Jacob Roberts

Influence of electron evaporative cooling on ultracold plasma expansion

The expansion of ultracold neutral plasmas (UCP) is driven primarily by the thermal pressure of the electron component and is therefore sensitive to the electron temperature. For typical UCP spatial extents, evaporative cooling has a significant influence on the UCP expansion rate at lower densities (less than 108/cm3). We studied the effect of electron evaporation in this density range. Owing to the low density, the effects of three-body recombination were negligible. We modeled the expansion by taking into account the change in electron temperature owing to evaporation as well as adiabatic expansion and found good agreement with our data. We also developed a simple model for initial evaporation over a range of ultracold plasma densities, sizes, and electron temperatures to determine over what parameter range electron evaporation is expected to have a significant effect. We also report on a signal calibration technique, which relates the signal at our detector to the total number of ions and electrons in...

Density-dependent response of an ultracold plasma to few-cycle radio-frequency pulses

Ultracold neutral plasmas exhibit a density-dependent resonant response to applied radio-frequency (RF) fields in the frequency range of several MHz to hundreds of MHz for achievable densities. We have conducted measurements where short bursts of RF were applied to these plasmas, with pulse durations as short as two cycles. We still observed a density-dependent resonant response to these short pulses. However, the too rapid timescale of the response, the dependence of the response on the sign of the driving field, the response as the number of pulses was increased, and the difference in plasma response to radial and axially applied RF fields are inconsistent with the plasma response being due to local resonant heating of electrons in the plasma. Instead, our results are consistent with rapid energy transfer from collective motion of the entire electron cloud to electrons in high-energy orbits. In addition to providing a potentially more robust way to measure ultracold neutral plasma densities, these measurements demonstrate the importance of collective motion in the energy transport in these systems.

Ultracold plasma expansion as a function of charge neutrality

Ultracold plasmas (UCPs) are created under conditions of near but not perfect neutrality. In the limit of zero electron temperature, electron screening results in non-neutrality manifesting itself as an interior region of the UCP with both electrons and ions and an exterior region composed primarily of ions. The interior region is the region of the most scientific interest for 2-component ultracold plasma physics. This work presents a theoretical model through which the time evolution of non-neutral UCPs is calculated. Despite Debye screening lengths much smaller than the characteristic plasma spatial size, model calculations predict that the expansion rate and the electron temperature of the UCP interior is sensitive to the neutrality of the UCP. The predicted UCP dependence on neutrality has implications for the correct measurement of several UCP properties, such as electron temperature, and a proper understanding of evaporative cooling of the electrons in the UCP.

Observation of a strong-coupling effect on electron-ion collisions in ultracold plasmas

Ultracold plasmas (UCPs) provide a well-controlled system for studying multiple aspects in plasma physics that include collisions and strong-coupling effects. By applying a short electric field pulse to an UCP, a plasma electron center-of-mass oscillation can be initiated. For accessible parameter ranges, the damping rate of this oscillation is determined by the electron-ion collision rate. We performed measurements of the oscillation damping rate with such parameters and compared the measured rates to both a molecular dynamics (MD) simulation that includes strong-coupling effects and a Monte Carlo binary collision simulation designed to predict the damping rate including only weak-coupling considerations. We found agreement between the experimentally measured damping rate and the MD result. This agreement did require including the influence of a previously unreported UCP heating mechanism whereby the presence of a dc electric field during ionization increased the electron temperature, but estimations and simulations indicate that such a heating mechanism should be present for our parameters. The measured damping rate at our coldest electron temperature conditions was much faster than the weak-coupling prediction obtained from the Monte Carlo binary collision simulation, which indicates the presence of a significant strong-coupling influence. The density averaged electron strong-coupling parameter Γ measured at our coldest electron temperature conditions was 0.35(8).

