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Physical Review A

Femtosecond measurement of fluorescence by two-photon interference

Ying Li, M. Baba, Masaya Matsuoka

Published Apr 1, 1997• 7 citations
Publisher

Abstract

We show that the coherence time and the pulse envelope of ultrashort fluorescence can be measured with two-photon interference. We have performed these measurements in a solution of Nile Blue in N,N-dimethylaniline in the femtosecond regime. The coherence time and the envelope contribute two components to a dip in the coincidence count rate. We have verified that these components can be measured separately at will. This technique provides the means to measure the pulse envelope of ultrashort pulses over a wide range of wavelengths and intensities.

DOI
10.1103/physreva.55.3177

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The Journal of Chemical Physics•Mar 1, 1999•19 citations
We show that the coherence time and the pulse envelope of ultrashort fluorescence can be measured with two-photon interference. We have performed these measurements in a solution of Nile Blue in N,N-dimethylaniline in the femtosecond regime. The coherence time and the envelope contribute two components to a dip in the coincidence count rate. We have verified that these components can be measured separately at will. This technique provides the means to measure the pulse envelope of ultrashort pulses over a wide range of wavelengths and intensities.
We extend the intensity interference experiments to a pulse envelope measurement of ultrashort incoherent optical pulses. The interfering fields can be mutually incoherent light pulses from independent sources. We show that the pulse envelope or width of ultrashort incoherent light pulses can be measured independently from the coherence time by using a combination of interferometer and two-photon coincidence counter. With this method we have measured the envelope autocorrelation of mutually incoherent femtosecond fluorescence from a solution of Nile Blue in dimethylaniline.
By using an intensity correlation, or photon coincidence, technique and a Mach-Zehnder interferometer with artificially applied phase fluctuations in one arm, we measured the coherence time of a thermal light beam. The light was derived from the filtered fluorescence of a rhodamine 6G dye solution. The two-photon coincidence rate was measured as a function of path difference and was found to dip 50% when the two arms of the interferometer are balanced. The coherence time of ~100 fs is determined from the measured width of the dip and is consistent with the passband of the filters. The method does not require the stability needed in second-order interference measurements, and its time resolution can be much shorter than that of the photodetectors. The theory of this type of fourth-order interference technique is discussed.
Multiphoton interference effects can be measured with a single detector when two input photons are temporally well separated when compared with the dead time of the single-photon avalanche detector. Here we experimentally demonstrate that the Hong-Ou-Mandel interference effect can be observed with a single non-photon-number resolving detector via a time-delayed coincidence measurement of successive electrical signals from the detector. The two-photon interference experiment is performed by utilizing temporally well-separated pairwise weak coherent pulses, and the interference fringes are successfully measured with high visibility in the range of the limited upper bound for the weak coherent photon source.
Spontaneous fluorescence has been used to measure the coherent femtosecond response of the organic, binary crystal pentacene/p-terphenyl. By using two-pulse excitation with phase-randomized pulses in an interferometric setup and analyzing the variance of the fluctuating intensity of correlated fluorescence photons, femtosecond beatings have been observed. The pattern of these terahertz oscillations is strongly dependent on the detuning frequency range of the exciting pulses, but is rather invariant with regard to the spectral position of the fluorescence probe window. In the interferometric regime of freely propagating pulses novel, ultrafast fluorescence carrier-wave oscillations superimposed to the beat structure have been obtained. The oscillatory signals evolve from a coherent superposition of optical free induction decays, caused by the different electronic transition energies of the pentacene absorber sites O1, O2, O3, and O4, respectively, are monitored as intrinsic, heterodyne beats by the fluorescence square detector. The major part of oscillations is thus considered to result from typical polarization interferences, but vibrational quantum beats are also extractable from the interferogram in the ultimate frequency regime of the pentacene S0→S1ν=1, S0→S1ν=2 resonances. The early picosecond-decay of the fluorescence beats reveals the mechanism of the loss of coherence to be mainly controlled by inhomogeneous dephasing at low phonon temperatures. A model of uncoupled two-level systems that includes Gaussian inhomogeneous broadening of the individual absorber sites and a Gaussian frequency distribution for the excitation pulses can account for the experimental data, quite adequately.