Natsuki Kanda

Efficient coupling of propagating broadband terahertz radial beams to metal wires

Bare metal wires have recently been demonstrated as waveguides for transporting terahertz (THz) radiation, where the guiding mode is radially polarized surface Sommerfeld waves. In this study, we demonstrate high-efficiency coupling of a broadband radially polarized THz pulsed beam, which is generated with a polarization-controlled beam by a segmented half-wave-plate mode converter, to bare copper wires. A total coupling efficiency up to 16.8% is observed, and at 0.3 THz, the maximum coupling efficiency is 66.3%. The results of mode-overlap calculation and numerical simulation support the experimental data well.

Terahertz wave polarization rotation with double layered metal grating of complimentary chiral patterns

We propose and demonstrate polarization rotation of a terahertz (THz) electromagnetic wave by using two-dimensional gratings consisting of two displaced layers of gold film with complimentary chiral patterns with four-fold symmetry. We develop a time domain THz polarimetry method with three wire grid polarizers and distinguish optical activity from optical anisotropy. We obtain the isotropic polarization rotation of a terahertz wave free from the birefringence of the structures. Results indicate the possibility of controlling THz polarization with artificial chiral structures fabricated with thin metal films.

The vectorial control of magnetization by light

Application of coherent light–matter interactions has recently been extended to the ultrafast control of magnetization. An important but unrealized technique is the manipulation of magnetization vector motion to make it follow an arbitrarily designed multidimensional trajectory. Here we demonstrate a full manipulation of two-dimensional magnetic oscillations in antiferromagnetic NiO with a pair of polarization-twisted femtosecond laser pulses. We employ Raman-type nonlinear optical processes, wherein magnetic oscillations are impulsively induced with a controlled initial phase. Their azimuthal angle follows well-defined selection rules that have been determined by the symmetries of the materials. We emphasize that the temporal variation of the laser-pulse polarization angle enables us to control the phase and amplitude of the two degenerate modes, independently. These results lead to a new concept of the vectorial control of magnetization by light.

Enantiomeric switching of chiral metamaterial for terahertz polarization modulation employing vertically deformable MEMS spirals

Active modulation of the polarization states of terahertz light is indispensable for polarization-sensitive spectroscopy, having important applications such as non-contact Hall measurements, vibrational circular dichroism measurements and anisotropy imaging. In the terahertz region, the lack of a polarization modulator similar to a photoelastic modulator in the visible range hampers expansion of such spectroscopy. A terahertz chiral metamaterial has a huge optical activity unavailable in nature; nevertheless, its modulation is still challenging. Here we demonstrate a handedness-switchable chiral metamaterial for polarization modulation employing vertically deformable Micro Electro Mechanical Systems. Vertical deformation of a planar spiral by a pneumatic force creates a three-dimensional spiral. Enantiomeric switching is realized by selecting the deformation direction, where the polarity of the optical activity is altered while maintaining the spectral shape. A polarization rotation as high as 28° is experimentally observed, thus providing a practical and compact polarization modulator for the terahertz range.

Terahertz vector beam generation using segmented nonlinear optical crystals with threefold rotational symmetry

We demonstrated a new method of Terahertz cylindrical vector beam generation using segmented nonlinear crystals with three-fold rotational symmetry. The polarization dependent in-plane images obtained with a Terahertz camera clearly showed axial-symmetric polarization distributions. The electro-optic sampling image clearly visualized the longitudinal electric field at the focal point.

Light-induced terahertz optical activity

We propose and demonstrate a simple method to induce optical activity in the terahertz (THz) region using photoexcited carriers in a semiconductor substrate with metal two-dimensional chiral masks, which does not show optical activity without photoexcitation. The three-dimensional chirality induced by the combination of photocarriers and metal mask gives rise to the optical activity. With this simple method, we can develop THz polarization modulation techniques applicable for biology, chemistry, and material sciences.

Spiral metamaterial for active tuning of optical activity

We propose an electrostatically actuated spiral structure as a metamaterial for circularly polarized light in the terahertz (THz) frequency range. An array of planar spiral structures was fabricated with micro electro mechanical system technology, and the geometry of the structures can be changed by electrostatic actuation. The actuation transforms the planar spirals into three-dimensional helices, resulting in optical activity in which the differential polarization response of the material depends on whether the incident light is left- or right-circularly polarized. THz spectroscopy confirmed that the optical activity can be tuned with the proposed structures.

Selection rules for light-induced magnetization of a crystal with threefold symmetry: the case of antiferromagnetic NiO.

We propose Raman-induced collinear difference-frequency generation (DFG) as a method to manipulate dynamical magnetization. When a fundamental beam propagates along a threefold rotational axis, this coherent second-order optical process is permitted by angular momentum conservation through the rotational analogue of the Umklapp process. As a demonstration, we experimentally obtained polarization properties of collinear magnetic DFG along a [111] axis of a single crystal of antiferromagnetic NiO with micro multidomain structure, which excellently agreed with the theoretical prediction. PACS numbers: 78.20.Ls, 42.65.-k, 76.50.+g

Generation of broadband terahertz vortex beams

We propose and demonstrate a method for generating broadband terahertz (THz) vortex beams. We convert a THz radially polarized beam into a THz vortex beam via achromatic polarization optical elements for THz waves and characterize the topological charge of the generated vortex beam by measuring the spatial distribution of the phase of the THz wave at its focal plane. For example, a uniform topological charge of +1 is achieved over a wide frequency range. We also demonstrate that the sign of the topological charge can be easily controlled. By utilizing the orbital angular momentum of the beam, these results open new THz wave technologies for sensing, manipulation, and telecommunication.

All-photoinduced terahertz optical activity

We proposed and demonstrated active control of terahertz optical activity via chiral patterned photoexcitation in a semiconductor with a spatial light modulator (SLM). Arbitrary patterns can be generated by a SLM, including completely symmetric enantiomer pairs. This technique provides a new route to terahertz polarization modulators.