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

Terahertz (THz) radiation can be used in an abundance of applications ranging from fundamental science to in-situ quality control of industrial processes. The electric field strengths of strong THzradiation allows for studying fundamental properties of matter, and ultrafast THz-spectroscopy is a promising tool for material analysis and security applications. Semi-transparent to many common objects that are opaque at optical frequencies, THz-radiation is used in non-invasive imaging applications. On top of this, THz-imaging offers the additional benefit over high-frequency methods like X-Rays by having very low photon-energies and thus being non-damaging to the materials exposed to it. The largest disadvantage of THz-radiation is its strong absorption by water molecules, which are present in many materials of interest (e.g. biological samples), and omnipresent in the atmosphere as humidity. Hence, for the practicality of the above mentioned applications, a strong THz-source is required. Currently, most high-efficiency, broadband THz-generation methods require strong pump-light energy, which is typically provided with large lasers operating at low repetition rates of a few hertz or kilohertz at most. Regretfully, this low repetition rate drastically affects measurement time or data acquisition speed. As a consequence, the signal-to-noise ratio tends to be low when fast measurements are required. To circumvent both of these issues, this thesis provides a feasible approach to generate highpower THz-radiation at MHz repetition rates by using a compact fibre-laser system and a simple external pulse compression method. The combination produces high peak power laser pulses which are applied to drive optical rectification (OR) in a highly efficient organic crystal to produce THz-radiation with a comparatively large efficiency. The output of a near-infrared femtosecond ytterbium-doped fibre-laser is passed through a polarisation maintaining large mode area photonic crystal fibre (LMA-PCF) inducing spectral broadening to a full-width half-maximum (FWHM) bandwidth of ∼100 nm from an initial FWHM of 8 nm. The strongly polarised output is sent
through a pair of SF11-glass prisms, compressing the spectrally broadened pulse from 250 fs to 22 fs at FWHM pulse duration. Ignoring fibre coupling loss, this method provides an almost tenfold increase in peak power to 13.8 MW at a repetition rate of 10 MHz. Further scaling of the repetition rate (and thus the average power) proved possible, as the damage mechanism were solely peak power dependent at >2 MW. The 22 fs beam is focussed onto the organic crystal HMQ-TMS, which provides a highly efficient
option for THz-generation through OR in a collinear setup, reducing complexity compared to other efficient methods for THz-generation, while also providing a far larger spectral bandwidth. It spans from below 1 THz to over 6 THz at an average power of 1.38 mW with peak electric field strengths exceeding 6 kV·cm⁻¹ in standard atmospheric conditions for fairly large THz-spot size of 369 µm in radius at 1/e²-intensity. Thus, the field strengths can easily exceed 20 kV·cm⁻¹ through tighter focussing and by operating in dry-air environments. The optical to THz-generation efficiency of 5.5·10⁻⁴ is more than an order of magnitude larger than what is typically achieved with inorganic crystals at similar pump parameters. The compression setup and THz-source described in this thesis is easily implemented and shows long-term stability. Thus, the work presented as part of this PhD thesis can greatly benefit a multitude of applications and propel THz-technology meaningfully out of the laboratories and into the commercial realm, while also providing a powerful tool for fundamental scientific applications.