A study on the effect of filling factor for laser action in dielectric random media

In a random system composed of dielectric materials, laser action is expected to occur from the light localization caused by multiple scattering and interference effect. The wavelengths at which the laser oscillation occurs in random media depend on the mean free path of continuous scattering. Since the mean free path of continuous scattering is dependent on the filling factor, it is important to investigate systematically the effect of the filling factor for the laser action. In the present study, we calculate the Poynting vectors of the light emitted from two-dimensional random media consisting of dielectric rods with population inversion modeled by the negative imaginary part of relative permittivity. The spectra of the radiated Poynting vectors from the two-dimensional random media are shown for several different filling factors and for various values of population inversion. We try to find laser generation which occurs at low population inversion and discuss about the threshold of laser action occurring in dielectric structures with different filling factors. We also study the spacial distribution of the electric field amplitude in several frequency regions where light strongly amplifies.     

Investigation of the center intensity of first- and second-order Laguerre-Gaussian beams with linear and circular polarization

The vectorial Debye integral shows that tightly focused Laguerre-Gaussian (LG) beams have a residual intensity at the focal point for linear polarization, for a topological charge of m=1 and 2. We measured the shapes of linearly and circularly polarized LG beams and found that a central intensity appeared at m=1 and 2 for linear and right-handed circular polarization, however, it is completely canceled for left-handed circular polarization. In general, when the orbital angular momentum of the LG beam is parallel to the spin angular momentum of the photons, zero intensity is always achieved at the focus.     

Lidar measurement of constituents of microparticles in air by laser-induced breakdown spectroscopy using femtosecond terawatt laser pulses

We experimentally demonstrated remote sensing of the constituents of microparticles in air by combining laser-induced breakdown spectroscopy (LIBS) and lidar, using femtosecond terawatt laser pulses. Laser pulses of 70 fs duration and 130 mJ energy generated filaments when focused at a focal length of 20 m and the pulses irradiated artificial saltwater aerosols in air at a 10 Hz pulse repetition rate. Na fluorescence was observed remotely at a distance of 16 m using a 318 mm diameter Newtonian telescope, a spectrometer, and an intensified CCD camera. These results show the possibility of remote measurement of the constituents of atmospheric particles, such as aerosols, clouds, and toxic materials, by LIBS-lidar using femtosecond terawatt laser pulses.     

Development of a high-power deep-ultraviolet continuous-wave coherent light source for laser cooling of silicon atoms

We developed a deep-UV single-mode coherent light source through two-stage highly efficient frequency conversions by use of external cavities. In the first stage, second-harmonic power of 500 mW was obtained by frequency doubling of a 746-nm Ti:sapphire laser with a conversion efficiency of 40%. In the second stage, 50-mW power at ~252nm was obtained by doubly resonant sum-frequency mixing of 373-nm light from the first-stage conversion and 780-nm light from a diode laser. The output performance of this deep-UV light source is sufficient for laser cooling of neutral silicon atoms.     

Study of γ d → π + π − d Reaction in an Energy Region of 0.67 ≤ E γ ≤ 1.08 GeV

Double charged pion photoproductions from the deuteron have been studied at Research Center for Electron Photon Science (ELPH), Tohoku University. Tagged photon beams in an energy range of 0.67 ≤ E _γ ≤ 1.08 GeV were impinged on the liquid deuteron target. Produced charged particles were observed with the Neutral Kaon Spectrometer2 (NKS2). The γ d → π ⁺ π ^? d process was clearly separated among the three charged tracks. We obtained the photon energy dependence of the normalized yields of this process.     

Hyperfine structure of the electron spin resonance of phosphorus-doped Si nanocrystals

Electronic states of P donors in Si nanocrystals (nc-Si) embedded in insulating glass matrices have been studied by electron spin resonance. Doping of P donors into nc-Si was demonstrated by the observation of optical absorption in the infrared region due to intraconduction band transitions. P hyperfine structure (hfs) was successfully observed at low temperatures. The observed splitting of the hfs was found to be much larger than that of the bulk Si:P and depended strongly on the size of nc-Si. The observed strong size dependence indicates that the enhancement of the hyperfine splitting is caused by the quantum confinement of P donors in nc-Si.