Abstract Recently, we introduced a resonant microwave cavity as a diagnostic tool for the study of ultracold plasmas (UCPs). This diagnostic allows us to study the electron dynamics of UCPs non-destructively, very fast, and with high sensitivity by measuring the shift in the resonance frequency of a cavity, induced by a plasma. However, in an attempt to theoretically predict the frequency shift using a Gaussian self-similar expansion model, a three times faster plasma decay was observed in the experiment than found in the model. For this, we proposed two causes: plasma–wall interactions and collisional microwave heating. In this paper, we investigate the effect of both causes on the lifetime of the plasma. We present a simple analytical model to account for electrons being lost to the cavity walls. We find that the model agrees well with measurements performed on plasmas with different initial electron temperatures and that the earlier discrepancy can be attributed to electrons being lost to the walls. In addition, we perform measurements for different electric field strengths in the cavity and find that the electric field has a small, but noticeable effect on the lifetime of the plasma. By extending the model with the theory of collisional microwave heating, we find that this effect can be predicted quite well by treating the energy transferred from the microwave field to the plasma as additional initial excess energy for the electrons.
We present the design and commissioning of a resonant microwave cavity as a novel diagnostic for the study of ultracold plasmas. This diagnostic is based on the measurements of the shift in the resonance frequency of the cavity, induced by an ultracold plasma that is created from a laser-cooled gas inside. This method is simultaneously non-destructive, very fast (nanosecond temporal resolution), highly sensitive, and applicable to all ultracold plasmas. To create an ultracold plasma, we implement a compact magneto-optical trap based on a diffraction grating chip inside a 5 GHz resonant microwave cavity. We are able to laser cool and trap (7.25 ± 0.03) × 107 rubidium atoms inside the cavity, which are turned into an ultracold plasma by two-step pulsed (nanosecond or femtosecond) photo-ionization. We present a detailed characterization of the cavity, and we demonstrate how it can be used as a fast and sensitive probe to monitor the evolution of ultracold plasmas non-destructively. The temporal resolution of the diagnostic is determined by measuring the delayed frequency shift following femtosecond photo-ionization. We find a response time of 18 ± 2 ns, which agrees well with the value determined from the cavity quality factor and resonance frequency.
We report a systematic experimental and numerical study of the expansion of ultracold Rydberg plasmas. Specifically, we have measured the asymptotic expansion velocities,
v0, of ultracold neutral plasmas (UNPs) which evolve from cold, dense samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the effective initial plasma electron temperature,
Te,0=mionv02/kB (where
mion is the
Rb+ ion mass), as a function of the original Rydberg atom density and binding energy,
Eb,i. We have also simulated numerically the interaction of UNPs with a large reservoir of Rydberg atoms to obtain data to compare with our experimental results. We find that for Rydberg atom densities in the range ${10}^{7}--{10}^{9} {\mathrm{cm}}^{\ensuremath{-}3}$, for states with principal quantum number
n>40,Te,0 is insensitive to the initial ionization mechanism which seeds the plasma. In addition, the quantity
kBTe,0 is strongly correlated with the fraction of atoms which ionize, and is in the range $0.6\ifmmode\times\else\texttimes\fi{}|{E}_{b,i}|\ensuremath{\lesssim}{k}_{B}{T}_{e,0}\ensuremath{\lesssim}2.5\ifmmode\times\else\texttimes\fi{}|{E}_{b,i}|$. On the other hand, plasmas from Rydberg samples with $n\ensuremath{\lesssim}40$ evolve with no significant additional ionization of the remaining atoms once a threshold number of ions has been established. The dominant interaction between the plasma electrons and the Rydberg atoms is one in which the atoms are deexcited, a heating process for electrons that competes with adiabatic cooling to establish an equilibrium where
Te,0 is determined by their Coulomb coupling parameter, ${\mathrm{\ensuremath{\Gamma}}}_{e}\ensuremath{\sim}0.01$.
