Dominik Maxein

Femtosecond time-resolved absorption processes in lithium niobate crystals

Femtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient beta(p) ranges from approximately 3.5 cm/GW for lambda(p) = 388 nm to approximately 0.1 cm/GW for 514 nm. In collinear pump-probe experiments the probe transmission at the double pump wavelength 2lambda(p) = 776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons (beta (r) = 0.9 cm/GW) leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump-probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of < or approximately equal to 0.1 ps.

Femtosecond holography in lithium niobate crystals

Spatial gratings are recorded holographically by two femtosecond pump pulses at 388 nm in lithium niobate (LiNbO3) crystals and read out by a Bragg-matched, temporally delayed probe pulse at 776 nm. We claim, to our knowledge, the first holographic pump-probe experiments with subpicosecond temporal resolution for LiNbO3. An instantaneous grating that is due mostly to the Kerr effect as well as a long-lasting grating that results mainly from the absorption caused by photoexcited carriers was observed. The Kerr coefficient of LiNbO3 for our experimental conditions, i.e., pumped and probed at different wavelengths, was approximately 1.0 x 10(-5) cm2/GW.

Light-induced scattering of femtosecond laser pulses in iron-doped lithium niobate crystals

Self-amplification of weak scattered coherent light waves in photorefractive crystals leads to losses, known as light-induced scattering or holographic scattering. We find with 532 nm light that it is reduced in LiNbO3:Fe for femtosecond laser pulses as compared to cw laser light. Light-induced scattering of pulses is completely absent in samples with sufficiently small Fe2+ content, in contrast to the scattering of cw light. Additional differences include a slower buildup time, a weaker Bragg selectivity, and a narrower angular distribution of the scattered light for pulsed illumination. The differences can be attributed mainly to the smaller temporal coherence of pulses.

Femtosecond recording and time-resolved readout of spatial gratings in lithium niobate crystals

Experimental and theoretical results on recording of spatial gratings with interfering femtosecond pulses at 388 nm and time-resolved Bragg-matched readout at 776 nm are presented for LiNbO3 crystals. These include dependences of the diffraction intensity on the time delay between probe and pump pulses, on the pump intensity and angle, and on the crystal composition. The grating buildup involves instantaneous and quasi-permanent changes of the refractive index and the absorption coefficient. These changes are due to Kerr and two-photon absorption effects and the modulation of photoexcited carriers, respectively. A good agreement between theory and experiment is achieved. Our analysis provides an understanding of nonlinear optical phenomena in LiNbO3 crystals on the femtosecond time scale. The theory is applicable to a large range of optical materials with modestly wide, ≈3to5 eV, bandgap.

Enhanced temporal resolution in femtosecond dynamic-grating experiments

Recording of gratings by interference of two pump pulses and diffraction of a third probe pulse is useful for investigating ultrafast material phenomena. We demonstrate, in theory and experiment, that the temporal resolution in such configurations does not degrade appreciably even for large angular separation between the pump pulses. Transient Kerr gratings are generated inside calcium fluoride (CaF2) crystals by two interfering femtosecond (pump) pulses at 388 nm and read out by a Bragg-matched probe pulse at 776 nm. The solution to the relevant coupled-mode equations is well corroborated by the experimental results, yielding a value of the Kerr coefficient of ~ 4.4×10^(–7) cm^2/GW for CaF2.

Photorefraction in LiNbO 3 :Fe crystals with femtosecond laser pulses

The photorefractive effect in lithium niobate crystals (LiNbO3) is investigated using femtosecond (fs) laser pulses. This research is triggered by an increasing interest in using LiNbO3 for nonlinear applications with high light intensities [1]. Such applications are affected by the photorefractive effect, since it can destroy the beam profiles and the phase matching conditions due to “optical damage” and “light-induced scattering”. To make measurements with ultra-high intensities, fs pulses are quite convenient. In addition, interesting photorefractive effects have already been discovered using short pulses and LiNbO3 [2, 3].