Mutual photon-conversion in backward OPO configuration between THz wave and near-infrared light

Hiroaki Minamide

Tera-Photonics Research Team, RIKEN Center for Advanced Photonics, RIKEN, 519-1399 Aramaki-Aoba, Sendai, Miyagi, 980-0845 Japan

The development of technologies in the sub-THz electromagnetic wave range is increasingly active, such as carrier frequencies for next-generation ultrahigh-speed wireless communication platforms (Beyond 5G/6G) and non-destructive tools for remote sensing. In addition, the development of various devices that generate, detect, and control sub-THz waves is becoming increasingly competitive worldwide because of the enormous social and economic value of standardized and patented technologies, such as connection to optical fiber networks. In such a background, methods to convert THz wave and near-infrared light efficiently and mutually are an extremely important research subject.
We have been developing highly bright sub-THz wave generation [1] and ultra-sensitive detection of sub-THz waves by using frequency up-conversion [2-5] based on nonlinear photonics technology under the backward optical parametric photonconversion [6] we first demonstrated [7-9]. In the experiment, two PPLN crystals with polarization reversal period of 53 um and slant angle of 67 degrees were used for sub-THz waves generation and up-conversion detection, respectively. The pulse energy of generated sub-THz waves with frequency of 308 GHz was evaluated by using a calibrated detector firstly. After that, 1-mm-thick glass plates were inserted step by step in front of the second PPLN crystal for up-conversion to attenuate the sub-THz pulse energy. The up-converted light from sub-THz to near infrared via backward parametric process is detected by an avalanche photodiode. The results show the intensity of the frequency up-converted light increases in proportional to incident sub-THz-wave pulse energy. As the input energy of the sub-THz-wave decrease, the up-converted light intensity also decreases.
When the incident pulse energy of the sub-THz waves down to 50 atto-Joule (10-18 J) level, the up-converted light intensity finally reaches a noise level. Currently, detectable limit is restricted by idler light generated in the second PPLN crystal. Reducing the pumping light power may improve the detectable limit.
In this talk, we will report on high-efficiency sub-THz-wave oscillation by original backward optical parametric photonconversion using slant-stripe-type PPLN crystals, and the development of optical elements necessary for miniaturization to make it a ubiquitous THz-wave source, such as the source mounted on robots. We also report on the detection of extremely weak sub-THz waves with pulse energies on the order of tens of attojoules by frequency up-conversion technology as a mutual photon conversion between THz wave and near-infrared light.

Acknowledgments
This work was supported by Innovative Science and Technology Initiative for Security Grant Number JPJ004596, ATLA, Japan. I would also like to thank Prof. H. Ito of RIKEN and Prof. M. Kumano of Tohoku University for fruitful discussions in this research.

References
[1] S. Hayashi, K. Nawata, T. Taira, J. Shikata, K. Kawase, and H. Minamide, “Ultrabright continuously tunable terahertz-wave generation at room temperature,” Sci. Rep., 4, 05045 (2014)
[2] R. Guo, S. Ohno, H. Minamide, T. Ikari, and H. Ito “Highly sensitive coherent detection of terahertz waves at room temperature using a parametric process” Appl. Phys. Lett. 93, 021106 (2008)
[3] F. Qi, S. Fan, T. Notake, K. Nawata, T. Matsukawa, Y. Takida, and H. Minamide, “10 aJ-level detection of ns pulse below 10 THz by frequency upconversion detection via DAST crystal: more than a 4 K bolometer” Optics Letters, 39, 001294 (2014)
[4] Y. Takida, K. Nawata, T. Notake, T. Otsuji, and H. Minamide, “Optical up-conversion-based cross-correlation for characterization of sub-nanosecond terahertz-wave pulses,” Opt. Express 30, 11217-11227 (2022)
[5] Y. Takida, K. Nawata, S. Suzuki, M. Asada, and H. Minamide, “Nonlinear optical detection of terahertz-wave radiation from resonant tunneling diodes,” Opt. Express 25, 5389-5396 (2017)
[6] Harris, S. E. “Proposed backward wave oscillation in the infrared.” Appl. Phys. Lett. 9(3), 114–116 (1966)
[7] K. Nawata, Y. Tokizane, Y. Takida, and H. Minamide, “Tunable backward terahertz-wave parametric oscillation,” Sci. Rep. 9, 726 (2019)
[8] Y. Takida, K. Nawata, and H. Minamide, “Injection-seeded backward terahertz-wave parametric oscillator,” APL Photonics 5, 061301 (2020)
[9] J. E. Muldera, K. Nawata, Y. Takida, D. Yadav, and H. Minamide, “Tunable backward terahertz-wave parametric oscillator centered at a high frequency of 0.87 THz with injection seeding,” Opt Express 31, 23966-23973 (2023)