Two-photon absorption spectrum of a side-chain polymer


Polymers with attached side-chains groups like DANS (4-dialkylamino-4′-nitrostilbene) are used in electro-optic applications and have potential for efficient second harmonic generation. It has also been shown that they have interesting third order nonlinearities. Therefore it is important to understand the physics of effects such as two-photon absorption which can lead to excessive high power losses, as well as structural changes in the material due to photobleaching. Here we report measurements of the two-photon absorption spectrum in order to identify the spectral positions and strengths of two-photon resonances, as well as to study the origins of two-photon active transitions. Although our previous tunable THG (third harmonic generation) study was quite successful in extracting information on overall resonances, the two-photon resonance was not clearly identified because of the broad linewidths involved and the presence of multiple overlapping resonances. In the current study we used direct TPA (two-photon absorption) measurements to identify the existence of a single two-photon resonance. The region between 780 nm and 970 nm was covered by a picosecond pulse, titanium:sapphire laser. The laser pulses were grated in 1 μs windows with a 10 Hz repetition rate giving typical input energies of 0.13 μJ per window (1.7 nJ/pulse). No flux-dependent effects such as bleaching, damage or excited state absorption were observed with such small input energies. A picosecond pulse Nd:YAG laser operating at 10 Hz was used to access 1.06 μm. The resulting spectrum of two-photon absorption coefficient α2(λ) which is defined as Δα = α2I, shown in Fig. 1, consists of a broad two-photon resonance centered around 920 nm which is slightly red-shifted from twice the wavelength of the linear absorption peak. The presence of such a two-photon allowed band is reasonable since one and two photon states are inherently mixed in molecules without a center of symmetry such as DANS. Calculations based on a simple two-level model are in good agreement with the data and with our previous THG and recent nondegenerate four-wave mixing nonlinear spectra (to be published). In previous experiments we did not see significant values of α2 at 1.06 μm and 1.32 μm, indicating that the peak at 900 nm is the only strong one present. From the peak value of α2 (= 5.5 ± 0.5 cm/GW) and the maximum linear absorption coefficient of 1.1 × 105 cm-1, we can deduce the ratio of the two-photon and one-photon oscillator strengths to be approximately 4, which indicates that the two-photon oscillator dominates in the spectral range with λ > 700 nm. In conclusion we have clearly identified the dominant two-photon state in DANS side-chain polymers with direct TPA experiments. The TPA spectrum shows a maximum α2 of 5.5 ± 0.5 cm/GW at a slightly red-shifted wavelength from twice the wavelength of the one-photon absorption peak at 430 nm.

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Proceedings of the International Quantum Electronics Conference (IQEC'94)

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Article; Proceedings Paper



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0028554221 (Scopus)

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