Title: Narrow-line Laser Cooling by Adiabatic Transfer,
Abstract: We propose and demonstrate a novel laser cooling mechanism applicable to
particles with narrow-linewidth optical transitions. By sweeping the frequency
of counter-propagating laser beams in a sawtooth manner, we cause adiabatic
transfer back and forth between the ground state and a long-lived optically
excited state. The time-ordering of these adiabatic transfers is determined by
Doppler shifts, which ensures that the associated photon recoils are in the
opposite direction to the particle's motion. This ultimately leads to a robust
cooling mechanism capable of exerting large forces via a weak transition and
with reduced reliance on spontaneous emission. We present a simple intuitive
model for the resulting frictional force, and directly demonstrate its efficacy
for increasing the total phase-space density of an atomic ensemble. We rely on
both simulation and experimental studies using the 7.5~kHz linewidth $^1$S$_0$
to $^3$P$_1$ transition in $^{88}$Sr. The reduced reliance on spontaneous
emission may allow this adiabatic sweep method to be a useful tool for cooling
particles that lack closed cycling transitions, such as molecules.