Andrea Jacassi

Hybridization in Three Dimensions: A Novel Route toward Plasmonic Metamolecules.

Plasmonic metamolecules have received much interest in the last years because they can produce a wide spectrum of different hybrid optical resonances. Most of the configurations presented so far, however, considered planar resonators lying on a dielectric substrate. This typically yields high damping and radiative losses, which severely limit the performance of the system. Here we show that these limits can be overcome by considering a 3D arrangement made from slanted nanorod dimers extruding from a silver baseplate. This configuration mimics an out-of-plane split ring resonator capable of a strong near-field interaction at the terminations and a strong diffractive coupling with nearby nanostructures. Compared to the corresponding planar counterparts, higher values of electric and magnetic fields are found (about a factor 10 and a factor 3, respectively). High-quality-factor resonances (Q ≈ 390) are produced in the mid-IR as a result of the efficient excitation of collective modes in dimer arrays.

Combination of scanning probe technology with photonic nanojets

Light focusing through a microbead leads to the formation of a photonic nanojet functional for enhancing the spatial resolution of traditional optical systems. Despite numerous works that prove this phenomenon, a method to appropriately translate the nanojet on top of a region of interest is still missing. Here, by using advanced 3D fabrication techniques we integrated a microbead on an AFM cantilever thus realizing a system to efficiently position nanojets. This fabrication approach is robust and can be exploited in a myriad of applications, ranging from microscopy to Raman spectroscopy. We demonstrate the potential of portable nanojets by imaging different sub-wavelength structures. Thanks to the achieved portability, we were able to perform a detailed optical characterization of the resolution enhancement induced by the microbead, which sheds light into the many contradictory resolution claims present in literature. Our conclusions are strongly supported by rigorous data analysis and by numerical simulations, all in perfect agreement with experimental results.

3D coaxial out-of-plane metallic antennas for filtering and multi-spectral imaging in the infrared range

We fabricated and investigated a new configuration of 3D coaxial metallic antennas working in the infrared which combines the strong lateral light scattering of vertical plasmonic structures with the selective spectral transmission of 2D arrays of coaxial apertures. The coaxial structures are fabricated with a top-down method based on a template of hollow 3D antennas. Each antenna has a multilayer radial structure consisting of dielectric and metallic materials not achievable in a 2D configuration. A planar metallic layer is inserted normally to the antennas. The outer dielectric shell of the antenna defines a nanometric gap between the horizontal plane and the vertical walls. Thanks to this aperture, light can tunnel to the other side of the plane, and be transmitted to the far field in a set of resonances. These are investigated with finite-elements electromagnetic calculations and with Fourier-transform infrared spectroscopy measurements. The spectral position of the resonances can be tuned by changing the lattice period and/or the antenna length. Thanks to the strong scattering provided by the 3D geometry, the transmission peaks possess a high signal-to-noise ratio even when the illuminated area is less than 2 × 2 times the operation wavelength. This opens new possibilities for multispectral imaging in the IR with wavelength-scale spatial resolution.

Plasmonic meta-electrodes allow intracellular recordings at network level on high-density CMOS-multi-electrode arrays

The ability to monitor electrogenic cells accurately plays a pivotal role in neuroscience, cardiology and cell biology. Despite pioneering research and long-lasting efforts, the existing methods for intracellular recording of action potentials on the large network scale suffer limitations that prevent their widespread use. Here, we introduce the concept of a meta-electrode, a planar porous electrode that mimics the optical and biological behaviour of three-dimensional plasmonic antennas but also preserves the ability to work as an electrode. Its synergistic combination with plasmonic optoacoustic poration allows commercial complementary metal–oxide semiconductor multi-electrode arrays to record intracellular action potentials in large cellular networks. We apply this approach to measure signals from human-induced pluripotent stem cell-derived cardiac cells, rodent primary cardiomyocytes and immortalized cell types and demonstrate the possibility of non-invasively testing a variety of relevant drugs. Due to its robustness and easiness of use, we expect the method will be rapidly adopted by the scientific community and by pharmaceutical companies.A combination of planar porous electrodes, optoacoustic poration and laser scanning allows large-scale recording of intracellular action potentials of cardiomyocyte cultures on CMOS-MEAs.

