ABSTRACT. Kinetic Inductance Detectors (KIDs) offer excellent sensitivity in the THz region combined with ease of operation. The SPACEKIDS project is working on developments needed to enable this technology for space. KID arrays have been developed for both low-background (astrophysical) and high-background (Earth-observing) applications. Two laboratory demonstrator systems are now being built to evaluate array characteristics and performance in an environment representative of both astronomy and Earth observing applications.

ABSTRACT. Due to their ease of fabrication, sensitivity and extremely high multiplexing ratios, Kinetic Inductance Detectors (KIDs) have become the detector of choice for a number of ground based astronomical instruments. By considering key future space-based experiments in astronomy and Earth observing, the SPACEKIDs project aims to develop and demonstrate KIDs technology towards space-based applications. Over the course of the project, developments have been made in broadband optical coupling, cross-talk mitigation, readout electronics and reducing the susceptibility of large format arrays to cosmic rays and stray light. Here we present the latest results from the SPACEKIDs project as well as the current status of two system demonstrators developed to evaluate KID arrays under the both high and low optical loads.

ABSTRACT. In this contribution, we will focus on the development of superconducting hot electron bolometer (HEB) mixers and novel quantum cascade lasers (QCLs) at super-THz frequencies (2-6 THz) at SRON although SRON is also working on superconducting transition edge sensors (TES) for SAFARI instrument on SPICA telescope and kinetic inductance detectors (KIDs) for ground based telescopes and future space telescopes. In the past years, excellent sensitivities of NbN HEB mixers1 have been reported by us in the frequencies from 1.5-6 THz, and also a number of key demonstrations of using a QCL as a local oscillator have been realized2-5. Those progresses are made due to international collaborations with the groups from all over the world, including, for example, CAS-PMO, MIT, University of Arizona, Nizhny Novgorod, and Moscow.

We will start with the research work on HEB and QCL in a close collaboration with Chinese CAS-PMO group and then the use of HEB and QCL technologies, as an application, for a NASA balloon borne telescope, called STO2.

A local oscillator using a THz QCL is crucial for a heterodyne receiver to observe astronomically important fine structured-molecular or -atomic lines at super-THz frequencies. So far no THz QCL has been demonstrated in a space telescope. We report on a 4.7 THz heterodyne receiver being developed to fly on the NASA Stratospheric Terahertz Observatory (STO-2) on a balloon platform. The receiver is based on a 4.7 THz 3rd order distributed feedback QCL as (LO) and is designed for high resolution spectroscopy of the astronomically important neutral oxygen [OI] line at 4.745 THz. STO2 consists of a 0.8 m diameter telescope, which will be launched from Antarctica and will fly at an altitude of 40 km for at least 14 days. Its first flight is scheduled for December 2015.

[1]. W. Zhang, P. Khosropanah, J. R. Gao, E. L. Kollberg, K. S. Yngvesson, T. Bansal, R. Barends, and T. M. Klapwijk “Quantum noise in a terahertz hot electron bolometer mixer”, Appl. Phys. Lett. 96, 111113 (2010). [2] Y. Ren, D.J. Hayton, J.N. Hovenier, M. Cui, J.R. Gao, T.M. Klapwijk, S.C. Shi, T-Y. Kao, Q. Hu, and J.L. Reno, Frequency and amplitude stabilized terahertz quantum cascade laser as local oscillator, Appl. Phys. Lett., 101, 101111 (2012). [3] J. L. Kloosterman, D. J. Hayton, Y. Ren, T. Y. Kao, J. N. Hovenier et al., “Hot electron bolometer heterodyne receiver with a 4.7-THz quantum cascade laser as a local oscillator,” Appl. Phys. Lett., 102, 011123 (2013). [4] M. Cui, J. N. Hovenier, Y. Ren, N. Vercruyssen, J. R. Gao, T. Y. Kao, Q. Hu, and J. L. Reno, Beam and phase distributions of a terahertz quantum cascade wire laser, Appl. Phys. Lett. 102, 111113 (2013). [5] D. J. Hayton, A. Khudchencko, D. G. Pavelyev, J. N. Hovenier, A. Baryshev, J. R. Gao, T. Y. Kao, Q. Hu, J. L. Reno, and V. Vaks, Phase locking of a 3.4 THz third-order distributed feedback quantum cascade laser using a room-temperature superlattice harmonic mixer, Appl. Phys. Lett. 103, 051115 (2013).

