Highlights Surface Photonics 2011
- Coupling Bright and Dark Plasmonic Lattice Resonances.
Low-loss plasmonic materials are highly desired in plasmonics-based light emitting devices, optical sensors, and solar cells. The periodic arrays of metallic nanorods we have investigated diffract light in a similar way as optical gratings do: Beautiful rainbows are reflected or transmitted under white-light illumination. Therefore, light shone on such an array not only excites localized surface-plasmon resonances on the individual nanorods, but also gets diffracted in an ordered fashion. The diffracted light, in fact, is able to couple the localized surface-plasmon modes on the individual nanorods to each other. This coupling leads to resonances at a higher level: collective ones among the modes on the nanorods, known already as surface lattice resonances. What we have discovered and understood are the following: (1) by varying the angle of incidence of the illuminating light, collective lattice resonances of different frequencies can be excited, depending on the order of the diffracted light that underlies their individual emergences; (2) these collective lattice resonances are, in turn, also coupled to each other; (3) as a result of that coupling, lattice resonances of certain frequencies become forbidden, in other words, a frequency stop gap appears; (4) these lattice plasmonic modes suffer very low losses. These findings offer a new line of possibilities to tailor plasmonic resonances in a frequency-, angle-, and polarization-dependent manner in devices and also hold a promise for the development of low-loss plasmonic devices. The results of this research are published in the new open-access journal of the American Physical Society, Physical Review X.
- Figure 1. Extinction of s-polarized light for arrays of gold nanorods. The nanorods in (b) are slightly bigger than those in (a). [See article for details]
- Figure 2. Near field enhancement (in color scale) and surface charge distribution (charges of opposite sign in black and white) at the mid-height of the nanorods, for the bright and dark mode.
- Active Control on Strong Coupling Regime by Molecular Activation.
Researchers from the AMOLF group Surface Photonics, together with colleagues from Holst Center IMEC Netherlands, have demonstrated the active control of the coupling regime of organic excitons and surface plasmon polaritons. Excitons refer to excited electron-hole pairs in the organic material, while surface plasmon polaritons are the coherent oscillation of electrons at the surface of a metal. The active control of the coupling is obtained by introducing a reacting gas into the organic layer. When nitrogen dioxide molecules react with the organic molecules, the "strength" of the excitonic layer increases; therefore, the coupling strength of these excitons to surface plasmon polaritons in a gold film beneath the organic layer also increases. This interaction, being reversible, provides a versatile way to tailor the degree of coupling between light and matter.
The ability to prepare the system in a given state of coupling, from weak coupling to strong coupling regimes, has both fundamental and practical applications. From a fundamental perspective, this tuning allows for a better quantitative understanding of the coupling than has previously been obtained for similar systems. From a more practical perspective, it is demonstrated that the tuning of the coupling acts on the velocity of surface plasmon polaritons. This research is part of the Industrial Partnership Program 'Improved solid-state light sources' of the Foundation for Fundamental Research on Matter (FOM) and Philips. It also received support from NanoNext.The result of this research has recently been published in the journal ACS Nano.
Reference
Active Control of the Strong Coupling Regime between Porphyrin Excitons and Surface plasmon Polaritons
Audrey Berrier, Ruud Cools, Christophe Arnolds, Peter Offermans, Mercedes Crego-Calama, Sywert H. Brongersma, and Jaime Gómez Rivas
ACS Nano (2011), 5(8), 6226-6232
- Enhanced sensitivity of plasmonic gas sensors using a nanoporous matrix.
