Invisibility cloak: Scientists achieve optical invisibility in visible light range of spectrum

ScienceDaily (May 18, 2011) — "Seeing something invisible with your own eyes is an exciting experience," say Joachim Fischer and Tolga Ergin. For about one year, both physicists and members of the team of Professor Martin Wegener at KIT's Center for Functional Nanostructures (CFN) have worked on refining the structure of the Karlsruhe invisibility cloak to such an extent that it is also effective in the visible spectral range.

In invisibility cloaks, light waves are guided by the material such that they leave the invisibility cloak again as if they had never been in contact with the object to be disguised. Consequently, the object is invisible to the observer. The exotic optical properties of the camouflaging material are calculated using complex mathematical tools.

These properties result from a special structuring of the material. It has to be smaller than the wavelength of the light that is to be deflected. For example, the relatively large radio or radar waves require a material "that can be produced using nail scissors," says Wegener. At wavelengths visible to the human eye, materials have to be structured in the nanometer range.

The minute invisibility cloak produced by Fischer and Ergin is smaller than the diameter of a human hair. It makes the curvature of a metal mirror appear flat, as a result of which an object hidden underneath becomes invisible. The metamaterial placed on top of this curvature looks like a stack of wood, but consists of plastic and air. These "logs" have precisely defined thicknesses in the range of 100 nm. Light waves that are normally deflected by the curvature are influenced and guided by these logs such that the reflected light corresponds to that of a flat mirror.

"If we would succeed again in halving the log distance of the invisibility cloak, we would obtain cloaking for the complete visible light spectrum," says Fischer.

Last year, the Wegener team presented the first 3-D invisibility cloak in the journal Science. Until that time, the only invisibility cloaks existed in waveguides and were of practically two-dimensional character. When looking onto the structure from the third dimension, however, the effect disappeared. By means of an accordingly filigree structuring, the Karlsruhe invisibility cloak could be produced for wavelengths from 1500 to 2600 nm. This wavelength range is not visible to the human eye, but plays an important role in telecommunications. The breakthrough was based on the use of the direct laser writing method (DLS) developed by CFN. With the help of this method, it is possible to produce minute 3-D structures with optical properties that do not exist in nature, so-called metamaterials.

In the past year, the KIT scientists continued to improve the already extremely fine direct laser writing method. For this purpose, they used methods that have significantly increased the resolution in microscopy. With this tool, they then succeeded in refining the metamaterial by a factor of two and in producing the first 3-D invisibility cloak for non-polarized visible light in the range of 700 nm. This corresponds to the red color.

"The invisibility cloak now developed is an attractive object demonstrating the fantastic possibilities of the rather new field of transformation optics and metamaterials. The design options that opened up during the last years had not been deemed possible before," emphasizes Ergin. "We expect dramatic improvements of light-based technologies, such as lenses, solar cells, microscopes, objectives, chip production, and data communication."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Karlsruhe Institute of Technology.

Journal Reference:

J. Fischer, T. Ergin, M. Wegener. Three-dimensional polarization-independent visible-frequency carpet invisibility cloak. Optics Letters, 2011; (in press)

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Scientists achieve guiding of electrons by purely electric fields

ScienceDaily (May 10, 2011) — The investigation of the properties of electrons plays a key role for the understanding of the fundamental laws of nature. However, being extremely small and quick, electrons are difficult to control. Physicists around Dr. Peter Hommelhoff, head of the Max Planck Research Group "Ultrafast Quantum Optics" at the Max Planck Institute of Quantum Optics (Garching near Munich), have now demonstrated efficient guiding of slow electrons by applying a microwave voltage to electrodes fabricated on a planar substrate.

The research is published online in Physical Review Letters.

This new technique of electron guiding -- which resembles the guiding of light waves in optical fibres -- promises a variety of applications, from guided matter-wave experiments to non-invasive electron microscopy.

Electrons have been the first elementary particles revealing their wave-like properties and have therefore been of great importance in the development of the theory of quantum mechanics. Even now the observation of electrons leads to new insight into the fundamental laws of physics. Measurements involving confined electrons have so far mainly been performed in so-called Penning traps, which combine a static magnetic field with an oscillating electric field.

