Creation Of Second-harmonic Optical Needle By Nonlinear Super-oscillatorv Lens

The focus of beam has always been a fascinating topic because focusing the light beam can improve the laser power density and reduce the spot diameter effectively.A focusing spot beyond the diffraction limit will play a key role in many field,for example,laser medicine,laser processing,laser nuclear fusion,optical data storage,nano optical imaging and so on.Furthermore,we can use axicon lens or other small aperture devices to create an ultralong optical needle with pure radially or azimuthally polarization,which has great importance to the field of control and acceleration of particles,light-matter interaction and nano photonics.However,the longitudinal component of optical needle created by the above method is too weak to be put into practice.Recently,with the discovery of super-oscillatory theory,the scientists begin to devote their energy to the focus of light beam though super-oscillatory lens(SOL).Super-oscillation is a complementary concept where a source of super-oscillatory fields can form,by precisely controlling the interference,an arbitrarily small spot at a fixed distance from the mask.The sub-wavelength spot created by amplitude or phase SOL is crucial to be applied on the super-resolution microscope.In principle,there are no limitations on the size of the hot-spot.With the development of super-oscillatory lens,scientists discover that SOL can not only used in super focusing experiment but also has significant effect on the formation of sub-wavelength optical needle.Scientists have achieved several optical needles beyond the diffraction limit with different lengths,however,they focused their attention on linear SOL and the length of achieved needles are below 20?.Depend on super-oscillatory theory and sub-diffraction theory,we have carried out several work based on nonlinear optics as follows:1.The design of SOL.Firstly,the desired sub-wavelength hot-spot is represented as a sum of prolate spheroidal wave functions.Secondly,the prolate spheroidal wave function is decomposed into plane waves.By using the angular spectrum theory,the required complex mask transmission function t(x)can be readily derived.Since we want to seek the extreme of N-dmensional distribution function,the genetic algorithm can be used to obtain the optimize structure of SOL.2.The experimental realization of second-harmonic(SH)optical needle.We experimentally achieved sub-diffracted optical needle by manipulating diffraction interference of the generated second-harmonic fields.Linear polarized light generated by a femtosecond mode-locked laser will pass though Q-plate to get a stable radially polarized beam.A conventional microscope with high NA objective and CCD(Nikon DS-Fi2)camera is used to capture the pattern generated by the designed mask at various distances away from the surface.We can experimentally achieve an optical needle with a sub-diffraction beam size of 0.42? and an ultra-long length of 40A.3.Setup of an optical probe scanning system.In order to further verify the experiment results above,we construct a scanning system with a probe.In the experiment,we capture the diffraction patterns generated by the designed mask at various distances away from the surface,which agree well with the imaging with a CCD camera.