Chen, Y.C.

Gupta, P.

Killian, T.C.

Laha, S.

Martinez, Y.N.

Mickelson, P.G.

Nagel, S.B.

Simien, C.E.

DSpace at Rice University

540 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 33, NO. 2, APRIL 2005Absorption Imaging of Ultracold Neutral PlasmasC. E. Simien, Y. C. Chen, P. Gupta, S. Laha, Y. N. Martinez, P. G. Mickelson, S. B. Nagel, and T. C. KillianAbstract—We report optical absorption imaging of ultracoldneutral plasmas. Imaging allows direct observation of the iondensity profile and expansion of the plasma. The frequency de-pendence of the plasma’s optical depth gives the ion absorptionspectrum, which is broadened by the ion motion. We use thespectral width to monitor ion equilibration in the first 250 ns afterplasma formation. On a microsecond time scale, we observe theradial acceleration of ions resulting from pressure exerted by thetrapped electron gas.Index Terms—Image analysis, plasma measurements, plasmas,spectral analysis.ULTRACOLD neutral plasmas are systems in which par-ticle temperatures can be 1 K or lower. Fundamental in-terest in these systems stems from a range of phenomena thatoccur in the ultracold regime. For instance, it may be possibleto form strongly coupled systems in which the electrical inter-action energy between the charged particles exceeds the averagekinetic energy. This reverses the traditional energy hierarchythat underlies our normal understanding of plasmas based onconcepts such as Debye screening and hydrodynamics.To produce ultracold plasmas, laser-cooled atoms [1], [2] areionized with photons from a laser whose wavelength is tunedbarely above the ionization threshold. Due to the small elec-tron mass, the initial electron kinetic energy approximatelyequals the difference between the photon energy and the ioniza-tion potential, which can be as low as a 100 mK. The initial ki-netic energy of the ions is in the millikelvin range. The resultingnonequilibrium plasma evolves rapidly, and many of its charac-teristics remain to be studied. Experiments [3]–[5] conducted sofar reveal surprising dynamics and recombination behavior thatshow us we are stretching the bounds of plasma physics. Manytheory papers have addressed the plasma behavior [6]–[10].We report results with a new diagnostic tool to study ultra-cold neutral plasmas: absorption imaging of a strontium plasmausing the transition at 422 nm [11]. Thistechnique provides in situ, nondestructive measurements andoffers excellent spatial, temporal, and spectral resolution. Theabsorption imaging reported here is particularly well adaptedfor small, cold, and relatively dilute plasmas that evolve veryquickly.A collimated laser beam, tuned near resonance with the prin-cipal transition in the ions, illuminates the plasma and falls onan image intensified charge-coupled device (CCD) camera. Fol-lowing Beer’s law, the optical depth (OD) is defined in terms ofManuscript received July 1, 2004; revised January 3, 2005.The authors are with the Department of Physics and Astronomy, Rice Uni-versity, Houston TX 77251 USA (e-mail: simien@rice.edu).Digital Object Identifier 10.1109/TPS.2005.845342the image intensity without and with theplasma presentwhere is the ion density, and is the ionabsorption cross section at the image beam frequency, . Theabsorption cross section is a function of temperature due toDoppler broadening, and since we expect the temperature tovary with density, we allow to vary with position.Fig. 1 shows a typical absorption image. The intensity of theprobe beam is much less than the saturation intensity of the tran-sition. The spatial resolution of typical images is 65 limited bypixel averaging performed to improve the signal-to-noise ratioof the images.To obtain the absorption spectrum, we plot the integral of theoptical depth over x and y as a function of the imaging laserfrequency. For longer delay times between formationof the plasma and camera exposure, the spectral lines becomesignificantly broader than the natural linewidth of the transition.The dominant contribution to the linewidth beyond the naturallinewidth is Doppler broadening, which makes the absorptionspectrum a very accurate probe of the ion velocity distribution.From a fit of each spectrum to a Voigt profile using a Loren-ztian