Showing 1–20 of 20 results for author: Lian, C

Ultrawide-angle diffraction-limited 2D beam steering via hybrid integrated metasurface-photonic circuit

Authors: Zhiping He, Luigi Ranno, Padraic Burns, Fan Yang, Hung-I Lin, Maarten R. A. Peters, Hanyu Zheng, Rui Chen, Yi Ji Tan, Chuanyu Lian, Nathan Dostart, Hyun Jung Kim, Carlos Ríos, Tian Gu, Juejun Hu

Abstract: Two-dimensional (2D) wide field-of-view (FOV) beam steering is a key enabling capability for emerging free-space optical systems, including inter-satellite optical links, airborne LiDAR, point-to-point optical wireless communications, and collaborative robotic platforms. These applications require rapid acquisition and tracking across both azimuth and elevation; architectures that offer wide scann… ▽ More Two-dimensional (2D) wide field-of-view (FOV) beam steering is a key enabling capability for emerging free-space optical systems, including inter-satellite optical links, airborne LiDAR, point-to-point optical wireless communications, and collaborative robotic platforms. These applications require rapid acquisition and tracking across both azimuth and elevation; architectures that offer wide scanning in only one dimension while maintaining limited coverage in the orthogonal direction constrain link availability, coverage uniformity, and system agility. Here, we demonstrate a chip-scale platform for ultrawide-angle, diffraction-limited 2D beam steering based on hybrid integration of a silicon photonic integrated circuit (PIC) and an optical metasurface. A free-form micro-optical reflector efficiently transforms the guided waveguide mode into an expanded free-space beam that illuminates an analytically optimized ultrawide-FOV metasurface. The integrated system achieves a measured FOV exceeding 160° while maintaining diffraction-limited beam quality over a broad angular range at telecom wavelengths. This hybrid PIC-metasurface architecture provides a compact and scalable route to high-quality 2D beam steering and establishes a practical pathway toward integrated optical projectors for space-based optical communications and other applications requiring agile, wide-angle, high-fidelity beam control. △ Less

Submitted 14 April, 2026; originally announced April 2026.

Comments: 17 pages, 6 figures

A Unified Heterogeneous Implementation of Numerical Atomic Orbitals-Based Real-Time TDDFT within the ABACUS Package

Authors: Taoni Bao, Yuanbo Li, Zichao Deng, Haotian Zhao, Denghui Lu, Yike Huang, Chao Lian, Lixin He, Mohan Chen

Abstract: We present a unified heterogeneous computing framework for real-time time-dependent density functional theory (RT-TDDFT) based on numerical atomic orbitals (NAOs), implemented in the ABACUS package. We introduce three co-designed abstraction layers, including unified data containers, unified linear algebra operators, and unified grid integration interfaces. These layers collectively accelerate the… ▽ More We present a unified heterogeneous computing framework for real-time time-dependent density functional theory (RT-TDDFT) based on numerical atomic orbitals (NAOs), implemented in the ABACUS package. We introduce three co-designed abstraction layers, including unified data containers, unified linear algebra operators, and unified grid integration interfaces. These layers collectively accelerate the two most demanding parts of NAO-based RT-TDDFT: explicit real-time wavefunction propagation and real-space grid operations such as Hamiltonian construction and force evaluation under external fields. We validate the method by computing optical properties for systems ranging from finite molecules to periodic solids, showing excellent agreement with standard benchmarks. Performance evaluations on bulk silicon demonstrate that a single GPU can achieve substantial wall-clock speedup over a fully utilized dual-socket CPU node. Furthermore, distributed multi-GPU strong-scaling tests confirm high parallel efficiency over tens of GPUs. This work establishes a high-performance, portable platform for large-scale first-principles simulations of ultrafast electron dynamics. △ Less

Submitted 23 March, 2026; originally announced March 2026.

