Mohammad Ali Khorrami

Terahertz plasmon amplification using two-dimensional electron-gas layers

In this study, we present an analytical model to investigate the possibility of guiding and amplifying terahertz (THz) plasmons in a two dimensional electron gas (2DEG) layer of a hetero-structure by applying a bias electric field. This analytical model solves Maxwell equations and semi-classical electronic transport equations inside the biased hetero-structure simultaneously. It is shown that the two dimensional plasmon’s properties alter vastly as the electrons are accelerated by the bias field. Four asymmetric plasmonic modes can propagate inside the un-gated 2DEG layer of the biased hetero-structure. One of these modes in the un-gated 2DEG layer is a growing mode which can be useful in the implementation of THz amplifiers. Since the modes characteristics can be controlled via biasing, design of new plasmonic devices such as modulators and switches is possible by this approach. Similar analysis has been performed in a gated 2DEG layer that shows clear changes in the two dimensional plasmon properties d...

Global Modeling of Active Terahertz Plasmonic Devices

In this study, a full wave numerical technique is employed to characterize the propagation properties of 2-D plasmons along two-dimensional electron gas (2DEG) layers of biased hetero-structures at terahertz frequencies. This method is based on a coupled solution of Maxwell and hydrodynamic transport equations. In this manner, a complete description of carrier-wave interactions inside the 2DEG layer is obtained. Particularly, this simulator is employed to investigate the 2-D plasmon variations initiated by the application of an external bias along the hetero-structure. Substantial changes in the plasmon characteristics such as wavelength and decay length are reported. It is also revealed that two symmetrical plasmonic modes along the unbiased 2DEG layer split into new asymmetrical ones after applying the bias voltage. The simulation has been performed in different structures to examine the effects of various electron densities and the presence of periodic metallic gratings on the plasmon properties. Moreover, the 2-D plasmon reflections from boundaries terminated by ohmic contacts are separately studied. This research demonstrates the potentials of the 2-D conductors in the design of novel active terahertz plasmonic devices as modulators and amplifiers while proposing a new approach for their modeling. The results of this simulation are verified independently with an analytical model.

Design and analysis of a silicon-based terahertz plasmonic switch.

In this paper, a novel terahertz (THz) plasmonic switch is designed and simulated. The device consists of a periodically corrugated n-type doped silicon wafer covered with a metallic layer. Surface plasmon propagation along the structure is controlled by applying a control voltage onto the metal. As will be presented, the applied voltage can effectively alter the width of the depletion layer appeared between the deposited metal and the semiconductor. In this manner, the conductivity of the silicon substrate can be successfully controlled due to the absence of free electrons at the depleted sections. Afterwards, the effectiveness of the proposed plasmonic switch is enhanced by implementing a p(++)-type doped well beneath the metallic indentation edges. Consequently, a P-Intrinsic-N diode is formed which can manipulate the plasmon propagation by modifying the electron and hole densities inside the intrinsic area. The simulation results are explained very concisely by the help of scattering matrix formalism. Such a representation is essential as employing the switches in the design of complex plasmonic systems with many interacting parts.

Optimized virtual ground fence for power delivery filtering of mixed-signal systems

In this paper, an efficient noise isolation technique in mixed-signal systems using semi-lumped element resonators is presented. The resonators act as a filter which can be implemented between the power delivery networks of RF and digital circuits. The power filtering is accomplished due to a short circuit that can be induced between the ground and power planes of the RF domain at the designed resonant frequency. Each resonator is consisted of a high-impedance micro-strip trace, acting as an inductor and a low-impedance open-circuited line providing a desired capacitor. As an array of the resonators surrounds the sensitive analog section of a mixed-signal system, a virtual ground fence (VGF) can be provided. It is shown that the semi-lumped element based VGFs require less PCB area compared to the ones implemented by quarter-wavelength open-circuited stubs. To show the effectiveness of the proposed method, a fast semi-analytical technique using a physics-based via model is employed. To this end, the semi-analytical method is altered accordingly to fit the problem of the plane-pair with the vias and the VGFs. To compare the simulated results, a full-wave finite-element based solver is utilized.

Analytical modeling of THz wave propagation inside ungated two dimensional electron gas layers

Plasma wave propagation along an ungated two Dimensional Electron Gas (2DEG) layer of a hetrostructure is studied. It is shown that the wave can be useful in amplification of THz signals. An analytical solution of Maxwell and Hydrodynamic equations is presented. This method provides an insight into electromagnetic modes allowed to propagate along the 2DEG as electrons are in motion with constant average drift velocity. Besides, wave impedances of the modes are illustrated. Afterwards, a simple matching network design for input and output ports of the 2DEGs is developed.

