A 3D Metamaterial

Posted on February 22, 2012 by Eva Rittweger

A 3D optical metamaterial can be made by replicating a block copolymer into gold.

Metamaterials have unusual optical properties arising from periodic nanostructures on a length scale much smaller than the wavelength of light. Such unexpected effects can be, for example, light focusing down to near atomic scales, or the cloaking of embedded objects toward invisibility. The required metal architectures with features on the order of 10 nm are extremely difficult to construct. Fabrication methods like focused ion beam lithography, direct laser writing, and atomic layer deposition provide design flexibility, but are limited with regard to accessible feature sizes as well as the scalability of samples.

Ulrich Steiner and co-workers (University of Cambridge, UK) present for the first time the fabrication of a truly 3D metamaterial using polymers that assemble themselves into repeating structures of the required size and symmetry. This organic template is then replicated into a gold network. For this purpose, block copolymers are unique as they can form complex 3D architectures with features on the nanometer length scale and provide scalability of the samples. The resulting material shows a reduced plasmon frequency and exhibits an orientation-dependent color change under linearly polarized light, as well as optical chirality across the visible region. The presented approach will pave the way for potential mass production of large-scale 3D optical metamaterials.

Ultrafast Polymerization Inhibition by Stimulated Emission Depletion for Three-dimensional Nanolithography

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Ultrafast Polymerization Inhibition by Stimulated Emission Depletion for Three-dimensional Nanolithography

Joachim Fischer and Martin Wegener

To identify the depletion mechanism in a stimulated-emission-depletion (STED) inspired photoresist composed of a ketocoumarin photoinitiator in pentaerythritol tetraacrylate, we perform lithography with pulsed excitation and tunable delayed depletion. A fast component can unambiguously be assigned to stimulated emission. Our results allow the systematical optimization of the conditions in next-generation STED direct-laser-writing optical lithography.

Self-Assembled Flexible Microlasers

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Self-Assembled Flexible Microlasers

Van Duong Ta, Rui Chen and Han Dong Sun

Hemispherical microresonators with tunable sizes are obtained based on the hydrophobic effect on distributed Bragg reflectors. Under optical excitation, whispering gallery mode lasing is observed from the dye-doped microresonators at room temperature. The results indicate the potential application of the flexible microresonators in photonic integrated circuits.

Certain Biominerals in Leaves Function as Light Scatterers

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Certain Biominerals in Leaves Function as Light Scatterers

Assaf Gal, Vlad Brumfeld, Steve Weiner, Lia Addadi and Dan Oron

Leaf minerals function as internal light scatterers inside leaves. They transfer light from the saturated upper tissue into the light deprived lower tissue. This eases the steep light gradient inside the leaf and improves photosynthetic efficiency on the tissue scale.

Homoepitaxial Growth of Single Crystal Diamond Membranes for Quantum Information Processing

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Homoepitaxial Growth of Single Crystal Diamond Membranes for Quantum Information Processing

Igor Aharonovich, Jonathan C. Lee, Andrew P. Magyar, Bob B. Buckley, Christopher G. Yale, David D. Awschalom and Evelyn L. Hu

Homoepitaxial growth of single crystal diamond membranes is demonstrated employing a microwave plasma chemical vapor deposition technique. The membranes possess excellent structural, optical, and spin properties, which make them suitable for fabrication of optical microcavities for applications in quantum information processing, photonics, spintronics, and sensing.

A Bright Future – for Optical Materials

Posted on January 31, 2012 by Eva Rittweger

Humankind has always been intrigued by light and optical phenomena. Today research in optics and photonics is a steadily growing field with more than several tens of thousands of papers published every year. Optical devices are natural tools in our everyday lives, and optical technologies are the foundation of a multi-million dollar business with many job opportunities. At the forefront of new discoveries in this exciting subject, various materials, from semiconductors to (noble) metals and novel carbon materials, are employed to make a significant contribution to progress for many applications in biomedical imaging, electronic devices, and laser technology, but also in fundamental research areas like quantum computing and precision measurements.

A lot of effort is put into traditional research areas like coatings, displays, light sources and detectors. Furthermore, nanofabrication and nanostructuring have enabled materials with features on a length scale below the wavelength of light. This opened up exploration of new types of materials with novel optical properties like negative refractive indices and new application areas such as optical cloaking and contact-free nanoprobes. Substantial progress has also been made in research where light is used to shape or manipulate materials, like new lithography techniques. Also, research focused on how light interacts and responds to a material or structure is progressing rapidly in realizing new plasmonic, magneto-optical or nonlinear responses as well as unique absorption, emission and transmission properties.