A Novel Pixel-Chip-Based Region-of-Interest Readout Circuit Design

Authors: Shi-Qiang Zhou, Li-Rong Xie, Dong Wang, Cheng Lian, Si-Ying Liu, Zi-Yi Zhang, Xiang-Ming Sun, Hong-Bang Liu, Chao-Song Gao, Jun Liu, Huan-Bo Feng, Di-Fan Yi

Abstract: This paper presents a novel pixel chip readout scheme: the Region-of-Interest Readout Circuit (ROIRC), which is designed for large area, large array pixel chips and Gas Pixel Detector (GPD). This design employs a sentinel pixel detection strategy, enabling rapid identification and prioritized readout of the pixel regions containing signal events. During the scanning readout of these signal events,… ▽ More This paper presents a novel pixel chip readout scheme: the Region-of-Interest Readout Circuit (ROIRC), which is designed for large area, large array pixel chips and Gas Pixel Detector (GPD). This design employs a sentinel pixel detection strategy, enabling rapid identification and prioritized readout of the pixel regions containing signal events. During the scanning readout of these signal events, ROIRC employs a Block-based readout approach, effectively minimizing the readout of non-signal pixels. The functionality of ROIRC has been successfully implemented on both the ASIC and FPGA platforms. In the tests of the ROIRC, the pixel chip embedded in the GPD is capable of detecting low-energy X-rays in the range of 2-10 keV and supports multiple event readouts, and the pixel chip can read out photo-electron signal events with the count rate up to 15k / (cm2 x s). △ Less

Submitted 19 November, 2025; originally announced November 2025.

Incorporating Si into Sb2Se3: Tailoring Optical Phase Change Materials via Nanocomposites

Authors: Chih-Yu Lee, Yi-Siou Huang, Felix Adams, Chuanyu Lian, Hongyi Sun, Jie Zhao, Zichao Ye, Nathan Youngblood, Juejun Hu, Leslie H Allen, Yifei Mo, Ichiro Takeuchi, Carlos A Rios Ocampo

Abstract: Chalcogenide-based optical phase change materials (OPCMs) exhibit a large contrast in refractive index when reversibly switched between their stable amorphous and crystalline states. OPCMs have rapidly gained attention due to their versatility as nonvolatile amplitude or phase modulators in various photonic devices. However, open challenges remain, such as achieving reliable response and transpare… ▽ More Chalcogenide-based optical phase change materials (OPCMs) exhibit a large contrast in refractive index when reversibly switched between their stable amorphous and crystalline states. OPCMs have rapidly gained attention due to their versatility as nonvolatile amplitude or phase modulators in various photonic devices. However, open challenges remain, such as achieving reliable response and transparency spanning into the visible spectrum, a combination of properties in which current broadband OPCMs (e.g., Ge2Sb2Se4Te1, Sb2Se3, or Sb2S3) fall short. Discovering novel materials or engineering existing ones is, therefore, crucial in extending the application scope of OPCMs. Here, we use magnetron co-sputtering to study the effects of Si doping into Sb2Se3. We employ ellipsometry, X-ray diffraction, Raman spectroscopy, and scanning and transmission electron microscopy to investigate the effects of Si doping on the optical properties and crystal structure and compare these results with those from first principles calculations. Moreover, we study the crystallization and melt-quenching of thin films via nano-differential scanning calorimetry (NanoDSC). Our experiments demonstrate that 20% Si doping increases the transparency window in both states, specifically to 800 nm (1.55 eV) in the amorphous phase, while reducing power consumption by lowering the melting temperature. However, this reduction comes at the cost of reducing the refractive index contrast between states and slowing the kinetics of the phase transition. △ Less

Submitted 2 October, 2025; originally announced October 2025.

