highlights

2010

1. Nanoscale Free-Carrier Profiling of Individual Semiconductor Nanowires by Infrared Near-Field Nanoscopy.

   In collaboration with the groups of Rainer Hillenbrand (CIC Nanogune Consolider, San Sebastian, Spain) and Erik Bakkers (Philips Research, Eindhoven), we have utilized infrared near-field nanoscopy to probe the free carrier concentration in semiconductor nanowires. With a spatial resolution of 25 nm, we determined the gradient of the free-carrier concentration at the interface between a non-doped and a doped segment and we quantified the free-carrier concentration in the doped region. Controlling the free-carrier concentration and the sharpness of the interface between regions with different doping concentrations allows for fabrication of novel nanowire diodes, transistors or light emitting devices. Please download the Nano Letters article for more information.

   

1. Generic nano-imprint process for fabrication of nanowire arrays.

   We have developed a generic process for growing defect-free ordered arrays of nanowire in collaboration with the group of Erik Bakkers from Philips Research. Substrate conformal imprint lithography has been used to pattern gold particle arrays on full 2 inch substrates, that catalyze the growth of the nanowires. From the imprint process, organic residues remain on the surface, which induce the growth of additional undesired nanowires. Cleaning the substrates before growth with a wet chemical etch in combination with a thermal anneal results in uniform nanowire arrays. The cleaning procedure is applicable to other lithographic techniques, and therefore represents a generic process. Please download the Nanotechnology article for more information.

   

1. Long range surface polaritons supported by phase change materials.

   Light can propagate along thin layers of materials with sub-wavelength confinement. These waves are called long-range surface polaritons (LRSPs). In the case of ultra-thin films, even very lossy materials can support these waves.
   
   In our recently published Applied Physics Letters, we have demonstrated experimentally a theoretical prediction that was made almost 20 years ago by Yang, Sambles and Bradberry (Phys. Rev. B, 1991): Long-range surface LRSPs are nearly independent of the real component of the permittivity of the thin layer for large values of its imaginary component.
   
   We were able to confirm this prediction by using layers of chalcogenide glasses. These are phase change materials commonly used for optical memory applications. The phase can be changed between amorphous and crystalline by heating up the material. Whereas the material in its amorphous phase is a lossy dielectric, in its crystalline phase behaves like a metal. A change of phase leads to a large modification of the real component of the permittivity at some frequencies in the visible, while leaving the imaginary component nearly unchanged. (for more information contact Christophe Arnold, Arnoldatamolf.nl or download our Applied Physics Letters article )

2009

1. Efficient light scattering by nanowires for random lasers and next-generation solar cells.

   One-dimensional nanowires from semiconductors like InP, GaP, and Si are promising nanomaterials for next-generation inorganic solar cell devices. Understanding their optical response is extremely important for achieving increased performance in solar energy collection. Researchers at the FOM Institute for Atomic and Molecular Physics (AMOLF) and Philips Research in the Netherlands have investigated strong light scattering in layers of nanowires. Examples of nanowire layers are shown in Figure 1. Using a specially developed broadband optical technique, they show that trapping of light by multiple scattering is important in the design of nanowire devices (Figure 2).
   
   The researchers have demonstrated that nanowires actually can be grown to form one of the most strongly scattering materials available today. Next to its technological relevance, this property opens exciting new prospects in fundamental research on random lasers and Anderson localization of light. By matching the nanowire diameter to the optical wavelength, light can be trapped for several periods inside the nanowire, leading to a resonant enhancement of their scattering efficiency. The high tunability of nanowire properties and alignment, and the general applicability to groups III-V, II-VI, and IV semiconductors, enable new possibilities for harvesting of the solar spectrum. The results of this research are published in the prestigious journal Nano Letters.

   

   Figure 1 High densities of GaP nanowires were grown with controlled sizes and alignment using vapor-phase epitaxy at Philips Research. (left to right) Cross-sectional Scanning Electron Microscopy images of nanowire layers with increasing nanowire diameters.

   

   Figure 2 Using a new technique of broadband enhanced backscattering, the transport mean free path of light in the nanowire medium was determined over a wide spectral range in the visible and infrared. The result, presented in the right panel, shows a strong variation of the mean free path from the weak to strong scattering regimes with characteristic oscillations corresponding to the guided modes of the wires.

1. Harvesting light using nanostructured surfaces.

   Researchers from the FOM Institute for Atomic and Molecular Physics (AMOLF), located at the High Tech Campus in Eindhoven, and Philips Research developed a method to drastically reduce the reflection of light at the interface between a high refractive index semiconductor and air. The researchers were inspired by the eyes of moths that are covered by tapered nanostructures. Thanks to these nanostructures, night moths are capable of seeing very well in the dark. The results of the research have recently been published in the prestigious journal Advanced Materials.
   
