光学莫尔晶格中超短脉冲的脉宽调控

Modulation of Ultrashort Pulse Width in Optical MoiréLattices

具有莫尔角的光学莫尔晶格使能带平坦化,为激光超短脉冲的脉宽调控提供了新的思路。通过组合两种不同周期的光子晶格,构建3种人工合成莫尔角逐渐增大的光学莫尔晶格,实现了莫尔晶格能带的平坦化。通过理论分析莫尔晶格的能带色散,发现人工合成莫尔角较大的莫尔晶格具有丰富的群速度色散,这导致了脉宽的剧烈变化。实验上,使用自相关仪测量了超短脉冲经莫尔晶格后的脉宽。在理论和实验上证明了莫尔晶格对超短脉冲脉宽的精准调控。提出的莫尔晶格对激光脉冲压缩器件的研究具有重要意义。

Objective Ultrashort pulses,with a pulse duration of tens of picoseconds(10–12 ps)or less,are timing tools with the highest precision available currently.Their narrow pulse width and high peak power characteristics have considerably advanced the development of nonlinear optics.However,ultrashort pulse lasers unavoidably suffer from the dispersion introduced by various optical elements during their operation,resulting in pulse deformation and power attenuation,which adversely affect the performance of ultrashort pulses.Therefore,extensive research has been conducted on pulse width regulation.The current solution is primarily based on the utilization of dispersion-compensation devices,which suffer from low integration characteristics and nonactive regulation.Recently,the optical Moiréstructure has become a widely discussed topic due to its high potential in the modulation of light field by changing Moiréangles.However,most published works introduce physical twisted angles,and optical Moiréstructures combined with the concept of synthetic dimensions have rarely been reported.Therefore,we propose an optical Moirélattice with artificially synthesized Moiréangles to achieve ultrashort pulse width modulation.Methods A method of constructing optical Moirélattices with artificially synthesized Moiréangles(nonphysical twisted angles)is proposed herein.Moirélattices comprise two simple photonic lattices with different periods,and the ratio of the arctangents of their periods represents the Moiréangle of the optical Moirélattice.Three optical Moirélattices with gradually increasing Moiréangles are theoretically designed,and the band structure of the optical Moirélattices is determined by the transfer matrix method;we observe that the increase in Moiréangles leads to the flattening of the band structure.The band dispersion is further analyzed,following which the group velocity and pulse width changes introduced by an ultrashort pulse through the Moirélattices are computed.The theoretical calculations demonstrate the effect of Moirélattices on the width of ultrashort pulses.Subsequently,an optical path based on an autocorrelator is built to experimentally verify the theoretical results.Further,we define the variation rate of pulse width to effectively illustrate the modulation of ultrashort pulses.Results and Discussions First,the effects of artificially synthesized Moiréangles on the band structure are analyzed:the increase in the Moiréangles results in higher band compression coefficients(Fig.2),which indicates a decrease in the bandwidth(Fig.3).Meanwhile,a narrow band leads to a decreased group velocity(Fig.4)and considerable second-and third-order group velocity dispersion(Fig.5).We demonstrate a theoretical model to explain the effect of group velocity dispersion on the width of ultrashort pulses(Equations 3-8).The equations imply that Moirélattices with high group velocity dispersion lead to intense pulse broadening and pulse compression of ultrashort pulses.The results of pulse width measurements before and after passing through the three Moirélattices were obtained using the autocorrelator(Table 1).As seen from Table 1,the variation in pulse width increases with the increase in the Moiréangle.The accurate modulation of the ultrashort pulse width by the optical Moirélattice is confirmed by the comparison of the theoretical and experimental values of the pulse width ratio(Fig.7).Conclusions The construction method for the optical Moirélattices in the synthetic space proposed herein can effectively realize the analogy of traditional optical Moirélattices with physical twisted angles.The regularity of the artificially synthesized Moiréangles affecting band structures has been clearly confirmed.Theoretical calculations show the following:a larger artificially synthesized Moiréangle leads to lower group velocity and more considerable second-and third-order group velocity dispersion,which results in larger variations in pulse widths.Moreover,the accurate modulation achieved by the optical Moirélattice indicates that the lattices can be predesigned to satisfy universal requirements such as a wide range of pulse width adjustments.In theory,we can design a series of Moirélattices with different artificially synthesized Moiréangles,resulting in rich and more predictable pulse width variations.In summary,first,we state that the optical Moirélattice in the synthetic dimension considerably simplifies the complexity of structural processing:traditional optical Moirélattices require precise control of the physical twisted angles of the two sublattices,while the optical Moirélattice in the synthetic space depends on strategic parameter definitions.The one-dimensional structure in the geometric space renders its processing highly convenient.Second,the optical Moirélattice provides a new degree of light field modulation,which means that a flattened band structure can be obtained by changing the Moiréangle.Finally,the overall thickness of our optical Moirélattice is at the micrometer level,which has the advantage of high integration.Our structure can become the key component in the manufacturing of laser pulse width compressors.