EMI control performance of the absorbing material for application on flexible cables

Common mode signals on flexible cables often cause unwanted EMI. Common mode currents can be controlled by applying electromagnetic absorbing materials to the flexible cables. This work presents a set of specially designed test apparatuses that allow the performance of absorbing materials to be estimated for three typical structures known to radiate: monopole antennas, loop antennas, and transmission lines. Two materials are compared for each of the apparatuses, and the resulting common mode and EMI metrics are presented. The amount and placement of absorbing material was optimized based on reduction of EMI and impact on signal integrity. The better-performing materials were further tested in the actual hardware (television), showing a lower reduction in EMI (of about 3 dB) compared to the test apparatuses. The possible reason for reduced performance is that the flexible cable may not be the only source of radiation in the more complex real system.

Study on the reduction of heatsink radiation by combining grounding pins and absorbing materials

This paper describes a study on the effectiveness of grounding pins and high-frequency absorbers to reduce the emissions of heatsinks up to 20 GHz. The study considers four different types of excitations, namely a direct excitation, an excitation by a patch antenna, an excitation by a microstrip below the center of the heatsink, and an excitation by a microstrip that was off-set with respect to the center of the heatsink. Grounding post are positioned equidistantly along the edges of the heatsink and the number of grounding pins per side is varied. Adding grounding pins significantly reduces the emission of the heatsink below 1-2 GHz. At higher frequencies, they lose their capability to reduce the heatsink emission and even introduce extra resonance frequencies where the emission is higher compared to the ungrounded heatsink. Three different possible arrangements for the application of absorbing material were considered, namely a thin flat absorber just below the heatsink, a ring absorber between the heatsink and the PCB, and a collar absorber around the heatsink. The absorber succeeds in reducing the heatsinks emission above 1-2 GHz, but has little of no positive effect on the emissions below 1-2 GHz. Only by the combination of the grounding pins and the absorber material, a reduction of the heatsinks emission is achieved over the full frequency range.

Noise reduction of bended differential transmission lines using compensation capacitance

In this paper, a novel technique to reduce the common mode noise of bended differential transmission lines is proposed. To this end, open circuited transmission lines are employed to compensate the undesirable capacitance of the bends. This method can remarkably decrease the differential to common mode conversion ratio in a very wide frequency range while offering an acceptable level of input return loss compared to un-compensated bended differential transmission lines. To present the effectiveness of this technique, time domain simulations have also been performed which show high level of common mode noise reduction offered by this method. This technique is examined on the right angle and 45 degree bended differential lines implemented inside microstrip and stripline structures.

Compact terahertz Surface Plasmon switch inside a Two Dimensional Electron Gas layer

The possibility of realizing a terahertz (THz) switch by employing Surface Plasmons (SPs) along a Two Dimensional Electron Gas (2DEG) layer of a hetero-structure is presented. It is shown that SPs properties may be easily controlled by changing the motion of the electrons inside the 2DEG. The electron drift velocity is controlled by applying an external bias voltage at the 2DEGs ends. A compact and efficient THz switch with high On/OFF signal ratio is reported using this concept. The control voltage of the switch is considerably low. A multi-physic simulator, based on numerical solution of Maxwells and Boltzmanns equations, is developed to analyze the switch appropriately. This micro-meter size plasmonic switch demonstrates a very promising method for navigating the sub-wavelength THz signals inside future plasmonic circuits.

Employment of microwave absorbers for EMI/RFI mitigations from high speed digital buses with signal integrity considerations

An analysis of electromagnetic radiation reduction achieved by placement of microwave absorbers over a serial high speed digital channel is performed. The study is performed on a second generation peripheral component interconnect express (PCIe Gen II) interface with 5.0 Gbps transfer rate. A set of full wave simulations are performed on a printed circuit board with the PCIe interface implemented in an embedded micro-strip line structure. Lossy material patches are applied onto the channel to reduce the electromagnetic radiations from differential pairs. This reduction might be required especially in compact mixed signal systems as noisy digital circuits are located close to radio-frequency receivers with stringent sensitivity limits. Using the numerical solver, it has been shown that the application of the absorber can reduce the magnitude of the received electromagnetic fields at the desired locations while unfortunately affecting the signal integrity performance of the bus. In order to reduce the unintentional discontinuity introduced along the differential pairs as applying the material on the PCB, the insertion of a dielectric spacer sheet between the absorber and the solder mask is investigated. It has been shown that the presence of a thin layer of the spacer (less than 0.2mm) can significantly decrease the introduced mismatch while still keeping the benefit of the EM reduction. Full wave simulation results are being confirmed by near field probing set-up and a PCIe compliance test board.