width equal to the sum of the natural width and the laserlinewidth, we extract the root mean square (rms) Doppler broad-ening, , where is the ion mass,and is the wavelength of the transition. This provides the effec-tive ion temperature as a function of time. Various effectscontribute to the ion kinetic energy and thus to .increases rapidly for ns. Two pieces ofinformation imply that this is thermalization of ions with them-selves after creation in a spatially disordered state [6]: 1) thetime scale is on the order in which ions respond to perturba-tions from their equilibrium spatial distribution, which is on theorder of the inverse plasma frequency of ions,ns, where and are the electric chargeand initial peak ion density; 2) the energy is also on the orderof the amount of potential energy inherent in theinitial disorder of the ions. Here, is theWigner–Seitz radius, and is the initial peak electron density.For ns, continues to increase, but at aslower rate. The slow increase results from an outward radialacceleration of the ions caused by pressure exerted by the gasof trapped electrons [9], [10]. This acceleration will lead to theexpansion of the plasma on a 10- s time scale.In conclusion, absorption imaging of ultracold neutral stron-tium plasma is a powerful diagnostic technique. We use this0093-3813/$20.00 © 2005 IEEESIMIEN et al.: ABSORPTION IMAGING OF ULTRACOLD NEUTRAL PLASMAS 541Fig. 1. Optical depth of an ultracold neutral plasma. Delay between the formation of the plasma and image exposure is 85 ns, and the initial peak ion density isn = 210 cm 3. Plasma contains 710 ions. Spatial resolution is about 65 m, limited by pixel averaging performed to improve the signal-to-noise ratio.technique to study the ion spectrum at various delay times. Thespectrum reveals rapid thermalization of the ions on a submi-crosecond time scale as predicted by theory [6], and the acceler-ation of ions due to electron pressure. In addition, this probe hasthe potential to study a host of phenomena such as ion collec-tive modes, shock waves, recombination, and particle–particlespatial correlations.REFERENCES[1] H. J. Metcalf and P. van der Straten, Laser Cooling and Trapping. NewYork: Springer-Verlag, 1999.[2] S. B. Nagel, C. E. Simien, S. Laha, P. Gupta, V. S. Ashoka, and T. C.Killian, “Magnetic trapping of metastable P atomic strontium,” Phys.Rev. A., Gen. Phys., vol. 67, pp. 011 401-1–011 401-4, 2003.[3] T. C. Killian, S. Kulin, S. D. Bergerson, L. A. Orozco, C. Orzel, and S.L. Rolston, “Creation of an ultracold neutral plasma,” Phys. Rev. Lett.,vol. 83, pp. 4776–4779, 1999.[4] S. Kulin, T. C. Killian, S. D. Bergeson, and S. L. Rolston, “Plasma oscil-lations and expansion of an ultracold neutral plasma,” Phys. Rev. Lett.,vol. 85, pp. 318–321, 2000.[5] T. C. Killian, M. J. Lim, S. Kulin, R. Dumke, S. D. Bergeson, and S. L.Rolston, “Formation of rydberg atoms in an expanding ultracold neutralplasma,” Phys. Rev. Lett., vol. 86, pp. 3759–3762, 2001.[6] M. S. Murillo, “Using fermi statistics to create strongly cou-pled ion plasmas in atom traps,” Phys. Rev. Lett., vol. 87, pp.115 003-1–115 003-4, 2001.[7] S. G. Kluzmin and T. M. O’Neil, “Numerical simulation of ultracoldplasmas: How rapid intrinsic heating limits the development of correla-tion,” Phys. Plasmas, vol. 9, pp. 0 605 003–0 605 004, 2002.[8] S. Mazevet, L. A. Collins, and J. D. Kress, “Evolution of ultracold neu-tral plasmas,” Phys. Rev. Lett., vol. 88, pp. 055 001-1–055 001-4, 2002.[9] F. Robicheaux and J. D. Hanson, “Simulation of the expan-sion of an ultracold neutral plasma,” Phys. Plasmas, vol. 10, pp.055 002-1–055 002-4, 2003.[10] T. Pohl, T. Pattard, and J. M. Rost, “Coulomb crystallization in ex-panding laser-cooled neutral plasmas,” Phys. Rev. Lett., vol. 92, pp.155 003-1–155 003-4, 2004.[11] C. E. Simien, Y. C. Chen, P. Gupta, S. Laha, Y. N. Martinez, P. G. Mick-elson, S. B. Nagel, and T. C. Killian, “Using absorption imaging to studyion dynamics in an ultracold neutral plasma,” Phys. Rev. Lett., vol. 92,pp. 143 001-1–143 001-4, 2004.

Absorption Imaging of Ultracold Neutral Plasmas

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Absorption Imaging of Ultracold Neutral Plasmas

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