Optical Limiting in Chromophore-Doped Gelatin Thin Films
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Copper Phthalocyanine Tetrasulfonate Optical power limiting was observed in both thin films and gels. The films exhibited a bleaching effect at high fluence levels, while the gels exhibited high damage resistance. MATERIALS AND METHODS Materials Porcine skin type A, 175 bloom gelatin(Sigma) and copper phthalocyanine tetrasulfonic acid(CPTA), sodium salt(Aldrich) were used without further purification. For spin coating experiments, materials were dissolved in deionized water. 99 Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
Thin Film Preparation and Characterization All thin films, gels and solutions were prepared in a certified class-100 clean room. A weighed gelatin/CPTA sample was added to water and dissolved by heating at 50 0 C for 30 minutes. The heated solution was applied to pre-heated microscope slides prior to spin coating(Solitec spin coater). The thin films were dried at room temperature prior to characterization. Film thickness was measured using a Dektak film profileometer. Absorption spectra were obtained using a Perkin-Elmer Lambda 9 spectrophotometer. Optical Limiting Experiments A 532 nm, 7 ns pulsed laser beam was spatially filtered to form an airy disk profile. The sample was placed at the intermediate focal point of a telescope consisting of two f/# 1.8 Nikon camera lenses with a 50 mm focal length spaced in a confocal geometry(Figure 1). The laser beam was focused to an 11 Igm diameter spot centered in the sample. For optical limiting, the incident and transmitted energies were measured for ten laser pulses at 10 Hz, and then repeated for increasing incident laser energies. The transmitted beam leaving the telescope was focused with a 100 mm focal length lens through a 100 gtm pinhole prior to entering the detector. Limiting curves are expressed as average over ten pulses. Wedge
f-150mm
Sample
f=100mm
Energy
532hnimn1 4 O
f• 100rm
f=200mm
LI
L2
100 mm Pinhole
Energy Meter
Figure 1. Diagram of optical limiting apparatus used in this study RESULTS AND DISCUSSION To assist in the preparation of thin films of known thickness, we performed a systematic study of the relation between thickness and concentration, spin speed and time. A steady-state film thickness was achieved for spin times greater than 20 sec. For all further experiments, the spin time was 30 sec. We then made a series of thin films where the gelatin concentration in weight percent units(5%, 10%, 15%, 20%, 25%, 30% and 40%) and spin speed in rpm(25, 50, 100, 150, 250) was varied. The data were fit to a function d=- kcaX
(1)
where d is the thickness in pM, c in the gelatin concentration in weight percent and D is the spin speed in rpm. The exponents were determined to be k = 0.182, (X= 2.96 and P = 1.26, with a multivariable correlation coefficient r = 0.86. A plot of calculated vs. measured film thickness is shown in Fig. 2.
100
S102 "-"
~d
•
=kcO/MP
01-
iz
101
PIW-
10
101
Calculated Film Thickness(pM)
Figure 2. Plot of measured film thickness vs. thickness calcu
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