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Ultrafast Phenomena and Optical Communications Group |
The Ultrafast Phenomena Program is devoted to the study of femtosecond processes in materials. The main setup uses 10 fs probe pulses, generated in a setup based on a CPM laser, copper vapor laser pumped amplifier and grating pair pulse compressor. The repetition rate is 6 kHz and the central wavelength is 625 nm.
Materials of present interest for our group are: semiconductor quantum-dots in glass (CdTe, CdSeS, CdTeS, PbTe) and AlGaAs alloys.
Femtosecond dynamics of the Optical Stark Effect in CdSexS1-x doped glasses
Ultrafast recovery in CdTe quantum dots in glass
Intervalley scattering in AlGaAs
Femtosecond dynamics of the Optical
Stark Effect in CdSexS1-x doped
glasses
The sample studied was the semiconductor doped glass CS-2.62 from Corning Glass with a thickness of 500 µm Measurements were performed using the pump-probe technique. The sample was pumped by 60 fs duration pulses of 7 nm FWHM width, centered at 626 nm (1.98 eV) and with an energy of 350 nJ and the response of the dynamical processes in the system were probed by 12 fs duration probe pulses. The detuning between the central wavelength of the pump and the band gap is 110 meV. The pump and probe polarizations are orthogonal and the probe energy is 1/150 of that in the pump. The transmittance of the probe through the excited sample is monitored by an optical multichannel analyzer. Differential transmittance spectra are recorded as a function of the time delay between the pump and probe beams. The change in the sample transmission is measured by the difference in the transmission of the weak probe pulse with and without the presence of the pump pulse.
Figure 1. Differential absorption spectra for a CdSeS doped glass excited below the band edge. The upper trace shows the absorption edge, the pump spectrum and the probe pulse spectrum.
Figure 1 shows the experimental absorption change for the CdSSe doped glass CS-2.62 at 300 K for different relative time delays between the pump and probe pulses. Negative time delays indicate situations in which the probe peak precedes the peak of the pump pulse. In Figure 1 we also plot, in the upper trace, the linear absorption and the pump and probe spectra from which we can see that the sample is being excited far below its band gap. It can be seen that for large negative delays the spectra are unchanged from the linear absorption until overlap between the two pulses starts to occur. The curves from Figure 5 show that there is a dynamical increase in the sample transmission. When the measured absorption change for each delay is subtracted from the linear absorption spectrum one can observe that the resulting absorption edge is quickly shifted to the blue as the pump travels through the medium, reaching a maximum at zero delay and then returning almost to the non perturbed position. For positive delays, changes in the spectra are still observable until 500 fs, which was the largest delay investigated.
Figures:
Band edge shift as a function of the pump intensity
(published in: S. Tsuda and C.H. Brito Cruz, Appl. Phys. Lett.68,1093 (1996);