Shiri Abbas, Yessenov Murat, Webster Scott, Schepler Kenneth L, Abouraddy Ayman F
CREOL, The College of Optics & Photonics, University of Central Florida, Orlando, FL, 32816, USA.
Department of Electrical and Computer Engineering, University of Central Florida, Orlando, FL, 32816, USA.
Nat Commun. 2020 Dec 8;11(1):6273. doi: 10.1038/s41467-020-20009-2.
Light is confined transversely and delivered axially in a waveguide. However, waveguides are lossy static structures whose modal characteristics are fundamentally determined by their boundary conditions. Here we show that unpatterned planar waveguides can provide low-loss two-dimensional waveguiding by using space-time wave packets, which are unique one-dimensional propagation-invariant pulsed optical beams. We observe hybrid guided space-time modes that are index-guided in one transverse dimension and localized along the unbounded dimension. We confirm that these fields enable overriding the boundary conditions by varying post-fabrication the group index of the fundamental mode in a 2-μm-thick, 25-mm-long silica film, achieved by modifying the field's spatio-temporal structure. Tunability of the group index over an unprecedented range from 1.26 to 1.77 is verified while maintaining a spectrally flat zero-dispersion profile. Our work paves the way to utilizing space-time wave packets in on-chip platforms, and enable phase-matching strategies that circumvent restrictions due to intrinsic material properties.
光在波导中横向受限并沿轴向传输。然而，波导是有损耗的静态结构，其模式特性从根本上由其边界条件决定。在此我们表明，无图案平面波导可以通过使用时空波包提供低损耗二维波导，时空波包是独特的一维传播不变脉冲光束。我们观察到混合引导时空模式，其在一个横向维度上是折射率引导的，并且沿无界维度局域化。我们证实，通过改变制造后2微米厚、25毫米长的二氧化硅薄膜中基模的群折射率，这些场能够超越边界条件，这是通过修改场的时空结构实现的。在保持光谱平坦的零色散分布的同时，验证了群折射率在前所未有的1.26至1.77范围内的可调性。我们的工作为在片上平台利用时空波包铺平了道路，并实现了规避由于固有材料特性而产生的限制的相位匹配策略。
