Spatio-temporal nonlinear optics with ultra short laser pulses

The control and harnessing of ultra short laser pulses in space and time is a big challenge in photonics because the temporal as well as the spatial nonlinearities have to be controlled simultaneously. As the material dispersion leads to a temporal and spatial spreading of the pulses nonlinear effects have to be used to compensate for dispersion. By these means a diffraction free propagating light pulse called soliton is generated. The vision to stabilize a light pulse in time and in all three dimensions - an optical bullet - can revolutionize the smallest piece of information - a bit - in signal processing. Up to now real (3+1) dimensional optical bullets have not been found experimentally because to find the right combination of material and nonlinearity for the appropriate range of wavelengths and pulse durations is a great challenge in nonlinear photonics.

We examine different numerical and experimental approaches for spatio-temporal control of light pulses that use different nonlinearities and especially the structuring of materials seems to be promising to tailor light. In our lab "ultra short laser pulses" we use a mode-locked Ti:sapphire laser, a regenerative amplifier and an optical parametric oscillator to generate laser pulses with a pulse duration of either 120 fs or 2 ps in a wavelength range between 470 nm and 2.6 µm. Due to this large range of pulse durations and wavelengths we can exploit different nonlinear effects and dispersion regions depending on the material. Besides the light propagation in homogeneous nonlinear media the interaction of light with periodic structures is a possibility to control ultra short laser pulses in space and time. Periodic structures can be induced optically in crystals by a reversibl change of the refractive index through the photorefractive effect. A permanent pericodic structure can also be induced in crystals and glasses using ultra short laser pulses by direct laser writing. These techniques are not limited to periodic structures but also quasi-periodic and even random structures are under investigations.    

We have realized the spatial control of an ultra short laser pulse in a photorefractive Strontium Barium Niobate (SBN) crystal. The spatial spreading of the pulses is compensated for by a nonlinear refractive index change leading to a self focusing of the pulse. For the first time we have generated spatial photorefractive screening solitons in SBN crystals with ultra short laser pulses and we have examined the special properties of these solitons. By these means we even could produce solitons with infrared pulses which reveals the large potential for further applications. Actuel the spatial nonlinear interaction of ultra short laser pulses with periodic refractive index structures is under investigation.

The temporal control of short laser pulses plays an important role in data communications. A superposition of light induced refractive index gratings in photorefractive crystals can lead to super structures with a definite photonic resonance. Using this resonance of the periodic structure one can influence the group velocity of pulses to slow down light pulses.  

Optical structuring of crystals is possible through the photorefractive effect. By coherently superimposing two pulses in space and time one can induce a periodic refractive index change in form of a hologram in a photorefractive crystal. As the photorefractive effect is reversible the stored hologram is erased during read-out. To overcome this problem one can use a two-step process by sensitizing the crystal with blue light and recording the hologram with infrared light. During read-out the sensitizing light is switched off an the hologram can be read-out nondestructively whereas the hologram can be still erased with the sensitizing light. We have implemented this method of nonvolatile holographic storage with ps pulses in lithium niobate (LiNbO3) crystals. By changing the recording wavelength it is possible to record many gratings at the same place inside the crystal (wavelength multiplexing). Thus even more complex structures can be optically induced.