Abstract Body

Communication using terahertz (~10¹² Hz) electromagnetic waves is critical for developing 6^th-generation wireless network infrastructures. Conflictions between stable radiation and the modulation frequency of terahertz sources impede the superposing of transmitting signals on carrier waves. The Josephson junctions included in a cuprate superconductor radiate terahertz waves [1] with frequencies proportional to the bias voltages [2][3]. Thus, the modulation of the bias voltage leads to the modulation of the Josephson plasma emission (JPE) frequency. This study aims to demonstrate the generation of frequency-modulated (FM) terahertz continuous waves from Josephson junctions. We achieved to demonstrate sinusoidal signal up to 4.5 GHz superimposed on sub-terahertz carrier waves in two JPE devices. One device radiates at 840–890 GHz with the maximum FM bandwidth of 40 GHz [4] and the other radiates at 400-500 GHz with high- and low bias regions. The results verify that the instantaneous JPE frequency follows the gigahertz-modulated bias voltage. The on-chip FM terahertz generation shows a sharp contrast to the mode-lock frequency comb constructed by highly sophisticated optics on a bench. A further increase of the modulation amplitude facilitates up- or down-frequency conversion over more than one octave. The obtained FM bandwidth exhibited an improvement of two orders of magnitude in the demodulation signal-to-noise ratio compared to the amplitude-modulated signal. The demonstrated FM-JPE stimulates further research on terahertz communication technology and metrology using superconducting devices.

References

Ozyuzer, L. et al. Emission of coherent THz radiation from superconductors. Science 318, 1291 (2007).

Tsujimoto, M. et al. Broadly Tunable Subterahertz Emission from Internal Branches of the Current-Voltage Characteristics of Superconducting Bi2Sr2CaCu2O8+δSingle Crystals. Phys Rev Lett 108, 107006 (2012).

Kakeya, I. & Wang, H. Terahertz-wave emission from Bi2212 intrinsic Josephson junctions: a review on recent progress. Supercond Sci Technol 29, 073001 (2016).

Miyamoto, M., Kobayashi, R., Kuwano, G., Tsujimoto, M. & Kakeya, I. Wide-band frequency modulation of a terahertz intrinsic Josephson junction emitter of a cuprate superconductor. Nat Photonics 18, 267 (2024).

Acknowledgment

The results were obtained by the collaboration with M. Miyamoto, R. Kobayashi, N. Yagyu, G. Kuwano, and M. Tsujimoto. This work has been partly supported by JSPS KAKENHI (grant numbers JP20H02606, 23K17747, and 24K00946) and ISHIZUE 2023 of Kyoto University.

Radiation spectrum (red lines and symbols) at temperature of 25 K, mesa voltage of 2.05 V, superimposed modulation frequency of 3 GHz, and modulation power of 25 dBm. The ideal FM spectrum for a carrier frequency of 856 GHz and modulation index of 5.7 is shown with blue bars.

Keywords: Cuprate, Intrinsic Josephson junction, terahertz radiation