Three - University of Arkansas Physics Department

APPLIED PHYS1C:S LETTERS VOLUME 79. NUMBER 10 3 SEPTEMBi'.R 2001
Fixing solitonic y junctions in photorefractive strontium-barium-niobate
Matthew Klotz, Mike Crosser, Aqiang Guo, Michael Henry,
and Gregory J. salarnoa)
Phy.uics Depurtment, Univel:si/y ofArkun.vu.s, Fuyetfeville, Arkunstr~ 72701
Mordechai Segev
Physics Deportment, Technion-I.~rctel In.stitute of Technology, Huifiz 32000, Israel und Electricul
Engineering Deparrment, Princeron University. Pr'rinceron, New Jersey 08544
Gary L. Wood
U.S. .4rmj, Reseurch Lahorutoi).: AhiSRL-SE-EO. Adelphi, Morylond 20783-1197
(Received 6 Marcli 2001; acccpted for publication 4 June 2001)
Two-dimensional solitonic orbital waveguides as y junctions were formed in a strontium barium
niobate crystal. The waveguides are 10-20 pm in diameter and propagate unpolarized light.
6 2001 American Insfittrte uf Physics. [DOI: 10.1063/1.1389824]
Optical spatial solitons' in photorcfractivc crystals2 havc
shown potential to fonn graded index waveguides which can
guide other A soliton forms when a photoinduced
index change in the material compensates exactly for the
diffraction of the beam; i.e., the beam creates its own waveguide.
In photorefractive materials, a screening soliton is
formed by the screening of an externally applied electric
field through the transport of photoinduced carries.536 However,
these induced waveguides disappear if the applied field
is removed from the material. Our motivation for this letter is
to demonstrate the use of soliton formation to create arrays
of permanent waveguides7-" and y junctions (by selectively
reorienting ferroelectric domains within the propagating light
bearnI4) that can be used to form optical wiring in thc bulk of
a crystal.
For thc experiment, the extraordinary polarized output of
an argon-ion laser is focused to a spot size of 12 pm on the
cal dircction. A voltage was applied and five 13 pm solitons
formed [Fig. I(c)] and were subsequently fixed [Fig. I (d)].
The fixed waveguides guided light independently of one another,
as evidenced by blocking one or more input beams and
observing no changc in the transmitted intensity of the rcmaining
waveguides. The waveguides reached equilibrium in
the same manner as the single fixed waveguide, transmitting
60+2% of the incident power at equilibriuin after approximately
60 min. The waveguides were monitored for an additional
140 n~in and showed no sign of decay.
In addition to fixing single and multiple solitons, a coherent
collision of two solitons was used to fixed a y junction
in the crystal. Typically, two beams were launched in parallel
with angles of less than 0.05" in both the horizontal and
vertical directions. As a result, two collincar beams were
focused to a spot size of 12 pm at the cntrancc face of the
front face of a 1 cm cube of strontium-barium-niobate
(SBN) crystal. When a 3 kV/cm electric field is applied to
the crystal along the dircction of spontaneous polarization,
(a)Entrance Face (b)Di ffracted Beams
the beam self-focuses to its input diameter. The external field
is then removed and a uniform background bean1 that fills
the crystal is switched on. The space charge field due to
photoinduced screening charges is larger than the coercive
field of the fersoelectric domains and causes the domaills in 0
the area of the incident beam to reverse their orientation. At
equilibrium, a new space charge field, due to the bound
charge of the domain boundaries, is locked into place. This
new field increases the index of refraction only in the area of
the original soliton, so that a waveguide is formed. The (c)Trapped Beams (d)Fixed Waveguides
waveguides are observed to have-the same size as the origi- -
nal soliton, exhibit single mode behavior, and last
indefinitely.14
To fuither demonstrate the feasibility of creating optical
circuitry, multiple independent waveguides were formed in a
SBN:75 crystal. A diffractive optic was inserted into the experimeiltal
apparatus behind the focusing lens to form fivc
replicas of incident beam on the entrance face of the crystal
[Figs. l(a) and l(b)]. The spacing between the beams was
230 pm in the horizontal direction and 225 pm in the verti-
"Electronic mail: salamo@comp.uark.edu
Pm
FIG. 1. (a) Input profilc of five beams; (b) profilc at the cxil face; (c) fivc
soliton hcam profilcs at thc exit facc; (d) fivc fixed wavcguidcs guiding light
to the exit face.
1.1'"
0003-695112001179(10)1142313I$18.00 1423 O 2001 American Institute of Physics
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