Tag Archives: femtoseconds

Nano-fireworks

Fig. 1: Nano-fireworks in an argon nanoparticle are ignited by a moderately intense and invisible XUV laser pulse. A subsequent visible laser pulse heats the nanoparticle very efficiently, resulting in its explosion. Electrons and ions move in different directions and send out fluorescence light in various colors. Without the XUV pulse the nanoparticle would remain intact. Courtesy: Max Born Institute in Berlin (Germany)

Fig. 1: Nano-fireworks in an argon nanoparticle are ignited by a moderately intense and invisible XUV laser pulse. A subsequent visible laser pulse heats the nanoparticle very efficiently, resulting in its explosion. Electrons and ions move in different directions and send out fluorescence light in various colors. Without the XUV pulse the nanoparticle would remain intact. Courtesy: Max Born Institute in Berlin [Germany]

You can see why these have been called nano-fireworks by the researchers. Here’s more from a Jan. 23, 2016 news item on Nanowerk,

A team of researchers from the Max Born Institute in Berlin and the University of Rostock demonstrated a new way to turn initially transparent nanoparticles suddenly into strong absorbers for intense laser light and let them explode.

Intense laser pulses can transform transparent material into a plasma that captures energy of the incoming light very efficiently. Scientists from Berlin and Rostock discovered a trick to start and control this process in a way that is so efficient that it could advance methods in nanofabrication and medicine. The light-matter encounter was studied by a team of physicists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin and from the Institute of Physics of the University of Rostock [Germany].

A Jan. 19, 2016 MBI press release, which originated the news item, offers more detail,

The researchers studied the interaction of intense near-infrared (NIR) laser pulses with tiny, nanometer-sized particles that contain only a few thousand Argon atoms – so-called atomic nanoclusters. The visible NIR light pulse alone can only generate a plasma if its electromagnetic waves are so strong that they rip individual atoms apart into electrons and ions. The scientists could outsmart this so-called ignition threshold by illuminating the clusters with an additional weak extreme-ultraviolet (XUV) laser pulse that is invisible to the human eye and lasts only a few femtoseconds (a femtosecond is a millionth of a billionth of a second). With this trick the researchers could “switch on” the energy transfer from the near-infrared light to the particle at unexpectedly low NIR intensities and created nano-fireworks, during which electrons, ions and colourful fluorescence light are sent out from the clusters in different directions .. . …

The experiments were carried out at the Max Born Institute at a 12 meter long high-harmonic generation (HHG) beamline. “The observation that argon clusters were strongly ionized even at moderate NIR laser intensities was very surprising”, explains Dr. Bernd Schütte from MBI, who conceived and performed the experiments. “Even though the additional XUV laser pulse is weak, its presence is crucial: without the XUV ignition pulse, the nanoparticles remained unaffected and transparent for the NIR light … .” Theorists around Prof. Thomas Fennel from the University of Rostock modelled the light-matter processes with numerical simulations and uncovered the origin of the observed synergy of the two laser pulses. They found that only a few seed electrons created by the ionizing radiation of the XUV pulse are sufficient to start a process similar to a snow avalanche in the mountains. The seed electrons are heated in the NIR laser light and kick out even more electrons. “In this avalanching process, the number of free electrons in the nanoparticle increases exponentially”, explains Prof. Fennel. “Eventually, the nanoscale plasma in the particles can be heated so strongly that highly charged ions are created.”

The novel concept of starting ionization avalanching with XUV light makes it possible to spatially and temporally control the strong-field ionization of nanoparticles and solids. Using HHG pulses paves the way for monitoring and controlling the ionization of nanoparticles on attosecond time scales, which is incredibly fast. One attosecond compares to a second as one second to the age of the universe. Moreover, the ignition method is expected to be applicable also to dielectric solids. This makes the concept very interesting for applications, in which intense laser pulses are used for the fabrication of nanostructures. By applying XUV pulses, a smaller focus size and therefore a higher precision could be achieved. At the same time, the overall efficiency can be improved, as NIR pulses with a much lower intensity compared to current methods could be used. In this way, novel nanolithography and nanosurgery applications may become possible in the future.

Here’s a link to and a citation for the paper,

Ionization Avalanching in Clusters Ignited by Extreme-Ultraviolet Driven Seed Electrons by Bernd Schütte, Mathias Arbeiter, Alexandre Mermillod-Blondin, Marc J. J. Vrakking, Arnaud Rouzée, and Thomas Fennel.
Phys. Rev. Lett. 116, 033001 – Published 19 January 2016 DOI:http://dx.doi.org/10.1103/PhysRevLett.116.033001

© 2016 American Physical Society

This paper is behind a paywall.

The birth of a molecule

This research comes from Korea’s Institute of Basic Science in a Feb. 27, 2015 news item on Azonano,

The research team of the Center for Nanomaterials and Chemical Reactions at the Institute for Basic Science (IBS) has successfully visualized the entire process of bond formation in solution by using femtosecond time-resolved X-ray liquidography (femtosecond TRXL) for the first time in the world.

A Feb. 18, 2015 IBS press release, which originated the news item, provides more details,

Every researcher’s longstanding dream to observe real-time bond formation in chemical reactions has come true. Since this formation takes less than one picosecond, researchers have not been able to visualize the birth of molecules.

The research team has used femtosecond TRXL in order to visualize the formation of a gold trimer complex in real time without being limited by slow diffusion.

They have focused on the process of photoinduced bond formation between gold (Au) atoms dissolved in water. In the ground (S0) state, Au atoms are weakly bound to each other in a bent geometry by van der Waals interactions. On photoexcitation, the S0 state rapidly converts into an excited (S1) state, leading to the formation of covalent Au-Au bonds and bent-to-linear transition. Then, the S1 state changes to a triplet (T1) state with a time constant of 1.6 picosecond, accompanying further bond contraction by 0.1 Å. Later, the T1 state of the trimer transforms to a tetramer on nanosecond time scale, and Au atoms return to their original bent structure.

“By using femtosecond TRXL, we will be able to observe molecular vibration and rotation in the solution phase in real time,” says Hyotcherl Ihee, the group leader of the Center for Nanomaterials at IBS, as well as the professor of the Department of Chemistry at Korea Advanced Institute of Science and Technology.

Here’s a link to and a citation for the paper,

Direct observation of bond formation in solution with femtosecond X-ray scattering by Kyung Hwan Kim, Jong Goo Kim, Shunsuke Nozawa, Tokushi Sato, Key Young Oang, Tae Wu Kim, Hosung Ki, Junbeom Jo, Sungjun Park, Changyong Song, Takahiro Sato, Kanade Ogawa, Tadashi Togashi, Kensuke Tono, Makina Yabashi, Tetsuya Ishikawa, Joonghan Kim, Ryong Ryoo, Jeongho Kim, Hyotcherl Ihee & Shin-ichi Adachi. Nature 518, 385–389 (19 February 2015) doi:10.1038/nature14163 Published online 18 February 2015

This paper is behind a paywall although there is a free preview via ReadCube access.

A compendium of attosecond nanophysics papers

A Feb.11, 2015 news item on Nanowerk features a new book on attosecond nanophysics,

A steadily growing treasure of knowledge has accumulated in the past years on attosecond nanophysics of nanostructured solids, which has, so far, not been sorted and structured.

This has now been rectified by two physics professors, Peter Hommelhoff and Matthias Kling. Together with numerous other authors, they have collected the studies conducted in this very young science field in their book Attosecond Nanophysics: From Basic Science to Applications.

The result is an overview for PhD students and interested students as well as other physicists, who like to gain an overview of ultrafast nanooptics and related fields.

A Feb. 9, 2015 Munich-Centre for Advanced Photonics Technische Universität München (TUM) press release by Thorsten Naeser, which originated the news item, describes the book further,

Elementary particles interacting with light move so fast, that they can only be observed with the help of sophisticated techniques. Typically, the motion of excited electrons in atoms or solids, for example, occurs on attosecond timescales. An attosecond is a billionth of a billionth of a second (10-18 s). Starting a few years ago, scientists around the world have been exploring how electrons in nanostructured solids behave when influenced by extremely short and intense light pulses. In order to observe such rapid electron motion, physicists used light pulses with durations of a few femtoseconds down to attoseconds (a femtosecond lasts 1000 attoseconds). These light flashes act, for example, as an ultrafast shutter, following the principles of conventional camera technology, to take pictures of the fast moving particles in the nanocosm.

