Towards realization of beyond-5G large-capacity and ultrahigh-speed terahertz (THz) wireless communication systems, we have developed InGaAs-channel high-electron-mobility transistor (HEMT) and graphene-channel field-effect transistor (G-FET) plasmonic THz detectors, featured by original asymmetric dual-grating-gate (A-DGG) structures. The A-DGG fingers act as a broadband deep-subwavelength grating coupler that converts incoming THz waves into 2D plasmons in the channel, and their strong hydrodynamic nonlinearities as well as their fast response enable fast, highly sensitive THz detection at room temperature.

In this talk, we review recent advances of the A-DGG plasmonic THz detectors and demonstrate that they are promising for use in beyond-5G wireless communication systems. First, we investigate a new way to read out the photovoltage from the gate electrode of an A-DGG InGaAs-channel HEMT that enables the enhancement of responsivity in proportion to the active area size as well as the impedance matching to 50-Ω interconnection systems. Second, we demonstrate that a new plasmonic THz detection mechanism called “3D rectification effect” in an A-DGG InGaAs-channel HEMT in the gate-readout configuration drastically enhances the responsivity. Third, we reveal that, in an A-DGG G-FET, both plasmonic and photothermoelectric effects coexist as THz detection mechanisms, resulting in the high responsivity and 10-ps-order fast response time.

We report the observation of (ultra)strong light-matter coupling, in the UV spectral region, between optical modes of a metal/dielectric bilayer nanocavity and the electronic excitations of spin-crossover (SCO) molecules. By thermally switching the SCO molecules between their low-spin and high-spin states, we demonstrate the possibility of fine-tuning the light-molecule hybridization strength, allowing a reversible switching between strong- (with Rabi splitting values of up to 550 meV) and weak-coupling regimes within a single photonic resonator. As a result, we show that spin-crossover molecular compounds constitute a novel, promising class of active nanomaterials in the context of tuneable and reconfigurable polaritonic devices.

We have designed and realized all-dielectric lossless nanoantennas, in which a thin SiO2 layer doped with erbium ions is placed inside slotted silicon nanopillars arranged in a square array. The modal analysis has evidenced that the slotted nanopillar array supports optical quasi-BIC resonances with ultra-high Q-factors (up to Q∼10^9), able to boost the electromagnetic local density of optical states in the optically active layer. Up to 3 orders of magnitude photoluminescence intensity increment and 2 orders of magnitude decay rate enhancement have been measured at room temperature when the Er^(3+) emission at about λ=1540 nm couples with the quasi-BIC resonances. Furthermore, by tailoring the nanopillar aspect ratio, the slot geometry has been exploited to obtain selective enhancements of the electric or magnetic dipole contribution to Er^(3+) radiative transitions in the NIR, keeping the emitter quantum efficiency almost unitary. Finally, by computing the angularly resolved electromagnetic field enhancement inside the nanoslot, the nanoantenna directivity has been designed, proving that strong beaming effects can be obtained. Our findings have a direct impact on the development of bright and efficient photon sources operating at telecom wavelength that are of primary importance for quantum nanophotonic applications.

The damage caused by irradiation of crystalline material with ions results in localized volume changes. Here, swelling is utilized to fabricate nanostructured gratings with heights below 10 nm for extreme ultraviolet radiation. Irradiations were performed through a structured layer of photoresist shadowing parts of the sample from a broad ion beam. This enabled much shorter fabrication times than comparable direct write processes with a focussed ion beam. The study presents results from first systematic investigations regarding the fabrication of nanostructured gratings by irradiation of silicon with a broad beam of helium ions with energies of 30 keV. A smaller, scanned beam is used for comparison. Fluence was varied from 0.4 to 7.5×1e16 ions/cm^2 . Fabricated structures were measured via atomic force microscopy. This yielded a controllable method to fabricate shallow gratings with heights in the range of 0 to 10 nm.

We investigated the effect of light on the DNA-driven self-assembly to form hierarchical patterns on two dimensional surfaces. We specifically focused on the self-assembly of DNA-functionalized quantum dots (QDs) onto DNA-functionalized glass substrate while illuminating the surface with a laser during coating process. The region illuminated with a green laser remained uncoated with red-emitting QDs while the not-illuminated region was successfully coated. Next, a red laser whose light cannot be absorbed by the red-emitting QDs did not avoid the DNA-driven self-assembly of the QDs onto glass substrate. Additionally, we demonstrated that silica nanoparticles that had been functionalized with DNA and are not able to absorb the light in the visible regime were again coated on the surface while being exposed to a green laser. These results prove that the absorption of the light is responsible for controlling the binding-unbinding process of QDs. Finally, we added DNA-functionalized green-emitting QDs onto the area that was not coated with red-emitting QDs under the green laser exposure. We observed successful coating of previously uncoated regions with the green QDs. This revealed the viability of our technique for building up hierarchical structures without using, sophisticated, or resource-intensive microfabrication methods.