e) Time-Resolved Spectroscopy of THz Coherent Transients

In recent years experiments with ultrashort pulses were almost exclusively restricted to the visi - ble and infrared spectral range. However, the newly developed terahertz beam sources, produ - cing sub picosecond pulses of terahertz radiation, have made accessible a new frequency range for time dom ain spectroscopy and the studies of fast transients. These pulses essentially consist of a single cycle over the pulse duration of typically 300 fs, and are characterized by a transform-limited white spectrum extending from low frequencies up to 5 THz. This frequency range, midway between the microwave and infrared frequencies, is im - portant owing both to the samples that can be investigated and to the unusual experimental conditions encoun tered with these THz pulses. They allow to study gases, liquids and solids in a frequency range, which in recent years was not or only hardly accessible. The time response of TeraHertz-matter interactions can be studied by obser ving the pulse reshaping of femtosecond THz-pulses propagating through resonant and nonreso nant media. By precisely modeling the pulse interaction with these samples the mea - sure ments can be analyzed directly in the time domain or parallel in the frequency domain by Fourier transforming the pulses and comparing them with theoretical spectra. This gives de - tailed insight into the material properties in the THz frequency range.

Generation and Detection of THz-Pulses

Meanwhile different techniques are known to generate and detect THz radiation, e.g., by mixing op tical frequencies on an opto electronic chip or using nonli n ear crystals as sour - ces and de tectors. A very successful technique for generating and detec - ting these pulses with ex - cellent signal-to-noise ratios is to use photo-conducting antenna chips irra diated with ultra short laser pul ses. When the transmitting chip is additionally supplied with a voltage, the incoming light pulses instan taneously create photo carriers which are accelerated in the sup plied electric field. The rapid chan ges in conductivity result in short pulses of the electric current, and it is well known from classical electrodynamics that a changing current causes a radiation field which is pro - portional to the time derivative of the current. The so-called Hertzian dipole antennas are fabricated on an ion-implanted silicon-on-sapphire wa fer and are located in the middle of two coplanar transmission lines separated from each other by 10 µm. The antenna gap is 5 µm wide and photoconductively shortened with 60 - 70 fs laser pulses coming at a rate of 100 MHz from a colliding pulse mode-locked (CPM) dye laser or a Ti:Sapphire la ser. The emitted TeraHz radi ation is collimated by a silicon lens and parabolo idal mirror into a highly directional beam with a 25 mrad divergence. After a 50 cm propaga tion distance this beam is fo - cused by an identical combination of mirror and lens onto the receiving antenna chip which in this case is of the same design as the transmitter. The electric field of the THz radiation induces a voltage across the gap and thus replaces the voltage source of the transmitting antenna. The actual electric field of a THz pulse is now samp - led by photo conductively shorting the antenna gap with laser pulses from a detection beam and monitoring the respective photocurrent in the antenna as a function of the time delay between the optical excita tion and probe pulse.

Special Properties and Advantages of Time Resolved THz Spectroscopy

• The measured average antenna current directly represents the electric field amplitude of the pulses in contrast to the optical region, where only intensities are registered and the phase in formation of the field is lost. • The pulses are measured with a time resolution of 65 fs. • It's particularly noteworthy that these pulses are detected with a signal-to-noise ratio of better than 5.000:1, which for the power corresponds to a ratio of 25 Mio:1. The average power of the THz radiation is approximately 10 nW and therefore the NEP of the system only 10 -15 Watt/Hz 1/2 . • Compared with a helium cooled bolometer this setup is more than 1.000x more sensitive, al though it is operated at room temperature. • The reason for this high sensitivity is mainly due to the subpicosecond gating at the detector measuring only events within this short time interval, and on the other hand the integration over more than 10 Mio pulses at a detection bandwidth of 1 Hz at the current amplifier. • This coherent detection almost completely suppresses thermal or shot noise, processes limi ting the conventional techniques. • The THz pulses are unique in terms of their pulse duration and corresponding bandwidth. • They essentially consist of a single cycle and are characterized by a transform-limited white spectrum, in this case extending up to 5 THz. • In this frequency range up to now no proper radiation for time resolved measurements of fast transients is available, and • no other system with this S/N ratio, high sensitivity and broad spectral range is known.