The researchers’ new book is a collection of their accumulated knowledge in this new research area, a first publication of its kind. The main attention here is not drawn to single atoms or molecules but rather to nanostructured solids, which are typically comprised of many millions of atoms. The main question is: how do electrons behave under the influence of intense light? The answer to this question is of fundamental importance. This research could lead to new technologies, where the electromagnetic field of the light wave can be used to carefully control minute electronic building blocks. With such lightwave-controlled electronics, switching speeds in the petahertz domain (1015 Hz, one million times a billion operations per second) may be reached. “With this technology we could accelerate current electronics by up to a million times”, explains Matthias Kling, one of the editors of the book.

“Attosecond nanophysics” contains the descriptions of experiments which have been conducted in the last years and resulted in groundbreaking scientific publications. The book also contains their mathematical and physical foundations. “All authors are pioneers in this field”, describes the second editor, Peter Hommelhoff. “We have compiled – for the first time – a book, which conveys a current overview of our knowledge and activities on currently the fastest phenomena in the area of small solids”, elaborates Hommelhoff. With this the authors provide students of upper level physics and PhD students a handy overview of the topic. But also interested colleagues from other disciplines can use this book to gain a first, comprehensive insight into this young field of attosecond nanophysics.

Here’s a link to where you can purchase it.

The evolution of molecules as observed with femtosecond stimulated Raman spectroscopy

A July 3, 2014 news item on Azonano features some recent research from the Université de Montréal (amongst other institutions),

Scientists don’t fully understand how ‘plastic’ solar panels work, which complicates the improvement of their cost efficiency, thereby blocking the wider use of the technology. However, researchers at the University of Montreal, the Science and Technology Facilities Council, Imperial College London and the University of Cyprus have determined how light beams excite the chemicals in solar panels, enabling them to produce charge.

A July 2, 2014 University of Montreal news release, which originated the news item, provides a fascinating description of the ultrafast laser process used to make the observations,

 “We used femtosecond stimulated Raman spectroscopy,” explained Tony Parker of the Science and Technology Facilities Council’s Central Laser Facility. “Femtosecond stimulated Raman spectroscopy is an advanced ultrafast laser technique that provides details on how chemical bonds change during extremely fast chemical reactions. The laser provides information on the vibration of the molecules as they interact with the pulses of laser light.” Extremely complicated calculations on these vibrations enabled the scientists to ascertain how the molecules were evolving. Firstly, they found that after the electron moves away from the positive centre, the rapid molecular rearrangement must be prompt and resemble the final products within around 300 femtoseconds (0.0000000000003 s). A femtosecond is a quadrillionth of a second – a femtosecond is to a second as a second is to 3.7 million years. This promptness and speed enhances and helps maintain charge separation.  Secondly, the researchers noted that any ongoing relaxation and molecular reorganisation processes following this initial charge separation, as visualised using the FSRS method, should be extremely small.

As for why the researchers’ curiosity was stimulated (from the news release),

The researchers have been investigating the fundamental beginnings of the reactions that take place that underpin solar energy conversion devices, studying the new brand of photovoltaic diodes that are based on blends of polymeric semiconductors and fullerene derivatives. Polymers are large molecules made up of many smaller molecules of the same kind – consisting of so-called ‘organic’ building blocks because they are composed of atoms that also compose molecules for life (carbon, nitrogen, sulphur). A fullerene is a molecule in the shape of a football, made of carbon. “In these and other devices, the absorption of light fuels the formation of an electron and a positive charged species. To ultimately provide electricity, these two attractive species must separate and the electron must move away. If the electron is not able to move away fast enough then the positive and negative charges simple recombine and effectively nothing changes. The overall efficiency of solar devices compares how much recombines and how much separates,” explained Sophia Hayes of the University of Cyprus, last author of the study.

… “Our findings open avenues for future research into understanding the differences between material systems that actually produce efficient solar cells and systems that should as efficient but in fact do not perform as well. A greater understanding of what works and what doesn’t will obviously enable better solar panels to be designed in the future,” said the University of Montreal’s Carlos Silva, who was senior author of the study.

