Prof. Dr. Erich ROHWER​

Erich Rohwer, Professor of Physics and Executive Head of the Physics Department at the University of Stellenbosch (South Africa), is visiting the Faculty of Physics and Astronomy as an Erasmus Mundus Visiting Professor in December 2011. During his stay he will give a series two lectures on modern problems in optics, starting on December 5th.

Prof. Erich Rohwer is Professor of Physics and Executive Head of the Physics Department at the University of Stellenbosch. He obtained his PhD in Laser physics from the University of Stellenbosch in 1992. Subsequently, he spent a sabbatical as a Postdoctoral Fellow of the Max Planck Gesellschaft (Max Planck Gesellschaft Stipend) at the Max Planck Institute for Extraterrestrial Physics in Garching. Since this time, one focus in his research activity in the Physics department of the Laser Research Institute has been on vacuum ultraviolet laser spectroscopy of molecules. As a result of this research, a number of publications have been produced, providing detailed information on ro-vibrational vuv spectra of the CO molecule, which is of interest to the astrophysics community.

Lecture 1: The Generation of Tunable vuv Radiation for Laser Spectroscopy

Time: June 9, 2011, 12:00
Place: Lecture hall 2, Helmholtzweg 5, 07743 Jena

Abstract: In this introductory lecture the development of a vuv laser source for vacuum ultraviolet laser spectroscopy of molecules will be discussed in detail. The tunable vuv output is obtained by four wave mixing of tunable dye lasers in metal vapour in a heatpipe oven. In this lecture some detail relevant to the four wave mixing process, such as resonance enhancement and phase matching will be discussed.

Audience: upper level undergraduate seniors, graduate students majoring in optics, physics
Pre-requisites: Electromagnetic waves, Optics, basic quantum mechanics

Lecture 2: State Selective ro-Vibrational Excitation of Electronic States in CO Using a Tunable VUV Laser Source

Time: June 16, 2011, 12:00
Place: Lecture hall 2, Helmholtzweg 5, 07743 Jena

Abstract: In this lecture the application of a tunable vuv laser source will be demonstrated focussing on the excitation of cold CO molecules. It will be shown that using Laser Induced Fluorescence techniques state selective excitation spectra (Rotationally resolved spectra of electronic excitations from the ground state) can be used generated. Moreover,  using this novel tuneable vacuum ultraviolet laser source with CO sample cooled in a supersonic gas jet even weak absorption signals of rare carbon monoxide (CO) isotopomers (in a natural sample) and forbidden singlet-triplet transitions of 12C16O could be studied. For the first time measurements of the spectra of the rare CO isotopomers could be obtained in a laboratory using this technique.
Furthermore, using the vuv laser source in conjunction with subsequent laser pulses, high lying Rydberg (Hydrogen like) states of CO could be measured. Molecules are prepared by excitation (absorption of the vuv photon), and subsequent visible dye laser photons are used to access targeted (allowed by selection rules) higher energy states via the intermediate states. The details of the techniques and methods used to attain the required resolution, selectivity and sensitivity will be discussed in detail.

Audience: upper level undergraduate seniors, graduate students majoring in optics, physics
Pre-requisites: Electromagnetic waves, Optics, basic quantum mechanics

Lecture 3: Second Harmonic Generation on Surfaces Generated by Femto Second Laser

Time: June 23, 2011, 12:00
Place: Lecture hall 2, Helmholtzweg 5, 07743 Jena

Abstract: In this lecture the relevant basic theory of Second Harmonic Generation will be discussed. The design of the setup used for the application of a femto second laser to generate Second Harmonic in a reflection geometry in order to be able to measure rotational anisotropy as well as a translational sample positioning for SH imaging will be discussed. SHG measurements performed on SiC thin films grown by chemical vapour deposition (CVD) on silicon for various polarization combinations will be discussed to illustrate the power of the rotational anisotropy technique to characterized crystalline structure of thin films.
A setup for spatially resolved imaging of SH from ZnO thin films on various substrates will be presented and results will be discussed. The imaging setup can also be used to probe PbxCd1-xTe wafers to show how SH imaging resolves a segregation into Pb- and Cd-rich sections. It will be shown that spatially resolved mappings of the wafer surface which are not only sensitive to sample composition, but also crystalline growth orientation can be obtained.

