Microwave Sensors; Active Sensors; Broadband Sensors; Sensor Intercalibration;
and a National Roadmap for Satellite Calibration.
The second report has two overarching recommendations. The first recommendation
is that satellite benchmark missions be initiated to measure Earth’s radiation budget, including Earth’s
spectral emission, its reflectance, and total solar irradiance. The second recommendation is for NIST, NOAA,
and NASA to establish a National Calibration Center to ensure climate-quality measurements in operational
and research satellite missions.
Quantum Dot Method Rapidly Identifies Bacteria
A rapid method for detecting and identifying
very small numbers of diverse bacteria, from anthrax to E. coli, has been developed
in collaboration with scientists from the National Cancer Institute (NCI). The work, described in Proceedings
of the National Academy of Sciences, could lead to the development of handheld devices for the rapid
identification of biological weapons and antibiotic-resistant or virulent strains
of bacteria.
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Figure 5.Summary of the new method. A bacterium bursts following infection with a phage.
The phages have been genetically engineered to express biotin on their surface.
The biotinylated phages have an affinity for quantum dots labeled with streptavidin.
The quantum dots fluoresce when illuminated as shown by the bright white spot in the square image. |
Traditional ways of identifying infectious bacteria are time consuming and laborious,
requiring the isolation and growth of a bacterial culture over hours to days. The new method speeds up the
process by using fast-replicating viruses (called bacteriophages or phages) that infect specific
bacteria of interest and are genetically engineered to bind to “quantum dots.” Quantum dots are nanoscale
semiconductor particles that provide an intense fluorescent signal and are less prone to deterioration than
conventional molecular tags when illuminated.
The phages were genetically engineered to produce a specific protein on their surface. When these phages infect
bacteria and reproduce, the bacteria burst and release many phage progeny attached to biotin which is present
in all living cells. The biotin-capped phages selectively attract specially treated quantum dots, which absorb
light efficiently over a wide frequency range and re-emit it in a single color that depends on particle size.
The resulting phage-quantum dot complexes can be detected and counted using microscopy, spectroscopy, or flow
cytometry, and the results used to identify the bacteria.
The new method, summarized graphically in Fig. 5, can detect and identify as few as ten target bacterial cells
per milliliter of sample in less than an hour. It is more sensitive than conventional optical methods,
and it can count how many viruses are infecting a single bacteria cell or how many quantum dots are attached
to a single virus.
The method could be extended to identify multiple bacterial strains simultaneously by pairing
different phages
with quantum dots that have different emission colors.
A provisional patent application was filed through NIST, and more recently a nonprovisional patent application
was filed through the National Institutes of Health. The NIST contributions to the work include experimental
design and fluorescence imaging. The non-NIST collaborators are from NCI, NIH, SAIC, and the National Cancer Institute.
This research spun off multiple new collaborations with agencies outside NIST, including the Navy Medical Research Center (a novel, fieldable method to detect biowarfare agents) and the Nanotechnology Characterization Laboratory
of the National Cancer Institute (flowcytometry standards).
Magnetic Nanoparticles Assembled into Long Chains
Division scientists have controllably assembled
and disassembled chains of magnetic nanoparticles suspended in a solution. The chains were composed of millions
of 12 nm diameter cobalt nanoparticles. Such particles and their chain structures may have applications in
medical imaging, hyperthermia treatment for cancer, and information storage, provided that techniques are
available to manipulate and control their physical and chemical properties and to assemble them into structures.
The NIST work is the first to demonstrate the formation and control of centimeter-long chains of magnetic
nanoparticles of a consistent size and quality in a solution.
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Figure 6.Tunneling electron microscope (TEM) image of 12 nm diameter surfactant-coated cobalt
nanoparticles ordered by magnet dipole-dipole couplings.
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The nanoparticles were induced to form linear chains by subjecting them to a weak
magnetic field—about the same strength as a refrigerator magnet. The particles line up because the
nanoparticles act like tiny bar magnets, all facing the same direction as the applied field. Once this
alignment occurs, the attraction between particles is so strong that reversing the direction of the applied
magnetic field causes the whole chain to rotate 180 degrees. When the magnetic field is turned off,
the chains fold into three-dimensional coils. When the solution is lightly shaken, the chains fall apart into
small rings. The chains were characterized by optical and transmission electron microscopy (TEM). A TEM image
is shown in Fig. 6.
Magnetic particles have already been used in medical imaging and information
storage, and nano-sized particles may offer unique or improved properties. For example, magnetic nanoparticle
dyes may improve contrast between healthy and diseased tissue in magnetic resonance imaging
(MRI), a possibility under study by a different NIST research group. Research is ongoing to improve the
biocompatibility of these magnetic nanoparticles.
A Highly Efficient Two-Photon Source
Division scientists have developed an approach to efficiently create pairs of photons over a wide range of
energy, while minimizing the production of extraneous photons. The approach promises to benefit applications
in physics and technology such as quantum information and telecommunications.
Paired photons can be generated from a monochromatic light source—albeit very inefficiently—in standard
optical media such as glass optical fibers. Most photons normally travel through glass independently,
without interacting. Occasionally, two of the input photons will interact, producing an output photon pair
with one higher in energy than the original photons and the other lower in energy by the same amount.
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Figure 7.A microstructured optical fiber in NIST’s new correlated-photon source delivers high numbers of
photon pairs over a broad spectral bandwidth with low noise in a compact device for quantum communication applications.
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Because the vast majority of photons go through the fiber unchanged, the relative intensity of these pairs is
low. Since the fiber generates the pairs randomly with a range of possible energies, selecting those photons
with some specific energy further reduces the number of useful photon pairs. Typically,
the photon pairs in the system must be detected against the photon noise in the system due primarily to Raman
scattering. In Raman scattering individual photons interact nonlinearly with the phonon modes of the glass to
change their energies. Such scattering produces undesirable photons with properties that mimic one of the
photons of a photon pair.
To increase the efficiency of photon pair production while minimizing noise from the extraneous Raman photons,
the NIST two-photon source relies on a microstructured optical fiber. The fiber has a slender glass core at the
center of an array of hollow channels, giving a cross-section that resembles a honeycomb. The geometrical
structure of the fiber affects the optical modes of the fiber, leading to an increase in the intensity of
light in the thin central core with a concomitant enhancement of the photon pair production.
This greater pair-production efficiency reduces the length of optical fiber required, from hundreds of meters to
a couple of meters. Moreover, optimization of the size of the channels in the microstructured fiber allows
reduction of the amount of Raman scattering relative to the desired two-photon light production.
The result is a source that produces significantly more pairs of photons over a wide frequency range,
with greatly reduced contamination by spurious Raman
photons. A picture of the source is shown in Fig. 7.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2005-2007" - Table of Contents
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