A pinboard by
this curator

I do research in large user facilities and manage a lab in one to help scientists from all fields.


Understanding how plasmonic properties of nanomaterials can be exploited in multiple applications.

Nanomaterials such as graphene, transition metal dichalcogenide monolayers, phosphorene, nanotubes, nanowires, nanoparticles etc. exhibit physical properties that allow their exploitation for many applications. One of their interesting properites is plasmonic activity generated by a collective motion of the electrons in the material due to external, oscillating electric field and leading to very large field enhancement. Understanding plasmonic properties is an exciting field on its own as fundamental science, but the effects can be exploited in many ways, such as biosensing, photovoltaics, catalysis, etc. The articles presented here try to give a starting point to those interested in this wide field of science.


Computational Sensing Using Low-Cost and Mobile Plasmonic Readers Designed by Machine Learning.

Abstract: Plasmonic sensors have been used for a wide-range of biological and chemical sensing applications. Emerging nano-fabrication techniques have enabled these sensors to be cost-effectively mass-manufactured onto various types of substrates. To accompany these advances, major improvements in sensor read-out devices must also be achieved to fully realize the broad impact of plasmonic nano-sensors. Here, we propose a machine learning framework which can be used to design low-cost and mobile multi-spectral plasmonic readers that do not use traditionally employed bulky and expensive stabilized light-sources or high-resolution spectrometers. By training a feature selection model over a large set of fabricated plasmonic nano-sensors, we select the optimal set of illumination light-emitting-diodes needed to create a minimum-error refractive index prediction model, which statistically takes into account the varied spectral responses and fabrication-induced variability of a given sensor design. This computational sensing approach was experimentally validated using a modular mobile plasmonic reader. We tested different plasmonic sensors with hexagonal and square periodicity nano-hole arrays, and revealed that the optimal illumination bands differ from those that are 'intuitively' selected based on the spectral features of the sensor, e.g., transmission peaks or valleys. This framework provides a universal tool for the plasmonics community to design low-cost and mobile multi-spectral readers, helping the translation of nano-sensing technologies to various emerging applications such as wearable sensing, personalized medicine, and point-of-care diagnostics. Beyond plasmonics, other types of sensors that operate based on spectral changes can broadly benefit from this approach, including e.g., aptamer-enabled nanoparticle assays and graphene-based sensors, among others.

Pub.: 28 Jan '17, Pinned: 12 Apr '17

Biomacromolecular stereostructure mediates mode hybridization in chiral plasmonic nanostructures.

Abstract: The refractive index sensitivity of plasmonic fields has been exploited for over 20 years in analytical technologies. While this sensitivity can be used to achieve attomole detection levels, they are in essence binary measurements which sense the presence / absence of a pre-determined analyte. Using plasmonic fields, not to sense effective refractive indices, but to provide more "granular" information about the structural characteristics of a medium, provides a more information rich output, which affords opportunities to create new powerful and flexible sensing technologies not limited by the need to synthesize chemical recognition elements. Here we report a new plasmonic phenomenon that is sensitive to the biomacromolecular structure without relying on measuring effective refractive indices. Chiral biomaterials mediate the hybridization of electric and magnetic modes of a chiral solid-inverse plasmonic structure, resulting in a measureable change in both reflectivity and chiroptical properties. The phenomenon originates from the electric-dipole - magnetic-dipole response of the biomaterial and is hence sensitive to biomacromolecular secondary structure providing unique fingerprints of -helical, -sheet and disordered motifs. The phenomenon can be observed for sub-chiral plasmonic fields (i.e. fields with a lower chiral asymmetry than circularly polarized light) hence lifting constraints to engineer structures that produce fields with enhanced chirality, thus providing greater flexibility in nanostructure design. To demonstrate the efficacy of the phenomenon we have detected and characterized picogram quantities of simple model helical biopolymers and more complex real proteins.

