Science - USA (2019-01-04)

The light-scattering force from the cooling
lasers also notably retarded the expansion of the
plasma, as shown in Fig. 3 for the same initial
plasma conditions as in Fig. 2. The kinetic model
discussed above provides a reasonable descrip-
tion of the evolution of the RMS size along the
laseraxis(Fig.3B).Forlargerinitialplasmasize
and detuning of the cooling laser, retardation
of the expansion was even more effective ( 36 ),
raising the intriguing possibility of confining a
neutral plasma with optical forces, perhaps with
the addition of spatially varying magnetic fields
in a hybrid magneto-optical trap ( 4 ) and mag-
netic cusp ( 40 ) configuration. For blue detun-
ing, the plasma bifurcated because ions were
accelerated away from the center until the force
diminished when the velocity exceeded the cap-
turerange.Themarkedchangeinexpansiondy-
namics for blue detuning highlights the potential
of laser forces for manipulating a plasma.
Figure 4 illustrates the variation of laser-
coolingandheatingeffects with detuningD.
After 135ms of laser application,Giin the central
region was minimized forD~−20 MHz. How-

ever, as the lasers were further red detuned, more

of the cloud experienced the light-scattering

forces for a longer time, creating a larger region

of cooled ions at the expense of slightly decreased

cooling efficiency (Fig. 4A, inset). Similarly, the

acceleration of plasma expansion for the heating

configuration was most effective for a relatively

large blue detuning,D~50MHz,asshowninthe

plot of RMS size along the laser axis (Fig. 4B).

WithGi= 11, laser-cooled UNPs are already in

an interesting regime for applying established

techniques for measuring collision rates, trans-

port, and dispersion relations ( 15 , 25 ). Laser forces

also open entirely new possibilities. For example,

by patterning the lasers spatially, it should be

possible to create reasonably sharp velocity gradi-

ents and measure shear viscosity. Heating and

cooling separate regions of the plasmas should

initiate heat flow and allow measurement of ther-

mal conductivity. There are also straightforward

opportunities to improve the laser-cooling pro-

cess, such as investigating cooling along three

dimensions, which might be effective for plasma

confinement. Increasing the number of ionized

atoms would allow for largers 0 andtexpfor a

given density. Laser cooling could then be ap-

plied for a longer time, leading to lower temper-

ature and higherGi. The lowest temperature

theoretically achievable with Doppler cooling

on this transition isTDoppler=hg/2kB= 0.5 mK

( 4 ), and even with present conditions it is likely

that cooling can continue longer than reported

here and below 50 mK. However, the natural

linewidth of the LIF transition is already large

compared with the Doppler broadening at this

temperature, limiting further application of this

temperature diagnostic. Improving the temper-

ature resolution by, for example, using a narrow

two-photon transition to a metastableDstate

should enable measurement of lower tempera-

ture. Then it will be possible to study the laser-

cooling and laser-confinement limits imposed

by electron-ion heating and three-body recom-

bination ( 14 , 15 , 32 ).

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Langinet al.,Science 363 ,61–64 (2019) 4 January 2019 3of4

Fig. 4. Effects of detuning.(A) Temperature atx= 0 versus detuning for 135ms of evolution in
the optical field. The inset showsTiversusxforD=−10 MHz (red) andD=−30 MHz (blue),
along with Gaussian fits. (B) Plasma size versus detuning at 135ms of evolution. Here, size is the
RMS width calculated numerically from the image. Error bars indicate SD.

Fig. 3. Influence of laser forces on plasma expansion.(A) Evolution of plasma density
distribution. Red-detuned optical molasses along^xretarded expansion, whereas blue-detuned light
accelerated it, eventually leading to bifurcation. The scale bar is 5 mm. The color bar is rescaled for
each time tonmax= (13, 8.5, 4.2, 2.2, 1.3) × 10^7 cm−^3 fort= (5, 30, 60, 90, 120)ms. (B) RMS radius
sx(t) from Gaussian fit to experimental data (same symbols as in Fig. 2). Error bars indicate SD.

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