The effects of Coherent excitation of a two level system with a linearly chirped pulse are studied theoretically and experimentally (in Rb (5s -5p)) in the low field regime. The Coherent Transients are measured directly on the excited state population on an ultrashort time scale. A sharp step corresponds to the passage through resonance. It is followed by oscillations resulting from interferences between off-resonant and resonant contributions. We finally show the equivalence between this experiment and Fresnel diffraction by a sharp edge.Coherent excitation of atomic transitions is a general phenomenon occurring whenever electromagnetic radiation interacts with atoms. This interaction may result in various kinds of processes, for example Rabi oscillations, free induction decay, adiabatic population transfer and Coherent Transients (CT) [1]. By simply varying the temporal shape of the pulses, a great variety of systems can be manipulated such as spins in Nuclear Magnetic Resonance [2] or atomic and molecular systems in coherent control schemes [3]. We consider here the simplest case of linearly chirped pulses where the "instantaneous frequency" drifts in time.Depending on the frequency sweep and on the intensity, adiabatic following can be observed with a significant population transfer. For example total population transfer has been achieved via multilevel ladder climbing [4][5][6][7][8][9] and Stimulated Raman Adiabatic Passage [10].On the contrary if the adiabaticity criterion is not fulfilled, the final population depends crucially on the pulse integral and CT dominate the interaction. This letter presents theoretical and experimental studies of these Coherent Transients. Although many works have dealt with the transfer efficiency into the final state, we report here the first direct observation of these transients on a subpicosecond time scale. The CT result in oscillating population. This is a signature of interferences between resonant and non-resonant excitation. A detailed understanding of this behaviour is crucial to analyze pumpprobe experiments in which dynamics takes place on the same time scale as laser interaction. Due to the difficulty to control properly the laser pulses, the effects of CT are most often ignored [11].Similarities can be found with optical CT observed in the experiments of free induction decay or photon echoes which provide relaxation rates [12][13][14]. In these experiments, they observed on the transmitted optical signal, a beat between initially induced polarisation of the medium at resonance and a near-resonant laser. In the particular case of a single chirped optical pulse, the beat frequency increases linearly after passage through resonance, following the increase of resonance mismatch [15]. On the contrary to these previous studies, the present report involves CT directly observed on the population before relaxation becomes significant. The beats result here from interferences between the resonant and non-resonance excitation paths. The passage through resonance does not ...

This paper reports on phase transition photo-induced by a nanosecond laser pulse in the molecular spin crossover material [Fe(NH(2)-trz)(3)] (Br)(2).3H(2)O (with NH(2)trz = 4-amino-1,2,4-triazole) around room temperature and in the close vicinity of the thermal hysteresis loop. The measurements are carried out using a time-resolved pump-probe experiment and by recording the reflectivity change at various temperatures and laser intensities. The dynamics of the optically induced reflectivity changes are presented and discussed. We propose a simple model that describes well the recorded phenomena. It takes into account the physical and optical properties of the sample that directly impact the amplitude and the dynamics of the laser-induced heating of the compound.