Dielectric rod waveguide components at sub-THz frequencies

This thesis is focused on the development of dielectric rod waveguide (DRW) components at sub-THz frequencies. DRWs proved themselves as low loss transmission lines at sub-millimeter wave and THz frequencies; they can be well matched with rectangular metal waveguides, and also used as antennas. In addition, the DRW allows integration of various components using standard microfabrication techniques, e.g. the bolometric power sensor can be integrated in the center of the DRW and measure the power travelling in the DRW, and a phase shifter based on a high impedance surface (HIS) can be manufactured on the surface of the DRW and can change the phase of the propagating wave inside the DRW. In the first part of this thesis the narrow band and wide band DRW antennas were designed, manufactured and tested. The DRW antennas are lightweight, compact and easy to manufacture. The narrow band DRW antenna proved to operate in the range of 220 – 325 GHz. The wideband DRW antenna showed a constant performance over the band of 75 – 1100 GHz according to numerical simulations and over the band of 75 – 325 GHz experimentally. The radiation patterns of the antenna were measured in co- and cross-polarization. The co-polarization radiation patterns are nearly independent of frequency. The 3 dB beamwidth is 50º - 60º, and the 10 dB beamwidth is about 95º. The return loss of the antenna is better than 15 dB. In the second part of this thesis the bolometric power sensor integrated into DRW was designed, manufactured and tested at frequencies 75 – 1010 GHz. The power sensor consists of a metallic antenna -like structure in the center of the DRW in E-plane suspended on a membrane over an airgap to improve the thermal insulation. The power sensor showed good matching with the rectangular metal waveguides and constant responsivity over the wide band of frequencies, as well as a linear responsivity up to 500 mW applied power. In the third part of this thesis the microelectromechanical system (MEMS) tunable HIS was developed for integration on to the surface of a DRW using suspended carbon nanotube (SWCNT) film as a movable element of the HIS. The implementation of a SWCNT network as a material for movable suspended film allows to significantly simplify the fabrication process of the HIS due to a simple technique of the SWCNT film deposition by dry transfer, and additionally it allows to reduce the actuation voltage of the HIS due to the low Young's modulus of the SWCNT network. The unique deposition technique of the SWCNT film allows to design a HIS phase shifter directly on the surface of the DRW. The suspended SWCNT film structure showed the tunability of the capacitance of 100% at 0 – 10 V applied bias voltage. Such properties allow to create a SWCNT MEMS HIS with a phase shift of 260° at 0 – 7 V bias voltage

A heterodyne graphene FET detector at 400 GHz

We present a THz heterodyne detector based on a single layer graphene field effect transistor (GFET) integrated with a bowtie antenna at 400 GHz. The heterodyne detection is achieved by coupling RF and LO signals quasi-optically to the same GFET. The down converted IF signal is extracted via a coplanar stripline connected to the GFET source and drain terminals. The measured IF bandwidth is 5 GHz

Geometrical magnetoresistance effect and mobility in graphene field-effect transistors

Further development of the graphene field-effect transistors (GFETs) for high-frequency electronics requires accurate evaluation and study of the mobility of charge carriers in a specific device. Here, we demonstrate that the mobility in the GFETs can be directly characterized and studied using the geometrical magnetoresistance (gMR) effect. The method is free from the limitations of other approaches since it does not require an assumption of the constant mobility and the knowledge of the gate capacitance. Studies of a few sets of GFETs in the wide range of transverse magnetic fields indicate that the gMR effect dominates up to approximately 0.55 T. In higher fields, the physical magnetoresistance effect starts to contribute. The advantages of the gMR approach allowed us to interpret the measured dependencies of mobility on the gate voltage, i.e., carrier concentration, and identify the corresponding scattering mechanisms. In particular, the range of the fairly constant mobility is associated with the dominating Coulomb scattering. The decrease in mobility at higher carrier concentrations is associated with the contribution of the phonon scattering. Analysis shows that the gMR mobility is typically 2-3 times higher than that found via the commonly used drain resistance model. The latter underestimates the mobility since it does not take the interfacial capacitance into account.Comment: The following article has been submitted to Applied Physics Letters. After it is published, the DOI will be found her

Broadband Flexible Graphene RF Power Detectors

With the development of wearable radios, foldable Wi-Fi devices, and conformal wireless sensors,flexible radio frequency (RF) electronics have become a highly active research field [1, 2]. As anessential component for both RF transmitters and receivers, a power detector is required to withstandhigh levels of strain. The flexible RF power detectors based on laminated Si and III-V membranes onpolymer substrates demonstrate poor mechanical reliability, restricting the range of applications [3, 4].In contrast, graphene is an ideal candidate for the use in flexible RF power detectors, because it offersoutstanding electrical and mechanical properties [5]. Furthermore, graphene can be grown over largeareas by chemical vapour deposition and transferable to various flexible substrates [6].In this work, we demonstrate RF power detection up to 67 GHz using coplanar access graphene field -effect transistors (GFETs) on flexible and transparent polyethylene terephthalate substrates. Thefrequency dependence of measured and modelled voltage responsivity at optimum gate bias is shownin Figure 1. At room temperature, this detector reveals voltage responsivity above 10 V/W over thefrequency range from 1 GHz to 67 GHz. The measured voltage responsivity results are well fitted by thenonlinear empirical model [7] with all parameters extracted from S-parameter and DC measurements.In addition, we study the effects of interfacial capacitance, associated with traps, on the hysteresis ofvoltage responsivity. Figure 2 shows the measured voltage responsivity as a function of the gate voltagewith reproducible hysteresis loop at 55 GHz

