arXiv subjects

Xiankai Sun

Publications and source records attributed to Xiankai Sun.

At least 19 recordsLinked to original sources

arXiv2023

Tunable chiral anomalies and coherent transport on a honeycomb lattice

The search for energy efficient materials is urged not only by the needs of modern electronics but also by emerging applications in neuromorphic computing and artificial intelligence. Currently, there exist two mechanisms for achieving dissipationless transport: superconductivity and the quantum Hall effect. Here we reveal that dissipationless transport is theoretically achievable on a honeycomb lattice by rational design of chiral anomalies tunable without any magnetic fields. Breaking the usual assumption of commensurability and applying an external electric field lead to electronic modes exhibiting chiral anomalies capable of dissipationless transport in the material bulk, rather than on the edge. As the electric field increases, the system reaches a cubic-like dispersion material phase. While providing performance comparable to other known honeycomb lattice-based ballistic conductors such as an armchair nanotube, zigzag nanoribbon and hypothetical cumulenic carbyne, this scheme provides routes to a strongly correlated localization due to flat band dispersion and to exotic cubic dispersion material featuring a pitchfork bifurcation and a critical slowing down phenomena. These results open a new research avenue for the design of energy efficient information processing and higher-order dispersion materials.

cond-mat.mes-hall↗

arXiv2022

Visual observation of optical Floquet-Bloch oscillations

Bloch oscillations, an important transport phenomenon, have extensively been studied in static systems but remain largely unexplored in Floquet systems. Here, we propose a new type of Bloch oscillations, namely the "Floquet-Bloch oscillations," which refer to rescaled Bloch oscillations with a period of extended least common multiple of the modulation and Bloch periods. We report the first visual observation of such Floquet-Bloch oscillations in femtosecond-laser-written waveguide arrays by using waveguide fluorescence microscopy. These Floquet-Bloch oscillations exhibit exotic properties, such as fractal spectrum and fractional Floquet tunneling. This new transport mechanism offers an intriguing method of wave manipulation, which has significant applications in coherent quantum transport.

physics.optics↗

arXiv2022

Surface acoustic microwave photonic filters on etchless lithium niobate integrated platform

Lithium niobate on insulator emerges as a promising platform for integrated microwave photonics because of its capability of ultralow-loss guidance and high-efficiency modulation of light. Chip-level integration of microwave filters is important for signal processing in the 5G/6G wireless communication. Here, we employed the principle of bound states in the continuum for low-loss waveguiding on an etchless lithium niobate integrated platform, and realized high-performance microwave photonic filters thereon. These microwave photonic filters consist of a high-quality photonic microcavity modulated by piezoelectrically excited surface acoustic waves. Acoustic time delays from 21 to 106 ns and passbands with bandwidth as narrow as 0.89 MHz were achieved in the fabricated filters operating at gigahertz frequencies. Our demonstration may open up new applications on the lithium niobate integrated platform, such as optical communication, signal processing, and beam steering.

physics.optics↗

arXiv2022

Realization of bound states in the continuum in anti-PT-symmetric optical systems

Novel physical concepts that originate from quantum mechanics, such as non-Hermitian systems (dealing mostly with PT and anti-PT symmetry) and bound states in the continuum (BICs), have attracted great interest in the optics and photonics community. To date, BICs and anti-PT symmetry seem to be two independent topics. Here, we for the first time propose a parallel cascaded-resonator system to achieve BICs and anti-PT symmetry simultaneously. We found that the requirements for the Fabry-Pérot BIC and anti-PT symmetry can both be satisfied when the phase shift between any two adjacent resonators is an integer multiple of π. We further analyzed the cascaded-resonator systems which consist of different numbers of resonators and demonstrated their robustness to fabrication imperfections. The proposed structure can readily be realized on an integrated photonic platform, which can have many applications that benefit from the advantages of both BICs and anti-PT symmetry, such as ultralow-linewidth lasing, enhanced optical sensing, and optical signal processing.

physics.optics↗

arXiv2021

Second-Harmonic Generation in Etchless Lithium Niobate Nanophotonic Waveguides with Bound States in the Continuum

