TECHNICAL ACTIVITIES 1998 - NISTIR 6268
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Figure 1. Germanium islands grown on Si(100) using arsenic as a surfactant. |
Figure 2. Fiber tips for infrared near-field microscopy. A new, two-step method, for pulling fluoride fibers leads to 200 nm tips (left) with enhanced infrared transmissivity (right).
Figure 3. Apparent measured frequency of the a10 component of
R(56) at 532 nm. The data of 4/18/98 are shown with their own baseline.
June measurements were made to see limits of offsets caused by pre-filter
mistuning, maladjustment of tracking filter gains, etc. Corrections for rf
standard frequency and 633 nm I2-stabilized laser offsets are
not yet applied.
Figure 4. (top) Stability of beat between I2 stabilized and HCCD-stabilized lasers. The improving ultrasensitive detection of a weak overtone resonance of molecular HCCD permits progressively better results on the laser stabilization. (bottom) The heterodyne reference laser is stabilized on an I2 transition at 532 nm using modulation transfer spectroscopy. This reference laser has a stability ~5 × 10-14 at 1 s, from beating experiments with two I2-stabilized systems.
The NICE-OHMS spectrometer naturally provides laser frequency discrimination
information from both the cavity resonance and the molecular transition. Thus,
it is an ideal system for simultaneously achieving good short- and long-term
frequency stabilization. The laser frequency basically tracks the cavity
resonance with a precision of a few mHz with a fast servo loop. The vibration
noise and the long-term drift of the cavity can be eliminated by stabilizing to
the intracavity molecular transition. (J.L. Hall)
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Figure 5. Apertureless AFM/NSOM fluorescence image of a dye doped polystyrene nanosphere (~80 nm) with the corresponding atomic force image below. |
Optical second harmonic generation has been shown to be sensitive to roughness at this interface. In materials with bulk inversion symmetry, second harmonic generation is only dipole allowed at an interface or surface, making it a highly interface/surface selective technique. However, the underlying physics that gives rise to the roughness sensitivity is not understood. We are investigating why second harmonic generation is sensitive to roughness. Our experiments are focused on measuring the spectral dependence of the signal. There are resonant structures that can be attributed to various features in the band structure of silicon.
The preliminary results indicate a previously unknown resonance in the second harmonic spectrum. It is manifest as a strong increase in the signal at the blue end of the spectral region that the incident laser can tune over. Most strikingly, this resonance shows different symmetry properties than adjacent spectral regimes. The symmetry properties suggest that the signal is arising from step edges at the interface. If confirmed, this would be a very interesting result because we will have identified interface features that influence the second harmonic spectrum. Efforts are underway to extend the tuning range of the incident pulses using an optical parametric oscillator. This will necessitate the use of a reference signal to allow comparison of results from the optical parametric oscillator and laser. (S.T. Cundiff)
Measurements of the fields of such pulses have been performed with frequency-resolved optical gating (FROG), providing a means to observe the evolution of both the temporal amplitude and phase of femtosecond pulses as they undergo rapid broadening and splitting while propagating in fused silica. These accurate measurements have initiated the development of a more complete, modified, non-linear Schrödinger equation (NLSE), which includes contributions of physical mechanisms such as Raman non-linearities, space-time coupling, nonlinear shock effects, and non-paraxiality. The improved model successfully predicts temporal asymmetries observed in the measurements. In addition, the technique of spectral interferometry has been applied to full beam measurements, permitting the measurement of the full (temporal plus spatial) electromagnetic field on a femtosecond time scale for the first time. (T.S. Clement).
Work at JILA on BEC in the past year has concentrated in two main areas, tunable interactions and mixed condensates.
Tunable interactions: Many of the properties of a Bose-Einstein condensate are determined by the interactions between atoms. To the extent one can deliberately modify the nature of the interactions, one has intimate control over the behavior of the condensate. JILA theorists predicted that in the presence of an ambient magnetic field of about 15.0 mT (150 gauss), two-body interactions between Rb-85 atoms should go through a resonance. Experiments at JILA recently confirmed that the rate of elastic collisions between two ultracold Rb-85 atoms in a 15.5 mT (155 gauss) field is at least a factor of 10,000 higher than it is in a 16.7 mT (167 gauss field). The 16.7 mT magnetic fields at the heart of an atomic collision are on the order of tens of Tesla (hundreds of thousands of gauss). That a mT change in the ambient field could have such a profound effect on a collision says a lot about the power of resonance effects in ultracold collisions. Efforts are now underway to create a condensate in Rb-85 in order to exploit this so-called "Feshbach resonance" in BEC studies.
Mixed Condensates: A series of studies on the behavior of condensate mixtures has been performed, with the two components corresponding to two different hyperfine states of Rb-87. Interestingly, the same atomic physics theory that predicts the "pressure" in the condensate (the self-repulsion of the condensate atoms) also yields a prediction for the pressure-shift of the Rubidium clock transition. Thus, some of JILA's fluid-dynamical studies on mixed condensates have provided a sensitive confirmation of the accuracy of the atomic theory that predicts the ultimate limit to the accuracy of Rubidium-based atomic clocks. (E.A. Cornell).
Mission | Organization | Current Directions | Technical Highlights | Future Directions
TECHNICAL ACTIVITIES 1998 - Contents
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Online: April 1999