Makoto Kuwata-Gonokami

Polymer microdisk and microring lasers

Dye-doped polymer microlasers have been fabricated by photolithography and self-assembly. Microdisk lasers 5 to 30 microm in diameter were photolithographically patterned on thin planar polymer waveguides. We formed polymer microring lasers on thinned silica fibers by dipping the fibers in polymers and allowing the polymer droplets to cure.

Laser Emission from Dye-Doped Polystyrene Microsphere*

Lasing of dye-doped polystyrene microspheres with diameters ranging from 10 µm to 92 µm is observed. Emission from an individual sphere placed on a glass plate is examined under optical pumping with nanosecond pulses whose frequency is tuned to the absorption band of the dye. Spectral and temporal responses, and spatial profile show the characteristics of lasing with spherical cavity modes.

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.

Polymer whispering gallery mode lasers

We study the emission properties of dye doped polymer microspheres. Under the weak excitation condition, emission exhibits resonant features characteristic to the spherical cavity modes. By the systematic analyses with the Lorenz-Mie calculation, we determine the mode indices of all the resonance lines. From the line width analyses, we determine the effective concentration of dye molecules. Under the pulsed pumping, we observe lasing. Spheres with dye molecule in the entire sphere show complicated spectra indicating the multi-mode lasing. We demonstrate the mode elimination by using a stratified structure. A 20 μm diameter sphere which has the dye doped shell shows very simple lasing by the lowest order number modes of spherical cavity mode. A small sphere of 5 μm in diameter shows single mode lasing behavior. The observed spontaneous coupling coefficient is about 30% which agrees with the calculated value.

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.

Whispering-gallery-mode microring laser using a conjugated polymer

We observed laser emission in whispering gallery modes using a microring composed of a semiconducting polymer poly[2,5-bis-(2′-ethylhexyloxy)-p-phenylenevinylene coated on an etched fiber under transient and quasisteady-state pumping conditions. The threshold for laser oscillation was 1 mJ/cm2 (0.1 MW/cm2) and 30 μJ/cm2 (300 MW/cm2) for nanosecond and femtosecond excitation, respectively. The laser output showed superlinear dependence on the excitation energy above the threshold. The demonstration of lasing under quasisteady-state pumping shows the possibility to develop electrically pumped polymer lasers.

Ultrafast optical nonlinearity in the quasi-one-dimensional mott insulator Sr2CuO3

We report strong instantaneous photoinduced absorption in the quasi-one-dimensional Mott insulator Sr2CuO3 in the IR spectral region. The observed photoinduced absorption is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculation based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of Sr2CuO3 for all-optical switching.

Circularly polarized light emission from semiconductor planar chiral nanostructures.

Circular anisotropy of vacuum filed modes is induced in on-waveguide semiconductor chiral photonic crystals. Quantum dots embedded in the waveguide layer emit circularly polarized light with 25% of circular degree of polarization.

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.

Photonic molecule lasing

Lasing at resonantly coupled whispering-gallery mode frequencies is observed in photonic molecules consisting of bispheres of 4.2 and 5.1 microm in diameter placed in a silicon V-groove. We examine spatial profiles of photonic molecule modes by use of frequency-resolved imaging and reveal bonding and antibonding mode features. From the lasing threshold characteristics, we quantitatively measure the quality factor and the spontaneous-emission coupling ratio of the photonic molecule modes and confirm that strong coherent coupling leads to photonic molecule modes.