Bound states in the continuum (BICs) have been extensively studied in various systems since its first proposal in quantum mechanics. Photonic BICs can enable optical mode confinement and provide field enhancement for nonlinear optics, but they have rarely been explored in nonlinear integrated photonic waveguides. Applying BICs in photonic integrated circuits enables low-loss light guidance and routing in low-refractive-index waveguides on high-refractive-index substrates, which is suitable for integrated photonics with nonlinear materials. Here, we report experimental demonstration of second-harmonic generation from telecom to near-visible wavelengths on an etchless lithium niobate platform by using a photonic BIC for the second-harmonic mode. The devices feature second-harmonic conversion efficiency of 0.175%W-1cm-2 and excellent thermal stability with a wavelength shift of only 1.7 nm from 25°C to 100°C. Our results represent a new paradigm of nonlinear integrated photonics on a cost-effective and convenient platform, which can enable a broad range of on-chip applications such as optical parametric generation, signal processing, and quantum photonics.

physics.optics↗

arXiv2021

Highly tunable broadband coherent wavelength conversion with a fiber-based optomechanical system

The modern information networks are built on hybrid systems working at disparate optical wavelengths. Coherent interconnects for converting photons between different wavelengths are highly desired. Although coherent interconnects have conventionally been realized with nonlinear optical effects, those systems require demanding experimental conditions such as phase matching and/or cavity enhancement, which not only bring difficulties in experimental implementation but also set a narrow operating bandwidth (typically in MHz to GHz range as determined by the cavity linewidth). Here, we propose and experimentally demonstrate coherent information transfer between two orthogonally propagating light beams of disparate wavelengths in a fiber-based optomechanical system, which does not require any sort of phase matching or cavity enhancement of the pump beam. The coherent process is demonstrated by phenomena of optomechanically induced transparency and absorption. Our scheme not only significantly simplifies the experimental implementation of coherent wavelength conversion, but also extends the operating bandwidth to that of an optical fiber (tens of THz), which will enable a broad range of coherent-optics-based applications such as optical sensing, spectroscopy, and communication.

physics.optics↗

arXiv2021

Observation of bound states in the continuum in a micromechanical resonator

Bound states in the continuum (BICs) refer to physical states that possess intrinsic zero dissipation loss even though they are located in the continuous energy spectrum. BICs have been widely explored in optical and acoustic structures, leading to applications in sensing and lasing. Realizing BICs in micromechanical structures is of significant importance for both fundamental research and engineering applications. Here, we fabricated, with CMOS-compatible processes on a silicon chip, a wheel-shaped micromechanical resonator, in which we experimentally observed the BIC in the micromechanical domain. Such BICs result from destructive interference between two dissipative modes of the mechanical structure under broken azimuthal symmetry. These BICs are found to be robust against size variations of the dissipation channels. The demonstrated mechanical BIC can be obtained with a large and robust supporting structure, which substantially reduces device fabrication difficulty and allows for its operation in versatile environments for broader application areas. Our results open a new way of phonon trapping in micromechanical structures with dissipation channels, and produce long phonon lifetimes that are desired in many mechanical applications such as mechanical oscillators, sensors, and quantum information processors.

physics.app-ph↗

arXiv2021

A topological Dirac-vortex parametric phonon laser

Nonlinear topological photonic and phononic systems have recently aroused intense interests in exploring new phenomena that have no counterparts in electronic systems. The squeezed bosonic interaction in these systems is particularly interesting, because it can modify the vacuum fluctuations of topological states, drive them into instabilities, and lead to topological parametric lasers. However, these phenomena remain experimentally elusive because of limited nonlinearities in most existing topological bosonic systems. Here, we experimentally realized topological parametric lasers based on nonlinear nanoelectromechanical Dirac-vortex cavities with strong squeezed interaction. Specifically, we parametrically drove the Dirac-vortex cavities to provide phase-sensitive amplification for topological phonons, and observed phonon lasing above the threshold. Additionally, we confirmed that the lasing frequency is robust against fabrication disorders and that the free spectral range defies the universal inverse scaling law with increased cavity size, which benefit the realization of large-area single-mode lasers. Our results represent an important advance in experimental investigations of topological physics with large bosonic nonlinearities and parametric gain.

cond-mat.mes-hall↗

arXiv2021

Room-temperature continuous-wave Dirac-vortex topological lasers on silicon

Robust laser sources are a fundamental building block for contemporary information technologies. Originating from condensed-matter physics, the concept of topology has recently entered the realm of optics, offering fundamentally new design principles for lasers with enhanced robustness. In analogy to the well-known Majorana fermions in topological superconductors, Dirac-vortex states have recently been investigated in passive photonic systems and are now considered as a promising candidate for single-mode large-area lasers. Here, we experimentally realize the first Dirac-vortex topological lasers in InAs/InGaAs quantum-dot materials monolithically grown on a silicon substrate. We observe room-temperature continuous-wave single-mode linearly polarized vertical laser emission at a telecom wavelength. Most importantly, we confirm that the wavelength of the Dirac-vortex laser is topologically robust against variations in the cavity size, and its free spectral range defies the universal inverse scaling law with the cavity size. These lasers will play an important role in CMOS-compatible photonic and optoelectronic systems on a chip.

