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Detectors made from sub-micron-sized bridges of the high- ${T}_{c}$ superconductor Y–Ba–Cu–O have unique properties useful for the detection of THz radiation, namely evidence of a detector response with an additional sensitivity to the direction of the electric field of short THz pulses when used without a transport current in the so-called “zero-bias” regime. In order to examine the influence of the antenna design on the detector response, we have conducted electromagnetic simulations and performed experiments at a synchrotron light source and with a photomixing setup as sources for short THz pulses. In this paper, we evaluate a narrow-band double-slot antenna as well as a broadband log-spiral antenna design.

We exploit information and communications tech-nologies to build a radio frequency-driven frequency comb spanning several hundred gigahertz. We investigated electro-optic modulators, which can serve as building blocks in frequency combs, terahertz generation and terahertz detection systems. These devices have high potential for applications in robust laser-based diagnostics at electron accelerators. During the last year, we have reduced the pulse length generated by a frequency-comb without moving parts by more than one order of magnitude to less than 150 fs, fitting a Lorentzian-type autocorrelation function.

Detectors made from the high- T c superconductor YBa2Cu3O7-x are well suited to detect the high-intensity pulsed-terahertz (THz) radiation emitted by synchrotron light sources and free-electron lasers due to their fast response times. We propose a detection system consisting of an integrated planar array with four superconducting detectors coupled to narrowband double-slot antennas for the frequencies 140 GHz, 350 GHz, 650 GHz, and 1.02 THz, respectively. With the ability for spectroscopic measurements, it has the potential to further improve the understanding of accelerator science and the future application of THz radiation. In this paper, we present a new antenna and array design that has been especially optimized to match the high impedance of sub- μ m-sized detector bridges needed for high sensitivities, and measurements performed at the Diamond Light Source.

We report on exploiting information and communications technologies to build reliable and durable terahertz diagnostics for electron accelerator-based light and laser sources. Terahertz and electro-optical diagnostics is an ideal tool to investigate short-bunch electron beams and their emitted photons with the potential to devise methods for controlled and tailored radiation emission. We investigate electro-optical modulators to evaluate their potential for laser-based diagnostics at accelerators, which can serve as building blocks in frequency combs, terahertz generation and detection systems. We built a frequency comb spanning at least 250 GHz with a pulse length of less than 3.7 ps.

Electron accelerators and synchrotrons can be operated to provide short emission pulses due to longitudinally compressed or sub-structured electron bunches. Above a threshold current, the high charge density leads to the micro-bunching instability and the formation of sub-structures on the bunch shape. These time-varying sub-structures on bunches of picoseconds-long duration lead to bursts of coherent synchrotron radiation in the terahertz frequency range. Therefore, the spectral information in this range contains valuable information about the bunch length, shape and sub-structures. Based on the KAPTURE readout system, a 4-channel single-shot THz spectrometer capable of recording 500 million spectra per second and streaming readout is presented. First measurements of time-resolved spectra are compared to simulation results of the Inovesa Vlasov-Fokker-Planck solver. The presented results lead to a better understanding of the bursting dynamics especially above the micro-bunching instability threshold., Comment: 12 pages, 11 figures

A key bottleneck for investigations of ultrafast processes is a detection scheme to record all individual spectra with high repetition rates to avoid averaging and to improve the signal-to-noise ratio. Here, we present spectral measurements of fs laser sources used for electro-optical detection with a KIT-developed linear sensor array and DAQ system adapted to light sources with MHz repetition rates. This system can be equipped with different sensor types covering a broad wavelength range. It can therefore be used for various applications and scientific questions. The presented exemplary applications range from accelerator-based diagnostics to table-top laser experiments.

At the KArlsruhe Research Accelerator (KARA), we use electro-optical sampling to measures profiles of compressed electron bunches during the microbunching instability. The observation of the complex dynamics of this instability is of special interest because it leads to intense THz radiation bursts. As the revolution frequency of the storage ring is 2.72 MHz, high detection rates are required to record the bunch profiles for every revolution with single-shot measurements. To achieve fast detection rates, we implemented a KIT-developed ultra-fast line array and recorded the electron bunch charge density for every revolution for 3.6 s with a data throughput of 1.4 GBytes/s.

