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/.     

Dithiane- and trithiane-based photolabile scaffolds for molecular recognition

[see structure]. A modular synthetic approach to novel dithiane- and trithiane-based photolabile molecular hosts equipped with elements of molecular recognition is developed. The approach provides ready access to a family of amino-derivatized photocleavable molecular systems capable of hydrogen-bonding-based recognition of biologically relevant molecules, e.g., ureas, barbiturates etc. These systems undergo efficient photofragmentation in the presence of external (e.g., benzophenone) or internal (e.g., nitropyridine) electron-transfer sensitizers.     

Two-photon photosensitized production of singlet oxygen.

Singlet molecular oxygen (a(1)Delta(g)) has been produced and optically detected upon two-photon nonlinear excitation of a sensitizer with a focused laser beam. The experiments were performed using toluene solutions with either a substituted difuranonaphthalene or a substituted distyryl benzene as the sensitizer. The data indicate that the two-photon absorption cross sections of the difuranonaphthalenes are comparatively large and depend significantly on the functional groups attached to the chromophore. The time-resolved 1270 nm phosphorescence signals used to characterize the production of singlet oxygen are limited in much the same way as signals from other two-photon spectroscopic studies (e.g., weak signals that can be masked by scattered radiation). Nevertheless, the two-photon singlet oxygen signals also reflect the unique advantages of this nonlinear optical technique (e.g., depth penetration in the sample afforded by irradiation in a spectral region void of the more dominant one-photon linear transitions and spatial resolution afforded by irradiation with a focused laser beam).     

An excited state paired interacting orbital method

A new method for analyzing and visualizing the molecular excited states, named "excited state paired interacting orbital (EPIO)," is proposed. The method is based both on the paired interacting orbital (PIO) proposed by Fujimoto and Fukui [J. Chem. Phys. 60, 572 (1974)] and the natural transition orbital (NTO) by Martin [J. Chem. Phys. 118, 4775 (2003)]. Within the PIO method, orbital interactions between the two fragmented molecules are represented practically only by a few pairs of fragment orbitals. The NTO method is a means of finding a compact orbital representation for the electronic transitions in the excited states. With the method, electronic transitions are expressed by a few particle-hole orbital pairs and a clear picture on the electronic transitions is obtained. EPIO method is designed to have both properties of the preceding two methods: electronic transitions in composite molecular systems can be expressed with a few pairs of EPIOs which are constructed with fragmented molecular orbitals (MOs). Excited state characters, such as charge transfer and local excitations, are analyzed by using EPIOs with their generation probabilities. Thus, the present method gives us clear information on the composition of MOs which play an important role in the molecular excitation processes, e.g., optical processes.     

Real-time time-dependent density functional theory approach for frequency-dependent nonlinear optical response in photonic molecules

We present ab initio calculations of frequency-dependent linear and nonlinear optical responses based on real-time time-dependent density functional theory for arbitrary photonic molecules. This approach is based on an extension of an approach previously implemented for a linear response using the electronic structure program SIESTA. Instead of calculating excited quantum states, which can be a bottleneck in frequency-space calculations, the response of large molecular systems to time-varying electric fields is calculated in real time. This method is based on the finite field approach generalized to the dynamic case. To speed the nonlinear calculations, our approach uses Gaussian enveloped quasimonochromatic external fields. We thereby obtain the frequency-dependent second harmonic generation beta(-2omega;omega,omega), the dc nonlinear rectification beta(0;-omega,omega), and the electro-optic effect beta(-omega;omega,0). The method is applied to nanoscale photonic nonlinear optical molecules, including p-nitroaniline and the FTC chromophore, i.e., 2-[3-Cyano-4-(2-{5-[2-(4-diethylamino-phenyl)-vinyl]-thiophen-2-yl}-vinyl)-5,5-dimethyl-5H-furan-2-ylidene]-malononitrile, and yields results in good agreement with experiment.     

Third‐order nonlinear optical properties of dendritic molecular aggregates: Effects of fractal architecture

We investigate the microscopic third‐order nonlinear optical properties, i.e., the second hyperpolarizabilities (γ), of two different sizes of molecular aggregates with a dendritic, i.e., Bethe‐lattice, structure. One possesses a nonfractal structure, while the other has a fractal structure. The aggregate is treated in a two‐exciton model composed of two‐state monomers coupled to each other by the dipole–dipole interaction. The off‐resonant γ of the aggregates are calculated by the numerical Liouville approach, including relaxation effects. The total γ value is partitioned into the contribution of virtual exciton generation, and its spatial contribution to γ is analyzed in relation to the virtual excitation processes in the perturbation theory. It is found that the intermolecular‐interaction effect enhances both one‐ and two‐exciton‐generation contributions, while the relaxation effect reduces those, although the one‐ and two‐exciton‐generation contributions have mutually opposite signs. From the comparison of spatial contributions to γ between the nonfractal and fractal aggregates, an enhancement of the contribution to γ from the periphery to the core is observed in the fractal structure, while such a feature is not observed in the nonfractal structure. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Multiparticle sintering dynamics: from fractal-like aggregates to compact structures

