Berardi Sensale-Rodriguez

Broadband graphene terahertz modulators enabled by intraband transitions.

Terahertz technology promises myriad applications including imaging, spectroscopy and communications. However, one major bottleneck at present for advancing this field is the lack of efficient devices to manipulate the terahertz electromagnetic waves. Here we demonstrate that exceptionally efficient broadband modulation of terahertz waves at room temperature can be realized using graphene with extremely low intrinsic signal attenuation. We experimentally achieved more than 2.5 times superior modulation than prior broadband intensity modulators, which is also the first demonstrated graphene-based device enabled solely by intraband transitions. The unique advantages of graphene in comparison to conventional semiconductors are the ease of integration and the extraordinary transport properties of holes, which are as good as those of electrons owing to the symmetric conical band structure of graphene. Given recent progress in graphene-based terahertz emitters and detectors, graphene may offer some interesting solutions for terahertz technologies.

InAlN/AlN/GaN HEMTs With Regrown Ohmic Contacts and

We report 30-nm-gate-length InAlN/AlN/GaN/SiC high-electron-mobility transistors (HEMTs) with a record current gain cutoff frequency (fT) of 370 GHz. The HEMT without back barrier exhibits an extrinsic transconductance (gm.ext) of 650 mS/mm and an on/off current ratio of 106 owing to the incorporation of dielectric-free passivation and regrown ohmic contacts with a contact resistance of 0.16 Ω·mm. Delay analysis suggests that the high fT is a result of low gate-drain parasitics associated with the rectangular gate. Although it appears possible to reach 500-GHz fT by further reducing the gate length, it is imperative to investigate alternative structures that offer higher mobility/velocity while keeping the best possible electrostatic control in ultrascaled geometry.

f_{T}

The modulation depth of two-dimensional electron-gas (2DEG) based terahertz (THz) modulators using AlGaAs/GaAs hetero-structures with metal gates is inherently limited to <30%. The metal gate not only attenuates the THz signal but also severely degrades modulation depth. Metal losses can be significantly reduced employing an alternative material with tunable conductivity. Graphene presents a unique solution to this problem due to its symmetric band structure and extraordinarily high hole mobility. In this work, we show that it is possible to achieve a modulation depth of >90% while simultaneously minimizing signal attenuation to <5% by tuning the Fermi level at its Dirac point. VC 2011 American Institute of Physics. [doi:10.1063/1.3636435]

of 370 GHz

We demonstrate a graphene-based electro-absorption modulator achieving extraordinary control of terahertz reflectance. By concentrating the electric field intensity in an active layer of graphene, an extraordinary modulation depth of 64% is achieved while simultaneously exhibiting low insertion loss (∼2 dB), which is remarkable since the active region of the device is atomically thin. This modulator performance, among the best reported to date, indicates the enormous potential of graphene for terahertz reconfigurable optoelectronic devices.

Unique prospects for graphene-based terahertz modulators

In this paper, we examine graphene as a material for reconfigurable terahertz (THz) optoelectronics. The ability of electrically tuning its optical properties in a wide range of THz frequencies, together with its 2-D nature and facile integration, leads to unique opportunities for inventing novel THz devices as well as for extending the performance of existing THz technologies. We first review progress in graphene THz active optoelectronic components to date, including large-area graphene, plasmonic, and metamaterial-based devices. Advanced designs and associated challenges are then discussed.

