Selection rules and symmetry relations for four-wave mixing measurements of uniaxial assemblies.

Uniaxial systems represent the next lowest symmetry below isotropic and are ubiquitous. The objective of the present work is to present a systematic foundation for interpreting polarization-dependent four-wave mixing measurements of oriented and aligned assemblies. Orientational averages connecting the molecular frame to the macroscopic frame in uniaxial assemblies were derived for several common molecular symmetry groups for coherent anti-Stokes Raman spectroscopy (CARS) measurements, coherent anti-Stokes two-photon spectroscopy (CATS) probing electronic transitions, resonant two-photon absorption (2PA), and traditional Raman measurements. First, the complete set of orientational averages connecting the molecular and macroscopic frames was compiled for the most general case of C1 molecular symmetry. Then, the orientational averages of a select few commonly occurring molecular symmetry groups (Cs, C2, C2v, and C3v) were explored in greater detail to illustrate the approach and to facilitate the interpretation of routine experimental measurements. One outcome of this analysis is the prediction of efficient electric dipole-allowed chiral-specific four-wave mixing in uniaxially oriented media.

Electronic and vibrational second-order nonlinear optical properties of protein secondary structural motifs.

A perturbation theory approach was developed for predicting the vibrational and electronic second-order nonlinear optical (NLO) polarizabilities of materials and macromolecules comprised of many coupled chromophores, with an emphasis on common protein secondary structural motifs. The polarization-dependent NLO properties of electronic and vibrational transitions in assemblies of amide chromophores comprising the polypeptide backbones of proteins were found to be accurately recovered in quantum chemical calculations by treating the coupling between adjacent oscillators perturbatively. A novel diagrammatic approach was developed to provide an intuitive visual means of interpreting the results of the perturbation theory calculations. Using this approach, the chiral and achiral polarization-dependent electronic SHG, isotropic SFG, and vibrational SFG nonlinear optical activities of protein structures were predicted and interpreted within the context of simple orientational models.

Controlling the orientation of terraced nanoscale "ribbons" of a poly(thiophene) semiconductor.

The large-scale manufacture of organic electronics devices becomes more feasible if the molecular orientation and morphology of the semiconductor can be controlled. Here, we report on a previously unidentified crystal shape of terraced nanoscale "ribbons" in thin films of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT). The ribbons form after a pBTTT film is heated above its highest temperature phase transition. In contrast to the wide terrace crystal shape previously reported, terraced ribbons have lateral widths of approximately 60 nm and lengths greater than 10 microm, with a common orientation between adjacent ribbons. Further, we report a simple and scalable flow coating process that can control the ribbon orientation without requiring special substrates or external fields. The degree of molecular orientation is small after coating but increases dramatically after the terraced ribbons are formed, indicating that an oriented minority templates the whole film structure. The large extent of orientation obtained in these polythiophene crystallites provides potential opportunities to exploit anisotropic electrical properties and to obtain detailed information about the structure of organic semiconductor thin films.

Visual methods for interpreting optical nonlinearity at the molecular level.

The emergence of nonlinear optical (NLO) measurement approaches has provided new windows into molecular and macromolecular structure within thin films and materials. The greatest barriers in mining this structural information increasingly appear in meaningfully relating these macroscopic results back to molecular-level descriptions, driven largely by the increasing complexity of the molecular systems and interfacial architectures under interrogation. As NLO methods continue their expansion into increasingly diverse disciplines, so grows the need for tools to guide this evolution without sacrificing the mathematical rigor of more traditional tensor representations. Recent developments reviewed in this Account are designed to facilitate interpretation of complex assemblies using relatively simple but still quantitatively accurate visual representations of the polarization-dependent optical nonlinearity, both for individual chromophores and for polymeric assemblies of coupled chromophores. Although the primary focus of this Account is on second-order nonlinear optical effects, including second harmonic generation and sum frequency generation, many of these same concepts also directly apply to higher-order phenomena.

NLOPredict: visualization and data analysis software for nonlinear optics.

A data analysis and visualization program was developed to assist in the interpretation of second-order nonlinear optical (NLO) processes, including vibrational sum-frequency generation and electronically resonant second harmonic generation. A novel diagrammatic approach allows concise visual representations of the resonant NLO molecular response. By mapping the predicted NLO response as a function of molecular orientation, molecular modeling results can be combined with experimental measurements for orientational analysis. A method is developed and implemented to predict the nonlinear optical properties of the amide backbones in complete proteins with known structures. NLOPredict is available for most computer operating systems from http://sda.iu.edu/nlopredict/.

Self-consistent approach for simplifying the molecular interpretation of nonlinear optical and multiphoton phenomena.

A method is developed for simplifying molecular interpretations of nonlinear optical phenomena. General sum-over-states expressions derived from perturbation theory can be written identically and self-consistently as simple products of lower-order effects. Electric dipole-allowed expressions for the nonlinear polarizability reduce to straightforward formulas directly connected to intuitive molecular properties without sacrificing mathematical rigor. This approach is sufficiently general to allow its application in treating electronic, vibrational, and vibronic interactions for both parametric (passive) processes (e.g., wave-mixing spectroscopies, sum- and difference-frequency generation, harmonic generation, etc.) and nonparametric (active) processes (e.g., hyper-Raman spectroscopy, multiphoton absorption, etc.). Explicit examples for sum-frequency generation and for four-wave mixing provide a convenient context for interpreting higher order nonlinear optical processes.

Direct determination of effective interfacial optical constants by nonlinear optical null ellipsometry of chiral films.

Nonlinear optical null ellipsometry (NONE) measurements of chiral interfaces allowed direct experimental measurement of the linear interfacial optical constants in surface second harmonic generation (SHG) measurements. Since phase information is retained in NONE measurements, the real and imaginary components of the interfacial refractive index (n and k, respectively) were uniquely obtained from the measured chiral chi((2)) tensor elements of a fluorescein-labeled bovine serum albumin film. The sensitivity of the calculated chi((2)) tensor elements on the assumed values of the interfacial optical constants allowed measurements of n and k to four significant figures with no additional adjustable parameters and independent of molecular symmetry. The optical constants measured by SHG agreed within a relative error of 0.8% with values predicted independently using a simple effective medium approximation, also with no adjustable parameters. Additionally, those same optical constants produced relationships between the achiral chi((2)) tensor elements in excellent agreement with predictions for systems exhibiting weak orientational order. This study suggests that the far-field intensity and polarization state of the nonlinear optical beam may be largely independent of the near-field optical constants within the interfacial layer in the limit of a film thickness much less than the wavelength of light.

Experimental confirmation of the importance of orientation in the anomalous chiral sensitivity of second harmonic generation.

Macromolecular interactions were demonstrated to yield large chiroptical effects in second harmonic generation measurements of ultrathin surface films. Second harmonic generation (SHG) has recently shown to be several orders of magnitude more sensitive to chirality in oriented systems than common linear methods, including absorbance circular dichroism (CD) and optical rotary dispersion (ORD). Numerous mechanisms have been developed to explain this anomalous sensitivity, with a general emphasis on understanding the molecular origins of the chromophore chirality. In this work, orientational effects alone are shown to be the dominant factor for generating large SHG chiral dichroic ratios in many surface systems. Three distinct uniaxial surface films of SHG-active achiral chromophores oriented at chiral templated surfaces were observed to yield chiral dichroic ratios as great as 40% in magnitude.