Three - University of Arkansas Physics Department

January 15,1999 1 Vol. 24, No. 2 / OPTICS LETTERS 77
Fixing the photorefractive soliton
Matt Klotz, Hongxing Meng, and Gregory J. Salamo
Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701
Mordechai Segev
Department of Electrical Engineering, Princeton University, Princeton, New jersey 08544
Steven R. Montgomery
Department of Physics, U.S. Naval Academy, Annapolis, Maryland 21402
Received June 29, 1998
We report the formation of permanent two-dimensional 12-pm waveguides in a bulk strontium barium niobate
crystal. The waveguides are made by formation of a photorefractive spatial soliton in which the space-charge
field induces ferroelectric domains that are permanently polarized opposite to the crystal c axis. The fixed
waveguide propagates light with 80% efficiency. These results make possible the permanent recording of
intricate optical circuitry in the volume of a bulk crystal. O 1999 Optical Society of America
OCIS codes: 190.0190.
Photorefractive soliton^'^^ have become a convenient
playground in which to study soliton phenomena, since
they are observed at low light powers (microwatts) and
exhibit robust trapping in both transverse dimensions.
Screening soliton^^-^ form when an electric field applied
to a photorefractive crystal is partially screened
within the incident light beam owing to transport
of photoexcited charge^.^ An internal field develops
within and around the beam and modifies the refractive
index through the Pockels effect. This index distribution
is in the form of a graded-index waveguide
that guides the beam that induced it. Such solitoninduced
waveguides are highly controllable: One can
control their numerical aperture, number of modes, etc.
by adjusting the parameters of the soliton existence
curve.6 Owing to a low dark current, these solitoninduced
waveguides survive in the dark but can be
erased by uniform illumination that restores a uniform
charge distribution and electric field. In fact, these
waveguides disappear if the applied field is turned off
while the crystal is illuminated, because the trapped
electrons are re-excited and undergo transport, giving
rise to a charge distribution that cannot support
solitons. Although the self-induced and easily erased
nature of photorefractive soliton-induced waveguides
is attractive for dynamic applications, e.g., reconfigurable
interconnects, for many applications it is advantageous
to permanently impress waveguides into the
crystalline structure, i.e., to have the induced waveguide
last indefinitely without an applied field, even
under intense illumination.
Two main methods can transform a photorefractive
electronic hologram into an ionic deformation: ion
drift and ferroelectric space-modulated poling.7 The
first method applies primarily to photovoltaic LiNbOa,
in which the optical beam can introduce long-lasting
optical damage. In recent experiments researchers
have reported fixing of waveguides by use of ion drift
either individually by scanning of a tightly focused
beam or as a two-dimensional (2D) array of waveguides
by imaging a nondiffracting pattern.' Using
the scanning method, one can fabricate single-
mode waveguides, but this requires subwavelength
movement control. A nondiffracting pattern, on
the other hand, employs linear optics to generate
a beam with an intensity peak that broadens
very little on a short enough length scale.g For a
single nondiffracting beam, the narrower the intensity
peak, the faster it diffracts, so a narrow waveguide
induced by such a beam broadens throughout propagation.
Nondiffracting arrays stay narrow for a
longer distance but, being periodic, do not allow
much flexibility in the optical circuitry. Finally, in
recent experiments waveguides induced by domain
reversal were demonstrated.1° However, these waveguides
were induced by diffracting beams and therefore
had to be very broad (highly multimode) or rapidly
varying throughout propagation. With all these new
results on inducing permanent waveguides in photorefractive
crystals, one thing is obvious: It is highly
desirable to induce waveguides with individual beams
that truly do not diffract by fixing the photorefractive
soliton.
~ - - --
Here we report on experiments in which ferroelectric
domain reversal was successfully employed to permanently
fix a waveguide induced by a photorefractive
soliton in a crystalline matrix. This waveguide survived
at room temperature when the applied field was
removed, even under intense illumination, but could
be easily erased by application of electric fields that
were larger than the coercive field of the crystal. This
method permits permanent recording of intricate optical
circuitry in the volume of a bulk crystal so that
integrated optics need no longer be limited to planar
geometry.
We used the standard setup for forming screening
soliton^^^^: a beam splitter divided by the output
of a 514.5-nm argon-ion laser into a soliton
and a background beam. The crystal was a 1-cm cube
of SBN:75 doped with 0.02% cerium by weight. The
soliton beam had a 12-pm FWHM input diameter
[Fig. l(a)], and the 35-mW background beam was
expanded to fill the entire crystal. Normally the
incident beam, which propagated along an a axis,
0146-95921991020077-03$15,0010 @ 1999 Optical Society of America