Research Horizons

Ultracold Molecules

In recent years, the scientific community has begun to recognize the potential of ultracold molecules (at temperatures less than 1 millikelvin). The manipulation of such molecules could lead to new advances in chemistry while paving the way for novel frequency standards, high-precision spectroscopy, and quantum computation and cryptography. Aerospace has significant expertise in laser-based cooling of atoms; however, molecules have more complex internal energy structures, and are therefore much harder to cool directly using a direct laser. To overcome this difficulty, Aerospace has been exploring new physical concepts and techniques. Initial efforts have yielded promising results.

For example, Aerospace achieved the simultaneous laser cooling and trapping of both rubidium (Rb) and cesium (Cs) atoms in a dual-species magneto-optical trap (dual MOT). According to He Wang of the Photonics Technology Department, the dual MOT employs the same technology that Aerospace uses to make cold-cesium atomic clocks. It uses laser beams at two different wavelengths and an inhomogeneous magnetic field to trap a cloud of two atomic species in a tiny volume of about 1 cubic millimeter. The two species are then mixed and simultaneously cooled to an extremely low temperature of about 100 microkelvin, at which point they could form ultracold diatomic molecules.

The ultrahigh-vacuum chamber where the dual MOT and ultracold molecules are produced.

In the course of this research, Aerospace scientists first constructed the basic lab apparatus and performed a theoretical analysis, which served as the guide to the experiment. Next, they developed a two-photon ionization time-of-flight mass spectrometer and demonstrated the production of ultracold RbCs molecules, Rb dimers, and Cs dimers in the dual MOT. According to Wang, the direct detection of ultracold heteronuclear molecules in the lab represented a breakthrough in the field. "The experimental demonstration of three cold-molecule species places Aerospace among only a few laboratories in the world to have prepared microkelvin molecular samples in the laboratory," Wang said.

Researchers conducted further experiments geared toward enhancing the cold-molecule production rate and observed strong photoassociative resonance of ultracold Cs molecules. A higher production rate will benefit applications requiring higher signal-to-noise ratio and higher signal intensity, Wang said.

Heteronuclear cold molecules like RbCs have multiple frequency ranges in microwave, terahertz, and optical frequencies. In contrast, cold atoms like Cs only have frequencies in the microwave and optical ranges. Thus, just as laser-cooled atoms and ions form the basis for ultraprecise microwave and optical frequency standards, vibrational transitions in ultracold molecules could form the basis of terahertz frequency standards. In addition, Wang said, ultracold heteronuclear molecules like RbCs are electric dipoles, which allows them to be manipulated as the qubits (quantum bits) in quantum computing applications. Atoms do not have this dipole property.

Researchers are now implementing a laser-based dipole-force trap to store cold molecules long enough to allow more useful observation. "If the cold molecules formed in the dual MOT are not trapped, they will drift away within one hundredth of a second, leaving no time to study or probe them," Wang explained. "A cold-molecule trap can store cold molecules for a few seconds—long enough to complete the measurements."

Testing Ultrafast Circuits for Space Applications

Spacecraft electronics must operate in an extremely harsh environment. The constant bombardment of heavy ions and space particles can affect the performance of electronic components and lead to temporary or permanent failure. Aerospace has been testing the spaceworthiness of circuits containing silicon-germanium heterojunction bipolar transistors (SiGe HBTs), which are expected to play an important role in advanced electronic systems for space. Potential applications include analog-to-digital converters operating at sampling rates in the 1–15 gigahertz range as well as digital logic circuits with data rates beyond 10 gigabits per second.

A high-speed SiGe RF circuit board containing the digital test device within a surface-mount package.

SiGe HBTs are potentially sensitive to total ionizing dose from protons as well as electrons, explained Donald Romeo, Senior Engineering Specialist in the Digital and Integrated Circuit Electronics Department. Ionizing radiation is primarily a surface effect, which can damage majority-carrier field-effect transistors, such as those used for CMOS; HBTs are bulk minority-carrier devices and are therefore largely immune to the effects of ionizing radiation within the device. Still, radiation can cause leakage paths between devices.

To study this technology, Aerospace designed testable logic circuits operating at gigabit-per-second rates using an in-house–designed custom digital logic cell family, which was used to implement the circuitry on a die fabricated by a wafer foundry using commercial 0.35-micron feature-size technology. More than 150 samples of the test die were supplied.

In addition, the research team used customized software to develop a high-speed circuit board containing the digital test device within a surface-mount package. Edge-mount connectors provided radio-frequency input and output signals. These boards are still being used to support accelerated radio-frequency life testing at elevated temperatures.

"Our forte is the ability to design special test circuits which will reveal the effects of aging and radiation damage well before ultimate failure," said Romeo. The sort of test data acquired from these special test circuits and test structures are not generally available from wafer foundry operations for the state-of-the-art fabrication technologies, he said. "From our testing, which monitors several critical parameters, we can generate parameter-shift trend lines to extrapolate the mean time to failure for the on-orbit environment," he said.

