Discipline:Condensed Matter Physics,Optics,Acoustics
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Space-Time Wave Packet in Dispersive Media
Our new paper titled “Nondispersive Space-Time Wave Packet Propagating in Dispersive Media” is published in Laser & Photonics Reviews.
In this work, we propose a new scheme to overcome chromatic dispersion in any dispersive medium at arbitrary wavelengths without using additional compensating elements. Specifically, we introduce a new type of propagation-invariant space-time wave packets that exhibit neither diffraction nor dispersion in a dispersive medium. Such wave packets have a special form of spatio-temporal spectral correlation that precisely compensates the chromatic dispersion under propagation. We also discover a modified law of refraction for such non-dispersive space-time wave packets.
Our work highlights the significant opportunities of spatio-temporal engineering in dispersion compensation and may enable potential applications of space-time wave packets. Our work also points to the significant opportunities for creating space-time wavepackets in other general wave systems with different dispersion relations, such as acoustic waves or quantum waves.
Fig 1：a-c Light cone of free space (a) and dispersive medium (b, c). d Gaussian wave packet spreads in dispersive medium while the space-time wave packet remains invariant. e-h Spectrum and intensity distribution of Gaussian wave packet. i-l Spectrum and intensity distribution of space-time wave packet.
Fig 2：Modified law of refraction for space-time wave packet.
For full text: https://onlinelibrary.wiley.com/doi/10.1002/lpor.202100634
Improving the Robustness of Bound States in the Continuum by Harnessing Higher-order Topological Charges
Bound states in the continuum (BICs) have attracted broad research interests owing to their excellent performance in light confinement, which can boost light-matter interaction. BICs can eliminate the radiation loss to achieve an infinity quality factor Q theoretically. However, in practical on-chip resonators, there are inevitable fabrication imperfections that would couple BIC to nearby radiative states by scattering, thus limiting the available Q. To suppress the scattering loss, the improvement of light confinement in the nearby radiative states is required. Multiple BICs can be tuned to the same position to form a merging BIC using the topological properties of BIC. This physical mechanism can significantly enhance the Q of nearby states over a broad wavevector range and improve the robustness of BICs against the scattering loss of fabrication imperfections.
For photonic crystal slabs, the polarizations of the far-field radiation form a polarization vortex around a BIC in momentum space. The BIC is located at the topological singularity, whose polarization direction cannot be defined, so there is no radiation loss. The winding number of polarizations along the counterclockwise direction defines the topological charge of BICs. BICs are topologically protected following the topological charge conservation. They are tunable in momentum space with the variation of structural parameters.
However, to date, the construction of merging BICs involves only the manipulation of fundamental topological charges.
In a new paper published in Light Science & Applications, Prof. Meng Xiao from Wuhan University, Prof. Hongxing Xu’s team from Wuhan University, and Prof. Che Ting Chan from the Hong Kong University of Science and Technology cooperated to propose a novel physical mechanism for realizing merging BICs through manipulating higher-order topological charges.
In the photonic crystal slab with a triangular lattice, there is a symmetry-protected BIC with a topological charge of -2 at the G points. When the thickness of the slab is appropriate, accidental BICs with topological charges of ±1 appear at off-G points, which are formed by the accidental cancellation of radiation loss because of destructive interference. By varying the thickness, multiple accidental BICs can be simultaneously tuned to the G points, forming a merging BIC with a higher-order charged BIC. The Q factors of the nearby radiative states have been significantly enhanced for nearby radiative states compared with isolated BICs or merging BICs with only fundamental charges. In short summary, merging BICs involving higher-order charges can further improve the light confinement and suppress the scattering loss caused by fabrication imperfections.
By replacing cylindrical holes with elliptical cylindrical holes, the symmetry of the structure is reduced and higher-order charged BICs are no longer allowed at the G point. Because of topological charge conservation, BICs with the higher-order topological charge split into off-G BICs. The split BICs can be tuned in momentum space by varying the structural parameters. They can be simultaneously tuned to the G point to form a merging BIC, or to the same position with accidental BICs and form a merging BIC at off-G points.
By rotating elliptical cylindrical holes, the mirror symmetry is further broken and BICs are turned away from the mirror plane. By choosing a suitable rotation angle and slab thickness, the merging BIC are steerable with a designed momentum, which is of great significance for improving the performance of direction-rrelated applications.
Figure| Merging BICs by harnessing higher-order topological charges.
For full text：https://www.nature.com/articles/s41377-022-00923-4
Acoustic Topological Dislocation Modes
Our new paper titled “Topological dislocation modes in three-dimensional acoustic topological insulators” is published in Nature Communications.
In this work, we experimentally construct a three-dimensional (3D) acoustic topological insulator with precisely-controlled dislocations and present an unambiguous evidence for the long-sought bulk-dislocation correspondence. The delicate design of our acoustic system enables us to observe not only the topological dislocation modes (TDMs) in momentum-resolved frequency spectroscopy but also their spatial localization in pressure-field distributions. The topological robustness of the TDMs is identified by introducing a spin-preserved defect to the dislocation. Significantly, the TDMs observed here exhibit flexibilities in wave manipulations since the dislocation lines can be deformed at will in 3D space. As such, we can design dislocation waveguides of arbitrary shapes and unidirectionally guide the TDMs along any prescribed routes inside the bulk materials, which are conclusively identified by our acoustic experiments.
