High refractive index dielectric nanoantennas strongly modify the decay rate via the Purcell effect through the design of radiative channels. Due to their dielectric nature, the field is mainly confined inside the nanostructure and in the gap, which is hard to probe with scanning probe techniques. Here we use single-molecule fluorescence lifetime imaging microscopy (smFLIM) to map the decay rate enhancement in dielectric GaP nanoantenna dimers with a median localization precision of 14 nm. We measure, in the gap of the nanoantenna, decay rates that are almost 30 times larger than on a glass substrate. By comparing experimental results with numerical simulations we show that this large enhancement is essentially radiative, contrary to the case of plasmonic nanoantennas, and therefore has great potential for applications such as quantum optics and biosensing.

Hot carrier based methods constitute a valuable approach for efficient and silicon compatible sub-bandgap photodetection. Although, hot electron excitation and transfer have been studied extensively on traditional materials such as Au and Ti, reports on alternative materials such as titanium nitride (TiN) are rare. Here, we perform hot hole photodetection measurements on a p-Si/metal thin film junction using Ti, Au and TiN. This material is of interest as it constitutes a refractory alternative to Au which is an important property for plasmonic applications where high field intensities can occur. In contrast to Au, a TiN/Si junction does not suffer from metal diffusion into the Si, which eases the integration with current Si-fabrication techniques. This work shows that a backside illuminated p-Si/TiN system can be used for efficient hot hole extraction in the IR, allowing for a responsivity of 1 mA/W at an excitation wavelength of 1250 nm and at zero bias. Via a comparison between TiN and other commonly used materials such as Au, the origin of this comparably high photoresponse can be traced back to be directly linked to a thin TiO_2-x interfacial layer allowing for a distinct hot-hole transfer mechanism. Moreover, the fabrication of TiN nanodisk arrays is demonstrated which bears great promise to further boost the device efficiency.

Despite the fact that analyte manipulation can have great applications in different research fields, most techniques used for this still present some limitations. Plasmonic nanostructures can be used to try and overcome this, but they lack the ability to manipulate objects over long distances. In this paper we therefore present different geometries for Brownian ratchet devices that are able to not only optically trap, but also to manipulate over long distances, dielectric nanometer-sized beads by periodically modulating an external light source.

Research on optically resonant dielectric nanostructures has accelerated the development of photonic applications, driven by their ability to strongly confine light on the nanoscale. However, as dielectric resonators are typically operated below their band gap to minimize optical losses, the usage of dielectric nanoantenna concepts for absorption enhancement has largely remained unexplored. In this work, we realize engineered nanoantennas composed of photocatalytic dielectrics and demonstrate increased light-harvesting capabilities in otherwise weakly absorptive spectral regions. In particular, we employ anapole excitations, which are known for their strong light confinement, in nanodisks of oxygen-vacancy-rich TiO2-x, a prominent photocatalyst that provides a powerful platform for exploring concepts in absorption enhancement in tunable nanostructures. The arising photocatalytic effect is monitored on the single particle level using the well-established photocatalytic silver reduction reaction on TiO2. With the freedom of changing the optical properties of TiO2 through tuning the abundance of VO states, we discuss the interplay between cavity damping and the anapole-assisted field confinement for absorption enhancement. This concept is general and can be extended to other catalytic materials with higher refractive indices.

Titanium oxynitride enables a range of plasmonic and optoelectronic functionality using long-lived photo-generated hot carriers. We explore the time scale of hot carriers in TiN and their use in photochemical reduction and Schottky detectors.

This Conference Presentation, "Oxide-enhanced IR hot-carrier-based photo detection in metal thin-film Si junctions" was recorded at Photonics West 2020 held in San Francisco, California, United States.

In this talk I will discuss some of the research directions of my group in the area of biosensing. Specifically, I will discuss: the development of hybrid plasmonic substrates that can be implemented into the Krechmann configuration for simultaneous qualitative and quantitative detection of analytes; the use of combinatory techniques based on plasmonic structures that enable control of analyte motion and thus concentration of analytes in the sensing area; and finally, the development of a theranostic agent based on conjugated polymer nanoparticles.