ABSTRACT. Detectors are among the critical components of terahertz imaging systems. To make such systems cost-effective it is desirable to use un-cooled detectors and arrays. For applications it is important to estimate, at certain radiation frequency, their ultimate possible noise equivalent power (NEP) to be applicable in active or passive imaging systems. Issues associated with the development of direct detection terahertz detectors on the base of rectifying (Si-MOSFETs, GaN/AlGaN heterojunction FETs) or hot electron bolometer (thin film mercury-cadmium-telluride) types detectors are discussed.Presented analytical equations allow describe parameters for zero biased rectifying type detectors with antennas in dependence on antenna and detector impedance, radiation frequency, some parasitic elements. E.g. estimated minimal value of optical NEP is ~2.6 pW/Hz^1/2 at 300 K for structures with rather typical antenna radiation resistance 100 Ohm and detector resistance 1000 Ohm. The measured values of the optical NEP are sufficiently worse compared to calculated ones, especially at high radiation friquancy (near and over 1 THz), as detectors sensitivity is reduced by parasitic elements (serial resistance and shunting capacity) and because of not optimal matching of the antenna and detector impedances. For comparison semiconductor mercury-cadmium-telluride (MCT) thin layers also were considered as uncooled hot electron bolometer (HEB) THz detectors.

ABSTRACT. We present a study of a cold-electron bolometer with a twin-slot antenna designed to couple radiation at 160 GHz operating at 350 mK. The detector’s absorbing element consists of degenerately-doped strained silicon and has Schottky contacts to superconducting aluminium leads. These contacts allow for direct electron cooling of the absorber to below the phonon temperature, enabling the cold-electron bolometer to achieve much faster time constants (τ < 1 μs) compared to conventional bolometric detectors while not sacrificing sensitivity. We measure both the dark and optically-loaded noise of the detector via a novel method of cross-correlating the outputs of two amplifiers in order to measure noise below the amplifier noise level. From this we calculate the noise-equivalent power of the detector to be 6 × 10^-17 W/rtHz when observing a 77-Kelvin source. We also present a study of the transmission spectrum of the strained-silicon material in the THz region.

ABSTRACT. For the THz imaging it is still a challenge to create multi-element detector focal plane array (FPA), which are operating at the ambient temperatures. Such type new generation of multi element focal plane arrays is expected to enable real-time imaging, reduce the scanning time and increase the information capacity and reliability of the video system by eliminating mechanical scanning components. Currently, there are no large-format THz-range FPAs with good spatial resolution capable of real-time operation. Among the number of various detectors proposed and realized for THz/sub-THz spectral region in recent time, there seem to be two most promising types of semiconductor detectors for the ambient temperature operation: Schottky barrier diodes (SBDs) and field effect transistors (FETs). Both of them have shown the capability of achieving noise equivalent temperature difference NETD~0.5 – 1 K [1], and were used as direct detectors and as nonlinear elements in heterodyne mixers [2,3]. Other types of broadband semiconductor detectors also show good potential of operation in THz/sub-THz range. E.g., antenna coupled AlGaN/GaN HEMT have been mounted on an aplanatic hemispherical silicon substrate lens [4]. The transistors are used as direct power detectors of 590-GHz radiation, achieving system noise-equivalent power values of NEP =1.6x10-10W/√Hz at 590 GHz. Narrow-gap mercury-cadmium-telluride (MCT) semiconductor thin layers with antennas were considered [5] both as sub-terahertz direct detection bolometers, and infrared (IR) photoconductors. NEP for detectors studied at   140 GHz reaches NEP300K  4.5x10-10 W*Hz-1/2. In this way, all these semiconductor detectors are perspective for multielement array implementation by MBE or CMOS technology on thick dielectric substrates. As we have shown earlier [6], the substrate electric thickness in THz FPAs plays a determinative role in the frequency characteristics of the system, and should be much lower than the conventional one for CMOS technology. When the substrate is thick and its permittivity is high, many side lobes appear, and the antenna patterns as well as its frequency dependencies become different for different FPA elements. The gain dependence on the detector position on the finite substrate is the main factor of optical responsivity difference for the detectors in the array. Тhe dependence on the impedance matching coefficient is much weaker. There was proposed to use electrically thin substrates. However there are some technological constrains on the substrate thickness. The goal of the current investigation is to find the alternative ways to improve antenna patterns on the thick substrate. The antenna pattern should be independent on frequency, small changes of substrate dimensions and relative permittivity. Here we propose and model three ways of the solution together with the focusing system: • Substrate micro lens on the top of planar antenna. • Holes in substrate. Decrease substrate modes by creating holes in places of substrates where electric field has maximum. • Conductive layer. Adding conductive layer on bottom of substrate to make modes attenuate faster.

