Theoretical And Experimental Study On The Generation Of Scalar And Vectorial Hollow Beams

Hollow Beams(HB)are the ring beams with zero center intensity in the propagation direction,which have a series of unique physical properties such as barrel strength distribution,smaller spot dark size(DSS),propagation invariance,spin and orbital angular momentum(OAM),and so on.The vectorial hollow beams have special electromagnetic field distributions,special polarizations and focusing characteristics.So,HBs can be widely use in laser optics,computer-generated holography,atomic and molecular optics,optical trapping of micro particles,material science and biomedicine.In this paper,we mainly study the theories and experiments on the generation of scalar and vectorial HBs.The main contents can be concluded as follows:Firstly,a nonlinear optical technique to generate a localized hollow laser beam and its array using crossed nonlinear ZnSe crystals is proposed.We calculate the intensity distributions of the HB and its propagating properties in free space,and study the dependences of the transverse and longitudinal DSS of the HB on the incident beam waist radius and the crystal length.Our study shows that 3D hollow space size of the beam can be tuned by altering the relative position of the crystals,the crystal length,and the waist radius.Such beam with a dark hollow region and a large intensity gradient can be used to trap cold neutral atoms,molecules and microscopic particles.The localized hollow beam array scheme proposed in our scheme not only enriches the diffraction-free optics,but also has potential applications for guiding and trapping of cold atoms,cold molecules,or micro particles.Then,we propose an interesting scheme to implement the nonlinear beam shaping using a thin organic salt 4-N,N-dimethylamino-4'-N'-methyl-Stilbazolium tosylate(DAST)film.Due to the nonlinear optical properties of the DAST film,the incident Gaussian laser beam can be transformed to a HB with a wavelength range from 780nm-810 nm.Experimental results show that the light beam after the DAST film can keep hollow for a certain long distance.The generation of the HB is in full agreement with our theoretical investigations.Also,we have created a localized hollow beam by vertically superposing two hollow beams output from a DAST film.This beam shaping scheme is simple,stable and compact.Also,it is easy to be integrated for the special applications,such as particle manipulations,material fabrication,quantum communications,and so on.Lastly,we present an experimental approach to convert linearly polarized Gaussian beams into elliptical and circular vector hollow beams(VHBs)with different polarization states.The scheme employed is based on a Mach–Zehnder-type optical path combined with a reflective spatial light modulator(SLM)in each path.The resulting VHBs have radial,azimuthal,and other polarization states.Our studies also show that the size of the generated VHBs remains constant during the propagation in free space over a certain distance,and can be controlled by the axial ratio of the SLM's binary phase plate.These studies deliver great optical parameters and hold promising applications in the fields of optical trapping and manipulation of particles.