【作者】 陈浩；

【导师】 王兵；

【作者基本信息】 华中科技大学 ， 光学， 2021， 博士

【摘要】 新型人工结构和材料的开发是当今光学领域的主要研究课题之一。在迄今为止的大多数研究中,此类结构主要通过操控折射率的空间分布来实现光子调控,其中典型的结构便是光子晶格。将空间光子晶格概念推广到时间、频率、相位和角动量等合成维度(synthetic dimensions)中,构成合成光子晶格(synthetic lattices),可以把能带理论应用于调控光子中,以更加灵活、可控的方式在多个维度中控制光子的输运。此外,在合成晶格中引入等效电场和等效磁场,不仅可以在光学系统中模拟电子中的量子效应,还可以实现光子的拓扑调控。目前,利用光学微腔、动态调制波导和光纤网络等光学结构和系统,针对时域和频域晶格中的光子调控研究已广泛展开,不仅为设计开发新型光通信和光信息处理器件开辟了新的途径,也为验证高维度空间中的物理现象和实现光子的拓扑效应提供了有效的平台。本文依托光纤环路系统的灵活性,利用相位调制和光纤色散,在合成时间和频率维度下构造等效光子晶格,调控光脉冲时域波形和频谱包络的演化,并实现光子的拓扑效应。本文研究内容分为以下几方面:1.采用具有失谐相位调制的光纤环路,在实验上实时观测了频域晶格中的布洛赫振荡。相位调制引起具有等频率间隔的模式之间的跃迁,形成离散的频域光子晶格。脉冲在光纤环路中的循环时间和调制信号周期之间的时间失谐量充当等效矢势提供恒定的等效电场力,实现了频率维度下的布洛赫振荡。利用色散傅里叶变换技术,将脉冲频谱映射到时间维度中,达到了对快速演化的脉冲频谱进行精确测量的目的。这项研究为在频率维度中模拟固体物理中的电子动力学过程奠定了基础,也为频谱成像、频率复用等光谱操控技术提供新的物理机制。2.利用带有相位调制器的光纤单环,在时间维度中研究光脉冲在异质界面上的折射和反射。其中,相位调制提供时变周期势场,构建一维时域光子晶格;光纤色散实现光子在势场中的隧穿,使光脉冲在时域晶格中发生离散衍射(discrete diffraction)。周期性地改变调制信号的强度和频率,可以构造出垂直界面并实现脉冲的全反射,界面处的反射可以通过改变入射脉冲的布洛赫波矢进行控制。当调制信号仅发生一次变化或将垂直与倾斜晶格拼接,则可形成水平界面,并实现时域脉冲的负折射。该研究将在脉冲整形和光信号处理等领域中有重要应用价值。3.将基频和倍频调制信号叠加,在时间维度中构造二聚体光子晶格(dimerized lattices)。通过构建垂直界面,在时间维度下研究拓扑边界态,抑制脉冲在光纤内的色散展宽。相对于倍频信号,连续改变基频信号中的相位,构造动态变化的时域光子晶格,使晶格内的耦合强度和在位势能在参数空间中形成包含奇点的闭合路径,借此研究光子绝热泵浦,实现光脉冲的量子化输运。该工作论证了在一维时域晶格中探索光子拓扑性质的可行性,为拓扑光子学研究提供了有效的平台。4.设计光纤网络构造二维时域光子晶格,在理论上探索了脉冲在晶格中的离散衍射和布洛赫振荡。耦合器实现脉冲间的耦合,结合光纤网络内的时延差,将相互耦合的脉冲映射到二维空间中,从而在合成时间维度中等效地构造出二维光子晶格。改变耦合器分光比形成界面,研究时域蜂窝晶格中的边界及相关的拓扑特性,并实现赝自旋依赖的拓扑边界态。该研究为在低维空间中探索高维度物理问题提供了可行方案,在二维成像、时分复用等领域具有应用价值。更多还原

