arXiv subjects

Chuanshan Tian

Publications and source records attributed to Chuanshan Tian.

16 recordsLinked to original sources

arXiv2026

Probing phonon chirality and circular lattice motion with symmetry-selective nonlinear optical spectroscopy

Truly chiral phonons are lattice eigenmodes that combine broken mirror symmetry with circular atomic motion. They can mediate angular-momentum-selective interactions in quantum materials, yet directly resolving both their chirality and underlying circular motion remains challenging, especially in high-symmetry crystals. Here we show that symmetry-selective terahertz difference-frequency spectroscopy provides a phase- and polarization-resolved route to identifying truly chiral phonons in a tabletop experiment. Using $α$-quartz as a benchmark, we validate this approach by resolving phonon chirality via chiral-sensitive $χ^{(2)}_{ijk}$ tensor elements ($i \neq j \neq k$), while vector-field detection directly reveals a time-dependent polarization rotation arising from circular ionic motion and thus nonzero angular momentum. Applying the same protocol to tetragonal $α$-TeO$_2$, we isolate chiral $E$-mode resonances below 5~THz and directly verify their circular lattice motion, thereby resolving a symmetry-imposed ambiguity in chiral-phonon identification in fourfold-symmetric crystals. Our results establish symmetry-selective nonlinear terahertz spectroscopy as a general route to identify truly chiral phonons in condensed matter systems.

physics.optics↗

arXiv2026

Gate- and Optically Controlled Nonlinear Optical Response in Graphene via Non-Perturbative Ultrafast Carrier Dynamics

While the Dirac band structure of graphene has established it as a leading platform for ultrafast optoelectronics, its non-perturbative nonlinear response under intense excitation remains poorly understood. Here, we report ultrafast spectral modulation of nonlinear optical signals in graphene. By utilizing a robust suspended-graphene platform that allows for both wide-range electrostatic gating and high optical damage thresholds, we observe dramatic frequency shifts (up to 8 THz) in third-harmonic generation (THG) and sum-frequency generation (SFG) driven by pump-induced nonequilibrium carrier dynamics. The magnitude and even the direction of this spectral shift can be reversibly controlled by the Fermi level and excitation conditions. A quasiequilibrium theoretical framework based on hot-carrier dynamics quantitatively reproduces the measured spectral evolution, elucidating the critical interplay between carrier heating and the Fermi level. These findings establish a universal mechanism for carrier-mediated spectral control, providing a practical route toward high-speed, gatetunable nonlinear photonic architectures.

physics.optics↗

arXiv2025

Observation of counterion binding in the inner Helmholtz layer at the ionic surfactant-water interface

Understanding specific ion adsorption within the inner Helmholtz layer remains central to electrochemistry yet experimentally elusive. Here we directly quantify counterion adsorption and extract the associated thermodynamic parameters within the inner Helmholtz layer using phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS). Using sodium dodecyl sulfate (SDS) as a model ionic surfactant, we determine the Na+ and DS- surface densities by simultaneously analyzing interfacial free OH response and the diffuse-layer SF signal, from which the adsorption thermodynamic parameters are derived. We then construct an adsorption phase diagram that maps the evolution of Na+ and DS- species in the compact layer as functions of bulk NaCl and SDS concentrations, revealing a continuous increase in surface ion pairing. The DS-: Na+ pairing ratio gradually decreases with increasing NaCl and approaches 2.8 at the supersaturation state prior to surface nucleation. These results establish PS-SFVS as a quantitative probe of ion-headgroup correlations in charged interfaces and reveal the thermodynamic mechanism underlying counterion-mediated interfacial ordering, with broad implications for electrolyte design, biomembrane stability, and soft-matter assembly.

physics.optics↗

arXiv2023

Nonrelativistic and nonmagnetic control of terahertz charge currents via electrical anisotropy in RuO2 and IrO2

Precise and ultrafast control over photo-induced charge currents across nanoscale interfaces could lead to important applications in energy harvesting, ultrafast electronics, and coherent terahertz sources. Recent studies have shown that several relativistic mechanisms, including inverse spin-Hall effect, inverse Rashba-Edelstein effect and inverse spin-orbit-torque effect, can convert longitudinally injected spin-polarized currents from magnetic materials to transverse charge currents, thereby harnessing these currents for terahertz generation. However, these mechanisms typically require external magnetic fields and suffer from low spin-polarization rates and low efficiencies of relativistic spin-to-charge conversion. In this work, we present a novel nonrelativistic and nonmagnetic mechanism that directly utilizes the photo-excited high-density charge currents across the interface. We demonstrate that the electrical anisotropy of conductive oxides RuO2 and IrO2 can effectively deflect injected charge currents to the transverse direction, resulting in efficient and broadband terahertz radiation. Importantly, this new mechanism has the potential to offer much higher conversion efficiency compared to previous methods, as conductive materials with large electrical anisotropy are readily available, whereas further increasing the spin-Hall angle of heavy-metal materials would be challenging. Our new findings offer exciting possibilities for directly utilizing these photo-excited high-density currents across metallic interfaces for ultrafast electronics and terahertz spectroscopy.

