Coherent Manipulation On Scattering Properties Of Non-Hermitian Optical Systems

Parity-Time(PT)symmetry theory was first put forward in quantum mechanics.It has attracted great attention since it was published,and then it was introduced into optics.By accurately adjusting the gain and loss of the medium,PT symmetry can be achieved in a variety of optical systems,and many exotic phenomena have been found,such as absorption-induced-transparency,coherent perfect absorption-laser and super Bloch oscillation.Inspired by this,people realize that controlling the gain and dissipation of the medium can realize the phenomena and functions that are difficult to achieve in traditional optics devices.Some interesting research directions also emerge,such as supersymmetry,spatial Kramers-Kronig relationship and PT antisymmetry.There is no doubt that non-Hermitian optics has become one of the most popular topics in modern optics.In this paper,we mainly consider the scattering characteristics,modulation methods and potential applications of three non-Hermitian scattering structures: heterostructure cavity,grating and atomic lattice.In Chapter 3,we propose a PT symmetric heterostructure cavity model and study its special properties and potential applications.PT symmetric simple heterostructures have special scattering properties,which can be used to fabricate optical devices to achieve special functions.However,they are hard to control.For this reason,we add a vacuum layer between the two layers to make it a PT symmetrical heterostructure cavity,so the whole system can be controlled by adjusting the length of the vacuum layer.Interestingly,the appearance of vacuum layer greatly changes the original characteristics of simple heterostructures.When the incident frequency changes,there will be multiple PT symmetrical phase transition in the heterostructure cavity,and the corresponding scattering properties will also change.Using high order PT symmetric phase,people can realize filtering.Subsequently,we discuss the causes of the multiple PT symmetric phase transition.Through numerical calculation,we find that the higher order PT symmetric phase is related to the unireflectionlessness of the right side of the vacuum layer.Finally,we discuss the potential applications of this structure.For a fixed frequency incident light,unireflectionlessness and unidirectional invisibility can be achieved by adjusting the width of the vacuum layer.In addition,ordinary simple heterostructures need to satisfy specific condition to produce coherent perfect absorption and laser phenomena at a single frequency;in simple heterostructure cavities,by adjusting the length of the vacuum layer,we can always make the system at the singularity of the energy spectrum and achieve coherent perfect absorption-laser at multiple frequenciesIn Chapter 4,we discuss the intrinsic relation between asymmetric reflection and diffraction caused by non-Hermitian periodic structures.The asymmetric reflection and diffraction phenomena of PT symmetric gratings have been discussed separately,but no one has discussed the relation between them,and our work fills this gap.Firstly,we study the PT symmetric sinusoidal grating.Using the coupled mode theory,we obtain the coupled equations corresponding to the reflection and diffraction processes under the appropriate approximation,and find that the two unbalanced coupling coefficients are proportional,which means that the degree of asymmetry of reflection and diffraction should be consistent.In order to quantitatively describe this relationship,we introduce two physical quantities: the ratio of reflection contrast to diffraction.Through numerical calculation,we find that the asymmetry of reflection and diffraction always has the same tendency,and this relation goes beyond the PT symmetry.So,further,we validate the above relation through a practical non-Hermite EMI grating model,and extend it to higher-order reflection and diffraction.In Chapter 5,we use Tripod-type atomic ensemble in one-dimensional optical lattice to realize an atomic lattice with asymmetric reflection.In order to destroy the in-phase of the real and imaginary parts of the refractive index,we introduce a spatial modulation of dynamic frequency shift of the empty ground state,and prove that this modulation can be achieved by adding a standing auxiliary driving field.Since in Tripod model there are two empty ground levels,we consider two situations: single modulation and double modulation.The results show that the properties of the system are obviously different under the two cases.In particular,under single modulation,the lattice has one bidirectional reflectionless point and two unidirectional reflectionless points,while under double modulation,it has only three unidirectional reflectionless points.In addition,the optical response of the periodic structure will also change significantly,such as transmission phase,fast-and slow-light,etc.Both cases can produce non-Hermitian degeneracy,so our model can be used to study the topological properties of non-Hermitian degeneracy.Because the model has high flexibility and good expansibility,it is helpful for the following research.