= Optical molasses
{wiki}
Optical molasses surrounds slow <chemical atoms> with opposing red-detuned <laser> beams, usually three pairs along perpendicular axes. In the elementary <Doppler cooling> picture, each pair damps one component of <velocity>. The result resembles viscous drag, even though the atoms are moving through a <vacuum> rather than a liquid.
Uniform balanced <optical molasses> supplies <velocity> <damping>, without a <restoring force> towards a chosen position. An atom at rest has zero mean <force> wherever it sits within the uniform illuminated region. Without noise or external <forces>, $m\dot v=-\alpha v$ gives
$$
v(t)=v_0e^{-\alpha t/m},\qquad
x(t)=x_0+\frac{mv_0}{\alpha}\bigl(1-e^{-\alpha t/m}\bigr).
$$
The atom approaches a position determined by its initial condition, not the beam intersection. Recoil-driven <diffusion> and <gravity> eventually remove atoms from finite beams. A long residence time in <optical molasses> is therefore compatible with the absence of position trapping. An <optical trap> provides spatial confinement by a different <force> mechanism; a <magneto-optical trap> adds a position-dependent <magnetic field> and suitable beam <electromagnetic polarization> to create a <restoring force>.
Back to article page
