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Physical Review Letters

Measurement of Exciton Diffusion Lengths in NdxLa1−xP5O14 by Four-Wave Mixing Techniques

Christopher M. Lawson, Richard C. Powell, Walter K. Zwicker

Published Apr 13, 1981• 32 citations
Publisher

Abstract

Transient four-wave mixing techniques have been used to measure the decay rate of the excited-state population grating in ${\mathrm{Nd}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{P}}_{5}{\mathrm{O}}_{14}$ samples as a function of grating spacing. The results show that at room temperature there is diffusive energy migration over distances of the order of 0.36 \ensuremath{\mu}m in the highly concentrated samples.

DOI
10.1103/physrevlett.46.1020

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Investigation of four-wave mixing in NdxLa1−xP5O14

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Four-wave mixing has been investigated in ${\mathrm{Nd}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{P}}_{5}{\mathrm{O}}_{14}$ crystals as a function of temperature, laser excitation wavelength, and power. The results are interpreted in terms of scattering from a population grating, and the relative intensity of the scattered signal is investigated as well as the grating decay time. The power dependence of the scattered intensity is explained by a model involving interaction with a three-level system, and the properties of the grating decay are associated with the exciton dynamics of the ions in the excited state. Decay patterns characteristic of both incoherent and partially coherent exciton migration can be observed under different experimental conditions. Enhanced migration due to vibronic generation of phonons resonant with the metastable state splitting is also observed. The temperature dependence of the results shows a weak coupling between the excitions and the acoustic phonons of the host. Local heating effects are found to affect the ion-ion and ion-phonon interaction parameters and produce a thermal grating under certain experimental conditions.

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Journal of Modern Optics•Mar 1, 1988•14 citations
Transient four-wave mixing techniques have been used to measure the decay rate of the excited-state population grating in NdxLa1\ensuremath−xP5O14{\mathrm{Nd}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{P}}_{5}{\mathrm{O}}_{14}Ndx​La1\ensuremath−x​P5​O14​ samples as a function of grating spacing. The results show that at room temperature there is diffusive energy migration over distances of the order of 0.36 \ensuremath{\mu}m in the highly concentrated samples.
Four-wave mixing has been investigated in ${\mathrm{Nd}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{P}}_{5}{\mathrm{O}}_{14}$ crystals as a function of temperature, laser excitation wavelength, and power. The results are interpreted in terms of scattering from a population grating, and the relative intensity of the scattered signal is investigated as well as the grating decay time. The power dependence of the scattered intensity is explained by a model involving interaction with a three-level system, and the properties of the grating decay are associated with the exciton dynamics of the ions in the excited state. Decay patterns characteristic of both incoherent and partially coherent exciton migration can be observed under different experimental conditions. Enhanced migration due to vibronic generation of phonons resonant with the metastable state splitting is also observed. The temperature dependence of the results shows a weak coupling between the excitions and the acoustic phonons of the host. Local heating effects are found to affect the ion-ion and ion-phonon interaction parameters and produce a thermal grating under certain experimental conditions.
Four-wave mixing signals from transient population gratings established in NdP5O14crystals were analyzed to obtain information on the pumping dynamics and exciton migration characteristics in this material. As the laser pump wavelength is scanned through an Nd3+absorption band, the exciton diffusion coefficient is found to vary in a manner consistent with site selection within the inhomogeneously broadened profile. Deviations from simple signal decay patterns are observed whenever the laser is tuned to excited state resonances. In addition, recent theoretical developments are applied to the grating decay data to determine the degree of coherence in the exciton motion.
We report what is to our knowledge the first direct measurement of spatial energy migration among the Tm(3+) ions in a Tm,Ho:YAG crystal at room temperature. Four-wave mixing techniques were used to establish and probe transient laser-induced population gratings in the Tm(3+) ions. The grating decay rate was measured as a function of the grating spacing. The results show the presence of spatial energy migration among the Tm(3+) ions, and the diffusion coefficient of this process was measured to be 4 x 10(-7) cm(2) s(-1) for a sample containing 8.3 x 10(20) cm(-3) Tm(3+) ions.
The spatial confinement of carriers in semiconductor quantum well structures leads to resolvable excitonic features at room temperature. Saturation of these due to phase space filling gives rise to a very large non-linear refractive index [1,2]. We have used this non-linearity in GaAs/AlGaAs multiple quantum wells in two separate degenerate four wave mixing configurations. The two arrangements, forward travelling and counter-propagating (see fig.1), produce transient phase gratings across and parallel to the plane of the wells respectively. Time resolved measurements of the decay of these gratings allow comparison of intra-well and cross-well contributions to carrier diffusion, which is highly anisotropic, and reveal decay rates dependent upon the angle of the sample to the normal. In addition such experiments give carrier relaxation times and the intensity at which the excitonic non-linear refraction starts to saturate.
Donor-donor and donor-acceptor dynamics in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{P}}_{5}{\mathrm{O}}_{14}:{{\mathrm{Nd}}_{x}}^{3+}$ are measured directly with a high-resolution, near-ir laser system. Time-resolved fluorescence line-narrowing measurements show an absence of rapid donor-donor transfer below 20 K and an onset of spectral diffusion at \ensuremath{\approx} 20 K for an x=0.75x=0.75x=0.75 sample. The linear concentration quenching and exponential decay in the absence of fast diffusion are explained with existing energy-transfer theories and are consistent with room-temperature studies.
We report degenerate four-wave mixing experiments at room temperature, using a three-beam geometry and sub-picosecond laser to study GaAs/GaAlAs multiple quantum well material. Diffraction efficiencies and grating decay times for two samples of different carrier lifetimes have been measured in the spectral region corresponding to excitonic resonances. The measurements provide values of the nonlinear refractive index per carrier pair (n eh) deriving from the excitonic saturation, and also the recombination times and diffusion coefficients for the samples.