Lap-Tak Andrew Cheng

Three coordinate phosphorus and boron as π-donor and π-acceptor moieties respectively, in conjugated organic molecules for nonlinear optics: crystal and molecular structures of E-Ph–CHCH–B(mes)2, E-4-MeO–C6H4–CHCH-B(mes)2, and E-Ph2P–CHCH–B(mes)2[mes = 2,4,6-Me3C6H2]

Hydroboration of π-donor substituted alkynes D–CC–H [D = Ph2P, 4-X–C6H4-(X = H, MeS, MeO, H2N), and (η5-C5H5)Fe(η5-C5H4-)] with dimesitylborane {[(mes)2BH]2, mes = 2,4,6-Me3C6H2} yields air-stable ‘push–pull’E-alkenes of the form E-D–CHCH–B(mes)2, which possess large molecular hyperpolarizabilities, β, as shown by electric-field induced second-harmonic generation (EFISH) experiments at 1.91 µm; single crystal X-ray diffraction studies indicate that E-Ph–CHCH–B(mes)2 and E-4-MeO–C6H4–CHCH–B(mes)2 crystallise in centrosymmetric space groups, whereas E-Ph2P–CHCH–B(mes)2 crystallises in the acentric space group P212121 and exhibits powder SHG of 1.064 µm laser light.

Nonresonant third‐order optical nonlinearity of quantum‐confined CdS clusters: A third harmonic generation study

We report the first study, to the best of our knowledge, of the nonresonant third‐order optical nonlinearity of quantum‐confined CdS clusters using third harmonic generation from 1.91 to 0.64 μm. We observe that the nonlinearity increases with the increase in the cluster size. The intrinsic, nonresonant χ(3)CdS for ∼30, ∼15 A thiophenolate‐capped CdS clusters and [Cd10S4(SPh)16]−4 molecular clusters are 3.3×10−10, 2.5×10−11, 4.7×10−12 esu, respectively. To utilize this large nonresonant nonlinearity it is necessary to incorporate these clusters into a sample in high concentrations.We report the first study, to the best of our knowledge, of the nonresonant third‐order optical nonlinearity of quantum‐confined CdS clusters using third harmonic generation from 1.91 to 0.64 μm. We observe that the nonlinearity increases with the increase in the cluster size. The intrinsic, nonresonant χ(3)CdS for ∼30, ∼15 A thiophenolate‐capped CdS clusters and [Cd10S4(SPh)16]−4 molecular clusters are 3.3×10−10, 2.5×10−11, 4.7×10−12 esu, respectively. To utilize this large nonresonant nonlinearity it is necessary to incorporate these clusters into a sample in high concentrations.

The Effect of varying Ground-state Aromaticity on the First Molecular Electronic Hyperpolarizabilites of Organic Donor-Acceptor Molecules

A series of compounds of the form 4-dimethylaminophenyl–polyene–acceptor, where the polyene ranges from nothing to all-trans-1,3,5-hexatriene and the acceptor is 2-nitrovinyl, formyl, or 2,2-dicyanovinyl has been prepared and their β values measured by solution electric-field-induced second-harmonic generation; these molecules, which lose only one aromatic resonance upon charge-transfer excitation, show enhanced β compared to bi-aromatic molecules with the same substitution and total conjugation length, such as 4-dimethylamino-4′-nitrostilbene (DANS), a well-known benchmark for high β organic molecules.

Linear and nonlinear optical properties of the arsenate isomorphs of KTP

Single-domain crystals of MTiOAsO4, where M equals K, Rb, & Cs were grown from various alkali arsenate self fluxes. Their refractive indices and Sellmeier coefficients were determined with critical angle of total reflection measurements. Type-II phase-matched SHG was used to check for crystal domains and to determine the accuracy of the Sellmeier coefficients. The dij coefficients for second harmonic generation were determined with Maker-fringe technique at 1.06 micrometers . Electro-optic coefficients rji were determined with interferometry. This paper reports the above measurement results and discusses the systematics of crystal structure, birefringence, and optical nonlinearities among the pure isomorphs and their solid-solutions. Calculated SHG cut-offs and tuning curves are also reported.