Comments: 5 Figures, 2 tables, 17 pages

Convective Nature of the Stimulated Raman Side Scattering in Inertial Confinement Fusion

Authors: F. -X. Zhou, C. -W. Lian, R. Yan, Y. Ji, J. Li, Q. Jia, J. Zheng

Abstract: The absolute growth of Stimulated Raman side scattering (SRSS) predicted by previous theories appeared to be surprisingly absent in the recent ignition-scale direct-drive experiments with the absence attributed to different reasons. We present evidence from simulations that the linear SRSS modes are naturally all convective (i.e., absolute SRSS does not exist at all) in an experimentally relevant… ▽ More The absolute growth of Stimulated Raman side scattering (SRSS) predicted by previous theories appeared to be surprisingly absent in the recent ignition-scale direct-drive experiments with the absence attributed to different reasons. We present evidence from simulations that the linear SRSS modes are naturally all convective (i.e., absolute SRSS does not exist at all) in an experimentally relevant regime where a finite-beam-width laser is incident into a non-uniform low-density plasma below a quarter of the critical density. The convective gain demonstrated by our newly proposed formula via numerical fitting monotonically increases with the beam width without saturation, which is significantly different from the prediction of previous convective SRSS theories. △ Less

Submitted 25 March, 2025; originally announced March 2025.

High-Speed Multifunctional Photonic Memory on a Foundry-Processed Photonic Platform

Authors: Sadra Rahimi Kari, Marcus Tamura, Zhimu Guo, Yi-Siou Huang, Hongyi Sun, Chuanyu Lian, Nicholas Nobile, John Erickson, Maryam Moridsadat, Carlos A. Ríos Ocampo, Bhavin J Shastri, Nathan Youngblood

Abstract: The integration of computing with memory is essential for distributed, massively parallel, and adaptive architectures such as neural networks in artificial intelligence (AI). Accelerating AI can be achieved through photonic computing, but it requires nonvolatile photonic memory capable of rapid updates during on-chip training sessions or when new information becomes available during deployment. Ph… ▽ More The integration of computing with memory is essential for distributed, massively parallel, and adaptive architectures such as neural networks in artificial intelligence (AI). Accelerating AI can be achieved through photonic computing, but it requires nonvolatile photonic memory capable of rapid updates during on-chip training sessions or when new information becomes available during deployment. Phase-change materials (PCMs) are promising for providing compact, nonvolatile optical weighting; however, they face limitations in terms of bit precision, programming speed, and cycling endurance. Here, we propose a novel photonic memory cell that merges nonvolatile photonic weighting using PCMs with high-speed, volatile tuning enabled by an integrated PN junction. Our experiments demonstrate that the same PN modulator, fabricated via a foundry compatible process, can achieve dual functionality. It supports coarse programmability for setting initial optical weights and facilitates high-speed fine-tuning to adjust these weights dynamically. The result showcases a 400-fold increase in volatile tuning speed and a 10,000-fold enhancement in efficiency. This multifunctional photonic memory with volatile and nonvolatile capabilities could significantly advance the performance and versatility of photonic memory cells, providing robust solutions for dynamic computing environments. △ Less

Submitted 20 September, 2024; originally announced September 2024.

Microheater hotspot engineering for repeatable multi-level switching in foundry-processed phase change silicon photonics

Authors: Hongyi Sun, Chuanyu Lian, Francis Vásquez-Aza, Sadra Rahimi Kari, Yi-Siou Huang, Alessandro Restelli, Steven A. Vitale, Ichiro Takeuchi, Juejun Hu, Nathan Youngblood, Georges Pavlidis, Carlos A. Ríos Ocampo

Abstract: Nonvolatile photonic integrated circuits employing phase change materials have relied either on optical switching mechanisms with precise multi-level control but poor scalability or electrical switching with seamless integration and scalability but mostly limited to a binary response. Recent works have demonstrated electrical multi-level switching; however, they relied on the stochastic nucleation… ▽ More Nonvolatile photonic integrated circuits employing phase change materials have relied either on optical switching mechanisms with precise multi-level control but poor scalability or electrical switching with seamless integration and scalability but mostly limited to a binary response. Recent works have demonstrated electrical multi-level switching; however, they relied on the stochastic nucleation process to achieve partial crystallization with low demonstrated repeatability and cyclability. Here, we re-engineer waveguide-integrated microheaters to achieve precise spatial control of the temperature profile (i.e., hotspot) and, thus, switch deterministic areas of an embedded phase change material cell. We experimentally demonstrate this concept using a variety of foundry-processed doped-silicon microheaters on a silicon-on-insulator platform to trigger multi-step amorphization and reversible switching of Sb$_{2}$Se$_{3}$ and Ge$_{2}$Sb$_{2}$Se$_{4}$Te alloys. We further characterize the response of our microheaters using Transient Thermoreflectance Imaging. Our approach combines the deterministic control resulting from a spatially resolved glassy-crystalline distribution with the scalability of electro-thermal switching devices, thus paving the way to reliable multi-level switching towards robust reprogrammable phase-change photonic devices for analog processing and computing. △ Less

Submitted 15 June, 2024; originally announced July 2024.

