A compact excimer-dye laser system is developed for subpicosecond pulse generation at 193 nm. In this setup a double-discharge excimer laser (EMG 150) is used both for pumping the seed-pulse generator and for amplification. The seed-pulse generator is a distributed-feedback dye-laser-based subpicosecond dye laser system, generating a 400 fs pulse at 537 nm and a broadband nanosecond pulse centered at 690 nm. By sending these pulses through a H2 Raman cell in a collinear beam, part of the red pulse is amplified, which coincides with the spectral and temporal gain window of the Raman amplifier pumped by the transform-limited 537-nm pulse. Phase-matched frequency mixing of these Raman-amplified pulses with the frequency-doubled 537-nm pulses results in seed-pulse energies of more than 0.1 jJ at 193 nm, with good spatial, spectral and amplitude stability. By double-pass amplification of the seed pulses in the amplifier section of the EMG 150 excimer laser, typically 0.5 mJ output pulses of -800 fs duration are obtained in a 6 mm diameter circular beam. Additional pulse compression in a prism compressor resulted in significant decrease of the pulse duration to less than 400 fs.

Generation and amplification of femtosecond laser pulses is discussed. We discribe different amplification schemes and present detailed results on the spatial and temporal properties of the amplified pulses. Pulses of approximately 100 fs and peak intensities of several times 1O are obtained.

This talk is not a survey of the numerous papers recently published but rather a brief description of some recent researches carried out by my closed colleagues and myself. I would like to popularize "electromagnetic optics" by giving an idea of its methods, its possibilities . . . and also its limits. -

Huge dynamic Stark shifts of atomic energy levels in xenon induced by high-intensity laser light have been mesured by a new method based upon multiphoton ionization by a tunable femtosecond laser source. Stark-induced multiphoton resonances appear as identifiable structures in the photoelectron energy spectra1. The change in the structures positions, when the photon energy is tuned, is directly related to the energy change of the atomic levels as a function of intensity. Shifts of the order of the electron-volt have thus been measured in xenon and found to vary linearly with intensity.

Microsecond pulse trains with pulse duration < 1 ps are generated by a Nd:glass laser oscillator with 15 Hz repetition rate. Application of these pulses to non-cw synchronous pumping of a dye laser yields intense pulses of 25 fs. Subsequent nonlinear frequency conversion provides femtosecond pulses tunable over the visible spectral range. The pulsed approach is compared with cw modelocking techniques.

In this paper we report the generation of femtosecond pulses in the near infrared by passive mode locking of a Rhodamine 700 dye laser.Two saturable absorbers were used to produced pulses from 775 to 800 nm. In a short cavity to avoid a multiple pulses regime, pulses shorter than 50 fsec have been obtained.

Two versions of the singly resonant parametric oscillator (OPO) are investi- gated, synchronously pumped by microsecond pulse trains of a feedback controlled Nd:glass laser. The first setup utilizing a LiNbO crystal generates narrow-band, sub-ps pulses with tuning range 1 .35 - 2. 1 1 pm and 5 % conversion efficiency. The second OPO-system applies a BBO specimen and a novel double pass pump geometry with partial group delay compensation. Broadly tunable pulses are generated in the wavelength range 0.7 - 1.8 pm with duration between 160 and 260 fs and energy conversion of 3 %. Steep pulse wings are observed with l/e decay time of 90 fs. Under special conditions parametric pulses as short as 65 7 fs are obtained.

We report on the design and performance of an improved copper vapor laser pumped amplifier for femtosecond optical pulses. Consideration of the large duration mismatch between pump and seed pulses, the short energy storage time of the gain dye, spatial mode matching, and polarization effects led to an amplifier design of 1.5% efficiency, which is greater than previous designs. Pulses of > 10 tJ energy and -65 fsec duration are generated that are focussable to less than twice the diffraction limit. The multiple pass amplifier incorporates optimized mode matching, an antireflection coated flowing dye cell, and a polarized, spatially smoothed pump beam.

