We report the creation of an ultracold neutral plasma by photoionization of laser-cooled xenon atoms. The charge carrier density is as high as 2 × 10 9 cm −3 , and the temperatures of electrons and ions are as low as 100 mK and 10 µK, respectively. Plasma behavior is evident in the trapping of electrons by the positive ion cloud when the Debye screening length becomes smaller than the size of the sample. We produce plasmas with parameters such that both electrons and ions are strongly coupled.The study of ionized gases in neutral plasma physics spans temperatures ranging from 10 16 K in the magnetosphere of a pulsar to 300 K in the earth's ionosphere [1]. At lower temperatures the properties of plasmas are expected to differ significantly. For instance, three-body recombination which is prevalent in high temperature plasmas, should be suppressed [2]. If the thermal energy of the particles is less than the Coulomb interaction energy, the plasma becomes strongly coupled, and the usual hydrodynamic equations of motion and collective mode dispersion relations are no longer valid [3]. Strongly coupled plasmas are difficult to produce in the laboratory and only a handful of examples exist [4], but such plasmas do occur naturally in astrophysical systems.In this work we create an ultracold neutral plasma with an electron temperature as low as T e = 100 mK, an ion temperature as low as T i = 10 µK, and densities as high as n = 2×10 9 cm −3 . We obtain this novel plasma by photoionization of laser-cooled xenon atoms. Within the experimentally accessible ranges of temperatures and densities both components can be simultaneously strongly coupled. A simple model describes the evolution of the plasma in terms of the competition between the kinetic energy of the electrons and the Coulomb attraction between electrons and ions. A numerical calculation accurately reproduces the data.Photoionization and laser-cooling have been used before in plasma experiments. Photoionization in a 600 K Cs vapor cell produced a plasma with T e ≥ 2000 K [5], and a strongly coupled non-neutral plasma was created by laser-cooling magnetically trapped Be + ions [6]. A plasma is often defined as an ionized gas in which the charged particles exhibit collective effects [7]. The length scale which divides individual particle behavior and collective behavior is the Debye screening length λ D . It is the distance over which an electric field is screened by redistribution of electrons in the plasma, and is given by λ D = ǫ 0 k B T e /e 2 n. Here, ǫ 0 is the electric permittivity of vacuum, k B is the Boltzmann constant, and e is the elementary charge. An ionized gas is not a plasma unless the Debye length is smaller than the size of the system [7]. In our experiment, the Debye length can be as low as 500 nm, while the size of the sample is σ ≈ 200 µm. The condition λ D < σ for creating a plasma is thus easily fulfilled.The atomic system we use is metastable xenon in the 6s [3/2] 2 state. This state has a lifetime of 43 s [8] and can be treated as the groun...

We report the observation of plasma oscillations in an ultracold neutral plasma. With this collective mode we probe the electron density distribution and study the expansion of the plasma as a function of time. For classical plasma conditions, i.e. weak Coulomb coupling, the expansion is dominated by the pressure of the electron gas and is described by a hydrodynamic model. Discrepancies between the model and observations at low temperature and high density may be due to strong coupling of the electrons.One of the most interesting features of neutral plasmas is the rich assortment of collective modes that they support. The most common of these is the plasma oscillation [1], in which electrons oscillate around their equilibrium positions and ions are essentially stationary. This mode is a valuable probe of ionized gases because the oscillation frequency depends solely on the electron density.In an ultracold neutral plasma as reported in [2], the density is nonuniform and changing in time. A diagnostic of the density is thus necessary for a variety of experiments, such as determination of the three-body recombination rate at ultralow temperature [3], and observation of the effects of strong Coulomb coupling [4] in a two-component system. A density probe would also aid in the study of the evolution of a dense gas of cold Rydberg atoms to a plasma [5], which may be an analog of the Mott insulator-conductor phase transition [6].In this work we excite plasma oscillations in an ultracold neutral plasma by applying a radio frequency (rf) electric field. The oscillations are used to map the plasma density distribution and reveal the particle dynamics and energy flow during the expansion of the ionized gas.The creation of an ultracold plasma has been described in [2]. A few million metastable xenon atoms are laser cooled to approximately 10 µK. The peak density is about 2 × 10 10 cm −3 and the spatial distribution of the cloud is Gaussian with an rms radius σ ≈ 220 µm. These parameters are determined with resonant laser absorption imaging [7]. To produce the plasma, up to 25% of the atoms are photoionized in a two-photon excitation. Light for this process is provided by a Ti:sapphire laser at 882 nm and a pulsed dye laser at 514 nm (10 ns pulse length). Because of the small electron-ion mass ratio, the resulting electrons have an initial kinetic energy (E e ) approximately equal to the difference between the photon energy and the ionization potential. In this study we vary E e /k B between 1 and 1000 K. The initial kinetic energy of the ions varies between 10 µK and 4 mK.For detection of charged particles, a small DC field (about 1 mV/cm) directs electrons to a single channel electron multiplier and ions to a multichannel plate detector. The amplitude of the rf field that excites plasma oscillations, F , varies between 0.2 − 20 mV/cm rms. All electric fields are applied to the plasma with grids located above and below the laser-atom interaction region.In the absence of a magnetic field, the frequency of plasma oscillation...

