Periodic Reporting for period 1 - TERAPLASM (Towards On-Chip Plasmonic Amplifiers of THz Radiation)
Berichtszeitraum: 2023-08-01 bis 2026-01-31
Discoveries in the spectral range of Terahertz (THz) electromagnetic waves/oscillations, made over the past 20 years, established an important scientific research axis bridging the photonics and electronics and unifying efforts of a large scientific community. However, the ability to develop THz technology for wireless communication, biosensing, security screening and other applications was hindered by the lack of compact,room temperature-operating, cost effective THz amplifiers and sources.
More than 40 years ago, theoretical and experimental studies of plasma oscillations in two-dimensional (2D) electron systems began, and plasmonic resonances at THz range were observed. M. Dyakonov and M. Shur theoretically predicted that the plasma waves (charge density waves) in nanostructures can lead to THz detection and generation.
The detection part of the “nanodevice plasmonics promise” has been proven and THz plasmonic detector arrays are nowadays widely used. The case of emitters turned out to be considerably more complicated. Only recently, room temperature, current driven amplification of incoming THz radiation has been demonstrated in graphene flake- based structures with an innovative double grating gate geometry (applicant result PRX 10, 031004, 2020). The observed effects, breakthrogh but yet unexplained, suggested that perhaps new 2D materials - or their heterojunctions with semiconductors exhibiting innovative geometries - may produce the much desired on-chip plasmonic amplifiers of THz radiation - the subject of “TERAPLASM” project.
The project proposed to fabricate structures of novel geometries, made of materials such as graphene, alternatives to graphene (HgTe and GaN-based systems) and their heterojunction with 2D materials featuring high-mobility 2D electron gas (2DEG). Through studies of these structures with various spectroscopic methods, TERAPLASM expected to uncover more examples of plasmonic amplification to create physical models of THz plasmonic amplification and select the optimal systems for practical on-chip THz devices, owing to radically scaled-down device dimensions, costs and requirements and to revolutionize THz wireless telecommunication, biosensing, security screening and other areas important for society.
More than 40 years ago, theoretical and experimental studies of plasma oscillations in two-dimensional (2D) electron systems began, and plasmonic resonances at THz range were observed. M. Dyakonov and M. Shur theoretically predicted that the plasma waves (charge density waves) in nanostructures can lead to THz detection and generation.
The detection part of the “nanodevice plasmonics promise” has been proven and THz plasmonic detector arrays are nowadays widely used. The case of emitters turned out to be considerably more complicated. Only recently, room temperature, current driven amplification of incoming THz radiation has been demonstrated in graphene flake- based structures with an innovative double grating gate geometry (applicant result PRX 10, 031004, 2020). The observed effects, breakthrogh but yet unexplained, suggested that perhaps new 2D materials - or their heterojunctions with semiconductors exhibiting innovative geometries - may produce the much desired on-chip plasmonic amplifiers of THz radiation - the subject of “TERAPLASM” project.
The project proposed to fabricate structures of novel geometries, made of materials such as graphene, alternatives to graphene (HgTe and GaN-based systems) and their heterojunction with 2D materials featuring high-mobility 2D electron gas (2DEG). Through studies of these structures with various spectroscopic methods, TERAPLASM expected to uncover more examples of plasmonic amplification to create physical models of THz plasmonic amplification and select the optimal systems for practical on-chip THz devices, owing to radically scaled-down device dimensions, costs and requirements and to revolutionize THz wireless telecommunication, biosensing, security screening and other areas important for society.
Our main work so far concerned fabrication and THz spectral analysis of plasmonic crystals. GaN-based 2D system was chosen because it achieves carrier density of the same order as graphene samples (>5E^12/cm2) and because at liquid nitrogen temperature the electron mobility is around 10 000 cm2/Vs (comparable to graphene used in pioneering experiments). The fabrication requires structures a few square mm in size (size of THz beam) with plasmonic cavities in the hundred nm size (size of cavity oscillating at THz range). Successful technology followed by THz optical experiments revealed multiple plasmon resonances tuneable by grating gates with demonstrating transitions between diffrent plasmonic crystal phases(modes) –localized and delocalized ones.
The main achievements were :
1) Demonstration of Electrical Tuning of THz Plasmonic Crystal Phases: Phys Rev X, 13, 041003 (2023)
We started from grating gate structures produced using a two-dimensional (2D) system based on GaN/AlGaN heterojunctions. Extensive studies of resonant two-dimensional (2D) plasmon excitations in grating-gated quantum well heterostructures enabled an electrical control of periodic charge carriers’ density profile. These were the first systematic studies of plasmonic crystals showing how their properties change as a function of geometry and voltage applied to grating gtae. The work was published.
2) Demonstration of double grating gate structures plasmonic crystals: J Appl Phys 135, 193103 (2024);
One of our main hypothesis was that a two gate (back and front) configuration is necessary to reach the condition of THz amplification. Plasma resonators with such a unique architecture have never been used before in the studies of THz plasma emission from any 2D semiconductor system. Fabrication of double gate structures with an additional bottom gate allowed to obtain any desirable profile of carrier density modulation as well as an independent activation of both delocalized and localized modes in the grating-gate AlGaN/GaN plasmonic crystals.
3) Comparative study of graphene and metal-based grating-gate THz plasmonic structures: J Appl Phys 137, 213103 (2025);
We considered another plasma instability excitation mechanism – that may lead to the amplification and generation of THz radiation in the grating gate structures when the the grating gates also have plasma oscillations in the THz spectral range.
