Investigations On The Atomic Coherence Effects Under Spatial Modulation

With the continuous development of the technologies of the laser,micro/nano-fabrication and atomic manipulation,much attention has been paid to the atomic coherence effects under spatial modulation.Note that the so-called spatial modulation refers to the modulation of the spatial distribution of the coherent atomic medium or coherent laser field.As so far,the atomic coherence effects under spatial modulation have been used to realize many interesting optical phenomena such as subluminal and superluminal pulse transmission,Goos-H?nchen shift,two-color lasing,radiation damping optical enhancement,atom localization,electromagnetically induced photonic band-gap,electromagnetically induced grating,parity-time?PT?symmetry and so on.A thorough and systematic study on these optical phenomena may be helpful to the developments of the fields of optical communication,optical sensing,optical information process,high-resolution imaging,etc.In this thesis for the Doctorate,by using the semiclassical theory of field-atom interaction,Fraunhofer diffraction theory and Goos-H?nchen shift theory,we have mainly realized the lopsided Raman-Nath diffraction with high diffraction efficiency in non-Hermitian atomic gratings and investigated the effect of disorder on the lopsided Raman-Nath diffraction in disordered atomic grating.Moreover,we have obtained the giant enhancement of Goos-H?nchen shift via the control of a broadband squeezed vacuum field.The main contents are as follows:1)We propose and analyze an efficient scheme for the lopsided Raman-Nath diffraction of one-dimensional?1D?and two-dimensional?2D?atomic gratings with PT-symmetric refractive index.The atomic grating is constructed by the cold atomic vapor with three-level?-typed⁸⁷Rb and⁸⁵Rb atoms.Using experimentally achievable parameters,we identify the conditions under which PT-symmetric atomic grating allows us to observe the lop-sided Raman-Nath diffraction phenomenon with high diffraction efficiency,i.e.,the exceptional point?EP?.The nontrivial atomic grating is a superposition of an amplitude grating and a phase grating.It is found that the lopsided Raman-Nath diffraction at the EP of PT-symmetric grating originates from constructive and destructive interferences between the amplitude and phase gratings.Furthermore,we show that the PT phase transition from unbroken to broken PT-symmetric regimes can modify the asymmetric distribution of diffraction spectrum.Finally,we also analyze the influence of the grating thickness on the diffraction spectrum.Then,using the similar approach,an efficient scheme for realizing the lopsided Raman-Nath diffraction is proposed in 2D gain-assisted atomic grating with PT-antisymmetric susceptibility.The atomic grating constructed by 2D cold atomic lattices with four-level N-typed configuration.We find that gain-assisted PT antisymmetry allows us to realize lopsided Raman-Nath diffraction with high diffraction efficiency at the EP.It is shown that the nontrivial phenomenon is related to non-Hermitian degeneracy of PT antisymmetry.In addition,we investigate the influence of the phase of the modulated frequency detuning.It is found that the phase can be exploited for the control of the diffraction direction of the atomic grating.These two schemes offer opportunities to design special beam splitters,converters and routers,which have potential applications in optical communication,optical storage,optical information process,etc.2)We investigate the effects of geometrical and structural disorders on the lop-sided Raman-Nath diffraction in Raman-Nath regime.The two types of disorders are realized by introducing random fluctuations in the position and width of 1D driven atomic lattices.Raman-Nath diffraction is modified differently with respect to the geometrical and structural disorders.It is shown that the lopsided Raman-Nath diffraction is destroyed with the increase of the disorder strength,while it is rather robust against the structural disorder.The different behaviors originate from the disorder-induced random variations of the spatial phase shifts of the standing-wave coupling field and atomic lattices with Gaussian profile.Furthermore,we find that,in the presence of geometrical disorder,the lop-sided Raman-Nath diffraction is more susceptible to correlated disorder than to uncorrelated disorder.Our scheme may be useful for understanding the diffraction behaviors of light and matter waves in disordered potentials.3)We propose an efficient scheme to enhance the Goos-H?nchen shifts of the reflected and transmitted beams in a cavity containing two-level atomic medium.A broadband squeezed vacuum field is injected into the cavity to interact with the atomic medium.In the bad cavity limit,the Bloch equations for the atomic operators are identical to those in the free space,but with the modification of the system parameters.Using experimentally achievable parameters,we identify the conditions under which the squeezed vacuum allows us to enhance the Goos-H?nchen shifts of the reflected and transmitted beams.It is shown that the enhanced Goos-H?nchen shifts originate from the coherent population oscillations?CPOs?controlled by the squeezed vacuum field.Furthermore,we also find that the Goos-H?nchen shifts of the reflected and transmitted beams depend sensitively on the relative phase between the control field and the squeezed vacuum field.Subsequently we propose a scheme for such a configuration of the Goos-H?nchen shift as a family of hypersensitized displacement sensors.Based on the numerical analysis,the detection sensitivity for the tiny displacement can reach approximately2340?m/nm,which is improved in comparison with previous schemes.In conclusion,this thesis deepens our awareness and understanding of the characteristics of the atomic coherence effects under spatial modulation.These investigations may have some reference value for the developments of correlative subjects such as atomic and molecular physics,laser physics,quantum optics,diffraction optics,etc.