Yoh Mita

Luminescence processes in Tm3+‐ and Er3+‐ion‐activated, Yb3+‐ion‐sensitized infrared upconversion devices

Frequency upconversion characteristics in Er3+‐ or Tm3+‐ion‐activated and Yb3+‐ion‐ sensitized luminescent materials have been investigated. The principal goal was to obtain high‐brightness, blue and green light sources under 980 nm emitting laser diode light excitation. Processes leading to the efficient upconversion have been investigated both by experimental and analytical methods. It has been shown that upconversion efficiencies are principally determined with Yb3+ ion excited state lifetime, which is highly influenced by device optical confinement as well as material characteristics. The effect of the optical confinement has been analyzed quantitatively on the basis of a rate equation model. Criteria for realizing efficient infrared upconversion devices have been presented. A strong tendency toward temperature quenching has been observed especially in blue emitting materials. This temperature dependence has been shown to be principally due to increase of nonradiative decay in the Yb3+ ion excited state.

Solid State Light Source Fabricated with YAG:Ce Single Crystal

A solid state, yellow-white emitting light source has been manufactured employing a YAG:Ce single crystal as a wavelength conversion material to analyze the conversion process quantitatively and also to explore the ultimate limit of the conversion efficiency. Excitation polarized coherent blue light is incident on to the crystal at the Brewster angle and is multiply reflected inside the crystal with internal total reflection. The generated light is shown to be efficiently extracted by providing a light guide with a suitable shape at the front surface of the crystal.

Luminescence Processes in Yb3+‐Sensitized Rare‐Earth Phosphors

Quantitative analyses were carried out on the infrared‐to‐visible conversion processes and related luminescent phenomena in Yb3+‐sensitized phosphors. The analyses were based on a rate equation model and the effect of host lattice was taken into account using the Miyakawa‐Dexter relations. The results were found to be in fair agreement with the experimental results on luminescence characteristics under ultraviolet and infrared excitations. Approximate values of parameters characteristic of the host materials were obtained for YF3 and YOF lattices by comparing the calculated and experimental results.

Energy transfer processes in Er3+‐ and Yb3+‐doped infrared upconversion materials

Luminescence characteristics in Er3+‐ion activated, Yb3+‐ion sensitized infrared upconversion materials have been investigated, especially for their energy back transfer processes from Er3+ to Yb3+ ions. Decay characteristics of Er3+ ion green emission have been examined in various fluoride crystals and fluoride glasses for the purpose of elucidating the energy transfer processes involved in the upconversion phenomena. It has been shown that decay characteristics of the green emission are sensitive to the host materials as well as Yb3+ concentration. The decay characteristics have been analyzed with a model taking into account configurations of Yb3+ ions around an Er3+ ion. It has been shown that the analyses present useful methods for characterizing energy transfer processes in materials containing higher amount of Yb3+ ions.

Energy transfer processes in Yb3+ and Tm3+ ion-doped fluoride crystals

Energy transfer processes from Yb3+ to Tm3+ ions have been investigated in various fluoride microcrystals to examine the application limit of a simple rate equation model and to obtain methods for characterizing the host materials as to energy transfer. It has been shown that the energy transfer coefficients from donor Yb3+ to acceptor Tm3+ ions differ by nearly one order of magnitude even for fluoride lattices that have similar luminescence characteristics. The energy transfer coefficients increase monotonically with Yb3+ concentration at low Yb3+ concentrations, while they take nearly constant values at high Yb3+ concentrations. The present experimental results were compared with computer simulation results as well as related analyses. It is shown that the apparently complicated behaviors are consistently explained by a microscopic model. As an example of the application of the present results, the optimization of the infrared emission of Tm3+ ions is shown.

High brightness blue and green light sources pumped with a 980 nm emitting laser diode

Efficient frequency upconversion from 980 nm infrared light to 480 nm wavelength blue emission has been observed in Tm3+‐activated, Yb3+‐sensitized fluoride glass. Similar upconversion to green emission has been obtained with higher efficiency in Er3+‐containing luminescent materials. The conversion efficiencies have been markedly improved when the luminescent material small in size is contained in a reflective cavity. The improvement effect is pronounced when transparent luminescent material is adopted. Processes leading to the generation of visible emission have been examined and criteria for optimizing the upconversion efficiency are presented.

Detection of 1.5‐μm wavelength laser light emission by infrared‐excitable phosphors

Laser emission from a 1.5‐μm wavelength InGaAsP/InP diode was shown to be efficiently converted to visible or to near‐infrared light by Er‐containing infrared‐excitable phosphors. The visible emission has been markedly enhanced in Yb‐sensitized phosphors under 0.97‐μm wavelength auxiliary light irradiation especially under weak 1.5‐μm wavelength excitation. The enhancement mechanism has proved to be Yb‐sensitized quantum counter action.

Surface ripples in laser‐photochemical wet etching of gallium arsenide

Surface ripples of submicrometer spatial period are observed in laser enhanced etching with liquid etchants. The surface ripple formation in the laser‐photochemical wet etching is not explained by the surface polariton model which needs melting of the semiconductor surface. To explain this ripple formation, we have to consider other mechanisms for ripple formation which does not need melting of the semiconductor surface, such as Raman excitation of surface polariton, polarization charge model, and bulk‐selvedge coupling model. We suggest that this low power and high speed laser‐photochemical etching provides a valuable one‐step process for producing the fine diffraction gratings required by integrated optics.

Extrinsic photoconductive characteristics of semi‐insulating GaAs crystals

Extrinsic photoconductive characteristics of undoped, semi‐insulating GaAs have been investigated, especially the characteristic enhancement effect at low temperatures. It has been shown that two quasistable photoconductive states exist, in which photoconductivity differs by orders of magnitude and that the two states are partly interchangeable by irradiating with an appropriate wavelength light. Spectra and temperature dependencies were investigated to elucidate the enhancement mechanism. Discussions are made on the basis of metastable states in As‐rich defects.

Efficient infrared‐to‐visible conversion in BaY2F8 : Yb,Er crystal by confinement of excitation energy

Efficient up‐conversion of near‐infrared energy was realized by confinement of infrared energy in an optical cavity containing a BaY2F8 : Yb,Er single crystal. The lifetime of the ytterbium excited state showed a marked increase and, in a typical instance, became as long as 3.6 msec. Combined with 8% efficiency GaAs : Si diode, an over‐all conversion efficiency of nearly 0.04% was obtained for a 50‐mA diode‐exciting current.