Here’s a link to and a citation for the paper,

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions by Françoise Provencher, Nicolas Bérubé, Anthony W. Parker, Gregory M. Greetham, Michael Towrie, Christoph Hellmann, Michel Côté, Natalie Stingelin, Carlos Silva & Sophia C. Hayes. Nature Communications 5, Article number: 4288 doi:10.1038/ncomms5288 Published 01 July 2014

This article is behind a paywall but there is a free preview available vie ReadCube Access.

Only for the truly obsessed: a movie featuring gold nanocrystal vibrations

Folks at the London Centre for Nanotechnology (at the University College of London) have released a film made with a pioneering 3D imaging technique that shows how gold nanocrystals vibrate. From the May 23, 2013 news release on EurekAlert,

A billon-frames-per-second film has captured the vibrations of gold nanocrystals in stunning detail for the first time.

The film, which was made using 3D imaging pioneered at the London Centre for Nanotechnology (LCN) at UCL [University College of London], reveals important information about the composition of gold. The findings are published in the journal Science.

Jesse Clark, from the LCN and lead author of the paper said: “Just as the sound quality of a musical instrument can provide great detail about its construction, so too can the vibrations seen in materials provide important information about their composition and functions.”

“It is absolutely amazing that we are able to capture snapshots of these nanoscale motions and create movies of these processes. This information is crucial to understanding the response of materials after perturbation. “

Caption: The acoustic phonons can be visualized on the surface as regions of contraction (blue) and expansion (red). Also shown are two-dimensional images comparing the experimental results with theory and molecular dynamics simulation. The scale bar is 100 nanometers. Credit: Jesse Clark/UCL

Caption: The acoustic phonons can be visualized on the surface as regions of contraction (blue) and expansion (red). Also shown are two-dimensional images comparing the experimental results with theory and molecular dynamics simulation. The scale bar is 100 nanometers. Credit: Jesse Clark/UCL

Here are more details from the news release,

Scientists found that the vibrations were unusual because they start off at exactly the same moment everywhere inside the crystal. It was previously expected that the effects of the excitation would travel across the gold nanocrystal at the speed of sound, but they were found to be much faster, i.e., supersonic.

The new images support theoretical models for light interaction with metals, where energy is first transferred to electrons, which are able to short-circuit the much slower motion of the atoms.

The team carried out the experiments at the SLAC National Accelerator Laboratory using a revolutionary X-ray laser called the “Linac Coherent Light Source”. The pulses of X-rays are extremely short (measured in femtoseconds, or quadrillionths of a second), meaning they are able to freeze all motion of the atoms in any sample, leaving only the electrons still moving.

However, the X-ray pulses are intense enough that the team was able to take single snapshots of the vibrations of the gold nanocrystals they were examining. The vibration was started with a short pulse of infrared light.

The real keeners can watch the movie if they click on the link to the May 23, 2013 news release on EurekAlert.

The team developing this movie was international in scope (from the news release),

The research team included contributors from UCL, University of Oxford, SLAC, Argonne National Laboratory [US] and LaTrobe University, Australia.

Adventures in time, mass, and topological insulators

Nano at a billionth (of a second, or a metre, or some other measure) is not the smallest unit of measurement, despite how we often talk about nano ‘anything’. But, as we continue to explore matter at ever more subtle levels, we need ever smaller units of measure and there are some ready for use.

I have a few excerpts from a Sept. 18, 2012  article (Explained: Femtoseconds and attoseconds) by David Chandler at the Massachusetts Institute of Technology (MIT) describing some of these smaller units of measure and how they were devised,

Back in the first half of the 20th century, when MIT’s famed Harold “Doc” Edgerton was perfecting his system for capturing fast-moving events on film, the ability to observe changes unfolding at a scale of microseconds — millionths of a second — was considered a remarkable achievement. This led to now-famous images such as one of a bullet piercing an apple, captured in midflight.

Nowadays, microsecond-resolution imagery is almost ho-hum. The cutting edge of research passed through nanoseconds (billionths of a second) and picoseconds (trillionths) in the 1970s and 1980s. Today, researchers can easily reach into the realm of femtoseconds — quadrillionths (or millionths of a billionth) of a second, the timescale of motions within molecules.

Femtosecond laser research led to the development, in 2000, of a system that revolutionized the measurement of optical frequencies and enabled optical clocks. Continuing the progress, today’s top-shelf technologies are beginning to make it possible to observe events that last less than 100 attoseconds, or quintillionths of a second.

Those prefixes — micro, nano, pico, femto and atto — are part of an internationally agreed-upon system called SI units (from the French Système International d’Unités, or International System of Units). The system was officially adopted in 1960, and has been updated periodically, most recently in 1991. It encompasses a total of 20 prefixes, 10 of them for decimal amounts, and 10 more for large multiples of the basic units (mega, giga, tera and so on).

As Chandler points out in more detail than I have, there’s a reason for developing these units of measure,

The ability to observe events on such timescales is important for basic physics — to understand how atoms move within molecules — as well as for engineering semiconductor devices, and for understanding basic biological processes at the molecular level.

But physicists and engineers are interested in pushing these limits ever further. To understand the movements of electrons, and eventually those of subatomic particles, requires attaining the attosecond and ultimately zeptosecond (sextillionths of a second) range, Kaertner says. Achieving that requires pushing technology to produce pulses using higher-wavelength sources, and also producing pulses that encompass a wider range of frequencies — a more broadband source.

I finally managed to conceptualize the nanoscale a few years ago but it appears I have more work to do. Chandler offers some suggestions for imagining the femtoscale,

So, just how short is a femtosecond? One way to think of it, Kaertner [Franz Kaertner, MIT adjunct professor of electrical engineering] says, is in terms of how far light can move in a given amount of time. Light travels about 300,000 kilometers (or 186,000 miles) in one second. That means it goes about 30 centimeters — about one foot — in one nanosecond. In one femtosecond, light travels just 300 nanometers — about the size of the biggest particle that can pass through a HEPA filter, and just slightly larger than the smallest bacteria.

Another way of thinking about the length of a femtosecond is this: One femtosecond is to one second as one second is to about 32 million years.

Chandler discuses in another MIT article (Watching electrons move at high speed) also posted on Sept. 18, 2012, a new electronic material, a topological insulator, and the importance of viewing the behaviour of electrons present in such an insulator,

Topological insulators are exotic materials, discovered just a few years ago, that hold great promise for new kinds of electronic devices. The unusual behavior of electrons within them has been very difficult to study, but new techniques developed by a team of researchers at MIT could help unlock the mysteries of exactly how electrons move and react in these materials, opening up new possibilities for harnessing them.

For the first time, the MIT team has managed to create three-dimensional “movies” of electron behavior in a topological insulator, or TI. [can be viewed here] The movies can capture vanishingly small increments of time — down to the level of a few femtoseconds, or millionths of a billionth of a second — so that they can catch the motions of electrons as they scatter in response to a very short pulse of light.

Electrons normally have mass, just like many other fundamental particles, but when moving along the surface of TIs they move as if they were massless, like light — one of the extraordinary characteristics that give these new materials such promise for new technologies. [emphases mine]

It’s the bit about mass and masslessness that caught my eye. Fascincating, non? Here’s a graphical representation of what the MIT scientists observed (I think it looks like a cup or a grail),

Three-dimensional graphical representations of the way electrons respond to an input of energy, delivered by a pulse of laser light. The horizontal axis represents the electrons’ momentum, and the vertical axis shows their energy. The time sequence runs from top left to bottom right, and the laser pulse arrives just before the second image, causing a sudden burst of higher energy levels. Images courtesy of Yihua Wang and Nuh Gedik [of MIT]

Here’s a bit more about TIs and possible future applications,

TIs are a class of materials with seemingly contradictory characteristics: The bulk of the material acts as an insulator, almost completely blocking any flow of electrons. But the surface of the material behaves as a very good conductor, like a metal, allowing electrons to travel freely. In fact, the surface is even more conductive than normal metals — allowing electrons to travel at almost the speed of light and to be unaffected by impurities in the material, which normally hinder their motion.

Because of these characteristics, TIs are seen as a promising new material for electronic circuits and data-storage devices. But developing such new devices requires a better understanding of exactly how electrons move around on and inside the TI, and how the surface electrons interact with those inside the material.

I highly recommend reading both of Chandler’s articles.