Terahertz Antenna Impedance Matched to a Graphene Photodetector

TL;DR

The paper tackles room-temperature terahertz detection with low power dissipation by leveraging a photo-thermal-electric (PTE) effect in graphene. It presents a detector built from quasi-free standing bilayer graphene on SiC, with an impedance-matched THz antenna and a quasi-optical coupling scheme using an aspherical lens, including capacitive coupling to a BLG p-n junction. The device achieves an external responsivity of about $R_{THz} ≈ 35$ V/W and a noise-equivalent power of approximately $NEP ≈ 300 pW/√Hz$ at 300 K, with a fractional bandwidth around $≈150$ GHz near a resonance at roughly $f ≈ 600$ GHz. This work demonstrates a scalable, wafer-scale graphene platform for room-temperature THz sensing with potential impact on future far-infrared detectors and heterodyne receivers.

Abstract

Developing low-power, high-sensitivity photodetectors for the terahertz (THz) band that operate at room temperature is an important challenge in optoelectronics. In this study, we introduce a photo-thermal-electric (PTE) effect detector based on quasi-free standing bilayer graphene (BLG) on a silicon carbide (SiC) substrate, designed for the THz frequency range. Our detector's performance hinges on a quasi-optical coupling scheme, which integrates an aspherical silicon lens, to optimize impedance matching between the THz antenna and the graphene p-n junction. At room temperature, we achieved a noise equivalent power (NEP) of less than 300 $pW/\sqrt{Hz}$. Through an impedance matching analysis, we coupled a planar antenna with a graphene p-n junction, inserted in parallel to the nano-gap of the antenna, via two coupling capacitors. By adjusting the capacitors and the antenna arm length, we tailored the antenna's maximum infrared power absorption to specific frequencies. The sensitivity, spectral properties, and scalability of our material make it an ideal candidate for future development of far-infrared detectors operating at room temperature.

Figures (4)

• Figure 1: a THz dipole antenna utilizing the PTE effect in BLG as a rectification mechanism. b presents a schematic illustrating the electrical connections to the antenna, where the photovoltage ($V_{PTE}$) is developed between the two antenna arms with the THz field concentrated at the antenna nanogap. Voltages $V_{gate 1,2}$ control the carrier densities on both sides of the gap. c shows the equivalent circuit model for the antenna.
• Figure 2: a Input resistance and reactance of the antenna are calculated from the equivalent circuit model when matched to a BLG p-n junction at 600GHz. In the impedance matching condition, the antenna's maximum absorbed power occurs near the resonance frequency, which is also verified using FEM simulations, plotted concurrently in b.
• Figure 3: Transport characterizations of dipole antenna ( a,c,e) and patch (b,d,f) PTE detectors at room temperature. a and b show resistance maps as a function of the voltage applied to two top gates, revealing four distinct regions corresponding to different doping configurations: p-n, p-p, n-p, and n-n. c and d depict direct current (d.c.) transport responsivity, measured in volts per watt (V/W), against gate voltage without THz excitation. e and f present a noise voltage map as a function of the gate voltage, with no terahertz excitation present.
• Figure 4: Responsivity maps and NEP spectra for patch and dipole antennas demonstrating PTE. a and b show THz responsivity maps for the patch and dipole antennas, respectively, each featuring a six-fold symmetry characteristic of the photo-thermal electric effect. c displays the antenna-enhanced responsivity spectra. For both NEP and $\mathbb{R}_{THz}$ spectra, the used set of gate voltages is indicated in the responsivity maps in a and b with circles. d illustrates the antenna-optimised NEP in $W/Hz^{1/2}$ for both dipole and patch antenna detectors, measured at optimal gate voltage configurations. Concurrent plots of the calculated antenna enhancement from the circuit model are provided for both types of antennas, alongside the NEP and responsivity of an unmatched antenna PTE detector (control).