arXiv subjects

Xianfeng Chen

Publications and source records attributed to Xianfeng Chen.

At least 55 records · Page 3Linked to original sources

arXiv2023

Observation of non-Hermitian antichiral edge currents

Non-Hermitian topological photonics is of great interest in bridging topological matter with gain/dissipation engineering in optics. A key problem in this direction is the interplay between the effective gauge potential and the non-Hermiticity. Here we tackle this problem in a synthetic non-Hermitian Hall ladder and experimentally observe antichiral edge currents (ACECs) of photons, by tuning the locally uniform effective magnetic flux and the on-site gain/loss. Such ACECs provide a topological method to probe the signatures of the non-Hermitian skin effect (NHSE) from steady-state bulk dynamics. The universality of this method is verified by its generalization to three dimensions. This study paves a way to investigate exotic non-Hermitian topological physics and has potential applications in topological photonics engineering.

physics.optics↗

arXiv2023

Topological Dissipative Photonics and Topological Insulator Lasers in Synthetic Time-Frequency Dimensions

The study of dissipative systems has attracted great attention, as dissipation engineering has become an important candidate towards manipulating light in classical and quantum ways. Here,we investigate the behavior of a topological system with purely dissipative couplings in a synthetic time-frequency space. An imaginary bandstructure is shown, where eigen-modes experience different eigen-dissipation rates during the evolution of the system, resulting in mode competition between edge states and bulk modes. We show that distributions associated with edge states can dominate over bulk modes with stable amplification once the pump and saturation mechanisms are taken into consideration, which therefore points to a laser-like behavior for edge states robust against disorders. This work provides a scheme for manipulating multiple degrees of freedom of light by dissipation engineering, and also proposes a great candidate for topological lasers with dissipative photonics.

physics.optics↗

arXiv2023

Extreme subradiance from two-band Bloch oscillations in atomic arrays

Atomic arrays provide an important quantum optical platform with photon-mediated dipoledipole interactions, which can be engineered to realize key applications in quantum information processing. A major obstacle for such application is the fast decay of the excited states. By controlling two-band Bloch oscillations in an atomic array under external magnetic field, here we show that exotic subradiance can be realized and maintained at a time scale upto 12 orders of magnitude larger than the spontaneous decay time in atomic arrays with the finite size. The key finding is to show a way for preventing the wavepacket of excited states scattering into the dissipative zone inside the free space light cone, which therefore leads to the excitation staying at a subradiant state for extremely long decay time. We show that such operation can be achieved by introducing a spatially linear potential from external magnetic field in atomic arrays and then manipulating interconnected two-band Bloch oscillations along opposite directions. Our results also point out the possibility of controllable switching between superradiant and subradiant states, which leads to potential applications in quantum storage.

quant-ph↗

arXiv2023

Optical neural network architecture for deep learning with the temporal synthetic dimension

The physical concept of synthetic dimensions has recently been introduced into optics. The fundamental physics and applications are not yet fully understood, and this report explores an approach to optical neural networks using synthetic dimension in time domain, by theoretically proposing to utilize a single resonator network, where the arrival times of optical pulses are interconnected to construct a temporal synthetic dimension. The set of pulses in each roundtrip therefore provides the sites in each layer in the optical neural network, and can be linearly transformed with splitters and delay lines, including the phase modulators, when pulses circulate inside the network. Such linear transformation can be arbitrarily controlled by applied modulation phases, which serve as the building block of the neural network together with a nonlinear component for pulses. We validate the functionality of the proposed optical neural network for the deep learning purpose with examples handwritten digit recognition and optical pulse train distribution classification problems. This proof of principle computational work explores the new concept of developing a photonics-based machine learning in a single ring network using synthetic dimensions, which allows flexibility and easiness of reconfiguration with complex functionality in achieving desired optical tasks.

physics.optics↗

arXiv2022

Simultaneous $ χ^{(2)} $- $ χ^{(2)} $ and $ χ^{(2)} $-$ χ^{(3)} $ nonlinear processes generation in thin film lithium tantalate microcavity

