Ultrafast carrier dynamics and radiative recombination in multiferroic BiFeO3_{3}

We report a comprehensive study of ultrafast carrier dynamics in single crystals of multiferroic BiFeO$_{3}$. Using femtosecond optical pump-probe spectroscopy, we find that the photoexcited electrons relax to the conduction band minimum through electron-phonon coupling with a $\sim$1 picosecond time constant that does not significantly change across the antiferromagnetic transition. Photoexcited electrons subsequently leave the conduction band and primarily decay via radiative recombination, which is supported by photoluminescence measurements. We find that despite the coexisting ferroelectric and antiferromagnetic orders in BiFeO$_{3}$, the intrinsic nature of this charge-transfer insulator results in carrier relaxation similar to that observed in bulk semiconductors

Enhanced third harmonic generation from the epsilon-near-zero modes of ultrathin films

We experimentally demonstrate efficient third harmonic generation from an indium tin oxide (ITO) nanofilm (lambda/42 thick) on a glass substrate for a pump wavelength of 1.4 um. A conversion efficiency of 3.3x10^-6 is achieved by exploiting the field enhancement properties of the epsilon-near-zero (ENZ) mode with an enhancement factor of 200. This nanoscale frequency conversion method is applicable to other plasmonic materials and reststrahlen materials in proximity of the longitudinal optical phonon frequencies.Comment: 13 pages, 5 figure

Coherent Spin-Phonon Coupling in the Layered Ferrimagnet Mn3Si2Te6

We utilize ultrafast photoexcitation to drive coherent lattice oscillations in the layered ferrimagnetic crystal Mn3Si2Te6, which significantly stiffen below the magnetic ordering temperature. We suggest that this is due to an exchange-mediated contraction of the lattice, stemming from strong magneto-structural coupling in this material. Additionally, simulations of the transient incoherent dynamics reveal the importance of spin relaxation channels mediated by optical and acoustic phonon scattering. Our findings highlight the importance of spin-lattice coupling in van der Waals magnets and a promising route for their dynamic optical control through their intertwined electronic, lattice, and spin degrees of freedom

Probing the Interplay between Quantum Charge Fluctuations and Magnetic Ordering in LuFe2O4

Ferroelectric and ferromagnetic materials possess spontaneous electric and magnetic order, respectively, which can be switched by the corresponding applied electric and magnetic fields. Multiferroics combine these properties in a single material, providing an avenue for controlling electric polarization with a magnetic field and magnetism with an electric field. These materials have been intensively studied in recent years, both for their fundamental scientific interest as well as their potential applications in a broad range of magnetoelectric devices [1, 2, 3, 4]. However, the microscopic origins of magnetism and ferroelectricity are quite different, and the mechanisms producing strong coupling between them are not always well understood. Hence, gaining a deeper understanding of magnetoelectric coupling in these materials is the key to their rational design. Here, we use ultrafast optical spectroscopy to show that quantum charge fluctuations can govern the interplay between electric polarization and magnetic ordering in the charge-ordered multiferroic LuFe2O4

Optically tuned terahertz modulator based on annealed multilayer MoS2

Controlling the propagation properties of terahertz waves is very important in terahertz technologies applied in high-speed communication. Therefore a new-type optically tuned terahertz modulator based on multilayer-MoS 2 and silicon is experimentally demonstrated. The terahertz transmission could be significantly modulated by changing the power of the pumping laser. With an annealing treatment as a p-doping method, MoS 2 on silicon demonstrates a triple enhancement of terahertz modulation depth compared with the bare silicon. This MoS 2 -based device even exhibited much higher modulation efficiency than the graphene-based device. We also analyzed the mechanism of the modulation enhancement originated from annealed MoS 2, and found that it is different from that of graphene-based device. The unique optical modulating properties of the device exhibit tremendous promise for applications in terahertz switch

Cavity-Altered Superconductivity

Is it feasible to alter the ground state properties of a material by engineering its electromagnetic environment? Inspired by theoretical predictions, experimental realizations of such cavity-controlled properties without optical excitation are beginning to emerge. Here, we devised and implemented a novel platform to realize cavity-altered materials. Single crystals of hyperbolic van der Waals (vdW) compounds provide a resonant electromagnetic environment with enhanced density of photonic states and superior quality factor. We interfaced hexagonal boron nitride (hBN) with the molecular superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br (κ-ET). The frequencies of infrared (IR) hyperbolic modes of hBN match the IR-active carbon-carbon stretching molecular resonance of (κ-ET) implicated in superconductivity. Nano-optical data supported by first-principles molecular Langevin dynamics simulations confirm resonant coupling between the hBN hyperbolic cavity modes and the carbon-carbon stretching mode in (κ-ET). Meissner effect measurements via magnetic force microscopy demonstrate a strong suppression of superfluid density near the hBN/( -ET) interface. Non-resonant control heterostructures, including RuCl3/(κ-ET) and hBN/Bi2Sr2CaCu2O8+x, do not display the superfluid suppression. These observations suggest that hBN/(κ-ET) realizes a cavity-altered superconducting ground state. This work highlights the potential of dark cavities devoid of external photons for engineering electronic ground-state properties of materials using IR-active phonons

Photocurrent-driven transient symmetry breaking in the Weyl semimetal TaAs

Symmetry plays a central role in conventional and topological phases of matter, making the ability to optically drive symmetry change a critical step in developing future technologies that rely on such control. Topological materials, like the newly discovered topological semimetals, are particularly sensitive to a breaking or restoring of time-reversal and crystalline symmetries, which affect both bulk and surface electronic states. While previous studies have focused on controlling symmetry via coupling to the crystal lattice, we demonstrate here an all-electronic mechanism based on photocurrent generation. Using second-harmonic generation spectroscopy as a sensitive probe of symmetry change, we observe an ultrafast breaking of time-reversal and spatial symmetries following femtosecond optical excitation in the prototypical type-I Weyl semimetal TaAs. Our results show that optically driven photocurrents can be tailored to explicitly break electronic symmetry in a generic fashion, opening up the possibility of driving phase transitions between symmetry-protected states on ultrafast time scales