Optical Bandwidth Determinants for Excited State Absorptive Limiters
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PERRY A. MILES Logicon RDA, P.O. Box 92500, Los Angeles, CA 90009, USA
ABSTRACT The performance of optical limiters depends on the ratio of the absorptive cross section for the transition from an excited state, ae , to that for transition from the ground state, cTg . Both these factors depend on wavelength. It is appropriate to define two bandwidths, one for high energy pulse suppression, depending on ae , and the other for linear transmission, depending on Gg . These bandwidths are affected differently by the use of molecular mixtures.
ANALYSIS We have recently" 2 put forward detailed designs to achieve optimal performance of optical pulse limiters whose action depends on the near complete population of highly absorptive excited singlet and triplet states of organic dye molecules (Fig. 1). As the ground state population is depleted, the effective molecular absorption cross section increases from its original ground- state value, cyg , towards the excited state limit Ge . Typically, the ratio ae /cyg at visible wavelengths ranges up to 30. For such a device of length, L , with a (nonuniform) concentration of dye molecules, N(z), along the z axis, the original transmittance, T, is given by 1L (1) T= exp [-AgNL], N = • LN(z) dz. For a high transmittance device, T II - ygNL. In the fully excited state, the corresponding transmittance, Tx, is given by Tx =exp[-aeRL]
or -In Tx =InS=aeNL.
(2)
We have proposed 3 that the ratio e, / ag = In S /- In T, be considered a figure-of-merit for the active molecules. Typically, the design goal would be to provide a high level signal suppression, S, in the range 103 _ 105 , together with a low level signal transmittance, T, of over 80 percent. For example, a device with S=10 3 , T > 80% calls for Ge / cg9 _ 31.
51 Mat. Res. Soc. Symp. Proc. Vol. 374 ©1995 Materials Research Society
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