On-chip efficient nonlinear functions are instrumental in escalating the utilities and performance of photonic integrated circuits (PICs), especially for a wide range of classical and quantum applications, such as tunable coherent radiation, optical frequency conversion, spectroscopy, quantum science, etc. Lithium tantalate (LT) has been widely used in nonlinear wavelength converters, surface acoustic wave resonators, and electro-optic, acoustic-optic devices owing to its excellent optical properties. Here, we fabricated a Z-cut lithium tantalate on insulator (LTOI) microdisk with high quality(Q) factors in both telecom (10$^{6}$) and visible (10$^{5}$) bands by optimizing the fabrication. With the Q factor of the LTOI microdisk increasing, we can obtain higher pump light intensity in the cavity which is beneficial to get more optical nonlinear effect easily. By making use of the mode phase matching of interacting waves and inputting high pump power, we experimentally observed on-chip near-infrared light, visible (red, green), and ultraviolet (UV) from microresonator-based $ χ^{(2)}-χ^{(2)}$, $ χ^{(2)}-χ^{(3)}$, and $χ^{(2)}$ nonlinear processes such as cascaded four-wave mixing (cFWM), cascaded sum-frequency generation (cSFG), third harmonic generation (THG), second harmonic generation (SHG). It is believed that the LTOI can support a variety of on-chip optical nonlinear processes, which heralds its new application potential in integrated nonlinear photonics.

physics.optics↗

arXiv2022

Doubly resonant photonic crystal cavity using merged bound states in the continuum

In this work, a doubly resonant photonic crystal (PhC) cavity using the merged bound states in the continuum (BICs) is proposed to obtain a higher second harmonic generation (SHG) efficiency. Firstly by scanning geometry parameters the accidental BICs and a band-edge mode outside the light cone can be obtained. Then as the lattice constant or the thickness of the slab is adjusted the accidental BICs will merge. A supercell with large and small holes is constructed and the band-edge mode outside the light cone can be mode-matched with the merged BICs mode. Finally the heterostructure PhC cavity is designed. The merged BICs show a high quality factor for the photonic crystal with finite size. Consequently, the SHG efficiency of the lattice constant near merged BICs of ~6000% W-1 is higher than the one of the isolated BIC.

physics.optics↗

arXiv2022

Large Quality Factor Enhancement Based on Cascaded Uniform Lithium Niobate Bichromatic Photonic Crystal Cavities

In this paper, by cascading several bichromatic photonic crystals we demonstrate that the quality factor can be much larger compared with that in an isolated cavity without increasing the total size of the device. We take lithium niobate photonic crystal as an example to illustrate that the simulated quality factor of the cascaded cavity can attain 10^5 with a 70° slant angle, which is an order of magnitude larger than that in isolated cavity. The device can be fabricated easily by current etching technique for lithium niobate. We have fabricated the proposed device experimentally including holes with 70° slant angle. This work is expected to provide guidance to the design of photonic crystal cavity with high-quality factor.

physics.optics↗

arXiv2022

Concentrated subradiant modes in one-dimensional atomic array coupled with chiral waveguides

Non-Hermitian systems have recently attracted broad interest and exhibited intriguing physical phenomena, in which the non-Hermitian skin effect is one of the most remarkable quantum phenomena desiring detailed investigations and has been widely studied in various fermionic and bosonic systems. Here we propose a non-Hermitian atom-waveguide system composed of a tilted one-dimensional atomic array coupled with two identical waveguides with opposite chiralities. Such system creates an effective lattice model including nonreciprocal long-range hoppings through the chiral-waveguide photon-mediated interactions. We find the excitation of the collective atomic states concentrates in the middle interface, pointing towards the non-Hermitian skin effect associated with subradiant modes, while, on the contrary, superradiant modes exhibit extended features. Simulation results present subradiant funneling effect, with robustness against small atomic position disorders. Our work underpins the fundamental comprehension towards the non-Hermitian skin effect in open quantum systems and also provide prospective paths to study non-Hermitian systems in the area of quantum optics.

quant-ph↗

arXiv2022

Diamagnetic composites for high-Q levitating resonators

Levitation offers extreme isolation of mechanical systems from their environment, while enabling unconstrained high-precision translation and rotation of objects. Diamagnetic levitation is one of the most attractive levitation schemes, because it allows stable levitation at room temperature without the need for a continuous power supply. However, dissipation by eddy currents in conventional diamagnetic materials significantly limits the application potential of diamagnetically levitating systems. Here, we present a route towards high $Q$ macroscopic levitating resonators by substantially reducing eddy current damping using graphite particle based diamagnetic composites. We demonstrate resonators that feature quality factors $Q$ above 450,000 and vibration lifetimes beyond one hour, while levitating above permanent magnets in high vacuum at room temperature. The composite resonators have a $Q$ that is more than 400 times higher than that of diamagnetic graphite plates. By tuning the composite particle size and density, we investigate the dissipation reduction mechanism and enhance the $Q$ of the levitating resonators. Since their estimated acceleration noise is as low as some of the best superconducting levitating accelerometers at cryogenic temperatures, the high $Q$ and large mass of the presented composite resonators positions them as one of the most promising technologies for next generation ultra-sensitive room temperature accelerometers.