Extraordinary Control of Terahertz Beam Reflectance in Graphene Electro-absorption Modulators

We propose and discuss terahertz (THz) electro-absorption modulators based on graphene plasmonic structures. The active device consists of a self-gated pair of graphene layers, which are patterned to structures supporting THz plasmonic resonances. These structures allow for efficient control of the effective THz optical conductivity, thus absorption, even at frequencies much higher than the Drude roll-off in graphene where most previously proposed graphene-based devices become inefficient. Our analysis shows that reflectance-based device configurations, engineered so that the electric field is enhanced in the active graphene pair, could achieve very high modulation-depth, even ∼100%, over a wide frequency range up to tens of THz.We propose and discuss terahertz (THz) electro-absorption modulators based on graphene plasmonic structures. The active device consists of a self-gated pair of graphene layers, which are patterned to structures supporting THz plasmonic resonances. These structures allow for efficient control of the effective THz optical conductivity, thus absorption, even at frequencies much higher than the Drude roll-off in graphene where most previously proposed graphene-based devices become inefficient. Our analysis shows that reflectance-based device configurations, engineered so that the electric field is enhanced in the active graphene pair, could achieve very high modulation-depth, even ∼100%, over a wide frequency range up to tens of THz.

Graphene for Reconfigurable Terahertz Optoelectronics

In this paper we propose and experimentally demonstrate arrays of graphene electro-absorption modulators as electrically reconfigurable patterns for terahertz cameras. The active element of these modulators consists of only single-atom-thick graphene, achieving a modulation of the THz wave reflectance > 50% with a potential modulation depth approaching 100%. Although the prototype presented here only contains 4x4 pixels, it reveals the possibility of developing reliable low-cost video-rate THz imaging systems employing single detector.

Efficient terahertz electro-absorption modulation employing graphene plasmonic structures

Switchable metamaterials offer unique solutions for efficiently manipulating electromagnetic waves, particularly for terahertz waves, which has been difficult since naturally occurring materials rarely respond to terahertz frequencies controllably. However, few terahertz modulators demonstrated to date exhibit simultaneously low attenuation and high modulation depth. In this letter we propose a new class of electrically-tunable terahertz metamaterial modulators employing metallic frequency-selective-surfaces (FSS) in conjunction with capacitively-tunable layers of electrons, promising near 100% modulation depth and < 15% attenuation. The fundamental departure in our design from the prior art is tuning enabled by self-gated electron layers that is independent from the metallic FSS. Our proposal is applicable to all possible electrically tunable elements including graphene, Si, MoS(2), oxides etc, thus opening up myriad opportunities for realizing high performance switchable metamaterials over an ultra-wide terahertz frequency range.

Terahertz imaging employing graphene modulator arrays

Depletion-mode high-electron mobility transistors (HEMTs) based on a quaternary barrier In_0.13Al_0.83Ga_0.04 N/AlN/GaN heterostructure on SiC substrate were fabricated. The 66-nm-long gate device shows a dc drain current density of 2.1 A/mm, a peak extrinsic transconductance of 548 mS/mm, and a record current gain cutoff frequency f_T of 220 GHz for quaternary barrier GaN-based HEMTs, which is also among the highest <i>f</i>_T for all GaN-based HEMTs. The large <i>L</i>_g ·<i>f</i>_T product of 14.5 GHz ·μm with a gate-length-to-barrier-thickness aspect ratio of 5.8 indicates a high effective electron velocity of 0.9 ×10⁷ cm/s, attributed to a high electron Hall mobility (1790 cm²/V ·s at an <i>n</i>_s of 1.8 ×10^13-2)-the highest reported in GaN-channel HEMTs with In-containing barriers. An intrinsic electron velocity of 1.7 ×10⁷ cm/s, extracted from conventional Moll delay-time analysis, is comparable to that reported in the state-of-art AlGaN/GaN HEMTs.

A new class of electrically tunable metamaterial terahertz modulators

We report on 30-nm-gate-length InAlN/AlN/GaN/SiC high-electron-mobility transistors (HEMTs) with a record current gain cutoff frequency (fT) of 400 GHz. Although the high drain-induced barrier lowering (DIBL) value is indicative of significant short-channel effects, more than seven orders of magnitude in the current on/off ratio was observed. The high fT is a result of minimized parasitic effects and at the expense of a low power gain cutoff frequency (fMAX). The gate length dependence and temperature dependence of fT were also measured.