Three - University of Arkansas Physics Department

September 1,1996 I Vol. 21, No. 17 I OPTICS LETTERS 1333
Self-trapping of planar optical beams by use
of the photorefractive effect in 1nP:Fe
M. Chauvet, S. A. Hawkins, and G. J. Salamo
Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701
M. Segev
Department of Electrical Engineering and Advanced Center for Photonics and Optoelectronic Materials,
Princeton University, Princeton, New Iersey 08544
D. F. Bliss and G. Bryant
U.S. Air Force, Rome Laboratory, Hanscom Air Force Rase, Massachusetts 01 731
Received January 24, 1996
We demonstrate what we believe to be the first experimental observation of self-trapping and self-deflection of
a planar optical beam by the photorefractive effect in a semiconductor. The semiconductor material is indium
phosphide doped with iron. We show that the observed focusing and defocusing effects follow the component
of the two-wave-mixing space charge field that is in phase with the intensity pattern, whereas the spatial beam
deflection effects follow the 90"-shifted component. O 1996 Optical Society of America
Spatial solitons in photorefractive materials have
been the subject of recent intere~tl-~ Compared
with Kerr spatial198 solitons, the most distinctive
features of spatial solitons are that they are observed
at low light intensitie~~.~~%nd that trapping
occurs in both transverse dimension^.^,^ Until now,
photorefractive spatial solitons have been observed
in the tungsten bronze ferroelectric o~ides~'~ and
in the nonferroelectric sillenite oxide^.^ Several
reasons have led us to carry out similar experiments
in the photorefractive semiconductor crystal
1nP:Fe. First, this material is sensitive in the range
of the near-infrared wavelengths used in optical
telecommunications. Second, the photorefractive
effect in semiconductors has the advantage of a
faster response time than that observed in either the
tungsten bronze or the sillenite crystal materials.
Third, the possibility of monolithic integration with
other optoelectronic components (lasers, detectors) is
also attractive.
We experimentally demonstrate in this Letter what
we believe to be the first self-trapping of optical beams
in a semiconductor, using the photorefractive effect.
The self-trapping is observed at steady state with a low
light intensity (50 mW/cm2) and with a moderate d.c.
applied field (9 kV/cm) in bulk 1nP:Fe. The output
diameter is observed to be trapped at the input value
after propagation of both 5 and 10 mm.
To observe the self-trapping effects, we focus a
continuous beam at 1.04 or at 1.3 pm with a 70-mm
focal-length cylindrical lens onto the entrance face of a
1nP:Fe crystal. A beam profile system and an imaging
lens are used to analyze the beam shape. The light
beam is linearly polarized along the (110) direction and
propagates along the (110) direction. Absorption is
-1.5 cm-' at the 1.04-pm wavelength and0.15 cm-' at
1.3 pm. A d.c. field (Eo) is applied between the (001)
faces. The temperature of the crystal is stabilized
at 297 K.
The resonance behavior of the two-wave-mixing
(TWM) gain in 1nP:Fe has been characterized both
experimentally and theoreti~ally.~*l~ In this model
electrons are dominantly thermally excited from Fe2+
to the conduction band, whereas holes are dominantly
optically excited from Fe3+ to the valence band. The
resonance in the gain as a function of intensity occurs
at an intensity I,,, for which the optical excitation of
holes is equal to the thermal excitation of electrons.
In addition, the relative phase between the intensity
spatial pattern and the space-charge-field pattern is
also intensity and temperature dependent. The reso-
\. , - I
0 40 80 120 160 200
Intensity (mWlcm2)
Fig. 1. Real and imaginary parts of the normalized TWM
space charge field in 1nP:Fe at 1.06 pm calculated with
formula 18 from Ref. 10. m is the modulation rate of
the intensity grating. Eo = 10 kV/cm; T = 297 K;
grating spacing = 10 pm; [Fez+] nTo = 0.5 x 1016 ~rn-~;
[Fe3+] p~o = 6 X 1016 ~rn-~; and hole and electron
photoionization cross sections an0 = 6 X 10-l8 cm2,
upo = 1 X lo-'" cm2. I,,, = 38 mW/cm2.
O 1996 Optical Society of America