Chowdhury Al-Amin

Tunable Room Temperature THz Sources Based on Nonlinear Mixing in a Hybrid Optical and THz Micro-Ring Resonator

We propose and systematically investigate a novel tunable, compact room temperature terahertz (THz) source based on difference frequency generation in a hybrid optical and THz micro-ring resonator. We describe detailed design steps of the source capable of generating THz wave in 0.5–10 THz with a tunability resolution of 0.05 THz by using high second order optical susceptibility (χ(2)) in crystals and polymers. In order to enhance THz generation compared to bulk nonlinear material, we employ a nonlinear optical micro-ring resonator with high-Q resonant modes for infrared input waves. Another ring oscillator with the same outer radius underneath the nonlinear ring with an insulation of SiO2 layer supports the generated THz with resonant modes and out-couples them into a THz waveguide. The phase matching condition is satisfied by engineering both the optical and THz resonators with appropriate effective indices. We analytically estimate THz output power of the device by using practical values of susceptibility in available crystals and polymers. The proposed source can enable tunable, compact THz emitters, on-chip integrated spectrometers, inspire a broader use of THz sources and motivate many important potential THz applications in different fields.

Deep Sub-Wavelength Multimode Tunable In-Plane Plasmonic Lenses Operating at Terahertz Frequencies

We report on focusing of terahertz (THz) plasmons in two-dimensional electron gas (2DEG) at III-N heterostructures and in graphene by using planar circular grating lenses. Propagation of a broadband pulse of EM waves in 0.5-10 THz is studied by using finite difference time-domain (FDTD) approach. The results show that plasmonic modes excited by incident THz radiation are concentrated into λ/180 area localized under the central disc. Electric field intensity under the central point is found to be orders of magnitude larger than the outer grating area. Optimal geometries for enhanced radiation coupling and plasmon focusing are investigated. Plasmonic lens modes supported by system has the advantage of tunability by an applied voltage to gratings. The large field enhancement by plasmonic confinement presents the potential for sub-wavelength imaging.

Improving High-Frequency Characteristics of Graphene FETs by Field-Controlling Electrodes

We propose and extensively analyze a novel graphene-FET (GFET) with two capacitively coupled field-controlling electrodes (FCE) at the access region. The dc and RF characteristics of the proposed device are studied using analytical and numerical techniques and compared with the baseline designs. The independently biased FCEs could control the electric field and sheet carrier concentration at the ungated region, and thus reduce the access resistance effectively. The reduction of source/drain access resistance results in improved fT and fMAX compared with those of conventional GFETs of the same geometry. The proposed device with improved characteristics of GFET can be used for high-frequency applications.

Subwavelength, multimode, tunable plasmonic terahertz lenses and detectors

We report on sub-wavelength THz plasmonic lenses based on 2 dimensional electron gas (2DEG) at AlGaN/GaN interface and also on few-layer graphene sheets. Circular gratings investigated in this study concentrate THz electric field into deep sub-wavelength scale by plasmonic excitations polarization independently. Propagation of a broadband pulse of EM waves in 0.5-10 THz was simulated by using a commercial FDTD simulation tool. The results show that concentric plasmonic grating structures can be used to concentrate THz into deep sub-wavelength down to λ/350 spot size and achieve very large field enhancements by plasmonic confinement which can be used for THz detection and possibly for sub-wavelength imaging. Electric field intensity under the central point can be orders of magnitude higher than the outer grating area. Moreover, plasmonic lens modes supported by system can be tuned with an applied voltage to gratings.

Bandgap engineering of single layer graphene by randomly distributed nanoparticles

In this paper, we have proposed and experimentally demonstrated engineering the bandgap of semi-metal zero bandgap single layer chemical vapor deposited graphene by decorating with randomly distributed nanoparticles. Decoration with two types of nanoparticles was investigated: zinc oxide nano-seed grown by sonication of zinc acetate dihydrate, and gold nanoparticles grown by thermal annealing of sputtered gold thin film. The proximity of nanoparticles and graphene break graphene’s sublattice symmetry and opens-up a bandgap. The grown nanoparticles were characterized by scanning electron microscopy and atomic force microscopy. The single layer graphene before and after decoration was characterized with Raman spectroscopy. The bandgap was engineered by simply adjusting the nanoparticle size and density, and the introduced bandgap was measured from the slope of temperature dependent conductivity characteristics. Graphene with significant bandgap introduced by the proposed methods could be used for devices intended for high speed digital and logic applications.