Our findings will stimulate the study on the highly-intriguing interaction between the real-space topology and band topology. The peculiar topological dislocation transport points to new possibilities for information communication and energy transportation.
Fig. 1. TDMs guided by the dislocations in a 3D topological insulator.
Fig. 2. Numerical demonstrations of the acoustic TDMs and their topological transports.
Fig. 3. Experimental characterizations of the acoustic TDMs and their topological robustness to defects.
For full text:
Photonic Dirac Nodal Line Semimetal
Our new paper titled “Double-bowl State in photonic Dirac nodal line semimetal” is published in Light: Science & Applications.
In this work we investigate a photonic Dirac nodal line semimetal (DNLS), which is not a trivial spinless extension of the electronic DNLS, and thus indicates the possibility of identifying new mechanisms unique for photonics to protect topological phases. We provide a stringent photonic realization of type-II DNLS with a ring shape four-fold band degeneracy. We measure the dispersion of the DNLS along relevant directions and thus provide the smoking gun for the realization of photonic DNLS. We deposit a silver film on top of this photonic DNLS and identify the double-bowl surface states possessed by this composite system through angle-resolved spectra.
Our system can serve as an ideal platform for investigating phenomena that require large field enhancement as well as manipulating light with an arbitrary polarization while studying polarization-sensitive light-matter interactions.
Fig. 1. Photonic Dirac nodal line semimetal.
Fig. 2. Experimental observation of photonic Dirac nodal ring.
Fig. 3. Observation of the double-bowl states.
For full text:
Second Harmonic Enhanced with Photonic Weyl “Fermi Arcs” on LNOI Chips
Our new paper titled “Probing rotated Weyl physics on nonlinear lithium niobate-on-insulator chips” is published in Physical Review Letters.
In this work, we report the first topological device on a lithium niobate-on-insulator (LNOI) chip. We design a 1D quaternary waveguide lattice on a LNOI chip. Based on this system, we obtain Weyl points and the Fermi arc edge states induced by them with the aid of a synthetic parameter space. We also explore the intriguing rotated Weyl physics in a 3D synthetic space. We find whether the interface between two Weyl structures can host gapless topological interface states or not, is determined by the relative rotational angle of the two Weyl structures. If the two Weyl structures rotate in the same (opposite) direction, topological interface states presence (absence). We also observed experimentally the second-order nonlinear enhancement of the topological interface state due to the second-order nonlinearity of the lithium niobate material.
Our work shows that LNOI provides a flexible platform for many other topological devices. It also opens up a new paradigm for experimentally exploring various applications in integrated nonlinear and quantum optics.
Fig. 1.(a) Cross section diagram of a unit cell that consists four waveguides which simulates the Weyl physics in (b). (c)Domain wall between two Weyl systems formed with waveguides on LNOI chips.
For full text:
Topological Rainbow Energy Concentrator
We propose a method to realize topological rainbow concentrator based on synthetic dimension. This work is published in Physical Review Letters.
Synthetic dimension provides a new platform for realizing topological photonic devices. We realize a topological rainbow concentrator by constructing a synthetic dimension. The synthetic dimension is constructed using a translational degree of freedom of the nanostructures inside the unit cell of a two-dimensional photonic crystal. The translational deformation induces a nontrivial topology in the synthetic dimension, which gives rise to robust interface states at different frequencies. The topological rainbow can trap states with different frequencies, controlled by tuning the spatial modulation of interface state group velocities. The operation frequency as well as the bandwidth of the topological rainbow can be easily tuned by controlling the band gap of the photonic crystal.
Our work provides a universal method for the realization of topological rainbow concentrators based on the notion of synthetic dimension, and it will enable practical applications such as topological routers and optical information processing.
Fig. 1. Topological rainbow concentrator constructed in the synthetic dimension.
For full text:
Free Merging BICs
Our new research on merging bound states in the continuum (BICs) has been published in Physical Review Letters.
BICs are localized resonances embedded in the continuum spectrum by eliminating radiation loss. They have unique advantages in trapping light. Merging BICs can suppress the scattering loss induced by the fabrication imperfections but they are only limited to Γ point before our work.
In this work, we propose a new scheme to construct merging BICs at almost an arbitrary point in the reciprocal space. Instead of merging only accidental BICs at Γ point in the previous work, we show the concurrence of an accidental BIC and a Friedrich-Wintgen BIC (FW-BIC) and then tune them to merge at an off-Γ point. We design a PCS made of Si3N4 and immersed in a liquid (common in lab) to demonstrate our idea and such a proposal can be readily verified experimentally. By breaking the in-plane mirror symmetry, the merging BIC can be tuned to almost an arbitrary position in the reciprocal space. Since FW-BICs and accidental BICs are quite common on the band structure, our proposal provides a general approach to realize off-Γ merging BICs.
Our work enriches the explorations of topological photonics and can boost applications desiring strong light-matter interactions, including nonlinear effects, sensors, lasing and optoelectronic devices.