Measurement of Gases

Almost all polar molecules have their rotational absorption lines in this spectral range and therefore can be investigated with these exceptional pulses. As an example of a measurement with symmetric top molecules is shown a plot with 10 hPa methyl fluoride in a gas cell. For the analysis of such measurement two scans are necessary, one without the vapor to determine the input pulse, and a second one with the vapor in the gas cell, to regist er the pulse being re - shaped and attenu ated due to the pro - pagation though the sample. At this low pressure the input pulse is only slightly affected, but some signi - ficant changes in the pulse structure are already observed. The main pulse is followed by a series of coherent transients reradiated by the vapor every 20 ps after the excitation pulse. This phenomenon is best under stood when switching to the frequency do - main. The spectrum of the input pulse is shown in the lower plot as the green line in direct comparison with the ab - sorption spectrum of MF. The THz pul - se which co vers this full spectrum, ex - cites the mole cu les simultaneously on a multitude of pure rotational lines and the molecules respond to this excita - tion as a phased array of oscil la tors by re-radi ating a free-induction decay sig - nal consisting of coherent tran si ents. Because the MF molecule is distin guished by absorp tion lines with an almost constant frequen - cy separation, a periodic rephasing and depha sing of the entire ensemble of more than 50 exci - ted transitions occurs during the free induc tion decay, mani fest as a train of subpsec THz pulses with a repetition rate equal to the frequency separation between adjacent lines. This situation is quite similar to mode-locking of lasers. Because of the rephasing of the dipoles and the commensurate frequencies involved, we term these periodic pulses " THz commensurate echoes ".

What New Informations are Derived from THz-Spectroscopy?

• In the THz frequency range we observe a transition from the classical collision theory with a Lorentzian lineshape to a Van Vleck-Weisskopf line shape of collisionally broadened lines. • From this lineshape we get informations on the temporal response of the polar molecules which can align to the electric field of the THz pulses in the presence of molecular colli - sions. • The alignment characterizes the thermalization to the THz-field in the presence of colli - sions. • From this time response we derive the molecular collision time. • Also can be measured the absorption and collisional broadening at zero frequency origi - nating from molecular tunneling. • New analyses for the molecular re spon - se time allow a generali zed unification of the basic collision and line-shape theories of Lorentz, van Vleck-Weiss - kopf and Debye which can be described by our molecular response theory (MRT). • This shows that the applied THz experi - mental set-up allows the direct obser - vation of the ulti mate time response of mole cules to an external applied electric field in the presence of molecular collisions. • This response is limited by the uncertainty principle and is determined by the inverse of the spitting frequency between adjacent levels.

Doctoral Theses

J. Zhao Zeitaufgelöste Terahertz-Spektroskopie an Polymeren, Silizium und Ammoniak School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 2003, Fortschr.-Ber. VDI Reihe 8 Nr. 1010, Düsseldorf, VDI-Verlag 2003, ISBN: 3-18-501008-6 B. Heinemann Weiterentwicklung und Einsatz eines Mess-Systems mit asynchroner Abtastung zur zeitaufgelösten Terahertz- Spektroskopie an Gasen School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 2014