Abstract Understanding Coulomb driven many-body dynamics in ultracold atomic systems far from equilibrium remains an open challenge, particularly when ultrafast excitation channels create competing pathways toward Rydberg gases or ultracold plasmas. Here, we investigate the many-body dynamics in a 87 Rb Bose-Einstein condensate after exposure to a single femtosecond laser pulse. By tuning the laser wavelength across the two-photon ionization threshold, we can control the initial state that is either dominated by free electrons and leads to an ultracold plasma or dominated by electrons in excited states which leads to a dense Rydberg gas. The large bandwidth enables overcoming the Rydberg blockade that limits the excitation density for narrow-linewidth lasers. We directly measure the kinetic energy of the released electrons and compare the final distribution of free, bound and plasma electrons to molecular dynamics simulations where the electrons are modeled as individual particles including collisional ionization and recombination processes. We find very good agreement between the simulated electron distribution and the experimental observation. We identify charge imbalance as main driver for the decay of a dense Rydberg gas into an ultracold plasma.
We report a systematic experimental and numerical study of the expansion of ultra-cold Rydberg plasmas. Specifically, we have measured the asymptotic expansion velocities,
v0, of ultra-cold neutral plasmas (UNPs) which evolve from cold, dense samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the effective initial plasma electron temperature,
Te,0=mionv02/kB (where
mion is the Rb
+ ion mass), as a function of the original Rydberg atom density and binding energy,
Eb,i. We have also simulated numerically the interaction of UNPs with a large reservoir of Rydberg atoms to obtain data to compare with our experimental results. We find that for Rydberg atom densities in the range
107−109 cm
−3, for states with principal quantum number
n>40,
Te,0 is insensitive to the initial ionization mechanism which seeds the plasma. In addition, the quantity
kBTe,0 is strongly correlated with the fraction of atoms which ionize, and is in the range
0.6×∣Eb,i∣≲kBTe,0≲2.5×∣Eb,i∣. On the other hand, plasmas from Rydberg samples with
n≲40 evolve with no significant additional ionization of the remaining atoms once a threshold number of ions has been established. The dominant interaction between the plasma electrons and the Rydberg atoms is one in which the atoms are deexcited, a heating process for electrons that competes with adiabatic cooling to establish an equilibrium where
Te,0 is determined by their Coulomb coupling parameter,
Γe∼0.01.
Using high-resolution laser-induced fluorescence spectroscopy, ion dynamics in\nan expanding ultracold neutral plasma (UNP) have been studied in unprecedented\ndetail. The evolution of the ion temperature is of great interest because the ions in\nan ultracold neutral plasma are strongly coupled, meaning their Coulomb interaction\nenergy exceeds the thermal energy. This leads to novel plasma properties that are\nhard to describe theoretically. Understanding all the factors that contribute to ion\ntemperature evolution is critical for designing schemes to cool the ions and achieve\neven stronger Coulomb coupling.\nThis work has also observed several phenomena that have not previously been\nstudied in ultracold plasmas. Ion adiabatic cooling has been observed in a UNP for\nthe first time, resulting in ion temperatures as low as 100mK and up to a tripling of\nthe Coulomb coupling parameter. The importance of electron-ion energy transfer is\ndemonstrated with the first observations of electron-ion collisional heating. Inclusion\nof this effect into existing numerical models provides better agreement with experimental\nresults. Results of molecular dynamics simulations of equilibrating plasmas\nhave been compared to early time experimental data, which provides a new method\nof density determination that is much less sensitive to experimental systematics and\nis accurate to within 10%. This improved certainty in the density allows accurate\ncomparison between data and a theoretical model of the plasma dynamics. The\nagreement is excellent, but on a hydrodynamic time scale a small amount of extra\nheat (on order of 100mK), not ascribable to electron-ion collisions or other sources in\nthe model, is detected in the ion component. Potential sources are discussed within\nas well as proposals for further studies and improvements.