PhD defence by Tobias Buchmann

Title: High-power terahertz generation at megahertz repetition rates using few-cycle pulses


Principal supervisor: Professor Peter Uhd Jepsen, DTU Fotonik
Co-supervisor: Associate Prof. Edmund Kelleher, DTU Fotonik

Evaluation Board
Professor Karsten Rottwitt, DTU Fotonik
Professor Clara Saraceno, Ruhr-Universität Bochum, Germany
Professor John Travers, Heriot-Watt University, United Kingdom

Master of the Ceremony,
Senior Researcher Binbin Zhou DTU Fotonik



This work addresses the combination of two fields of physics – femtosecond lasers and terahertz (THz) radiation.


Femtosecond laser pulses have two main advantages: The ultrashort pulse durations and the intense peak powers provided by confining energy into such a short pulse duration.The pulse durations allow for an extremely high temporal resolution when probing physical processes,The extreme peak powers in turn allow for controlling nonlinear optical processes. One such nonlinear process involves the conversion of the laser frequencies to other frequencies in the THz-regime. The method is termed optical rectification. 

Several applications for THz-radiation are envisioned, with some already being used commercially, e.g. security scanners at airports. THz-radiation is transparent to many materials where visible light is blocked, making it a great choice for imaging and security applications. 

Another anticipated application-field of THz-radiation lies in scientific research. The field-strengths of ultrashort THz pulses and the frequencies covered by its bandwidth open up several interesting applications. For example, terahertz scanning tunneling microscopy allows for incredible temporal and spatial resolution, potentially allowing for mapping out the movement of electrons in chemical processes. A second example is THz-time-domain spectroscopy, which allows for material analysis, gas-sensing, or thickness measurements of thin layers in industrial processes.

However, a significant drawback of this technology is that THz-radiation sources have low conversion efficiencies, producing only small amounts of power. This makes detection and commercial applications difficult. Furthermore, the strong field strengths required for several applications is typically only achieved at very low repetition rates, so that measurement times are significantly prolonged. 

This thesis shows a strong improvement on these two drawbacks. The first part addresses compressing the ultrashort pulses of a high repetition rate laser source down to 22 femtoseconds at 3 Watts of average power. This is done with an external pulse compression setup using a polarisation maintaining large mode area photonic crystal fibre. The hereby compressed pulses with their increased peak power of over 10 megawatts are then used to generate THz-radiation in the highly efficient organic crystal HMQ-TMS. It results in producing a pulse

train of ultrashort THz-pulses at several megahertz repetition rate. Their bandwidth spans over 6 THz and the pulse durations are a few hundreds of femtoseconds short. Most importantly, the average power crosses the milliwatt threshold and the field strengths achieved can exceed several kilovolt per meter.

In conclusion, a method is presented that allows for the generation high power, broadband THz pulses at megahertz repetition rates. It is easily implemented and offers a significant increase of conversion efficiency.





ons 28 jul 21
13:30 - 16:30



The defence will take place online via this Zoomlink. and in Building 341, auditorium 22, where attendances are required to have a valid corona passport or alternatively show a negative COVID 19 test result not older than 72 hours for quick test or 96 hours for PCR test.


Contact information will also be requested by all attendances in accordance to DTU's COVID 19 regulations for corona tracing.