Researchers from the AMOLF group Surface Photonics, together with colleagues from Holst Center IMEC Netherlands, have demonstrated that surface plasmon resonance based sensors can be improved for gas detection if a nanoporous layer is used. Surface plasmon resonance based sensors are a versatile and widely recognized platform for detection of biological and chemical substances. The sensing principle is based on the detection of refractive index changes. In the case of gas sensors, the use of an intermediate molecule capturing the gas molecules is often used. In the particular case of nitrogen containing gasses such as nitrogen dioxide, the capturing molecule is an OH substituted tetraphenyl porphyrine. It is shown that the detection sensitivity of such a sensor is increased, when the overlap between the evanescent field decay of the surface plasmon polaritons with the porphyrine layer is increased. In order to reach the optimum overlap the porphyrin molecules are embedded in a porous silica matrix. The fractional free volume beiong larger than in the case of a conventional polymer, the gas molecules can diffuse through the porphyrin layer more easily. The result is that the detectivity of the whole sensor is improved. This paves the way for better gas sensors, with applications such as autonomous systems for environment monitoring. This interaction of the NO2 with the porphyrin layer being reversible, the novel concept of sensor presented in this work provides a versatile way to enhance the detection sensitivity of plasmonic sensors. The result of this research has recently been published in the journal Sensors and Actuators B: Chemical. This research is part of the Industrial Partnership Program 'Improved solid-state light sources' of the Foundation for Fundamental Research on Matter (FOM) and Philips. It also received support from NanoNext.
Reference
Enhancing the gas sensitivity of surface plasmon resonance with a nanoporous silica matrix
Audrey Berrier, Peter Offermans, Ruud Cools, Bram van Megen, Wout Knoben, Gabriele Vecchi, Jaime Gómez Rivas, Mercedes Crego-Calama, and Sywert H. Brongersma
Sensors and Actuators B: Chemical (2011), doi: 10.1016/j.snb.2011.07.030
- Nanowires offer opportunities for improved LEDs.
We have made special nanostructures that could be used as light-emitting diodes (LEDs). In a collaboration with colleagues from Philips Research, Eindhoven University of Technology and Delft University of Technology, we have made special nanostructures that could be used as light-emitting diodes (LEDs). These nanostructures can be used to control the direction of the emission. Controlling the direction of the light is vitally important for increasing the efficiency of LEDs. It is also a step towards a new generation of LEDs that are based on semiconducting nanowires. The results of this research are recently published in the prestigious journal ACS Nano.
The direction in which a LED emits light is mainly determined by the surface between the LED and the surrounding air. As light can only escape from the LED at small angles, the direction of emission is usually straight on (perpendicular to the surface). However this can be influenced by nanostructures in the surface of the LED. Inspired by these nanostructures, the researchers have developed a new technology with which the direction of the light can be changed.
Photonic crystal
The new method consists of growing partially-emitting nanowires in an ordered pattern. This pattern forms a ‘photonic crystal’ that sends the light in specific directions. Furthermore, the researchers have shown that the emission can be optimised by a smart positioning of the emitting part within the nanowire. This knowledge could lead to an increased efficiency of LEDs. Moreover it provides opportunities for a next generation of LEDs, based on semiconducting nanowires.
Reference
'Controlling the directional emission of light by periodic arrays of heterostructured semiconductor nanowires',
Silke L. Diedenhofen, Olaf T.A. Janssen, Moïra Hocevar, Aurélie Pierret, Erik P.A.M. Bakkers, H. Paul Urbach, and Jaime Gómez Rivas
ACS Nano (2011), doi: 10.1021/nn201557h
- Strong Geometrical Dependence of the Absorption of Light in Arrays of Semiconductor Nanowires.
We demonstrate experimentally that arrays of base-tapered InP nanowires on top of an InP substrate form a broad band and omnidirectional absorbing medium. These characteristics are due to the specific geometry of the nanowires.
Almost perfect absorption of light (higher than 97%) occurs in the system. We describe the strong optical absorption by finite-difference timedomain simulations and present the first study of the influence of the geometry of the nanowires on the enhancement of the optical absorption by arrays. Cylindrical nanowires present the highest absorption normalized to the volume fraction of the semiconductor. The absolute absorption in layers of conical nanowires is higher than that in cylindrical nanowires but requires a larger volume fraction of semiconducting material. Base-tapered nanowires, with a cylindrical top and a conical base, represent an intermediate geometry. These results set the basis for an optimized optical design of nanowire solar cells.
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