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Xianfeng Chen

Publications and source records attributed to Xianfeng Chen.

At least 37 records · Page 2Linked to original sources

The Magnetic Dislocation in Photonics

The dislocation created in the topological material lays the foundation of many significant findings to control light but requires delicate fabrication of the material. To extend its flexibility and reconfigurability, we propose the magnetic dislocation concept and unveil its properties in a representative model, which effectively combines the topological defect and edge mode at the magnetic domain wall. The results include distinct localization modes and robust light trapping phenomena with the rainbow feature where the eigen-energy of each light-trapping state can be linearly tuned by the magnetic dislocation. The conversion from the trapping state to edge modes can be harnessed by further adiabatically pumping light across an amount of the magnitude of the magnetic dislocation. Our work solves a fundamental problem by introducing magnetic dislocation with new light-manipulation flexibility, which may be implemented in a variety of platforms in photonic, acoustics, and optomechanics with dynamic modulations and synthetic dimensions.

physics.optics↗

Extreme narrow band in Moiré Photonic time crystal

The Moiré superlattice has attracted growing interest in the electromagnetic and optical communities. Here, we extend this concept to time-varying photonic systems by superposing two binary modulations on the refractive index with different modulation periods, i.e., the Moiré photonic time crystal (PTC). Such a Moiré PTC leads to extreme narrow bands in momentum space which supports temporal localized modes, exhibiting periodically self-reconstructing pulse in time domain. We investigate how the modulation parameters change the bandstructure of the Moiré PTC and the temporal localization behavior. Moreover, we explore mode-locking mechanism in frequency space in the Moiré PTC, which points towards potential applications in mode-locked lasers with tunable time width of the emitted pulses. Our work therefore extends the study of PTC to complex modulation patterns, and unveils new possibility in wave manipulation with time-varying systems.

physics.optics↗

Construction of various time-dependent Hamiltonians on a single photonic chip

Integrated photonics provides an important platform for simulating physical models with high-performance chip-scale devices, where the lattice size and the time-dependence of a model are key ingredients for further enriching the functionality of a photonic chip. Here, we propose and demonstrate the construction of various time-dependent Hamiltonian models using a single microresonator on thin-film lithium niobate chip. Such an integrated microresonator holds high quality factor to 10^6, and supports the construction of the synthetic frequency lattice with effective lattice sites up to 152 under the electro-optic modulation. By further applying a bichromatic modulation composed of two radio-frequency signals oppositely detuned from the resonant frequency in the microresonator, we build different time-dependent Hamiltonians with the time-varying nearest-neighbor coupling strength in synthetic frequency lattice. We measure the temporal features from capturing the dynamic band structures of the lattice and demonstrate a variety of time-dependent synthetic lattice models by engineering the driven pattern of the modulation, highlighting great flexibility of the microresonator. Our work shows a photonic chip for simulating versatile time-dependent Hamiltonians, which pushes forward quantum simulations in integrated photonics with great experimental tunability and reconfigurability.

physics.optics↗

3D Orbital Angular Momentum Nonlinear Holography

Orbital angular momentum (OAM), due to its theoretically orthogonal and unbounded helical phase index, has been utilized as an independent physical degree of freedom for ultrahigh-capacity information encryption. However, the imaging distance of an OAM hologram is typically inflexible and determined by the focal length of the Fourier transform lens placed behind the hologram. Here, 3D orbital angular momentum holography is proposed and implemented. The Fourier transform between the holographic plane and imaging plane is performed by superimposing Fresnel zone plates (FZP) onto the computer-generated holograms (CGH). The CGH is binarized and fabricated on the birefringence lithium niobate crystal by femtosecond laser micromachining. Experimental verification demonstrates the feasibility of the encoding method. Moreover, by superimposing FZPs with different focal lengths into various OAM channels, OAM-multiplexing holograms are constructed. Target images are separately projected to different planes, thereby enabling 3D multi-plane holographic imaging with low crosstalk. The interval between adjacent imaging planes can be uniform and minimal, free from depth of field constraints, thus achieving high longitudinal resolution. This work achieves OAM holography in a more compact manner and further expands its applicability.

physics.optics↗

Enhanced Second-Harmonic Generation in Thin-Film Lithium Niobate Circular Bragg Nanocavity

