Reuben M. Bakker

Demonstration of a spaser-based nanolaser

One of the most rapidly growing areas of physics and nanotechnology focuses on plasmonic effects on the nanometre scale, with possible applications ranging from sensing and biomedicine to imaging and information technology. However, the full development of nanoplasmonics is hindered by the lack of devices that can generate coherent plasmonic fields. It has been proposed that in the same way as a laser generates stimulated emission of coherent photons, a ‘spaser’ could generate stimulated emission of surface plasmons (oscillations of free electrons in metallic nanostructures) in resonating metallic nanostructures adjacent to a gain medium. But attempts to realize a spaser face the challenge of absorption loss in metal, which is particularly strong at optical frequencies. The suggestion to compensate loss by optical gain in localized and propagating surface plasmons has been implemented recently and even allowed the amplification of propagating surface plasmons in open paths. Still, these experiments and the reported enhancement of the stimulated emission of dye molecules in the presence of metallic nanoparticles lack the feedback mechanism present in a spaser. Here we show that 44-nm-diameter nanoparticles with a gold core and dye-doped silica shell allow us to completely overcome the loss of localized surface plasmons by gain and realize a spaser. And in accord with the notion that only surface plasmon resonances are capable of squeezing optical frequency oscillations into a nanoscopic cavity to enable a true nanolaser, we show that outcoupling of surface plasmon oscillations to photonic modes at a wavelength of 531 nm makes our system the smallest nanolaser reported to date—and to our knowledge the first operating at visible wavelengths. We anticipate that now it has been realized experimentally, the spaser will advance our fundamental understanding of nanoplasmonics and the development of practical applications.

Enhanced localized fluorescence in plasmonic nanoantennae

Pairs of gold elliptical nanoparticles form antennae, resonant in the visible. A dye, embedded in a dielectric host, coats the antennae; its emission excites plasmon resonances in the antennae and is enhanced. Far-field excitation of the dye-nanoantenna system shows a wavelength-dependent increase in fluorescence that reaches 100 times enhancement. Near-field excitation shows enhanced fluorescence from a single nanoantenna localized in a subwavelength area of ∼0.15μm2. The polarization of enhanced emission is along the main antenna axis. These observed experimental results are important for increasing light extraction from emitters localized around antennae and for potential development of a subwavelength sized laser.

Magnetic and electric hotspots with silicon nanodimers

The study of the resonant behavior of silicon nanostructures provides a new route for achieving efficient control of both electric and magnetic components of light. We demonstrate experimentally and numerically that enhancement of localized electric and magnetic fields can be achieved in a silicon nanodimer. For the first time, we experimentally observe hotspots of the magnetic field at visible wavelengths for light polarized across the nanodimers primary axis, using near-field scanning optical microscopy.

Nanoantenna array-induced fluorescence enhancement and reduced lifetimes

Enhanced fluorescence is observed from dye molecules interacting with optical nanoantenna arrays. Elliptical gold dimers form individual nanoantennae with tunable plasmon resonances depending upon the geometry of the two particles and the size of the gap between them. A fluorescent dye, Rhodamine 800, is uniformly embedded in a dielectric host that coats the nanoantennae. The nanoantennae act to enhance the dye absorption. In turn, emission from the dye drives the plasmon resonance of the antennae; the nanoantennae act to enhance the fluorescence signal and change the angular distribution of emission. These effects depend upon the overlap of the plasmon resonance with the excitation wavelength and the fluorescence emission band. A decreased fluorescence lifetime is observed along with highly polarized emission that displays the characteristics of the nanoantennas dipole mode. Being able to engineer the emission of the dye?nanoantenna system is important for future device applications in both bio-sensing and nanoscale optoelectronic integration.

Near-field excitation of nanoantenna resonance

An array of paired elliptic nanoparticles designed to enhance local fields around the particle pair is fabricated with gold embedded in quartz. Light excites a coupled plasmon resonance in the particle pair and the system acts like a plasmonic nanoantenna providing an enhanced electromagnetic field. Near-field scanning optical microscopy and finite element modeling are used to study the local field effects of the nanoantenna system. Local illumination shows similar resonant properties as plane wave illumination: a strong, localized optical resonance for light polarized parallel to the main, center-to-center axis.

