新型人工材料中量子系统的动力学演化及非经典性质

发布时间：2019-05-28 18:37

【摘要】：处于真空中的激发原子,由于受到均匀涨落电磁模式的影响,将自发的辐射出光子,从激发态跃迁至基态。在认识到材料能够修饰原子的自发辐射过程之后,人们开始探索各种新型材料对真空环境的影响。随着实验生产技术的发展,特异材料(metamaterials)、拓扑绝缘体(topological insulators)和石墨烯(graphene)等新型人工材料在2004年前后得以制备。其中,特异材料是基于自然材料的认知框架上加以设计和制备的,具有负折射率及电磁透明等独特的光学性质。在拓扑绝缘体中,当无能隙表面态的时间反演对称性遭到破坏时,其光学性质将受到拓扑量的修饰而表现出类似法拉第旋光效应等特殊的电磁现象。而在石墨烯薄层中,通过调节门电压或化学掺杂的浓度及类型,能够调控其在不同频段的光学性质。在太赫兹频段,石墨烯将表现出类金属的性质,并支持表面等离子体模式的传播。结合上述新型材料的特性,在本文中我们研究了不同材料中原子自发辐射的性质,并根据材料不同的光学性质实现原子间的纠缠,量子干涉及共振荧光谱压缩等-系列非经典现象。首先,我们研究了两个二能级体系在零折射材料中的纠缠特性。其中,零折射材料由两种不同类型,厚度相同的单负材料板组成。为了能够激发两平板材料交界处的表面场并与之产生较强的耦合,我们将原子对置于交界面附近。假设系统处于单激发态,根据薛定谔方程,在不做马尔科夫近似的情况下我们得到了系统的几率幅演化方程。当材料厚度远大于表面场的特征长度时,格林函数能够得到简化并具有解析表达式。通过求解系统的运动方程可以得知,存在一个临界值,当对称模式、反对称模式与表面场的相互作用强度处于其两端时,系统将表现出不同的动力学性质。具体来说,对应于不同情况,系统的演化将分别表现出马尔科夫行为及强相互作用下的非马尔科夫行为。而体系的纠缠受到初态的影响,将表现出从纠缠态逐渐衰减直至消失或随时间逐渐增大,并长时间保持等特性。此外,当原子的跃迁频率与表面场的共振频率发生失谐时,若原子问的相互关联较强,依然能够产生纠缠。其次,我们研究了由拓扑绝缘体组成的光学微腔中,三能级Zeeman原子的量子干涉效应。由于拓扑电磁效应的存在,当微腔的长度小于半个真空波长时,原子平行于腔镜的偶极跃迁将受到抑制,而垂直方向的偶极跃迁则得到加强。而当拓扑电磁效应极强时,平行于腔镜的偶极辐射将完全消失,此时处于腔中的原子能够产生极强的量子干涉效应。若微腔的长度增大,由于腔内电磁场的不均匀分布,将使得此时量子干涉的强度取决于原子在腔中所处的位置,呈现出相干电磁波叠加后的波动特性。实际情况下,材料将存在一定的能量损耗。结果表明,该损耗仅在腔镜附近一段很小的区域内对原子的自发辐射有着较大的影响。因而当原子处于该区域时,由于其它电磁模式对自发辐射的贡献小于耗散对原子的影响,量子干涉效应将受到破坏而大幅度下降。在原子远离腔镜后,损耗对量子干涉的影响将逐渐消失,对应的情况与无损耗时基本一致。最后,我们讨论了处于石墨烯表面附近的二能级量子点的辐射性质。在太赫兹频段,量子点的Purcell系数随频率变化而表现出近似洛伦兹型的分布。并且,随着环境温度的增加,该分布将趋于平均,同时Purcell系数较零温时有所下降。将量子点置于表面等离子体场的工作区域内,通过调节泵浦激光场的强度和中心频率,当修饰量子点对应的两条跃迁通道以不同的速率进行衰变时,共振荧光谱中将出现压缩现象。适当的减小量子点到石墨烯的距离,使得量子点与表面场的耦合强度增大,可以克服量子点退相干作用对压缩的破坏。当修饰量子点的布居差极大时,合理地选取实验参数,室温下的压缩强度将有可能大于零温时的情况。此外,即便是在室温下,通过调节石墨烯的费米能及泵浦光场的强度及中心频率,量子点荧光谱中的压缩现象将能够得到极大增强。
[Abstract]:The excitation atoms in the vacuum, due to the influence of the uniform fluctuation electromagnetic mode, will spontaneously radiate the photons and transition from the excited state to the ground state. After recognizing the self-emitting process of the material capable of modifying the atoms, the effect of a variety of new materials on the vacuum environment has been initiated. With the development of experimental production technology, new types of artificial materials, such as metal materials, topological insulators, and graphenene, were prepared before and after 2004. In which the specific material is designed and prepared on a cognitive framework based on natural materials, and has unique optical properties such as negative refractive index and electromagnetic transparency. In the topological insulator, when the time inversion symmetry of the inept surface state is destroyed, the optical property of the topological insulator is affected by the modification of the topological quantity, and special electromagnetic phenomena such as the Faraday rotation effect can be displayed. In the graphene thin layer, the optical properties in different frequency bands can be controlled by adjusting the concentration and the type of the gate voltage or the chemical doping. In the terahertz frequency band, the graphene will exhibit the nature of the metalloid and support the propagation of the surface plasma mode. In this paper, the properties of atom self-emission in different materials are studied in this paper, and the entanglement, quantum interference and resonance fluorescence spectrum compression of atoms in different materials are studied in this paper. First, we have studied the entanglement of two two-level system in the zero-refractive material. Wherein the zero-refraction material consists of two different types of single-negative material plates with the same thickness. In order to be able to excite the surface field at the junction of the two plate materials and to generate a strong coupling, we place the atomic pairs near the interface. It is assumed that the system is in a single excited state. According to the Schrodinger equation, the probability amplitude evolution equation of the system is obtained without the Markov approximation. When the thickness of the material is much larger than the characteristic length of the surface field, the green function can be simplified and has an analytical