Efficient waveguiding inside low refractive index media is of key importance for a great variety of applications that demand strong light-matter interaction on small geometric footprints. Here, we demonstrate efficient light guidance in single-defect dual-ring light cages over millimeter distances that are integrated on silicon chips via direct laser writing. The cages consist of two rings of high aspect-ratio polymer strands (length 5 mm, aspect ratio >1000) hexagonally arranged around a hollow core. Clear-core mode formation via the photonic band gap effect is observed, with the experiments showing pronounced transmission bands with fringe and polarization contrasts of >20  dB and >15  dB, respectively. Numerical simulations confirm our experiments and reveal the dual-ring arrangement to be the optimal geometry within the light cage concept. Particularly, the side-wise access to the core regions and the chip integration makes the light cage concept attractive for a great number of fields such as bioanalytics or quantum technology.

Lithographically-produced plasmonic structures, while offering exceptional control of light through light-matter interactions, have seen limited implementation into sensing technologies. This has been due to several factors, such as large ohmic losses and high cost of fabrication. The use of self-assembled superclusters of chemically-synthesized metallic nanoparticles has been proposed as means of overcoming some of these limitations, by exploiting their collective modes. In this work, the existence of such modes is successfully verified by 3D Raman tomography. Proof-of-principle experiments are also conducted to demonstrate the potential of superclusters in sensing schemes.

Plasmonic hot carriers have been recently identified as key elements for photocatalysis at visible wavelengths. The possibility to transfer energy between metal plasmonic nanoparticles and nearby molecules depends not only on carrier generation and collection efficiencies but also on their energy at the metal-molecule interface. Here an energy screening study was performed by monitoring the aniline electro-polymerization reaction via an illuminated 80 nm gold nano-particle. Our results show that plasmon excitation reduces the energy required to start the polymerization reaction as much as 0.24 eV. Three possible photocatalytic mechanisms were explored: the enhanced near field of the illuminated particle, the temperature increase at the metal liquid-interface, and the excited electron-hole pairs. This last phenomenon is found to be the one contributing most prominently to the observed energy reduction.

In article number 1804798, Max Grell, Firat Guder, and co-workers describe a method for depositing a wide range of metals inside fabrics which they call Si ink-enabled autocatalytic metallization. Without requiring surface modification, the method provides access to devices as diverse as antennas, batteries, biosensors, or Raman spectroscopy substrates. Future research might bring many more applications for this highly adaptable method.

We investigate titanium nitride (TiN) thin film coatings on silicon for CMOS-compatible sub-bandgap charge separation upon incident illumination, which is a key feature in the vast field of on-chip photodetection and related integrated photonic devices. Titanium nitride of tunable oxidation distributions serves as an adjustable broadband light absorber with high mechanical robustness and strong chemical resistivity. Backside-illuminated TiN on p-type Si (pSi) constitutes a self-powered and refractory alternative for photodetection, providing a photoresponsivity of about similar to 1 mA/W at 1250 nm and zero bias while outperforming conventional metal coatings such as gold (Au). Our study discloses that the enhanced photoresponse of TiN/pSi in the near-infrared spectral range is directly linked to trap states in an ultrathin TiO2-x interfacial interlayer that forms between TiN and Si. We show that a pSi substrate in conjunction with a few nanometer thick amorphous TiO2-x film can serve as a platform for photocurrent enhancement of various other metals such as Au and Ti. Moreover, the photoresponse of Au on a TiO2-x/pSi platform can be increased to about 4 mA/W under 0.45 V reverse bias at 1250 nm, allowing for controlled photoswitching. A clear deviation from the typically assumed Fowler-like response is observed, and an alternative mechanism is proposed to account for the metal/semiconductor TiO2-x interlayer, capable of facilitating hole transport.

Plasmon excitation in metal nanoparticles triggers the generation of highly energetic charge carriers that, when properly manipulated and exploited, can mediate chemical reactions. Single-particle techniques are key to unearthing the underlying mechanisms of hot-carrier generation, transport and injection, as well as to disentangling the role of the temperature increase and the enhanced near-field at the nanoparticle-molecule interface. Gaining nanoscopic insight into these processes and their interplay could aid in the rational design of plasmonic photocatalysts. Here, we present three different approaches to monitor hot-carrier reactivity at the single-particle level. We use a combination of dark-field microscopy and photoelectrochemistry to track a hot-hole driven reaction on a single Au nanoparticle. We image hot-electron reactivity with sub-particle spatial resolution using nanoscopy techniques. Finally, we push the limits by looking for a hot-electron induced chemical reaction that generates a fluorescent product, which should enable imaging plasmonic photocatalysis at the single-particle and single-molecule levels.