Refereed Publications in Journals and Conference Digests

H. Harde, S. Keiding, D. Grischkowsky THz Commensurate Echoes: Rephasing of Molecular Transitions in Free-Induction Decay Physical Review Letters 66, p. 1834 (1991) H. Harde, S. Keiding, D. Grischkowsky THz Coherent Transients from N 2 O Vapor in ''Quantum Electronics Laser Science'', 1991 Technical Digest Series (Optical Society of America, Washington, D.C. 1991), p. 248 (1991) H. Harde, D. Grischkowsky Coherent Transients Excited by Subpicosecond Pulses of Terahertz Radiation Journal Optical Society America B 8, p. 1642 (1991) H. Harde, D. Grischkowsky Time-Domain Spectroscopy of Molecular Vapors with Subpicosecond Pulses of THz Radiation in:''Ultrafast Processes in Spectroscopy'', Institute of Physics Conference Series No. 126, ed. by A. Laubereau and A. Seilmeier (IOP Publishing Ltd, Bristol, 1992), pp. 217-222 (1992) D. Grischkowsky, N. Katzenellenbogen, H. Harde Time-Domain Spectroscopy with Femtosecond Pulses of THz Radiation in: ''Nonlinear Optics: Materials, Fundamentals and Applications Technical Digest, 1992'' (Optical Society of America, Washington, D.C., 1992), Vol. 18, pp. 187-188 (1992) H. Harde, N. Katzenellenbogen, D. Grischkowsky THz Time-Domain Spectroscopy of Methyl Chloride Vapor in: ''Quantum Electronics and Laser Science Conference'', 1993 OSA Technical Digest Series, Vol. 3 (Optical Society of America, Washington DC), p. 89 (1993) H. Harde, N. Katzenellenbogen, D. Grischkowsky TeraHz Coherent Transients from Methyl Chloride Vapor Journal Optical Society America B 11, 1018 (1994) H. Harde, N. Katzenellenbogen, D. Grischkowsky Time-Domain Studies in Dense Molecular Vapors with Femtosecond Pulses of TeraHz Radiation in: ''5th European Quantum Electronics Conference'', Technical Digest, IEEE Catalog No. 94TH0615-5, ISBN: 0-7803-1791-2, Piscataway, p. 253 (1994) H. Harde, N. Katzenellenbogen, D. Grischkowsky Line-Shape Transition of Collision Broadened Lines Physical Review Letters 74, 1307 (1995) H. Harde, R. A. Cheville, D. Grischkowsky Collision Induced Orientation of Polar Molecules in the Field of Femtosecond Terahertz Pulses Proceedings of the 1996 European Quantum Electronics Conference, IEEE Catalog No. 96TH8162, ISBN: 0-7803-3171-0, p. 24 (1996) D. Grischkowsky, R. A. Cheville, H. Harde THz-Studies of Rotational Line-Shapes Quantum Electronics and Laser Science Conference, Vol. 12,1997 OSA Technical Digest Series (Optical Society of America), Washington DC, p. 82 (1997) H. Harde, R. A. Cheville, D. Grischkowsky Collision-Induced Tunneling in Methyl Halides Quantum Electronics and Laser Science Conference, Vol. 12, 1997 OSA Technical Digest Series (Optical Society of America), Washington DC, p. 115 (1997) H. Harde, R. A. Cheville, D. Grischkowsky Terahertz Studies of Collision-Broadened Rotational Lines Feature Article in: Journal of Physical Chemistry A, 3646-3660 (1997) H. Harde, R. A. Cheville, D. Grischkowsky Collision Induced Tunneling in Methyl Halides J. Opt. Soc. Am. B 14, 3282 (1997) H. Harde, R.A. Cheville, M. Wolff, D. Grischkowsky Line-Shape Studies of Zero-Frequency Transitions Proceedings of the 1998 European Quantum Electronics Conference, IEEE Catalog No. 98TH8326, ISBN 0-7803-4233, p. 177 (1998) H. Harde, J.-X. Zhao, M. Wolff, A. Cheville, D. Grischkowsky THz Time-Domain Spectroscopy on Ammonia 7. European Conference on Atomic and Molecular Physics (2001) H. Harde, J. Zhao, M. Wolff, R. A. Cheville, D. Grischkowsky THz Spectroscopy on Ammonia Conference on Lasers and Electrooptics (CLEO), Technical Digest Series 15, pp. 324-325 (2001) H. Harde, J. Zhao, M. Wolff, R.A. Cheville, D. Grischkowsky THz-Time-Domain-Spectroscopy on Ammonia J. Phys. Chem. A, 105 (25), pp. 6038-6047 (2001) H. Harde, J. Zhao, J. Pfuhl Ausbreitung von THz-Impulsen in resonanten und nichtresonanten Medien Symposium „Simulation in Physik, Informatik und Informationstechnik“, 66. Physikertagung, Leipzig 2002, Herausg. H. Hofman, ISSN: 0944-7121, 27-30 (2002) B. Heinemann, C. J. Fox, H. Harde Improved Fast Scanning TeraHz Pulse System Proceedings of XVIth Intern. Conference on Ultrafast Phenomena, Stresa, Italy, 09-13 June, 2008 H. Harde, D. Grischkowsky Molecular Response Theory in Terms of the Uncertainty Principle J. Phys. Chem. A 119 (34), pp 9085–9090 (2015). https://pubs.acs.org/doi/10.1021/acs.jpca.5b05909