Pub.: 23 Aug '16, Pinned: 11 Apr '17

Highly Controlled Synthesis and Super-Radiant Photoluminescence of Plasmonic Cube-in-Cube Nanoparticles

Abstract: The plasmonic properties of metal nanostructures have been heavily utilized for surface-enhanced Raman scattering (SERS) and metal-enhanced fluorescence (MEF), but the direct photoluminescence (PL) from plasmonic metal nanostructures, especially with plasmonic coupling, has not been widely used as much as SERS and MEF due to the lack of understanding of the PL mechanism, relatively weak signals, and the poor availability of the synthetic methods for the nanostructures with strong PL signals. The direct PL from metal nanostructures is beneficial if these issues can be addressed because it does not exhibit photoblinking or photobleaching, does not require dye-labeling, and can be employed as a highly reliable optical signal that directly depends on nanostructure morphology. Herein, we designed and synthesized plasmonic cube-in-cube (CiC) nanoparticles (NPs) with a controllable interior nanogap in a high yield from Au nanocubes (AuNCs). In synthesizing the CiC NPs, we developed a galvanic void formation (GVF) process, composed of replacement/reduction and void formation steps. We unraveled the super-radiant character of the plasmonic coupling-induced plasmon mode which can result in highly enhanced PL intensity and long-lasting PL, and the PL mechanisms of these structures were analyzed and matched with the plasmon hybridization model. Importantly, the PL intensity and quantum yield (QY) of CiC NPs are 31 times and 16 times higher than those of AuNCs, respectively, which have shown the highest PL intensity and QY reported for metallic nanostructures. Finally, we confirmed the long-term photostability of the PL signal, and the signal remained stable for at least 1 h under continuous illumination.

Pub.: 07 Nov '16, Pinned: 11 Apr '17

Imaging Intra- and Interparticle Acousto-plasmonic Vibrational Dynamics with Ultrafast Electron Microscopy

Abstract: We report real-space, time-resolved imaging of coherently excited acoustic phonon modes in plasmonic nanoparticles via femtosecond electron imaging with an ultrafast electron microscope. The particles studied were cetyl trimethylammonium bromide stabilized Au nanorods (40 × 120 nm), and the particular specimen configurations for which photoinduced vibrational modes were visualized consisted of a single, isolated nanocrystal and a cluster of four irregularly arranged and randomly oriented particles, all supported on an amorphous Si3N4 membrane. In both configurations, we are able to resolve discrete intraparticle acoustic phonon modes via diffraction-contrast modulation with bright-field femtosecond electron imaging. For the single nanorod, we spatiotemporally mapped the intraparticle vibrational energy distribution and decay times. With Fourier filtering, acoustic phonons ranging from 4 to 30 GHz (250 to 33 ps periods, respectively) were visualized, corresponding to bending, extensional, and higher-order modes. Furthermore, heterogeneously distributed intraparticle decay times, ranging from 3 to 10 ns, were spatially mapped, indicating a strong dependence on coupling of the mode to the underlying substrate. For a cluster of four randomly oriented nanorods, we are able to image acoustic phonon modes that are strongly localized to particular particle–particle contact regions within the aggregate. A vibrational mode occurring at 27 GHz (37 ps period) was observed to occur at a 10 nm side-to-end contact region, with other intraparticle points at distances of 20 and 50 nm from the region showing no such dynamics, although the initial few-picosecond diffraction-contrast response was observed changing sign in moving from the end to the center of the particle. Excellent agreement is found between the spatiotemporally mapped vibrational-mode symmetries and finite-element simulations of supported modes in a polymer-coated Au nanorod supported on a Si3N4 membrane. This experiment resolves both the structure and dynamic properties of the plasmonic assembly, providing insight into the characteristics of complex plasmonic assemblies that ultimately determine their response to ultrafast excitation.

Pub.: 24 Oct '16, Pinned: 11 Apr '17

Al-Pd Nanodisk Heterodimers as Antenna-Reactor Photocatalysts.

