Suchismita Sarker

The electro‐optic effect powers microwave‐to‐optical quantum transducers, photonic quantum computing switches, and high‐speed optical interconnects. A larger electro‐optic coefficient directly reduces the drive voltage and device footprint of electro‐optic modulators, making the search for materials with large coefficients a central challenge in photonics. Here, guided by phase‐field modeling, we engineer an electro‐optic coefficient of 43 000 pm V ⁻¹ in monoclinic KNbO ₃ thin films, the largest ever measured in any material, exceeding industry‐standard LiNbO ₃ by 1000×. This exceptional response arises from electric field‐driven “lateral swaying” of the ferroelectric polarization vector between symmetry‐equivalent monoclinic variants, which reorients the optical index ellipsoid far beyond the conventional Pockels response that relies on a linear coupling between field and polarization. The ultralow coercive fields of the monoclinic phase (∼1 kV cm ⁻¹ ) place this giant response within reach of standard drive electronics, enabling drive voltages orders of magnitude below the current state‐of‐the‐art. The response is stable to within 1% − 2% over hours, robust under thermal and electrical cycling, and demonstrated up to 1 MHz, establishing low‐symmetry ferroelectric phases as an unexploited design space for next‐generation energy‐efficient electro‐optic modulators for classical and quantum photonic integrated circuits.

The absence of a native electro‐optic effect in silicon remains a fundamental limitation for integrated photonics, motivating the integration of ferroelectric oxides as active materials. Here, we report the first epitaxial integration of KNbO ₃ thin films on silicon, establishing a new materials platform for silicon‐integrated electro‐optics. Silicon integration of KNbO ₃ has remained unexplored due to potassium volatility challenges during synthesis. Employing suboxide molecular‐beam sources assisted by in situ RHEED monitoring and guided by thermodynamic simulations, we establish an adsorption‐controlled growth window for phase‐pure KNbO ₃ films. Temperature dependent x‐ray diffraction, optical second‐harmonic generation, piezo‐response force microscopy, and transmission electron microscopy aided by phase‐field simulations confirm high‐quality epitaxial films with sharp interfaces and structural phase transitions mirroring KNbO ₃ single crystals. Electro‐optic measurements demonstrate a linear Pockels response with an effective electro‐optic response of 146 ± 14 pm V ⁻¹ at 1550 nm. Phase‐field simulations further predict that modest compressive strain can drive the electro‐optic response to ∼900 pm V ⁻¹ . Beyond KNbO ₃ , this integration pathway provides access to the broader (K,Na)(Ta,Nb)O ₃ family, opening pathways to harness their strong electro‐optic response, nonlinear optical response, piezoelectric response and ferroelectric properties for next‐generation silicon photonics and electronics beyond current BaTiO ₃ ‐based approaches.

The advent of two-dimensional moiré systems has revolutionized the exploration of phenomena arising from strong correlations and nontrivial band topology. Recently, a moiré superstructure formed by two coexisting charge density waves with slightly mismatched wavevectors has been realized. These incommensurate charge density waves can collectively exhibit commensurability, forming a jointly commensurate charge density wave that provides a paradigm for controlling moiré potential and periodicity. Here, we use time-resolved diffraction to probe the light-induced dynamics of the jointly commensurate charge density wave in EuTe ₄ . Our measurements distinguish the instantaneous quenching of its amplitude, as verified by time-resolved photoemission spectroscopy, from the slower evolution of phase fluctuations. Moreover, while the wavevector remains locked upon photoexcitation, the correlation length decreases exclusively perpendicular to the wavevector, revealing the formation of shear-type defects. Together, our multimodal approach reconstructs the temporal evolution of the jointly commensurate charge density wave, highlighting its robustness out of equilibrium and providing insights into the optical manipulation of moiré materials through defect control.

Vortical motion naturally occurs on various scales, from galaxies to skyrmions, influencing the dynamics of mass and information transport. Yet its impact on lattice dynamics in solids is rarely explored so far, primarily owing to the strong interatomic bonds in real materials that inhibit the actual transport of their constituent species. Here we investigate the effect of vortical modes of polarized phonons on the heat insulation of solid materials using synchrotron X-ray diffuse scattering and inelastic neutron scattering along with ab initio molecular dynamics simulations. We demonstrate this vortical vibration as well as its effect on thermal insulation in CsAg ₂ I ₃ , where the anisotropically bonded iodine atoms form the low-lying transverse optical phonons for enhancing scattering phase space. Moreover, the vortical modes, mainly arising from the chiral symmetric operations acting on the iodine sites, induce two-dimensional short-range-ordering of atomic clusters, lead to a substantially increased phonon scattering centers with a diameter on the order of ~2 primitive cells. This eventually results in extraordinary heat insulation in the single-crystalline CsAg ₂ I ₃ , exhibiting a thermal conductivity as low as 0.08 W/m-K at room temperature. This strategy establishes a material design paradigm for heat insulation and unveils an unexplored avenue for phonon transport manipulation.