physics.optics↗

arXiv2023

Probing Interface of Perovskite Oxide Using Surface-specific Terahertz Spectroscopy

The surface/interface species in perovskite oxides play an essential role in many novel emergent physical phenomena and chemical processes. With low eigen-energy in the terahertz region, such species at buried interfaces remain poorly understood due to the lack of feasible experimental techniques. Here, we show that vibrational resonances and two-dimensional electron gas at the interface can be characterized using surface-specific nonlinear spectroscopy in the terahertz range. This technique uses intra-pulse difference frequency mixing (DFM) process, which is allowed only at surface/interface of a medium with inversion symmetry. Sub-monolayer sensitivity can be achieved using the state-of-the-art detection scheme for the terahertz emission from surface/interface. As a demonstration, Drude-like nonlinear response from the two-dimensional electron gas emerging at LaAlO3/SrTiO3 or Al2O3/ SrTiO3 interface was successfully observed. Meanwhile, the interfacial vibrational spectrum of the ferroelectric soft mode of SrTiO3 at 2.8 THz was also obtained that was polarized by the surface field in the interfacial region. The corresponding surface/interface potential, which is a key parameter for SrTiO3-based interface superconductivity and photocatalysis, can now be determined optically via quantitative analysis on the polarized phonon spectrum. The interfacial species with resonant frequencies in the THz region revealed by our method provide more insights into the understanding of physical properties of complex oxides.

physics.optics↗

arXiv2022

Efficient Generation of Intense Broadband Terahertz Pulses from Quartz

The intense terahertz (THz) pulses facilitate the observation of various nonlinear optical effects and manipulation of material properties. In this work, we report a convenient approach that can produce strong broadband terahertz pulses with center frequency tunable between 2-4 THz. The coherent THz light source with pulse energy of 1.2 microjoule can be generated from a low-cost crystalline quartz pumped by an ultrashort tilted wave-front pulse. Thanks to the wide transparent spectral window and high damage threshold, our theoretical analysis and experiment show that the optical rectification in quartz is as efficient as that in LiNbO3, but covers much broader spectral range. This work not only provides the light source that is urgently needed for nonlinear THz spectroscopy beyond 1 THz, but offers an alternative route in the selection of nonlinear optical crystals for optical frequency conversion.

physics.optics↗

arXiv2022

Low-frequency Phonon at Perovskite Oxide Interface Studied by Surface-specific Nonlinear Terahertz Spectroscopy

The low-frequency collective excitations, which often occur in the terahertz or multi-terahertz spectral region, play an essential role in many novel emergent phenomena. Despite numerous studies in the bulk, detection of such excitations at interfaces remains challenging owing to the lack of feasible experimental techniques. Here, we show that interfacial low-frequency modes can be characterized using surface-specific nonlinear terahertz spectroscopy. This technique uses intra-pulse difference frequency mixing (DFM) process that can extend the second-order optical spectroscopy to the terahertz range. As a demonstration, the surface phonon of SrTiO3(001) at 2.8 THz was successfully measured. This surface polarization originates from the excess of oxygen vacancies or charge transfer at the interface. We have also developed an analytical procedure for remote measurement of the interfacial potential of complex oxides in a practical environment. Our method offers new opportunities for in situ studies of the low-frequency excitations at interfaces in broad disciplines.

cond-mat.mtrl-sci↗

arXiv2022

Measurement of DC Magneto-Optical Kerr Effect with Sensitivity of $10^{-7} \text{Rad}/\sqrt{\text{Hz}}$

A high-sensitive DC Magneto-Optical Kerr Effect (MOKE) apparatus is described in this letter. Via detailed analysis on several dominating noise sources, we have proposed solutions that significantly lower the MOKE noise, and a sensitivity of $1.5\times10^{-7} \text{rad}/\sqrt{\text{Hz}}$ is achieved with long-term stability. The sensitivity of the apparatus is tested by measuring a wedge-shaped Ni thin film on $\text{SiO}_2$ with Ni thickness varying from 0 to 3 nm. A noise floor of $1.5\times10^{-8}$ rad is demonstrated. The possibility of further improving sensitivity to $10^{-9}$ rad via applying ac modulation is also discussed.

