Polymethine Dyes with Exceptionally Large Third-Order Optical Nonlinearities and Loss Figures of Merit for All-Optical Switching

Figure A: (Left) Photoexcitation in metalated polymers creates electron-hole pairs, a small (~1%) fraction become charge carriers with <100 ps lifetimes, and the bulk become bound excitons. (Right) The frequency-dependent conductivity of metalated polymers is described well by the Drude-Smith model at early delays and shows only excitonic features at long delays.

The Marder, Perry, and Brédas groups have recently published in Science their findings that a carefully designed selenopyrylium-terminated heptamethine dye exhibits an exceptionally large real part of the third-order polarizability of –2.2 x 10^-31 esu and an exceptionally large optical switching figure-of-merit in the telecommunications wavelength band of 1.3 to 1.55 µm. The large nonlinearity and low linear and nonlinear losses would, if successfully translated to the solid state, make the possibility of low-power high-contrast all-optical switching a reality.

Third-order NLO chromophores with the characteristics necessary for low-power all-optical switching (AOS) in the telecommunications bands (1.3–1.55 µm) have been elusive. While molecules with moderately large values of the molecular third-order polarizability, γ , have been obtained, a serious obstacle to the development of AOS is that most molecular materials examined to date exhibit unacceptable optical losses, resulting from linear and/or nonlinear absorption and from scattering. While it is relatively straightforward to avoid linear losses by working at energies well below that of the lowest lying one-photon absorption (1PA) transition, addressing the issue of nonlinear loss is far more challenging as 2PA can be appreciable in the near-IR for materials with large γ. Consequently, a challenge for AOS is to identify materials that possess good optical switching figures-of-merit (FOM), i.e. |Re(γ)/Im(γ)|.

2PA can be minimized in a similar way to 1PA, by working at photon energies (ħ ω) less than half the energy of the lowest lying 2PA transition (see Figure A), but the magnitude of Re(?) at this wavelength cannot reach its full potential as a consequence of a large energy detuning term, D = (E – ħ ω), reduction of which can lead to pre-resonant enhancement of ? by well over an order-of-magnitude. An alternative involves operating at photon energies relatively close to the lowest-lying 1PA transition such that ? can be pre-resonantly enhanced while twice the photon energy falls between the energies that would excite potentially detrimental 2PA bands (Figure 31b). This approach requires careful molecular design to achieve favorably spaced narrow 2PA features. The Marder, Perry, and Brédas groups have recently reported in Science (Hales, J. M.; Matichak, J.; Barlow, S.; Ohira, S.; Yesudas, K.; Brédas, J. L.; Perry, J. W.; Marder, S. R., Science, 2010, 327, 1485, DOI: 10.1126/science.1185117) a series of molecules that simultaneously possess very large non-resonant values of Re(?) and, for one member of the series, very low nonlinear losses, as demonstrated by values of |Re(?)/Im(?)| that exceed 100 throughout the 1.3-1.55 µm range.

Previous work suggested that cyanine dyes should possess the largest magnitude of γ for a discrete polymethine/polyene molecule of a given chain length and that γ for these dyes should exhibit a very steep power-law dependence on conjugation length. However, cyanine-type dyes are subject to a Peierls-type symmetry breaking that adversely affects ? at long chain lengths. Another route for achieving large nonlinearities while avoiding symmetry breaking is to maximize Re(γ) for a fixed conjugation length, by identifying cyanine-type dyes for which absorptions occur at low energy (E), even at moderate chain lengths, and are very strong (large transition dipole, M_ge). An appropriate terminal group should provide good energy matching of terminal group orbitals to those of both the highest occupied and lowest unoccupied p orbitals of the polymethine bridge and orbital overlap sufficient for significant mixing with the bridge p-electron wavefunctions. Thus, the terminal group should (1) participate effectively in extending the overall conjugation length and (2) increase the transition densities towards the periphery, increasing Mge, while maintaining a bridge of modest length that could help prevent the onset of symmetry breaking. Selenopyrylium-terminated polymethines with varying bridge length appear to meet these requirements. The Marder group synthesized that dyes shown in Figure B, using solubilizing 4-n-butylphenyl groups, and tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (BAr'4) counter ions and the Perry group has characterized their third-order NLO properties. The |Re(γ)/Im(γ)| values for one example were reported in last year's report: here we summarize our more recent work on characterizing the nonlinearity of a range of these compounds and understanding how the nonlinear loss is minimized in these systems.

Figure B: Selenopyrylium dyes examined in this work.

Im(γ) and Re(γ) at several near-IR wavelengths were measured using fs Z-scan measurements (data for 1.3 µm in Table 1), while degenerate four-wave mixing was used to determine | γ | and to verify the ultrafast nature of its temporal response. Re(γ) increases by an order of magnitude with the addition of each vinylene group, leading to an exceptionally high value of –2.2 x 10-31 esu for dye Se-7C. These values of Re(γ) compare very favorably to those reported for other organic systems in this spectral region. Se-5C and Se-7C exhibit exceptional values of |Re(γ)/Im(γ)| >> 12 at 1.3 µm, i.e. much larger molecular optical switching FOMs for AOS than other chromophores previously reported. To gain further insight into the low 2PA losses exhibited at telecom wavelengths, non-degenerate 2PA spectra were measured. All spectra exhibit a favorable region over which 2PA is relatively weak; this low-loss window appears to be tunable simply by modifying the position of E₀₁ (or equivalently molecular length). As is typical for cyanines, the 2PA transitions lie at E02 ≈ 1.1 x E₀₁ and E₀₃ ≈ 1.7 x E₀₁, leading to a large energy gap between these resonances. The bands are also reasonably narrow (for example, compared to 2PA spectra for dipolar compounds). The strengths of the 2PA transitions are also modest.

Although challenges, especially the development of high chromophore density materials that are photochemically robust, remain before such chromophores would be ready for use in AOS applications, the molecular design flexibility afforded by cyanine-like chromophores should provide pathways to address these issues. However, the strategy developed by the Marder, Perry, and Brédas groups is a key step towards making low-power high-contrast AOS a reality.

(a) Linear and nonlinear optical properties were extracted from solutions of the chromophores in chloroform. (b) Obtained from integration of the absorption spectra (in Debye) as Mge = 0.9584(∫ ϒdn / n_max)^0.5 where ϒ is in M^-1cm^-1 and n is in cm^-1. (c) Measurements performed using the femtosecond-pulsed Z-scan technique at λ_exc = 1300 nm. Experimental uncertainties were estimated to be ±10%.