Highly efficient THz generation by optical rectification of mid-IR pulses in DAST

We report on efficient THz generation in DAST by optical rectification of intense mid-IR pulses centered at (i) 3.9 μm and (ii) its second harmonic at 1.95 μm. Suppression of multi-photon absorption shifts the onset of saturation of the THz conversion efficiency to pump energy densities, which are almost an order of magnitude higher as compared to conventional pump schemes at 1.5 μm. Despite strong linear absorption at 3.9 μm, DAST exhibits a high optical-to-THz conversion efficiency, which we attribute to resonantly enhanced nonlinearity and advantageous phase matching of the THz phase velocity and group velocity of the driving pulse. At 1.95 μm, we find that low linear and multi-photon absorption in combination with cascaded optical rectification lead to record optical-to-THz conversion efficiencies approaching 6%. The observed high sensitivity of the THz generation to the parameters of the mid-IR driving pulses motivates an in-depth study of the underlying interplay of nonlinear wavelength- and intensity-dependent effects

Vibronic collapse of ordered quadrupolar ice in the pyrochore magnet Tb2+x_{2+x}Ti2−x_{2-x}O7+y_{7+y}

While the spin liquid state in the frustrated pyrochlore Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ has been studied both experimentally and theoretically for more than two decades, no definite description of this unconventional state has been achieved. Using synchrotron based THz spectroscopy in combination with quantum numerical simulations, we highlight a significant link between two previously unrelated features: the existence of a quadrupolar order following an ice rule and the presence of strong magneto-elastic coupling in the form of hybridized Tb$^{3+}$ crystal-field and phonon modes. The magnitude of this so-called vibronic process, which involves quadrupolar degrees of freedom, is significantly dependent on small off-stoichiometry $x$ and favors all-in all-out like correlations between quadrupoles. This mechanism competes with the long range ordered quadrupolar ice, and for slightly different stoichiometry, is able to destabilize it.Comment: Main text: 7 pages, 3 figures ; Supplemental Material: 6 pages, 2 figure

Helical bunching and symmetry lowering inducing multiferroicity in Fe langasites

International audienceThe chiral Fe-based langasites represent model systems of triangle-based frustrated magnets with a strong potential for multiferroicity. We report neutron scattering measurements for the multichiral Ba3MFe3Si2O14 (M = Nb, Ta) langasites revealing new important features of the magnetic order of these systems: the bunching of the helical modulation along the c-axis and the in-plane distortion of the 120° Fe-spin arrangement. We discuss these subtle features in terms of the microscopic spin Hamiltonian, and provide the link to the magnetically-induced electric polarization observed in these systems. Thus, our findings put the multiferroicity of this attractive family of materials on solid ground

Unusual magnetoelectric effect in paramagnetic rare-earth langasite

Violation of time reversal and spatial inversion symmetries has profound consequences for elementary particles and cosmology. Spontaneous breaking of these symmetries at phase transitions gives rise to unconventional physical phenomena in condensed matter systems, such as ferroelectricity induced by magnetic spirals, electromagnons, non-reciprocal propagation of light and spin waves, and the linear magnetoelectric (ME) effect - the electric polarization proportional to the applied magnetic field and the magnetization induced by the electric field. Here, we report the experimental study of the holmium-doped langasite, Ho$_{x}$La$_{3-x}$Ga$_5$SiO$_{14}$, showing a puzzling combination of linear and highly non-linear ME responses in the disordered paramagnetic state: its electric polarization grows linearly with the magnetic field but oscillates many times upon rotation of the magnetic field vector. We propose a simple phenomenological Hamiltonian describing this unusual behavior and derive it microscopically using the coupling of magnetic multipoles of the rare-earth ions to the electric field.Comment: 8 pages, 3 figure

Terahertz spectroscopy of magnetic metal oxides

The optical excitation and control of spins in magnetic materials promises new avenues for devices that couple photonic and spintronic functionality, with the prospects of a new paradigm in information processing. An understanding of the behaviour of the magnetic interactions in candidate materials is essential to the design of devices that exhibit these desirable properties. Terahertz radiation presents an ideal medium for the study of spin dynamics in magnetic materials. Its low energy in the meV range is equivalent to the many weak magnetic coupling mechanisms present. Terahertz radiation is therefore also complementary to established methods that probe these mechanisms such as inelastic neutron scattering and magnetometry. The electromagnetic nature of terahertz radiation means that the magnetic field component can couple directly to the ordered spin moments. Furthermore, the electric field component may interact with lattice coupled magneto-electric excitations. The metal oxides, which combine magnetic super-exchange, strongly correlated electrons and low symmetry crystalline environments, are good candidates for technological applications invoking these interactions. This is because they often feature exploitable properties such as antiferromagnetism, ferroelectricity, semiconducting behaviour and magneto-electric coupling. They are also typically insulating allowing for terahertz transmission investigations. In this thesis terahertz transmission spectroscopy is performed on a number of magnetic metal oxides as well as a non-magnetic semiconductor. Specifically, a non-magnetic semiconductor (ZnTe); a geometrically frustrated metamagnet (Cu3Bi(SeO3)2O2Cl); a canted antiferromagnet (NdFeO3) and a quantum spin ladder system (Sr14Cu24O41) are investigated. Collectively, the work establishes terahertz radiation as an e↵ective probe of material properties. The results reveal a diverse range of magnetic excitations, characterising their temperature and external magnetic field dynamics. The physical interactions probed are commented on regarding their relevance to the emerging field of spintronics

The emission of visible radiation by peeling adhesive tape

We have observed the emission of visible radiation on peeling adhesive tape from a spool. We have found that there is a fluctuation of light intensity in phase with the rotation of the spool from which the tape is being unwound. We have further found that the amount of light emitted initially increases as the unwinding speed is increased, then decreases at higher speeds

THz absorption bands in Sr14Cu24O41 by synchrotron radiation

We use unpolarized synchrotron radiation to probe the temperature dependent transmittance of single crystal Sr14Cu24O41 in the spectral range 15-110 cm-1 (0.45-3.3 THz). At high temperature we identify a conductivity band, which we assign to hole hopping between neighboring chains. At low temperatures we identify 14 resonance bands, 11 of which are previously unreported. Analysis of the absorption band spectral weight indicates mode coupling accompanying a transition in the vicinity of 100-200 K

Stacked-and-drawn metamaterials with magnetic resonances in the terahertz range

We present a novel method for producing drawn metamaterials containing slotted metallic cylinder resonators, possessing strong magnetic resonances in the terahertz range. The resulting structures are either spooled to produce a 2-dimensional metamaterial monolayer, or stacked to produce three-dimensional multi-layered metamaterials. We experimentally investigate the effects of the resonator size and number of metamaterial layers on transmittance, observing magnetic resonances between 0.1 and 0.4 THz, in good agreement with simulations. Such fibers promise future applications in mass-produced stacked or woven metamaterial

Far-infrared spectroscopy of quantum spin chain: PbCuSO4(OH)2

This work presents far-infrared transmission spec-troscopy on single crystal PbCuSO4(OH)2 using synchrotron radiation. The study covers the spectral region 150-400 cm-1 with electric field polarisation parallel to either the a or b crystal directions. The results reveal a number of anisotropic absorption features tentatively attributed to phonon modes