Terahertz Detection and Mixing Using Two-Dimensional Materials
Terahertz Detection and Mixing Using Two-Dimensional Materials
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Date
2016
Authors
Leong, Edward Siokei
Advisor
Murphy, Thomas E
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Abstract
The discovery of graphene in 2004 and the rise of the two-dimensional materials
provide an excellent platform for developing optoelectronics for terahertz (THz)
detection and mixing. For graphene and black phosphorus, their high carrier mobility,
attractive electrical and thermal properties show promises for tunable terahertz
technology.
In the first part of the thesis, we discuss the THz photoresponse of black phosphorus
(BP). The rediscovery of BP as a two-dimensional material in 2014 has drawn
attention to its near and mid-infrared application. Here, we present an ultrafast THz
detector based on black phosphorus with high efficiency at room temperature. The
detector has a noise equivalent power of 130 µW/√
Hz at 2.5 THz, and an ultrafast
photoresponse of about 20 ps was observed. Two detection mechanisms were observed:
a strong photothermoelectric effect and a weak bolometric effect. The high
carrier mobility allows BP to absorb THz radiation through free-carrier absorption,
even thought the photon energy (10 meV) is far below the band gap (300 meV) of
the material. The intrinsic responsivity of BP is also estimated via a Joule-heating
experiment and using a free-carrier model.
In the second part of the thesis, we discuss down frequency mixing and the
fabrication process of a graphene FET mixer. Graphene exhibits a significant change
in conductivity when the Fermi energy is altered by applying a gate voltage. Near
the Dirac point, graphene field effect transistors (FETs) show a strongly nonlinear
response (i.e. a nonlinear change in resistivity with applied gate voltage) that can
be exploited to provide efficient rectification and mixing of THz signals. Although
rectification in graphene field-effect transistors has been demonstrated, heterodyne
mixing in the THz band has not been explored. We examine a THz graphene
mixer using an antenna-coupled graphene FET configuration. We will discuss the
antenna and graphene device design optimized for heterodyne mixing at 0.35 THz.
In addition, fabrication and preliminary measurements of a microwave frequency
prototype will be presented to demonstrate the principle of the operation.