Comments: 20 pages, 7 figures, 1 table

Two-Plasmon-Decay Instability Stimulated by a Normal- and Large-Angle-Incidence Laser Pair

Authors: C. -W. Lian, Y. Ji, R. Yan, J. Li, S. -H. Cao, C. Ren, L. -F. Wang, Y. -K. Ding, J. Zheng

Abstract: The two-plasmon-decay instability (TPD) is a critical target preheating risk in direct-drive inertial confinement fusion. In this paper, TPD collectively driven by a normal-incidence laser beam (Beam-N) and a large-angle-incidence laser beam (Beam-L) is investigated via particle-in-cell simulations. Significant TPD growth is found able to develop in this regime at previously unexpected low laser i… ▽ More The two-plasmon-decay instability (TPD) is a critical target preheating risk in direct-drive inertial confinement fusion. In this paper, TPD collectively driven by a normal-incidence laser beam (Beam-N) and a large-angle-incidence laser beam (Beam-L) is investigated via particle-in-cell simulations. Significant TPD growth is found able to develop in this regime at previously unexpected low laser intensities if the intensity of Beam-L exceeds the large-angle-incidence threshold. Both beams contribute to the growth of TPD in a "seed-amplification" manner where the absolute instability driven by Beam-L provides the seeds that get convectively amplified by Beam-N, making TPD energetically important and causing significant pump depletion and hot electron generation. △ Less

Submitted 12 May, 2024; originally announced May 2024.

Comments: 16 pages, 5 figures, submitted

Theory of excitonic polarons: From models to first-principles calculations

Authors: Zhenbang Dai, Chao Lian, Jon Lafuente-Bartolome, Feliciano Giustino

Abstract: Excitons are neutral excitations that are composed of electrons and holes bound together by their attractive Coulomb interaction. The electron and the hole forming the exciton also interact with the underlying atomic lattice, and this interaction can lead to a trapping potential that favors exciton localization. The quasi-particle thus formed by the exciton and the surrounding lattice distortion i… ▽ More Excitons are neutral excitations that are composed of electrons and holes bound together by their attractive Coulomb interaction. The electron and the hole forming the exciton also interact with the underlying atomic lattice, and this interaction can lead to a trapping potential that favors exciton localization. The quasi-particle thus formed by the exciton and the surrounding lattice distortion is called excitonic polaron. Excitonic polarons have long been thought to exist in a variety of materials, and are often invoked to explain the Stokes shift between the optical absorption edge and the photo-luminescence peak. However, quantitative ab initio calculations of these effects are exceedingly rare. In this manuscript, we present a theory of excitonic polarons that is amenable to first-principles calculations. We first apply this theory to model Hamiltonians for Wannier excitons experiencing Fröhlich or Holstein electron-phonon couplings. We find that, in the case of Fröhlich interactions, excitonic polarons only form when there is a significant difference between electron and hole effective masses. Then, we apply this theory to calculating excitonic polarons in lithium fluoride ab initio. The key advantage of the present approach is that it does not require supercells, therefore it can be used to study a variety of materials hosting either small or large excitonic polarons. This work constitutes the first step toward a complete ab initio many-body theory of excitonic polarons in real materials. △ Less

Submitted 17 January, 2024; originally announced January 2024.

