Juliane Raasch

Highly Responsive Y–Ba–Cu–O Thin Film THz Detectors With Picosecond Time Resolution

High-temperature superconducting YBa2Cu3O7 - δ (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.

Investigation of the Electrical Field Sensitivity of Sub-μm Y–Ba–Cu–O Detectors

The behavior of submicrometer-sized thin-film YBa2Cu3O7-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.

Integrated Four-Pixel Narrow-Band Antenna Array for Picosecond THz Spectroscopy

One of the most promising sources for pulsed terahertz (THz) radiation is coherent synchrotron radiation (CSR). Due to their response time in the picosecond range, direct THz detectors based on YBa2Cu3O7-x hot-electron bolometers are ideally suited for the analysis of electron-beam instabilities occurring in the synchrotron storage ring and affecting the emitted CSR spectrum. Response times of down to 16 ps full-width at half-maximum (FWHM) have been already achieved with wideband antenna-coupled detectors. We are presenting an integrated array combining this fast detector response time with the capability of single-shot spectral resolution for individual CSR pulses. The detector array consists of an integrated four-pixel array that contains four narrow-band double-slot antennas with embedded detectors.

An Integrated Planar Array of Ultrafast THz Y–Ba–Cu–O Detectors for Spectroscopic Measurements

Detectors made from the high-Tc 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-μm-sized detector bridges needed for high sensitivities, and measurements performed at the Diamond Light Source.

Microbunching Instability in Relativistic Electron Bunches: Direct Observations of the Microstructures Using Ultrafast YBCO Detectors

Relativistic electron bunches circulating in accelerators are subjected to a dynamical instability leading to microstructures at millimeter to centimeter scale. Although this is a well-known fact, direct experimental observations of the structures, or the field that they emit, remained up to now an open problem. Here, we report the direct, shot-by-shot, time-resolved recording of the shapes (including envelope and carrier) of the pulses of coherent synchrotron radiation that are emitted, and that are a “signature” of the electron bunch microstructure. The experiments are performed on the UVSOR-III storage ring, using electrical field sensitive YBa2Cu3O7−x thin-film ultrafast detectors. The observed patterns are subjected to permanent drifts, that can be explained from a reasoning in phase space, using macroparticle simulations.

An ultra-fast digitizer with picosecond sampling time for Coherent Synchrotron Radiation

Since a few years Coherent Synchrotron Radiation (CSR) generated by short electron bunches is provided at the ANKA synchrotron light source. To study the THz emission characteristics over multiple revolutions superconducting YBa2Cu3O7-δ (YBCO) thin-film detectors can be used. The intrinsic response time of YBCO thin films is in the order of a few picoseconds only. For fast and continuous sampling of this individual ultra-short terahertz pulses a novel digitizer system has been developed with programmable sampling times in the range of 3 to 100 ps. The Real-time system is based on a heterogeneous FPGA and GPU architecture for on-line pulse reconstruction and evaluations of the peak amplitudes and the time between consecutive bunches. The data is transmitted to a GPU computing node by a fast data transfer link based on a bus master DMA engine connected to PCI express endpoint logic. This new DMA architecture ensures a continuous high data throughput of up to 4 GByte/s. The presented DAQ system is able to resolve the bursting behavior of single bunches even in a multi-bunch environment and to study the bunch-bunch-interactions.