LMU/MPQ-scientists can image the optical properties of individual nanoparticles with a novel microscope.
Nanomaterials play an essential role in many areas of daily life. There is thus a large interest to gain detailed knowledge about their optical and electronic properties. Conventional microscopes get beyond their limits when particle size falls to the range of a few ten nanometers where a single particle provides only a vanishingly small signal.
As a consequence, many investigations are limited to large ensembles of particles. Now, a team of scientists of the Laser Spectroscopy Division of Prof. Theodor W. Hänsch (Director at the Max Planck Institute of Quantum Optics and Chair for Experimental Physics at the Ludwig-Maximilians-Universität Munich) has developed a technique, where an optical microcavity is used to enhance the signals by more than 1000-fold and at the same time achieves an optical resolution close to the fundamental diffraction limit.
The possibility to study the optical properties of individual nanoparticles or macromolecules promises intriguing potential for many areas of biology, chemistry, and nanoscience (Nature Communications, DOI: 10.1038/ncomms8249, 24 June 2015).
Spectroscopic measurements on large ensembles of nanoparticles suffer from the fact that individual differences in size, shape, and molecular composition are washed out and only average quantities can be extracted. There is thus a large interest to develop single-particle-sensitive techniques. “Our approach is to trap the probe light used for imaging inside of an optical resonator, where it circulates tens of thousands of times. This enhances the interaction between the light and the sample, and the signal becomes easily measurable”, explains Dr. David Hunger, one of the scientists working on the experiment. “For an ordinary microscope, the signal would be only a millionth of the input power, which is hardly measurable. Because of the resonator, the signal gets enhanced by a factor of 50000.”
In the microscope, built by Dr. David Hunger and his team, one side of the resonator is made of a plane mirror that serves at the same time as a carrier for the nanoparticles under investigation. The counterpart is a strongly curved mirror on the end facet of an optical fibre. Laser light is coupled into the resonator through this fibre. The plane mirror is moved point by point with respect to the fibre in order to bring the particle step by step into its focus. At the same time, the distance between both mirrors is adjusted such that the condition for the appearance of resonance modes is fulfilled. This requires an accuracy in the range of picometers.
For their first measurements, the scientists used gold spheres with a diameter of 40 nanometers. “The gold particles serve as our reference system, as we can calculate their properties precisely and therefore check the validity of our measurements” says David Hunger. “Since we know the optical properties of our measurement apparatus very accurately, we can determine the optical properties of the particles from the transmission signal quantitatively and compare it to the calculation”. In contrast to other methods relying on direct signal enhancement, the light field is limited to a very small area, such that by using only the fundamental mode, a spatial resolution of 2 micron is achieved. By combining higher order modes, the scientists could even increase the resolution to around 800 nanometers.
The method becomes even more powerful when both absorptive and dispersive properties of a single particle were determined at the same time. This is interesting especially if the particles are not spherical but e.g. elongated. Then, the corresponding quantities depend on the orientation of the polarization of light with respect to the symmetry axes of the particle. “In our experiment we use gold nanorods (34x25x25 nm to the 3) and we observe how the resonance frequency shifts depending on the orientation of the polarization. If the polarization is oriented parallel to the axes of the rod, the shift of the resonance is larger than if the polarization is oriented orthogonally, resulting in two different resonance frequencies for both orthogonal polarizations” explains Matthias Mader, PhD student at the experiment. “This birefringence can be measured very precisely and is a very sensitive indicator for the shape and orientation of the particle.”
“As an application of our method, we could think of e.g. investigating the temporal dynamics of macro molecules, such as the folding dynamics of proteins” says David Hunger. “Overall we see a large potential for our method: from the characterization of nanomaterials and biological nanosystems to spectroscopy of quantum emitters.” [OM/DH]
Matthias Mader, Jakob Reichel, Theodor W. Hänsch, and David Hunger
A Scanning Cavity Microscope
Nature Communications, DOI: 10.1038/ncomms8249, 24 June 2015
Dr. David Hunger
Max Planck Institute of Quantum Optics,
Schellingstr. 4 /III, 80799 Munich, Germany
Phone: +49 (0)89 / 21 80 -3937
Prof. Dr. Theodor W. Hänsch
Professor of Experimental Physics
Direktor at the Max Planck Institute of Quantum Optics
85748 Garching, Germany
Phone: +49 (0)89 / 32 905 -712
Dr. Olivia Meyer-Streng
Press & Public Relations
Max Planck Institute of Quantum Optics, Garching, Germany
Phone: +49 (0)89 / 32 905 -213
Dr. Olivia Meyer-Streng | Max-Planck-Institut für Quantenoptik
Molecule flash mob
19.01.2017 | Technische Universität Wien
Magnetic moment of a single antiproton determined with greatest precision ever
19.01.2017 | Johannes Gutenberg-Universität Mainz
An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by...
Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...
Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.
While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...
Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales
Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
19.01.2017 | Event News
10.01.2017 | Event News
09.01.2017 | Event News
19.01.2017 | Earth Sciences
19.01.2017 | Life Sciences
19.01.2017 | Physics and Astronomy