Journal ref: Physical Review B 109, 045202 (2024)

Excitonic polarons and self-trapped excitons from first-principles exciton-phonon couplings

Authors: Zhenbang Dai, Chao Lian, Jon Lafuente-Bartolome, Feliciano Giustino

Abstract: Excitons consist of electrons and holes held together by their attractive Coulomb interaction. Although excitons are neutral excitations, spatial fluctuations in their charge density couple with the ions of the crystal lattice. This coupling can lower the exciton energy and lead to the formation of a localized excitonic polaron, or even a self-trapped exciton in the presence of strong exciton-phon… ▽ More Excitons consist of electrons and holes held together by their attractive Coulomb interaction. Although excitons are neutral excitations, spatial fluctuations in their charge density couple with the ions of the crystal lattice. This coupling can lower the exciton energy and lead to the formation of a localized excitonic polaron, or even a self-trapped exciton in the presence of strong exciton-phonon interactions. Here, we develop a theoretical and computational approach to compute excitonic polarons and self-trapped excitons from first principles. Our methodology combines the many-body Bethe-Salpeter approach with density-functional perturbation theory, and does not require explicit supercell calculations. As a proof of concept, we demonstrate our method for a compound of the halide perovskite family. △ Less

Submitted 17 January, 2024; originally announced January 2024.

Journal ref: Physical Review Letters 132, 036902 (2024)

Nonvolatile Tuning of Bragg Structures Using Transparent Phase-Change Materials

Authors: Nicholas A. Nobile, Chuanyu Lian, Hongyi Sun, Yi-Siou Huang, Brian Mills, Cosmin Constantin Popescu, Dennis Callahan, Juejun Hu, Carlos A. Ríos Ocampo, Nathan Youngblood

Abstract: Bragg gratings offer high-performance filtering and routing of light on-chip through a periodic modulation of a waveguide's effective refractive index. Here, we model and experimentally demonstrate the use of Sb2Se3, a nonvolatile and transparent phase-change material, to tune the resonance conditions in two devices which leverage periodic Bragg gratings: a stopband filter and Fabry-Perot cavity.… ▽ More Bragg gratings offer high-performance filtering and routing of light on-chip through a periodic modulation of a waveguide's effective refractive index. Here, we model and experimentally demonstrate the use of Sb2Se3, a nonvolatile and transparent phase-change material, to tune the resonance conditions in two devices which leverage periodic Bragg gratings: a stopband filter and Fabry-Perot cavity. Through simulations, we show that similar refractive indices between silicon and amorphous Sb2Se3 can be used to induce broadband transparency, while the crystalline state can enhance the index contrast in these Bragg devices. Our experimental results show the promise and limitations of this design approach and highlight specific fabrication challenges which need to be addressed in future implementations. △ Less

Submitted 26 June, 2023; originally announced June 2023.

Generating axial magnetic fields via two plasmon decay driven by a twisted laser

Authors: Yu Ji, Chang-Wang Lian, Yin Shi, Rui Yan, Shihui Cao, Chuang Ren, Jian Zheng

Abstract: We propose a new way of axial magnetic fields generation in a non-relativistic laser intensity regime by using a twisted light carrying orbital angular momentum (OAM) to stimulate two-plasmon decay (TPD) in a plasma. The growth of TPD driven by an OAM light in a Laguerre-Gauss (LG) mode is investigated through three dimensional fluid simulations and theory. A theory based on the assumption that th… ▽ More We propose a new way of axial magnetic fields generation in a non-relativistic laser intensity regime by using a twisted light carrying orbital angular momentum (OAM) to stimulate two-plasmon decay (TPD) in a plasma. The growth of TPD driven by an OAM light in a Laguerre-Gauss (LG) mode is investigated through three dimensional fluid simulations and theory. A theory based on the assumption that the electron plasma waves (EPWs) are locally driven by a number of local plane-wave lasers predicts the maximum growth rate proportional to the peak amplitude of the pump laser field and is verified by the simulations. The OAM conservation during its transportation from the laser to the TPD daughter EWPs is shown by both the theory and the simulations. The theory predicts generation of ~40T axial magnetic fields through the OAM absorption via TPD, which has perspective applications in the field of high energy density physics. △ Less

Submitted 9 November, 2022; originally announced November 2022.