Audience: upper level undergraduate seniors, graduate students majoring in optics, physics
Pre-requisites: Electromagnetic waves, Optics, basic quantum mechanics

Lecture 4: Electric Field Induced Second Harmonic Generation

Time: June 30, 2011, 12:00
Place: Lecture hall 2, Helmholtzweg 5, 07743 Jena

Abstract: In this lecture the relevant basic theory of Electric Field Induced Second Harmonic (EFISH) technique will be discussed. In the case of of centrosymmetric crystalline structure such as the diamond like structure of silicon SHG in the bulk material is forbidden. The design of the setup used for EFISH measurements on Si/SiO2 interface will be presented. SH measurements were performed on natively oxidized silicon wafers oriented in the 100 surface direction. Since the ultra-thin (<5 nm) SiO2 layer is transparent for the employed near infrared (NIR) laser frequencies, the incident laser light is transmitted through the SiO2 layer and the SH signal is generated at the buried Si/SiO2 interface. The experimentally observed SH signal response of the Si/SiO2 interface appears to be a function of time. This observation can be explained by laser induced electron and hole injection from the silicon substrate into the SiO2 layer. This photo induced charge separation process results in an electric field across the interface, which significantly affects the interfacial SH response. The measured temporal SH evolution thus reflects the dynamics of the charge carrier separation process. Furthermore, investigation of the time dependent SH response reveals that pre-irradiated samples show a drastically accelerated SH response. This observation can be exploited for defect imaging by SHG and it will be shown that EFISH measurements can be used for the spatially resolved visualization of electrically active defects apparent in ultra-thin (< 5nm) Si/SiO2 interfaces.

Audience: upper level undergraduate seniors, graduate students majoring in optics, physics
Pre-requisites: Electromagnetic waves, Optics, basic quantum mechanics

Lecture 5: Probing the Si/SiO2 Interface Using EFISH Technique

Time: December 5, 2011, 16:00
Place: Seminar room 6, Helmholtzweg 5, 07743 Jena

Abstract: In this lecture the results from Electric Field Induced Second Harmonic (EFISH) measurements of SHG for the Si/SiO2 interface in reflection and transmission will be discussed. In the case of of centrosymmetric crystalline structure such as the diamond like structure of silicon SHG in the bulk material is forbidden. SH measurements were performed on natively oxidized silicon wafers oriented in the 100 surface direction. Since the ultra-thin (<5 nm) SiO2 layer is transparent for the employed near infrared (NIR) laser frequencies, the incident laser light is transmitted through the SiO2 layer and the SH signal is generated at the buried Si/SiO2 interface. The experimentally observed SH signal response of the Si/SiO2 interface appears to be a function of time. This observation can be explained by laser induced electron and hole injection from the silicon substrate into the SiO2 layer. This photo induced charge separation process results in an electric field across the interface, which significantly affects the interfacial SH response due to the χ3 of Si. The measured temporal SH evolution thus reflects the dynamics of the charge carrier separation process. Measurements observing SHG in reflection geometry reveal that multiphoton processes determining the electric field across the Si/SiO2 interface due to laser irradiation.
Recently measurements were performed where the SH was measured in transmission through thin Si membranes. Measurements revealed unexpectedly high yield in SH signal, generated at the exit surface Si/SiO2 interface.

Audience: upper level undergraduate seniors, graduate students majoring in optics, physics
Pre-requisites: Electromagnetic waves, Optics, basic quantum mechanics

Lecture 6: Pump Probe Experiments with Photons and Electrons 

Time: December 12, 2011, 16:00
Place: Seminar room 6, Helmholtzweg 5, 07743 Jena

Abstract: In this lecture the concept of pump-probe spectroscopy will be briefly discussed. The experimental setups developed by the ultrafast spectroscopy group at the Laser Research Institute will be introduced. The setup for ultrafast transient absorption spectroscopy will be discussed. The excitation by fs light generated in a NOPA is used as a pump and white light generated in CaF provides the probe pulse for the transient absorption experiments. Results of experiments will be briefly discussed. The development of a white light source, generated in a photonic crystal fiber, will also be discussed briefly. The development of an ultra-short pulsed electron gun has been a recent development which provides a novel facility. The layout of the ultrafast electron diffraction setup will be introduced and first results will be discussed.

Audience: upper level undergraduate seniors, graduate students majoring in optics, physics
Pre-requisites: Electromagnetic waves, Optics, basic quantum mechanics