Fig. 1. Merging the FW BIC and accidental BIC with opposite topological charge
For full text:
Nature Collection for the 40th Anniversary of the Quantum Hall Effect
The discovery of quantum Hall effect in 1980 has inspired new theories and led to experimental discoveries in a range of fields. To celebrate the diverse legacy of this discovery, Nature selects its related review, news and commentary articles and makes a collection. Two review articles from our group have been included in this collection. One is published in Nature Communications Physics in 2018, entitled “Topological sound”. The other is published in Nature Reviews Physics in 2019, entitled “Topological phases in acoustic and mechanical systems”.
Link to this Nature collection:
For the two works mentioned above:
Higher Order Topological Insulator: Acoustic Quadrupole Topological Insulator
Our recent experimental work about acoustic quadrupole topological insulators is published in Physical Review Letters.
Recently, tremendous efforts have been devoted to realizing high-order topological insulators (HOTIs). Quadrupole topological insulators (QTIs), featured with nontrivial bulk quadrupole moments, are proposed as the first type of HOTIs. However, the tight-binding model proposed for such emergent topological insulators demands both positive and negative hopping coefficients, which poses a great challenge in practical realizations.
In this Letter, we introduce a simple mechanism to construct positive and negative hoppings in acoustics. We present the first experimental realization of the acoustic QTI that stringently fulfills the tight-binding model proposed for QTI theory. The hierarchy topology of our acoustic QTI has been conclusively identified through detecting the acoustic responses at the bulk, edge and corner. The arbitrary controlling of the real-valued hoppings enables the further investigation of rich physics inherent in Z2-gauge transformation. Our study can also be extended to other HOTIs such as three-dimensional octupole topological insulators and semimetals, and open new application avenues for the robust and highly confined topological in-gap states.
Fig. 1. Acoustic realizations of negative and positive hoppings.
Fig. 2. Realizations of acoustic quadrupole topological insulators and experimental demonstration.
For full text:
Topologically Charged Nodal Surface Identified with Acoustic Metacrystal
Our new research on semimetal with topologically charged nodal surface is published in Science Advances, entitled “Experimental demonstration of acoustic semimetal with topologically charged nodal surface”.
This paper proposes, for the first time, the concept of topologically charged nodal surface, and shows that such an object can be implemented in a tight-binding model as well as a suitably designed 3D phononic crystal that works for acoustic waves. Furthermore, we have experimentally measured the acoustic analogue of “Femi-arcs” for the topologically-protected surface states, which serves as an evidence for the nonzero charge possessed by the nodal surface. Strikingly different from those well-known Weyl semimetals where the surface state arcs are pinned to the Weyl points, here the end of the arcs depend sensitively on the surface truncation of the acoustic semimetal.
Our work indicates that topologically charged objects in a band structure is not restricted to 0D, and points to abundant unexplored features of the novel phenomena associated with the topological charges such as chiral anomaly and negative magnetoresistance in condensed matter systems.
Fig. 1. A 3D phononic crystal designed with a charged nodal surface.
For full text:
Meron Spin Textures Found in Momentum Space
We reveal the intrinsic meron pseudospin texture in momentum space in a photonic crystal slab, which can be directly observed as meron and antimeron spin texture by polarimetric study of high-order diffracted light from the system. This work is published in Physics Review Letters.
Spin textures, the spin configurations in either real space or momentum space, are of great interest in various fields of physics and are relevant to many nontrivial physical phenomena. Skyrmions, merons and antimerons are non-singular topological spin textures that have been extensively studied in various systems. Photons are massless spin-1 particles, thus skyrmion-related objects can emerge as transverse spin textures, i.e., polarization of photons as well.
In this work, we report meron and antimeron in momentum space using a honeycomb photonic crystal slab. The existence of such objects has not been previously noted either in electronic or photonic systems. Breaking the inversion symmetry of a honeycomb photonic crystal gaps out the Dirac cones at the corners of Brillouin zone. The spin textures of photonic bands near the gaps exhibit a meron or antimeron. Unlike the electronic systems, the spin texture of the photonic modes manifests directly in the polarization of the leakage radiation, as the Dirac points can be above the light line.
Our work highlights the significant opportunities of using photonic structures for the exploration of topological spin textures, with potential applications towards topologically robust ways to manipulate polarizations and other modal characteristics of light.
Fig. 1. The spin textures of the leakage radiation on the iso-frequency contours of the photonic band structure.
For full text:
Our latest review article is published in Nature Reviews Physics, entitled “Topological phases in acoustic and mechanical systems”.
The study of classical wave physics has been reinvigorated by incorporating the concept of the geometric phase, which has its roots in optics, and topological notions that were previously explored in condensed matter physics. Recently, sound waves and a variety of mechanical systems have emerged as excellent platforms that exemplify the universality and diversity of topological phases.
In this Review, we introduce the essential physical concepts that underpin various classes of topological phenomena realized in acoustic and mechanical systems: Dirac points, the quantum Hall, quantum spin Hall and valley Hall effects, Floquet topological phases, 3D gapless states, Weyl crystals and etc.
For full text: