Megapixel CCD Can See Terahertz

Photo: Markus Fischer/Paul Scherrer Institute

By Alexander Hellemans

Terahertz waves, a frequency band squeezed in between the far infrared and the very short-wave radio frequency region of the electromagnetic spectrum, are not only difficult to create but also difficult to detect. So making a good imager for them is quite a difficult task.  Still, in 2012 researchers reported an experimental 1000-pixel CMOS terahertz camera.

The SwissFEL laser team led by Christoph Hauri at the Paul Scherrer Institute near Zurich has now shown that you can use a common megapixel  CCD device, as found in electronic cameras or in smartphones, to capture images produced by terahertz waves.

In research published today in Nature Communications, the team describes how with a silicon CCD of 1360 x 1024 pixels they obtained images of THz beams with a resolution that is 25 times better than currently available bolometer-based terahertz imaging systems, and at a fraction of the cost. Microbolometer imagers are two dimensional arrays of metal-and-insulator pixels. The pixels heat up in response to terahertz radiation and change their resistances. However, these arrays are not only slower than CCDs but their pixels are several times larger (24 micrometers instead of 4.65 µm), so microbolomters have worse resolution.  

In common CCDs individual photons of visible light liberate individual electrons, a phenomenon known as the internal photoelectric effect. These electrons, have sufficient energy to cross silicon’s band gap, and end up stored in a potential well, from which they can be read out. Terahertz photons, with their longer wavelengths, carry much less energy and the dislodged electrons simply don't make it across the band gap.

"We used kind of an experimental trick, the mode of CCD operation we use is different from the mode of operation at optical frequencies," says Mostafa Shalaby, the lead author of the Nature Communications paper and member of the SwissFEL Laser Group at PSI. It was previously known that low frequency radiation, when intense enough, could lead to dramatic changes in the semiconductor band structure. (For the intense light source they turned to a new and uniquely powerful source the SwissFEL team developed.) Long terahertz wavelengths force electrons to tunnel through the bandgap and the charge carriers start multiplying, leading to huge sensitivity, explains Shalaby.

Although materials with a band gap corresponding to the energy of terahertz photons don't exist, it is clear that any reduction of the band gap would increase the sensitivity. Using a photoactive layer with a smaller band gap would most likely improve the sensitivity of the imaging device, but "it is really hard for us to convince big companies with mass production to fabricate something for us custom," says Carlo Vicario of SwissFEL.

The researchers obtained images by exposing the CCD directly to the terahertz beam. Improving the CCD’s sensitivity, the subject of their immediate research plans, will require much less powerful terahertz lasers, says Vicario. The researchers found that by using 2 to 5 percent of their laser power they could still obtain visible images.

There are a wide range of other improvements possible as well. "If you use CMOS instead of CCD you have a much higher sensitivity,” says Shalaby. “Also with structured metal or metamaterials on top of the substrate one can enhance the sensitivity of the imager." With the first results of this proof of principle the researchers have filed for a patent, and they say they’ve attracted interest from industry, including a CCD manufacturer.

Observed live with X-ray laser: Electricity controls magnetism

Principle of the experiment. The motion of the magnetic moments in TbMnO3 (shown as arrows on the right hand side) is excited by a terahertz pulse (red beam) and probed by a pulse from the x-ray laser LCLS (blue beam). Credit: Teresa Kubacka

by Paul Piwnicki

 http://phys.org/news/2014-03-x-ray-laser-electricity-magnetism.html#jCp

Researchers from ETH Zurich and the Paul Scherrer Institute PSI demonstrate how the magnetic structure can be altered quickly in novel materials. The effect could be used in efficient hard drives of the future.

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 fieldcan 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 arrangement of magnetic moments in TbMnO3.Neighbouring moments are tilted in respect to each other. There are two possible directions in which the moments can turn that might correspond to the two values of a bit in future storage …more

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 Linac Coherent Light Source (LCLS), a large-scale research facility located 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.