physics.optics↗

arXiv2021

Unraveling the Angular Symmetry of Optical Force in a Solid Dielectric

The textbook-accepted formulation of electromagnetic force was proposed by Lorentz in the 19th century, but its validity has been challenged due to incompatibility with the special relativity and momentum conservation. The Einstein-Laub formulation, which can reconcile those conflicts, was suggested as an alternative to the Lorentz formulation. However, intense debates on the exact force are still going on due to lack of experimental evidence. Here, we report the first experimental investigation of angular symmetry of optical force inside a solid dielectric, aiming to distinguish the two formulations. The experiments surprisingly show that the optical force exerted by a Gaussian beam has components with the angular mode number of both 2 and 0, which cannot be explained solely by the Lorentz or the Einstein-Laub formulation. Instead, we found a modified Helmholtz theory by combining the Lorentz force with additional electrostrictive force could explain our experimental results. Our results represent a fundamental leap forward in determining the correct force formulation, and will update the working principles of many applications involving electromagnetic forces.

physics.optics↗

arXiv2020

Nanomechanical topological insulators with an auxiliary orbital degree of freedom

Discrete degrees of freedom, such as spin and orbital, can provide intriguing strategies to manipulate electrons, photons, and phonons. With a spin degree of freedom, topological insulators have stimulated intense interests in condensed-matter physics, optics, acoustics, and mechanics. However, orbital as another fundamental attribute in crystals has seldom been investigated in topological insulators. Here, we invent a new type of topological insulators with an auxiliary orbital degree of freedom on a nanomechanical platform. We experimentally realized nanomechanical topological insulators where the orbital can arbitrarily be manipulated by the crystal. Harnessing this unique feature, we demonstrated adiabatic transition between distinct topological edge states, which is a crucial functionality for complicated systems that involve distinct topological edge channels. Beyond the one-dimensional edge states, we further constructed zero-dimensional Dirac-vortex states. Our results have unveiled unprecedented strategies to manipulate topological phase transitions and to study topological phases of matter on an integrated platform.

cond-mat.mes-hall↗

arXiv2020

Gigahertz acousto-optic modulation and frequency shifting on etchless lithium niobate integrated platform

Acousto-optic interactions involving propagating phonons can break the time-reversal and frequency-modulation symmetry of light. However, conventional acousto-optic modulators based on bulk materials have frequency bandwidth limited to hundreds of megahertz due to their large structural sizes. Here, we experimentally demonstrate gigahertz single-sideband acousto-optic modulation on an etchless lithium niobate integrated platform by using photonic bound states in the continuum. The upper- or lower-sideband modulation of light can be obtained conveniently by choosing specific combinations of input and output channels. Under this scheme, we have realized a 3-GHz frequency shifter, which operates in the C-band with a 3-dB bandwidth of ~35 nm. The extinction ratios of the upper(lower)-sideband modulated light to the lower(upper)-sideband modulated and unmodulated light are >44 (47) and 25 (23) dB in the 3-dB operating bandwidth. The frequency-shifted light can be further processed with amplitude and frequency modulation. Therefore, the demonstrated gigahertz single-sideband acousto-optic modulation can enable many photonic applications such as optical signal processing, sensing, and ion trapping.

physics.app-ph↗

arXiv2019

Hyperuniform disordered photonic crystal polarizers

An ultra-broadband transverse magnetic (TM) pass hyperuniform disordered photonic crystal (HUDPC) polarizer is proposed and demonstrated on a silicon-on-insulator platform. Propagation of the transverse electric mode is blocked by three combined effects, including the photonic bandgap (PBG) effect, diffusive (non-resonant) scattering, and bandedge resonances. Specially, the designed 30-dB bandwidth in polarization extinction ratio (PER) of 265 nm is much larger than the spectral width of the PBG (149 nm) due to using the bandedge resonances. The TM mode is in the subwavelength regime of the HUDPC and thus has a low insertion loss (IL). An ultrawide 30-dB bandwidth in PER of 210 nm (1.44-1.65 um) is experimentally demonstrated in a 12.9-um-long HUDPC polarizer with spectrally averaged PER of 39.6 dB and IL for the TM mode of 1.1 dB (IL = 0.6 dB at 1.55 um). The HUDPC polarizers can be an excellent candidate for ultra-broadband polarization filtering in the silicon photonic platform.