Advanced Materials has a twenty-year tradition of publishing research at the forefront of materials science, including the fabrication and study of novel optical materials and their applications. We are therefore delighted to announce a new section of Advanced Materials: Advanced Optical Materials. Advanced Optical Materials will be answering key questions in the active field of optical materials. It will publish papers of the highest standard that have undergone the same rigorous peer review evaluation that readers expect from Advanced Materials.

Advanced Optical Materials will contain short Communications, more detailed Full Papers, and comprehensive Reviews on breakthrough discoveries and fundamental research in photonics, plasmonics, metamaterials and more.

You can submit your papers now for consideration in Advanced Optical Materials. Visit Advanced Optical Materials: Call for Papers for more details. Articles in the section will be cited as normal Advanced Materials articles and will also be prominently featured on our websites.

As section editors, we will be supported by an Editorial Advisory Board. We thank the following outstanding scientists for their enthusiasm in agreeing to join the Board of Advanced Optical Materials:
•   Hatice Altug (Boston University)
•   Richard Averitt (Boston University)
•   Paul Braun (University of Illinois at Urbana-Champaign)
•   Mark Brongersma (Stanford University)
•   Timothy J. Bunning (Air Force Research Laboratory)
•   Harald Giessen (University of Stuttgart)
•   Peter Günter (ETH Zürich)
•   Holger Moench (Philips)
•   Zhi-Yuan Li (Institute of Physics Chinese Academy of Sciences)
•   David Lidzey (University of Sheffield)
•   Luis Liz-Marzán (Universidade de Vigo)
•   Cefe López (Instituto de Ciencia de Materiales de Madrid)
•    Albert Polman (FOM-Institute for Atomic and Molecular Physics)
•   Ullrich Steiner (University of Cambridge)
•   Jianfang Wang (Chinese University of Hong Kong)
•   Ralf Wehrspohn (Fraunhofer IWM)

We are looking forward to the development of this exciting section into a communication forum for high quality research regarding all aspects of light–matter interactions. Enjoy the content of this first section, and continue submitting your best manuscripts in this important field to Advanced Optical Materials.

Tim Adams and Eva Rittweger
(Section Editors)

Spatially Resolved Photodetection in Leaky Ferroelectric BiFeO3

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Spatially Resolved Photodetection in Leaky Ferroelectric BiFeO3

Won-Mo Lee, Ji Ho Sung, Kanghyun Chu, Xavier Moya, Donghun Lee, Cheol-Joo Kim, Neil D. Mathur, S.-W. Cheong, C.-H. Yang and Moon-Ho Jo

Potential gradients due to the spontaneous polarization of BiFeO₃ yield asymmetric and nonlinear photocarrier dynamics. Photocurrent direction is determined by local ferroelectric domain orientation, whereas magnitude is spectrally centered around charged domain walls that are associated with oxygen vacancy migration. Photodetection can be electrically controlled by manipulating ferroelectric domain configurations.

Perfectly spherical gold nanodroplets produced with smallest-ever nanojets

Posted on January 19, 2012 by Dave Flanagan

Similar to the way water backjets eject droplets of water on the surface of a pond, powerful laser pulses can locally melt gold nanostructures and produce gold nanojets, ejecting perfectly spherical gold nanodroplets.

KU Leuven researcher Ventsislav Valev and an international team of scientists have developed a new method for optical manipulation of matter at the nanoscale. Using ‘plasmonic hotspots’ – regions with electric current that heat up very locally – gold nanostructures can be melted and made to produce the smallest nanojets ever observed. The tiny gold nanodroplets formed in the nanojets are perfectly spherical, which makes them interesting for applications in medicine.

The ‘backjet’ phenomenon on which the method turns can be compared to a pebble being dropped into water. Tightly focused ultrafast laser pulses carry sufficient energy to locally melt the surface of a gold film. When a laser pulse of light hits the film, a nanoscale backjet – a nanojet – of molten gold surges upward.

As the name suggests, nanojets on the surface of a homogeneous gold film are incredibly small, their size being determined by the distribution of energy in the light pulse. This distribution of energy is in turn dependent on the wavelength of light. Initially, scientists anticipated that nanojets could not be significantly smaller than the wavelength of light. In this study however, Ventsislav Valev and his colleagues show that nanojets can in fact be made much smaller with the help of ‘plasmonic hotspots’.