Nonlinear optical resonators for tunable THz emission

We designed and theoretically investigated nonlinear optical micro-ring resonators for tunable terahertz (THz) emission in 1-10 THz range by using difference frequency generation (DFG) phenomenon with large second order optical nonlinearity (χ(2)). Our design consists of a nonlinear ring resonator and another ring underneath capable of sustaining high-Q resonant modes for infrared pump beams and the generated THz radiation, respectively. The nonlinear ring resonator generates THz through DFG by mixing the input waves carried by a pair of waveguides. The proposed device can be a viable platform for tunable, compact THz emitters and on-chip integrated spectrometers.

Graphene FETs with Low-Resistance Hybrid Contacts for Improved High Frequency Performance

This work proposes a novel geometry field effect transistor with graphene as a channel—graphene field-effect transistor (GFET), having a hybrid contact that consists of an ohmic source/drain and its extended part towards the gate, which is capacitively coupled to the channel. The ohmic contacts are used for direct current (DC) biasing, whereas their capacitive extension reduces access region length and provides the radio frequency (RF) signal a low impedance path. Minimization of the access region length, along with the paralleling of ohmic contact’s resistance and resistive part of capacitively coupled contact’s impedance, lower the overall source/drain resistance, which results in an increase in current gain cut-off frequency, fT. The DC and high-frequency characteristics of the two chosen conventional baseline GFETs, and their modified versions with proposed hybrid contacts, have been extensively studied, compared, and analyzed using numerical and analytical techniques.

Lowering contact resistance of graphene FETs with capacitive extension of ohmic contacts for enhanced RF performance

In this work, we propose a novel Graphene field effect transistor (GFET) with ohmic Source/Drain contacts having capacitive extension towards the Gate. The ohmic contacts of the proposed GFET are used for DC biasing as like as conventional GFETs whereas their extended parts which are capacitively coupled to the channel reduce access region length as well as the access resistance and provide a low impedance route for the high frequency RF signal. Reduction of access resistance along with the paralleling of ohmic contact resistance and real part of capacitive impedance result in an overall lower Source/Drain resistance which eventually increases the current gain cut-off frequency, fT. We have studied and compared the DC and RF characteristics of the baseline conventional GFET and proposed GFET using analytical and numerical techniques.

Microdisk resonators for difference frequency generation in THz range

We theoretically investigated and designed a tunable, compact THz source in 1-10 THz range based on a nonlinear optical microdisk resonator. The lack of tunable THz source operating at room temperature is still one of the major impediments for the applications of THz radiation. The proposed device on an insulated borosilicate glass substrate consists of a nonlinear optical disk resonator on top of another disk capable of sustaining THz modes. A pair of Si optical waveguides is coupled to the nonlinear microdisk in order to carry the two input optical waves. Another pair of Si THz waveguides is placed beneath the input optical waveguides to couple out the generated THz radiation from the disk to receiver antenna. Both optical and THz disks are engineered optimally with necessary effective mode indices in order to satisfy the phase matching condition. We present the simulation results of our proposed device using a commercial finite element simulation tool. A distinguished THz peak coincident exactly with the theoretical calculations involving DFG is observed in frequency spectrum of electric field in the microdisk resonator. Our device has the potential to enable tunable, compact THz emitters and on-chip integrated spectrometers.

Dispersion studies in THz plasmonic devices with cavities

Analytical and numerical studies of the dispersion properties of grating gated THz plasmonic structures show that the plasmonic crystal dispersion relation can be represented in terms of effective index of the dielectric medium around the 2DEG for the plasmons. Forbidden energy band gaps are observed at Brillion zone boundaries of the plasmonic crystal. FDTD calculations predict the existence of the plasmonic modes with symmetrical, antisymmetrical and asymmetrical charge distributions. Breaking the translational symmetry of the crystal lattice by changing the electron concentration of the two dimensional electron gas (2DEG) under a single gate line in every 9th gate induces a cavity state. The induced cavity state supports a weekly-coupled cavity mode.