physics.app-ph↗

arXiv2022

Chip-scale Spontaneous Quasi-Phase-Matched Micro-Racetrack Resonator

Due to their capacity for non-classical light generation, high-efficiency second-order nonlinear parametric processes play an important role in quantum photonic technology, and chip-scale realization of these processes is recognized as the key to building efficient light sources for integrated quantum photonic circuits. To achieve ultra-high nonlinear conversion efficiency, traditional method uses quasi-phase matching (QPM) technology. However, QPM requires electric field poling, which is incompatible with the CMOS fabrication process, and this hinders the wafer-scale production of integrated photonic circuits. In this paper, we demonstrate efficient spontaneous quasi-phase matched (SQPM) frequency conversion in a micro-racetrack resonator. Our approach does not involve poling, but exploits the anisotropy of the ferroelectric crystals to allow the phase-matching condition to be fulfilled spontaneously as the TE-polarized light circulates in a specifically designed racetrack resonator. SQPM second harmonic generation is observed with a normalized intracavity conversion efficiency of 0.85%/W, corresponding to the 111st-order QPM. This could theoretically reach 186,000%/W by first-order QPM. In this case such high intracavity conversion efficiency can be implemented in practice with an optimized outward coupling. Our configurable SQPM approach will benefit the application of nonlinear frequency conversion in chip-scale integrated photonics with CMOS-compatible fabrication processes, and is applicable to other on-chip nonlinear processes such as quantum frequency conversion or frequency-comb generation.

physics.optics↗

arXiv2022

Diamagnetically levitating resonant weighing scale

Diamagnetic levitation offers stable confinement of an object from its environment at zero power, and thus is a promising technique for developing next generation unclamped resonant sensors. In this work, we realize a resonant weighing scale using a graphite plate that is diamagnetically levitating over a checkerboard arrangement of permanent magnets. We characterize the bending vibrations of the levitating object using laser Doppler vibrometry and use microgram glass beads to calibrate the responsivity of the sensor's resonance frequency to mass changes. The sensor is used for real-time measurement of the evaporation rate of nano-litre droplets with high-accuracy. By analyzing the resonator's frequency stability, we show that the millimeter graphite sensor can reach mass resolutions down to 4.0ng, relevant to biological and chemical sensing concepts.

physics.ins-det↗

arXiv2022

Rigid body dynamics of diamagnetically levitating graphite resonators

Diamagnetic levitation is a promising technique for realizing resonant sensors and energy harvesters, since it offers thermal and mechanical isolation from the environment at zero power. To advance the application of diamagnetically levitating resonators, it is important to characterize their dynamics in the presence of both magnetic and gravitational fields. Here we experimentally actuate and measure rigid body modes of a diamagnetically levitating graphite plate. We numerically calculate the magnetic field and determine the influence of magnetic force on the resonance frequencies of the levitating plate. By analyzing damping mechanisms, we conclude that eddy current damping dominates dissipation in mm-sized plates. We use finite element simulations to model eddy current damping and find close agreement with experimental results. We also study the size-dependent Q-factors (Qs) of diamagnetically levitating plates and show that Qs above 100 million are theoretically attainable by reducing the size of the diamagnetic resonator down to microscale, making these systems of interest for next generation low-noise resonant sensors and oscillators.

physics.app-ph↗

arXiv2021

All-optical control of the photonic Hall lattice in a pumped waveguide array

Quantum Hall system possesses topologically protected edge states which have enormous theoretical and practical implications in both fermionic and bosonic systems. Harnessing the quantum Hall effect in optical platforms with lower dimensionality is highly desirable with synthetic dimensions and has attracted broad interests in the photonics society. Here, we introduce an alternative way to realize the artificial magnetic field in a frequency dimension, which is achieved in a pump-probe configuration with cross-phase modulations in a one-dimensional four waveguide array. The dynamics of the topological chiral edge state has been studied and the influence from the crosstalk of the pump fields has been explored. Our work shows an all-optical way to simulate the quantum Hall system in a photonic system and holds potential applications in manipulating light in waveguide systems.

physics.optics↗

arXiv2021

Excite atom-photon bound state inside the coupled-resonator waveguide coupled with a giant atom

It is of fundamental interest in controlling the light-matter interaction for a long time in the field of quantum information processing. However, usual excitation with the propagating photon can hardly excite a localized state of light while keeping the atom under a subradiant decay in an atom-waveguide system. Here, we propose a model of coupling between a giant atom and the dynamically-modulated coupled-resonator waveguide and find that a bound state, where the light shows the localization effect and atom exhibits a subradiant decay time, can be excited by a propagating photon. An analytical treatment based on the separation of the propagating states and localized states of light has been used and provides inspiring explanation of our finding, i.e., a propagating photon can be efficiently converted to the localized light through the light-atom interactions in three resonators at frequency difference precisely equivalent to external modulation frequency. Our work therefore provides an alternative method for actively localizing the photon in a modulated coupled-resonator waveguide system interacting with giant atom, and also points out a way to study the light-atom interaction in a synthetic frequency dimension that holds the similar Hamiltonian.