The behavior of submicrometer-sized thin-film YBa 2 Cu 3 O 7-x (YBCO) detectors under illumination with picosecond terahertz (THz) pulses was investigated. Real-time measurements with a temporal resolution of 15 ps full width at half maximum were performed at ANKA, the synchrotron facility of Karlsruhe Institute of Technology, and the UVSOR-III facility at the Institute for Molecular Science in Okazaki, Japan. The capability of YBCO detectors to reproduce the shape of a several picosecond long THz pulse was demonstrated. Single-shot measurements adhering to a reversal of the direction of the electrical field of the THz radiation were carried out. They provided evidence for the electrical field sensitivity of the YBCO detector. Exploiting the electrical field sensitivity of the YBCO detector, the effect of microbunching was observed at UVSOR-III.

The photonic time-stretch technique allows electric field pulse shapes to be recorded with picosecond resolution, at megahertz acquisition rates. Using this strategy, we could directly record spatial patterns that spontaneously appear in relativistic electron bunches, and follow their dynamical evolution over time. We present recent results obtained using two strategies. At SOLEIL, we present the shapes of the THz pulses which are emitted by the structures, and detected far from the emission point, at the end of a beamline. At ANKA, we present how it has been possible to monitor directly the electron bunch near-field. These new types of single-shot recordings allow direct and stringent tests to be performed on electron bunch dynamical models in synchrotron radiation facilities.

Dedicated optics with extremely short electron bunches enable synchrotron light sources to generate intense coherent THz radiation. The high degree of spatial compression in this so-called low-alpha optics entails a complex longitudinal dynamics of the electron bunches, which can be probed studying the fluctuations in the emitted terahertz radiation caused by the micro-bunching instability ("bursting"). This article presents a "quasi-instantaneous" method for measuring the bursting characteristics by simultaneously collecting and evaluating the information from all bunches in a multi-bunch fill, reducing the measurement time from hours to seconds. This speed-up allows systematic studies of the bursting characteristics for various accelerator settings within a single fill of the machine, enabling a comprehensive comparison of the measured bursting thresholds with theoretical predictions by the bunched-beam theory. This paper introduces the method and presents first results obtained at the ANKA synchrotron radiation facility., Comment: 7 pages, 7 figures, to be published in Physical Review Accelerators and Beams

Using arbitrary periodic pulse patterns we show the enhancement of specific frequencies in a frequency comb. The envelope of a regular frequency comb originates from equally spaced, identical pulses and mimics the single pulse spectrum. We investigated spectra originating from the periodic emission of pulse trains with gaps and individual pulse heights, which are commonly observed, for example, at high-repetition-rate free electron lasers, high power lasers, and synchrotrons. The ANKA synchrotron light source was filled with defined patterns of short electron bunches generating coherent synchrotron radiation in the terahertz range. We resolved the intensities of the frequency comb around 0.258 THz using the heterodyne mixing spectroscopy with a resolution of down to 1 Hz and provide a comprehensive theoretical description. Adjusting the electron's revolution frequency, a gapless spectrum can be recorded, improving the resolution by up to 7 and 5 orders of magnitude compared to FTIR and recent heterodyne measurements, respectively. The results imply avenues to optimize and increase the signal-to-noise ratio of specific frequencies in the emitted synchrotron radiation spectrum to enable novel ultrahigh resolution spectroscopy and metrology applications from the terahertz to the x-ray region.

Photonic time-stretch enables to record electric field pulse shapes with picosecond resolution, at megahertz acquisition rates. Using this strategy, we could record the complex spatiotemporal evolution of relativistic electron bunches circulating in storage ring accelerators. We tested two complementary approaches. At SOLEIL, we monitored the THz pulses of coherent synchrotron that are emitted by the electrons. At ANKA, we monitored directly the electron bunch near-field.