Contributions on National Conferences and Meetings

H. Harde, D. Grischkowsky Zeitaufgelöste THz-Spektroskopie an dichten Gasen Poster Lasertag 1994, Hannover, 7. Dezember 1994 H. Harde, D. Grischkowsky, R. A. Cheville Messungen und Modellbildung zur stoßinduzierten Thermalisierung von Molekülen Poster Lasertag 1995, Hannover, 6. Dezember 1995 H. Harde Erweiterte Theorie für molekulare Stöße Lasertag 1996, Hannover, 11. Dezember 1996 H. Harde, D. Grischkowsky, R. A. Cheville Stoß-induziertes Tunneln in Methylhalogeniden Frühjahrstagung der DPG, Sektion Quantenoptik, Mainz, 6. März 1997, Verhandl. DPG (VI) 32, Q 39.5, 349 (1997) H. Harde, D. Grischkowsky, R. A. Cheville Untersuchungen mit Femtosekunden THz-Impulsen zum stoßinduzierten Tunneln in Molekülen Poster Norddeutsches Laserkolloquium 1999, Hannover, 9. Dezember 1999 H. Harde, J. Zhao, M. Wolff, A. Cheville, D. Grischkowsky THz Time Domain Spectroscopy on Ammonia Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Fachverband Molekülphysik, Berlin, 2.-4. April 2001, Verhandl. DPG (VI) 36, M-Methoden (2001) J. Zhao, H. Harde Characterization of Plastic Materials in the Terahertz Frequency Range Poster, Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Fachverband Quantenoptik, Berlin, 2.-4. April 2001, Verhandl. DPG (VI) 36, Q-Ultrakurze Pulse (2001) J. Zhao, H. Harde Untersuchungen von Polymeren im THz-Frequenzbereich Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Osnabrück, 4.-8. März 2002, Verhandl. DPG (VI), Q 434.19 (2002) H. Harde, J. Zhao, M. Wolff, R. A. Cheville, D. Grischkowsky Stoßprozesse und (NH 3 ) 2 –Dimerenbildung auf einer fs-Zeitskala Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik, Verhandl. (DPG) (VI) 37, Q 434.20 (2002)

g) Surface Cleaning and Micro-Structuring with Lasers

Previous studies have shown that excimer lasers are efficient and safe tools to remove securely and controlled layers of a certain thickness from the surface of icons and paintings. Excimer lasers offer significant advantages for sur - face cleaning processes, because their short wave - lengths (e.g. 248 nm) can be focused to a smaller spot size than visible or infrared light. With each pulse the UV light can remove quite pre - cisely about 0.2 µm of dirty material . In com parison: A human hair is about 50 - 100 µm thick. And for some applications even more important (different to some other laser types), excimer lasers do not release greater fractions of heat, which could cause thermal damage to surroun - ding material. These attributes make excimer lasers well-suited for cleaning sensitive surfaces like pigment lac quer, finishing varnish or art paintings, but also for manufacturing of all-poly - mer electronic compo nents.

