The current status of organic low-molecular weight and polymeric materials for third-order nonlinear optics is reviewed. The importance of organic materials lies in their promise of large nonlinear optical figure of merit, high optical damage thresholds, ultrafast optical responses, architectural flexibility, and ease of fabrication. Organic materials exhibiting interesting third-order nonlinear optical properties are discussed to illustrate the importance of structure–property correlations. Results on emerging organic materials that include liquids, dyes, fullerenes, charge-transfer complexes, π-conjugated polymers, dye-grafted polymers, organometallic compounds, composites, and liquid crystals are presented. Organic nonlinear optical materials seem promising for a wide range of applications and their potential for integrated optics should be further explored.

Organic Materials for Third‐Order Nonlinear Optics

The relaxation dynamics of photoexcitations in cast films of the polydiacetylenes, poly[4,6-decadiyne-1,10-diol bis([(n-butoxycarbonyl)methyl]urethane)] (PDA-3BCMU) and poly[5,7-dodecadiyne-1,12-diol bis([n-butoxycarbonyl)methyl]urethane)] (PDA-4BCMU), and in electrochemically prepared films of poly(3-methylthiophene) (P3MT) were investigated by femtosecond absorption and picosecond luminescence spectroscopies. 1Bu free excitons, generated by 100-fsec pump pulses, relaxed to the self-trapped (ST) state with time constants of 150 ± 50 fsec in PDA-3BCMU and 70 ± 50 fsec in P3MT. The difference was explained in terms of the relative sizes of the side chains. PDA-3BCMU has bulky side chains that are linked by hydrogen bonds, while P3MT has a repeating unit that contains a ring structure with a small methyl group. The lifetimes of the ST excitons at 10 K (290 K) were 2.0 ± 0.1 (1.5 ± 0.1) psec in PDA-3BCMU, 3.0 ± 0.3 (2.1 ± 0.2) psec in PDA-4BCMU, and 800 ± 100 (800 ± 100) fsec in P3MT. The decay kinetics of the ST exciton were explained by potential crossing and tunneling between two potential curves of the ST exciton and the ground state. The weak temperature dependence indicates that the activation process over the potential barrier between the ST exciton and the ground state is not dominant in the radiationless relaxation of the ST exciton. The relaxation dynamics of fluorescence from the PDA-4BCMU film cannot be represented by a single exponential decay; however, it can be described in terms of a random walk in the fractal dimension. By applying the fractal-dimension model, the spectral dimension was determined to be between 0.50 and 0.85. The spectral changes that are due to several nonlinear-optical processes, i.e., hole burning, Raman gain, and the dynamic Stark effect, were also observed in femtosecond-resolved spectra. The third-order susceptibility was determined for these nonlinear processes, such as absorption saturation at the exciton transition, Raman gain, and the resonant Kerr effect from the observed spectral change. The third-order susceptibility that corresponds to absorption saturation in PDA-3BCMU and in P3MT was obtained as Im[χ1111(3)(−ω;ω,ω,−ω)] equal to −2.6 × 10−9 and −3.5 × 10−10 esu, respectively, for ħω = 1.97 eV. The third-order susceptibility that corresponds to the Raman gain in PDA-3BCMU and in P3MT was determined to be Im[χ1111(3)(−ω2;ω2,ω1,−ω1)] equal to 5.8 × 10−10 and −1.5 × 10−10 esu, respectively, for ħω1 = 1.97 eV and ħω2 = 1.79 eV. From the resonant Kerr experiment in the P3MT film, |Δχ(3)|≡|χ1111(3)(−ω2;ω2,ω1,−ω1)−χ1122(3)(−ω2;ω2,ω1,−ω1)|=3.9×10−11 esu was determined for ħω1 = 1.97 eV and ħω2 = 1.88 eV.

