## Monday, March 10, 2014

### Observed live with x-ray laser: electricity controls magnetism

http://www.chemeurope.com/en/news/147248/observed-live-with-x-ray-laser-electricity-controls-magnetism.html
0-03-2014: Data on a hard drive is stored by flipping small magnetic domains. Researchers from the Paul Scherrer Institute PSI and ETH Zurich have now changed the magnetic arrangement in a material much faster than is possible with today’s hard drives. The researchers used a new technique where an electric field triggers these changes, in contrast to the magnetic fields commonly used in consumer devices. This method uses a new kind of material where the magnetic and electric properties are coupled. Applied in future devices, this kind of strong interaction between magnetic and electric properties can have numerous advantages. For instance, an electrical field can be generated more easily in a device than a magnetic one. In the experiment, the changes in magnetic arrangement took place within a picosecond (a trillionth of a second) and could be observed with x-ray flashes at the American x-ray laser LCLS. The flashes are so short that you can virtually see how the magnetisation changes from one image to the next – similar to how we are able to capture the movement of an athlete with a normal camera in a series of images with a short exposure time. In future, such experiments should also be possible at PSI’s new research facility, the x-ray laser SwissFEL. The results will be published in the journal Science. They appear online in advance of print in Science Express on 6 March.
One common method of data storage uses materials in which different magnetic domains can be oriented in different directions. In other words, the tiny elementary magnets inside the material are aligned along two possible directions, which enables one bit to be saved in the material. A bit is the smallest unit of information, for which there are two possibilities, often referred to as 0 and 1. In the storage device, these correspond to the two different magnetic directions. In a real hard drive, which must store a large amount of information, there are many small areas that correspond to single bits. To change the information on the hard drive, the direction of the magnetism in one domain must be flipped. In modern consumer devices this is achieved using a small magnetic field.
An electric field can be generated in a small space more easily than a magnetic field, which means that, in principle, smaller storage devices can be constructed if magnetism is switched by electric fields. A strong connection between magnetic and electric properties is exhibited by so-called multiferroic materials, which have been one of the hottest topics in materials research for a number of years. Researchers from the Paul Scherrer Institute PSI and ETH Zurich have now studied the material TbMnO3 and demonstrated that its magnetic arrangement can be changed by an electric field in a matter of picoseconds (10-12 s = one trillionth of a second), which is considerably shorter than the time it takes for today’s hard drives to be switched. “This shows that multiferroic materials can be switched quickly enough electrically for them to be used in magnetic storage devices,” explains Urs Staub, a research group leader at PSI and one of the research project supervisors. “Electric switching could have numerous advantages. In order to generate a magnetic field, you need a coil through which a current flows. An electric field can be generated without current.
“The material we studied can’t be used in technical devices – you need very low temperatures and strong electrical fields to observe the relevant phenomena. However, the basic result probably also applies for materials that are more suitable for applications and will presumably consist of a combination of thin layers of different materials.”

### Exposure time: 0.000 000 000 000 1 seconds

The experiment is based on the interaction between pulsed light produced by two lasers – terahertz light generated by a laser which can easily fit into a lab, and the radiation from the x-ray laser LCLS, a large-scale research facility at SLAC National Accelerator Laboratory in Menlo Park, California, that is roughly three kilometres in length. In the experiment, the material was illuminated with short flashes of terahertz-frequency light which were only a few picoseconds long. Light consists of an electric and a magnetic field, which periodically become stronger and weaker. The terahertz flashes were so short that the electric fields in them were only able to perform a few oscillations. With experiments at the LCLS, the researchers were able to demonstrate that the magnetic arrangement was distorted by the flash of light and – with a slight delay – this distortion followed the oscillation of the electrical field within the flash. The magnetic component of the light was too weak to influence the magnetic structure. The x-ray laser generates very short (100 femtoseconds = 0.000 000 000 000 1 seconds) and intense flashes of x-ray light which are so much shorter than the terahertz flash. This allows the x-rays to measure the magnetic distortion along the different stages of its motion, similar to how a camera with a fast shutter speed captures still images of rapid motions. Today, the LCLS is one of two facilities where such experiments are possible. In the future, they will also be possible at the x-ray laser SwissFEL, which is currently under construction at the Paul Scherrer Institute. “An experiment like this can only be conducted at an x-ray laser because only the pulses from the x-ray laser show the magnetic order and are short enough for you to follow the chronological sequences,” explains Staub.

