High-power few-cycle THz generation at MHz repetition rates in an organic crystal

Ultrafast terahertz (THz) spectroscopy is a potent tool for studying the fundamental properties of matter. Limitations of current THz sources, however, preclude the technique being applied in certain advanced configurations or in the measurement of, e.g., strongly absorbing samples. In response to this problem, here we demonstrate the generation of 1.38 mW broadband THz radiation at 10 MHz repetition rate by combining the highly efficient nonlinear organic crystal HMQ-TMS with ultrafast pump pulses generated using a simple and stable external pulse compression of a high power, near-infrared (NIR) femtosecond ytterbium-doped fiber (Yb:fiber) laser. Utilizing spectral broadening in a large core, polarization maintaining photonic crystal fiber and a pair of SF11 prisms, we achieve a tenfold pulse compression of the Yb:fiber laser, yielding compressed 0.35 µJ pulses with a full-width at half maximum pulse duration of 22 fs, exerting a peak power of 13.8 MW at a repetition rate of 10 MHz. THz generation through optical rectification of the NIR pulses is explored in two distinct thicknesses of the organic crystal, leading to a maximum conversion efficiency of ∼5.5 · 10−4, an order of magnitude higher than that achieved with inorganic nonlinear crystals, e.g., gallium phosphide, for similar pump parameters. The focused THz beam has a peak on-axis field strength greater than 6.4 kV cm−1 in unpurged atmosphere. We believe that our moderately strong-field THz source is well suited to a variety of applications in ultrafast THz spectroscopy, in particular THz-enabled scattering-type near-field, and scanning tunneling spectroscopy, where multi-MHz repetition rate sources are required

MHz-repetition-rate, sub-mW, multi-octave THz wave generation in HMQ-TMS

We demonstrate the first megahertz (MHz) repetition-rate, broadband terahertz (THz) source based on optical rectification in the organic crystal HMQ-TMS driven by a femtosecond Yb:fibre laser. Pumping at 1035 nm with 30 fs pulses, we achieve few-cycle THz emission with a smooth multi-octave spectrum that extends up to 6 THz at -30 dB, with conversion efficiencies reaching 10-4 and an average output power of up to 0.38 mW. We assess the thermal damage limit of the crystal and conclude a maximum fluence of ∼1.8 mJ·cm-2 at 10 MHz with a 1/e2 pump beam diameter of 0.10 mm. We compare the performance of HMQ-TMS with the prototypical inorganic crystal gallium phosphide (GaP), yielding a tenfold electric field increase with a peak on-axis field strength of 7 kV·cm-1 and almost double the THz bandwidth. Our results further demonstrate the suitability of organic crystals in combination with fibre lasers for repetition-rate scaling of broadband, high-power THz sources for time-domain spectroscopic applications

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 sentthrough 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 &gt;2 MW. The 22 fs beam is focussed onto the organic crystal HMQ-TMS, which provides a highly efficientoption 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.<br/

Terahertz sensing of sub-nm dielectric film via electron tunnelling

Terahertz (THz) sensing of ultrathin layers has been a longstanding challenge due to limitations in conventional detection techniques. In this study, we present a novel approach for sensing sub-1 nm thin dielectric layers based on Fowler-Nordheim (FN) tunneling. Our method exploits the FN tunneling effect at a metal-dielectric interface, enabling sensitive detection of changes in dielectric layer thickness within the THz frequency range. To validate our FN tunneling-based THz sensing technique, we carried out a comprehensive analysis of experimental and simulated data. Our findings demonstrate that this approach exhibits exceptional sensitivity, capable of detecting dielectric layers with thicknesses down to the sub-nanometer scale. Such sensitivity has significant implications for various applications, including nanoscale dielectric characterization, advanced material development, and quality control in microelectronics manufacturing. The FN tunneling-based THz sensing methodology not only overcomes the limitations of traditional detection techniques but also paves the way for novel ultrathin layer sensing capabilities in the rapidly advancing field of terahertz technology. Our study showcases the potential of this groundbreaking technique to revolutionize the THz sensing landscape, offering new opportunities for research and development in various fields.</p

Terahertz detectors based on vacuum electronics

We report on various metasurfaces for the purpose of THz driven electron field emission and subsequent detection using vacuum electronics. The underlying principle is based on strong localised field enhancement at metal and semimetal emission points, which bends the vacuum potential temporarily to allow for field emission of electrons from the parent material. The structures are investigated for varying electric field strength using electron time-of-flight measurements as well as electron multiplication and visualisation on a phosphor screen. Measured properties include the emitted electron energy, their count, and the emission threshold. From the recorded data, the local field enhancement for each structure is extracted and compared to simulated values. Subsequently, optimised metasurfaces are implemented into handheld devices that serve as easy-to-use THz detectors. These devices include photomultiplier tubes which operate at frequencies from THz to infrared, as well as live imaging devices with kilohertz framerates. The investigated metallic structures include standard dipole antennas, double split-ring resonators, bow-tie designs, hybrid split-ring and dipole designs, and logarithmic spirals. Semimetallic structures are based on structured and unstructured graphene, which show different emission characteristics. All samples are investigated using strong-field THz radiation generated using lithiumniobate tilted pulse front setup, as well as commercial THz-TDS instruments. In conclusion, we present a holistic overview of the current state-of-the-art THz-PMTs and image intensifiers

A novel terahertz detector technology based on vacuum electronics

A THz detector with both high sensitivity and fast time response has been required for industrial applications such as nondestructive testing (NDT), security, and spectroscopy. Through a collaboration with the Technical University of Denmark (DTU), we have recently developed a THz-sensitive point detector and imager based on metasurface and photomultiplier tube (PMT) and image intensifier (I.I.) technologies, respectively. A fast time response is one of the unique characteristics of these devices: the PMT-based point detector provides a nanosecond response time while the I.I.based imager is capable of frame rates up to 1000 fps. These devices have a double split-ring resonator (DSRR) at the photocathode for THz-electron conversion (metasurface). In this paper, we discuss the two devices and report on the development and results for increasing their sensitivity for ultrafast, broadband THz pulses by sharpening the field-enhancing antenna tips. This leads to a smaller tip diameter, which increases the electric field confinement and thus intensity at the tip, making the field emission more likely to occur at lower field strengths as a result. Both devices thus offer a sensitive and simple method to detect THz frequencies easily, with the I.I. offering a handheld, 9V battery-powered device.</p

High Sensitivity Spectroscopic Measurement with a Highly Nonlinear THz-PMT and an is-TPG

We use an injection-seeded terahertz (THz)-wave Parametric Generator (is-TPG) source with a THz-PMT detector for spectroscopic measurements on water vapor concentration in air and doping levels of semiconductor wafers. The THz-PMT detector has a highly non-linear response to changes in the electric field strength from the is-TPG source, which allows for precise characterization of miniscule changes in the absorption spectrum. We perform a frequency sweep from 0.7 to 1.2 THz and show that our THz-PMT signal drops significantly at water vapor absorption lines. In addition, we measure changes in the carrier density of Si wafers through reflection mode measurements. This proposed technique, operated with our novel THz instrumentation, is expected to enable highly sensitive and high-speed spectroscopic measurements