Self-referenced terahertz refractive index sensor based on a cavity resonance and Tamm plasmonic modes
Mehdi Keshavarz, Abbas Alighanbari
Abstract
A self-referenced terahertz (THz) refractive index sensor is proposed. The structure consists of two opposite-facing, graphene-covered distributed Bragg reflectors (DBRs), with a cavity formed in between. The cavity is filled with the ambient medium, and its resonance frequency is sensitive to the changes of the ambient refractive index. On the other hand, Tamm-plasmonic modes, which are excited at the DBR-graphene boundaries, are insensitive to the ambient refractive index and thus provide a frequency reference. The proposed structure is studied using a semi-analytical transfer matrix method (TMM). The sensor, studied for gas sensing, achieves a sensitivity of 0.982 THz per refractive index unit (THz/RIU), and a figure of merit (FoM) of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>142</mml:mn> <mml:mspace width="thinmathspace"/> <mml:mspace width="thinmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">R</mml:mi> <mml:mi mathvariant="normal">I</mml:mi> <mml:mi mathvariant="normal">U</mml:mi> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> at the cavity resonance frequency of 1.1 THz. The effects of different parameters on the sensor’s performance are also investigated. Compared to the previous high-performance THz refractive index sensing approaches, the proposed structure is simpler because it requires no phase- or polarization-matching devices, such as polarizers, prisms, and gratings. Moreover, it provides a self-referenced operation, which was rarely achievable using previous methods.