### Tilted elementary magnets

Magnetic materials which can be used to store data can have different magnetic arrangements. In today’s hard drives, the magnetic areas are arranged ferromagnetically, which means that the elementary magnets or, to use the technical term, magnetic moments are all pointing in same direction within the area encoding one bit. In the material studied in the experiment, the moments are arranged in rows but in such a way that two neighbouring moments are slightly rotated with respect to each other as opposed to being parallel. If you move from one moment to the next, the direction of the moments keeps turning and overall the sequence of magnetic moments forms a cycloid. Generally speaking, there are two directions in which the moments can turn, clockwise and anticlockwise – and these could correspond to the two values of a bit. To change between “0” and “1”, the magnetic moments would have to change the turning direction within the sequence, which is equivalent to rotating the entire sequence of magnetic moments by 180 degrees.

### Positive and negative – offset from each other

The multiferroic material also has another property: electric polarisation, which means that the positive and negative charges are shifted slightly against each other. The interior of the material is constructed from atoms that have fixed positions in a three-dimensional structure. As there are just as many negative charges (electrons) as positive ones (atomic nuclei) in the atoms, the entire material is electrically neutral. Some of the electrons, however, are not bound rigidly to the atomic nuclei. These electrons can be displaced with respect to the atomic nuclei, which means that one side of the material is positively charged, the other negatively. In other words, the material is electrically polarised. In everyday life, electrically polarised materials are primarily known thanks to the piezoelectric effect used to produce sparks in lighters or sound in loudspeakers, for instance.

In TbMnO3, the electrical polarisation is linked to the magnetic arrangement, which means that if the magnetic moments turn in one direction, this always corresponds to an alignment of the electric polarisation; if you reverse the polarisation, the rotational direction of the magnetic moments also turns around. The researchers studied this coupling in their experiment. Using the alternating electric field of the terahertz pulse, they influenced the electric polarisation and observed the extent to which the magnetic arrangement followed the alternating field. Although the electric field was too weak to actually turn the sequence of magnetic moments by 180 degrees, the scientists were able to observe that it was turned by around four degrees in time with the electrical field. “This procedure is also important for possible applications,” explains Teresa Kubacka, a doctoral student in the Ultrafast Dynamics Group at ETH Zurich and first author of the paper. “The terahertz pulse is designed in such a way that it influences the magnetic arrangement only in this particular way. If the magnetic arrangement in a device could be changed so specifically, much less energy would be wasted and the material would not heat up as much.”

### Precision measuring

It is the first time that it was possible to measure such a rapid change in a multiferroic material so precisely. The angle by which the magnetic moments were turned, was determined using the short flashes from the LCLS x-ray laser in a scattering experiment. It involved sending the x-ray beam through the sample studied and observing the directions in which the x-ray light was deflected by the sample. In the case of this material, there are directions in which the light is deflected by the atomic structure and others where the deflection is caused by the magnetic moments. If the magnetic arrangement is changed, the intensity of the deflected x-ray light changes. In the experiment, the researchers measured the intensity of the deflected x-ray beam at different times for a selected direction. Then they calculated how the magnetic moments react to the electric field within the terahertz flash.

### Experimental challenges

“One of the challenges of the experiment was to create the terahertz flashes with the correct frequency and guarantee that enough of their intensity reaches the sample. Such pulses were not created directly by a laser, but rather with the aid of special organic crystals hit by laser pulses with another frequency. At ETH Zurich, we are also working on facilities that generate terahertz pulses and working together with the specialists from PSI and LCLS we were able to adapt the lasers available at the LCLS to our experiment’s needs,” says Kubacka.
Original publication:
Large-amplitude spin dynamics driven by a THz pulse in resonance with an electromagnon T. Kubacka et al., Science Express, 6 March 2014

## Saturday, March 8, 2014

### Author Information

• 1Institute of Photonics, University of Eastern Finland, P. O. Box 111, FI-80101 Joensuu, Finland. Electronic address: prince.bawuah@uef.fi.
• 2Institute of Photonics, University of Eastern Finland, P. O. Box 111, FI-80101 Joensuu, Finland.
• 3School of Pharmacy, Promis Centre, University of Eastern Finland, P. O. Box 1617, FI-70211, Joensuu, Finland.
• 4Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge CB2 3RA, United Kingdom.