Electrically and magnetically linked

The deflection of the magnetic moments (black line) follows the electric field of the terahertz pulse (red line) with a short delay. The blue dots show the results of the measurement. Credit: Kubacka et al., Science Express (2014) DOI: 10.1126/science.1242862

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.

 Explore further: 'Electromagnon' effect couples electricity and magnetism in materials

More information: "Large-Amplitude Spin Dynamics Driven by a THz Pulse in Resonance with an Electromagnon." T. Kubacka, et al. Science DOI: 

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.

Electrically and magnetically linked

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

Magnetization Controlled at Picosecond Intervals by Terahertz laser

A pulse from a terahertz laser (blue) controls the magnetisation of a material: the magnetisation (red - determined via the magneto-optic Kerr effect MOKE) follows the laser pulse's magnetic field with a slight delay. The black curve shows the prediction of a computer simulation. (Credit: Image courtesy of Paul Scherrer Institut (PSI)

http://www.sciencedaily.com/releases/2013/08/130811150605.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily%2Ftop_news+(ScienceDaily%3A+Top+News)

Aug. 11, 2013 — A terahertz laser developed at the Paul Scherrer Institute makes it possible to control a material's magnetisation at a timescale of picoseconds (0.000,000,000,001 seconds). In their experiment, the researchers shone extremely short light pulses from the laser onto a magnetic material, where the magnetic moments -- "elementary magnets" -- were all aligned in parallel. The light pulse's magnetic field was able to deflect the magnetic moments from their idle state in such a way that they exactly followed the change of the laser's magnetic field with only a minor delay.

---

These days, the majority of data is stored magnetically, such as on hard disc drives. Thus, a bit, the smallest amount of information, is stored in the magnetisation direction of a small section of the storage medium. One might imagine that such a magnetic material contains many miniscule magnets -- the magnetic moments. If one wants to change the information, one has to reverse the direction of the moments. And in order to be able to store large amounts of data, one needs processes that enable the magnetisation direction in a material to be changed quickly.

The terahertz laser used in the experiment is one of the strongest of its kind in the world.One special feature is the fact that it is phase-stable, which enables the exact change in the electrical and magnetic field within the individual pulses to be defined reliably for each laser pulse. As the majority of data is stored magnetically these days, the possibility to quickly change a material's magnetisation is crucial for new, rapid storage systems. The researchers report on their results the journal Nature Photonics.

Magnetisation in time with the terahertz laser

Researchers at the Paul Scherrer Institute (PSI) and the Swiss Federal Institute of Technology in Lausanne (EPFL) have now studied a new approach in conjunction with French colleagues at the Université Pierre et Marie Curie in Paris that enables the magnetisation of a material to be controlled at the timescale of picoseconds (0.000,000,000,001 seconds). To do so, they used a newly developed laser that generates very short pulses of light in the terahertz range. Like all electromagnetic radiation, the light consists of an electrical and a magnetic field that both alter their directions very quickly -- in the light of a terahertz laser, the direction changes around 1,000,000,000,000 times a second. If you shine this light onto a magnetic material, the variable magnetic field in the laser light can change the direction of the material's magnetisation -- much like if you hold a magnet to one side of a compass needle, then to the other, the difference here being that this realignment takes place within an extremely short space of time: less than one picosecond.

In their experiment, the researchers used extremely short "flashes" of terahertz light. Unlike the light from conventional lasers, terahertz light does not heat up the magnetic sample, which turns out to be essential for an exact manipulation of the magnetisation. The terahertz flashes used were so short that the magnetic field just about had time to point in one direction, then in the other. In the illuminated material, the magnetic moments were deflected as a result: first in one direction, then in the other. They thus followed the change of the magnetic field in the terahertz flash exactly with a tiny delay.

Identical pulses

The terahertz laser was developed by the laser group within the SwissFEL project at the Paul Scherrer Institute. Until a few years ago, there were barely any strong terahertz lasers -- there was even talk of a terahertz gap. "We use special organic crystals for our lasers that reduce the frequency of laser light," says Christoph Hauri, head of the laser group and professor at EPFL, explaining the idea behind the equipment. "If we shine onto the crystal using a strong laser with a high frequency, it emits radiation on a terahertz scale." The laser is one of the strongest in the world. Another of the laser's properties important for the experiments is its phase stability, which means that one can specify exactly how the change in the magnetic field within the individual pulse takes place and that this pulse form can be reproduced time and again. The development was made possible thanks to a successful collaboration with Swiss industrial partner Rainbow Photonics AG.