Quadratic hyperpolarizabilities of fluorinated sulfonyl and carbonyl aromatics: optimization of nonlinearity and transparency trade-off

We present electric-field induced second-harmonic generation measurement results on the hyperpolarizabilities of donor-acceptor substituted aromatics. Those with strongly inductive accepting groups, such as the fluoromethyl carbonyl, perfluoroalkyl sulfonyl, and perfluoroalkyl sulfonylsulfimide groups, afford favorable trade-off between quadratic optical nonlinearity and optical transparency. Their relatively large molecular dipole moment and hyperpolarizability products (4) as well as high transparency in the visible region suggest promising applications in the frequency conversion of short wavelength diode lasers. The inductive contribution is found to be short ranged, being most effective in the relatively compact benzene derivatives but less so in the extended stilbene structures. Dispersive enhancements of hyperpolarizabiities at 1064 and 810 nm are also reported.

Optimizing the second-order optical nonlinearities of organic molecules: asymmetric cyanines and highly polarized polyenes

We recently reported that there is an optimal combination of donor and acceptor strengths for a given molecular length and bridge structure that maximizes (beta) . For this combination, there is the correct degree of bond length alternation and asymmetry in the molecule. Our recent findings suggest that molecules that can be viewed as asymmetric cyanines with relatively small amounts of bond length alternation are nearly optimal. In this manner, we have identified molecules with nonlinearities many times that of conventional chromophores for a given length. In this paper, we will present a new computational analysis that allows the correlation of bond length alternation with hyperpolarizabilities and will present EFISH data on simple donor-acceptor polyene chromophores.

Molecular first hyperpolarizabilities of a new class of asymmetric squaraine dyes

The molecular first hyperpolarizabilities (β) of a series of asymmetric squaraine dyes have been measured by electric-field-induced second harmonic generation; the dyes have negative β-values whose magnitudes are comparable to 4-N,N-dimethylamino-4′-nitrostilbene (DANS).

Structure/property relationships for molecular second‐order nonlinear optics

Using a two‐state model, it was recently shown that there is an optimal combination of donor and acceptor strengths for a given bridge that will maximize the first molecular electronic hyperpolarizability (β). Implementation of this strategy relies on the realization that molecules with strongly aromatic end groups will not have the correct balance of two limiting charge transfer resonance forms in the ground state to achieve the degree of bond alternation required to optimize the molecular hyperpolarizability. Electric field induced second harmonic generation studies on organic molecules with various bridge structures support our hypothesis.

Relationship between conjugation length and third-order nonlinearity in bis-donor substituted diphenyl polyenes

In the design of new organic materials with enhanced nonlinear optical activity, it has been predicted that (gamma) , the third order hyperpolarizability (second hyperpolarizability) will not increase indefinitely in a regular fashion as the conjugation length increases. In regular polyenes, for example, (gamma) has been predicted to level off after circa 10 - 15 repeat units. However, very few experimental systems have been investigated in which the number of repeat units approach these limits. Synthetic difficulties and the lack of solubility of longer oligomers has precluded accurate measures of (gamma) in polyenylic series of well- characterized compounds. In the present study, we discuss how long, and branched-chain alkyl groups attached via alkoxy-substituted phenyl rings can greatly increase the solubility of long polyene oligomers for D,D- and D,A- substituent patterns. This allows, for the first time, a comprehensive study of how (gamma) varies with conjugation length and symmetrical versus asymmetrical substitution.

The dependence of the molecular first hyperpolarizabilities of merocyanines on ground-state polarization and length

We report here the dipole moment (µ) and first hyperpolarizability (β) determined by electric field-induced second harmonic generation, for several merocyanine dyes containing an 1,3,3-trimethylindoline heterocycle as a ‘donor’ in which the ‘acceptor’ end of the molecule and the polyene bridge length was systematically varied; dyes with hexamethine bridges gave positive β, while that with a dimethine bridge gave a negative β value.