### http://www.ncbi.nlm.nih.gov/pubmed/24530384

We report on the non-destructive quantification of the porosity of pharmaceutical compacts (microcrystalline cellulose tablets) by using both optical and terahertz techniques. For the full analysis of the porosity of pharmaceutical tablets, the results obtained in both cases have shown that optical and terahertz techniques are complementary. The intrinsic refractive index of microcrystalline cellulose was estimated using the effective refractive index obtained from the time delay of the THz pulse together with the Bruggeman model for effective media. Once this intrinsic refractive index is known, the unknown porosity of the tablet can be estimated with the aid of the measured effective refractive index as well as the thickness of the pharmaceutical tablet. The method was tested using a set of thirteen tablets having different porosities. It is shown that the error in the estimation of the unknown tablet's porosity is less than 1%. In addition, surface roughness was measured by using an optical interferometer and gloss by using a diffractive-optical-element based glossmeter. The measurement was achieved by scanning the tablets with a probe beam and detecting the reflected light. The surface roughness and gloss data show relatively good correlation with the porosities of the tablets.

### Abstract-Photonic generation of millimeter and terahertz waves with high phase stability

Dongning Sun, Yi Dong, Lilin Yi, Siwei Wang, Hongxiao Shi, Zongyang Xia, Weilin Xie, and Weisheng Hu  »View Author Affiliations
Optics Letters, Vol. 39, Issue 6, pp. 1493-1496 (2014)
http://dx.doi.org/10.1364/OL.39.001493
Optical generation of highly stable millimeter and terahertz waves is proposed and experimentally demonstrated. The optical-fiber-path-induced phase fluctuation is identically transferred to a 40 MHz intermediate frequency by using dual-heterodyne phase error transfer, then canceled by a phase-locked loop. Based on the scheme, highly stable signals within the frequency range from 25 GHz to 1 THz are generated, and the phase jitter is decreased from 2.05 rad to 4.7 mrad in the frequency range from 0.01 Hz to 1 MHz. For 1 THz, the residual phase noise reaches 60dBc/Hz at 1 Hz frequency offset from the carrier, and the relative timing jitter is reduced to 0.7 fs.
© 2014 Optical Society of America

## Friday, March 7, 2014

### Abstract-Terahertz Plasmonic Structures Based on Spatially Varying Conductivities

1. Barun Gupta1
2. Shashank Pandey1,
3. Sivaraman Guruswamy2
4. Ajay Nahata1,*
Article first published online: 7 MAR 2014
Terahertz plasmonic structures are demonstrated in which the conductivity of the metallic film is varied spatially in order to further enhance the response. Using a commercially available inkjet printer, in which one cartridge is filled with conductive silver ink and a second cartridge is filled with resistive carbon ink, computer generated drawings of plasmonic structures are printed in which the individual printed dots can have differing amounts of the two inks, thereby creating a spatial variation in the conductivity. The silver ink has a DC conductivity that is only a factor of six lower than bulk silver, while the carbon ink acts as a lossy dielectric at THz frequencies. Both inks sinter at room temperature immediately after contact with the plastic film. Using a periodic array of subwavelength apertures as a test structure, patterns printed with different fractional amounts of the two inks show dramatically different enhanced optical transmission properties. These differences arise from changes in the propagation loss properties as a function of conductivity. This data is used to design and fabricate aperture arrays in which the conductivity varies spatially. The resulting plasmonic effect is found to dramatically alter the spatial beam profile of the transmitted THz radiation, as measured by THz imaging.