Among the circuits tested was a SiGe HBT "flight-like" maximum-length binary sequence circuit; Aerospace verified its operation up to 5 gigahertz (the test-fixture limit). Researchers also demonstrated logic-element gate-delay times of 18 picoseconds with a circuit containing a serial connection of 101 logic gates, which predicts the feasibility of radiation-hardened digital logic for operation beyond 10 gigahertz. The preliminary results derived from the accelerated life testing have demonstrated an estimated service life beyond 100,000 hours. Total-dose testing with gamma rays at Aerospace showed no change in performance at 500 kilorads (Si), and proton testing at UC Davis showed no increase in bit-error rate after 1.5 megarads (Si) when operating up to 5 gigahertz.

Glass Micromachines

Aerospace has developed a novel process for making microelectromechanical systems (MEMS) from silicon dioxide and implanting them on or within a silicon substrate.

Interdigitated glass fingers over a sculpted hole in the silicon wafer.

As explained by Meg Abraham of the Aerospace Center for Microtechnology, the new process entails placing a mask over a silicon wafer and firing oxygen ions at it. The ions pass through the mask and burrow into the wafer to varying depths, based on their acceleration energy. When the wafer is annealed at a high temperature, the oxygen and silicon combine to form a glass-like silicon dioxide, which shrinks in the process. The technique can therefore produce nanoscale devices using the comparatively inexpensive microscale masks common in most silicon processing.

These devices can range in size from tens of microns down to the tens of nanometers. "As far as we can tell," said Abraham, "the lower limit of the device scale is defined by the ability of the material to remain structurally viable." The devices can be released after processing or packaging via laser-assisted chemical etching. This aspect makes them easy to integrate with electronics, because the etch releases only the devices, conserving the rest of the silicon for other uses.

Traditionally, MEMS are manufactured by growing successive layers of material and patterning them via photolithography and wet etching, Abraham explained; however, these layers tend to be uneven over large areas. Also, in trying to produce very thin layers—on the order of a few hundred nanometers— the process can result in discontinuous layers. Furthermore, said Abraham, the wet etch processes are hard to control. "All these problems make the traditional methods hard to scale down to the nanometer level," she said.

Nanoscale glass beam over sculpted silicon collector.

Traditional MEMS are also fabricated from silicon—but this material is not always a good choice for electro-optical and optical devices because it does not transmit light in some important wavelengths. Silicon dioxide, the chemical compound found most frequently in glass, would be more useful in these applications, said Abraham. But MEMS manufacturers have not found many effective ways to release the glassy MEMS component without destroying the rest of the silicon wafer, which is needed for both structural support and for microelectronics. The Aerospace technique overcomes this difficulty.

The process can be used to make a host of devices, Abraham said. For example, Aerospace has shown that efficient radio-frequency microreceivers could be fabricated if they were constructed in integrated arrays; the Aerospace MEMS technique could enable this construction. Other potential applications include optical ring resonators, which could be used in integrated electro-optics or to generate Raman spectra for spectrometers; electromechanical ring resonators, which are commonly used as radio-frequency filters for cellular technology; on-chip fiber optics, which could lead to improved optical communications in both terrestrial and space environments; and on-chip bioassay microlabs, which could be useful in national defense and crime investigation as well as in medical diagnostics.

Data Fusion and Satellite Observation

The correlation of data from satellite sensors and ground-based monitors can yield significant insights into phenomena occurring near or on Earth's surface. Aerospace is investigating how data from defense satellite sensors can augment data from weather satellites, nuclear-event sensors, imagery assets, ground-based infrasonic and seismic arrays, ground-based cameras, and other recording and monitoring devices. According to Dee Pack, director of the Remote Sensing Department, this work could enhance missions such as early detection and long-term monitoring of serious fires, early warning of volcanic-ash plumes that can threaten jet aircraft, and reduction of false alarms concerning atmospheric nuclear detonations.

For example, in one recent study, Aerospace helped analyze data obtained from satellites and ground-based systems to characterize a large meteor that exploded over Park Forest, Illinois, on March 27, 2003. "These large meteors are of concern to the Department of Defense due to their ability to mimic nuclear events," Pack said. These meteor events occur 50–60 times per year globally and are the single largest source of false alarms for infrasonic nuclear monitoring stations operated by the Comprehensive Test Ban Treaty organization, he said.

Track of the Park Forest meteor projected into three planes. The blue circles are azimuth and elevation points from a ground-based video camera. A fit is forced between the first flash in the video and the space sensor high-altitude infrared intensity peak.

Working with Sandia National Laboratory, Aerospace correlated data from visible-light satellite sensors and video recordings from ground-based cameras to generate light curves. These indicated that the total energy release of the meteor was equivalent to a 0.34 kiloton nuclear event. An accurate trajectory was generated from measurements taken by infrared satellite sensors that scanned the emissive track of the meteor as it passed through the atmosphere. The trajectory was used to derive an initial velocity of roughly 20 kilometers per second, decelerating to 14 kilometers per second at lower altitude. The calculated velocity and total kinetic energy were then used to derive the meteor's initial mass, which was estimated at 7.8 tons. The mass calculation, in turn, was used to obtain a diameter estimate of 1.6 meters.

"Thorough study of these bolides is warranted so no confusion results should one explosively disintegrate at an inopportune time in a region where military tensions are high," Pack said, adding, "Further analysis of the Park Forest event will add to our knowledge base of the infrared, visible, infrasonic, and seismic signatures of these extraordinary Earth-crossing objects and serve to train global observers to better recognize and characterize these naturally occurring huge explosive events."

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