Planck distribution of phonons in a Bose-Einstein condensate.

The Planck distribution of photons emitted by a blackbody led to the development of quantum theory. An analogous distribution of phonons should exist in a Bose-Einstein condensate. We observe this Planck distribution of thermal phonons in a 3D condensate. This observation provides an important confirmation of the basic nature of the condensate's quantized excitations. In contrast to the bunching effect, the density fluctuations are seen to increase with increasing temperature. This is due to the nonconservation of the number of phonons. In the case of rapid cooling, the phonon temperature is out of equilibrium with the surrounding thermal cloud. In this case, a Bose-Einstein condensate is not as cold as previously thought. These measurements are enabled by our in situ k-space technique.

Observing atom bunching by the Fourier slice theorem.

By a novel reciprocal space analysis of the measurement, we report a calibrated in situ observation of the bunching effect in a 3D ultracold gas. The calibrated measurement with no free parameters confirms the role of the exchange symmetry and the Hanbury Brown-Twiss effect in the bunching. Also, the enhanced fluctuations of the bunching effect give a quantitative measure of the increased isothermal compressibility. We use 2D images to probe the 3D gas, using the same principle by which computerized tomography reconstructs a 3D image of a body. The powerful reciprocal space technique presented is applicable to systems with one, two, or three dimensions.

Examination of the epicentral waveform for laser ultrasound in the melting regime.

A laser ultrasonic source just below the ablation regime is examined by recording an epicentral waveform in a high purity tungsten sample. Using pulse energy as a parameter, a slight delay in the shear wave arrival time is observed upon transition to the melting regime. This phenomenon is attributed to a change in character of the ultrasonic source. In the thermoelastic regime, shear waves are generated by mode conversion at the sample surface of longitudinal waves emanating from subsurface sources. Just above the melting threshold, a molten pool forms in the center of the generation volume. Shear waves are not supported by the molten pool. As a result, shear waves generated from off-axis thermoelastic sources are weighted more heavily. This results in a delay of the shear wave arrival time.

Phonon dispersion relation of an atomic Bose-Einstein condensate.

We measure the time oscillations of a freely evolving standing wave of phonons in a Bose-Einstein condensate. We present the technique of short Bragg pulses, which stimulates the standing wave. The subsequent oscillations are observed in situ. The frequency of the oscillations gives the dispersion relation, the amplitude gives the static structure factor, and the decay gives the dephasing time. The new technique gives orders of magnitude more sensitivity than Bragg spectroscopy, allowing for the observation of deviations from the local density approximation. Specifically, it is seen that the phonons undergo a transition from three dimensions to one dimension, when their wavelength becomes longer than the transverse radius of the condensate. The one-dimensional regime contains an inflection point in the dispersion relation, a decrease in the superfluid critical velocity, a minimum in the group velocity, and an increase in the lifetime of the standing wave oscillations.

Static and dynamic two-wave mixing in GaAs.

We studied the two-wave mixing anisotropic diffraction process in GaAs for demodulation of static and dynamic phase encoded signals. The static results quantitatively agreed with a previous theoretical model for cubic crystals. This model has been described explicitly for all beam polarizations and crystal rotation angles with respect to the plane of incidence. Dynamic phase modulation, in which the signal beam was phase modulated at frequency f(s) and the reference beam at f(r) = f(s) + Deltaf, produced a signal at Deltaf that was proportional to the difference between the static beam intensities with and without two-wave mixing under all conditions of polarization and crystal orientation studied. A significant dynamic output signal was produced even when only a shift in polarization but no energy transfer occurred as a result of the anisotropic two-wave mixing process. Therefore not only is the two-wave mixing gain important when the photorefractive effect is used for dynamic phase demodulation, but so are the polarization shifts occurring from the mixing process.