Yosuke Minowa

Achromatic THz wave plate composed of stacked parallel metal plates

We propose an achromatic wave plate in the high THz frequency region based on the artificial medium. It is composed of the stacked parallel metal plates with a pillar array, which bring in the controllable birefringence in an effectively lossless medium. We experimentally demonstrate that the artificial medium operates as the quarter wave plate from 2.05 to 3.26 THz.

Inner structure of ZnO microspheres fabricated via laser ablation in superfluid helium

ZnO microspheres fabricated via laser ablation in superfluid helium were found to have bubble-like voids. Even a microsphere demonstrating clear whispering gallery mode resonances in the luminescence had voids. Our analysis confirmed that some voids are located away from the surface and have negligible or little effect on the whispering gallery mode resonances since the electromagnetic energy localizes near the surface of these microspheres. The existence of the voids indicates that helium gas or any evaporated target material was present within the molten microparticles during the microsphere formation.

Achromatic wave plate in THz frequency region based on parallel metal plate waveguides with a pillar array

We demonstrated an achromatic wave plate based on parallel metal plate waveguides in the high THz frequency region. The metal plates have periodic rough structures on the surface, which allow slow transverse magnetic wave propagation and fast transverse electric wave propagation. A numerical simulation showed that the height of the periodic roughness is important for optimizing the birefringence. We fabricated stacked metal plates containing two types of structures by chemical etching. An array of small pillars on the metal plates allows higher frequency optimization. We experimentally demonstrated an achromatic quarter-wave plate in the frequency region from 2.0 to 3.1 THz.

Coherent transitions between the shallow acceptor levels in germanium using intense THz pulses

Nonlinear transmission spectroscopy was performed on a doped Ge:Ga semiconductor using intense THz pulses with different cycle numbers. When single-cycle pulses were used, non-perturbative phenomena, such as the ionization of shallow impurities, competed with the conventional coherent transition, whereas the coherent transition was dominant when multi-cycle pulses were used.

White-light lasing in ZnO microspheres fabricated by laser ablation

We succeeded in fabricating ZnO microspheres with high sphericity by laser ablation in superfluid helium. Such microspheres enable efficient lasing in the whole visible region due to defects with a CW laser at room temperature. The lasing threshold is found to be around 100 W/cm2. This value is much smaller than those of the recent reports on the lasing in ZnO microwire. Cathodoluminescence of single ZnO microspheres was also measured.

Soft X-ray Angle Resolved Photoemission with Micro Positioning Techniques for Metallic V2O3

Soft X-ray micro-ARPES measurements have successfully been applied to band mapping of the strongly correlated metallic V2O3.

In situ tuning of whispering gallery modes of levitated silica microspheres

We demonstrated the tuning of whispering gallery modes (WGMs) of a silica microsphere during optical levitation through the annealing process. We determined the annealing temperature from the power balance between the CO2 laser light heating and several cooling processes. Cooling caused by heat conduction through the surrounding air molecules is the dominant process. We achieved a blueshift of the WGMs as large as 7 nm at 660 nm covering 40% of the free spectral range, which was observed in the white-light scattering spectrum from the levitated microsphere.

Carrier envelope phase control of monocycle THz pulses using an artificial dispersive medium

To control the carrier envelope phase (CEP) is very important when we employ a few cycle electro-magnetic pulses for nonlinear phenomena such as higher-order harmonic generation [1], strong-field ionization and dissociation[2], and population transfer between two bound states [3]. Hence it should also be essential for nonlinear spectroscopy in the terahertz (THz) frequency region. Recently, the generation of extremely intense monocycle THz pulses has been established with the nonresonant optical rectification process and various THz nonlinear spectroscopies have been demonstrated [4]. The CEP of such THz pulses is originally locked in principle. However, to change the CEP arbitrarily has been impossible so far. To overcome the task, we focused our attention on the novel property of a series of parallel plate waveguides. For the TE mode in the waveguide [5], the phase velocity vp is larger than the light velocity c in the vacuum while the group velocity vg is smaller. If the chirp caused by the group velocity dispersion is negligible, the CEP of the THz pulse is changed during its propagation in the dispersive medium.The schematic of the dispersive medium, which we used for the CEP control is shown in Fig. 1 (a). It consists of tens of 50×10×0.1mm3 stainless plates aligned with an equal spacing of 3, 2, and 1mm. The cut-off frequencies of the media are c/2g =0.05, 0.07, and 0.15 THz, respectively. Figure 1(b) shows the temporal electric-field profile of the transmitted THz pulse with the polarization parallel (TE mode; bold) and perpendicular (TEM mode; thin) to the steel plates. In the case of g=1mm, the profile for the TE mode pulse is chirped due to the group velocity dispersion. The chirp is negligible for g≥2mm and we can clearly see the carrier phase of the pulse slightly shifts towards earlier time keeping the envelope phase stable. This means that the CEP of the THz pulse is obviously modulated; the shifted value is ~π/2 CEP for g=2mm. Figure 1 (c) shows the complex transmission coefficient of these medium, and the transmissivity of this optics is above 50%. Such arbitrary-CEP-controlled THz pulses will give us a new field of phase-sensitive THz nonlinear spectroscopy.

Fabrication of cadmium selenide quantum dots with laser ablation in superfluid helium

We fabricated semiconductor cadmium selenide (CdSe) quantum dots via the pulsed laser ablation in the superfluid helium. The fabricated quantum dots showed blue-shifted fluorescence due to the strong quantum confinement effect. The fluorescence blinking phenomena were also observed indicating the single photon emission process. Our proposed scheme is a simple, robust, and reliable method to fabricate quantum dots and to introduce the highly fluorescence nanoparticles into superfluid helium appropriate for resonant optical manipulation and nano-tracers for liquid helium visualization.

Fabrication of semiconductor microspheres with laser ablation in superfluid helium

We fabricated semiconductor ZnO microspheres via the pulsed laser ablation in the superfluid helium. The scanning electron microscope observation revealed the high sphericity and smooth surface. We also observed whispering gallery mode resonances, the electromagnetic eigenmode resonances within the microspheres, in the cathodoluminescence spectrum, verifying the high symmetry of the fabricated microspheres. Further, we cross-sectioned the microspheres with using focused ion beam. The scanning electron microscope observation of the cross section uncovers the existence of small holes within the microspheres. The inner structure examination helps us to understand the microscopic mechanism of our fabrication method.