The emerging field of plasmonics has yielded methods for guiding and localizing light at the nanoscale, well below the scale of the wavelength of light in free space. Now plasmonics researchers are turning their attention to photovoltaics, where design approaches based on plasmonics can be used to improve absorption in photovoltaic devices, permitting a considerable reduction in the physical thickness of solar photovoltaic absorber layers, and yielding new options for solar-cell design. In this review, we survey recent advances at the intersection of plasmonics and photovoltaics and offer an outlook on the future of solar cells based on these principles.

Plasmonics for improved photovoltaic devices

Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.

Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy

The unprecedented ability of nanometallic (that is, plasmonic) structures to concentrate light into deep-subwavelength volumes has propelled their use in a vast array of nanophotonics technologies and research endeavours. Plasmonic light concentrators can elegantly interface diffraction-limited dielectric optical components with nanophotonic structures. Passive and active plasmonic devices provide new pathways to generate, guide, modulate and detect light with structures that are similar in size to state-of-the-art electronic devices. With the ability to produce highly confined optical fields, the conventional rules for light-matter interactions need to be re-examined, and researchers are venturing into new regimes of optical physics. In this review we will discuss the basic concepts behind plasmonics-enabled light concentration and manipulation, make an attempt to capture the wide range of activities and excitement in this area, and speculate on possible future directions.

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Plasmonics for extreme light concentration and manipulation.

We present an experimental and theoretical study of the fluorescence rate of a single molecule as a function of its distance to a laser-irradiated gold nanoparticle. The local field enhancement leads to an increased excitation rate whereas nonradiative energy transfer to the particle leads to a decrease of the quantum yield (quenching). Because of these competing effects, previous experiments showed either fluorescence enhancement or fluorescence quenching. By varying the distance between molecule and particle we show the first experimental measurement demonstrating the continuous transition from fluorescence enhancement to fluorescence quenching. This transition cannot be explained by treating the particle as a polarizable sphere in the dipole approximation.

https://www.photonics.ethz.ch/fileadmin/user_upload/FILES2SHARE/anger06a.pdf

Enhancement and quenching of single-molecule fluorescence.

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

We investigate the coupling of a single molecule to a single spherical gold nanoparticle acting as a nanoantenna. Using scanning probe technology, we position the particle in front of the molecule with nanometer accuracy and measure a strong enhancement of more than 20 times in the fluorescence intensity simultaneous to a 20-fold shortening of the excited state lifetime. Comparisons with three-dimensional calculations guide us to decipher the contributions of the excitation enhancement, spontaneous emission modification, and quenching. Furthermore, we provide direct evidence for the role of the particle plasmon resonance in the molecular excitation and emission processes.

https://www.iap.uni-jena.de/iapmedia/de/Lecture/Nanomaterials1367272800/NanoMat13_Paper_05.pdf

Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna.

The existing optical microscopes form an image by collecting photons emitted from an object. Here we report on the experimental realization of microscopy without the need for direct optical communication with the sample. To achieve this, we have scanned a single gold nanoparticle acting as a nanoantenna in the near field of a sample and have studied the modification of its intrinsic radiative properties by monitoring its plasmon spectrum.

Optical microscopy via spectral modifications of a nanoantenna.

We demonstrate a tomographic method for determining the degree of ellipticity and the orientation of a gold nanoparticle. To do this, we attach a single nanoparticle to the end of a sharp glass fiber tip and record its plasmon spectra for different incident polarizations and angles of incidence. Our measurements allow us to identify the plasmon spectra along the three main axes of the particle, therefore resolving its “internal” spectral inhomogeneity. Knowledge of the plasmon resonances and the orientation of a metallic nanoparticle is an important asset for controlled and quantitative studies of its interaction with a single molecule.

Tomographic Plasmon Spectroscopy of a Single Gold Nanoparticle

Optical microscopy using the spectral modifications of a nano-antenna

The combination of integrated optics and microfluidics in planar optofluidic devices carries the potential for novel compact and ultra-sensitive detection in liquid and gaseous media. Single molecule fluorescence detection sensitivity in planar beam geometry was recently demonstrated in liquid-core antiresonant reflecting optical waveguides (ARROWs) fabricated on a silicon chip. A key component of a fully integrated single-molecule sensor is the addition of an optical filtering capability to separate excitation beams from much weaker generated fluorescence or scattering signals. This capability will eventually allow for integration of the photodetector on the same chip as the optofluidic sensing part. It has been theoretically shown that the wavelength-dependent transmission of liquid-core ARROWs can be tailored to efficiently separate excitation and fluorescence. Here, we present the wavelength dependent transmission of air-core ARROW waveguides, using a highly nonlinear photonic crystal fiber to generate a broadband excitation spectrum, and the design of liquid-core ARROW waveguides with integrated filter function. The air-core waveguide loss shows pronounced wavelength dependence in good agreement with the design, demonstrating the potential of tailoring the optical properties of liquid-core waveguides to accommodate single-molecule sensing on a chip. We also present an ARROW design to produce wavelength-dependent transmission that is optimized for fluorescence resonance energy transfer (FRET) studies with high transmission at 573 nm and 668nm, and low transmission at 546 nm.

Ulf Hakanson

Papers

Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna.

Optical microscopy via spectral modifications of a nanoantenna.

Tomographic Plasmon Spectroscopy of a Single Gold Nanoparticle

Optical microscopy using the spectral modifications of a nano-antenna

Tailoring the transmission of liquid-core waveguides for wavelength filtering on a chip