Commercially available metal inks are mainly designed for planar substrates (for example, polyethylene terephthalate foils or ceramics), and they contain hydrophobic polymer binders that fill the pores in fabrics when printed, thus resulting in hydrophobic electrodes. Here, a low-cost binder-free method for the metallization of woven and nonwoven fabrics is presented that preserves the 3D structure and hydrophilicity of the substrate. Metals such as Au, Ag, and Pt are grown autocatalytically, using metal salts, inside the fibrous network of fabrics at room temperature in a two-step process, with a water-based silicon particle ink acting as precursor. Using this method, (patterned) metallized fabrics are being enabled to be produced with low electrical resistance (less than 3.5 Ω sq-1). In addition to fabrics, the method is also compatible with other 3D hydrophilic substrates such as nitrocellulose membranes. The versatility of this method is demonstrated by producing coil antennas for wireless energy harvesting, Ag-Zn batteries for energy storage, electrochemical biosensors for the detection of DNA/proteins, and as a substrate for optical sensing by surface enhanced Raman spectroscopy. In the future, this method of metallization may pave the way for new classes of high-performance devices using low-cost fabrics.

Assembly of nanoscale building blocks into hierarchical superstructure by self-assembly is one of the most pursued topics in nanoparticles chemistry. The possibilities obtainable when individual components arrange themselves into an ordered structure are limitless and of great interest. Spherical colloidal clusters have been proposed to possess remarkable collective supermodes with large local field enhancements, and a spectral response extending from the near-infrared to deep into the mid-infrared region. As such, these superstructures hold great promise as a sensing platform that is capable of addressing the whole spectral domain of vibrational molecular fingerprints, while simultaneously exploiting the advantages of their plasmonic constituents [1], [2]. By exploiting these properties of the self-assembled metamaterials supercluster, we report the experimental measurement of near and mid- IR plasmonic collective modes by monitoring the Raman scattering of 4-Mercaptobenzoic acid with a confocal microscope. The strongly enhanced Raman signal allows measurement of the plasmonic mode with a lateral resolution lower than 300 nm and a vertical one of 300 nm. As the supercluster structure possesses tunable optical modes, different plasmonic responses are mapped according to the cluster size and the excitation wavelength. Moreover, as SERS allows sensitive detection of molecules, the remarkable and tunable modes excitable inside the spherical colloidal clusters can provide an efficient platform for ultra-sensitive molecular spectroscopy. To this end proof-of-principle implementation of the superclusters as an efficient platform for pH sensing of the surrounding medium is reported [2]. REFERENCES [1] V. A. Turek, L. N. Elliott, A. I. I. Tyler, A. Demetriadou, J. Paget, M. P. Cecchini, A. R. Kucernak, A. A. Kornyshev, and J. B. Edel, “Self-assembly and applications of ultraconcentrated nanoparticle solutions,” ACS Nano, vol. 7, no. 10, pp. 8753–8759, 2013. [2] A. Lauri, L. Velleman, X. Xiao, E. Cortes, J. B. Edel, V. Giannini, A. Rakovich, and S. A. Maier, “3D Confocal Raman Tomography to Probe Field Enhancements inside Supercluster Metamaterials,” ACS Photonics, vol. 8, no. 4, p. pp 2070-2077, 2017.

Spherical colloidal superclusters, composed from sub-100 nm plasmonic nanoparticles, have been proposed to possess collective plasmonic modes imbued with large field enhancements and tunable spectral response extending from the visible to infrared regions. Here, we report the experimental verification of collective near-IR plasmonic modes inside single superclusters, with dimensions ranging from 0.77 μm up to 2 μm. Raman reporters, coated onto the nanoparticle building blocks, were used as local probes of the electric field enhancement inside the metamaterial. By performing diffraction-limited 3D Raman tomography we were able to build up the electric field intensity distribution within the superclusters. We demonstrate that plasmonic responses of superclusters vary according to their size and excitation wavelength, in accordance with theoretical predictions of their tunable optical properties. The existence of three-dimensional internal collective modes in these superclusters enables the excitation of a l...

We demonstrate simultaneous wide-field (>1000 μm2) measurements of magnetic fields and temperature on GaN HEMTs using nitrogen vacancy centers in nanodiamonds.