Conferences1,2, viele
AwardsOhm Preis der Universität Erlangen-Nürnberg als Auszeichnung der Diplomarbeit; Ohm Preis der Universität Erlangen-Nürnberg als Auszeichnung der Dissertation; Otto Hahn Medaille für den wissenschaftlichen Nachwuchs der Max-Planck Gesellschaft
FellowshipsAlumni der Studienstiftung des deutschen Volkes; Alumni IMPRS; Humboldt Stiftung
Current research / employment

I am currently working at the University of California, Santa Barbara, USA

Research interests: Terahertz sources and detectors Terahertz optoelectronics Semiconductor physics Optics

Short abstract of IMPRS PhD project:

A main goal of this thesis was the development of an efficient, continuous-wave, room-temperature operating, tunable n-i-pn-i-p superlattice Terahertz (THz) photomixer in the (Al)GaAs and In(Al)GaAs material system.

Photomixers generate a THz AC current by absorbing the beat signal of two lasers that are offset by the THz frequency. The current is fed into an antenna for radiation of a free space THz signal. Photomixers are limited at high frequencies by the time required for separation of optically generated carriers and by their electrical antenna (R) and device (C) RC time constant. Each term causes a ν-² roll-off at high frequencies. A n-i-pn-i-p photomixer consists of a stack of N high speed p-i-n diodes. This high speed is achieved by transit-time optimization of the intrinsic layer length and by incorporating the concept of quasi-ballistic transport of electrons. At the same time, a number, N, of p-i-n superlattice periods is grown in series. This shifts the RC time constant to smaller values. Both transit- and RC-roll-off can independently be optimized. At the np-junction that forms between two adjacent p-i-n periods, efficient recombination diodes are implemented by introducing low-temperature grown layers (LT-GaAs) or a few ErAs monolayers (ErAs:GaAs, ErAs:InGaAs). A three period InGaAs sample provided 0.27 μW at 1 THz with a broadband antenna, driven by amplified, continuous-wave telecom laser diodes with a total optical power of 190 mW. At 66 GHz, a similar device provided 0.18 mW of output power. In the second part of the thesis, n-i-pn-i-p superlattice photomixers were used for THz experiments for the first time. The photomixers were driven by a home built erbium doped fiber amplifier photomixing setup. THz optical elements such as wire grid polarizers, a blazed grating and waveguides were characterized. Their performance agreed well with the respective theoretical expectations. A demonstration of the capability of the setup and the photomixers was given by the example of the spectroscopy of water vapor in air. We also investigated the coherence of the THz signal with a Young’s double slit experiment and a THz Mach Zehnder setup. The coherence length of the THz signal is close to that of the mixed optical lasers. We exploited the mutual coherence of THz emitters that are driven by the same pair of lasers for a chip-sized 3x3 photomixer array. The photomixers were emitting in phase, increasing the directivity and intensity. Their phase relation was controlled by including a small angle of a few mrad between the two lasers. This allowed for steering the THz beam. The layout of a tens of cm scale, fiber-based array was theoretically investigated. The promising results were experimentally verified during the diploma thesis of Sebastian Bauerschmidt. Furthermore, THz whispering gallery mode resonators were investigated. They store light by total internal reflection in a convex, dielectric medium. Interference within the cavity generates very narrow resonances. Similar to two approaching atoms, the modes split up when resonators are coupled. We demonstrated mode splitting by coupled resonator systems of 2 and 3 cavities with matched mode spectra.

Publications S. T. Bauerschmidt, G. H. Döhler, H. Lu, A. C. Gossard, S. Malzer, S. Preu, Arrayed free space continuous-wave terahertz photomixers, Optics Letters, 38, 3673-3676, (2013)

G. H. Döhler, L. E. Garcia-Muñoz, S. Preu, S. Malzer, S. Bauerschmidt, J. Montero-de-Paz, E. Ugarte-Muñoz, A. Rivera-Lavado, V. Gonzalez-Posadas, D. Segovia-Vargas, From Arrays of THz Antennas to Large-Area Emitters, IEEE Transactions on Terahertz Science and Technology, 3, 532-544, (2013)

A, Rivera-Lavado, L. E. García-Muñoz, G. Dohler, S. Malzer, S. Preu, S. Bauerschmidt, J. Montero-de-Paz, E. Ugarte-Muñoz, B. Andrés-García, V. Izquierdo-Bermúdez, D. Segovia-Vargas, Arrays and New Antenna Topologies for Increasing THz Power Generation Using Photomixers, Journal of Infrared, Millimeter, and Terahertz Waves, 34, 97-108, (2013)

A. R. Criado, C. de Dios, G. H. Döhler, S. Preu, S. Malzer, S. Bauerschmidt, H. Lu, A. C. Gossard, P. Acedo, Ultra-narrow linewidth CW sub-THz generation using GS based OFCG and n-i-pn-i-p superlattice photomixers, Electronics Letters, 48, 1425 - 1426, (2012)

B. Andres-Garcia, E. Garcia-Munoz, S. Bauerschmidt, S. Preu, S. Malzer, G. H. Döhler, L. Wang, D. Segovia-Vargas, Gain Enhancement by Dielectric Horns in the Terahertz Band, IEEE Transactions on Antennas and Propagation, 59, 3164 - 3170, (2011)

S. Preu, S. Malzer, G. H. Döhler, H. Lu, A. C. Gossard, L. J. Wang, Efficient III-V tunneling diodes with ErAs recombination centers, submitted to Semicond. Sci. Technol., , , (2010)

S. Preu, G. H. Döhler, S. Malzer, L. J. Wang, A. C. Gossard, Tunable, continuous-wave terahertz photomixer sources and applications, invited review article for the J. Appl. Phys., , submitted, (2010)

S. Preu, H. G. L. Schwefel, G. H. Döhler, L. J. Wang, M. Hanson, J. D. Zimmerman, A. C. Gossard, Coupled whispering gallery mode resonators in the terahertz frequency range, Opt. Express, 16, 7336-7343, (2008)

S. Preu, S. Malzer, G. H. Döhler, Q. Z. Zhao, M. Hanson, J. D. Zimmerman, A. C. Gossard, L. J. Wang, Interference between two coherently driven monochromatic terahertz sources, Appl. Phys. Lett., 92, 2211107, (2008)

S. Preu, F. H. Renner, S. Malzer, G. H. Döhler, L. J. Wang, T. L. J. Wilkinson, E. R. Brown, M. Hanson, A. C. Gossard, Efficient terahertz emission from ballistic transport enhanced n-i-p-n-i-p superlattice photomixers, Appl. Phys. Lett., 90, 212115, (2007)