Three - University of Arkansas Physics Department

15 January 1997
OPT lcs
COMMUNICATIONS
ELSMER Optics Communications 134 (1 997) 2 1 1 - 2 17
Phase shift between index and intensity patterns
in photorefractive two-wave mixing experiments in 1nP:Fe
M. Chauvet ", T. Towe ", G.J. Salamo D.F. Bliss b, G. Bryant b, G. Iseler
Uniuersiry of'Arhn~us, Physics Drpartmmr. Fayerreuiiie. AR 72701, USA
Rome Lohororory. USAF, Hanscnm AFB, MA 01731. US4
MITLincoln Lahorarory. Le.ringion. MA 02173. USA
Received 16 July 1996; revised version received 9 August 1996; accepted 12 August 19%
Abstract
The phase shift between the intensity pattern and the index pattem in two-wave mixing experiments has been determined
from measured values of photorefractive two-wave mixing gain and diffraction efficiency in a bulk 1nP:Fe crystal as a
function of the total incident intensity. Remarkably, the phase shift varies from 0 to 180" with a value of 90" at the intensity
for which the two-wave mixing gain peaks. Comparison between theory and experiment yields values for sample ~ e" and
Fe3+ concentrations.
Kry~aord~v: Two-wave mixing; Indium phosphide; Photorefracrive effect; Photorefractive phase shift; Impurity concentration determination
1. Introduction
The photorefractive effect in semiconductors has been the subject of many papers [l-61. Studies of the
photorefractive effect in 1nP:Fe show unusual behavior [1,5,7,8]. The two-wave mixing (TWM) gain coefficient,
in the presence of an applied dc electric field shows a sharp resonance as a function of the total incident
intensity. In addition, the resonance peak is found to shift to higher intensities with increasing crystal
temperature. These observations are unexpected since a characteristic feature of the photorefractive effect in
oxides is that its magnitude is independent of pump intensity for an incident intensity exceeding a critical value
depending on the dark irradiance of the material. This intensity dependent behavior is, however, successfully
explained by an elegant model developed by Picoli et al. [8]. The resonance peak is seen when thermally
generated electrons are matched by photogenerated holes. In addition to the observed intensity dependent TWM
gain, the model also predicts an intensity dependent phase shift between the intensity spatial pattem and the
corresponding index grating. The predicted phase varies from 0 to 180' as the incident intensity is changed. This
prediction is also unexpected since another characteristic feature of the photorefractive effect in oxides is that
the phase is intensity independent, although its value is predicted to depend on the magnitude of the applied
field. This predicted intensity dependent phase behavior in 1nP:Fe has, to our knowledge, never been observed.
In this paper we report the first observation of an intensity dependent phase shift between the photorefractive
~ndex pattem and the intensity pattern for 1nP:Fe crystals. The phase shift varies from near 0' to near 180°,
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