Three - University of Arkansas Physics Department

Optically induced birefringence in bacteriorhodopsin as an optical limiter
George E. Dovgalenko, Matthew Klotz, and Gregory J. Salamo
Physics Department, Universitj ofdrkansas, Fayetfeville, Arkansas 72701
Gary L. Wood
Army Research Laborafoq For Belvoi~ Virginia 22060
(Received 18 September 1995; accepted for publication 1 November 1995)
Experimental data are presented, which demonstrates an optical limiter based on a large
birefringence which is optically induced in bacteriorhodopsin. The induced birefringence is
observed to be a function of incident intensity, but saturates at a value of about 0.454 ~ lcm A
measured value of An of 6.6X at a wavelength of 514 nm is reported. The observed
birefringence is found to be in good agreement with a proposed model. O 1996 American Institute
of Physics. [SOOO3-6951(96)00303-61
~acteriohodo~sin' (BR) has received much attention recently
due to its potential for real-time holographic recording,
optical pattern recognition, and nonlinear optical
effect^.^-^ BR is the light harvesting protein in the purple
membrane of the micro-organism called halobacterium holbium,
produced in salt marshes, and is closely related to the
visual pigment rhodopsin. Tt displays a characteristic broadband
absorption profile in the visible spectral region, and
when the molecule absorbs light it undergoes several structural
transformations in a well-defined photocycle. One of its
long list of interesting attributes and clever applications is its
inherent photoinducible optical anisotropy.I0 It is this anisotropy
which is explored in this letter.
The apparatus for our investigation is shown in Fig. 1.
The output beam from a linearly polarized argon-ion laser at
514.5 nm is split into two beams. One beam is polarized as
P, and directed through the BR sample. The second beam is
polarized using polarizer P2 at 45" relative to the first beam
and is directed through the BR sample at a small crossing
angle (-2") relative to the first beam. The first beam then
passes through a second polarizer P3, which is parallel to the
first. When the second (pump) beam is blocked, the first
(probe) beam passes through the BR without polarization
rotation, hence with minimal loss through the second polarlzer
(P2). When the pump beam is unblocked the probe
beam experiences a change in polarization and is partially
blocked by the second polarizer. As the intensity of the pump
beam is increased, the polarization change of the probe beam
is observed to increase and then saturate. For a particular Polarlzer Rolalor
pump intensity and sample thickness, the polarization change
can be made to be a 90" rotation. Since the saturating effect
is observed to occur at low light intensities, a 90' rotation is
in practice quickly independent of the incident intensity of
either beam and the degree of rotation is "fixed." Figure 2
shows a plot of the observed polarization versus the incident
pump intensity. For a thickness of 50 pm we observe a minimum
transmission at an angle of 18". The thickness of the
BR sample can be easily selected to produce a 90" rotation
for a large range of incident intensities. For a rotation of 9O0,
for example, a thickness of about 0.2 mm is required for the
samples we have examined. For an effective 0.2 mm sample,
used in the apparatus shown in Fig. 1, a 90" polarization
rotation limits the intensity of the probe beam to at least
of the incident intensity depending on the quality of the
polarizers, P, and P,. Although our sample was only 50 pm
thick, we used a multiple pass scheme to produce a 90" rotation.
To understand the cause of the observed polarization
change, we must consider the effect of absorption of a photon
by BR. A cycle of conformational changes is initiated by
the absorption of a photon by bacteriorh~do~sin.~' The
BR(570) form, absorbs at 570 nm, giving the molecule its
purple color. The absorbed photon isomerizes the all-trans
retinal group of BR(570) to the 13-cis form of K(610) in
picosecond times. The transient red-absorbing intermediate
then undergoes a series of conformational transitions
[L(550), M(412), N(520), and 0(640)] in microsecond to
millisecond times, and then a conformational transition back
to the BR(510) form in millisecond times. The cycle of conformational
changes is depicted in the inset of Fig. 2. As a
result of the conformational changes, we can use a simple
three molecular form picture to model the induced birefringence
in bacteriorhodopsin. The incident light excites the
trans-molecular form, It), to molecular form 11) which immediately
decays nonradiatively to the cis-molecular form (c).
The long-lived cis form then rapidly accumulates a popula-
GIE~-T~O~PSO~ Polarization
FIG. I. Apparatus used to measure the induced birefringence and limiting
Appl. Phys. Lett. 68 (3). 15 January 1996 0003-6951196168(3)128713/$6.00 O 1996 American Instibte of Physics 287