Comments: 6 pages, 3 figures,

Measurement of Stimulated Raman Side-Scattering Predominance in Directly Driven Experiment

Authors: Kevin Glize, Xu Zhao, Yihang Zhang, Changwang Lian, Shang Tan, Fuyuan Wu, Chengzhuo Xiao, Rui Yan, Zhe Zhang, Xiaohui Yuan, Jie Zhang

Abstract: Due to its particular geometry, stimulated Raman side-scattering (SRSS) drives scattered light emission at non-conventional directions, leading to scarce and complex experimental observations. Direct-irradiation campaigns at the SG-II UP facility have measured the scattered light driven by SRSS over a wide range of angles. It indicated an emission at large polar angles over a broad azimuthal range… ▽ More Due to its particular geometry, stimulated Raman side-scattering (SRSS) drives scattered light emission at non-conventional directions, leading to scarce and complex experimental observations. Direct-irradiation campaigns at the SG-II UP facility have measured the scattered light driven by SRSS over a wide range of angles. It indicated an emission at large polar angles over a broad azimuthal range, sensitive to the plasma profile and laser polarization, resulting in a loss of about 5\% of the total laser energy. Direct comparison with back-scattering measurement has evidenced SRSS as the dominant Raman scattering process. The predominance of SRSS was confirmed by 2D particle-in-cell simulations, and its angular spread has been corroborated by ray-tracing simulations. The main implication is that a complete characterization of the SRS instability and an accurate measurement of the energy losses require the collection of the scattered light in a broad range of directions. Otherwise, spatially limited measurement could lead to an underestimation of the energetic importance of stimulated Raman scattering. △ Less

Submitted 10 October, 2023; v1 submitted 17 September, 2022; originally announced September 2022.

Journal ref: Phys. Plasmas 30, 122706 (2023)

Direct numerical simulations of the modified Poisson-Nernst-Planck equations for the charging dynamics of cylindrical electrolyte-filled pores

Authors: Jie Yang, Mathijs Janssen, Cheng Lian, René van Roij

Abstract: Understanding how electrolyte-filled porous electrodes respond to an applied potential is important to many electrochemical technologies. Here, we consider a model supercapacitor of two blocking cylindrical pores on either side of a cylindrical electrolyte reservoir. A stepwise potential difference $2Φ$ between the pores drives ionic fluxes in the setup, which we study through the modified Poisson… ▽ More Understanding how electrolyte-filled porous electrodes respond to an applied potential is important to many electrochemical technologies. Here, we consider a model supercapacitor of two blocking cylindrical pores on either side of a cylindrical electrolyte reservoir. A stepwise potential difference $2Φ$ between the pores drives ionic fluxes in the setup, which we study through the modified Poisson-Nernst-Planck equations, solved with finite elements. We focus our discussion on the dominant timescales with which the pores charge and how these timescales depend on three dimensionless numbers. Next to the dimensionless applied potential $Φ$, we consider the ratio $R/R_b$ of the pore's resistance $R$ to the bulk reservoir resistance $R_b$ and the ratio $r_{p}/λ$ of the pore radius $r_p$ to the Debye length $λ$. We compare our data to theoretical predictions by Aslyamov and Janssen ($Φ$), Posey and Morozumi ($R/R_b$), and Henrique, Zuk, and Gupta ($r_{p}/λ$). Through our numerical approach, we delineate the validity of these theories and the assumptions on which they were based. △ Less

Submitted 4 April, 2022; originally announced April 2022.

Comments: 14 pages, 13 figures

Computing the Local Ion Concentration Variations for Electric-Double-Layer-Modulation Microscopy