Inter-isotope effects in optimal dual-isotope loading into a shallow optical trap

Examination of loading the isotopes 85Rb and 87Rb simultaneously into a shallow far-off-resonance trap (FORT) has revealed an unexpected decrease in maximum atom number loaded as compared to loading either isotope alone. The simultaneous loading of the FORT will be affected by additional homonuclear and heteronuclear light-assisted collisional losses. However, these losses are measured and found to be insufficient to explain the observed drop in the total number of atoms loaded into the FORT. We find that our observations are consistent with a decrease in loading rate caused by inter-isotope disruptions of the efficient laser cooling required to load atoms into the optical trap.

Damping of electron center-of-mass oscillation in ultracold plasmas

Applying a short electric field pulse to an ultracold plasma induces an electron plasma oscillation. This manifests itself as an oscillation of the electron center of mass around the ion center of mass in the ultracold plasma. In general, the oscillation can damp due to either collisionless or collisional mechanisms, or a combination of the both. To investigate the nature of oscillationdamping in ultracold plasmas, we developed a molecular dynamics model of the ultracold plasma electrons. Through this model, we found that depending on the neutrality of the ultracold plasma and the size of an applied DCelectric field, there are some parameter ranges where the damping is primarily collisional and some primarily collisionless. We conducted experiments to compare the measureddamping rate with theory predictions and found them to be in good agreement. Extension of our measurements to different parameter ranges should enable studies for strong-coupling influence on electron-ion collision rates.

Suppression of Reabsorption via Modulation of Light

Reabsorption, the multiple scattering of spontaneously emitted photons in optically thick gases, is a major limitation to efficient optical pumping and laser cooling in ultracold gases. We report mitigation of reabsorption using spatial and frequency modulation of laser light illuminating such gases. We developed a semiclassical model that successfully describes the reabsorption process when frequency-modulated light is present. It was necessary to extend the treatment in the model beyond a simple two-atom picture in order to reproduce our experimental results.

Evaluation of charged particle evaporation expressions in ultracold plasmas

Electron evaporation plays an important role in the electron temperature evolution and thus the expansion rate of low-density ultracold plasmas. In addition, evaporation is useful as a potential tool for obtaining colder electron temperatures and characterizing plasma parameters. A theoretical treatment of evaporation has been developed for atomic gases and has been applied to a one-component plasma system. We numerically investigate whether such an adapted theory is applicable to ultracold neutral plasmas. We find that it is not due to the violation of fundamental assumptions of the model. The details of our calculations and a discussion of the implications for a simple description of the electron evaporation rate in ultracold plasmas are presented.

Enhanced light-assisted-collision rate via excitation to the long-lived5S1/2-5D5/2molecular potential in anRb85magneto-optical trap

We report measurements of a significant increase in the two-body loss rate in an {sup 85}Rb magneto-optic trap (MOT) caused by the addition of light resonant with the 5P{sub 3/2}-to-5D{sub 5/2} transition (776 nm) in Rb. Exposure to the additional light resulted in up to a factor of 5 decrease in the steady-state number of atoms in the MOT. This loss is attributed to more than an order of magnitude increase in the light-assisted collision rate brought about by the 776-nm light. By measuring the intensity dependence of the loss rate, the loss channel was identified to be the relatively long-lived 5S{sub 1/2}-5D{sub 5/2} molecular potential.

Influence of optical molasses in loading a shallow optical trap

We have examined loading of 85Rb atoms into a shallow Far-Off-Resonance Trap (FORT) from an optical molasses and compared it to loading from a Magneto-Optical Trap (MOT). We found that substantially more atoms could be loaded into the FORT via an optical molasses as compared to loading from the MOT alone. To determine why this was the case, we measured the rate of atoms loaded into the FORT and the losses from the FORT during the loading process. For both MOT and molasses loading, we examined atom load rate and losses over a range of detunings as well as hyperfine pump powers. We found that the losses induced during MOT loading were essentially the same as the losses induced during molasses loading at the same MOT/molasses detuning. In contrast, load rate of the molasses was higher than that of a MOT at a given detuning. This caused the optical molasses to be able to load more atoms than the MOT. Optimization of FORT loading form an optical molasses improved the number of atoms we could trap by a factor of two over that of optimal loading from a MOT.