Second-order nonlinearity gives rise to many distinctive physical phenomena, e.g., second-harmonic generation, which plays an important role in fundamental science and various applications. Lithium niobate, one of the most widely used nonlinear crystals, exhibits strong second-order nonlinear effects and electro-optic properties. However, its moderate refractive index and etching sidewall angle limit its capability in confining light into nanoscales, restricting its application in nanophotonics. Here, we exploit nanocavities formed by second-order circular Bragg gratings, which support resonant anapole modes to achieve highly enhanced SHG in thin film lithium niobate. The CBG nanocavity exhibits a record-high normalized conversion efficiency of $1.21\times10^{-2}\mathrm{cm^2/GW}$ under the pump intensity of $1.9$ $\mathrm{MW/cm^2}$. An SHG enhancement of $42,000$ is realized compared to TFLN. Besides, we also show s- and p-polarization independent SHG in elliptical Bragg nanocavities. This work could inspire studying nonlinear optics at the nanoscale on TFLN as well as other novel photonic platforms.

physics.optics↗

Controlling quasi-parametric amplifications: From multiple PT-symmetry phase transitions to non-Hermitian sensing

Quasi-parametric amplification (QPA) is a nonlinear interaction in which the idler wave is depleted through some loss mechanism. QPA plays an important role in signal amplification in ultrafast photonics and quantum light generation. The QPA process has a number of features characterized by the non-Hermitian parity-time ($\mathcal{PT}$) symmetry. In this report, we explore new interaction regimes and uncover multiple $\mathcal{PT}$-symmetry phase transitions in such QPA process where transitions are particularly sensitive to external parameters. In particular, we demonstrate the feasibility of detection of $10^{-11}$ inhomogeneities of the doped absorber, which is order of magnitude more sensitive than similar measurements performed in a linear absorption regime. In doing so, we reveal a family of $\mathcal{PT}$-symmetry phase transitions appearing in the QPA process and provide a novel nonlinear optical sensing mechanism for precise optical measurements.

physics.optics↗

Cascaded multi-phonon stimulated Raman scattering near second-harmonic-generation in thin-film lithium niobate microdisk

High-quality microresonators can greatly enhance light-matter interactions and are excellent platforms for studying nonlinear optics. Wavelength conversion through nonlinear processes is the key to many applications of integrated optics. The stimulated Raman scattering process can extend the emission wavelength of a laser source to a wider range. Lithium niobate, as a Raman active crystalline material, has remarkable potential for wavelength conversion. Here, we demonstrate the generation of cascaded multi-phonon Raman signals near the second-harmonic-generation peak in X-cut thin-film lithium niobate microdisk. Fine tuning of the specific cascaded Raman spectral lines has also been made by changing the pump wavelength. Raman lines can reach wavelength up to about 80 nm away from the SHG signal. We realize the SFG process associated with Raman signals in the visible range as well. Our work extends the use of WGM microresonators as effective optical upconversion wavelength converters in nonlinear optical applications.

physics.optics↗

Enhanced Nonlinear Frequency Conversion Bandwidth through Birefringence induced Mode Hybridization

On-chip quantum information network requires qubit transfer between different wavelengths while preserving quantum coherence and entanglement, which needs broadband up-conversion available. Herein, we demonstrate a mode-hybridization based broadband nonlinear frequency conversion on X-cut thin film lithium niobate. With the spontaneous quasi-phase matching and quasi groupvelocity matching being simultaneously satisfied, broadband second harmonic generation with a 3-dB bandwidth up to 13 nm has been achieved in a micro-racetrack resonator. The same mechanism can work on the frequency conversion of the ultra-short pulse in the bent waveguide structure. This work will be beneficial to on-chip tunable frequency conversion and quantum light source generation on integrated photonic platforms, and further enable on-chip large-capacity multiplexing, multichannel optical information processing, and large quantum information networks.

physics.optics↗

Dual-color Coherent Perfect Absorber

Perfect absorption of light critically affects light-matter interaction for various applications. Coherent perfect absorbers (CPA) gain the unique capability of controlling light with light in a linear fashion. Multi-color CPAs [Phys. Rev. Lett. 107, 033901] are highly desirable for broadband and nonlinear light-to-light coherent control, however, the experimental demonstration has still remained elusive. Here we experimentally observe a dual-color version of CPA (DC-CPA) through a second harmonic generation in a single whispering-gallery-mode microcavity. The DC-CPA enables simultaneous perfect absorption of both the incoming fundamental wave and its second harmonic. Similar to its linear counterpart, coherent control in the DC-CPA can be also realized by tuning the relative phase and intensity between the two-colored waves through nonlinear interference instead of the linear one. This scheme breaks the linear boundary of the traditional CPA into a multi-frequency domain and paves the way toward all-optically signal processing and quantum information.

