Monolayer transition-metal dichalcogenides (TMDCs) have attracted a lot of research attention recently, motivated by their remarkable optical properties and potential for strong light-matter interactions. Realization of strong plasmon-exciton coupling is especially desirable in this context because it holds promise for the enabling of room-temperature quantum and nonlinear optical applications. These efforts naturally require investigations at a single-nanoantenna level, which, in turn, should possess a compact optical mode interacting with a small amount of excitonic material. However, standard plasmonic nanoantenna designs such as nanoparticle dimers or particle-on-film suffer from misalignment of the local electric field in the gap with the in-plane transition dipole moment of monolayer TMDCs. Here, we circumvent this problem by utilizing gold bi-pyramids (BPs) as very efficient plasmonic nanoantennas. We demonstrate strong coupling between individual BPs and tungsten diselenide (WSe) monolayers at room temperature. We further study the coupling between multilayers of WSe and BPs to elucidate the effect of the number of layers on the coupling strength. Importantly, BPs adopt a reduced-symmetry configuration when deposited on WSe, such that only one sharp antenna tip efficiently interacts with excitons. Despite the small interaction area, we manage to achieve strong coupling, with Rabi splitting exceeding ∼100 meV. Our results suggest a feasible way toward realizing plasmon-exciton polaritons involving nanoscopic areas of TMDCs, thus pointing toward quantum and nonlinear optics applications at ambient conditions.
Transition metal dichalcogenides (TMDCs) have attracted significant attention recently in the context of strong light–matter interaction. To observe strong coupling using these materials, excitons are typically hybridized with resonant photonic modes of stand-alone optical cavities, such as Fabry–Pérot microcavities or plasmonic nanoantennas. Here, we show that thick flakes of layered van der Waals TMDCs can themselves serve as low-quality resonators due to their high background permittivity. Optical modes of such “cavities” can in turn hybridize with excitons in the same material. We perform an experimental and theoretical study of such self-hybridization in thick flakes of four common TMDC materials: WS2, WSe2, MoS2, and MoSe2. We observe splitting in reflection and transmission spectra in all four cases and provide angle-resolved dispersion measurements of exciton-polaritons as well as thickness-dependent data. Moreover, we observe significant enhancement and broadening of absorption in thick TMDC multilayers, which can be interpreted in terms of strong light–matter coupling. Remarkably, absorption reaches >50% efficiency across the entire visible spectrum, while simultaneously being weakly dependent on polarization and angle of incidence. Our results thus suggest formation of self-hybridized exciton-polaritons in thick TMDC flakes, which in turn may pave the way toward polaritonic and optoelectronic devices in these simple systems.
Plasmonic nanoantennas emit two-photon photoluminescence, which is much stronger than their second harmonic generation. Unfortunately, luminescence is an incoherent process and therefore generally not explored for nanoscale coherent control of the antenna response. Here, we demonstrate that, in resonant gold nanoantennas, the two-photon absorption process can be coherent, provided that the excitation pulse duration is shorter than the dephasing time of plasmon mode oscillation. Exploiting this coherent response, we show the pure spectral phase control of resonant gold nanoantennas, with effective read-out of the two-photon photoluminescence.
We show that submicron-sized patterns can be imprinted into soft, recombinant-engineered protein hydrogels (here elastin-like proteins, ELP) by transferring wavy patterns from polydimethylsiloxane (PDMS) molds. The high-precision topographical tunability of the relatively stiff PDMS is translated to a bio-responsive, soft material, enabling topographical cell response studies at elastic moduli matching those of tissues. Aligned and unaligned wavy patterns with mold periodicities of 0.24–4.54 µm were imprinted and characterized by coherent anti-Stokes Raman scattering and atomic force microscopy. The pattern was successfully transferred down to 0.37 µm periodicity (width in ELP: 250±50 nm, height: 70±40 nm). The limit was set by inherent protein assemblies (diameter: 124–180 nm) that formed due to lower critical solution temperature behavior of the ELP during molding. The width/height of the ELP ridges depended on the degree of hydration; from complete dehydration to full hydration, ELP ridge width ranged from 79± 9% to 150 ± 40% of the mold width. The surface of the ridged ELP featured densely packed protein aggregates that were larger in size than those observed in bulk/flat ELP. Adipose-derived stem cells (ADSCs) oriented along hydrated aligned patterns with periodicities ≥0.60 µm (height ≥170±100 nm), while random orientation was observed for smaller distances/amplitudes, as well as flat and unaligned wavy ELP surfaces. Hence, micro-molding of ELP is a promising approach to create tissue-mimicking, hierarchical architectures composed of tunable micron-sized structures with nano-sized protein aggregates, which opens the way for orthogonal screening of cell responses to topography and cell-adhesion ligands at relevant elastic moduli.
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