We fabricated and compared plasmonic structures formed by metal and graphene large area (∼4 mm2) gratings integrated with AlGaN/GaN heterostructures of identical grating geometry. We have shown that the structures with metal gratings exhibit electrically tuneable plasmon resonances in the THz spectral range. On the contrary, those structures with graphene gratings showed only broadband absorption. We found that the latter is due to the relatively low conductivity of the graphene. The simulations also predicted that for graphene with higher carrier mobility, the emergence of different plasmon modes is possible.
The main achievements were :
1) Demonstration of Electrical Tuning of THz Plasmonic Crystal Phases: Phys Rev X, 13, 041003 (2023)
We started from grating gate structures produced using a two-dimensional (2D) system based on GaN/AlGaN heterojunctions. Extensive studies of resonant two-dimensional (2D) plasmon excitations in grating-gated quantum well heterostructures enabled an electrical control of periodic charge carriers’ density profile. These were the first systematic studies of plasmonic crystals showing how their properties change as a function of geometry and voltage applied to grating gtae. The work was published.
2) Demonstration of double grating gate structures plasmonic crystals: J Appl Phys 135, 193103 (2024);
One of our main hypothesis was that a two gate (back and front) configuration is necessary to reach the condition of THz amplification. Plasma resonators with such a unique architecture have never been used before in the studies of THz plasma emission from any 2D semiconductor system. Fabrication of double gate structures with an additional bottom gate allowed to obtain any desirable profile of carrier density modulation as well as an independent activation of both delocalized and localized modes in the grating-gate AlGaN/GaN plasmonic crystals.
3) Comparative study of graphene and metal-based grating-gate THz plasmonic structures: J Appl Phys 137, 213103 (2025);
We considered another plasma instability excitation mechanism – that may lead to the amplification and generation of THz radiation in the grating gate structures when the the grating gates also have plasma oscillations in the THz spectral range.
We fabricated and compared plasmonic structures formed by metal and graphene large area (∼4 mm2) gratings integrated with AlGaN/GaN heterostructures of identical grating geometry. We have shown that the structures with metal gratings exhibit electrically tuneable plasmon resonances in the THz spectral range. On the contrary, those structures with graphene gratings showed only broadband absorption. We found that the latter is due to the relatively low conductivity of the graphene. The simulations also predicted that for graphene with higher carrier mobility, the emergence of different plasmon modes is possible.
I consider all three achievements reported above as “Results beyond the state of the arts ”. They are showing for the first time the physical nature of the plasma modes in grating gate structures and report significant novel results on plasmonic crystals. We had to overcome two main challenges: technological and theoretical.
The first stems from simple fact: the manipulation of THz beams requires devices of a few mm size (size of THz beam) wheras plasmonic cavities should have sub-micrometer size (size of cavity oscillating at THz range). Thus the device should contain thousands of metal grating fingers with good dielectric isolation to control carrier density. To our best knowledge we were first to achieve electrical grating gate control, what enabled voltage tuning between diffrent plasmonic crystal phases.
Regarding the second challenge, analytical formulas existed that allowed prediction of plasma frequency for a single plasmonic cavity. But does the plasmonic crystal respond to THz radiation as sum of individual acvities or as a whole system, where plasmonic cavities are interconected and act as one oscillator? We have shown that in majority of cases the grating gate structures behave as crystals and exact prediction of plasma modes require cumbersome electromagnetic simulations. We have elaborated the theoretical model for such calculations, but importantly we also established a simplified analytical model that could successfully predict frequency of plasma resonance, what enables designing plasmonic crystals for THz filters, detectors, and amplifiers (Dub et al, Nanomaterials 2024, 14, 1502)
The experiments with resonance tuned by the voltage lead us to the idea of new spectroscopy: notch filter THz spectroscopy that may replace conventional spectrometers avoiding mechanical movements, where spectrum can be determined using voltage tuneable filters. This might require development of spécial mathematical tools based probably on machine learning methodologies. We consider this discovery as a potential proposition for prolongation of the TERAPLASM Project that may ensure uptake of the project results.
The first stems from simple fact: the manipulation of THz beams requires devices of a few mm size (size of THz beam) wheras plasmonic cavities should have sub-micrometer size (size of cavity oscillating at THz range). Thus the device should contain thousands of metal grating fingers with good dielectric isolation to control carrier density. To our best knowledge we were first to achieve electrical grating gate control, what enabled voltage tuning between diffrent plasmonic crystal phases.
Regarding the second challenge, analytical formulas existed that allowed prediction of plasma frequency for a single plasmonic cavity. But does the plasmonic crystal respond to THz radiation as sum of individual acvities or as a whole system, where plasmonic cavities are interconected and act as one oscillator? We have shown that in majority of cases the grating gate structures behave as crystals and exact prediction of plasma modes require cumbersome electromagnetic simulations. We have elaborated the theoretical model for such calculations, but importantly we also established a simplified analytical model that could successfully predict frequency of plasma resonance, what enables designing plasmonic crystals for THz filters, detectors, and amplifiers (Dub et al, Nanomaterials 2024, 14, 1502)
The experiments with resonance tuned by the voltage lead us to the idea of new spectroscopy: notch filter THz spectroscopy that may replace conventional spectrometers avoiding mechanical movements, where spectrum can be determined using voltage tuneable filters. This might require development of spécial mathematical tools based probably on machine learning methodologies. We consider this discovery as a potential proposition for prolongation of the TERAPLASM Project that may ensure uptake of the project results.