Takashi Kushida

PHONON-ASSISTED ENERGY TRANSFER BETWEEN TRIVALENT RARE EARTH IONS.

Nonresonant phonon-assisted energy transfer between trivalent rare earth ions is studied using Y 2 O 3 as the host crystal. The transfer probabilities are obtained from the analysis of the decay rates of donor luminescence for various combinations of donor and acceptor ions in which energy gap, i.e. mismatch of energy, is varied in a wide range up to 4000 cm -1 . The probabilities are decreased exponentially with the increase of energy gap in agreement with the theory of Miyakawa and Dexter. The observed value of the parameter expressing the exponential dependence agrees with the theoretically estimated value. The temperature dependence of the probabilities presents the direct evidence that the mismatch of energy is compensated by the emission of phonons. It is revealed that the phonon of about 400 cm -1 which produces the highest intensity in vibronic sidebands contributes dominantly to the phonon-assisted process.

Mechanism and Controlling Factors of Infrared-to-Visible Conversion Process in Er3+ and Yb3+-Doped Phosphors

Luminescence and related optical properties of Er 3+ and Yb 3+ -doped YF 3 , Y 3 OC 7 and YOCl phosphors are precisely and extensively measured in the range from 4.2 K to room temperature, in order to investigate the mechanism and controlling factors of infrared-to-visible conversion. It is concluded that the red luminescence emitted from Er: 4 F 9/2 is produced by two energy transfers from Yb 3+ to Er 3+ ions, and that 4 F 9/2 is directly populated from Er: 4 I 13/2 by the second energy transfer. The cutoff phonon energy and electron-phonon coupling constant of the hosts, which dominate the probabilities of internal relaxation and phonon-assisted energy transfer, are indicated to be important controlling factors for the conversion process, in addition to the relative energy positions of Yb: 2 F 5/2 and Er: 4 I 11/2 . The hosts having larger values of these factors, such as Y 3 OCl 7 , are advantageous to the red luminescence, while those with the opposite tendency, such as YF 3 , are favorable to the gre...

Excited-state absorption spectra of ZnS : Mn

Abstract Absorption bands have been found at 655 and 830 nm in a Mn2+-doped ZnS crystal under intense optical excitation. From the close correlation to the orange fluorescence, these bands are ascribed to the transitions from the emitting state 4 T 1 ( 4 G) of Mn2+. This observation of the excited-state absorption makes it possible for the first time to locate higher excited levels of impurity ions hidden behind the strong fundamental absorption of the host material by a direct optical absorption method. Assignments for the terminal states of the observed absorption transitions are discussed.

Broadly tunable, repetitive, picosecond parametric oscillator

Abstract A LiNbO3 parametric oscillator system based on a repetitively mode-locked YAG:Nd laser has been developed. Fairly stable, highly directional, intense, monochromatic, ultrashort pulses have been obtained, whose wavelengths are tunable between 630 nm and 3.6 μm. Second harmonic generation of the output of the parametric oscillator and sum and difference frequency mixing with the fundamental laser beam have been achieved to extend the tunable range. It is shown that the whole spectral region from 240 nm to 3.6 μm can be covered continuously by this system using such frequency conversion techniques.

Stimulated emission effects in excitonic molecule luminescence of CuCl under resonant two-photon excitation

Abstract Luminescence spectra of excitonic molecules in CuCl resonantly created by nano-second dye laser excitation are measured at 4.2 K changing the lenghts of the excited region. The spectra are dominated by stimulated emission. The gain coefficient for the MT line is saturated more easily than that for ML, though the former is larger under unsaturated conditions. These gain characteristics explain well various stimulated emission effects observed.

Luminescence due to metallic electron-hole liquid in GaAs

Abstract Emission spectra of high-purity GaAs have been studied at 4.2K under very intense optical excitation. The results give the first experimental evidence for an important contribution of the electron-hole liquid phase in the luminescence of a direct-allowed semiconductor. Theoretical fit of the luminescence band-shape is satisfactory. The effects of applied electric field on the emission spectra are explained well using a concept of the metallic electron—hole plasma state.

Memories of photon energy, polarization and phase in luminescence of rare earth ions under resonant light excitation

Abstract Luminescence is studied in Ca(PO3)2 glass doped with Eu3+ under resonant cw dye-laser excitation. From the breadth of the resonance fluorescence line, the homogeneous spectral width of the inhomogeneously broadened 7F0−5D0 transition is determined directly. Further, this experiment gives the first example of the identification of the transition character by the polarization memory in an amorphous material. Relation between luminescence and scattering under resonant light excitation is also discussed.

Luminescence due to exciton-exciton collision in GaAs

Abstract Emission spectra of high-purity GaAs have been studied at 4.2 K under N2 laser excitation. The slope of the low-energy tail of the main band has been found to fit well with the theoretical prediction for radiative Auger recombination of free excitons. Measurements under electric field support the dominant contribution of this process in the luminescence of highly excited GaAs.

Pico-second time-resolved luminescence spectra of excitonic molecules in CuCl

Abstract Transient luminescence behavior of excitonic molecules in CuCl is studied at 4.2 K with a time resolution of ∼ 10 ps. The lifetime of the excitonic molecule state is determined to be several hundred pico-seconds. The spectral shape and the time characteristics of the luminescence intensity depend on excitation wavelength and also on the direction of observation. This is explained by the effect of the stimulated emission process.

Pico-second transient luminescence of resonantly excited excitonic molecules in CuCl

Abstract Time-resolved luminescence spectra of excitonic molecules in CuCl created by the two-photon resonance excitation are studied at 4.2 K with a time resolution of ∼ 60 psec. Two sharp emission lines observed are ascribed to the radiative decay of “cold” excitonic molecules with k ∼ 2K 0 . Stimulated emission process gives a considerable influence on luminescence properties.