For a number of experiments with propagating electrons, like interferometry with slow electrons, it would be advantageous to confine the electrons by a purely electric field. This can be done in an alternating quadrupole potential similar to the standard technique that is used for ion trapping. These so-called Paul traps are based on four electrodes to which a radiofrequency voltage is applied. The resulting field evokes a driving force which keeps the particle in the centre of the trap. Wolfgang Paul received the Nobel Prize in physics for the invention of these traps in 1989.

For several years by now scientists realize Paul traps with micro structured electrodes on planar substrates, using standard microelectronic chip technology. Dr. Hommelhoff and his group have now applied this method for the first time to electrons. Since the mass of these point-like particles is only a tenth of a thousandth of the mass of an ion, electrons react much faster to electric fields than the rather heavy ions. Hence, in order to guide electrons, the frequency of the alternating voltage applied to the electrodes has to be much higher than for the confinement of ions and is in the microwave range, at around 1 GHz.

In the experiment electrons are generated in a thermal source (in which a tungsten wire is heated like in a light bulb) and the emitted electrons are collimated to a parallel beam of a few electron volts. From there the electrons are injected into the "wave-guide." It is being generated by five electrodes on a planar substrate to which an alternating voltage with a frequency of about 1 GHz is applied. This introduces an oscillating quadrupole field in a distance of half a millimetre above the electrodes, which confines the electrons in the radial direction. In the longitudinal direction there is no force acting on the particles so that they are free to travel along the "guide tube." As the confinement in the radial direction is very strong the electrons are forced to follow even small directional changes of the electrodes.

In order to make this effect more visible the 37mm long electrodes are bent to a curve of 30 degrees opening angle and with a bending radius of 40mm. At the end of the structure the guided electrons are ejected and registered by a detector. A bright spot caused by guided electrons appears on the detector right at the exit of the guide tube, which is situated in the left part of the picture. When the alternating field is switched off a more diffusively illuminated area shows up on the right side. It is caused by electrons spreading out from the source and propagating on straight trajectories over the substrate.

"With this fundamental experiment we were able to show that electrons can be efficiently guided be purely electric fields," says Dr. Hommelhoff. "However, as our electron source yields a rather poorly collimated electron beam we still lose many electrons." In the future the researchers plan to combine the new microwave guide with an electron source based on field emission from an atomically sharp metal tip. These devices deliver electron beams with such a strong collimation that their transverse component is limited by the Heisenberg uncertainty principle only.

Under these conditions it should be feasible to investigate the individual quantum mechanical oscillations of the electrons in the radial potential of the guide. "The strong confinement of electrons observed in our experiment means that a "jump" from one quantum state to the neighbouring higher state requires a lot of energy and is therefore not very likely to happen," explains Johannes Hoffrogge, doctoral student at the experiment. "Once a single quantum state is populated it will remain so for an extended period of time and can be used for quantum experiments." This would make it possible to conduct quantum physics experiments such as interferometry with guided slow electrons. Here the wave function of an electron is first split up; later on, its two components are brought together again whereby characteristic superpositions of quantum states of the electron can be generated. But the new method could also be applied for a new form of electron microscopy.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Max Planck Institute of Quantum Optics.

Journal Reference:

J. Hoffrogge, R. Fröhlich, M. Kasevich, P. Hommelhoff. Microwave Guiding of Electrons on a Chip. Physical Review Letters, 2011; 106 (19) DOI: 10.1103/PhysRevLett.106.193001

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Scientists achieve guiding of electrons by purely electric fields

ScienceDaily (May 10, 2011) — The investigation of the properties of electrons plays a key role for the understanding of the fundamental laws of nature. However, being extremely small and quick, electrons are difficult to control. Physicists around Dr. Peter Hommelhoff, head of the Max Planck Research Group "Ultrafast Quantum Optics" at the Max Planck Institute of Quantum Optics (Garching near Munich), have now demonstrated efficient guiding of slow electrons by applying a microwave voltage to electrodes fabricated on a planar substrate.