expression. By solving the motion equation of the system, it is known that there is a critical value, and when the interaction intensity of the symmetric mode, the antisymmetric mode and the surface field is at both ends, the system will exhibit different dynamic properties. In particular, the evolution of the system will show the Markov behavior and the non-Markov behavior under strong interaction, respectively. The entanglement of the system is affected by the initial state, which will show the gradual attenuation from the entangled state until it disappears or gradually increases with time, and keeps the characteristics for a long time. In addition, when the transition frequency of the atom and the resonance frequency of the surface field are detuned, entanglement can still be generated if the inter-correlation of the atoms is strong. Secondly, we study the quantum interference effect of three-level Zeeman atom in the optical microcavity composed of topological insulator. Due to the existence of the topological electromagnetic effect, when the length of the micro-cavity is less than half a vacuum wavelength, the dipole transition of the atoms parallel to the cavity mirror is suppressed, and the dipole transition in the vertical direction is strengthened. And when the topological electromagnetic effect is very strong, the dipole radiation parallel to the cavity mirror disappears completely, and the atoms in the cavity can generate extremely strong quantum interference effect. If the length of the micro-cavity is increased, due to the non-uniform distribution of the electromagnetic field in the cavity, the intensity of the quantum interference at this time is dependent on the position of the atom in the cavity, and the wave characteristic after the superposition of the coherent electromagnetic wave is presented. In practice, a certain amount of energy loss will be present in the material. The results show that this loss has a great effect on the spontaneous emission of atoms in a small area near the cavity mirror. Thus, when the atom is in the region, the contribution of other electromagnetic modes to the spontaneous emission is less than the effect of the dissipation on the atoms, and the quantum interference effect will be destroyed and greatly reduced. After the atom is far from the cavity mirror, the effect of loss on the quantum interference will gradually disappear, and the corresponding condition is basically the same as that of no loss. Finally, we discuss the radiation properties of two-level quantum dots near the surface of the graphene. In the terahertz frequency band, the Purcell coefficient of the quantum dot shows an approximate lorentz-type distribution along with the frequency change. And, with the increase of the ambient temperature, the distribution will tend to average while the Purcell coefficient decreases at zero temperature. The quantum dots are placed in the working area of the surface plasma field, and by adjusting the intensity and the center frequency of the pumping laser field, when the two transition channels corresponding to the modified quantum dots decay at different rates, the resonance fluorescence spectrum will exhibit a compression phenomenon. And the distance between the quantum point and the graphene is appropriately reduced, so that the coupling strength of the quantum dot and the surface field is increased, and the damage to the compression can be overcome by overcoming the quantum dot rejection coherence effect. When the distribution of the modified quantum dots is great, the experimental parameters can be reasonably selected, and the compression strength at room temperature will be more than zero temperature. In addition, even at room temperature, by adjusting the Fermi energy of the graphene and the intensity and the center frequency of the pump light field, the compression phenomenon in the quantum dot fluorescence spectrum can be greatly enhanced.
【学位授予单位】：华中师范大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O413

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