ABSTRACT. TeraSCREEN is an EU FP7 Security project aimed at developing and demonstrating in a live border control environment a safe, privacy respecting, high throughput security screening system which automatically detects and classifies potential threat objects concealed on a person. This will significantly improve both efficiency and security at border checks. TeraSCREEN will develop passive and active operation at several millimeter and submillimeter-wave frequencies to achieve this. The system developed will demonstrate, at a live control point, the safe automatic detection and classification of objects concealed under clothing, whilst respecting privacy and increasing current throughput rates. This innovative screening system will combine multi-frequency, multi-mode images taken by passive and active subsystems which will scan the subjects and obtain complementary spatial and spectral information, thus allowing for automatic threat recognition. This technology was developed for applications in Earth observation and astronomy and is now of increasing interest in ground based applications. The TeraSCREEN project will be an important demonstrator of this technology migrating into the commercial sector. This paper describes the development work at STFC-RAL on the subharmonic mixer operating at 360GHz for both active and passive subsystems. The 360GHz subharmonic mixer has worked as predicted and exhibiting global state of the art performance. A radiometer sensitivity measurement has also been carried out with outstanding performance.

ABSTRACT. We have developed a laboratory prototype camera for passive THz imaging with ultra-high temperature resolution and fast frame capture capability. Lumped Element Kinetic Inductance Detectors (LeKIDs) are constructed in thin film Aluminium on a Silicon substrate. They offer both high sensitivity and high time resolution. The detector array operating temperature is 250mK but this is achieved in an entirely cryogen-free and automatically controlled cooling platform. We present the result of recent experiments to show the capability of this technology and we discuss our plans for improved instruments for civil theatre applications, including video frame rate instruments with simultaneous multi-band capability.

ABSTRACT. This paper describes the development and characterisation of broadband radiometers centred at 94 and 183 GHz for passive millimetre wave imaging and NDT applications. The W-band radiometer has been built and tested using both fundamental and sub-harmonic mixer configurations at the front end. The 183 GHz radiometer uses a sub-harmonic mixer at the front-end and has an LO that is common with the fundamentally pumped W-band receiver. In this case the LO is supplied via a ~30 GHz source which is frequency tripled to 90 GHz with a conversion efficiency of around 10%. The IF bandwidth for both fundamental and sub-harmonic W-band mixers is 18 GHz while the 183 GHz mixer is about 10-12 GHz. The DSB mixer noise temperature was measured to be around 500 K with a conversion loss of approximately 5 dB. The measurement of NETD for these radiometers is also presented.