quant-ph↗

arXiv2021

Single Pulse Manipulations in Synthetic Time-frequency Space

Synthetic dimensions in photonic structures provide unique opportunities for actively manipulating light in multiple degrees of freedom. Here, we theoretically explore a dispersive waveguide under the dynamic phase modulation that supports single pulse manipulations in the synthetic (2+1) dimensions. Compared with the counterpart of the conventional (2+1) space-time, we explore temporal diffraction and frequency conversion in a synthetic time-frequency space while the pulse evolves along the spatial dimension. By introducing the effective gauge potential well for photons in the synthetic time-frequency space with the control of the modulation phase, we show that a rich set of pulse propagation behaviors can be achieved, including confined pulse propagation, fast/slow light, and pulse compression. With the additional nonlinear oscillation subject to the effective force along the frequency axis of light, we provide an exotic approach for actively manipulating the single pulse in both temporal and spectral domains, which shows the great promise for applications of the pulse processing and optical communications in integrated photonics.

physics.optics↗

arXiv2021

Tunable single-mode laser on thin film lithium niobate

Erbium-doped lithium niobate on insulator (LNOI) laser plays an important role in the complete photonic integrated circuits (PICs). Here, we demonstrate an integrated tunable whisper galley single mode laser (WGSML) by making use of a pair of coupled microdisk and microring on LNOI. A 974 nm single-mode pump light can have an excellent resonance in the designed microdisk, which is beneficial to the whisper gallery mode (WGM) laser generation. The WGSML at 1560.40 nm with a maximum 31.4 dB side mode suppression ratio (SMSR) has been achieved. By regulating the temperature, WGSMLs output power increased and the central wavelength can be changed from 1560.30 nm to 1560.40 nm. What's more, 1560.60 nm and 1565.00 nm WGSMLs have been achieved by changing the coupling gap width between microdisk and microring. We can also use the electro-optic effect of LNOI to obtain more accurate adjustable WGSMLs in further research.

physics.optics↗

arXiv2021

Quasi-edge states and topological Bloch oscillation in the synthetic space

In physics, synthetic dimensions trigger great interest to manipulate light in different ways, while in technology, lithium niobate shows important capability towards on-chip applications. Here, based on the state-of-art technology, we propose and study a theoretical model of dynamically-modulated waveguide arrays with the Su-Schrieffer-Heeger configuration in the spatial dimension. The propagation of light through the one-dimensional waveguide arrays mimics time evolution of field in a synthetic two-dimensional lattice including the frequency dimension. By adding the effective gauge potential, we find quasi-edge state that the intensity distribution manifests not at the boundary as the traditional edge state, which leads to an exotic topologically protected one-way transmission along adjacent boundary. Furthermore, a cosine-shape isolated band exhibits, supporting the topological Bloch oscillation in the frequency dimension under the effective constant force, which is localized at the spatial boundary and shows the topological feature. Our work therefore points out further capability of light transmission under topological protections in both spatial and spectral regimes, and provides future on-chip applications in the lithium niobate platform.

physics.optics↗

arXiv2021

Observation of flat-band and band transition in the synthetic space

Constructions of synthetic lattices in photonics attract growingly attentions for exploring interesting physics beyond the geometric dimensionality, among which modulated ring resonator system has been proved as a powerful platform to create different kinds of connectivities between resonant modes along the synthetic frequency dimension with many theoretical proposals. Various experimental realizations are investigated in a single ring resonator, while building beyond simple synthetic lattices in multiple rings with different types remains lacking, which desires to be accomplished as an important step further. Here, we implement the experimental demonstration of generating the one-dimensional Lieb lattice along the frequency axis of light, realized in two coupled ring resonators while the larger ring undergoing dynamic modulation. Such synthetic photonic structure naturally exhibits the physics of flat band. We show that the time-resolved band structure read out from the drop-port output of the excited ring is the intensity projection of the band structure onto specific resonant mode in the synthetic momentum space, where gapless flat band, mode localization effect, and flat to non-flat band transition are observed in experiments and verified by simulations. Our work gives a direct evidence for the constructing synthetic Lieb lattice with two rings, which hence makes a solid step towards experimentally constructing more complicated lattices in multiple rings associated with synthetic frequency dimension.

physics.optics↗