Nonlinear Absorption in Organic Dyes
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K.R. Welford*, S.N.R. Swatton*, S. Hughes"*, S.J Till*, G. Spruce**, R.C. Hollins*, and B.S. Wherrett** *DRA, St. Andrews Road, Malvern, Worcestershire, WR14 3PS, UK "**Departmentof Physics, Heriot-Watt University, Edinburgh EH14 4AS, UK
ABSTRACT The non-linear optical properties of chloro-aluminium phthalocyanine and 1,3,3,1',3',3'hexamethylindotricarbocyanine have been studied as model systems using z-scan and time resolved experimental techniques. Excited state lifetimes and absorption cross sections have been measured as well as the spectral response of the induced absorption. Excited state lifetimes are critical in determining the performance of a reverse saturable absorber. Even though a first excited state absorption cross section may be large compared with the ground state absorption cross section the lifetimes and transition probabilities of higher lying states can lead to constraints upon how effective organic dyes are as optical limiting materials. Both novel saturation of the induced absorption at high laser fluences and intermolecular quantum beats in 1,3,3, 1',3',3'-hexamethylindotricarbocyanine are demonstrated experimentally and accounted for theoretically. Quantum chemical calculations have been developed to propose a probable inter-molecular mechanism for the observed quantum beats.
INTRODUCTION Non-linear processes have long been known in organic dyes. Of significant interest has been the saturation and reverse saturation of the linear absorption at high laser fluences. Reverse saturable absorption (RSA) has been known to exist since the work of Giuliano and Hess 1 in 1967. Relatively straight forward measurements of the nonlinear transmission can be used to monitor excited state lifetimes and excited state absorption cross-sections 2 ,3,4 . More sophisticated experiments are necessary to examine the vibronic lifetimes 5 and broad band spectral behaviour of the excited species using white light probe beams 6 ,7 . Understanding the excited states of organic dyes, such as phthalocyanines and polymethines, has been important for laser development and for optical limiting 8 . Only recently have measurements been made on novel optical limiting materials such as Buckminsterfullerenes 9 . Of primary interest in this paper will be excited state absorption and the relaxation dynamics of the excited species. Many complex dynamic processes operate simultaneously. Care is needed in interpreting the experimental data. By way of example the novel effect of saturation of the induced absorption seen in a carbocyanine dye 1,3,3,1',3',3'-hexamethylindotricarbocyanine, HITCI, using a single high fluence laser pulse along with the time resolved nonlinear response will be reviewed. Excited state lifetimes, absorption cross-sections, rotational reorientation times, and intermolecular quantum beats will be discussed. The time resolved excited state properties of HITCI contrast with the dynamics seen in chloro-aluminium phthalocyanine, CAP. For both HITCI and CAP time resolved measurements have been used to determine
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