# Microwave Photodetection in an Ultraclean Suspended Bilayer Graphene pn Junction

## TL;DR

This study demonstrates that bilayer graphene pn junctions can serve as effective cryogenic microwave photodetectors, with a novel photothermal mechanism leading to significant photocurrent signals at specific gating conditions.

## Contribution

It introduces a new photothermal detection mechanism in bilayer graphene pn junctions and shows how device engineering can enhance microwave photodetection.

## Key findings

- Photocurrent peaks when one side is at the Dirac point.
- Significant photocurrent observed only in the pn regime.
- Device transport mechanisms influence photocurrent magnitude.

## Abstract

We explore the potential of bilayer graphene as a cryogenic microwave photodetector by studying the microwave absorption in fully suspended clean bilayer graphene pn junctions in the frequency range of $1-5$ GHz at a temperature of 8 K. We observe a distinct photocurrent signal if the device is gated into the pn regime, while there is almost no signal for unipolar doping in either the nn or pp regimes. Most surprisingly, the photocurrent strongly peaks when one side of the junction is gated to the Dirac point (charge-neutrality point CNP), while the other remains in a highly doped state. This is different to previous results where optical radiation was used. We propose a new mechanism based on the phototermal effect explaining the large signal. It requires contact doping and a distinctly different transport mechanism on both sides: one side of graphene is ballistic and the other diffusive. By engineering partially diffusive and partially ballistic devices, the photocurrent can drastically be enhanced.