Optical Limiting in Protonic Doped Bis-Benzothiazole
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N. TANG,*+ R. W. HELLWARTH,* J. P. PARTANEN,* S. SUN,** AND L. R. DALTON** *Department of Physics and Electrical Engineering, University of Southern California, Los Angeles, CA 90089-0484 "**Departmentof Chemistry, University of Southern California, Los Angeles, CA 90089-0482. +Current address: 3005 P St., Ste. 1, WL/MLPJ, Bldg. 651, Area B, WPAFB, OH 45433.
ABSTRACT A -13% increase in the absorption coefficient is seen by a weak probe beam to up exponentially in -50 ps and decay in -1.4 ns after a -30 ps intense pump pulse mj/cm 2 ) at 532 nm goes through a bis-benzothiazole/H 2 SO4 solution. We propose a level model to explain the effect and obtain the basic parameters by fitting our data model.
build(-220 threeto the
SAMPLE PREPARATION AND EXPERIMENTS Protonic doping is done by adding acid to an originally neutral molecule. Most of the protonic doping work was done in connection with conducting polymers. 1-5 Optical properties in protonic doped species have been less studied. In this paper we present our investigation into this class of materials by studying one protonic doped sample through a pump-probe experiment. The synthesis and basic characterization of the neutral bis-benzothiazole (BBT) can be found elsewhere. 6 The BBT/H 2 SO 4 sample used in this experiment is prepared by dissolving dried BBT powder into 98% pure H2 SO4 . We determine the concentration of doped BBT (DBBT hereafter) in the solution to be 2.9x10- 3 M. The sample is contained in a 1 mm spectral photometer cell during our experiments. UV-visible absorption spectra show the typical red shift of the maximum absorption from 376 nm in the neutral samples to 454 nm in the protonic doped sample. In our experiments, the laser wavelength of 532 nm is at the absorption band edge where the sample has a low intensity transmission of -0.18. DBBT exhibits luminance under illumination of either a UV source or our laser pulse at 532 nm. NMR experiments indicate that the end group R is substituted by hydrogen atom on both sides upon doping by the sulfuric acid. Combining the result of NMR and EPR, we believe that the ground state DBBT is a mixture of polaron and bipolaron species with polaron dominance. The pump-probe experimental layout is depicted in Fig. 1 where a strong pump pulse at 532 nm with pulse duration -30 ps and fluence up to -220 mJ/cm 2 enters the sample from one side. A weak probe beam P, which we estimate to be hundreds of times weaker than the pump, goes in from the other side and forms a small angle of -1.5' with the pump beam F. Three polarizers P1 to P3 control the polarization in the pump, the probe, and the transmitted 225 Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
probe. Part of the probe beam P is directed onto a photodiode PD2 by a microslide BS. The transmitted probe beam is reflected by a mirror M into another photodiode PD1 after going through a polarizer P2 and an aperture AP. A set of neutral density filters ND is used to control the pump fluence.
Pump F P1 ND
M
A--P P
PC: 1
•Sample
BS 33
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