Ryo Shimano

Above-room-temperature ferroelectricity in a single-component molecular crystal

Ferroelectrics are electro-active materials that can store and switch their polarity (ferroelectricity), sense temperature changes (pyroelectricity), interchange electric and mechanical functions (piezoelectricity), and manipulate light (through optical nonlinearities and the electro-optic effect): all of these functions have practical applications. Topological switching of pi-conjugation in organic molecules, such as the keto-enol transformation, has long been anticipated as a means of realizing these phenomena in molecular assemblies and crystals. Croconic acid, an ingredient of black dyes, was recently found to have a hydrogen-bonded polar structure in a crystalline state. Here we demonstrate that application of an electric field can coherently align the molecular polarities in crystalline croconic acid, as indicated by an increase of optical second harmonic generation, and produce a well-defined polarization hysteresis at room temperature. To make this simple pentagonal molecule ferroelectric, we switched the pi-bond topology using synchronized proton transfer instead of rigid-body rotation. Of the organic ferroelectrics, this molecular crystal exhibits the highest spontaneous polarization ( approximately 20 muC cm(-2)) in spite of its small molecular size, which is in accord with first-principles electronic-structure calculations. Such high polarization, which persists up to 400 K, may find application in active capacitor and nonlinear optics elements in future organic electronics.Ferroelectrics are electro-active materials that can store and switch their polarity (ferroelectricity), sense temperature changes (pyroelectricity), interchange electric and mechanical functions (piezoelectricity), and manipulate light (through optical nonlinearities and the electro-optic effect): all of these functions have practical applications. Topological switching of π-conjugation in organic molecules, such as the keto-enol transformation, has long been anticipated as a means of realizing these phenomena in molecular assemblies and crystals. Croconic acid, an ingredient of black dyes, was recently found to have a hydrogen-bonded polar structure in a crystalline state. Here we demonstrate that application of an electric field can coherently align the molecular polarities in crystalline croconic acid, as indicated by an increase of optical second harmonic generation, and produce a well-defined polarization hysteresis at room temperature. To make this simple pentagonal molecule ferroelectric, we switched the π-bond topology using synchronized proton transfer instead of rigid-body rotation. Of the organic ferroelectrics, this molecular crystal exhibits the highest spontaneous polarization (∼20 μC cm-2) in spite of its small molecular size, which is in accord with first-principles electronic-structure calculations. Such high polarization, which persists up to 400 K, may find application in active capacitor and nonlinear optics elements in future organic electronics.

Light-induced collective pseudospin precession resonating with Higgs mode in a superconductor

Optically manipulating superconductors In superconductors, electrons of opposite momenta pair to form a highly correlated state that manages to flow without encountering any resistance. Matsunaga et al. manipulated the wavefunction of these pairs in the superconductor NbN with an electromagnetic pulse that they transmitted through a thin layer of the material (see the Perspective by Pashkin and Leitenstorfer). The superconducting gap, which is the energy needed to break the pairs apart, oscillated at twice the frequency of the pulses electric field. When they matched this frequency to half the gap, the authors excited a collective mode in the superconductor called the Higgs mode, a relative of the Higgs boson in particle physics. Science, this issue p. 1145; see also p. 1121 Terahertz pump-probe spectroscopy reveals the oscillations of a superconducting order parameter at twice the pump frequency. [Also see Perspective by Pashkin and Leitenstorfer] Superconductors host collective modes that can be manipulated with light. We show that a strong terahertz light field can induce oscillations of the superconducting order parameter in NbN with twice the frequency of the terahertz field. The result can be captured as a collective precession of Anderson’s pseudospins in ac driving fields. A resonance between the field and the Higgs amplitude mode of the superconductor then results in large terahertz third-harmonic generation. The method we present here paves a way toward nonlinear quantum optics in superconductors with driving the pseudospins collectively and can be potentially extended to exotic superconductors for shedding light on the character of order parameters and their coupling to other degrees of freedom.

Higgs amplitude mode in the BCS superconductors Nb1-xTi(x)N induced by terahertz pulse excitation.

Ultrafast responses of BCS superconductor Nb(1-x)Ti(x)N films in a nonadiabatic excitation regime were investigated by using terahertz (THz) pump-THz probe spectroscopy. After an instantaneous excitation with the monocycle THz pump pulse, a transient oscillation emerges in the electromagnetic response in the BCS gap energy region. The oscillation frequency coincides with the asymptotic value of the BCS gap energy, indicating the appearance of the theoretically anticipated collective amplitude mode of the order parameter, namely the Higgs amplitude mode. Our result opens a new pathway to the ultrafast manipulation of the superconducting order parameter by optical means.

