Interferometric Plasmonic Lensing with Nanohole Arrays

Strong Surface-Enhanced Coherent Phonon Generation in van der Waals Materials

Terahertz (THz) coherent phonons have emerged as promising candidates for the next generation of high-speed, low-energy information carriers in atomically thin phononic or phonon-integrated on-chip devices. However, effectively manipulating THz coherent phonons remains a significant challenge. In this study, we investigated THz coherent phonon generation in exfoliated van der Waals (vdW) flakes of Fe3GeTe2, Fe5GeTe2, and FePS3. We successfully generated the THz A1g coherent phonon mode in these vdW flakes. An innovative approach involved partially exfoliating vdW flakes on a gold substrate and partially on a silicon (Si) substrate to compare the THz coherent phonon generation between both sides. Interestingly, we observed a significantly enhanced THz coherent phonon in the vdW/gold area compared with that in the vdW/Si area. Frequency-domain Raman mapping across the vdW flakes corroborated these findings. Numerical simulations further indicated a stronger enhanced surface field in vdW/gold structures than in vdW/Si structures. Consequently, we attribute the observed enhancement in THz coherent phonon generation to the increased surface field on the gold substrate. This enhancement was consistent across the three different vdW materials studied, suggesting the universality of this strategy. Our results hold promise for advancing the design of THz phononic and phonon-integrated devices.

Focusing of surface plasmon polaritons propagating at the SiO2/Ag interface with 2-level and 4-level Fresnel phase zone pad structures

We propose and demonstrate dielectric Fresnel phase zone pad (FPZP) structures for focusing surface plasmon polaritons (SPPs) propagating at the SiO2/Ag interfaces. We exploited up-conversion fluorescence microscopy to characterize the SPP focusing. We first report on the SPP focusing with 2-level FPZP structures that introduced a π-phase shift in the SPP wavefront between adjacent zones. We optimized the SPP focusing by fine-tuning the longitudinal width of the FPZP structure. This led to the enhancement of the peak intensity of the SPP focal spot and the reduction of the focal spot size in both the longitudinal and transverse directions. Such focusing was also demonstrated with different focal lengths. To further improve the SPP focusing, we developed a 4-level FPZP structure, which introduced a π/2-phase shift in the SPP wavefront between adjacent zones. With the optimized 4-level FPZP structure, the SPP focal spot peak intensity is further improved, and the spot size is reduced. To assist the design of the FPZP structures, we carried out theoretical analysis and numerical calculations to determine the SPP wavelengths at various oxide/Ag interfaces. We also carried out finite difference time domain (FDTD) calculations to simulate the SPP focusing with the FPZP structures. The results of the FDTD simulation agree with the experimental results qualitatively.

Femtosecond photoemission electron microscopy of surface plasmon polariton beam steering via nanohole arrays

Directional control over surface plasmon polariton (SPP) waves is a prerequisite for the development of miniaturized optical circuitry. Here, the efficacy of single and dual component SPP steering elements is explored through photoemission electron microscopy. Our imaging scheme relies on two-color photoemission and counter-propagating SPP generation, which collectively allow SPPs to be visualized in real space. Wave-vector difference mixing between the two-dimensional nanohole array and photon momenta enables SPP steering with directionality governed by the array lattice constant and input photon direction. In our dual component configuration, separate SPP generation and Bragg diffraction based steering optics are employed. We find that array Bragg planes principally influence the SPP angles through the array band structure, which allows us to visualize both positive and negative refractory waves.

Direct Visualization of Counter-Propagating Surface Plasmons in Real Space-Time

We deploy two-dimensional nanohole arrays as resonant surface plasmon polariton (SPP) couplers that enable counter-propagation and excitation field interference-free imaging of SPP wave packets. We monitor the spatiotemporal evolution of the resulting SPPs using two-color photoemission electron microscopy. The measurements track the electric field envelope of the SPP in real space and time and enable direct characterization of their spatiotemporal properties in a regime where the SPP wave packet is the principal observable. We provide an analysis of the observables for both the co- and counter-propagating directions via SPP trajectories that are recorded in tandem. Our results highlight the advantages of isolating SPPs through counter-propagation, where excitation field-SPP interactions are suppressed.

Gold nanoholes fabricated by colloidal lithography: novel insights into nanofabrication, short-range correlation and optical properties

Colloidal lithography is widely used as a low cost and large-area deposition approach, alternative to the conventional small-area expensive lithographic techniques, for the fabrication of short-range ordered sub-wavelength metallic nanostructures. This paper contributes to the understanding of the impact of the fabrication protocol of a colloidal mask on the optical and sensing properties of short range-ordered nanohole (NH) distributions fabricated by colloidal lithography in optically thin (20 nm thick) gold films. We consider polystyrene nanospheres (PS-NSPs) with a nominal diameter of 80 nm, electrostatically adsorbed from a salt-free colloidal solution onto a polydiallyldimethylammonium (PDDA) countercharged monolayer. By avoiding the conventional polyelectrolyte multilayer and based on the interplay between the deposition times of both PDDA and PS-NSPs, we demonstrate effective simplification of the commonly applied deposition protocol and effective tuning of the NH-to-NH spacing (dNN) with negligible agglomeration. Comparison with NH samples prepared by salt-containing colloidal solutions points out the negative impact of salt addition on the optical properties. The effective tuning of dNN obtained by our protocol demonstrates highly correlated disorder under unsaturated adsorption and allows a discussion on the analogies of the optical response between long- and short- range ordered NH systems, which is a still debated topic. By Fast Fourier Transform of autocorrelation images of scanning electron microscopy micrographs we demonstrate quantitatively, rather than in principle, the correspondence between an inherent ordering length-scale and dNN. As optical transducers for detecting refractive index changes, our samples exhibit significant bulk sensitivity (∼309 nm RIU-1) in the framework of short range ordered NH systems.

Ultrafast imaging of surface plasmons propagating on a gold surface

We record time-resolved nonlinear photoemission electron microscopy (tr-PEEM) images of propagating surface plasmons (PSPs) launched from a lithographically patterned rectangular trench on a flat gold surface. Our tr-PEEM scheme involves a pair of identical, spatially separated, and interferometrically locked femtosecond laser pulses. Power-dependent PEEM images provide experimental evidence for a sequential coherent nonlinear photoemission process, in which one laser source launches a PSP through a linear interaction, and the second subsequently probes the PSP via two-photon photoemission. The recorded time-resolved movies of a PSP allow us to directly measure various properties of the surface-bound wave packet, including its carrier wavelength (783 nm) and group velocity (0.95c). In addition, tr-PEEM images reveal that the launched PSP may be detected at least 250 μm away from the coupling trench structure.