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Interdigitated THz metamaterial sensors and high power THz emission from resonant tunneling diode (RTD) oscillator arrays

Fanqi Meng¹, Lei Cao², Yannik Loth³, Merle Richter³, Anna Katharina Wigger³, Maira Pérez Sosa⁴, Alaa Jabbar Jumaah⁴, Shihab Al-Daffaie⁴, Zhenling Tang⁵, Jahnabi Hazarika¹, Petr Ourednik⁶, Michael Feiginov⁶, Safumi Suzuki⁵ Peter Haring Bolívar³, and Hartmut G. Roskos¹

¹Physikalisches Institute, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany

²State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan, China

³University of Siegen, Institute for High Frequencies and Quantum Electronics, Siegen, Germany

⁴Department of Electrical Engineering, Eindhoven University of Technology, 5612 AE Eindhoven, Netherlands

⁵Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Tokyo, Japan

⁶Department of Electrical Engineering and Information Technology, TU Wien, Vienna 1040, Austria

In the first part of this abstract, we present a strategy for designing metamaterial sensors in detecting small amounts of dielectric materials and trace molecules. The amount of frequency shift depends on intrinsic properties (electric field distribution, Q-factor, and mode volume) of the bare resonator, as well as the overlap volume of its high-electric-field zone(s) and the analyte [1]. Guided by the simplified dielectric perturbation theory [2], we designed interdigitated electric split-ring resonators (ID-eSRR) to significantly enhance the detection sensitivity compared to eSRRs without interdigitated fingers [3]. ID-eSRR’s fingers redistribute the electric field, creating strongly localized enhancements which boost analyte interaction. The periodic change of the inherent anti-phase electric field reduces radiation loss, leading to a higher Q-factor. Figure 1 (a) shows the SEM image of one unit cell of fabricated ID-eSRR and the enlarged finger area. The fabricated ID-eSRR sensors operates at around 300~GHz. The proof-of-principle experiments were carried out to monitor the thickness of thin SiO2 films which cover the whole surface of metamaterials. Figure 1(b) shows the measured frequency shift for ID-eSRR and eSRR as a function the thickness of SiO₂ film. The measurements demonstrate a remarkable 33.5 GHz frequency shift upon depositing a 150~nm SiO2 layer, with a figure of merit (FOM) improvement of over 50 times compared to structures without interdigitated fingers. This rational design offers a promising avenue for highly sensitive detection of thin films and trace biomolecules.

Figure 1: (a) SEM image of the fabricated interdigitated electric split-ring-resonator (ID-eSRR) MMs for THz sensing applications. The finger area is enlarged. (b) Simulated ( red and black solid lines) and measured resonance frequency shifts (red and black stars) in dependence of the SiO₂ layer thickness.

In the second part of the abstract, I will discuss the high power THz emission from resonant-tunneling-diode (RTD) oscillator arrays. The RTD oscillators possess the highest oscillation frequency among all electronic THz emitters [4]. This makes RTD oscillator a prominent candidate for bridging the so-called ‘THz gap’. However, the emitted power from RTD oscillators remains limited [5]. Here, we propose a novel linear RTD oscillator arrays to achieve high power coherent emission. In principle, the proposed linear array can contain a large number of coupled slot antennae. The inset of the Figure 1 (a) shows a sketch of coupled two RTD oscillators embedded into two slot antennae. The two slot antennae share a common resistor. The proposed linear RTD-oscillator arrays is capable of supporting coherent emission from both odd and even coupled modes. And both modes exhibit constructive interference in the far field, enabling high power emission.

Experimental demonstrations of coherent emission from 11-RTD linear arrays are presented. The measured oscillation frequency as a function of mesa area is shown in Figure 2 (a). The experiments confirm the spectral distribution predicted from the modeling and simulations. The high-frequency emission corresponds to the even mode and the low-frequency emission corresponds to the odd mode. The odd mode oscillates at approximately 450 GHz, while the even mode oscillates at around 750 GHz. We can roughly estimate the total emitted power of RTD devices as a function of emission frequency, which is shown in Figure 2 (b). For the single RTD oscillator, the emission power is in the range of several tens of μW. The emitted power from the 11-RTD-oscillator array is strongly increased: For the odd mode that oscillates at ~0.45 THz, the power is about 400 μW; For the even mode that oscillates at ~0.75 THz, the estimated power is about 1 mW.

Moreover, certain RTD oscillator arrays demonstrate dual-band oscillation under different biases, allowing for controllable switching between two coupled modes. In addition, during bias sweeping in both directions, a notable hysteresis feature is observed in the switching bias for the odd and even modes. Our linear RTD oscillator array represents a significant step forward in the realization of high-power large RTD oscillator arrays at high frequencies, and enables large-scale applications of RTD devices.

Figure 2: (a) Measured oscillation frequency of the single RTD devices and RTD oscillator arrays, as a function of mesa area. The magenta double crosses symbolize the emissions originating from individual RTD oscillators, while the red triangles denote emissions from RTD oscillator arrays. The inset provides a schematic representation illustrating the coupling between two RTD oscillators. (b) Estimated output power of single RTD devices and arrays, as a function of oscillation frequency. The magenta circles represent the emissions from single RTD devices. The blue triangles denote the emissions from the odd mode oscillation of the arrays. The red stars indicate the emissions from the even mode oscillation of the arrays.

Funding
These research works were funded by DFG projects RO 770/46-1, RO 770/50-1 and HA3022/15 (the latter two being part of the DFG-Schwerpunkt “INtegrated TERahErtz sySTems enabling novel functionality” (INTEREST – SPP 2314)).

References
[1] M. Gupta and R. Singh. “Terahertz sensing with optimized Q/Veff metasurface cavities”. Adv. Opt. Mater. 8(16), 1902025 (2020)
[2] L. Cao, S. S. Jia, M. D. Thomson, F. Q. Meng, and H. G. Roskos. “Can a terahertz metamaterial sensor be improved by ultra-strong coupling with a high- Q photonic resonator?,” Opt. Express 30(8) 13659 (2022)
[3] L. Cao, F. Meng, E. Özdemir, Y. Loth, M. Richter, A. K. Wigger, M. Pérez Sosa, A. J. Jumaah, S. Al-Daffaie, P. Haring Bolívar, and H. G. Roskos. “Interdigitated terahertz metamaterial sensors: Design with the dielectric perturbation theory,” arXiv:2311.14493 (2023)
[4] S. Suzuki, M. Shiraishi, H. Shibayama, and M. Asada, “High-power operation of terahertz oscillators with resonant tunneling diodes Using Impedance-Matched Antennas and Array Configuration,” IEEE J. Sel. Top. Quantum Electron. 19, 8500108 (2013)
[5] T. V. Mai, Y. Suzuki, X. Yu, S. Suzuki, and M. Asada, “Structure dependence of oscillation characteristics of structure-simplified resonant-tunneling-diode terahertz oscillator,” Appl. Phys. Express 15, 042003 (2022)