### Squeezing light into metals: Team controls conductivity with inkjet printer

University of Utah electrical engineers Ajay Nahata and Barun Gupta used a $60 inkjet printer with silver and carbon ink cartridges to create a new, widely applicable way to make microscopic structures that use light in metals to carry information. This new technique could be used to rapidly fabricate superfast components in electronic devices, make wireless technology faster or print magnetic materials. Credit: Dan Hixson, University of Utah College of Engineering. http://phys.org/news/2014-03-metals-team-inkjet-printer.html#jCp Using an inexpensive inkjet printer, University of Utah electrical engineers produced microscopic structures that use light in metals to carry information. This new technique, which controls electrical conductivity within such microstructures, could be used to rapidly fabricate superfast components in electronic devices, make wireless technology faster or print magnetic materials. The study appears online March 7 in the journal Advanced Optical Materials. High-speed Internet and other data-transfer techniques rely on transported through optical fibers with very high bandwidth, which is a measure of how fast data can be transferred. Shrinking these fibers allows more data to be packed into less space, but there's a catch: optical fibers hit a limit on how much data they can carry as light is squeezed into smaller and smaller spaces. In contrast, electronic circuits can be fashioned at much smaller sizes on silicon wafers. However, electronic data transfer operates at frequencies with much lower bandwidth, reducing the amount of data that can be carried. A recently discovered technology called plasmonics marries the best aspects of optical and electronic data transfer. By crowding light into metal structures with dimensions far smaller than its wavelength, data can be transmitted at much higher frequencies such as terahertz frequencies, which lie between microwaves and infrared light on the spectrum of electromagnetic radiation that also includes everything from X-rays to visible light to gamma rays. Metals such as silver and gold are particularly promising plasmonic materials because they enhance this crowding effect. "Very little well-developed technology exists to create terahertz plasmonic devices, which have the potential to make wireless devices such as Bluetooth – which operates at 2.4 gigahertz frequency – 1,000 times faster than they are today," says Ajay Nahata, a University of Utah professor of electrical and computer engineering and senior author of the new study. Using a commercially available and two different color cartridges filled with silver and carbon ink, Nahata and his colleagues printed 10 different plasmonic structures with a periodic array of 2,500 holes with different sizes and spacing on a 2.5-inch-by-2.5 inch plastic sheet. The four arrays tested had holes 450 microns in diameter – about four times the width of a human hair – and spaced one-25th of an inch apart. Depending on the relative amounts of silver and carbon ink used, the researchers could control the plasmonic array's electrical conductivity, or how efficient it was in carrying an electrical current. Using a$60 inkjet printer, we have developed a low-cost, widely applicable way to make plasmonic materials," Nahata says. "Because we can draw and print these structures exactly as we want them, our technique lets you make rapid changes to the plasmonic properties of the metal, without the million-dollar instrumentation typically used to fabricate these structures."
Plasmonic arrays are currently made using microfabrication techniques that require expensive equipment and manufacture only one array at a time. Until now, controlling conductivity in these arrays has proven extremely difficult for researchers.
Nahata and his co-workers at the University of Utah's College of Engineering used  to measure the effect of printed plasmonic arrays on a beam of light. When light with terahertz frequency is directed at a periodic array of holes in a metal layer, it can result in resonance, a fundamental property best illustrated by a champagne flute shattering when it encounters a musical tone of the right pitch.
Terahertz imaging is useful for nondestructive testing, such as detection of anthrax bacterial weapons in packaging or examination of insulation in spacecraft. By studying how terahertz light transmits through their printed array, the Utah team showed that simply changing the amount of carbon and silver ink used to print the array could be used to vary transmission through this structure.
With this new printing technique, Nahata says, "we have an extra level of control over both the transmission of light and  in these devices – you can now design structures with as many different variations as the printer can produce." Nahata says these faster plasmonic arrays eventually could prove useful for:
• Wireless devices, because the arrays allow data to be transmitted much more quickly. Many research groups are actively working on this application now.
• Printing magnetic materials for greater functionality (lower conductivity, more compact) in different devices. This technology is more than five years away, Nahata says.
Although the Utah team used two different kinds of ink, up to four different inks in a four-color inkjet printer could be used, depending on the application.