Lead-free perovskites have emerged as environmentally benign alternatives to lead halide counterparts for photovoltaic and optoelectronic applications. Among them, the double perovskite Cs ₂ AgInCl ₆ family, with proper composition engineering, exhibits remarkable white-light emission characteristics enabled by strong electron-phonon coupling and the formation of self-trapped excitons (STEs). Despite these advantages, the fundamental photophysics and structural dynamics governing their excited-state behavior remain poorly understood. Here, we report a long-lived metastable phase in the Cs ₂ AgInCl ₆ double perovskite family and unravel this process and the concomitant electronic and structural evolution using a suite of tools including transient optical spectroscopy, time-resolved x-ray diffraction (TR-XRD), time-resolved x-ray absorption spectroscopy (TR-XAS), and inelastic x-ray scattering (IXS). We show that the photoinduced, transient metastable phase is associated with B-site [silver (Ag)–indium (In)] disorder, which induces a markedly reduced optical bandgap. Supported by TR-XRD and first-principles calculations, the Ag-In disorder drives the formation of Ag-rich and In-rich domains with millisecond lifetimes, with lifetimes increasing at lower temperatures. TR-XAS further reveals that the photogenerated STEs oxidize Ag ⁺ to Ag ²⁺ , which facilitates this highly temporally asymmetric order-disorder transition. Our findings demonstrate a previously unidentified mechanism, mediated by hole-localized STE formation, that enables prolongation of transient light-induced states to the multimillisecond regime in halide double perovskites, opening possibilities to harvesting the functional properties of metastable phases of these material systems.

Tantalum dioxide (TaO2) is a metastable tantalum compound. Here, we report the epitaxial stabilization of TaO2 on Al2O3 (11̄02) (r-plane sapphire) substrates using suboxide molecular-beam epitaxy and thermal laser epitaxy, demonstrating single-oriented, monodomain growth of anisotropically strained thin films. Microstructural investigation is performed using synchrotron x-ray diffraction and scanning transmission electron microscopy. The tetravalent oxidation state of tantalum is confirmed using x-ray absorption and photoemission spectroscopy as well as electron energy-loss spectroscopy. Optical properties are investigated via spectroscopic ellipsometry and reveal a 0.3 eV Mott gap of the tantalum 5d electrons. Density-functional theory and group theoretical arguments are used to evaluate the limited stability of the rutile phase and reveal the potential to unlock a hidden metal–insulator transition concomitant with a structural phase transition to a distorted rutile phase, akin to NbO2. Our study expands the understanding of tantalum oxides and paves the way for their integration into next-generation electronic and photonic devices.

Among the family of intercalated transition-metal dichalcogenides (TMDs), Fe_{x}NbS_{2} is found to possess unique current-induced resistive switching behaviors, tunable antiferromagnetic states, and a commensurate charge order, all of which are tied to a critical Fe doping of x_{c}=1/3. However, the electronic origin of such extreme stoichiometry sensitivities remains unclear. Combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations, we identify and characterize a dramatic eV-scale electronic restructuring that occurs across the x_{c}. Moment-carrying Fe 3d_{z^{2}} electrons manifest as narrow bands within 200 meV of the Fermi level, distinct from other transition metal intercalated TMD magnets. These states strongly hybridize with itinerant electrons in the TMD layer and rapidly lose coherence above x_{c} due to correlation-driven effects. This sudden quasiparticle decoherence collapses the Fe-Nb hybridization, which explicitly suppresses the out-of-plane effective Fe-Fe exchange interaction, driving the transformation of the magnetic ground state from an antiferromagnetic stripe phase to a zigzag phase. These observations resemble the exceptional electronic and magnetic sensitivity of strongly correlated systems, and demonstrate that quantifying orbital-specific hybridization via ARPES offers an alternative pathway to evaluate effective magnetic exchange in metallic magnets, complementing inelastic neutron and resonant x-ray scattering probes.