Theoretical Study On Optical Dipole Traps For Cold Atoms Or Molecules Using A Small-hole Diffracted Light Field

We review the basic principles and methods for guide cold atoms and the experimental progresses by using red-detuned Gaussian beam, Gaussian mode in hollow optical fibers (HOF), blue-detuned evanescent-wave light in the HOFs andblue-detuned TE₀₁ mode in hollow metallic waveguide as well as blue-detuned darkhollow beams. Also, we introduce the basic principles and experimental schemes for trap cold atoms and the relevant experimental results, including the red-detuned Gaussian laser trap, blue-detuned laser trap and a controllable double-well optical dipole trap as well as the optical lattice trap.We propose a novel scheme to form an optical dipole trap for cold neutral atoms (or cold molecules) by using a small-hole diffracted light field. From the Reylaigh-Sommerfeld diffraction theory and the Fresnel diffraction one, we calculatethe intensity distribution of the optical trap for cold CH₄ molecules and the opticalpotential, and derive some analytical relations between the characteristic parameters of the optical trap (including geometric parameters, intensity gradients and their curvatures) and the parameters of the optical system (including laser wavelength A and aperture radius a). Our study shows that the volume of the optical trap isproportional to a⁴.Moreover, we propose a new scheme to improve the red-detuned Gaussian optical trap by using a binary phase plate. When the laser beam passes through binary phase plate and the circular aperture, the constructive interference or destructive one will be induced due to the phase modulation of the phase plate. Our study finds thatwith the change of the phaseφ, the maximal absolute intensity, optical potential andthe trapping volume will be changed. When a -Ï€. phase plate is used, the maximal absolute diffracted intensity will be increased by 4 times, the trapping volume will be increased by～8.6 times, and the corresponding optical potential will be increased by about 4 times. Our study shows that a binary -Ï€phase plate can be used to improve the intensity distribution of the optical trap. Also, if our small-hole diffracted method is extended to the case of 1D or 2D array of circular holes and 1D or 2D -Ï€phase grating, a 1D or 2D array of optical traps for cold atoms (or molecules), that is, a novel 1D or 2D optical lattice, will be formed.