No abstract

Four series of dyes with dimethylamino electron donor groups and N‐R‐pyridinium (R = methyl Me, phenyl Ph, 2,4‐dinitrophenyl 2,4‐DNPh, or 2‐pyrimidyl 2‐Pym) electron acceptors are studied as their hexafluorophosphate salts. The intramolecular charge‐transfer (ICT) energies (Emax) of these compounds decrease within each of the series in the order R = Me > Ph > 2,4‐DNPh > 2‐Pym, as the electron‐accepting ability of the pyridinium ring increases. Hyper‐Rayleigh scattering with femtosecond 1300 or 800 nm lasers yields fluorescence‐free first hyperpolarizabilities β, and static first hyperpolarizabilities β0[H] are obtained via the two‐state model. Dipole moment changes Δμ12 for the ICT transitions obtained from Stark spectroscopy afford β0[S] values by using β0 = 3Δμ12(μ12)2/2(Emax)2 (μ12 = transition dipole moment). The β0[S] data show that the combination of pyridyl N‐arylation with conjugation extension affords large increases in β0. The β0[H] data generally agree with this conclusion, but resonance effects may explain some apparent anomalies. X‐ray structural studies on various salts reveal that the use of tosylate anions is not a generally applicable approach to engineering noncentrosymmetric structures of pyridinium salts. However, trans‐N‐phenyl‐4‐(4‐dimethylaminophenyl‐4‐buta‐1,3‐dienyl)pyridinium hexafluorophosphate adopts the polar space group Cc, and shows a very large powder second harmonic generation efficiency from a 1907 nm laser, which is similar to that of the well‐studied material trans‐4′‐(dimethylamino)‐N‐methyl‐4‐stilbazolium tosylate (DAST).

A series of five fluorescent and ionic dimethylamino stilbazolium homologues with increasing conjugation length (from ethenyl to decapentaenyl) is investigated by high-frequency, amplitude-modulated femtosecond hyper-Rayleigh scattering at 1300 nm. A hyperpolarizability value that is not overestimated by the presence of a multiphoton fluorescence contribution is obtained from the Fourier analysis of the hyper-Rayleigh scattering signal. The demodulation curve (decrease of Fourier amplitude versus modulation frequency) is characterized by both the hyperpolarizability value and the fluorescence decay parameters. The fluorescence decay parameters are accurately determined independently by single-photon counting. A detailed analysis of the fluorescence decay parameters from the hyper-Rayleigh scattering demodulation curve and of their relation to the fluorescence decay parameters obtained from single-photon counting experiments is presented. The inherent hyperpolarizability value for these chromophores shows a maximum of (2045 Ϯ 35) ϫ 10 Ϫ30 esu or (760 Ϯ 13) ϫ 10 Ϫ50 C 3 m 3 J Ϫ2 for the hexatrienyl conjugation length. A comparison with theoretical calculations suggests the importance of trans-cis isomerization in the excited state.

No abstract

Colloidal crystals with three-dimensional periodicities in the refractive index have a photonic band gap (PBG) in which electromagnetic waves are forbidden. We present a method to fabricate stacked colloidal crystals containing a two-dimensional defect as a middle layer by combining vertical deposition method with the Langmuir–Blodgett (LB) technique. The defect layer introduces an impurity mode within the optical stop band, which is observed as a defect peak (pass band) in the optical density spectrum. The result shows that the combination of vertical deposition with LB technique provides a way for introducing defect modes in PBG materials.

Suppression of the multiphoton fluorescence contribution to the hyper-Rayleigh (second-order nonlinear optical) scattering signal was recently achieved by intrinsic demodulation of the fluorescence at high amplitude-modulation (AM) frequencies [Olbrechts et al., Rev. Sci. Instrum. 69, 2233 (1998)]. These high AM frequencies were obtained from the high harmonic content in the Fourier spectrum of a repetitive train of femtosecond pulses from a Ti:sapphire laser emitting at 800 nm. We have used a femtosecond parametric oscillator to shift the fundamental wavelength to 1.3 mum . By further improving the detection electronics, we can now obtain fluorescence suppression at AM frequencies up to 600 MHz. Fluorescence-free hyperpolarizability values were obtained for fluorescent dipolar compounds as well as for an ionic fluorophore. The results also indicate that shifting the fundamental wavelength to the near infrared only is not a general solution to the multiphoton fluorescence problem in hyper-Rayleigh scattering.

Uniaxial stressors have received much interest over the last few years as a method to enhance carrier mobility and, hence, drive current with minimal modification to the structure of the transistor. However, the shift in device design to complex structures with multiple crystallographic orientations like advanced bulk-FinFETs has significantly complicated the incorporation of mobility enhancing stressors. For the n-FinFET in particular, it turns out that the crystal quality and growth rate of Si:P and Si:C:P films can be strongly dependent upon the crystallographic orientation of the starting surface. Both for raised and recessed epi we find that formation of (111) facets and twin defects occurs already after a limited growth on the fin. Besides the growth on raised and recessed fins, we also discuss the resistivity increase in Si:C:P layers as a function of carbon content and demonstrate that laser annealed Si:P films with high phosphorus content (e.g. 4% or higher) can be considered as potential alternatives to Si:C:P with a lower resistivity for the same strain.

No abstract