Multiparticle sintering is encountered in almost all high temperature processes for material synthesis (titania, silica, and nickel) and energy generation (e.g., fly ash formation) resulting in aggregates of primary particles (hard- or sinter-bonded agglomerates). This mechanism of particle growth is investigated quantitatively by mass and energy balances during viscous sintering of amorphous aerosol materials (e.g., SiO(2) and polymers) that typically have a distribution of sizes and complex morphology. This model is validated at limited cases of sintering between two (equally or unequally sized) particles, and chains of particles. The evolution of morphology, surface area and radii of gyration of multiparticle aggregates are elucidated for various sizes and initial fractal dimension. For each of these structures that had been generated by diffusion limited (DLA), cluster-cluster (DLCA), and ballistic particle-cluster agglomeration (BPCA) the surface area evolution is monitored and found to scale differently than that of the radius of gyration (moment of inertia). Expressions are proposed for the evolution of fractal dimension and the surface area of aggregates undergoing viscous sintering. These expressions are important in design of aerosol processes with population balance equations (PBE) and/or fluid dynamic simulations for material synthesis or minimization and even suppression of particle formation.     

基于硒化镓纳米片填充的空芯光纤超高二次谐波增强

与传统的光学晶体相比,全光纤功能器件由于和光纤系统的天然兼容性,被认为是下一代集成光学的重要研究方向,吸引了人们的广泛关注。然而,由于二氧化硅固有的中心反演对称性质,光纤中的二阶非线性光学过程仍有待探索,这在可调谐超快激光、全光信号处理、成像和光通信等商业全光纤非线性光学应用中具有重要意义。因此,我们提出了一种新的溶液填充方法,可有效地将具有高非线性的硒化镓纳米片直接沉积在长度达半米的空芯光纤(HCF)的内孔壁上。此外,采用制备的硒化镓纳米片-空芯光纤(GaSe-HCF)作为光频率转换器,其二次谐波(SHG)比嵌入MoS₂的HCF和普通HCF分别提高了2个数量级和3个数量级。我们的研究成果将拓展其它非线性材料在全光纤高端非线性光学和光电子学中的应用,并提供新的制备思路。     

The calculation of frequency-dependent hyperpolarizabilities including electron correlation effects

The theory for the calculation of the frequency-dependent hyperpolarizabilities β(?2ω; 0, ω), β(?ω; 0, ω), and β(0; ω, ?ω) is discussed. New relations between these tensors are derived for those wave functions that obey the time-dependent Hellmann–Feynman theorem (e.g., the self-consistent field [SCF] or the exact wave function). Using second-order Møller–Plesset perturbation theory (MP2), expressions are obtained for the hyperpolarizabilities in terms of derivatives of appropriately defined linear polarizability tensors with respect to a static electric field. Results are presented for ammonia and formaldehyde for the optical Kerr effect and for secondharmonic generation. These results indicate that it is desirable to determine the frequency-dependent contribution to the hyperopolarizability at the MP2 rather than the SCF level of theory, in cases where the static hyperpolarizability has a large contribution from electron correlation and/or where the frequency-dependent contribution may be more significant, such as for secondharmonic generation.     

Direct-write multiphoton photolithography: a systematic study of the etching behaviors in various commercial polymers

The nonlinear optical processes involved in etching thin polymer films by direct-write multiphoton photolithographic methods (Higgins et al. Appl. Phys. Lett. 2006, 88, 184101) are systematically explored. Power-dependent etching data are obtained for thin films of several commercial polymers, including poly(methyl methacrylate) (PMMA), polystyrene (PS), poly(butyl methacrylate) (PBMA), and poly[2-(3-thienyl)ethyloxy-4-butylsulfonate] (PTEBS). Femtosecond pulses of light from a Ti:sapphire laser are focused to a diffraction limited spot of approximately 570 nm 1/e2 diameter in the films to induce etching. The power dependence of etching in each polymer is used to determine the order of the nonlinear optical process involved. The results for PMMA and PBMA, both of which absorb to the blue of 240 nm, demonstrate that etching involves absorption of several (i.e., 4-6) photons by the polymer, whereas PS, which absorbs wavelengths shorter than 280 nm, is etched by a lower-order process involving fewer (i.e., 3-4) photons. PTEBS, a conducting polymer that absorbs in the visible, is etched by a two-photon process. The results are consistent with an etching mechanism that involves multiphoton-induced depolymerization of the polymer, followed by vaporization of the resulting fragments. The etching resolution is found to be highest for polymers having high glass transition temperatures, low molecular weights, and no visible absorption. Among the polymers examined, low molecular weight PMMA is concluded to be the best polymer for use with this lithographic method. Finally, soft lithography is used to transfer patterns produced in a PMMA film onto poly(dimethylsiloxane), demonstrating a simple means for fabricating submicrometer-scale structures for use in micro- and nanofluidic devices.     