ABSTRACT. Abstract—This work is focused on wideband terahertz (THz) beam/focus scanning reflectarrays/mirrors, to provide beam scanning solutions to THz real-time imaging systems. For the THz reflectarrays, frequency beam scanning method previously proposed by our group is experimentally verified in a frequency band of 200 ~ 300GHz. Meanwhile, the common mechanical scanning by offsetting the feed is also numerically provided. Based on similar idea, a THz mirror with plane wave incidence is also investigated in order to obtain a frequency scanning focus over a large frequency band. The simulated results are given in this paper. The proposed idea can also be extended to a mirror with frequency scanning focus as it is illuminated by a feed close to the mirror aperture. These achievements are highly desired by the real-time THz imaging systems.

ABSTRACT. This paper presents the development of a 240 GHz source – a GaAs Schottky diode based frequency tripler - for high power G-band Doppler radar application. First results will be presented at the Workshop.

ABSTRACT. We discuss a numerical simulation of high current density InGaAs/AlAs/InP resonant tunneling diodes (RTDs) with a view to their optimization for application in THz emitters. We introduce a figure of merit as a design guideline. A description of the interplay of various parameters follows and we highlight the importance of the emitter and collector doping scheme. An optimized structure utilizing asymmetric barriers and a graded quantum well is discussed.

ABSTRACT. The 346GHz back-ward wave oscillation (BWO) with 100mW power is proposed for a source for fusion plasma diagnostics. The output power of traditional folded waveguide slow wave structure (SWS) is low that can’t reach 100mW, a new SWS with high impedance is designed in the paper. The dispersion curve of the traditional folded wave guide slow wave structure as shown in fig.1 displays that the fundamental wave is back-ward wave, if the back-ward wave is chosen for operation, the operation voltage is very high. So the 1st special harmonic is back-ward wave and chosen for operation, however, the impedance of the 1st special harmonic is too low that the output power is very low. The traditional folded wave guide slow wave structure is improved in the paper, the new folded wave guide slow wave structure is shown in fig.2, and the dispersion curve shows that the fundamental wave is forward wave, so -1st special harmonic is back-ward wave with high impedance, as shown in fig.3. The dispersion, impedance and the tuning characteristic are simulated, and the interaction is also simulated too. The impedance of the traditional FWG and the novel FWG is shown in fig.4. The setting In the PIC module is as flows, the operation current is 15mA, the operation voltage is about 12.3kV, the intensity of permanent magnetic field is 0.8T, the loss of the lossy material is 1.6×107S/m, the result showed that the output power is above 100mW.

ABSTRACT. A Backward Wave Oscillator (BWO) based on Double Corrugated Waveguide (DCW) is designed to operate at 0.346 THz. The dimension of the DCW is selected in order to have maximum interaction impedance and minimum loss at beam voltage of 13 kV. An efficient coupler is designed to connect the DCW to the WR-2 standard rectangular waveguide. Particle in Cell (PIC) simulations are performed using CST particle studio. The results show that the BWO can produce 500mW average output power.

ABSTRACT. Latest developments of a W-band gyrotron travelling wave amplifier (gyro-TWA) with a helically corrugated waveguide and a cusp electron gun for operation at a high pulse repetition rate are presented. Performance upgrades of input coupler, pulsed power system and beam collector with water-cooling capability were realized for the high power wide band gyro-TWA.

ABSTRACT. A novel type of strong-coupling five-gap extended output cavity is investigated for the W-band SBEIK to verify its resonant advantages of mode separation, gap fields coupling and RF energy exporting etc. And the degeneracy mode competition for the strong-coupling five-gap cavity is studied due to its axial period structures. The result shows that the frequency interval between the work mode and vicinal non-working mode is above 600MHz, which is higher enough than the required 100MHz bandwidth for the SBEIK. Then, the 3D PIC simulation software is used to calculate the beam-wave interaction for the strong-coupling five-gap extended output cavity, the expected high output is observed with the corresponding performance and favorable stability.

ABSTRACT. This paper presents the design of a Brewster window for a W-band gyrotron travelling wave amplifier (gyro-TWA). To maintain the Gaussian-like HE₁₁ mode from the corrugated horn, a corrugated waveguide was optimized to host the Brewster window. The Brewster window was simulated and measured to have a lower than -20 dB reflection over the frequency band 85 - 101 GHz.