Plasmonic hotspots are regions on the surface of metal nanostructures where light causes very strong oscillation of the electrons. Because electron oscillations constitute an electric current and because electric currents heat up the material the same way an electric stove heats up in the kitchen, the plasmonic hotspots are extremely hot. So hot that they can melt the gold in a spot much smaller than the wavelength of light. Dr. Valev and his colleagues were successfully able to demonstrate that this tiny little pool of molten gold can give rise to the smallest nanojets ever observed.

The gold nanodroplets propelled upward by the nanojets solidify in flight, producing perfectly spherical nanoparticles. These gold nanodroplets can be collected and used for medical applications including cancer treatment. The nanoparticles can be attached to molecules and injected in the blood. Once the molecules attach to cancer cells, light can be used to heat up the gold nanodroplets and destroy the cancer cells. Currently, the gold nanoparticles used in medications are chemically synthesised. These chemically synthesised gold nanoparticles have an unavoidably granular aspect. Conversely, gold nanodroplets created by the plasmonic nanojet method detailed by Dr. Valev and his colleagues are perfectly spherical, increasing efficiency.

The research is published in Advanced Optical Materials, a new section in Advanced Materials (2010 Impact Factor: 10.880) dedicated to breakthrough discoveries and fundamental research in photonics, plasmonics, metamaterials, and more. Advanced Optical Materials will cover all aspects of light-matter interactions. It will include communications, full papers, and reviews. Look for the first edition in Spring 2012.

Giant optical gain in rare-earth-ion-doped fiber amplifiers

Posted on January 12, 2012 by Dave Flanagan

Giant optical gain in rare-earth-ion-doped amplifiers

Scanning electron micrograph of a typical waveguide amplifier structure with an overlay of the guided mode profile.

Prof. Markus Pollnau and co-workers at the MESA+ Institute for Nanotechnology at the University of Twente have developed a rare-earth-ion-doped optical amplifier with performance comparable to semiconductor amplifiers.

Signal amplification

Amplification of optical signals is critical in photonics applications. Semiconductor optical waveguide amplifiers have high gain per unit length (~1000 dB/cm), but suffer from spatial and temporal gain pattering effects.

In comparison, fiber amplifiers doped with trivalent rare-earth ions like Er³⁺ combine good overall gain with low noise and negligible non-linearities. However, this comes at the cost of having to use several meters of fiber length, making them unsuitable for on-chip applications.

By engineering the host material, dopant concentration, and geometry the MESA+ scientists were able to increase the modal gain per unit length of rare-earth-ion-doped waveguide amplifiers to ~1000 dB/cm.

As good as semiconductor amplifiers

“Our highest measured gain of 935 dB/cm is two orders of magnitude higher than previously demonstrated in any rare-earth-ion-doped amplifier and similar to the best results reported for semiconductor amplifiers,” says Dimitri Geskus, lead author on the paper.

The approach uses the family of monoclinic potassium double tungstates KY(WO₄)₂, KGd(WO₄)₂, and KLu(WO₄)₂. Yb³⁺ ions doped into these materials possess some of the highest transition cross-sections observed in dielectric materials.

Besides their applicability as on-chip amplifiers for high-bit-rate data transmission at signal wavelengths around 1 μm, these new rare-earth-ion-doped amplifiers may be used to provide optical gain in nanophotonic devices, such as nanoamplifiers and nanolasers, and may enable lossless propagation in plasmonic nanostructures.

The research is reported in the first issue of Advanced Optical Materials, a new section in Advanced Materials (2010 IF: 10.880) dedicated to exploring light-matter interactions.

Plasmon-Enhanced Sub-Wavelength Laser Ablation: Plasmonic Nanojets

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Plasmo n-Enhanced Sub-Wavelength Laser Ablation: Plasmonic Nanojets

Ventsislav K. Valev, Denitza Denkova, Xuezhi Zheng, Arseniy I. Kuznetsov,
Carsten Reinhardt, Boris N. Chichkov, Gichka Tsutsumanova, Edward J. Osley,
Veselin Petkov, Ben De Clercq, Alejandro V. Silhanek, Yogesh Jeyaram, Vladimir Volskiy, Paul A. Warburton, Guy A. E. Vandenbosch, Stoyan Russev, Oleg A. Aktsipetrov, Marcel Ameloot, Victor V. Moshchalkov, and Thierry Verbiest

In response to the incident light’s electric field, the electron density oscillates in the plasmonic hotspots producing an electric current. Associated Ohmic losses raise the temperature of the material within the plasmonic hotspot above the melting point. A nanojet and nonosphere ejection can then be observed precisely from the plasmonic hotspots.