We study the formation of Rydberg atoms in expanding plasmas at temperatures of 1-1000 K and densities from 10(5)-10(10) cm(-3). Up to 20% of the initially free charges recombine in about 100 micros, and the binding energy of the Rydberg atoms approximately equals the increase in the kinetic energy of the remaining free electrons. Three-body recombination is expected to dominate in this regime, yet most of our results are inconsistent with this mechanism.

We report new detailed density profile measurements in expanding strongly-coupled neutral plasmas. Using laser-induced fluorescence techniques, we determine plasma densities in the range of 10 5 to 10 9 cm −3 with a time resolution limit as small as 7 ns. Strong-coupling in the plasma ions is inferred directly from the fluorescence signals. Evidence for strong-coupling at late times is presented, confirming a recent theoretical result. [2], and in some astrophysical settings. A new class of strongly-interacting neutral plasmas was recently demonstrated using the tools of laser-cooling and trapping [3,4,5,6,7]. These "ultracold" neutral plasmas occupy a unique position in phase space. In these plasmas it is possible to create strongly-interacting Coulomb systems at modest densities because the initial electron and ion temperatures can be in the milliKelvin range. The initial ion-ion and ion-electron interaction strength can also be selected with great precision.Recent experimental work in this field has used absorption imaging techniques to make temperature and density measurements in expanding ultracold neutral plasmas [6,7]. This work explored the 50 to 1000 ns time period after plasma formation in great detail. Correlationinduced heating was observed in the plasma ions. The ion coupling parameter, given as the ratio of nearestneighbor Coulomb energy to the kinetic energy, equilibrated just inside the strongly-coupled regime, with a coupling parameter around 2.Radio-frequency (RF) excitation techniques have also been used to determine the average ion density and the electron temperature in these systems [4,8,9]. These studies confirm theoretical predictions regarding the generally self-similar Gaussian expansion of the ions and the clamping of the electron temperature in the weaklycoupled regime.In this letter we report laser-induced-fluorescence measurements of ions in expanding strongly-coupled plasmas as a tool to study the spatial and temporal evolution of the ion temperature and density. This measurement technique has a 7 ns temporal resolution limit. We measure plasma densities as low as 10 5 cm −3 at effective plasma temperatures of 100 K. The maximum density that can be measured is limited by radiation trapping, and for spherically-symmetric systems in the milliKelvin range is limited to around 10 9 cm −3 . The temporal resolution * Present Address: Lockheed Martin Space Systems Company, Sunnyvale, CA 94089 † Electronic address: scott.bergeson@byu.edu and dynamic range of this method in ultracold plasma measurements surpass those currently seen in absorption spectroscopy, and rival the sensitivity of RF spectroscopic methods.Much of the experimental setup has been described previously [10]. We create a calcium magneto-optical trap (MOT) using the resonance transition at 423 nm. Up to 50 mW of 423 nm radiation is generated by frequency-doubling a diode laser system at 846 nm using a periodically-poled KTP crysal in a resonant build-up cavity. The 423 nm MOT light is detuned one line-width below the atom...

We report a new absolute frequency measurement of the Cs 6s-8s two-photon transition measured using frequency comb spectroscopy. The fractional frequency uncertainty is 5x10(-11), a factor of 6 better than previous results. The comb is derived from a stabilized picosecond laser and referenced to an octave-spanning femtosecond frequency comb. The relative merits of picosecond-based frequency combs are discussed, and it is shown that the AC Stark shift of the transition is determined by the average rather than the much larger peak intensity.