Unique prospects for graphene-based terahertz modulators

Abstract

The modulation depth of 2-D electron gas (2DEG) based THz modulators using AlGaAs/GaAs heterostructures with metal gates is inherently limited to < 30%. The metal gate not only attenuates the THz signal (> 90%) but also severely degrades the modulation depth. The metal losses can be significantly reduced with an alternative material with tunable conductivity. Graphene presents a unique solution to this problem due to its symmetric band structure and extraordinarily high mobility of holes that is comparable to electron mobility in conventional semiconductors. The hole conductivity in graphene can be electrostatically tuned in the graphene-2DEG parallel capacitor configuration, thus more efficiently tuning the THz transmission. 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 the Dirac point in graphene.

Broadband graphene terahertz modulators enabled by intraband transitions

“…4, the theoretical 27 and experimental (from refs 25,26 and this work) intensity transmittance is plotted for several 2DEG-based terahertz modulator devices as a function of conductivity, showing a good agreement between our theory and experiments and predicting a nearly 100% modulation by single-layer graphene, using the highest experimental conductivity values reported in the literature 29 . The excellent match between the simulation and the experimental results using the DC graphene conductivity also suggests that the carrier scattering time in graphene τ is < 250 fs as σ(ω) = σ DC (E F )/(1 + ω 2 τ 2 ).…”

“…1b) or multiple (Fig. 1c) capacitively coupled graphene-semiconductor or graphenegraphene pairs, so that holes are induced in one layer whereas electrons are induced in the second layer 27 . At zero bias, the Fermi level is at the Dirac point of all graphene layers, thus introducing minimum insertion loss or signal attenuation.…”

“…Graphene supports a high carrier concentration (up to 1×10 14 cm − 2 ) 28 as well as outstanding carrier mobility ( > 20,000 cm 2 V − 1 s − 1 for n s ~5×10 12 cm − 2 ) 29 for both holes and electrons owing to its symmetric conical band structure, which leads to a good tunable conductivity range and a high maximum hole/electron conductivity comparable to that of conventional epitaxial semiconductors achievable only by electrons. Our theoretical calculations 27 show that transmission can be modulated between nearly zero and unity by a single-layer graphene over a broad range of carrier frequencies up to a few terahertz, which is in sharp contrast with its absorption of 2.3% in the infrared/visible range. Simultaneously, extremely low signal attenuation at room temperature, induced by the intrinsic device ( < 0.2 dB per graphene layer), can be realized by taking advantage of its reasonably low minimum conductivity, when the Fermi level is tuned at its Dirac point.…”

“…For the terahertz signal intensity investigated in this study, the detector output voltage is linearly proportional to the detected signal power. 27 . Because of the adverse influence of the metal gate, modulation of terahertz transmission by tuning the 2DEG conductivity is severely reduced in the traditional 2DEG modulators 27 .…”

“…However, terahertz wave modulators studied to date generally exhibit compromised performance between modulation depth, broadband operation, signal attenuation, polarization dependence, on-chip integrability and structural complexity [23][24][25][26] . We have recently proposed a novel room-temperature broadband terahertz intensity modulator using graphene, which is capable of avoiding these trade-offs 27 . By tuning the Fermi level in graphene, we tune the density of states available for intraband transitions under terahertz illumination and therefore terahertz transmission.…”