A. Antonetti

Femtosecond optical nonlinearities of CdSe quantum dots

Attract-Femtosecond differential absorption measurements of the quantum confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The bole width increases with pump intensity and the magnitude of the induced transparency saturates at our highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition.

Ultrafast all‐optical gate with subpicosecond ON and OFF response time

An all‐optical logic gate consisting of a GaAs‐GaAlAs multiple quantum well structure inserted in a 1.3‐μm‐thick Fabry–Perot cavity is demonstrated to perform with subpicosecond on and off switching time. The use of a purely optical field effect allows for a recovery time as rapid as the switch‐on time.

Resonant multiphoton ionisation of xenon with high-intensity femtosecond pulses

The authors report on the intensity and polarisation dependences of electron energy spectra from multiphoton ionisation of xenon with 120 fs pulses at 615 nm. The spectra show a large number of narrow structures assigned to resonances induced by the AC Stark shift. The structures are suppressed with circular polarisation. The maximum shift is observed for the 6s state which is up-shifted by 1.6 eV. The assignments are based on a lowest-order perturbative calculation of the AC Stark shifts.

Ultrabroadband second‐harmonic generation in organic and inorganic thin crystals

Organic thin crystals of 2‐methyl‐4‐nitroaniline (MNA) have been used to frequency double simultaneously all components of a broadband continuum using a non‐phase‐matched configuration. A 1 μm sample of MNA permits to convert, in a single pulse, wavelengths in the band 600–1400 nm to the 300–700 nm band. Using the same technique, we also evidence the remarkably large bandwidth of a 100‐μm‐thick KDP, when used in the wavelength noncritical phase‐matched region centered around 1.03 μm. The conversion efficiencies of the 1‐μm‐thick MNA sample and of the 100‐μm‐thick phase‐matched KDP crystal are shown to be comparable. The results are of interest to the processing and measurement of femtosecond pulses.

Dynamics and Fourier transform studies of the excitonic optical Stark effect

The authors study the optical Stark effect on the exciton of bulk gallium arsenide using high-repetition-rate femtosecond laser pulses. They distinguish three regimes: quasi-permanent, dynamic, and nonadiabatic. In the dynamic case they develop and demonstrate experimentally a technique of dynamics measurement based on the Fourier transform of the absorption spectra. This method has the advantages of being single-shot, of discriminating between a shift and an absorption strength change, and of separating the contribution of a single narrow line from the other spectral components. >

K-shell emission dynamics of Be-like to He-like ions from a 100 fs laser-produced aluminium plasma

The authors present experimental results and numerical simulations of K-shell emission from Be-like, Li-like and He-like ions from a 100 fs laser-produced aluminium plasma at intensities in excess of 1016 W cm-2. The use of thin film targets of thickness comparable to the laser skin depth greatly facilitates the interpretation of time-integrated spectra and permits the study of quasi-homogeneous temperature plasmas with limited plasma line broadening effects. Time-dependent modelling shows that the X-ray pulse duration of the Be-like and Li-like emissions is much shorter than the laser pulse. Time-integrated model spectra are in good agreement with experiment.

Characterization of a femtosecond-laser-produced plasma x-ray source by electronic, optical, and x-ray diagnostic techniques

Short-pulse laser-produced plasmas look very promising for the generation of sub-picosecond X-rays. By combining several experimental techniques, we have significantly progressed towards a better understanding of ultrafast laser-matter interaction. The X-ray yield is a sensitive function of the electron density gradient scale length of the target plasma. In this work, the scale length has been changed by varying the temporal separation between the main laser pulse and a lower intensity prepulse. X-ray spectroscopic diagnostics of the plasma parameters have been used from the analysis of resonance and dielectronic satellite lines. The angular and energy distribution of suprathermal electrons emitted during the ultrafast laser- plasma interaction have been measured as a function of laser polarization and prepulse delay. Frequency-domain interferometry and optical measurements of the reflected probe pulse have been used to study the velocity and the gradient scale length of the expanding plasma. The Kα emission yield peaks for a scale length where resonant absorption is optimized. Hydrodynamic simulations have been performed to investigate the plasma dynamics and the basic processes which control the X-ray emission duration and intensity. Applications of ultrashort Kα X-rays to the diagnostic of solid plasma conditions and as a source for time-resolved diffraction and spectroscopy of transient chemical, biological or physical phenomena are underway.

Observation of X-ray fluorescence resulting from photoionization by a femtosecond laser-produced plasma X-ray source

We demonstrate the first observation of X-ray fluorescence resulting from inner-shell photoionization by a subpicosecond laser-produced plasma X-ray source. 8 keV electrons produced by a 100 fs laser at 3*1016 W cm-2 intensity create K-shell vacancies in a Ti layer. This generates a 4.51 keV K alpha X-ray pulse which is used to photoionize a Ca sample. Fluorescence of Ca at 3.69 keV (K alpha ) and 4.01 keV (K beta ) is observed as a result of Ti K alpha photoionization.

Contrasted behaviour of Stark-induced resonances in multiphoton ionization of krypton

Electron energy spectra from multiphoton ionization of krypton with 617 nm, 70 fs pulses have been recorded over a large intensity range. Near the appearance intensity, the spectra display Stark-induced resonances with s and d states. Up to about 8*1013 W cm-2, the spectra broaden and the structures are progressively washed out. Above this intensity, a strong resonance emerges, corresponding to the 4f state for which the static detuning is 2.94 eV, a value larger than the photon energy. The influence of a large static detuning and populations of excited states with shifts different from the threshold shift are discussed.

Independent and ambipolar tunneling in asymmetric‐coupled quantum well structures

We report femtosecond pump‐probe transmission spectroscopy experiments on GaAs/AlGaAs asymmetric‐coupled quantum wells, using a spectrally narrow optical excitation at 740 nm. The time‐resolved evolution of the bleaching demonstrates the tunneling of both carriers through the barrier. At high densities a single tunneling time is observed in opposition to the low density regime, where the electron and hole contributions are well separated.