New optical microscope makes images with nanoscale resolution
Using a newly developed microscope, researchers at the FOM Institute AMOLF, King's College (London) and ICFO (Barcelona) have succeeded in imaging light at the true nanoscale. Normally the resolution of an optical microscope is limited by Abbe's law of diffraction, but the researchers were able to break this limit using a new technique 'angle-dependent cathodoluminescence imaging spectroscopy', which was developed and refined at AMOLF. The researchers used this technique to probe the inside of a photonic crystal and study the confinement of light with a spatial resolution more than ten times smaller than the diffraction limit for light. The research was published 19 August on the website of Nature Materials.
Photonic crystals are nanostructures in which two materials with different refractive index are arranged in a regular pattern, giving rise to exotic optical properties. Natural photonic crystals can be found in certain species of butterflies, birds and beetles as well as in opal gemstones where they give rise to beautiful iridescent colors. Due to major advances in nanofabrication techniques it has become possible to fabricate artificial photonic crystals with optical properties that can be accurately engineered. These structures can be used to make high-quality nanoscale optical waveguides and cavities, which are important in telecommunication and sensing applications.
Photonic crystal cavities
The researchers constructed a two-dimensional photonic crystal by etching a hexagonal pattern of holes in a very thin silicon nitride membrane. The photonic crystal inhibits light propagation for certain colors of light, which leads to strong reflection of those colors. By leaving out one hole a very small cavity can be defined where the surrounding crystal acts as a mirror for the light, making it possible to strongly confine light within such a 'crystal defect cavity'.
These resonant cavities are so small that it is impossible to resolve all the important features of the confined light using a conventional optical microscope. The researchers therefore used a five-nanometer-wide electron beam from a scanning electron microscope to study the photonic crystal. The electron beam generates light inside the cavity. A small fraction of the light escapes and is collected by an optical collection system, which consists of a parabolic mirror aligned with a piezoelectric stage that is mounted inside the electron microscope. Subsequently the color of the light and the angle at which it is emitted is accurately determined using CCD cameras.
Unprecedented level of detail
The emitted light carries the optical signature of the cavity and was used by the researchers to study its properties with an unprecedented level of detail, resolving features down to 30 nanometers. In the future, this experimental approach can be used to investigate nanoscale optical properties in communication technology, lighting technology, photovoltaics and in several other research fields, including materials science, biology and medical sciences.
Prof.dr. Albert Polman, the principal investigator of the project: "In the past few years we have worked hard with several technicians and researchers to develop and refine this new instrument. Due to the unique expertise in the technical support divisions at AMOLF we managed to get this done. In the Nature Materials paper, we demonstrate for the first time an instrument resolution of 30 nanometers, but I think in the future 10 nanometers is achievable".
Start-up
The angle-resolved cathodoluminescence imaging spectroscopy instrument is being commercialized by the start-up company Delmic, and will appear on the market in the fall of 2012. This development has been made possible by a Valorization Grant from Technology Foundation STW and the FOM Foundation.
The research was carried out by Riccardo Sapienza, Toon Coenen, Jan Renger, Martin Kuttge, Niek van Hulst, and Albert Polman and was made possible by funding from the FOM Foundation, NanoNextNL and the European Research Council.
Reference
Deep-subwavelength imaging of the modal dispersion of light
R. Sapienza, T. Coenen, J. Renger, M. Kuttge, N.F. van Hulst, and A. Polman
Nature Materials DOI: 10.1038/NMAT3402
Further information
Albert Polman +31 (0)20 754 74 02
www.erbium.nl/arcis.html