Geert Olbrechts

High-frequency demodulation of multiphoton fluorescence in long-wavelength hyper-Rayleigh scattering

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.

Enhancement of the molecular hyperpolarizability by a supramolecular amylose–dye inclusion complex, studied by hyper-Rayleigh scattering with fluorescence suppression

Abstract The first hyperpolarizability β of a free hemicyanine dye and a homologue dye included in a supramolecular complex have been determined by hyper-Rayleigh scattering. Since the inclusion complex is fluorescent, high-frequency demodulation of the time-delayed multiphoton fluorescence has been used to retrieve a fluorescence-free inherent value for its first hyperpolarizability. The free dye does not exhibit fluorescence; the inclusion induces fluorescence with a lifetime of 4.8±0.1 ns; and the inclusion complex has a fluorescence-free value for its dispersion-free first hyperpolarizability β0 of approximately twice that for the free dye ((200±5)×10−30 vs. (100±10)×10−30 esu)). The enhanced polar orientation of this complex in thin films, and better thermal and mechanical stability, together with this increase in molecular nonlinearity confirm inclusion as a way to engineer efficient macroscopic arrangements for nonlinear optics.

Multiphoton fluorescence free hyperpolarizabilities of subphthalocyanines

Subphthalocyanines have potential applications in the field of second-order nonlinear optics. However, multiphoton fluorescence has seriously complicated the accurate retrieval of their molecular first hyperpolarizability b by hyper-Rayleigh scattering. We report here hyper-Rayleigh scattering results on subphthalocyanines with experimental evidence of effective multiphoton fluorescence suppression. It is demonstrated that multiphoton fluorescence does indeed contribute to the scattering signal but that accurate values for the first hyperpolarizabilities can be deduced when taking this properly into account. More generally, this shows that experimental evidence of the absence of a multiphoton fluorescence contribution to hyper-Rayleigh scattering should be included whenever hyperpolarizability values of fluorescent compounds are reported. q 1999 Elsevier Science B.V. All rights reserved.

Novel columnar mesogen with octupolar optical nonlinearities: synthesis, mesogenic behavior and multiphoton-fluorescence-free hyperpolarizabilities of subphthalocyanines with long aliphatic chains†

Novel subphthalocyanines with long thioalkyl chains exhibit hexagonal columnar mesophases at room temperature; their inherent first hyperpolarizability values (β) measured by hyper-Rayleigh scattering (HRS) with fluorescence suppression are (189 ± 30) × 10–30 esu at 1300 nm.

Probing of spatial orientational correlations between chromophores in polymer films by femtosecond hyper-Rayleigh scatterring

Abstract Femtosecond hyper-Rayleigh scattering has been used to probe the spatial orientational fluctuations between nonlinear optical chromophores as dopants in spincoated polymer films. The fluctuation in the second-order incoherently scattered light intensity upon micro-translating the solid sample is indicative of the degree of spatial correlation between the individual chromophores. The decay of the autocorrelation function of this fluctuating signal is characterized by a spatial correlation length. Electric-field poling of dipolar chromophores is shown to increase this correlation length. The longer correlation length after poling results from a higher degree of spatial orientational correlation between the individual chromophores.

Improved fitting equation for frequency-resolved femtosecond hyper-Rayleigh scattering experiments

One approach to the experimental determination of the molecular second-order nonlinear polarizability, or the first hyperpolarizability, of fluorescent species by hyper-Rayleigh scattering is based on high-frequency demodulation of the time-delayed fluorescence contribution to the immediate scattering signal [Rev. Sci. Instrum.69, 2233 (1998)]. For typical fluorescence lifetimes of less than a nanosecond a detection bandwidth of more than 1 GHz is necessary. This bandwidth has not yet been realized. Measurements at successively higher modulation frequencies are performed instead. A fitting of the apparent hyperpolarizability as a function of the modulation frequency then reveals the inherent hyperpolarizability without the fluorescence contribution. An improved fitting function has been derived, resulting in the elimination of a small systematic error and in the reduction of the larger statistical uncertainty in the deduced value. Possible implications of the improved accuracy and precision are discussed.

Comparison between optical nonlinearity relaxation times from coherent second‐harmonic generation and from incoherent hyper‐Rayleigh scattering

Femtosecond hyper‐Rayleigh scattering has recently been used to determine the spatial correlation length between nonlinear optical chromophores in polymer films [G. Olbrechts et al., Chem. Phys. Lett. 253, 135 (1996)]. This correlation length corresponds to the effective interaction length for frequency doubling in waveguide format, as derived from phase‐matching curves. The temporal characteristics of this correlation length have now been studied and compared with thermal relaxation times, obtained with coherent second‐harmonic generation. The correlation length decays much faster than the second‐harmonic intensity, indicating that the efficiency of a second‐order waveguide process could decay much faster than generally anticipated.

Study of domain formation and relaxation in thin polymeric films by femtosecond hyper-Rayleigh scattering

Femtosecond hyper-Rayleigh scattering has been used to probe the spatial orientational fluctuations between nonlinear optical chromophores as dopants in spin-coated polymer films. The fluctuation in the second-order incoherently scattered light intensity on microtranslation of the solid sample is indicative of the degree of spatial correlation between the individual chromophores. The decay of the autocorrelation function of this fluctuating signal is characterized by a spatial correlation length. Electric-field poling of dipolar chromophores is shown to increase this correlation length. The temporal characteristics of the correlation length have been studied and compared with thermal relaxation times obtained with coherent second-harmonic generation. The correlation length decays much faster than the second-harmonic intensity. Possible implications of this fast relaxation are addressed.

Difference in relaxation time between coherent and incoherent second-harmonic generation

Abstract A significant difference in relaxation time between coherent and incoherent second-harmonic generation in electric-field poled thin polymeric films has been observed experimentally. Based on the intrinsic symmetry requirements for even-order non-linear optical effects, rotational relaxation of the chromophore only was invoked in combination with translational diffusion of free volume in the polymer matrix over the coherence length for coherent second-harmonic generation and over the wavelength in the incoherent case.

High-frequency demodulation of multiphoton fluorescence for hyper-Rayleigh scattering in solution

We demonstrate a novel technique in the frequency-domain for the suppression of the multiphoton fluorescence contribution in hyper-Rayleigh scattering experiments. The technique takes advantages of the demodulation and the phase-shift in the frequency-domain of the time-delayed (multiphoton) fluorescence in the time-domain. For this technique, we use the improved temporal resolution offered by the femtosecond pulse versus the nanosecond pulse. We demonstrate the effectiveness of the demodulation of the fluorescence at high modulation frequencies of the fundamental laser beam by determining the molecular second-order nonlinear polarizability for a reference molecule under fluorescent conditions. The value that was obtained for the /spl beta/ (second-order nonlinear optical polarizability) of crystal-violet in methanol with 9,10-diphenylanthracene added as a centrosymmetric fluorophore, compares very well with the values that were previously obtained. The frequency-dependence of the retrieved value shows both the overestimation at DC and low modulation frequencies and the convergence towards the fluorescence-free value at high modulation frequencies.