Abstract: Photocatalysis uses light energy to drive chemical reactions. Conventional industrial catalysts are made of transition metal nanoparticles that interact only weakly with light, while metals such as Au, Ag, and Al that support surface plasmons interact strongly with light but are poor catalysts. By combining plasmonic and catalytic metal nanoparticles, the plasmonic "antenna" can couple light into the catalytic "reactor". This interaction induces an optical polarization in the reactor nanoparticle, forcing a plasmonic response. When this "forced plasmon" decays it can generate hot carriers, converting the catalyst into a photocatalyst. Here we show that precisely oriented, strongly coupled Al-Pd nanodisk heterodimers fabricated using nanoscale lithography can function as directional antenna-reactor photocatalyst complexes. The light-induced hydrogen dissociation rate on these structures is strongly dependent upon the polarization angle of the incident light with respect to the orientation of the antenna-reactor pair. Their high degree of structural precision allows us to microscopically quantify the photocatalytic activity per heterostructure, providing precise photocatalytic quantum efficiencies. This is the first example of precisely designed heterometallic nanostructure complexes for plasmon-enabled photocatalysis and paves the way for high-efficiency plasmonic photocatalysts by modular design.

Pub.: 28 Sep '16, Pinned: 11 Apr '17

Probing the Near-Field of Second-Harmonic Light around Plasmonic Nanoantennas.

Abstract: We introduce a new concept that enables sub-wavelength polarization-resolved probing of the second-harmonic near-field distribution of plasmonic nanostructures. As a local sensor this method utilizes aluminum nanoantennas, which are resonant to the second-harmonic wavelength and which allow to efficiently scatter the local second-harmonic light to the far-field. We place these sensors into the second-harmonic near-field generated by plasmonic nanostructures, and carefully vary their position and orientation. Observing the second-harmonic light resonantly scattered by the aluminum nanoantennas provides polarization-resolved information about the local second-harmonic near-field distribution. We then investigate the polarization-resolved second-harmonic near-field of inversion symmetric gold dipole nanoantennas. Interestingly, we find strong evidence that the second-harmonic dipole is predominantly oriented perpendicular to the gold nanoantenna long axis, although the excitation laser is polarized parallel to the nanoantennas. We believe that our investigations will help to disentangle the highly debated origin of the second-harmonic response of inversion symmetric plasmonic structures. Furthermore, we believe that our new method, which enables the measurement of local nonlinear electric fields, will find widespread implementation and applications in nonlinear near-field optical microscopy.

Pub.: 10 Feb '17, Pinned: 11 Apr '17

In-plane plasmonic antenna arrays with surface nanogaps for giant fluorescence enhancement.

Abstract: Optical nanoantennas have a great potential for enhancing light-matter interactions at the nanometer scale, yet fabrication accuracy and lack of scalability currently limit ultimate antenna performance and applications. In most designs, the region of maximum field localization and enhancement (i.e., hotspot) is not readily accessible to the sample since it is buried into the nanostructure. Moreover, current large-scale fabrication techniques lack reproducible geometrical control below 20 nm. Here, we describe a new nanofabrication technique that applies planarization, etch back and template stripping to expose the excitation hotspot at the surface, providing a major improvement over conventional electron beam lithography methods. We present large flat surface arrays of in-plane nanoantennas, featuring gaps as small as 10 nm with sharp edges, excellent reproducibility and full surface accessibility of the hotspot confined region. The novel fabrication approach drastically improves the optical performance of plasmonic nanoantennas to yield giant fluorescence enhancement factors up to 10(4)-10(5) times, together with nanoscale detection volumes in the 20 zeptoliter range. The method is fully scalable and adaptable to a wide range of antenna designs. We foresee broad applications by the use of these in-plane antenna geometries ranging from large-scale ultra-sensitive sensor chips, to microfluidics and live cell membrane investigations.

Pub.: 10 Feb '17, Pinned: 11 Apr '17

Optical nano-imaging of gate-tunable graphene plasmons.

Abstract: The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons—coupled excitations of photons and charge carriers—in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can readily be tuned electrically by varying the graphene carrier density. Although evidence of optical graphene plasmon resonances has recently been obtained spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmon fields, and find that the extracted plasmon wavelength is very short—more than 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of active subwavelength-scale optics and a plethora of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light–matter interactions for quantum devices and biosensing applications.

Pub.: 23 Jun '12, Pinned: 11 Apr '17