physics.ins-det↗

arXiv2021

Self-suppression of the Giant CARS Background for Detection of Buried Interface with Sub-monolayer Sensitivity

The past decades have witnessed marked progresses on the research of interfacial science in complex systems promoted by the advances in novel experimental techniques. Despite its success in many fields, implementation of coherent anti-Stokes Raman spectroscopy (CARS) for tackling the problems at interfaces was hindered by the huge resonant and non-resonant background from the bulk. Here we have developed a novel CARS scheme that is capable of probing a buried interface via suppression of the non-resonant and resonant bulk contribution by at least $10^5$ times. The method utilizes self-destructive interference between the forward and backward CARS generated in the bulk near the Brewster angle. As a result, we are able to resolve the vibrational spectrum of sub-monolayer interfacial species immersed in the surrounding media with huge CARS responses. We expect our approach not only opens up the opportunity for interrogation of the interfaces that involve apolar molecules, but also benefits other nonlinear optical spectroscopic techniques in promoting signal-to-background noise ratio.

physics.optics↗

arXiv2021

Spatially homogeneous few-cycle compression of Yb lasers via all-solid-state free-space soliton management

The high power and variable repetition rate of Yb femtosecond lasers make them very attractive for ultrafast science. However, for capturing sub-200 fs dynamics, efficient, high-fidelity, and high-stability pulse compression techniques are essential. Spectral broadening using an all-solid-state free-space geometry is particularly attractive, as it is simple, robust, and low-cost. However, spatial and temporal losses caused by spatio-spectral inhomogeneities have been a major challenge to date, due to coupled space-time dynamics associated with unguided nonlinear propagation. In this work, we use all-solid-state free-space compressors to demonstrate compression of 170 fs pulses at a wavelength of 1030nm from a Yb:KGW laser to ~9.2 fs, with a highly spatially homogeneous mode. This is achieved by ensuring that the nonlinear beam propagation in periodic layered Kerr media occurs in soliton modes and confining the nonlinear phase through each material layer to less than 1.0 rad. A remarkable spatio-spectral homogeneity of ~0.87 can be realized, which yields a high efficiency of >50% for few-cycle compression. The universality of the method is demonstrated by implementing high-quality pulse compression under a wide range of laser conditions. The high spatiotemporal quality and the exceptional stability of the compressed pulses are further verified by high-harmonic generation. This work represents the highest efficiency and the best spatio-spectral quality ever achieved by an all-solid-state free-space pulse compressor for few-cycle-pulse generation.

physics.optics↗

arXiv2021

Active spintronic-metasurface terahertz emitters with tunable chirality

The ability to manipulate the electric-field vector of broadband terahertz waves is essential for applications of terahertz technologies in many areas, and can open up new possibilities for nonlinear terahertz spectroscopy and coherent control. Here, we propose a novel laser-driven terahertz emitter, consisting of metasurface-patterned magnetic multilayer heterostructures. Such hybrid terahertz emitters can combine the advantages of spintronic emitters for being ultrabroadband, efficient and flexible, as well as those of metasurfaces for the unique capability to manipulate terahertz waves with high precision and degree of freedom. Taking a stripe-patterned metasurface as an example, we demonstrate the generation of broadband terahertz waves with tunable chirality. Based on experimental and theoretical studies, the interplay between the laser-induced spintronic-origin currents and the metasurface-induced transient charges/currents are investigated, revealing the strong influence on the device functionality originated from both the light-matter interactions in individual metasurface units and the dynamic coupling between them. Our work not only offers a flexible, reliable and cost-effective solution for chiral terahertz wave generation and manipulation, but also opens a new pathway to metasurface-tailored spintronic devices for efficient vector-control of electromagnetic waves in the terahertz regime.

physics.optics↗

arXiv2020

Solitary beam propagation in a nonlinear optical resonator enables high-efficiency pulse compression and mode self-cleaning

Generating intense ultrashort pulses with high-quality spatial modes is crucial for ultrafast and strong-field science. This can be accomplished by controlling propagation of femtosecond pulses under the influence of Kerr nonlinearity and achieving stable propagation with high intensity. In this work, we propose that the generation of spatial solitons in periodic layered Kerr media can provide an optimum condition for supercontinuum generation and pulse compression using multiple thin plates. With both the experimental and theoretical investigations, we successfully identify these solitary modes and reveal a universal relationship between the beam size and the critical nonlinear phase. Space-time coupling is shown to strongly influence the spectral, spatial and temporal profiles of femtosecond pulses. Taking advantage of the unique characters of these solitary modes, we demonstrate single-stage supercontinuum generation and compression of femtosecond pulses from initially 170 fs down to 22 fs with an efficiency ~90%. We also provide evidence of efficient mode self-cleaning which suggests rich spatial-temporal self-organization processes of laser beams in a nonlinear resonator.

