We have fabricated the first optical quality polymer films of aromatic ladder polymers having an unbroken conjugated backbone, and find third-order nonlinear susceptibilities two orders of magnitude larger than CS2. The nonlinearities respond in less than 6 ps. Although we find their space symmetry to agree with that expected for a nonresonant electronic nonlinearity, the mechanism may well depend on lattice motion. The polymers are stable and rugged at room temperature. We also develop and analyze a simple form of degenerate four-wave mixing for the measurements, finding expressions for the effects of uncertainties in the spatial and temporal pulse profiles.

Subpicosecond transient absorption ͑TA͒ spectroscopy was used to identify excited states, and to measure their relaxation kinetics as a function of excitation wavelength in the halogen-bridged transition-metal (M X) compound PtCl. For pump photon energies E pump between 1.3 and 1.8 eV, below the threshold for exciton formation, intrinsic polarons are photoexcited and decay within 1-3 ps. Excitation into the exciton absorption tail ͑2.0-2.3 eV͒ yields singlet excitons that rapidly decay into an even-parity state, which has a lifetime ͑ϳ100 ps͒ varying with the intial photon energy. Excitation ͑3.1 eV͒ well above the exciton threshold appears to generate more slowly decaying excitons, as well as polarons that are formed upon exciton dissociation. The TA spectra also indicate that relaxation of excitons for E pump Ͼ2 eV is mediated by large-amplitude vibrations perpendicular to the chains. Using third-harmonic generation spectroscopy in addition to TA, we develop an energy-level diagram for the excited states, including polaron levels, and the lowest odd-and even-parity excitons.

We have measured the third harmonic generation spectra of epitaxial chloro-indium phthalocyanine (ClInPc) thin films. The χ(3) spectrum in the wavelength range of 970–1650 nm reveals resonance features of ClInPc. Theoretical analysis of the third-order nonlinear optical channels in phthalocyanine allows the assignment of the observed resonances to three- and two-photon resonances of χ(3).