physics.optics↗

Spin Hamiltonians in the Modulated Momenta of Light

Photonic solvers that are able to find the ground states of different spin Hamiltonians can be used to study many interactive physical systems and combinatorial optimization problems. Here, we establish a real-and-momentum space correspondence of spin Hamiltonians by spatial light transport. The real-space spin interaction is determined by modulating the momentum-space flow of light. This principle is formulated as a generalized Plancherel theorem, allowing us to implement a simple optical simulator that can find the ground states for any displacement-dependent spin interactions. Particularly, we use this principle to reveal the exotic magnetic phase diagram from a J1-J2-J3 model, and we also observe the vortex-mediated Berezinskii-Kosterlitz-Thouless dynamics from the XY model. These experiments exhibit high calculation precision by subtly controlling spin interactions from the momentum space of light, offering a promising scheme to explore novel physical effects.

physics.optics↗

Effective Sorting of Fractional Optical Vortex Modes

Mode sorter is the crucial component of the communication systems based on orbital angular momentum (OAM). However, schemes proposed so far can only effectively sort integer OAM (IOAM) modes. Here, we demonstrate the effective sorting of fractional OAM (FOAM) modes by utilizing the coordinate transformation method, which can convert FOAM modes to IOAM modes. The transformed IOAM modes are subsequently sorted by using a mode conversion method called topological charge matching. The validation of our scheme is verified by implementing two sorting processes and corresponding mode purity analyses, both theoretically and experimentally. This new sorting method exhibits a huge potential of implementing a highly confidential and high-capacity FOAM-based communication system, which may inspire further applications in both classical and quantum regimes.

physics.optics↗

Subwavelength Photorefractive Grating in a Thin-Film Lithium Niobate Microcavity

Subwavelength gratings play a fundamental and pivotal role in numerous science and applications for wave manipulation, exhibiting distinctive features such as filtering, phase manipulation, and anti-reflection. However, conventional fabrication methods for ultrasmall periodic structures are constrained by the fundamental optical diffraction limit, making it challenging to produce subwavelength gratings for optics. Here, we demonstrate a novel technique to build a reconfigurable subwavelength photorefractive grating (SPG) in a thin-film lithium niobate on the platform of an optical microcavity. Such SPGs are optically induced through the photorefractive effect and the subwavelength features originate from the spatial phase modulations of the pump's standing wave. The resulting SPGs lead to the mode splitting of two counter-propagating modes inside the microcavity, exhibiting an Electromagnetically Induced Transparency (EIT)-like transmission spectrum. Moreover, the unique subwavelength characteristic of SPGs enables first-order quasi-phase-matching for backward second-harmonic generation, a long-standing problem in nonlinear optics. Also, free-space-to-chip vertical nonlinear frequency conversion can be achieved in a similar manner. These results provide a flexible approach towards fabricating subwavelength gratings, which holds significant potential in various applications such as nonlinear frequency conversion, optical communication, sensing, and quantum technologies.

physics.optics↗

Enhanced Frequency Conversion in Parity-Time Symmetry Line

Non-Hermitian degeneracies reveal intriguing and non-trivial behaviors in open physical systems. Examples like Parity-Time (PT) symmetry breaking, topological encircling chirality, and enhanced sensing near an exceptional point (EP) are often associated with the abrupt nature of the phase transition around these degeneracies. Here we experimentally observe a cavity-enhanced second-harmonic frequency (SHG) conversion on a PT symmetry line, i.e. a set consisting of open-ended isofrequency or isoloss lines, both terminated at EPs on the Riemann surface in parameter space. The enhancement factor can reach as high as 300, depending on the crossing point whether in the symmetry or the broken phase of the PT line. Moreover, such enhancement of SHG enables sensitive distance sensing with a nanometer resolution. Our works may pave the way for practical applications in sensing, frequency conversion, and coherent wave control.