We report the generation of a variety of periodically modulated pulse trains in the CPM dye laser. These pulses are reminiscent of high-order solitons. We have identified the nonlinearity which prevents the formation of pulses which are solutions of the nonlinear Schrodinger's equation. A modification of the laser cavity removes this nonlinearity and true N = 2 solitons have been generated.

We want to make the best use of the mesoscopic or macroscopic transition dipolemornent of excitons by controling the size and dimension of the materials. First, the semiconductor microcrystallites embedded in insulators or glasses are shown to have the mesoscopic transition dipolemoment of excitons1 . As a result, the exciton in CuC1 microcrystallite with the radius 80 A superradiatively decays in 100 pico-second2. Futhermore, the exciton-exciton interaction and the decay and relaxation of the exciton work effectively in deviating the excitons in the microcrystallite from ideal bosons and this system shows the large third-order optical susceptibility"3 of an order of iO esu when we assume the 0.1% packing of the CuC1 microcrystallites. These predictions have been confirmed expriment ally4'5. Second, the two-dimensional excitons in quantum wells can superradiatively decay in the direction perpendicular to the surfaces2 . This brings about the quick response and the deviation of the excitons from ideal bosons at the same time. The latter effect as well as the two-dimensional macroscopic enhancement of the exction transition dipolemoment enhance the third-order optical susceptibility3. Both the superradiative decay rate and increase as does the coherent length, which is inversely proportional to the square root of the exciton line width6.

We report time-resolved pump-probe measurements of both absorption and refractive index for GaAs thin films with 80 fs time resolution. The probe photon energies are near the band-edge and near the initial excited states, and photoexcited carrier densities are varied from 1017 to 1019 cm3. Our results reveal the instantaneous band gap renormalization and the importance of carrier-carrier scattering and intervalley scattering. Mechanisms which cause an ultrafast change in refractive index are also discussed.

We study new structures peaked at the giant two-photon absorption (GTA) of biexcitons and peaked at h1P in the exciton region which appear in the excitation spectrum for the emission at h w of the biexciton origin in CuC1 at 2 K in the excitation photon density of ' 5Mw/cm2. An energy relation, thw1P . + ALT is obtained, where b and ALT are the binding energy of a biexciton and the L-T splitting energy of an exciton, respectively. Existence of a complex of biexciton-polariton or triexciton is implied as an interpretation of this phenomenum. Discussion is presented in relation to the broad band at GTA.

Spatial and polarization dependence of the beam diffracted by a transient phase grating induced in a transparent glass enables, for the first time, evidence of a fifth order nonlinear process. Discrimination between direct high order phenomena and cascade contributions due to lower nonlinearities is discussed in view of an anharmonic oscillator model expanded up to sixth order.

We have used the picosecond optoelectronic switching technique to study carrier recombination times and mobilities at different conditions in semiconductors. Recombination times were measured by correlation technique, e.g. linear and non'inear recombination rates. The photoconductivitv method was also used to measure the dependence of the carrier mobility on the applied field strength in GaAs The mobility was determined from the slope of the v(E curve. Using this technique we were able to distinguish between the carrier mobilities in GaAs in different valleys of the conduction band at different preparation conditions. The PS optoelectronic switching technique was also used to measure the response of a fast Dhotodiode of a 6 8Hz transistor and the dispersion of a ps electrical pulse 1on a cable.

The interest in fast risetime, high power photoconductive devices stems from several potential applications including pulse, impulse, and high power microwave generation [1, 2]. As the voltage becomes large and the switching speed becomes extremely fast, the generation of high power electrical pulses using photoconductive devices becomes very complicated because of effects such as surface flash-over, stray inductance, stray capacitance, and abrupt impedance transitions. Two ap- proaches were taken to produce maximum output voltage amplitude without degradation in switch risetime. A coaxial transmission line with a switch holder filled with a high dielectric strength liquid was fabricated for long pulsewidths ( 2 ns) . For the narrow pulsewidths ( 2 ns) a quasi-radial transmission line was fabricated. By incorporating a photoconductive bulk GaAs device with opposite gridded elec- trodes into these specially designed energy storage elements, and illuminating the switch with a mode-locked Nd:YAG laser, electrical pulses with risetimes of several picoseconds have been realized.