Perspectives in Supramolecular Chemistry—From Molecular Recognition towards Molecular Information Processing and Self-Organization

The selective binding of a substrate by a molecular receptor to form a supramolecular species involves molecular recognition which rests on the molecular information stored in the interacting species. The functions of supermolecules cover recognition, as well as catalysis and transport. In combination with polymolecular organization, they open ways towards molecular and supramolecular devices for information processing and signal generation. The development of such devices requires the design of molecular components performing a given function (e.g., photoactive, electroactive, ionoactive, thermoactive, or chemoactive) and suitable for assembly into an organized array. Light-conversion devices and charge-separation centers have been realized with photoactive cryptates formed by receptors containing photosensitive groups. Eleclroactive and ionoactive devices are required for carrying information via electronic and ionic signals. Redox-active polyolefinic chains, like the “caroviologens”, represent molecular wires for electron transfer through membranes. Push-pull polyolefins possess marked nonlinear optical properties. Tubular mesophases, formed by organized stacking of suitable macro-cyclic components, as well as “chundle”-type structures, based on bundles of chains grafted onto a macrocyclic support, represent approaches to ion channels. Lipophilic macrocyclic units form Langmuir-Blodgett films that may display molecular recognition at the air-water interface. Supramolecular chemistry has relied on more or less preorganized molecular receptors for effecting molecular recognition, catalysis, and transport processes. A step beyond preorganization consists in the design of systems undergoing self-organization, that is, systems capable of spontaneously generating a well-defined supramolecular architecture by self-assembling from their components under a given set of conditions. Several approaches to self-assembling systems have been pursued: the formation of helical metal complexes, the double-stranded helicates, which result from the spontaneous organization of two linear polybipyridine ligands into a double helix by binding of specific metal ions; the generation of mesophases and liquid crystalline polymers of supramolecular nature from complementary components, amounting to macroscopic expression of molecular recognition; the molecular-recognition-directed formation of ordered solid-state structures. Endowing photo-, electro-, and ionoactive components with recognition elements opens perspectives towards the design of programmed molecular and supramolecular systems capable of self-assembly into organized and functional supramolecular devices. Such systems may be able to perform highly selective operations of recognition, reaction, transfer, and structure generation for signal and information processing at the molecular and supramolecular levels.     

Molecular origins of the remarkable chiral sensitivity of second-order nonlinear optics.

Recent observations of remarkably large chiroptical effects in second-harmonic generation (SHG) and sum-frequency generation (SFG) measurements suggest exciting possibilities for the development of new chiral-specific spectroscopies and novel chiral materials for nonlinear optics. Several fundamental studies designed to elucidate the molecular and macromolecular origins of the chiral responses are reviewed to provide a framework for development of this emerging field. In general, the chiral activity in SHG and SFG has the potential to arise from complex interactions between hosts of different competing effects. Fortunately, relatively simple electric dipole-allowed mechanisms routinely dominate the nonlinear optical chiral activities of most practical systemsexpressions can often be generated to link the. This substantial reduction in complexity allows for the development of simple models connecting the macroscopic nonlinear optical response to intuitive molecular and supramolecular properties.     

Nanosensing at the single cell level

This article presents an overview of the development, operation, and applications of optical nanobiosensors for use in in vivo detection of biotargets in individual living cells. The nanobiosensors are equipped with immobilized bioreceptor probes (e.g., antibodies, enzyme substrate) selective to specific molecular targets. Laser excitation is transmitted into the fiber producing an evanescent field at the tip of the fiber in order to excite target molecules bound to the bioreceptors immobilized at the fiber tips. A photometric system detects the optical signal (e.g., fluorescence) originated from the analyte molecules or from the analyte–bioreceptor reaction. Examples of detection of biospecies and molecular signaling pathways of apoptosis in a living cell are discussed to illustrate the potential of the nanobiosensor technology for single cell analysis.     