Optimization of THz graphene FET detector integrated with a bowtie antenna

This paper discusses the integration of the split bowtie antenna with a graphene FET THz detector to maximize the detector efficiency at 1 THz. The detector utilizes the principle of distributed resistive self-mixing in GFET, and the split bowtie antenna provides an asymmetric feed to the GFET. The antenna is placed on a substrate lens to improve the directivity and can be used to create an imaging array. The dimensions of the split bowtie antenna are optimized for the best impedance matching with the GFET and to improve the pixel density of the array. The off-axis pixel performance is improved by modifying the edge-pixel antennas. The improvement in directivity of corresponding pixels is up to 1.3 dB

Graphene Field-Effect Transistors With High Extrinsic fT{f}_{T} and fmax{f}_{\mathrm{max}}

In this work, we report on the performance of graphene field-effect transistors (GFETs) in which the extrinsic transit frequency (fT) and maximum frequency of oscillation (fmax) showed improved scaling behavior with respect to the gate length (Lg). This improvement was achieved by the use of high-quality graphene in combination with successful optimization of the GFET technology, where extreme low source/drain contact resistances were obtained together with reduced parasitic pad capacitances. GFETs with gate lengths ranging from 0.5 μm to 2 μm have been characterized, and extrinsic fT and fmax frequencies of up to 34 GHz and 37 GHz, respectively, were obtained for GFETs with the shortest gate lengths. Simulations based on a small-signal equivalent circuit model are in good agreement with the measured data. Extrapolation predicts extrinsic fT and fmax values of approximately 100 GHz at Lg=50 nm. Further optimization of the GFET technology enables fmax values above 100 GHz, which is suitable for many millimeter wave applications

A flexible graphene terahertz detector

We present a flexible terahertz (THz) detector based on a graphene field-effect transistor fabricated on a plastic substrate. At room temperature, this detector reveals voltage responsivity above 2 V/W and estimated noise equivalent power (NEP) below 3 nW/Hz1/2 at 487 GHz. We have investigated the effects of bending strain on DC characteristics, voltage responsivity, and NEP of the detector, and the results reveal its robust performance. Our findings have shown that graphene is a promising material for the development of THz flexible technology

A 400-GHz Graphene FET Detector

This letter presents a graphene field effect transistor (GFET) detector at 400 GHz, with a maximum measured optical responsivity of 74 V/W, and a minimum noise-equivalent power of 130 pW/Hz1/2. This letter shows how the detector performance degrades as a function of the residual carrier concentration in the graphene channel, which is an important material parameter that depends on the quality of the graphene sheet and contaminants introduced during the fabrication process. In this work, the exposure of the graphene channel to liquid processes is minimized resulting in a low residual carrier concentration. This is in part, an important contributing factor to achieve the record high GFET detector performance. Thus, our results show the importance to use graphene with high quality and the importance to minimize contamination during the fabrication process

Wavelength-dependent photoconductivity of single-walled carbon nanotube layers

A number of electronic devices such as phase shifters, polarizers, modulators, and power splitters are based on tunable materials. These materials often do not meet all the requirements namely low losses, fast response time, and technological compatibility. Novel nanomaterials, such as single-walled carbon nanotubes, are therefore widely studied to fill this technological gap. Here we show how the dielectric constant of single-walled carbon nanotube layers can be substantially modified by illuminating them due to unique light–matter interactions. We relate the optical excitation of the nanotube layers to the illumination wavelength and intensity, by resistance and capacitance measurements. The dielectric constant is modified under laser illumination due to the change of material polarization and free carrier generation, and is shown to not be temperature-related. The findings indicate that SWCNT layers are a prospective tunable optoelectronic material for both high and low frequency applications.QC 20190515</p

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A general framework for extending means to higher orders. (English summary) Colloq. Math. 113 (2008), no. 2, 191–221. The authors develop a theory of higher-order means in general metric spaces and then they apply the results to Hilbert-space positive operators. In particular, they solve a problem by D. Petz and R. Temesi [SIAM J. Matrix Anal. Appl. 27 (2005), no. 3, 712–720 (electronic); MR2208330 (2006j:47032)] by proving that the well-known logarithmic mean admits an extension to any order n for each n ≥ 3