Slowing, Cooling, and Spectroscopy with Stimulated Optical Forces

University Of Connecticut, Storrs CT

Investigators

Abstract

Under normal conditions, molecules in the air move at speeds of nearly 500 meters per second, and if they were not constantly bumping into one another, they would travel an inch in just 50 millionths of a second. This grant will support research on new methods for using lasers to bring atoms or molecules to a standstill within this same one-inch range, by producing decelerations a million times the force of gravity. For the case of atoms, the objective is to improve upon existing methods, by greatly increasing the force and the velocity range over which the force is usable. For molecules, it has proven to be extremely difficult to apply standard methods, owing to the relative complexity of the internal quantum structure of molecules. Thus a particular emphasis will be placed on effective schemes for decelerating and cooling molecules, using as a test system the calcium monofluoride molecule, CaF. Optimal approaches will be determined by a combination of numerical modeling and experimental tests using laser beams that contain light at multiple colors simultaneously. The resulting cold atoms and molecules will be investigated using lasers to probe their internal energy structure. The apparatus will also be used to study the interaction of molecules with each other under unusual conditions. There are possible applications to chemical physics, cold plasma formation, electric field calibration, and quantum computation. The project will help train and educate undergraduate and graduate students. The proposed research addresses three main motivations: to develop and understand the physics of stimulated optical forces, to achieve large improvements in direct laser slowing and cooling of atomic and molecular beams, and to investigate excited-state structure and dynamics in atomic He and molecular CaF. Helium and CaF are chosen partially for their fundamental interest, and partially because they are excellent candidates for manipulation by stimulated optical forces. The stimulated bichromatic force (BCF) and the newly-proposed polychromatic force (PCF) are rectified coherent optical forces from multi-frequency laser beams that can be hundreds of times stronger than the conventional radiative force. As part of the proposed research, a prototype chirped-BCF decelerator for He will be completed, and a promising new four-color PCF version will be tested and compared with calculations. A similar decelerator will be developed for the CaF molecule, a species that has attracted considerable recent interest as a candidate for direct laser slowing and cooling. For molecules, there is an additional advantage to utilizing coherent optical forces beyond merely increasing the force. Because hundreds of BCF momentum exchanges can occur for each radiative decay event, the BCF can reduce the velocity by hundreds of times more than incoherent radiative forces before the molecule decays into an inaccessible "dark" state. Several spectroscopic experiments will be initiated on the resulting cold atoms and molecules, including studies of cold gases of highly-excited Rydberg states of CaF, and precise laser spectroscopy of both He and CaF. The study of cold molecules in Rydberg states is a little-explored topic likely to involve new physical phenomena. Precise spectroscopy of helium will facilitate tests of basic theory and pave the way for future ultra-precise measurements. Novel applications of metastable helium beams will also be explored.