   Light travels as a straight beam in homogeneous media. The direction of the light beam changes when it arrives at an interface between two different media. A fraction of light is reflected into the first medium and the rest is refracted into the second medium. At large angles of incidence at an interface between air and a solid material such as a semiconductor, nearly 100 % of the light is reflected. Interfaces which reduce this reflection exist in nature. For example, the eyes of a moth are covered with tapered nanostructures, which increase the eye sight of the moth in the dark by allowing more light to enter the eyes.
   
   Inspired by these biostructures, researchers from AMOLF and Philips Research developed a method which drastically reduces the reflection between air and a semiconductor. This method consists of the growth of nanowires with different lengths or with the same length, but conically shaped. Using this method, a gradual change from air to semiconductor is achieved, which leads to an efficient coupling of light into the semiconductor and minimizes the reflection. These layers show a large reduction of the reflection over a broad range of colors and angles of incidence. The reduction of the reflection is of importance for different applications. A low reflection can not only increase the sensitivity of a light detector, it can also increase the efficiency of solar cells and LEDs. The results of this research are published in the prestigious journal Advanced Materials.

   

2008

1. FOM program Nanophotovoltaics granted with 2.1 Meuro from 2009 till 2013.

   The group Nanowire Photonics participates in this program with one PhD student and one Postdoc. The groups of Prof. dr. A. Polman (program coordinator) and Prof. dr. M. Bonn also participate in this program.
   
   Program description: Solar cells are still to expensive for large scale application as a consequence of the high material costs and low efficiency. New concepts for solar cells, which make use of nanotechonology, are investigated in this program. In these concepts surface plasmons, polaritons and quantum dots play an important role.

1. Semiconductor nanowire metamaterials for photonic applications.

   A novel class of optical metamaterials is presented consisting of high densities of aligned gallium phosphide (GaP) nanowires fabricated using metal-organic vapor phase-epitaxy. Starting from a gold island film as a catalyst for nanowire growth, a sequential combination of vapor–liquid–solid and lateral growth modes is employed to obtain a continuous tunability of the nanowire volume fraction from 7% to over 35%. By choosing different crystallographic orientations of the GaP substrate, metamaterials are designed with different nanowire orientations. The anisotropy of the nanowire building blocks results in strong optical birefringence. Polarization interferometry demonstrates a very large polarization extinction contrast of 4_103 combined with a sharp angular resonance which holds promise for optical sensing. Nanowire metamaterials may find applications in photonics, optoelectronics, non-linear and quantum optics, microfluidics, bio-, and gas sensing.
   You can read more in our recently published Advanced Functional Materials.

   

2. Optical Switching of the surface plasmon enhanced THz transmission through a subwavelength slit.

   In collaboration with the groups of dr. E. Hendry of the University of Exeter, Prof. F.J. García-Vidal of the Universidad Autonoma of Madrid and Prof. L. Martín-Moreno of the University of Zaragoza we have demonstrated and three-fold increase of the THz transmission through a single slit structured on a Si wafer with a corrugated surface when the semiconductor is photoexcited by a laser pulse. The increase of the transmission originates by the modification of the propagation length of SPPs on the semiconductor surface upon excitation.
   You can read more in our recently published Physical Review Letter.

   

3. Excitation of surface plasmons polaritons by semiconductor nanowires.

   By placing InP nanowires inside gold bullseye grating we have demonstrated the anisotropic excitation of surface plasmon polaritons SPPs by semiconductor nanowires. This is the first demonstration of the efficient generation of SPPs by nanowires. The polarized emission of nanowires leads to the excitation of SPPs along the direction of their long axis.
   You can read more in our recently published Optics Express.

   

4. 5-fold enhancement of the radiative decay rate of dye molecules close to nanoantennas.

   We have demonstrated a strong enhancement of the radiative decay rate of dye-molecules when these are on the proximity of dimer nanoantennas. This enhancement is caused by the excitation and emission of particle plasmon polaritons. As a consequence of this modification of the decay rate, we are able to enhance the quantum efficiency of dye molecules from 40% up to 60%.
   You can read more in our recently published Nano Letter.

   

5. Enhanced THz transmission through arrays of holes.

   We have investigated the transition between the transmission through a periodic array of wires and a periodic array subwavelength holes at THz frequencies, unraveling the role of surface waves on this transmission when the hole gets smaller.
   You can read more in our recently published Physical Review B (rapid communication).

   

2007

1. 2^nd Workshop of the Center for Nanophotonics, Retie (Belgium) 3-5 October 2007.

   The Center for Nanophotonics has organized its 2nd Workshop in Retie (Belgium). During this workshop all the members had to present and discuss novel and breakthrough ideas that could be carried out in the Center. There was also a poster session in which everybody could present their current work.
   View photos of this event.

   

2. Giant birefringence in artificial nanowire materials.

   Nanowires are nanostructures with a large optical anisotropy. An effective birefringent medium is made by ensembles of nanowires grown along certain directions. We have demonstrated giant birefringence in ensembles of vertically aligned GaP nanowires and shown how this birefringence can be tuned by varying the semiconductor filling fraction.
   You can read more in the AMOLF scientific highlight and in our recently published Applied Physics Letter.