Design and fabrication of random silver films as substrate for SERS based nano-stress sensing of proteins

We report a simple and easy to fabricate random silver film (RSF) as a highly sensitive Surface Enhanced Raman Scattering (SERS) substrate which can be fabricated directly onto a dielectric substrate such as glass. An electron beam evaporation system was used for substrate fabrication. The SERS activity is attributed to the formation of electromagnetic ‘hot-spots’ on the film. Substrate performance is analyzed by studying the reproducibility and signal enhancement from the Raman active molecule, 2-naphthalene thiol (NT), which is covalently anchored to the substrate. The metal thickness is optimized to achieve the highest SERS enhancement. Based on this study we found that a 7 nm RSF substrate gave the best SERS activity. The SERS signal intensity exhibited by 7 nm RSF is found to be at least 3 orders of magnitude higher than that of a commercial substrate. The SERS enhancement factor is estimated to be ∼1 × 107 with a point-to-point intensity variation of about 12% and it reaches a maximum of 15% for batch-to-batch comparison. The efficacy of this substrate for biosensing is demonstrated by detecting H1 influenza protein, and the detection limit is found to be ∼10 pM when it is used along with a recently established nano-stress SERS sensor, 4-ATP (4-amino-thiophenol), as linker molecule. This detection limit shows a performance superior to conventional ELISA (which has a nM detection limit). These results show promise for the development of a biosensing platform based on the marriage of RSF with nano-stress sensors.

A Metalens with a Near-Unity Numerical Aperture

The numerical aperture (NA) of a lens determines its ability to focus light and its resolving capability. Having a large NA is a very desirable quality for applications requiring small light-matter interaction volumes or large angular collections. Traditionally, a large NA lens based on light refraction requires precision bulk optics that ends up being expensive and is thus also a specialty item. In contrast, metasurfaces allow the lens designer to circumvent those issues producing high-NA lenses in an ultraflat fashion. However, so far, these have been limited to numerical apertures on the same order of magnitude as traditional optical components, with experimentally reported NA values of <0.9. Here we demonstrate, both numerically and experimentally, a new approach that results in a diffraction-limited flat lens with a near-unity numerical aperture (NA > 0.99) and subwavelength thickness (∼λ/3), operating with unpolarized light at 715 nm. To demonstrate its imaging capability, the designed lens is applied in a confocal configuration to map color centers in subdiffractive diamond nanocrystals. This work, based on diffractive elements that can efficiently bend light at angles as large as 82°, represents a step beyond traditional optical elements and existing flat optics, circumventing the efficiency drop associated with the standard, phase mapping approach.

Enhanced transmission in near-field imaging of layered plasmonic structures.

Near-field imaging of an engineered double layer structure in transmission mode has shown enhancement of light intensity through the structure. An array created by an optically thick double layer structure of a total thickness of 165 nm containing twin 50 nm Au layers was imaged using a near-field scanning optical microscope in illumination mode. The resulting transmission image shows an increased local transmission at the position of each particle in the array. This viewable enhancement is due to a nanoantenna effect that is created by a resonant plasmon oscillation between the two layers.

Resonant Light Guiding Along a Chain of Silicon Nanoparticles

Subwavelength confined waveguiding is experimentally demonstrated with high refractive index dielectric nanoparticles with photon energy propagation at distances beyond 500 μm. These particles have naturally occurring electric and magnetic dipole resonances. When they are placed in a 1D chain, the magnetic resonances of adjacent elements couple to each other, providing a means to transport energy at visible or NIR wavelengths in a confined mode. Chains of nanoparticles made of silicon were fabricated and guided waves were measured with near-field scanning optical microscopy. Propagation loss is quantified at 34 dB/mm for 720 nm and 5.5 dB/mm for 960 nm wavelengths with 150 and 220 nm diameter particles, respectively. Simulations confirm the unique properties of this waveguiding in comparison with photonic crystals. The resonant nature of the waveguide lays a foundation for integrated photonics beyond nanowire waveguides of silicon and silicon nitride. This technology is promising for more compact and deeper photonic integration such as right angle bends, more compact modulators, slow light and interfacing with single photon emitters for photonic integrated circuits, quantum communications, and biosensing.

Fabricating Plasmonic Components for Nano- and Meta-Photonics

Different fabrication approaches for realization of metal-dielectric structures supporting propagating and localized surface plasmons are described including fabrication of nanophotonic waveguides and plasmonic nanoantennae.