Lukasz Bednarz

Carbon nanotube composites for broadband microwave absorbing materials

In this paper, we present a new shielding and absorbing composite based on carbon nanotubes (CNTs) dispersed inside a polymer dielectric material. The extremely high aspect ratio of CNTs and their remarkable conductive properties lead to good absorbing properties with very low concentrations. A broadband characterization technique is used to measure the microwave electrical properties of CNT composites. It is shown that a conduction level of 1 S/m is reached for only 0.35 wt % of a CNT, while, for a classical absorbing composite based on carbon black, 20% concentration is mandatory. The conductive properties are explained by a phenomenological electrical model and successfully correlated with rheological data aiming at monitoring the dispersion of conductive inclusions in polymer matrices

Microwave absorbers based on foamed nanocomposites with graded concentration of carbon nanotubes

A multilayered foamed nanocomposite with graded concentration of carbon nanotubes is proposed as a novel microwave absorber. The dispersion and foaming process enables to control the gradation in conductivity of each layer. Absorption performances are demonstrated through measurements of reflectivity and shielding effectiveness over the frequency range [8-16 GHz]. An improvement of about 5 dB in reflectivity is observed with respect to a foamed composite having a similar uniform concentration, while in both cases the shielding effectiveness is kept higher than 15 dB.

Negative Differential Transconductance and Nonreciprocal Effects in a Y-Branch Nanojunction: High-Frequency Analysis

We report on negative differential transconductance (NDT) effect in Y-branch nanojunctions which leads to small-signal nonreciprocity of the device for certain biasing schemes. We present the dc analysis of the device from which we identify the bias regions where NDT effect occurs. We compare experimental dc measurements with results of Monte Carlo simulations, which yields very good qualitative and quantitative agreement. We perform a high-frequency analysis of the NDT effect up to 110 GHz which shows that the effect discussed is still present at such high frequencies

Theoretical and experimental characterization of Y-branch nanojunction rectifier up to 94 GHz

A double Y-branch ballistic junction is proposed for rectification of signals up to 94 GHz. A nonlinear model is developed to predict the frequency dependence of its RF to DC conversion performances, and agrees very well with experiment. The model shows the importance of minimizing extrinsic parasitics when designing HF ballistic nanodevices.

Nanoscaled double Y-branch junction operating as room temperature RF to DC rectifier

Room temperature DC and broadband HF measurements of a double Y-branch junction are presented and discussed. Nonlinear DC characteristics of the devices at room temperature are observed and HF to DC conversion up to 20 GHz at room temperature is presented. Small-signal equivalent circuit of YBJ is proposed. The HF to DC conversion efficiency degradation is found to be due to parasitic crosstalk capacitance between the 2DEG access reservoirs of the device.

Solutions for input impedance matching of nanodevices: Application to Y-Branch Junction HF to DC rectifier

The problem of high input impedance of nanoscaled devices is analyzed for the case of a Y-Branch Junction. An electrical nonlinear model of YBJs validated on measured data is used to show influence of source impedance on HF to DC detection performance in YBJ. An impedance matching network is proposed and is proven to increase the detection sensitivity. Multiple 2DEG channels material used to fabricate parallel YBJs stacked on one another is also proposed as an alternative solution to mismatch problem. It is shown that using multiple 2DEG the input impedance and reflection coefficient can be decreased but at the price of decrease in sensitivity

Experimental studies of heterodyne mixing in Y-branch ballistic nano-junction

We analyze the potential of nanosized Y-branch junctions (YBJ) as devices for heterodyne mixers. We present the results of heterodyne conversion experiments showing frequency independent conversion loss up to 40 GHz input HF frequency. The high conversion loss (-55 dB for LO power PLO = 0 dB) is explained as a result of poor matching of high-impedance YBJ and 50 Omega equipment. The use of multiple YBJs in parallel is proposed as a solution for improvement of conversion loss. The solution is analyzed theoretically using a behavioral model of YBJ. It is found that conversion loss can be reduced by 20 dB if 24 devices are used in parallel.

Analysis of Bandwidth and Nonlinear Effects in InAlAs/InGaAs/InP Based Ballistic Nanodevices for Applications up to THz Range

Analysis of nonlinear effects in InAlAs/InGaAs-based channels and three-terminal ballistic junctions (TBJs) is performed for applications up to THz range. Results show that InGaAs-based channels designed for ballistic operation exhibit high intrinsic cut-off frequency (fT ~ 10 THz). Nonlinear effects in TBJs, which result from device geometry and space charge distribution, show good qualitative agreement with published results.