Laser Cooling without Spontaneous Emission

This Letter reports the demonstration of laser cooling without spontaneous emission, and thereby addresses a significant controversy. It works by restricting the atom-light interaction to a time short compared to a cycle of absorption followed by natural decay. It is achieved by using the bichromatic force on an atomic transition with a relatively long excited state lifetime and a relatively short cooling time so that spontaneous emission effects are minimized. The observed width of the one-dimensional velocity distribution is reduced by $\times 2$ thereby reducing the “temperature” by $\times 4$. Moreover, our results comprise a compression in phase space because the spatial expansion of the atomic sample is limited. This accomplishment is of interest to direct laser cooling of molecules or in experiments where working space or time is limited.

Figure 1

Part (a) shows the eigenvalues of the two-frequency dressed state system separated by $\hslash \delta$ where the two fields have equal strength, along with a typical path followed by an atom moving to the right. Circled transition points of the type marked “$A$” are exact crossings because they occur at the nodes of the standing wave fields shown in part (b). For our case, this occurs for a spatial phase offset of $\lambda /8$ and $\mathrm{\Omega }=\sqrt{3/2}\delta$. Point “$B$” in the vertical ellipse marks the location of a possible Landau-Zener transition.

Figure 2

Part (a) shows the two atomic levels and the detuning $\delta \gg \gamma$. Part (b) shows the $\mathrm{\Delta }m=0$ transitions driven by the linearly polarized light between the $J=1$ and $J=2$ He levels and their relative strengths.

Figure 3

Data taken with an optical slit width $\approx 230\mu \mathrm{m}$, corresponding to interaction time of 220 ns $<t_{SE}$ for longitudinal velocity $\approx 1050\mathrm{m}/\mathrm{s}$. The detuning was $\delta =2\pi \times 25\mathrm{MHz}$, corresponding to $\mathrm{\Delta }{v}_B\approx 4.9\mathrm{m}/\mathrm{s}$. In part (a), the dark regions represent reduced atomic intensity and the lighter regions increased intensity. The image has been corrected for detector inhomogeneity as discussed in the text. Part (b) shows a profile taken near the top of part (a) at $\mathrm{\Omega }\approx 2\pi \times 36\mathrm{MHz}$. The initial ($\mathrm{\Delta }{v}_i$) and final velocity widths ($\mathrm{\Delta }{v}_f$) are indicated.