Program Directions

• Research on Atomic Properties. We are continuing to use our unique ultra-high resolution Fourier transform spectrometer (FTS) for measurements of spectra of importance for microlithography, lighting industry research, and space astronomy. With our new capability of an extended ultraviolet response we are attacking problems of wavelength calibration of ArF lasers being used for microlithography at 193 nm.

  As part of our ongoing Cooperative Research and Development Agreement (CRADA) with the Electric Power Research Institute (EPRI), the FTS is also being used to measure wavelengths and transition probabilities of spectra of rare earth elements that are used as additives in high-intensity discharge lamps. As part of this CRADA we are also utilizing the Advanced Photon Source at the Argonne National Laboratory to map the spatial distribution of emitters in high intensity discharge (HID) lamps by means of x-ray absorption and fluorescence measurements. HID lamps are often made with translucent envelopes, and it is not possible to obtain this information by conventional optical methods

  On the theoretical side, we are continuing to calculate cross sections for the excitation and ionization of atoms and molecules by electron impact. These cross sections are used in plasma processing of semiconductors and in modeling of tokamak plasmas. New scaling methods are being developed to convert atomic excitation cross sections calculated from simple collision theories into high quality data comparable to the exact solutions that can be obtained for the hydrogen atom.

• Compilations of Atomic Properties. Data centers on atomic spectra located in the Division are the principal resources for such atomic reference data in the world. We are continuing critical evaluations and compilations of wavelengths, atomic energy levels, and transition probabilities. The critical assessments benefit from the experience our scientists have gained through original research and the data that have been obtained in our own laboratory. The compilations are disseminated through published papers and databases on the World Wide Web, as for example the online interactive Atomic Spectroscopic Database, which is queried about 60,000 times per month by outside users, including many technology companies.

• Properties of Nanoscale Systems. Quantum mechanical and electromagnetic methods are being developed and applied for calculating the electronic and optical properties of quantum nanostructures and for modeling the optics of nanoscale objects. Such systems have a wide variety of technological applications, including semiconductor lasers and advanced semiconductor devices. Applications of nano-optics modeling include scanning near field optical microscopy for use in nanometer-scale optical metrology, single molecule spectroscopy and optical nanostructures for novel uses in atom trapping, quantum information and intradevice optical communications.

  Also, an experimental activity to generate and characterize novel types of nanoscale fractures on surfaces is underway at the Electron Beam Ion Trap (EBIT) facility. Here, we are using STM, AFM, and photoluminescence techniques to characterize the response of surfaces to highly charged ion beams. In order to understand the formation and decay of the "hollow atoms" that underlie the production of the nanoscale features, we are using x-ray spectroscopy to observe the surfaces during ion bombardment, in collaboration with the University of Paris, Harvard University, and the University of Stockholm.

• Physics of Cold, Trapped Gases of Neutral Atoms. We are investigating, both experimentally and theoretically, the properties of cold dense gases in the quantum degenerate regime. We are using atom optics techniques to study the properties of Bose-Einstein condensates (BECs). We are investigating the coherent transport of condensate atoms confined in a one-dimensional optical lattice. We are also investigating the formation of molecules from BEC atoms using photo-association techniques. Comprehensive, predictive theoretical models are being developed and tested to understand the experimental results and guide further investigations.

• Quantum Information. We are investigating the feasibility of using ultracold neutral atoms in optical lattices for quantum information studies. The conceptual starting point for such studies is a three-dimensional optical lattice with one atom per lattice site, in the ground vibrational state. Such a system will serve as an initialized register for quantum information processing. In a recently constructed apparatus generating Bose-Einstein condensates of rubidium atoms, we are investigating the adiabatic loading of the atoms into the ground state of the optical lattice as well as the loading of one atom per lattice site. The latter should be achieved through a Mott-insulator transition. We will investigate the factors influencing the decoherence of atoms in the optical lattice, which will determine the fidelity of this system for quantum information processing. By introducing well characterized noise, such as laser intensity noise, we can study decoherence in a controlled manner. In a parallel theoretical effort we are developing models and running simulations of the atoms in an optical lattice in order to understand the experimental results and determine the best strategy for achieving the highest fidelity possible.

• Optical Manipulation of Biological Objects. We are developing techniques for the remote manipulation and control of biological objects. We use lasers to trap cells and microspheres coated with biochemical molecules in order to study bio-molecular interactions. We are investigating the use of lasers for manipulation of liposomes as sub-nanoliter reaction vessels. We are using laser-trapping techniques to bring two liposomes, each encapsulating some biochemical molecules of interest, into contact. A focused UV laser beam is used to remove the interstitial lipid bilayer membranes, allowing the contents of the liposomes to mix and a biochemical reaction to take place. This elementary sequence applied to a sample of liposomes containing a variety of biochemical molecules will allow us to perform combinatorial chemistry with very small quantities of reagents. Applications of these investigations include genetic testing, pharmaceutical development and targeted drug delivery.

• Optical Properties of VUV Materials. Using the unique VUV phase-shifting Twyman-Green interferometer and VUV polarimetry facilities that we developed, we study VUV optical properties, such as stress-induced birefringence, intrinsic birefringence, and index homogeneity of materials important for VUV optics. These at-wavelength measurement capabilities, not presently available to the industry, are recognized as critical for 157 nm photolithography. With the help of these facilities we are also working on solving the intrinsic birefringence problem in 157 nm lens materials that we first pointed out. We are developing, in collaboration with crystal production companies, mixed crystals such as Ca_1-xBa_xF₂ and Ca_1-xSr_xF₂ that we calculate will have no intrinsic birefringence.

• X-Ray and Gamma-ray Measurements. We use crystal diffraction to study the properties of x- and gamma-ray radiation. For measurements requiring high precision, flat diffraction crystals of known lattice spacing are employed in both reflection (Bragg) and transmission (Laue) geometries. The thrusts of this program are x- and -ray wavelength standards, material properties at x- and -ray wavelengths, and the determination of neutron binding energies. For lower-precision measurements, curved crystals with position-sensitive detectors are used. This program will assist spectral studies of highly-ionized species at the NIST EBIT, the calibration of medical radiographic x-ray sources, and x-ray diagnostics for laser fusion plasmas. For the latter, to characterize the hot electron energy distribution of such plasmas, we are designing, fabricating, assembling, and calibrating a cluster of five curved crystal x-ray spectrometers which will be packaged together in one diagnostic module and deployed at the National Ignition Facility at Lawrence Livermore National Laboratory.

• High Resolution X-Ray Probes of Thin-Film Electronic Structures. The sub-nanometer wavelengths of x-ray probes and their relatively weak interactions with materials make them a nearly ideal means for determining the geometry of the thin film and multilayer structures that underlie modern semiconductor manufacturing. We are developing an advanced metrological capability in this area including high performance instrumentation and advanced forms of modeling and analysis. Also, we are establishing a new industrial consortium (the Consortium for High-resolution x-ray Calibration Strategies) to better address the long term needs of users of high resolution x-ray scattering instrumentation in the semiconductor industry, with the core task to deliver NIST-documented prototype calibration samples to consortium members in a timely manner.

• Optical and X-Ray interferometry. We are carrying out intercomparisons of Michelson and Fabyry-Perot interferometry for displacement measurements and observe the effect of diffraction on interferometric measurements. We will also use our expertise to apply advanced interferometric techniques to important problems for which they are well suited. One such project concerns measurement of the coefficient of thermal expansion for low-thermal-expansion materials using Fabry-Perot interferometry, a matter of critical importance to next-generation EUV lithography.