Femtosecond pump-probe experiments have been performed on asymmetric GaAs/AIGaAs quantum well structures, using a spectrally narrow excitation beam whose center wavelength could be tuned between 740 and 800 nm. The respective widths of the wells were chosen to exhibit n=1 and n=2 excitonic absorptions of the wide well clearly separated from the n'=l narrow well excitonic absorption line, which lies in between the two others. We monitor the time evolution of the photoexcited carrier population in both wells by measuring the variations of the sample transmission. This method is another approach to the study of the tunneling transfer between coupled quantum wells, mostly addressed by use of time-resolved photoluminescence techniques. The sample recovery after excitation shows drastic changes depending on the pump intensity. At high intensities, all three transitions are bleached and we observe an important band-filling. At low intensities we see a decrease with time of the carrier population in the narrow well, along with a simultaneous increase in the wide well. This shows that carriers tunnel through the barrier with a characteristic time much smaller than the recovery time measured at high intensities, and thus demonstrates that the presence of a large number of carriers dramatically reduces the tunneling efficiency between the two wells.

Transient pump-probe reflectivity measurements are performed on crystalline and amorphous Silicon samples with 50 fs optical pulses at 2 eV. The excited carrier densities range from 1017cm3 up to a few 1021cm3. In both cases the reflectivity signal is dominated by a Drude-like carrier response. Crystalline Silicon shows a distinct subpicosecond feature due to the cooling of the optically excited hot carriers with a time constant of 200-300 fs. Diffusion and Auger-recombination come into play at higher carrier densities. A superlinear increase of instant reflectivity signal with excitation fluence is due to two-photon absorption (TPA) with a TPA-coeffiecient f:37+-5 cm/GW. In amorphous Silicon the TPA process is not observable. The recovery of the induced negative reflectivity changes is dominated by trapping into bandtail and defect states at lower carrier densities. At higher densities a non-radiative recombination process dominates the relaxation of free carriers in both materials. Comparison with crystalline Silicon clearly demonstrates the enhancement of the Auger-recombination process in disordered materials by more than an order of magnitude.

Time resolved measurements of the hot carrier relaxation in nP have been performed at room temperature using four different femtosecond techniques. At carrier densities between 1016cm3 and a few times 1018cm3 carrier-carrier scattering has been found to dominate the initial relaxation ensuring the internal thermalization of electrons and holes on a time scale of 1OO'2OO fs. At high excitation densities the subpicosecond cooling of the electrons is found to be clearly slower than expected from simple calculations of the e-LO-phonon interaction. The different relaxation behaviour in GaAs is attributed to strong intervalley scattering mechanisms.

Electron and hole tunneling transfer processes in asymmetric double quantum well structures are investigated by time-resolved picosecond photoluminescence. Change from nonresonant to resonant tunneling is achieved with a perpendicular eleitric field. Electron transfer times decrease if the two electron subbands in the two quantum wells are energetically aligned. Both nonresonant and resonant electron transfer times decrease strongly with the barrier thickness. The resonant times are more than one order of magnitude smaller than the nonresonant times for the same barrier thickness. The buildup of delocalized coherent states at resonance would lead to much shorter resonant transfer times than we observe experimentally. This discrepancy is discussed in terms of state broadening. Hole transfer times also decrease at specific electric fields showing a resonance feature, which can be attributed either to the resonant transfer of n=1 heavy holes to an n=1 light hole level or to the onset of the optical phonon scattering from the n=1 heavy hole level to the neighbouring n=1 heavy hole level.