Shubhendu Bhardwaj

Novel Phaseless Gain Characterization for Circularly Polarized Antennas at mm-Wave and THz Frequencies

We propose a new method of gain characterization for circularly polarized antenna arrays especially effective for mm-wave/THz frequencies. The method does not require phase-measurement or rotation of the antenna under test (AUT). It is thus ideal for waveguide-based frequency bands. In contrast to conventional methods, we use reflection-only measurements, utilizing readily available geometries, such as a PEC-plate and a PEC-dihedral corner reflector, to estimate the co- and cross-polarized gain of the AUT. Predicted error using this approach is less than 0.07 dB for the co-pol and 0.26 dB for the cross-pol for an AUT with 17 dBi gain at 100 GHz. Experimental results for a radial line slot array antenna, operating in F-band, show good agreement between the conventional method, and that proposed using phase-less method.

Resonant tunneling assisted propagation and amplification of plasmons in high electron mobility transistors

A rigorous theoretical and computational model is developed for the plasma-wave propagation in high electron mobility transistor structures with electron injection from a resonant tunneling diode at the gate. We discuss the conditions in which low-loss and sustainable plasmon modes can be supported in such structures. The developed analytical model is used to derive the dispersion relation for these plasmon-modes. A non-linear full-wave-hydrodynamic numerical solver is also developed using a finite difference time domain algorithm. The developed analytical solutions are validated via the numerical solution. We also verify previous observations that were based on a simplified transmission line model. It is shown that at high levels of negative differential conductance, plasmon amplification is indeed possible. The proposed rigorous models can enable accurate design and optimization of practical resonant tunnel diode-based plasma-wave devices for terahertz sources, mixers, and detectors, by allowing a precise representation of their coupling when integrated with other electromagnetic structures.

Full-wave hydrodynamic model for predicting THz emission from grating-gate RTD-gated plasma wave HEMTs

We provide an initial perturbation to the channel, by applying a small E-field pulse of amplitude Ex=10-3 V/cm at the center. Power emitted due to positive feedback is then recorded at the measurement planes, depicted in Fig. 1b. Shown in Fig. 6a is the simulated emitted spectrum for two devices. Sharp self-sustained oscillations are observed at the plasmonic resonance frequencies. As expected, these resonance frequencies are functions of grating period (L), and the electron density (n). The simulations predict stronger resonances for higher orders at low temperatures, thus higher carrier mobilities (see Fig 6b). Our simulations are able to predict for the first time the terahertz power-emission levels of RTD-gated plasma wave HEMTs.

Numerical Analysis of Terahertz Emissions From an Ungated HEMT Using Full-Wave Hydrodynamic Model

In this paper, we show how plasma-wave instability in an asymmetrically biased ungated InGaAs high-electron mobility transistor (HEMT) leads to terahertz emissions. Numerical calculations are provided using a new Maxwell-hydrodynamic solver. Using this solver, an accurate plasma-wave model is presented, accounting for nonuniform surroundings and finite dimensions of the 2D electron gas (2DEG) layer within the HEMT. We estimate that hundreds of nanowatts of power can be expected from such devices under ideal boundary conditions and sufficient channel mobility. Effects due to variations of carrier velocity, carrier concentration, and 2DEG confinement on the emitted power levels are also considered to provide design guidelines.

Room temperature detection of plasma resonances using multiple 2DEG channels in HEMT

A high electron mobility transistor (HEMT) configuration is proposed using multiple mutually coupled 2DEG layers to detect plasma waves at room temperatures. We show a 6 dB improvements in the transmission spectra, as opposed to single 2DEG layer, using a 4 channel HEMT configuration. To conduct this analysis, we developed and present a full-wave numerical model for self-consistent solution of the hydrodynamic equations coupled with the Maxwells equations.

Analytical and multiphysial-numerical models for plasma-waves in double and multiple channel HEMTs

We explore the possibilities of plasma-wave devics using multiple channel HEMT (high electron mobility transistor) structures. The multiple channel HEMTs are studied using rigorous analytical methods and full-wave-hydrodynamic numerical models. Using the electromagnetic field profiles and hydrodynamic conductivity, we first derive dispersion relations for the plasma-wave propagation within double channel systems. Further, using the numerical solutions, we extend the study to multiple (>2) channel systems. Results for specific applications of resonant and non-resonant plasma-wave devices are then discussed.

Analysis of plasma-modes of a gated bilayer system in high electron mobility transistors

We present rigorous analytical and computational models to study the plasma-waves in a gated-bilayer system present in a double-channel high electron mobility transistor. By analytically deriving the dispersion relations, we have identified the optical and acoustic modes in such systems. We find that the presence of the metal gate selectively modifies the optical plasmons of an ungated-bilayer, while the acoustic plasmons remain largely unchanged. Analysis shows that these modified optical plasmons could be advantageous for resonant and non-resonant plasma-wave devices. The paper further serves to verify our analytical formulae using a full-wave hydrodynamic numerical solver, based on finite difference time domain algorithm. Using the solver, we examine these modes in the gated/ungated bilayers under a plane wave excitation. We observe that, most incident power couples to the optical mode for such an excitation. Nevertheless, acoustic modes can also be excited, if the discontinuity dimensions are optimized accordingly. These observations are also explained using 2D field-plots for the first time, thus providing intuitive understanding of the plasmon excitation in the bilayers.

Septum-less, hexagonal waveguide based circularly polarized horn antenna for mm-wave and terahertz band

T Traditional circularly polarized (CP) horn antennas use a septum (or a partition) based feed, which is difficult to fabricate beyond 60 GHz. As an alternative, here we propose hexagonal waveguide based CP antenna design that is fabrication friendly and low-cost. Design, fabrication and characterization is shown for an F-band prototype. The prototype exhibits 18 dBi mid-band gain with three dB axial-ratio (AR) bandwidth beyond thirty percent.

Full-wave-hydrodynamic modeling of 2DEG graphene channels for terahertz devices using ADI-FDTD algorithm

We show time-efficient terahertz modeling of Graphene-layer and other 2D electron gas (2DEG) systems using the alternating-direction-implicit finite-difference-time-domain (ADI-FDTD) scheme coupled with hydrodynamic solver. The proposed approach allows significant reduction in simulation-times with acceptable accuracy levels. Having developed the solver, we present its accuracy and time-efficiency related results for plasmonic oscillations in graphene at terahertz frequencies.

Resonant tunneling diodes for feeding antenna-structures for terahertz source applications

Resonant tunneling diodes (RTDs) have negative differential resistance (NDR) that enables them to produce power at several terahertz (THz). In this paper, an AlAs/GaAs/AlAs RTD is optimized for maximum output power and large negative differential resistance to improve matching to the antenna. Simulations for the designed RTD are based on the quantum-mechanical global coherent model.