Authors: Zhu Zhang, Jie Yang, Cheng Lian, Sanli Faez

Abstract: Modulating the electric potential on a conducting electrode is presented to generate an optical contrast for scattering microscopy that is sensitive to both surface charge and local topography. We dub this method Electric-Double-Layer-Modulation microscopy. We numerically compute the change in the local ion concentration that is the origin of this optical contrast for three experimentally relevant… ▽ More Modulating the electric potential on a conducting electrode is presented to generate an optical contrast for scattering microscopy that is sensitive to both surface charge and local topography. We dub this method Electric-Double-Layer-Modulation microscopy. We numerically compute the change in the local ion concentration that is the origin of this optical contrast for three experimentally relevant geometries: nanosphere, nanowire, and nanohole. In absence of plasmonic effects and physical absorption, the observable optical contrast is proportional to the derivative of the ion concentration with respect to the modulated potential. We demonstrate that this derivative depends on the size of the object and, less intuitively, also on its surface charge. This dependence is key to measuring the surface charge, in an absolute way, using this method. Our results help to identify the experimental conditions such as dynamic range and sensitivity that will be necessary for detecting the elementary charge jumps. We conclude that the nanohole is the most suitable geometry among these three for achieving elementary charge sensitivity. △ Less

Submitted 16 April, 2021; originally announced April 2021.

Comments: 17 pages, 10 figures

Journal ref: J. Phys. D: Appl. Phys. 54 384005 (2021)

A Blessing and a Curse: How a Supercapacitor's Large Capacitance Causes its Slow Charging

Authors: Cheng Lian, Mathijs Janssen, Honglai Liu, René van Roij

Abstract: The development of novel electrolytes and electrodes for supercapacitors is hindered by a gap of several orders of magnitude between experimentally measured and theoretically predicted charging timescales. Here, we propose an electrode model, containing many parallel stacked electrodes, that explains the slow charging dynamics of supercapacitors. At low applied potentials, the charging behavior of… ▽ More The development of novel electrolytes and electrodes for supercapacitors is hindered by a gap of several orders of magnitude between experimentally measured and theoretically predicted charging timescales. Here, we propose an electrode model, containing many parallel stacked electrodes, that explains the slow charging dynamics of supercapacitors. At low applied potentials, the charging behavior of this model is described well by an equivalent circuit model. Conversely, at high potentials, charging dynamics slow down and evolve on two relaxation time scales: a generalized $RC$ time and a diffusion time, which, interestingly, become similar for porous electrodes. The charging behavior of the stack-electrode model presented here helps to understand the charging dynamics of porous electrodes and qualitatively agrees with experimental time scales measured with porous electrodes. △ Less

Submitted 25 February, 2020; v1 submitted 22 November, 2019; originally announced November 2019.

Comments: C.L. and M.J. contributed equally to this work. 11 pages, 14 figures

Journal ref: Phys. Rev. Lett. 124, 076001 (2020)

Indirect but Efficient: Laser-Excited Electrons Can Drive Ultrafast Polarization Switching in Ferroelectric Materials

Authors: Chao Lian, Zulfikhar A. Ali, Hyuna Kwon, Bryan M. Wong

Abstract: To enhance the efficiency of next-generation ferroelectric (FE) electronic devices, new techniques for controlling ferroelectric polarization switching are required. While most prior studies have attempted to induce polarization switching via the excitation of phonons, these experimental techniques required intricate and expensive terahertz sources and have not been completely successful. Here, we… ▽ More To enhance the efficiency of next-generation ferroelectric (FE) electronic devices, new techniques for controlling ferroelectric polarization switching are required. While most prior studies have attempted to induce polarization switching via the excitation of phonons, these experimental techniques required intricate and expensive terahertz sources and have not been completely successful. Here, we propose a new mechanism for rapidly and efficiently switching the FE polarization via laser-tuning of the underlying dynamical potential energy surface. Using time-dependent density functional calculations, we observe an ultrafast switching of the FE polarization in BaTiO3 within 200 fs. A laser pulse can induce a charge density redistribution that reduces the original FE charge order. This excitation results in both desirable and highly directional ionic forces that are always opposite to the original FE displacements. Our new mechanism enables the reversible switching of the FE polarization with optical pulses that can be produced from existing 800 nm experimental laser sources. △ Less

Submitted 7 June, 2019; originally announced June 2019.