physics.optics↗

On-chip Lithium Niobate Heterogeneous Photonic Crystal Nanocavity Laser

Thin film lithium niobate (TFLN) has become an platform for modern integrated circuits due to its excellent optical properties. With the development of rare earth ion doped TFLN, important breakthroughs of on-chip microlasers has emerged and show significant application for optical communication, computing and quantum photonics. However, challenges still remain in developing compact lasers with small mode volumes and low threshold on rare earth ion doped TFLN for highly efficient dense integration. In this letter, we fabricated a heterogeneous photonic crystal (PhC) nanobeam cavity on erbium-doped TFLN with a quality factor of 1.2\times10^4 and firstly demonstrated a single-mode integrated PhC laser with submicron mode volume on TFLN platform. Laser at 1559.63 nm is achieved with the 974 nm single-mode pump. The effective mode volume is 1.44 (λ/n)^3 (~ 0.645 μm^3) and the threshold power is 163 μW. Enhanced photorefractive effect in the PhC cavity is measured. This lithium niobate photonic crystal nanocavity laser, as a compact telecommunication C-band on-chip light source, would benefit the progress of high-speed and low-cost optical community on TFLN integrated photonics.

physics.optics↗

Quantum Fusion of Independent Networks Based on Multi-user Entanglement Swapping

With the advance development in quantum science, constructing a large-scale quantum network has become a hot area of future quantum information technology. Future quantum networks promise to enable many fantastic applications and will unlock fundamentally new technologies in information security and large-scale computation. The future quantum internet is required to connect quantum information processors to achieve unparalleled capabilities in secret communication and enable quantum communication between any two points on Earth. However, the existing quantum networks are basically constructed to realize the communication between the end users in their own networks. How to bridge different independent networks to form a fully-connected quantum internet becomes a pressing challenge for future networks. Here, we demonstrate the quantum fusion of two independent networks for the first time based on multiuser entanglement swapping, to merge two 10-user networks into a larger network with 18 users in quantum correlation layer. By performing the Bell state measurement between two nonneighboring nodes, the users from different networks can establish entanglement and ultimately every pair of the 18 users are able to communicate with each other using the swapped states. Our approach opens attractive opportunities for the establishment of quantum entanglement between remote nodes in different networks, which facilitates versatile quantum information interconnects and has great application in constructing large-scale intercity quantum communication networks.

quant-ph↗

Self-Amplification-Assisted Highly Efficient Integrated Laser

Light source is indispensable component in on-chip system. Compared with hybrid or heterogeneous integrated laser, monolithically integrated laser is more suitable for high density photonic integrated circuit (PIC) since the capability of large-scale manufacturing, lower active-passive coupling loss and less test complexity. Recent years have seen the spark of researches on rare-earth ion doped thin film lithium niobate (REI:TFLN), demonstrations have been made both in classical and quantum chips. However, low output power and limited quantum emitting efficiency hinder the application of the chip-scale laser source based on REI:TFLN. Here a highly efficient integrated laser assisted by cascaded amplifiers is proposed and experimentally prepared on Erbium-doped TFLN. A slope efficiency of 0.43% and a linewidth of 47.86 kHz are obtained. The maximum integrated laser power is 7.989 μW. Our results show a viable solution to improve efficiency by self-amplification without changing the intrinsic quantum emitting efficiency of the material, and our design has potential application in incorporating with functional devices such as optical communications, integrated quantum memory and quantum emission.

physics.optics↗

Enhanced temperature sensing by multi-mode coupling in an on-chip microcavity system

The micro-cavity is a promising sensor platform, any perturbation would disturb its linewidth, cause resonance shift or splitting. However, such sensing resolution is limited by the cavity's optical quality factor and mode volume. Here we propose and demonstrate in an on-chip integrated microcavity system that resolution of a self referenced sensor could be enhanced with multi mode coupling.

physics.optics↗

Spatiotemporal coupled-mode equations for arbitrary pulse transformation

Spatiotemporal modulation offers a variety of opportunities for light manipulations. In this paper, we propose a way towards arbitrary transformation for pulses sequentially propagating within one waveguide in space via temporal waveguide coupling. The temporal waveguide coupling operation is achieved by spatiotemporally modulating the refractive index of the spatial waveguide with a traveling wave through segmented electrodes. We derive the temporal coupled-mode equations and discuss how systematic parameters affect the temporal coupling coefficients. We further demonstrated a temporal Mach-Zehnder interferometer and universal multiport interferometer, which enables arbitrary unitary transformation for pulses. We showcase a universal approach for transforming pulses among coupled temporal waveguides, which requires only one spatial waveguide under spatiotemporal modulation, and hence provide a flexible, compact, and highly compatible method for optical signal processing in time domain.

physics.optics↗