Quantum Faraday and Kerr rotations in graphene

Graphene, a monolayer sheet of carbon atoms, exhibits intriguing electronic properties that arise from its massless Dirac dispersion of electrons. A striking example is the half-integer quantum Hall effect, which endorses the presence of Dirac cones or, equivalently, a non-zero (π) Berrys (topological) phase. It is curious how these anomalous features of Dirac electrons would affect optical properties. Here we observe the quantum magneto-optical Faraday and Kerr effects in graphene in the terahertz frequency range. Our results detect the quantum plateaus in the Faraday and Kerr rotations at precisely the quantum Hall steps that hallmark the Dirac electrons, with the rotation angle defined by the fine-structure constant. The robust quantum Hall plateaus in the optical regime, besides being conceptually interesting, may open avenues for new graphene-based optoelectronic applications.

Numerical phase correction method for terahertz time-domain reflection spectroscopy

We propose a numerical method for the misplacement phase error correction in terahertz time-domain reflection spectroscopy (THz-TDRS). The developed algorithm is based on the maximum entropy principle and can be readily implemented into data processing, allowing one to reveal material parameters of the opaque materials from the THz reflection measurements. The method resolves the phase retrieval problem in the THz-TDRS and dramatically simplifies the experimental procedure.

Terahertz frequency Hall measurement by magneto-optical Kerr spectroscopy in InAs

We report on the time-domain terahertz (THz) magneto-optical Kerr spectroscopy in the frequency range from 0.5 to 2.5 THz. The developed technique employs reflection geometry, enabling high-frequency noncontact Hall measurements in opaque samples. We also present a method to reveal the off-diagonal component of the complex dielectric tensor from the measured polarization-dependent THz wave forms. At a static magnetic field of 0.48 T, a large Kerr rotation over 10° originating from magnetoplasma resonance is observed in an n-type undoped InAs wafer at room temperature. This indicates the strong potential of this material for the polarization modulator in the THz regime.

Study of orthoexciton-to-paraexciton conversion in Cu(2)O by excitonic Lyman spectroscopy.

Using time-resolved 1s-2p excitonic Lyman spectroscopy, we study the orthoexciton-to-paraexciton transfer, following the creation of a high density population of ultracold 1s orthoexcitons by resonant two-photon excitation with femtosecond pulses. An observed fast exciton-density dependent conversion rate is attributed to spin exchange between pairs of orthoexcitons. Implication of these results on the feasibility of Bose-Einstein condensation of paraexcitons in Cu(2)O is discussed.

Chirality of matter shows up via spin excitations

Chirality is usually manifested by differences in a material’s response to left- and right-circularly polarized light. This difference is the result of the specific distribution of charge within chiral materials. A similar response has now been found to result from the chiral spin structure of an antiferromagnet.

Intense terahertz pulse induced exciton generation in carbon nanotubes.

We have investigated the highly nonlinear terahertz (THz) light-matter interaction in single-walled carbon nanotubes (SWNTs). The high-peak THz electric-field (∼0.7 MV/cm) and the low effective mass of carriers result in their ponderomotive energy exceeding the bandgap energy of semiconducting SWNTs. Under such an intense THz pulse irradiation, the interband excitation that results in the generation of excitons occurs, although the THz photon energy (∼4 meV) is much smaller than the gap energy of SWNTs (∼1 eV). The ultrafast dynamics of this exciton generation process is investigated by THz pump and optical probe spectroscopy. The exciton generation mechanism is described by impact excitation process induced by the strong THz E-field. Such intense THz pulse excitation provides a powerful tool to study nonlinear terahertz optics in non-perturbative regime as well as nonlinear transport phenomena in solids with ultrafast temporal resolution.

Optical Hall effect in the integer quantum Hall regime.

Optical Hall conductivity σ{xy}(ω) is measured from the Faraday rotation for a GaAs/AlGaAs heterojunction quantum Hall system in the terahertz-frequency regime. The Faraday rotation angle (∼ fine-structure constant ∼ mrad) is found to significantly deviate from the Drude-like behavior to exhibit a plateaulike structure around the Landau-level filling ν=2. The result, which fits with the behavior expected from the carrier localization effect in the ac regime, indicates that the plateau structure, although not quantized, still exists in the terahertz regime.