Manuel Joffre

Linear techniques of phase measurement by femtosecond spectral interferometry for applications in spectroscopy

Although nonlinear methods can provide only the amplitude and the phase of an isolated ultrashort pulse, linear techniques can yield such measurements with a much better sensitivity and reliability when a reference pulse is available. We demonstrate two such methods, dual-quadrature spectral interferometry and Fourier-transform spectral interferometry. These techniques are simple to implement, very sensitive, and provide a complete measurement of the complex electric field, E(ω), as a continuous function of frequency.

Generation of ultrabroadband femtosecond pulses in the mid‐infrared by optical rectification of 15 fs light pulses at 100 MHz repetition rate

Quasi‐single‐cycle near‐infrared light pulses with a measured spectrum extending from 7 to 15 μm have been generated, opening up new perspectives in IR spectroscopy. The method is based on the rectification of 0.8 μm 10–15 fs light pulses from a 100 MHz oscillator, using the instantaneous second‐order polarizability of bulk semiconductors such as GaAs.

Use of coherent control for selective two-photon fluorescence microscopy in live organisms

We demonstrate selective fluorescence We demonstrate selective fluorescence excitation of specific molecular species in live organisms by using coherent control of two-photon excitation. We have acquired quasi-simultaneous images in live fluorescently-labeled Drosophila embryos by rapid switching between appropriate pulse shapes. Linear combinations of these images demonstrate that a high degree of fluorophore selectivity is attainable through phase-shaping. Broadband phase-shaped excitation opens up new possibilities for single-laser, multiplex, in-vivo fluorescence microscopy.

Two-dimensional nonlinear optics using Fourier-transform spectral interferometry

The second-order nonlinear-optical response of a material is measured in two dimensions of frequency. This new spectroscopy technique, based on the use of Fourier-transform spectral interferometry, is a transposition to optics of two-dimensional nuclear magnetic resonance. Although we demonstrate the technique by measuring the second-order phase-matching map of a nonresonant nonlinear crystal, this method should find useful applications in the measurement of second-order nonlinear susceptibility and in photon-echo experiments.

Fourier-transform coherent anti-Stokes Raman scattering microscopy

We report a novel Fourier-transform-based implementation of coherent anti-Stokes Raman scattering (CARS) microscopy. The method employs a single femtosecond laser source and a Michelson interferometer to create two pulse replicas that are fed into a scanning multiphoton microscope. By varying the time delay between the pulses, we time-resolve the CARS signal, permitting easy removal of the nonresonant background while providing high resolution, spectrally resolved images of CARS modes over the laser bandwidth (approximately 1500 cm(-1)). We demonstrate the method by imaging polystyrene beads in solvent.

Measurement of photon echoes by use of femtosecond Fourier-transform spectral interferometry

Using Fourier-transform spectral interferometry, we demonstrate the measurement of both amplitude and phase of photon echoes in GaAs multiple-quantum-well structures. The complete measurement of the electric field thus achieved makes possible the determination of the corresponding Wigner spectrograms.

Coherent effects in pump–probe spectroscopy of excitons

Femtosecond measurements of coherent effects arising from exciton bleaching in bulk GaAs are reported. This phenomenon, which is characterized by spectral oscillatory structures, is general and appears whenever the temporal resolution is shorter than the material coherence time. This is shown to be a manifestation of the uncertainty relation in time-resolved spectroscopy.

Two-dimensional infrared spectroscopy detected by chirped pulse upconversion

A two-dimensional (2D) infrared spectrum of Mn2(CO)10 is measured by using chirped-pulse upconversion (CPU) of the nonlinear signal field plus a reference local oscillator. By converting the spectrum to the visible, a silicon CCD camera can be used. The method offers an attractive alternative to direct IR detection due to the technological maturity of silicon and its greater intrinsic detectivity over HgCdTe. Using CPU, we acquired a rephasing 2D IR spectrum in a few seconds.

Fourier transform measurement of two-photon excitation spectra: applications to microscopy and optimal control.

We report a novel Fourier transform method for measuring two-photon excitation spectra. We demonstrate this method using simple dye molecules and discuss its applications in two-photon fluorescence microscopy and optimal control. This method facilitates an intuitive interpretation of recent control experiments in terms of tuning the nonlinear spectrum of the exciting laser source.

Femtosecond pulse phase measurement by spectrally resolved up-conversion: application to continuum compression

The authors have developed an extension of the usual cross-correlation method in order to get the amplitude and phase of a femtosecond light pulse. This is done by analyzing the spectra of the upconverted pulse obtained by mixing the initial pulse with the reference pulse delayed with different times. The theory is worked out in closed form for Gaussian pulses and then checked with simulations for arbitrary fields. It is shown that the phase derivative of any probed pulse can be directly recovered in the limit where the reference is narrowband. This technique is used to measure the chirp induced at the output of a femtosecond Ti:sapphire oscillator. This technique is then applied to the determination of the exact pulse shape of a 21-fs compressed continuum at 770 nm using, as a reference, the initial fundamental 65-fs pulse at 620 nm. >