Nichtlineare Optik

Institut für Optik und Quantenelektronik Jena

Friedrich-Schiller-Universität Jena

The Nonlinear Optics group currently works on three research thrusts:

1. Quantum optics with single optical cycles:

The latest generation of femtosecond lasers is capable of generating light pulses that consist of less than two optical cycles in full-width at half-maximum. In addition, the phase of the field with respect to the envelope, the so-called absolute phase, can be controlled. This allows tailoring the temporal evolution of the electric field of the laser. Given that atomic and molecular processes induced by light are governed by the field rather than by the envelope, tuning the absolute phase offers an unprecedented degree of control for steering atomic and molecular processes by light. Moreover, investigating the phase-dependence of light matter interaction offers a new approach to the attosecond dynamics of strong-field laser matter interaction.

In previous experiments, we have shown this for photoionization. In fact, recording photoelectrons emitted in opposing directions can be used to measure the absolute phase. Another interesting experiment is based on the fact that, depending on emission direction, two or just one optical cycles may contribute to the ionization signal at a particular photoelectron energy. Entirely analogous to the case of the double slit, this leads to interference and the absence of interference. However, interference takes place in time, not space. An intriguing feature of this temporal version of the double slit is that interference and the absence of interference can be observed for the same electron depending on emission direction.

For this research, we are using a phase-stabilized few-cycle laser system. The laser is equipped with a booster amplifier and an optical parametric amplifier allowing experiments in the mid-infrared spectral region. In addition to that, there are several photoelectron spectrometers with various types of target gas sources.

2. Strong-field laser physics with ions beams:

Strong-field laser physics studies the interaction of intense laser fields with matter, in particular atoms and small molecules. A field is considered intense if ionization saturates within a few or even a single optical cycle. For neutral particles or singly charged ions, this means intensities below 1015W/cm2, whereas relativistic intensities beyond 1018W/cm2 are required for highly charged ions. Strong-field laser physics is known for several characteristic effects for which quite intuitive theoretical models have been developed.

More importantly, strong-field laser physics is the basis for attosecond laser physics. The very effects that are characteristic of strong- field laser physics are the basis for the generation of electron and soft-X-ray attosecond pulses and for attosecond metrology. At IOQ we study strong-field laser physics with different laser system and experimental setups. The most sophisticated approach is the use of fast, cold ion beams. Here it is possible to produce also molecular ions, e.g. H2+, which are of fundamental importance due to their simplicity. Using the most advanced detector technology, the momenta of the fragments of ionization and dissociation events can be measured in coincidence. Resembling the situation in high-energy physics, a (nearly) complete kinematic reconstruction of the reaction is possible.

3. Ionization dynamics at relativistic intensities

The project will study ionization dynamics at relativistic intensities with a novel approach. Using a fast, cold ion beam will eliminate all major experimantal limitations that have hamperes such research for a long time , most notably ionization of background gas. By measuring the recoil on a time- and position-senistive detector, the sum momentum of the electrons can be determined as well as the charge state of the ion. Particles from background ionization are spatially and temporally discriminated. A new regime of collective ionization is expected to be discovered under conditions where more than one electron is ionized during one optical cycle.

This project is part of Transregio 18 (B8).

4. Surface harmonic generation Laser intensities in the relativistic regime offer a variety of new approaches for solving long-standing problems.

One of them is generation of coherent extreme ultraviolet and X-ray radiation. To this end, the laser is simply focussed onto a solid surface. There, the laser creates a plasma interface that reflects the incident radiation. However, the plasma interface also oscillates at relativistic velocities thus modulating the reflected light. As a consequence, very high harmonics can be generated. The process has a relatively high efficiency and is fully coherent. For sufficiently high intensities, the harmonic emission is even diffraction limited, i.e. can be focussed to high intensities. In addition, the harmonics are phase locked thus making possible attosecond pulse generation.Generating a steep plasma interface at relativistic intensities calls for extremely clean laser pulses. We are working on improving our JETI terawatt laser in order to achieve a pulse contrast exceeding 1010 in the near future. This will enable the generation of surface harmonics and thus also attosecond pulses. One of our goals is surface harmonic generation in the relativistic regime at the full repetition rate of our laser.

So far, this has been possible only in single-shot operation. The next goal is a thorough characterization of the harmonics including their temporal structure, i.e. the measurement of attosecond light pulses. Finally, the harmonics can be applied for innershell spectroscopy. Even disregarding attosecond light bunching, surface harmonics are expected to belong to the most intense sources of X-ray radiation. Therefore, it is very likely that new classes of effects induced by such radiation will be discovered. Our activities in strong-field laser physics provide us with the equipment and experience to make use of the discovery potential. This project is part of Transregio 18 (A7).

5. From Compton scattering to strong-field electrodynamics

The objective of this project is the first detection of a nonlinear response of vacuum to macroscopic electromagnetic fields. The key observable is vacuum birefringence induced by a high-intensity laser field. Reaching the required sensitivity calls for a novel quasi-monoenergetic x-ray probe beam which will be generated via nonlinear Thomson scattering from laser accelerated electrons. The project is carried out in close collaboration with Profs. Gies and Wipf, who are responsible for in-depth theoretical analysis, and with Prof. Kaluza who is leading the POLARIS group.

Also this project is part of Transregio 18 (B7).