The research is published online in Physical Review Letters.

This new technique of electron guiding -- which resembles the guiding of light waves in optical fibres -- promises a variety of applications, from guided matter-wave experiments to non-invasive electron microscopy.

Electrons have been the first elementary particles revealing their wave-like properties and have therefore been of great importance in the development of the theory of quantum mechanics. Even now the observation of electrons leads to new insight into the fundamental laws of physics. Measurements involving confined electrons have so far mainly been performed in so-called Penning traps, which combine a static magnetic field with an oscillating electric field.

For a number of experiments with propagating electrons, like interferometry with slow electrons, it would be advantageous to confine the electrons by a purely electric field. This can be done in an alternating quadrupole potential similar to the standard technique that is used for ion trapping. These so-called Paul traps are based on four electrodes to which a radiofrequency voltage is applied. The resulting field evokes a driving force which keeps the particle in the centre of the trap. Wolfgang Paul received the Nobel Prize in physics for the invention of these traps in 1989.

For several years by now scientists realize Paul traps with micro structured electrodes on planar substrates, using standard microelectronic chip technology. Dr. Hommelhoff and his group have now applied this method for the first time to electrons. Since the mass of these point-like particles is only a tenth of a thousandth of the mass of an ion, electrons react much faster to electric fields than the rather heavy ions. Hence, in order to guide electrons, the frequency of the alternating voltage applied to the electrodes has to be much higher than for the confinement of ions and is in the microwave range, at around 1 GHz.

In the experiment electrons are generated in a thermal source (in which a tungsten wire is heated like in a light bulb) and the emitted electrons are collimated to a parallel beam of a few electron volts. From there the electrons are injected into the "wave-guide." It is being generated by five electrodes on a planar substrate to which an alternating voltage with a frequency of about 1 GHz is applied. This introduces an oscillating quadrupole field in a distance of half a millimetre above the electrodes, which confines the electrons in the radial direction. In the longitudinal direction there is no force acting on the particles so that they are free to travel along the "guide tube." As the confinement in the radial direction is very strong the electrons are forced to follow even small directional changes of the electrodes.

In order to make this effect more visible the 37mm long electrodes are bent to a curve of 30 degrees opening angle and with a bending radius of 40mm. At the end of the structure the guided electrons are ejected and registered by a detector. A bright spot caused by guided electrons appears on the detector right at the exit of the guide tube, which is situated in the left part of the picture. When the alternating field is switched off a more diffusively illuminated area shows up on the right side. It is caused by electrons spreading out from the source and propagating on straight trajectories over the substrate.

"With this fundamental experiment we were able to show that electrons can be efficiently guided be purely electric fields," says Dr. Hommelhoff. "However, as our electron source yields a rather poorly collimated electron beam we still lose many electrons." In the future the researchers plan to combine the new microwave guide with an electron source based on field emission from an atomically sharp metal tip. These devices deliver electron beams with such a strong collimation that their transverse component is limited by the Heisenberg uncertainty principle only.

Under these conditions it should be feasible to investigate the individual quantum mechanical oscillations of the electrons in the radial potential of the guide. "The strong confinement of electrons observed in our experiment means that a "jump" from one quantum state to the neighbouring higher state requires a lot of energy and is therefore not very likely to happen," explains Johannes Hoffrogge, doctoral student at the experiment. "Once a single quantum state is populated it will remain so for an extended period of time and can be used for quantum experiments." This would make it possible to conduct quantum physics experiments such as interferometry with guided slow electrons. Here the wave function of an electron is first split up; later on, its two components are brought together again whereby characteristic superpositions of quantum states of the electron can be generated. But the new method could also be applied for a new form of electron microscopy.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Max Planck Institute of Quantum Optics.

Journal Reference:

J. Hoffrogge, R. Fröhlich, M. Kasevich, P. Hommelhoff. Microwave Guiding of Electrons on a Chip. Physical Review Letters, 2011; 106 (19) DOI: 10.1103/PhysRevLett.106.193001

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here