physics.optics↗

arXiv2019

Topological photonic integrated circuits based on valley kink states

Valley pseudospin, a new degree of freedom in photonic lattices, provides an intriguing way to manipulate photons and enhance the robustness of optical networks. Here we experimentally demonstrated topological waveguiding, refracting, resonating, and routing of valley-polarized photons in integrated circuits. Specifically, we show that at the domain wall between photonic crystals of different topological valley phases, there exists a topologically protected valley kink state that is backscattering-free at sharp bends and terminals. We further harnessed these valley kink states for constructing high-Q topological photonic crystal cavities with tortuously shaped cavity geometries. We also demonstrated a novel optical routing scheme at an intersection of multiple valley kink states, where light splits counterintuitively due to the valley pseudospin of photons. These results will not only lead to robust optical communication and signal processing, but also open the door for fundamental research of topological photonics in areas such as lasing, quantum photon-pair generation, and optomechanics.

physics.app-ph↗

arXiv2019

Photonic integrated circuits in the continuum

Waves that are perfectly confined in the continuous spectrum of radiating waves without interaction with them are known as bound states in the continuum (BICs). Despite recent discoveries of BICs in nanophotonics, full routing and control of BICs are yet to be explored. Here, we experimentally demonstrate BICs in a fundamentally new photonic architecture by patterning a low-refractive-index material on a high-refractive-index substrate, where dissipation to the substrate continuum is eliminated by engineering the geometric parameters. Pivotal BIC-based photonic components are demonstrated, including waveguides, microcavities, directional couplers, and modulators. Therefore, this work presents the critical step of photonic integrated circuits in the continuum, and enables the exploration of new single-crystal materials on an integrated photonic platform without the fabrication challenges of patterning the single-crystal materials. The demonstrated lithium niobate platform will facilitate development of functional photonic integrated circuits for optical communications, nonlinear optics at the single photon level as well as scalable photonic quantum information processors.

physics.optics↗

arXiv2016

Hybrid graphene/silicon integrated optical isolators with photonic spin-orbit interaction

Optical isolators are an important building block in photonic computation and communication. In traditional optics, isolators are realized with magneto-optical garnets. However, it remains challenging to incorporate such materials on an integrated platform because of the difficulty in material growth and bulky device footprint. Here, we propose an ultracompact integrated isolator by exploiting graphene's magneto-optical property on a silicon-on-insulator platform. The photonic nonreciprocity is achieved because the cyclotrons in graphene experiencing different optical spin exhibit different response to counterpropagating light. Taking advantage of cavity resonance effects, we have numerically optimized a device design, which shows excellent isolation performance with the extinction ratio over 45 dB and the insertion loss around 12 dB at a wavelength near 1.55 um. Featuring graphene's CMOS compatibility and substantially reduced device footprint, our proposal sheds light to monolithic integration of nonreciprocal photonic devices.

physics.optics↗

arXiv2013

Nonlinear optical effects of ultrahigh-Q silicon photonic nanocavities immersed in superfluid helium

Photonic nanocavities are a key component in many applications because of their capability of trapping and storing photons and enhancing interactions of light with various functional materials and structures. The maximal number of photons that can be stored in silicon photonic cavities is limited by the free-carrier and thermo-optic effects at room temperature. To reduce such effects, we performed the first experimental study of optical nonlinearities in ultrahigh-Q silicon disk nanocavities at cryogenic temperatures in a superfluid helium environment. At elevated input power, the cavity transmission spectra exhibit distinct blue-shifted bistability behavior when temperature crosses the liquid helium lambda point. At even lower temperatures, the spectra restore to symmetric Lorentzian shapes. Under this condition, we obtain a large stored intracavity photon number of about 40,000, which is limited ultimately by the local helium phase transition. These new discoveries are explained by theoretical calculations and numerical simulations.

physics.optics↗

arXiv2013

Cavity piezooptomechanics: piezoelectrically excited, optically transduced optomechanical resonators

We present a monolithic integrated aluminum nitride (AlN) optomechanical resonator in which the mechanical motion is actuated by piezoelectric force and the displacement is transduced by a high-Q optical cavity. The AlN optomechanical resonator is excited from a radio-frequency electrode via a small air gap to eliminate resonator-to-electrode loss. We observe the electrically excited mechanical motion at 47.3 MHz, 1.04 GHz, and 3.12 GHz, corresponding to the 1st, 2nd, and 4th radial-contour mode of the wheel resonator respectively. An equivalent circuit model is developed to describe the observed Fano-like resonance spectrum.

cond-mat.mes-hall↗