physics.optics↗

arXiv2020

Study of Thermal Expansion Coefficients of 2D Materials via Raman Micro-spectroscopy: Revisited

We report a joint study, using Raman micro-spectroscopy and molecular dynamics simulations, on the substrate effect on thermal properties of 2D materials and revisit measurement of thermal expansion coefficient (TEC) of supported 2D film. Graphene is employed as a representative. We find that the out-of-plane coupling between graphene and substrate strongly affects the temperature-dependent vibrational modes and TEC of graphene. Density of states for long-wavelength out-of-plane oscillations is significantly reduced when graphene is supported on an alkane substrate. To account for the contribution of the out-of-plane coupling to TEC, a Raman micro-spectroscopic scheme is developed. The TEC of graphene on octadecyltrichlorosilane substrate is found to be (-0.6+-0.5)*10-6/K at room temperature, which is fundamentally smaller than that of free-standing graphene. Our results shed light on the understanding of the interaction between 2D material and substrate, and offer a general recipe for optical measurement of TEC of a supported 2D film.

cond-mat.mtrl-sci↗

arXiv2019

Mapping Dynamical Magnetic Responses of Ultra-thin Micron-size Superconducting Films using Nitrogen-vacancy Centers in Diamond

Two-dimensional superconductors have attracted growing interest because of their scientific novelty, structural tunability, and useful properties. Studies of their magnetic responses, however, are often hampered by difficulties to grow large-size samples of high quality and uniformity. We report here an imaging method that employed NV- centers in diamond as sensor capable of mapping out the microwave magnetic field distribution on an ultrathin superconducting film of micron size. Measurements on a 33nm-thick film and a 125nm-thick bulk-like film of $Bi_2Sr_2CaCu_2O_{8+δ}$ revealed that the ac Meissner effect (or repulsion of ac magnetic field) set in at 78K and 91K, respectively; the latter was the superconducting transition temperature (Tc) of both films. The unusual ac magnetic response of the thin film presumably was due to thermally excited vortex-antivortex diffusive motion in the film. Spatial resolution of our ac magnetometer was limited by optical diffraction and the noise level was at 14 $μT/Hz^{1/2}$. The technique could be extended with better detection sensitivity to extract local ac conductivity/susceptibility of ultrathin or monolayer superconducting samples as well as ac magnetic responses of other two-dimensional exotic thin films of limited lateral size.

cond-mat.supr-con↗

arXiv2018

Mechanism of Electric Power Generation from Ionic Droplet Motion on Polymer Supported Graphene

Graphene-based electric power generation that converts mechanical energy of flow of ionic droplets over the device surface into electricity has emerged as promising candidate for a blue-energy network. Yet the lack of a microscopic understanding of the underlying mechanism has prevented ability to optimize and control the performance of such devices. This requires information on interfacial structure and charging behavior at the molecular level. Here, we use sum-frequency vibrational spectroscopy (SFVS) to probe the interfaces of devices composed of aqueous solution, graphene and supporting polymer substrate. We discover that the surface dipole layer of the polymer is responsible for ion attraction toward and adsorption at the graphene surface that leads to electricity generation in graphene. Graphene itself does not attract ions and only acts as a conducting sheet for the induced carrier transport. Replacing the polymer by an organic ferroelectric substrate could enhance the efficiency and allow switching of the electricity generation. Our microscopic understanding of the electricity generation process paves the way for the rational design of scalable and more efficient droplet-motion-based energy transducer devices.

physics.app-ph↗

arXiv2016

Harmonic-seeded resonant Raman amplification in strong-field ionized nitrogen molecules

Generation of free-space laser-like emissions of high spatiotemporal coherence and narrow bandwidth in atmosphere opens promising opportunities for remote spectroscopic sensing. Here, we report on generation of such laser-like emissions, which results from the combined contributions of perturbative and non-perturbative nonlinear optical effects in nitrogen molecules exposed to intense mid-infrared laser fields. We systematically investigate the dependence of the generated free-space laser spectrum on wavelength and power of the driver laser. It is revealed that the free-space laser is produced by resonant Raman amplification of the fifth harmonic of the driver pulses in rotational wavepacket of the molecular nitrogen ions.

physics.optics↗