The laser flash in the experiment presented is not yet intensive enough to be able to flip over the magnetisation completely; you can merely observe the dynamics, i.e. the movement of the magnetisation. The experiment, however, is an extremely important milestone for demonstrating the concept of the ultrafast and exact manipulation of magnetism with a laser. Hauri is confident that a complete flip-over of the magnetism can be achieved. "There are tricks to enhance the fields of a weak laser to such an extent that they could switch the magnetisation." This would also involve selecting a special pulse form and generating a pulse where the magnetic field initially points in one direction weakly, then strongly in the other, then point back in the original direction weakly again. If only the middle strong part of the pulse is strong enough to flip over the magnetisation, one could use such pulses to re-magnetise materials. Such precisely defined pulses are now available at the PSI.

Part of the SwissFEL project

At the Paul Scherrer Institute, the development of the terahertz laser is part of the SwissFEL project, where the SwissFEL X-ray laser is being constructed as the PSI's new large-scale facility. It will produce laser light on an x-ray scale and thus make many processes visible in the material that are not accessible using today's study methods. Terahertz lasers are due to be used in two places there. On the one hand, they will be employed for measuring the properties of the x-ray beam in operation. On the other hand, they could be used to initiate reactions in experiments where the intermediate state is to be determined later with the x-ray laser.

Organic crystals put laser focus on magnetism

This thin green crystal, developed by a company in Switzerland, is used to convert near-infrared laser light into terahertz frequencies, which are useful for a range of experiments. (Photo by Glenn Roberts Jr.)
 http://phys.org/news/2012-07-crystals-laser-focus-magnetism.html#jCp






by Glenn Roberts Jr.
(Phys.org) -- In the first successful experiment of its type at SLAC's Linac Coherent Light Source, scientists used terahertz frequencies of light to change the magnetic state of a sample and then measured those changes with ultrafast pulses from a powerful X-ray laser.
Invisible to human eyes, terahertz describes a band of frequencies between microwave and infrared light. These frequencies are alluring to scientists because they can be used to control and study magnetic and electric states in materials, and have been applied to fields ranging from data storage to biological imaging and explosives detection. They provide an atomic-scale window into fundamental processes such as magnetism, molecular motion and protein vibrations. But observations in the terahertz range were until recently largely out of reach for scientists, said Matthias Hoffman, a SLAC scientist specializing in terahertz laser research who worked on the latest experiments. "There were no real efficient sources and detectors. It was relatively difficult to do science" at terahertz frequencies, he said. The experiment in July involved a technique called pump-probe in which one laser, the "pump," is used to stimulate changes in the sample – in this case a material with exotic magnetic properties – while the X-ray laser probes these changes. A team led by Urs Staub of the Paul Scherrer Institute in Switzerland and Steven Johnson of the Institute for Quantum Electronics at ETH Zurich in Switzerland generated the terahertz laser pulses by aiming an infrared laser beam at a specialized crystal. The passage through the crystal changed the frequency of light from near-infrared to terahertz light. The terahertz pulses then hit a sample, and the researchers measured changes in the sample using closely synchronized pulses from the LCLS X-ray laser. The crystal they used was a special type of thin organic crystal known by the acronym DAST, grown by a private company in Switzerland. DAST crystals tend to be more fragile and susceptible to damage than some non-organic crystals designed for terahertz conversion, Hoffmann noted. He is working with the LCLS laser group to develop better sources of intense terahertz pulses as a regular option for LCLS users conducting pump-probe experiments. Hoffman said another possible way to generate terahertz frequencies is with the electron beams that power advanced synchrotrons and LCLS. "This can produce even higher pulse energies," he said, but has proven more challenging for use in experiments.