Laser Research

General

The vast majority of these activities goes back to common studies with graduate and doctoral students at Helmut-Schmidt-University and also to sabbaticals at IBM-Watson Research Center, Yorktown Heights, and the Oklahoma State University. These activities were mainly focusing on laser sources, laser spectroscopy and laser applications. Coherent light sources allow completely new approaches to study the structure and properties of materials. Over recent years this has contributed to a much deeper insight and more complete understanding of solids, fluids and gases. Lasers can be used as extremely narrow-bandwidth light sources for high resolution laser spec- troscopy in the frequency domain, and on the other hand - under special operation conditions - they can generate extremely short and bandwidth limited light pulses in the picosecond and femtosecond range, which allows time resolved studies on these time scales. In combination with coherent excitation phenomena in materials this also makes possible ultrahigh resolution measurements in the frequency domain. In recent years the Laser-Physics-Group (LPG) was mainly focusing on pulsed laser operation and respective applications. But also special continuous-wave laser set-ups were developed for gas sensor applications and experiments with trapped ions.

a) Ultrashort Laser Pulses in the Pico- and Femtosecond Range

Nonlinear interaction of laser radiation in optical crystals and glasses are successfully used for generating and detecting ultrashort light pulses. Particularly in micro-structures and glass fibers with peak intensities up to several GW/cm 2 nonlinear effects can favorably be used for reshaping or generating pulses on new wavelengths and to realize compact fiber lasers, which are wavelength tunable and can generate extremely short laser pulses. On the other hand these effects can seriously limit the bandwidth and transmission distance of optical fiber communication systems. Therefore, a deeper understanding of these effects and their mutual interrelations is a necessary prerequisite for these appli- cations.

b)

Time Resolved Laser Spectroscopy of Optical Transients

Optically excited atomic and molecular states spontaneously decay to lower states or the ground state, typically on a nanosecond time scale. But a sufficiently short excitation can also prepare coherent superpositions of sub-levels in the excited and lower state, which can be observed in the time domain as coherent transients on the free induction decay. These transients are oscillating with the respective sub-level splitting frequencies and are known as quantum beats. They allow Doppler-free measurements of splitting frequencies.

c)

Coherence Spectroscopy with Pulse Trains

A train of short light pulses can create an enhanced coherent superposition of nearly degenerate atomic states or sub-states, when the excitation rate or a higher harmonic coincides with the level splitting frequency. By slightly tuning the laser repetition rate ground state hyperfine splittings with extremely sharp resonances of only 30 Hz can be measured directly in the frequency domain.

d)

Spectroscopy with Trapped Ions

Ion-storage techniques have disclosed new dimensions of fundamental and high-accuracy experiments. So, with single ions the phenomenon of quantum jumps was demonstrated, and optically cooled ions were observed to form a crystal structure in the trap. Ion traps are also promising tools for ultrahigh resolution laser spectroscopy because ions can be observed over long periods without perturbations. Therefore, transit time broadening and collisional effects can largely be eliminated.

e)

Time-Resolved Spectroscopy of THz Coherent Transients

The newly developed technique of THz time-domain spectroscopy allows to study gases, liquids and solids in a frequency range which in recent years was only hardly accessible. The time response of TeraHertz-matter interactions can be studied by observing the pulse reshaping of femtosecond THz-pulses propagating through resonant and non-resonant media. By precisely modeling the pulse interaction with these samples one gets detailed insight into the material properties in the THz frequency range.

f)

Optical Sensors for Environmental and Medical Applications

Photo-acoustic gas sensors can successfully be applied in medical diagnostics as breath test analyzer or for environmental measurements as sensitive detector of trace gases and pollutants in the atmosphere. In the meantime photo-acoustic sensors allow detection of special gas impurities with concentrations less than 1 ppb (part per billion).

g)

Surface Cleaning and Micro-Structuring with Lasers

For cleaning sensitive surfaces like pigment lacquer, finishing varnish or art paintings and also for manufacturing all-polymer electronic components a prototype set-up based on a KrF-excimer laser with adapted beam homogenizer and scanning unit was developed.

h)

Opto-Electronic Components for Metrological Applications

Laser based systems were developed to measure velocities of gases and liquids with a laser anemometer , shifts and vibrations with interferometers or longer distances with laser range finders.

i)

Optical Communication through the Atmosphere

Optical communication systems have the potential of a higher transmission bandwidth and a higher directionality than radiowave and microwave systems. But they suffer from stronger atmospheric perturbation effects. The propagation and transmission of optical and infrared radiation through the atmosphere was studied under the influence of molecular absorption, turbulence and scattering processes.

j)

Spectral Calculation

A program platform, called MolExplorer, has been developed for fast computation and display of molecular spectra on a PC Microsoft Windows system. It takes advantage of a database, in which the relevant parameters for the calculation of a spectrum are stored. It provides survey spectra from mm-waves up to the ultra violet as well as selected parts with highest spectral resolution for relevant gases and pollutants in the atmosphere.

Physics & Climate