Coupling corona discharge for ambient extractive ionization mass spectrometry

Unlike the extractive electrospray ionization (EESI) technique described elsewhere, a corona discharge instead of electrospray ionization has been utilized to charge a neutral solvent spray under ambient conditions for the generation of highly charged microdroplets, which impact a neutral sample plume for the extractive ionization of the analytes in raw samples without any sample pretreatment. Using the positive ion mode, molecular radical cations were easily generated for the detection of non-polar compounds (e.g., benzene, cyclohexane, etc.), while protonated molecular ions of polar compounds (e.g., acetonitrile, acetic ether) were readily produced for the detection. By dispensing the matrix in a relatively large space, this method tolerates highly complex matrices. For a given sample such as lily fragrances, more compounds were detected by the method established here than the EESI technique. An acceptable relative standard deviation (RSD 8.9%, n = 11) was obtained for the direct measurement of explosives (10 ppb) in waste water samples. The experimental data demonstrate that this method could simultaneously detect both polar and non-polar analytes with high sensitivity, showing promising applications for the rapid detection of a wide variety of compounds present in complex matrices.     

Hyperpolarizabilities for the one-dimensional infinite single-electron periodic systems. II. Dipole-dipole versus current-current correlations

Based on Takayama-Lin-Liu-Maki model, analytical expressions for the third-harmonic generation, dc Kerr effect, dc-induced second-harmonic optical Kerr effect, optical Kerr effect or intensity-dependent index of refraction, and dc-electric-field-induced optical rectification are derived under the static current-current (J0J0) correlation for one-dimensional infinite chains. The results of hyperpolarizabilities under J0J0 correlation are then compared with those obtained using the dipole-dipole correlation. The comparison shows that the conventional J0J0 correlation, albeit quite successful for the linear case, is incorrect for studying the nonlinear optical properties of periodic systems.     

Synthesis,chiral resolution and optical properties of amphiphilic oxa[9]helicene derivatives

A new class of amphiphilic oxa[9]helicene derivatives were synthesized in excellent-to-fair yields, and successfully characterized by different analytical techniques (e.g., FT-IR, ¹H and ¹³C NMR, and HRMS). The compounds were enantiomerically separated by chiral HPLC and the absolute configurations of the enantiomers were confirmed by circular dichroism spectroscopy. Other optical properties (e.g., UV, photoluminescence, specific optical rotation and molecular optical rotation) of the newly synthesized compounds were also examined.     

Application of Nonlinear Optical Microscopy for Imaging Skin†

Recent advances in the use of nonlinear optical microscopy (NLOM) in skin microscopy are presented. Nonresonant spectroscopies including second harmonic generation, coherent anti‐Stokes Raman and two‐photon absorption are described and applications to problems in skin biology are detailed. These nonlinear techniques have several advantages over traditional microscopy methods that rely on one‐photon excitation: intrinsic 3D imaging with <1 μm spatial resolution, decreased photodamage to tissue samples and penetration depths up to 1000 μm with the use of near‐infrared lasers. Thanks to these advantages, nonlinear optical spectroscopy has become a powerful tool to study the physical and biochemical properties of the skin. Structural information can be obtained using the response of endogenous chemical species in the skin, such as collagen or lipids, indicating that optical biopsy may replace current invasive, time‐consuming traditional histology methods. Insertion of specific probe molecules into the skin provides the opportunity to monitor specific biochemical processes such as skin transport, molecular penetration, barrier homeostasis and ultraviolet radiation‐induced reactive oxygen species generation. While the field is quite new, it seems likely that the use of NLOM to probe structure and biochemistry of live skin samples will only continue to grow.     

Topology of intermolecular nanostructures of adsorbed and liquid ethanol

Based on the combination of molecular-dynamic calculations with the graph theory, a method for identifying and remembering all molecular nanostructures observed in each snapshot of a molecular-dynamic trajectory; averaging data for any number of such snapshots, i.e., estimating averaged concentrations of associates (dimers, trimers, etc.); and determining the concentrations and structural characteristics (bond lengths, angles, etc.) of corresponding structures (e.g., chains, branched chains, cycles, etc.) in each group of the associates is developed for the first time. The method is implemented as a software package, which is applied for the topological analysis of adsorbed and liquid ethanol and molecular nanostructures of different substances occurring in adsorbed or liquid states.     

Open‐Shell‐Character‐Based Molecular Design Principles: Applications to Nonlinear Optics and Singlet Fission

Open‐shell character, e. g., diradical character, is a quantum chemically well‐defined quantity in ground‐state molecular systems, which is not an observable but can quantify the degree of effective bond weakness in the chemical sense or electron correlation strength in the physical sense. Because this quantity also correlates to specific excited states, physicochemical properties concerned with those states are expected to strongly correlate to the open‐shell character. This feature enables us to open a new path to revealing the mechanism of these properties as well as to realizing new design principles for efficient functional molecular systems. This account explains the open‐shell‐character‐based molecular design principles and introduces their applications to the rational design of highly efficient nonlinear optical and singlet fission molecular systems.