Infrared radiation from the near- to the far-infrared (terahertz) region of the electromagnetic spectrum is extensively used for many different purposes. Accelerator-based sources of infrared and terahertz radiation have become well-accepted research tools. This article introduces the physics processes underlying the generation of infrared radiation by different methods. Special emphasis is put on the interesting properties of coherent radiation, and the corresponding accelerator requirements are discussed. Finally, the various available accelerator-based sources are compared in view of different figures of merit.

The region of the electromagnetic spectrum between 0.3 and 20 THz is a “frontier region” of spectroscopy in physics, chemistry, biology, material sciences, and medicine [1]. Radiation in the THz range allows the investigation and excitation of phenomena with time constants of about 1 ps. Examples on this time scale are atomic electron orbits in highly excited Rydberg states, rotations of small molecules and modes of collective oscillations of proteins and polar liquids like water. Resonances of electrons in semiconductor nanostructures and band gaps of superconductors can equally be studied with THz radiation. The THz region lies above the frequencies of traditional electronics but below the range of optical and infrared generators. Until now, possibilities to generate sufficiently intense and brilliant radiation at the wavelengths in question were scarce and the region was therefore named the “THz gap” (Figure 1). In accelerators such highly intense coherent THz radiation can be generated under special c...

This paper presents a novel data acquisition system for continuous sampling of ultra-short pulses generated by terahertz (THz) detectors. Karlsruhe Pulse Taking Ultra-fast Readout Electronics (KAPTURE) is able to digitize pulse shapes with a sampling time down to 3 ps and pulse repetition rates up to 500 MHz. KAPTURE has been integrated as a permanent diagnostic device at ANKA and is used for investigating the emitted coherent synchrotron radiation in the THz range. A second version of KAPTURE has been developed to improve the performance and flexibility. The new version offers a better sampling accuracy for a pulse repetition rate up to 2 GHz. The higher data rate produced by the sampling system is processed in real-time by a heterogeneous FPGA and GPU architecture operating up to 6.5 GB/s continuously. Results in accelerator physics will be reported and the new design of KAPTURE be discussed.

Accelerators can serve as sources for intense THz radiation by producing sub-ps long electron bunches, which generate synchrotron radiation coherently in the THz regime. In this paper, we present an analytic method to calculate the electric field pulse emitted by an electron bunch with arbitrary bunch shape. Our method is applied to a Gaussian bunch as well as a complex bunch profile resulting from a nonlinear beam dynamics simulation.

A new compact versatile linear accelerator named FLUTE is currently being designed at the Karlsruhe Institute of Technology. This paper presents the status of this 42 MeV machine. It will be used to generate strong (several 100 MV/m) ultra-short (∼1 ps) THz pulses (up to ∼4–25 THz) for photon science experiments, as well as to conduct a variety of accelerator studies. The latter range from comparing different coherent THz radiation generation schemes to compressing electron bunches and studying the electron beam stability. The bunch charge will cover a wide range (∼100 pC–3 nC). Later we plan to also produce ultra-short x-ray pulses from the electron bunches, which, for example, could then be combined for THz pump–x-ray probe experiments.

High-temperature superconducting YBa2Cu3O7-d (YBCO) thin-film detectors with improved responsivities were developed for fast time-domain measurements in the THz frequency range. YBCO thin films of 30 nm thickness were patterned to micro- and nanobridges and embedded into planar log-spiral THz antennas. The YBCO thin-film detectors were characterized with continuous wave radiation at 0.65 THz. Responsivity values as high as 710 V/W were found for the YBCO nanobridges. Pulsed measurements in the THz frequency range were performed at the electron storage ring ANKA from the Karlsruhe Institute of Technology (KIT). Due to the high responsivities of the nanobridges no biasing was required for the detection of the coherent synchrotron radiation pulses achieving very good agreement between the measured pulse shapes and simulations.