【Abstract】 The development of novel artificial structures and materials is one of the major research topics in optics.In most studies so far,such structures have been applied to modulate photon flow by designing the refraction index in real space.Analogies of the artificial structures,the concept of photonic lattices has been extended to synthetic dimensions of time,frequency,phase and angular momentum and constructed the synthetic lattices.Based on band theory in solids,the synthetic lattices provide a flexible and controllable method to manipulate photons in different dimensions.Additionally,it is possible to emulate the quantum effects of electrons and investigate the topological properties of photons by introducing the effective scalar potential and vector potential.At present,the synthetic time and frequency lattices have been studied in various optical structures and systems,such as optical micro-resonators,modulated waveguides and optical fiber network.The above studies not only provide a new exploration direction for the design and development of novel optical communication and optical information processing devices,but offer an effective platform for studying the physical phenomena and topological properties of photons in a high dimensional space.In the work,the synthetic time and frequency lattices are constructed by using optical fiber-loop circuits with phase modulation and fiber dispersion,which can be used to manipulate the evolution of pulse waveform and spectrum envelope.The main contents are listed as follows:Firstly,we experimentally observe the frequency Bloch oscillations in a modulated fibre-loop circuit with detuned time.The spectrum evolution occurs in a frequency lattice which is formed by the phase modulation.The detuned time between the circulating time of the fibre loop and modulation period plays the role of an effective vector potential and gives rise to the Bloch oscillations in frequency dimension.Additionally,the transient evolution of the spectrum can be precisely measured by the dispersive Fourier transformation(DFT)technique which maps the pulse spectrum into the temporal waveform.The study not only introduces a novel platform to emulate the electron dynamics in solid state physics,but provides a new application scenario for Bloch oscillations such as the spectral imaging and multiplexing.Secondly,by using a fiber loop with phase modulation,the refraction and reflection of optical pulse have been studied at several kinds of heterointerface in time domain.The phase modulation provides the periodic potential and the fiber dispersion enables the tunneling of photons in the potential.Thus the one-dimension lattices can be constructed in time dimension.By periodically changing the modulation depth or frequency,the heterointerface is vertical and may induces total internal reflection.The temporal refraction can be adjusted by setting the initial Bloch wave vector at incidence.As the variation of modulation occurs at a specific moment,a horizontal interface could be introduced.The combination between straight and tilted lattice could provide another way to control the temporal refraction.The pulse negative refraction in time dimension could also be observed.The study may finds applications in optical communication and signals processing.Thirdly,we construct a dimerized lattices in time dimension by composing a fundamental frequency and frequency-doubled modulation signals.Based on the dimerized lattices,the topological boundary states can be achieved at a vertical interaface to suppress the fiber dispersion.Relative to frequency-doubled signal,the phase of fundamental frequency modulation is linearly adjusted continuously and slowly,so that the coupling strength and on-site potential of the lattices are changed and form a closed path around a degeneracy point in parameter space.Then the charge adiabatic pumping can be extended to the time lattices and induces a quantized transport of optical pulse.The study demonstrates the feasibility for exploring the topological properties of photons in one-dimensional time lattices and provides a novel platform for topological photonics.Finally,we design a fiber network to construct the two-dimensional synthetic time lattices to realize discrete diffraction and Bloch oscillations of optical pulses.Here the length difference among the fibers in network is used to introduce time delay,which generate a series of pulse trains.Each pulse corresponds to the site in the two-dimensional lattices.The coupling between the adjacent sites is supported by the optical couplers in the network.Using a variable optical coupler,two different boundaries and the related topological properties are studied in a honeycomb lattice.Then a contracted and expanded honeycomb lattices can also be constructed.By combining the lattices and forming a vertical interface,we construct pseudospin topological edge states which induce a directional transport of optical pulse in time domain.The works provide a feasible method for the studies of high-dimensional physical problems and have applications in two-dimensional imaging and time division multiplexing.更多还原