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# Strong mid-infrared photoresponse in small-twist-angle bilayer graphene

## Abstract

Small-twist-angle (<2°) bilayer graphene has received extraordinary attention recently due to its exciting physical properties1,2,3,4,5,6,7,8,9,10,11. Compared with monolayer graphene, the Brillouin zone folding in twisted bilayer graphene (TBG) leads to the formation of a superlattice bandgap and substantial modification to the density of states4,6,7,12,13. However, these emerging properties have rarely been leveraged to realize new optoelectronic devices. Here, we demonstrate the strong, gate-tunable photoresponse in the mid-infrared wavelength range of 5 to 12 μm. A maximum extrinsic photoresponsivity of 26 mA W−1 has been achieved at 12 μm when the Fermi level in 1.81° TBG was tuned to its superlattice bandgap. Moreover, the strong photoresponse critically depends on the formation of a superlattice bandgap, and it vanishes in the gapless case with an ultrasmall twist angle (<0.5°). Our demonstration reveals the promising optical properties of TBG and provides an alternative material platform for tunable mid-infrared optoelectronics.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors on reasonable request.

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## Acknowledgements

We acknowledge financial support from the National Science Foundation EFRI-NewLAW programme (grant no. 1741693). We also thank the Office of Naval Research for partial support in the experimental set-ups. The theoretical work at UTD is supported by the Army Research Office under grant no. W911NF-18-1-0416 and the Natural Science Foundation under grant no. DMR-1921581 through the DMREF programme. Growth of hexagonal boron nitride crystals was supported by the Elemental Strategy Initiative conducted by the MEXT, Japan and the CREST (grant no. JPMJCR15F3), JST. We also acknowledge L. Wang, D. Hynek, J. Woods, J. Cha at Yale West Campus and our previous group member X. Chen for their support.

## Author information

Authors

### Contributions

B.D., C.M. and S.Y. fabricated and characterized the devices. Q.W. and F.Z. performed the theoretical calculations. K.W. and T.T. synthesized the hBN crystals. F.X., F.Z., B.D. and Q.W. drafted the manuscript. All the authors discussed the results and commented on the manuscript.

### Corresponding authors

Correspondence to Fan Zhang or Fengnian Xia.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–7 and discussion.

## Rights and permissions

Reprints and Permissions

Deng, B., Ma, C., Wang, Q. et al. Strong mid-infrared photoresponse in small-twist-angle bilayer graphene. Nat. Photonics 14, 549–553 (2020). https://doi.org/10.1038/s41566-020-0644-7

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• DOI: https://doi.org/10.1038/s41566-020-0644-7

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