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24 May 2011 - Shine on crazy diamond
Shine on crazy diamond
It has been predicted that the propagation of light can be radically controlled with nanostructures for which certain colors of light are rigorously forbidden to exist. It is an outstanding challenge to demonstrate that light is reflected, no matter from what direction light is incident.
A team of scientists from the University of Twente and the FOM institute for Atomic and Molecular Physics in the Netherlands has observed that many colors of light are always reflected, independent of the position and orientation of the nanostructure. The structures consist of composites of the well-known semiconductor silicon and of air in such a way that the materials are arranged similar to the atoms in diamond crystals, yet ten thousand times magnified.
The results are published online on 23 May in the leading American journal Physical Review B, and are expected to lead to tiny on-chip light sources and lasers, efficient solar cells, invisibility cloaks, trapped light in extremely small volumes, and information-processing with light.
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Figure 1: A diamond-like photonic crystal nanostructure that is illuminated by white light (red arrow). Some colors of the light are strongly reflected (green arrows), indicating that these colors are radically forbidden to exist inside the structure. In this example it would be the color green. |
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Background:
Three-dimensional photonic crystals have attracted considerable attention due to their ability to radically control propagation and emission of light. Photonic crystals are periodically ordered non-absorbing materials that have a periodicity of the order of the wavelength of light. As a result of the periodic order, frequency ranges emerge in which light is forbidden to propagate in particular directions due to Bragg diffraction. In specific 3D crystals, a common frequency range is formed for which light is not allowed to propagate in any direction, called the photonic band gap. The photonic band gap has been predicted in 1987 by American scientist Yablonovitch and is expected to result into integrated light sources and lasers, efficient solar cells, invisibility cloaks, trapped light in extremely small volumes, and information-processing with light. It is an outstanding challenge to create photonic band gap crystals and experimentally demonstrate the forbidden frequency range for light.
The researchers have made an important step by demonstrating a clear experimental signature of the presence of the photonic band gap. They have fabricated high-quality photonic crystals in silicon using semiconductor industry (CMOS) compatible methods, which make them suitable for on-chip integration. The crystals are inspired by diamonds. Diamonds radically modify the properties of electric currents. The researchers have used crystals that have similar properties on the flow of light. These crystals are expected to have a broad photonic band gap at telecom wavelengths. Fig. 2(a) shows an electron microscope image of one of the crystals. The crystal is formed by two sets of crossing pores drilled in silicon, forming a 3D structure.
The presence of the band gap was studied by focusing a novel white light laser beam on the crystal. Light that cannot enter the crystals is reflected, indicating that these colors cannot enter the crystals. Fig. 2(b) shows an example of an obtained reflectivity spectrum. Light was focused on the crystals along multiple orientations for different polarizations and positions on the crystals. The researchers observe that light at specific colors is always reflected in every measurement. This reveals that these colors are forbidden inside the crystals, which is the signature of the photonic band gap.
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Figure 2: (a) Electron microscope image of a 3D inverse woodpile photonic crystal fabricated in silicon. The purple dashed line indicates a part of the boundary of the inverse woodpile crystal. (b) Example of the measured reflectivity spectrum (black) for an inverse woodpile photonic crystal. The reflectivity peak between 5300 and 7100 cm-1 indicates that light cannot enter the crystal for these colors. When the reflectivity spectra obtained along multiple directions are combined, the researches observe that light is always strongly reflected between 5900 and 6900 cm-1. This is the signature of a photonic band gap, marked by the yellow area. The dashed lines indicate the boundary of the predicted band gap. |
The team:
The research has been performed by Simon Huisman M.Sc., Dr. Rajesh Nair, Dr. Léon Woldering, Dr. Merel Leistikow, Dr. Allard Mosk and Prof. Dr. Willem Vos from the Complex Photonic Systems (COPS) Chair, MESA+ Institute for Nanotechnology, University of Twente, The Netherlands. Dr. Leistikow was also affiliated with the FOM Institute AMOLF in Amsterdam, the Netherlands.
Further information can be obtained from:
Simon Huisman M.Sc., University of Twente, Enschede, The Netherlands, email: s.r.huisman@utwente.nl phone: +31-53-489-5448.
Prof. Dr. Willem Vos, University of Twente, Enschede, The Netherlands, email: w.l.vos@utwente.nl phone: +31-53-489-5388.
Information on the world wide web:
The paper is entitled “Signature of a three-dimensional photonic band gap observed on silicon inverse woodpile photonic crystals” and is being published in Physical Review B on 23 May (Phys. Rev. B 83, 205313 (2011)).
A preprint of the paper is available at www.photonicbandgaps.com