Excimer Laser Scanning Device for Sensitive Surface Cleaning

Automated, non-polluting cleaning devices are of rapidly growing interest in science and in - dustry. Particularly laser cleaning units avoiding wet chemical reactions and by this significantly reducing environmental contamination, have proven successfully and have found many new applications. Most of these laser cleaning units are designed to completely remove the top layer of a surface or the surface itself, e.g., removing paint as a preparatory step for a new sealing of the sur - face (see aeroplane or car industry). In these cases it is important to make sure, that no re - maining lacquer is left. Likewise this applies to the cleaning of stone faca des, antiques or other objects suffering from dust and environmental impacts. The statue at the left was completely black like the base. Lasers, however, offer additional significant advantages compared to conventional cleaning me - chanisms: • So, soft and sensitive surfaces like paint or plastics may be treated to ablate a few micro - meters of the upper layer together with some adherent impurities without destroying the protecting function of the layer or the underlying structure. • In semiconductor epitaxi or optical coating fabrication substrates can be irradiated to re - move monolayers of water or oxygen from the surface even under ultrahigh vacuum con di - tions. • Toxic chemicals or hazardous bacteria sticking at the surface can be cracked and vaporized to sterilize instruments. • Lasers can be used to decontaminate some equipment which was radioactively conta - minated. We developed a laser cleaning unit de sig - ned for an automated processing and cleaning of conta minated samples. Particularly dust, che micals or radioactive isotopes sticking at var nish, lacquer or plastic sealing are removed together with a few µm thin film of the surface without destroying the sealing itself. While for complete removal of a sealing it may be favorable when the laser radiation pene trates into deeper layers and causes some depth effect, ablation of only a thin film requires deposition of the laser energy into the top layer of the surface. This is best realized with lasers operating in the UV, where paint and plastics show sig nificantly increasing absorption. Therefore, we are working with a KrF-excimer laser. The key element of the cleaning device is a beam forming and scanning unit. To improve the laser beam quality for homogeneous irra - di ation of sam ples, a beam homogenizer was developed which forms a rectangular cross se - ction with top-hat beam profile. The beam is fo cused down to 2x2 mm 2 and directed across the polluted surface by a beam scanner which uses two galvo mirrors. The scanning rate and by this the deflection of the beam is adapted to the pulse repetition rate (up to 1 kHz) and therefore allows an extremely fast automated processing. An accurately controlled beam positioning together with the top-hat profile ensures a very ho - mo geneous, uniform and reliable irradiation of each surface element with an almost identical laser fluence. Our investigations with this set-up show, that after optimizing the relevant laser parameters like pulse energy, peak intensity, laser repe ti - tion rate, num ber of pulses and beam profile, an efficient clea ning without damage of the surface is obtained. A computer controlled operation of the system gua rantees the uni - form and continuous cleaning as well as a comfortable handling. The unit is equip ped with an adapted suction nozzle to dispose the detached particles from the surface. To quantify the cleaning efficiency and to proof the applicability of laser cleaning even for radio - actively contaminated samples varnished plastic plates were prepared, contaminated with 152 Eu at the surface. 152 Eu is favorably used as contaminating test material, since under normal con - ditions it is not radioactive but can be activated well-aimed by neutron radiation to a meta stable configura tion with a half-life of 9.5 h. Therefore, any handling of the plates during pre paration and cleaning can be done without danger of radioactivity. The laser cleaning efficiency is determined by measuring the radioactivity of the plates before and after laser irradiation. We found that with a fluence of 1 J/cm² only one or two pulses per position are necessary to bring the initial contamination level down to less than 5%. The same set-up can also be used for cleaning of art paintings and restoration of antiques. The scanner unit directs the excimer laser radi ation onto the painting and sweeps the beam across an area of 6x10 cm². For processing of larger areas the painting can be shifted auto mati - cally by step per motor controlled displace units. This figure shows an oil painting as test ob - ject ir radiated with different concentrations (pulses per position). It demonstrates the well con trollable, homogeneous treatment and the efficiency of ex cimer laser cleaning of art ob - jects. It results in an appealing and bright appearance of the processed area and shows the suspicious potential for future excimer laser cleaning appli cations.

Doctoral Theses

B. Warm Einsatz Adaptiver Optik zur Formung von Laserstrahlen School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 1994 R. Meixner Herstellung und Charakterisierung wellenlängenselektiver organischer Feldeffekttransistoren School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 2007 K.-H. Steglich Reinigung und Strukturierung empfindlicher Oberflächen mittels Excimerlaserstrahlung - Entwicklung und Anwendung eines Applikationsgerätes - School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 2008

