Optoelectrical Cooling of Polar Molecules to Submillikelvin Temperatures

We demonstrate direct cooling of gaseous formaldehyde (H2CO) to the microkelvin regime. Our approach, optoelectrical Sisyphus cooling, provides a simple dissipative cooling method applicable to electrically trapped dipolar molecules. By reducing the temperature by 3 orders of magnitude and increasing the phase-space density by a factor of similar to 10(4), we generate an ensemble of 3 x 10(5) molecules with a temperature of about 420 mu K, populating a single rotational state with more than 80% purity.