Laser cooling to sub-Doppler temperatures by optical molasses is thought to be inhibited in atoms with unresolved, near-degenerate hyperfine structure in the excited state. We demonstrate that such cooling is possible in one to three dimensions, not only near the standard D2 line for laser cooling, but over a wide range extending to the D1 line. Via a combination of Sisyphus cooling followed by adiabatic expansion, we reach temperatures as low as 40 µK, which corresponds to atomic velocities a factor of 2.6 above the limit imposed by a single photon recoil. Our method requires modest laser power at a frequency within reach of standard frequency locking methods. It is largely insensitive to laser power, polarization and detuning, magnetic fields, and initial hyperfine populations. Our results suggest that optical molasses should be possible with all alkali species.Sisyphus cooling of neutral atoms is vital for reaching the ultracold temperatures needed in experiments ranging from metrology [1] to quantum information [2]. It is simple to apply for species with resolved hyperfine structure, e.g. sodium [3], cesium [4], and rubidium [5], which have thus become workhorses in atomic physics. A new generation of experiments, however, requires atoms offering lighter mass or bosonic and fermionic isotopes. Achieving sub-Doppler temperatures with these atoms has relied upon sympathetic and/or evaporative cooling -methods that are intrinsically lossy, require timescales of seconds, and favorable collisional properties -or optical lattices that require high laser intensities [6] or detunings of several hundred gigahertz [7]. Sisyphus cooling has been demonstrated with potassium [8,9], which has partially resolved hyperfine structure, but the same method does not apply to lithium, which has inverted and unresolved hyperfine structure [9]. Standard Sisyphus cooling of lithium, which has not been demonstrated [10][11][12][13][14][15], would open the door to simpler, faster, and more efficient experiments in ultracold chemistry, quantum gas microscopy, quantum simulation, and tests of the equivalence principle [16][17][18][19][20][21][22][23][24]. Recently, a sub-Doppler cooling scheme for lithium has been demonstrated using a bichromatic lattice and enhancement from a Λ-type level structure [25]. Here, we demonstrate simple, efficient (up to ∼ 45% cooled fraction), Sisyphus cooling of lithium to temperatures as low as 40 µK in one dimension to 100 µK in three dimensions using polarization-gradient cooling beams with a detuning of 1-9 GHz. These detunings can be produced using a standard offset laser lock. The cooling process operates on a timescale of milliseconds and requires only modest laser power. It can thus be integrated easily into experiments using existing diode lasers.Historically, when lithium was first laser cooled, sub-