Relaxation dynamics of photoexcitations in polydiacetylenes and polythiophene

Degenerate four-wave mixing measurements, using 45 ps pulses at 1064 nm, have been used to determine the magnitude of the third-order optical susceptibility tensor for thin films of a conjugated porphyrin polymer. The time dependence of the signals indicates that the dominant response is fast relative to the duration of the optical pulses. It is shown that a response on this time scale cannot be consistent with a mechanism in which resonant absorption is significant, and therefore that the primary component of the susceptibility must correspond to an instantaneous electronic polarization. The microscopic polarizability per macrocycle of the polymer is approximately 3 orders of magnitude greater than that of the monomera result that indicates the role of inter-macrocycle conjugation in the nonlinearity. This appears to be the largest one-photon−off-resonance third-order optical susceptibility reported for any organic material.

Large Third-Order Electronic Polarizability of a Conjugated Porphyrin Polymer

In the present study, a new diacetylene compound (PDATAZ), which readily forms a complementary hydrogen bonding self-assembly at the air−water interface or in the solid state with barbituric acid (BA) or cyanuric acid (CA), was designed and synthesized. The photopolymerization studies of PDATAZ and its assembly with BA or CA have revealed some important insights on the chromatic properties of polydiacetylenes. It was found that the chromatic property of polydiacetylenes is determined by whether the polymer chain is capable of adopting a linear chainlike shape. With the continuous increase of the length of the polymer chain, the original linear polyenyne backbone starts to “self-fold” to a “zigzag” structure due to the free rotation of single bonds within the polymer chain. The efficient π-electron delocalization along the polyenyne backbone is interrupted by this process, leading to a chromatic change from the blue to red form of polydiacetylenes. If there are strong intermolecular interactions existing b...

Chromatic studies of a polymerizable diacetylene hydrogen bonding self-assembly : A self-folding process to explain the chromatic changes of polydiacetylenes

Recognition that organic solids possess some of the largest all-optical nonlinearities of all known materials has resulted in an interdisciplinary effort directed at both the basic understanding and exploitation of these effects. Parallel efforts on ionrganic semiconductors have already reached a mature stage whereby the origin of the effects, together with the prospects for device applications, are well known and appreciated. In this article, a unified picture of nonlinear optical phenomena in both classes of materials is presented. The specific implications for organic-based optical devices are discussed.

https://science.sciencemag.org/content/sci/247/4943/679.full.pdf

All-Optical Nonlinearities in Organics

The temporal response of the third‐order nonlinear optical susceptibility in poly bis( p‐toluene sulfonate) of 2,4‐hexadiyne‐1,6 diol was determined by time‐resolved degenerate four‐wave mixing using tunable dye laser pulses with 300 fs duration. In the material’s absorption region these experiments were combined with saturated absorption techniques yielding a measurement for the excited state lifetime of 1.8 ps. Off resonance the nonlinear response was determined to be optical pulse width limited.

Nonlinear optical processes in a polydiacetylene measured with femtosecond duration laser pulses

In order to realize certain ultrafast optical signal processing and logic concepts, materials are needed with both a large and fast responding (in times less than a picosecond) third-order nonlinear optical susceptibility x(3) [1,2]. The speed of the nonlinear process is typically attained by using non-resonant excitation [1]. Further. one must be able to obtain the material in the form required for the anticipated application, e.g. waveguides [2,3] or etalons [1] with acceptable linear optical losses. Organic materials may be well suited for such signal processing schemes for several reasons. Firstly, the concept of declocalized electronic wavefunctions for one-dimensional organic systems has lead to the realization of extremely large nonresonant values of the molecular third — order hyperpolarizability [4]. This concept was extended to the solid state resulting in a large nonresonant x(3) for the organic polymeric family the polydiacetylenes (PDAs) [5]. Secondly, a large number of polydiacetylenes can be synthesized, and a wide variety of material forms(e.g. single crystals, liquid crystals, and multilayers) are available. In the following we will describe recent progress in the interdisciplinary research being carried out in our laboratory aimed at ultilizing the PDA’s for these signal processing applications.

S. E. Meyler

Papers

Nonlinear optical processes in a polydiacetylene measured with femtosecond duration laser pulses

Large Third Order Nonlinear Optical Susceptibilities in Organic Materials