Refereed Publications in Journals and Conference Digests

K.-H. Steglich, H. Harde Laser Cleaning of Radioactively Contaminated Surfaces Proceedings of the Conference on Lasers and Electro Optics Europe 1998, IEEE Catalog No. 98TH8326, ISBN: 0-7803-4233, Glasgow, Scotland, UK, p. 24 (1998) K.-H. Steglich, H. Harde Laser Scanner for Cleaning of Sensitive Surfaces Conference on Lasers and Electro-Optics/Europe (CLEO/EUROPE 2000), IEEE Conference Digest, CWF 57, Cat. No. 00TH8505, ISBN: 0-7803-6319-3, Nice, France, p. 215 (2000) K.-H. Steglich, H. Harde Sensitive Surface Cleaning using an Excimer Laser Scanner Conference on Lasers and Electro-Optics (CLEO 2001), Technical Digest, CTuT 7, Cat. No. 01CH37170, ISBN: 1-55752-676-1, Baltimore, MD, USA, CTuT 7, p. 233 (2001) K.-H. Steglich, H. Harde Sensitive Surface Cleaning Using an Excimer Laser Scanner Conference on Lasers, Applications and Technologies (IQEC/LAT 2002), Technical Digest, LMF 45, Moscow, Russia, p. 283 (2002) K.-H. Steglich, H. Harde Automated Surface Cleaning with an Excimer Laser Conference on Laser-assisted Micro- and Nanotechnologies (ILATA-III 2003/LAM-X), Technical Digest, PSII-1, 2003, St. Petersburg, Russia, p. 38 (2003) K.-H. Steglich, H. Harde Rapid Excimer Laser Cleaning of Plane Objects Conference on Lasers and Electro-Optics/Europe (CLEO/EUROPE 2003), Conference Digest (CD-ROM, Volume 27E), CK5W, ISBN 2-914771-15-0, Munich, Germany, p. 106 (2003) K.-H. Steglich, H. Harde Art Cleaning by Excimer Lasers Eurolaser 1/2004, Verlagsgesellschaft Grütter GmbH & Co. KG, Coburg, ISSN: 1430-8274 (2004) P.H. Thiesen, R. Meier, T. Fedorkova, K.-H. Steglich, H. Harde, B. Niemeyer Die Bestimmung der Dicke laserbehandelter Polymerfilme mittels Ellipsometrie Chemie Ingenieur Technik 2004, 76 No.9, Wiley-VCH, ISSN 0009-286X, p. 1385 (2004) P.H. Thiesen, R. Meier, T. Fedorkova, K.-H. Steglich, H. Harde, B. Niemeyer The Thickness of a Modified Polymere Film - An Ellipsometric Study 12. Wolfgang-Ostwald-Kolloquium, Kiel (2004) R. Meixner, R. Wille, P. Schertling, H. Goebel, H. Harde, K.-H. Steglich, F.A. Yildirim, W. Bauhofer, W. Krautschneider Bottom Gate Organic Field Effect Transistors Made by Laser Structuring Organic Electronics 7, pp. 586–591 (2006) K.-H. Steglich, H. Harde Oberflächenreinigung mit Lasern Uniforschung, Forschungsmagazin der Helmut-Schmidt-Universität Hamburg, 14. Jahrgang, S. 12-15 (2004), ISSN: 0940-8061

Contributions on National Conferences and Meetings

K.-H. Steglich, H. Harde Reinigung kontaminierter Oberflächen mit Hilfe eines Excimer-Lasers Frühjahrstagung der DPG, Sektion Kurzzeitphysik, Bayreuth, 10. März 1998, Verhandl. DPG (VI) 33, 305 (1998) K.-H. Steglich, H. Harde Excimer-Applikationsverfahren zur sensitiven Reinigung radioaktiv kontaminierter Oberflächen Frühjahrstagung der DPG, Fachverband Kurzzeitphysik, Heidelberg, 18. März 1999, Verhandl. DPG (VI) 34, K 7.6, 288 (1999) K.-H. Steglich, H. Harde Reinigung radioaktiv kontaminierter empfindlicher Oberflächen mittels eines Excimer-Lasers Poster Norddeutsches Laserkolloquium 1998, Hannover, 9. Dezember 1999 K.-H. Steglich, H. Harde Excimer-Laser-Scanner zur sensitiven Reinigung radioaktiv kontaminierter Oberflächen Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Fachverband Kurzzeitphysik, Bonn, 6. April 2000, Verhandl. DPG (VI) 35, D3 (2000) K.-H. Steglich, H. Harde Excimer-Laser-Scanner zur Reinigung empfindlicher Oberflächen Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Fachverband Kurzzeitphysik, Bochum, 18.-21. März 2002, Verhandl. DPG (VI) 37, D3 (2002)