Experimental Realization of a Reflections-Free Compact Delay Line Based on a Photonic Topological Insulator

Abstract

Electromagnetic (EM) waves propagating through an inhomogeneous medium are generally scattered whenever the medium's electromagnetic properties change on the scale of a single wavelength. This fundamental phenomenon constrains how optical structures are designed and interfaced with each other. Recent theoretical work indicates that electromagnetic structures collectively known as photonic topological insulators (PTIs) can be employed to overcome this fundamental limitation, thereby paving the way for ultra-compact photonic structures that no longer have to be wavelength-scale smooth. Here we present the first experimental demonstration of a photonic delay line based on topologically protected surface electromagnetic waves (TPSWs) between two PTIs which are the EM counterparts of the quantum spin-Hall topological insulators in condensed matter. Unlike conventional guided EM waves that do not benefit from topological protection, TPSWs are shown to experience multiwavelength reflection-free time delays when detoured around sharply-curved paths, thus offering a unique paradigm for compact and efficient wave buffers and other devices.The existence of localized TPSWs at the PTI's 1-13 edge 6,7,10 , or at an interface between two PTIs with different electromagnetic properties 1,3,11,12,14 , holds great promise for photonic applications. Their scattering-free propagation along sharply-curved paths 14 opens exciting opportunities across the electromagnetic spectrum, including optical isolators 15,16 multiple-input multiple-output communications systems 17 , and topologically robust broadband optical buffers and time delay lines 18,19 . Remarkably, while the latter set of applications was the original motivation 6 for PTI development, an experimental demonstration of such functionality has been elusive. For example, in one successful implementation of topologically protected edge transport that utilized an ensemble of high-Q resonators 7 , statistical properties of time delays were measured 20 . However, the combination of finite disorder and sharp resonances makes the development of a single-channel delay line in a given photonic structure extremely challenging.An alternative PTI platform emulates the quantum spin Hall (QSH) 21-24 effect by introducing a photonic analog of spin-orbital interaction using bianisotropic metamaterials 11 , as well as uniaxial 12 or bianisotropic 14 metawaveguides. Unlike nonreciprocal PTIs 3-5 , it does not require an external magnetic field or ferromagnetic materials. The spin degree of freedom can then be interpreted as the phase relationship between transverse electric (TE) and magnetic (TM) modes of the metamaterial-in-phase for the spin-up and out-of-phase for the spin-down states. In this Letter, an interface between two QSH PTIs is used to experimentally demonstrate a single-channel topologically protected delay line that employs only the edge modes and is not influenced by the bulk modes. Strong suppression of the bulk modes with respect to the edge modes is crucial because the forme...

Recent advances in 2D, 3D and higher-order topological photonics

“…Spin-polarized edge states with topological protection have been demonstrated numerically using a metamaterial composed of a hexagonal array of bianisotropic rods 8 and experimentally using a slightly modified design 146 . Bianisotropy has also stimulated studies on topologically nontrivial 2D photonic crystals, called bianisotropic metawaveguides with relaxed design complexity [147][148][149][150] .…”

Quantitative robustness analysis of topological edge modes in C6 and valley-Hall metamaterial waveguides

“…The recent progress in the field of photonic topological insulators (TIs) has led to the discovery of several schemes of inducing edge states at topological interfaces between photonic insulators [1][2][3][4][5][6][7][8][9]. Not only the presence of edge modes is guaranteed by the topology of the surrounding bulk insulators, but also in some cases the edge modes are immune to backscattering even when defects are present along the interface, such as geometrical disorder or interface bends [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. This is always the case for Chern insulators, a class of photonic insulators whose topology relies on time-reversal symmetry breaking, and induces non-reciprocal unidirectional transport on the topological boundary [6,20,21].…”

Near-field imaging of spin-locked edge states in all-dielectric topological metasurfaces

“…The band structure plotted in Fig. 1(b) reveals two pairs of Dirac bands, stemming from electric and magnetic dipolar modes of the disks, respectively, and their degeneracy effectively restores the dual symmetry responsible for the topological properties we exploit here 17,28,26,39 . The field configurations for the corresponding dipolar modes carrying angular momentum 𝑙 = ±1 are shown in Fig.…”

“…The experimental realization of photonic topological systems have demonstrated fascinating properties for their electromagnetic modes and opened avenues for their use in practical systems and devices 11,15,14,22,23,24,25,26,27 . Unfortunately, many of these original proposals have been based on metal containing metamaterials, 22,28,29,30,26 whose functionality cannot be extended into IR and visible domains. In this respect, all-dielectric topological structures based on arrays of ring resonators 14,31,32 or chiral waveguides 15,27,33 have demonstrated desirable properties at optical frequencies.…”

“…Interestingly, only two of the modes appear to cross the entire topological band gap, while the second pair touches the light line and disappears in the middle of the gap after emerging from the continuum of the bottom bulk modes. This is important distinction of the open metasurface from its closed photonic counterparts in which 4 topological edge modes would also cross the entire bandgap due to the absence of the light cone and the continuum of modes within it 17,29,26 . The inspection of the field profiles for the two sets of modes also shows that the ones crossing the gap represent a set of even modes symmetric across the domain wall (Fig.…”