Search

Complex-conjugate-resolved Fourier-domain optical coherence tomography, where the quadrature components of the interferogram are obtained by simultaneous acquisition of the first and second harmonics of the phase-modulated interferogram, is applied to multisurface test targets and biological samples. The method provides efficient suppression of the complex-conjugate, dc, and autocorrelation artifacts. A complex-conjugate rejection ratio as high as 70 dB is achieved.

A method of resolving the complex conjugate ambiguity in Fourier-domain OCT is described. The complex differential spectral interferogram is obtained by simultaneous acquisition of the first and second harmonics of the ac component of the phase-modulated interferogram. The harmonics represent the imaginary and real parts of the interferogram, respectively. The complex conjugate rejection ratio is found to be at least 45 dB and is limited by background noise.

Differential spectral interferometry (DSI), a novel method of biomedical imaging that combines the high dynamic range of optical coherence tomography (OCT) with inherently parallel low-bandwidth image acquisition of spectral interferometry (SI), is described. DSI efficiently removes the deleterious dc background inherent in SI measurements while maintaining the parallel nature of SI. We demonstrate DSI on both synthetic and biological samples. Because DSI preserves the low-bandwidth, parallel nature of SI, it is competitive with OCT for biomedical applications in terms of image quality and acquisition rate.

The transport of electronic excitations among chromophores which are randomly tagged or end tagged on finite flexible polymer chains in the amorphous solid state is described. First, the previously reported first order cumulant expansion treatment of excitation transport among chromophores randomly distributed in solution is extended to treat finite volume systems with nonrandom chromophore distributions. The method is demonstrated by considering chromophores randomly distributed in a finite sphere. The results are in good agreement with a previous treatment of this problem using a density expansion. The probability of finding the initially excited chromophore, still excited at time t, Gs(t), is calculated. Gs(t) is directly related to fluorescence depolarization and other observables. Gs(t) is then calculated for a finite flexible polymer chain in a polymer blend which is randomly tagged in low concentration with chromophores. The method permits any form of the polymer pair correlation function to be use...

Time-resolved fluorescence depolarization is used to examine electronic excitation transport among naphthalene chromophores in a polymeric solid as a function of excitation wavelength between 300 and 50 K. The characteristics of the wavelength and temperature dependences reveal, for the first time, that dispersive transport occurs at and near room temperature. The rate of excitation transport depends on the wavelength of excitation. A theoretical treatment is able to reproduce the essential features of the wavelength and temperature dependences without recourse to adjustable parameters by avoiding the complexities of the relationship between the difference in energy of a pair of molecules and the pairwise transfer rate.

Time-resolved fluorescence depolarization experiments are reported for systems of 1-pentylpyrene in polystyrene solid solutions. Two sources of fluorescence depolarization are examined hindered rotational depolarization and electronic excitation transport induced depolarization. At low concentrations of 1- pentylpyrene, rotational depolarization is examined for free 1-pentylpyrene and for 1-butylpyrene end-tagged on polystyrene chains ranging in molecular weight from 3000 to 32000. At high concentrations of free 1-pentylpyrene, electronic excitation transport increases the rate of depolarization with increasing concentration. In these room temperature samples, although the inhomogeneous optical line width is comparable to the available thermal energy, kT, dispersive transport is not in evidence for experiments conducted at a single wavelength. The dynamics of excitation transport can be accurately modeled by theories developed for chromophores distributed randomly in homogeneous liquid solution.

The problem of the distribution of distances between the ends of a polymer chain for short chains (a small number of statistical segments) is considered. A formalism developed by Rayleigh is utilized to calculate the exact end to end distribution functions for random walks with a small number of steps and variable step lengths. In particular, distribution functions for walks in which the first and last step differ in length from the intervening steps are obtained. These are used as models for low molecular weight polymers in which the first and last statistical segments are different from internal segments because of chain end effects or chromophores attached to the chain termini. The results are used to calculate the ensemble averaged time dependence of end to end electronic excitation transport. G8((t), the part of the transport Green function which yields the time-dependent fluorescence depolarization observable, is calculated with exact random walk distribution functions and approximate distribution functions. It is demonstrated that the results are similar but that experiments which examine several distance ranges have the capability of distinguishing the distribution functions.