Schottky diodes are well-known nonlinear elements allowing for effective detection and mixing of electromagnetic radiation in the range through microwave to terahertz. Although less sensitive than their superconducting counterparts, they generally do not require cooling that makes them the devices of choice for applications where the ultimate sensitivity is not needed. In the emerging field of terahertz technology, there is a long-time quest for cheap and handy detectors for laboratory use, as well as for serial compact and midsize instruments. We describe the use of a quasi-optically coupled zero-bias planar Schottky-diode detector for monitoring picosecond pulses of synchrotron terahertz radiation and weak continuous-wave emission from an array of Josephson junctions.

In synchrotron radiation sources short pulses of coherent THz radiation is emitted when the length of electron bunches becomes sufficiently small. A detector system based on a superconducting NbN ultra-fast bolometer with an intrinsic response time of about 100 ps was jointly developed by the University of Karlsruhe (Institute of Micro- and Nanoelectronic Systems) and German Aerospace Center (Berlin) and implemented at the storage ring ANKA to time-resolve radiation emitted by single electron bunches.

A synchrotron radiation source emits coherent infrared (IR) radiation when the electron bunch length is comparable to the wavelength of the emitted radiation (see for example [1, 2]). To generate coherent radiation in the far IR (THz) region, a ’low alpha mode’ has been devised at the ANKA storage ring operating at 1.3 GeV. The corresponding lattice has a significantly reduced momentum compaction factor. The spectral dependence of the emitted radiation is recorded at the ANKA-IR beam line, where the synchrotron light is produced in the fringe field of a bending magnet [3]. This edge radiation has the advantage of being more collimated than constant field radiation. This allows the observation of frequencies down to 1 cm-1through a modest vertical aperture, which would not be possible with classical constant field emission due to the increasing beam divergence with decreasing frequency. The onset of coherent emission is found at a synchrotron frequency of about 10 kHz. At 5 kHz, an intensity enhancement of up to 5 orders of magnitude, with respect to the incoherent emission, is observed in the spectral range between 1 and 65 cm-1.

The recording of coherent synchrotron radiation requires data acquisition systems with a temporal resolution of tens of picosecond. This paper describes a new real-time and high-accuracy data acquisition system suitable for recording individual ultra-short pulses generated by a fast terahertz (THz) detector (e.g. YBCO, NbN, Zero Biased Schottky Diode). The system consists of a fast sampling board combined with a high data throughput readout. The first board is designed for sampling the fast pulse signals with a full width half maximum (FWHM) between a few tens to one hundred picoseconds with a minimum sampling time of 3 ps. The high data throughput board consists of a PCIe-Bus Master DMA architecture used for fast data transfer up to 3 GByte/s. The full readout chain with fast THz detectors and the acquisition system has been successfully tested at the synchrotron ANKA. An overview of the electronics system and preliminary results with multi-bunch filling pattern will be presented.

High-speed communication systems rely on spectrally efficient modulation formats that encode information both on the amplitude and on the phase of an electromagnetic carrier. Coherent detection of such signals typically uses rather complex receiver schemes, requiring a continuous-wave (c.w.) local oscillator (LO) as a phase reference and a mixer circuit for spectral down-conversion. In optical communications, the so-called Kramers-Kronig (KK) scheme has been demonstrated to greatly simplify the receiver, reducing the hardware to a single photodiode. In this approach, an LO tone is transmitted along with the signal, and the amplitude and phase of the complex signal envelope are reconstructed from the photocurrent by digital signal processing. This reconstruction exploits the fact that the real and the imaginary part, or, equivalently, the amplitude and the phase of an analytic signal are connected by a KK-type relation. Here, we transfer the KK scheme to high-speed wireless communications at THz carrier frequencies. We use a Schottky-barrier diode (SBD) as a nonlinear element and generalize the theory of KK processing to account for the non-quadratic characteristics of this device. Using 16-state quadrature amplitude modulation (16QAM), we transmit a net data rate of 115 Gbit/s at a carrier frequency of 0.3 THz over a distance of 110 m., 9 pages Main Paper + 7 pages Supplementary Information