Journal ref: J. Phys. Chem. Lett.2019, 10, 3402

Ultrafast charge ordering by self-amplified exciton-phonon dynamics in TiSe$_2$

Authors: Chao Lian, Sheng-Jie Zhang, Shi-Qi Hu, Meng-Xue Guan, Sheng Meng

Abstract: The origin of charge density waves (CDW) in TiSe$_2$ has long been debated, mainly due to the difficulties in identifying the timescales of how and when the excitonic pairing and electron-phonon coupling (EPC) come into play. Without a proper time resolution and microscopic mechanism, one has to assume simultaneous appearance of CDW and periodic lattice distortions (PLD). Here, we accomplish a com… ▽ More The origin of charge density waves (CDW) in TiSe$_2$ has long been debated, mainly due to the difficulties in identifying the timescales of how and when the excitonic pairing and electron-phonon coupling (EPC) come into play. Without a proper time resolution and microscopic mechanism, one has to assume simultaneous appearance of CDW and periodic lattice distortions (PLD). Here, we accomplish a complete separation of exciton and PLD dynamics and unravel their interplay in the ultrafast time domain in our real-time time-dependent density functional theory simulations. We find that laser pulses knock off the exciton order and induce a homogeneous bonding-antibonding transition in the initial 20 fs, then the weakened electronic order triggers ionic movements antiparallel to the original PLD. The EPC comes into play after the initial 20~fs, and the two processes mutually amplify each other leading to a complete inversion of CDW ordering. The self-amplified dynamics reproduces the evolution of band structures in excellent agreement with ultrafast photoemission experiment. Hence we resolve the key processes in the initial dynamics of CDW that help elucidate the mechanism underlying the long debated problem. △ Less

Submitted 4 January, 2020; v1 submitted 2 January, 2019; originally announced January 2019.

Comments: Supplementary Material Included

Journal ref: Nat Commun 11, 43 (2020)

Ultrafast carrier relaxation and its Pauli drag in photo-enhanced melting of solids

Authors: Chao Lian, S. B. Zhang, Sheng Meng

Abstract: Ultrafast light-matter interaction is a powerful tool for the study of solids. Upon laser excitation, carrier multiplication and lattice acceleration beyond thermal velocity can occur, as a result of far-from-equilibrium carrier relaxation. The roles of electron-electron and electron-phonon scatterings are identified by first-principles dynamic simulations, from which a unified phase diagram emerg… ▽ More Ultrafast light-matter interaction is a powerful tool for the study of solids. Upon laser excitation, carrier multiplication and lattice acceleration beyond thermal velocity can occur, as a result of far-from-equilibrium carrier relaxation. The roles of electron-electron and electron-phonon scatterings are identified by first-principles dynamic simulations, from which a unified phase diagram emerges. It not only explains the experimentally-observed "inertial" melting but also predicts abnormal damping by Pauli Exclusion Principle with a new perspective on ultrahigh-intensity laser applications. △ Less

Submitted 2 January, 2019; originally announced January 2019.

Comments: 8 pages, 5 figures, with SUPPLEMENTARY MATERIALS

Momentum-resolved TDDFT algorithm in atomic basis for real time tracking of electronic excitation

Authors: Chao Lian, Shi-Qi Hu, Meng-Xue Guan, Sheng Meng

Abstract: Ultrafast electronic dynamics in solids lies at the core of modern condensed matter and materials physics. To build up a practical ab initio method for studying solids under photoexcitation, we develop a momentum-resolved real-time time dependent density functional theory (rt-TDDFT)algorithm using numerical atomic basis, together with the implementation of both the length and vector gauge of the e… ▽ More Ultrafast electronic dynamics in solids lies at the core of modern condensed matter and materials physics. To build up a practical ab initio method for studying solids under photoexcitation, we develop a momentum-resolved real-time time dependent density functional theory (rt-TDDFT)algorithm using numerical atomic basis, together with the implementation of both the length and vector gauge of the electromagnetic field. When applied to simulate elementary excitations in two-dimensional materials such as graphene, different excitation modes, only distinguishable in momentum space, are observed. The momentum-resolved rt-TDDFT is important and computationally efficient for the study of ultrafast dynamics in extended systems. △ Less

Submitted 15 October, 2018; v1 submitted 27 February, 2017; originally announced February 2017.

Journal ref: J. Chem. Phys. 149, 154104 (2018)