THz Response of GaN/AlGaN 2D Plasmonic Nanostructures

W.Knap1,2, P. Sai1,2, M.Dub1,2. V. Korotyeyev2,3, G. Cywiński1,2

1CENTERA LAB -CEZAMAT Warsaw University of Technology, Warsaw Poland
2Institute of High Pressure Physics Polish Academy of Sciences, Warsaw, Poland
3V. Ye. Lashkaryov Institute of Semiconductor Physics (ISP), NASU, Kyiv, Ukraine

We summarize recent experimental and theoretical efforts towards understanding if and how GaN based plasmonic nanostructures oscillating at THz frequencies can be electrically driven towards regime of amplification and generation.

The first demonstration of THz amplification by graphene nanostructures achieved by RIEC- Sendai- Tohoku University team [1,2] raised many fundamental questions like: i) is the graphene really necessary ii) can one scale results obtained on sub-wavelength (μm scale) graphene flakes up to practical size (mm scale) THz amplifier devices. In this work we present recent results on GaN/AlGaN based plasmonic structures obtained in CENTERA LAB – Warsaw, showing that in moderate cryogenic conditions (temperatures ~80K) in many aspects (i.e. carrier density, mobility and optical phonon energy) these structures mimic graphene nanostructures used in the first amplification experiments. The results allow us to identify main challenges/difficulties on the way towards realistic (practical size) plasmonic amplifiers of THz radiation [3,4,5].

Plasmonic crystals versus multi cavity THz resonators
Recently we have presented an extensive study of resonant two-dimensional (2D) plasmon excitations in grating-gated quantum well heterostructures, which enable an electrical control of periodic charge carrier density profile [3,4]. Our study revealed that main terahertz (THz) plasmonic resonances in these structures can be explained only within the framework of the plasmonic crystal model. We identified two different plasmonic crystal phases that can be switched on and off by application of the grating gate potential. However, in the pioneer work on THz amplification by graphene grating gates the plasma resonances could be fully interpreted as a sum of individual cavity (grating gate fingers) resonances.
We show that these two observations are not contradictory. In fact, with an increase in the distance between the grating gate fingers, the plasmonic crystal approximation tends to its limit in which the grating gate structures respond to external radiation as an ensemble of independent resonators.

Figure 1: Example of THz plasmonic crystal modes absorption in grating gate GaN/AlGaN based 2D nanostructure. The change of the phase (phase transition) close to gate voltage swing Vg = 0 is clearly seen – after Ref. [3].

Electrically driven experiments and modulation of room temperature thermal radiation
We show that terahertz plasmons in AlGaN/GaN grating-gate structures efficiently modulate the reflection of room temperature thermal radiation, leading to spectra that are in agreement with the measurements of plasmon absorption using high-power external sources 5. For typical samples of a few square millimeters in size, the reflected radiation intensity is relatively weak, and measurements need the use of gate voltage plasmon modulation and lock-in detection techniques. We show that unintentional use of lock-in techniques may lead to artifacts and demonstrate what kind of special precautions need to be taken into account. We show that drain voltage modulation also leads to modulation of the reflected thermal radiation by plasmons. These results are of key importance for the research on new resonant plasmon-based terahertz amplifiers and sources because of the always present superposition of electrically excited terahertz emission and background radiation reflected from the structures.

Although we study a specific case of plasmons in AlGaN/GaN grating-gate structures, our results have a general character and are applicable to any other semiconductor-based plasmonic crystal structures. Our work represents a crucial step towards a deeper understanding of THz plasma physics and the development of all-electrically tunable devices for THz optoelectronics.

The presented Research was partially funded by the European Union (ERC TERAPLASM #101053716). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. The work was also supported by the “International Research Agendas” Program of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund (Grant No. MAB/2018/9) for CENTERA.

[1] S. Boubanga-Tombet, W. Knap, D. Yadav, A. Satou, D. B. But, V. V. Popov, I. V. Gorbenko, V. Kachorovskii, and T. Otsuji, “Room Temperature Amplification of Terahertz Radiation by Grating-Gate Graphene Structures,” Phys. Rev. X 10, 031004 (2020); https://journals.aps.org/prx/abstract/10.1103/PhysRevX.10.031004
[2] T. Otsuji, S. A. Boubanga-Tombet, A. Satou, D. Yadav, H. Fukidome, T. Watanabe, T. Suemitsu, A. A. Dubinov, V. V. Popov, W. Knap, V. Kachorovskii, K. Narahara, M. Ryzhii, V. Mitin, M. S. Shur and V. Ryzhii, “Graphene-based plasmonic metamaterial for terahertz laser transistors,” Nanophotonics 11(9) (2022)
[3] P. Sai, V. V. Korotyeyev, M. Dub, M. Słowikowski, M. Filipiak, D. B. But, Yu. Ivonyak, M. Sakowicz, Yu. M. Lyaschuk, S. M. Kukhtaruk, G. Cywiński and W. Knap, “Electrical Tuning of Terahertz Plasmonic Crystal Phases,” Phys. Rev. X 13, 041003 (2023); https://doi.org/10.1103/PhysRevX.13.041003
[4] P. Sai, M. Dub, V. Korotyeyev, S. Kukhtaruk, G. Cywinski, and W. Knap, “THz Properties of Grating-Gate Plasmonic Crystals,” Lithuanian Journal of Physics 63(4), 251–263 (2023); https://doi.org/10.3952/physics.2023.63.4.7
[5] M. Dub, D. B. But, P. Sai, Yu. Ivonyak, M. Słowikowski, M. Filipiak, G. Cywinski, W. Knap and S. Rumyantsev, “Plasmons in AlGaN/GaN grating-gate structure probing with 300 K background illumination,” AIP Advances 13, 095017 (2023); https://pubs.aip.org/aip/adv/article/13/9/095017/2911692/Plasmons-in-AlGaN-GaN-grating-gate-structure

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