Scanning Probe Photonic Nanojet Lithography

The use of nano/microspheres or beads for optical nanolithography is a consolidated technique for achieving subwavelength structures using a cost-effective approach; this method exploits the capability of the beads to focus electromagnetic waves into subwavelength beams called photonic nanojets, which are used to expose the photoresist on which the beads are placed. However, this technique has only been used to produce regular patterns based on the spatial arrangement of the beads on the substrate, thus considerably limiting the pool of applications. Here, we present a novel microsphere-based optical lithography technique that offers high subwavelength resolution and the possibility of generating any arbitrary pattern. The presented method consists of a single microsphere embedded in an AFM cantilever, which can be controlled using the AFM motors to write arbitrary patterns with subwavelength resolution (down to 290 nm with a 405 nm laser). The performance of the proposed technique can compete with those of commercial high-resolution standard instruments, with the advantage of a one-order-of-magnitude reduction in costs. This approach paves the way for direct integration of cost-effective, high-resolution optical lithography capabilities into several existing AFM systems.

Author Correction: Plasmonic meta-electrodes allow intracellular recordings at network level on high-density CMOS-multi-electrode arrays

In the version of this Article originally published, the affiliation for the author Francesca Santoro was incorrectly given; it should have been ‘Center for Advanced Biomaterials for Healthcare, Istituto Italiano di Tecnologia, Napoli, Italy’. This has now been corrected in all versions of the Article.

Enhanced Raman Investigation of Cell Membrane and Intracellular Compounds by 3D Plasmonic Nanoelectrode Arrays

Abstract 3D nanostructures are widely exploited in cell cultures for many purposes such as controlled drug delivery, transfection, intracellular sampling, and electrical recording. However, little is known about the interaction of the cells with these substrates, and even less about the effects of electroporation on the cellular membrane and the nuclear envelope. This work exploits 3D plasmonic nanoelectrodes to study, by surface‐enhanced Raman scattering (SERS), the cell membrane dynamics on the nanostructured substrate before, during, and after electroporation. In vitro cultured cells tightly adhere on 3D plasmonic nanoelectrodes precisely in the plasmonic hot spots, making this kind of investigation possible. After electroporation, the cell membrane dynamics are studied by recording the Raman time traces of biomolecules in contact or next to the 3D plasmonic nanoelectrode. During this process, the 3D plasmonic nanoelectrodes are intracellularly coupled, thus enabling the monitoring of different molecular species, including lipids, proteins, and nucleic acids. Scanning electron microscopy cross‐section analysis evidences the possibility of nuclear membrane poration compatible with the reported Raman spectra. These findings may open a new route toward controlled intracellular sampling and intranuclear delivery of genic materials. They also show the possibility of nuclear envelope disruption which may lead to negative side effects.

Microsphere embedded in cantilever opens the AFM to high resolution optical microscopy

The combination of the AFM technique and the sphere-mediated microscopy (SMM) [1] opens a new opportunity to the Atomic Force Microscopy (AFM). With the help of a tipless AFM cantilever is possible to place and scan a microspheres (MS) close to the surface. From the optical point of view, when a MS is close to a surface act as high NA nanolenses whose optical characteristics define the maximum attainable resolution.

Efficient OAM generation at the nanoscale level by means of plasmonic vortex lens

Surface plasmon waves carry an intrinsic transverse spin angular momentum, which is locked to their propagation direction. On the other hand, helical plasmonic distributions may also carry an orbital angular momentum that is linked to the field topology. Apparently, when such a singular plasmonic mode propagates on a surface or is guided on a conic structure its helicity and the transverse spin can be coupled to the far-field spin and orbital angular momentum. We discuss the mechaism of such a coupling by using 2D and 3D guiding architetures. We analyze the coupling efficiency in each case as well as the intriguing spin-locking phenomenon occurring in our system. Finally we experimentally demonstrate the efficient beaming of a single-handed mode decorated by a desired orbital angular using accurately fabricated nanostructures.

Nanoporous gold decorated with silver nanoparticles as large area efficient SERS substrate

Nanoporous gold is a very promising material platform for several plasmonic applications. Nanoporous gold can be formed by dealloying Au–Ag alloys, previously grown by means of Ag-Au co-sputtering. The optical response is completely determined by the nanostructured film features, that only depend on the initial alloy composition. It has been extensively used as SERS substrate both as thin film and nanofabricated fancy designs. Here we explore the potential application of nanoporous gold as SERS substrate as it is coupled and decorated with Ag nanoparticles. Significant enhancement has been observed in comparison with bare nanoporous film.