In the attoworld, ‘in the blink of an eye’ doesn’t cut it as a description for just how rapid processes are. If an attosecond were stretched out to a second, a blink would take almost the entire age of the Universe. It’s a billionth of a billionth of a second, a nano nanosecond.
What is the purpose of detecting such unfathomably fast processes? It just so happens that the attosecond is the natural timescale of electron motion in atoms, molecules and solids. “We’re talking about not only very short timescales, but also very small spatial scales,” says Michael Chini , from the University of Central Florida, USA. “And what that means is that attosecond science is fundamentally a science that involves quantum mechanics.” Directly measuring quantum processes is now possible. Attosecond science may even one day provide the means of controlling them.
As a result, attosecond science is widely regarded as having huge potential to advance fundamental research, not only in quantum physics but also in biology, chemistry, medicine and others. More generally, the scientific community is in agreement that attosecond tools open up new possibilities in various critical sectors.
Already, researchers have probed the fast interactions of electrons in, for example, organic photovoltaic materials, which hold promise as new solar cell materials or catalysts but are currently either unstable or inefficient. A detailed understanding of the charge transfer pathway could help optimise such next-generation photosensitive elements.
Another promising application is electronics. In today’s electronic circuits, electrons are driven by microwave voltages to switch the current on and off in a fraction of a nanosecond. Theoretically, the fastest switching time possible is the time it takes an electron to travel between neighbouring atoms, a process occurring at the attosecond scale. For this reason, smaller structures allowing faster switching speeds are being explored with the overarching aim of realising petahertz electronics, in which the direction of electric current can be changed several trillion times per second; roughly 100,000 times faster than permitted by today’s electronics.
Towards attosecond science
What has made such feats enter the realm of possibility has been advances in photonics. From the 1960s onwards, laser pulse durations shrank from roughly microsecond durations to nanoseconds and, by the mid-1980s, the several femtosecond regime. There, progress stagnated for a decade and a half. But in the meantime, the foundations for attosecond science were being laid.
Among numerous advances, chirped pulse amplification (CPA) stands out as being pivotal in allowing researchers to reach the attosecond scale. Introduced by Donna Strickland and Gérard Mourou (who received the 2018 Nobel Prize in Physics for their work), CPA stretches, amplifies and then compresses again ultrashort laser pulses to produce enormously high optical intensities. “What that means is that now you have the ability to use a laser field to kind of control or move around the electron in the way that you would like,” explains Chini.
However, the shortest pulses available were (and still are) from titanium-doped sapphire lasers emitting in the near-infrared (NIR), producing at-best few-femtosecond pulses. Attosecond pulses required shorter wavelength, higher energy laser light. The solution came when researchers began experimenting with high-order harmonic generation (HHG). HHG is a process where CPA produces an intense infrared laser pulse that is focused on a target, causing strong nonlinear interactions that lead to a tiny fraction of the laser power being converted to very high harmonics of the optical frequency of the pulse.
With the later advent of carrier envelope phase stabilisation providing the ability to control the carrier envelope phase of an amplified laser pulse, in 2001, researchers generated a train of pulses with a temporal duration of 250 attoseconds – and, in the same year, the first single attosecond pulse with duration of 650 attoseconds. Attosecond science was born.
Recent advances
Since then, huge progress has been made. In isolating single attosecond pulses, several gating techniques have been developed to confine the HHG process to a single event. These developments led to shorter and shorter attosecond flashes, with the record tumbling from an 80 attoseconds pulse in 2008 to a 67 attosecond pulse generated by Chini and colleagues in 2012 and more recently 53 attoseconds in 2017. Later the same year, ETH Zurich researchers succeeded in shortening the pulse duration to only 43 attoseconds, representing the shortest controlled event that has ever been created by humans.
In addition, researchers have explored laser technology and other laser source spectral regions, such as the mid-infrared. “The mid-infrared is very good for attosecond science because the oscillation cycle is longer so you have more control over what the electron does,” explains Chini. “And that allows attosecond pulses to be generated not just in the extreme ultraviolet, but now into the X-ray regime.” Only recently have fully coherent, soft X-ray attosecond pulses through HHG driven by MIR femtosecond laser sources become available.
Other researchers have advanced attosecond spectroscopy to investigate various physical processes of interest, mainly employing pump-probe spectroscopy first developed by Ahmed Zewail (1999 Nobel Prize in Chemistry) using longer femtosecond pulses. But using attosecond pulses for both the pump and probe has proven challenging, so most practical approaches have used only one attosecond pulse, pump or probe, with a femtosecond pulse employed for the other step. Just this year, an international team (Max Born Institute, Germany, University College London, UK, and ELI-ALPS in Hungary) managed to use an attosecond-pump attosecond-probe method to study nonlinear multiphoton ionisation of atoms.
Yet more have focused on extending the applicability of attosecond techniques to different targets. “When attosecond science came out, it was all about gases, but now 50 per cent if not more of the research is done in solids,” explains Giulio Vampa at the National Research Council of Canada & University of Ottawa. “My first major contribution in 2011 was to discover that the physics that underlies high harmonic generation in gases is mirrored in solids, which meant we could pour all the technology developed for gases into solids to investigate what happens.”
In a completely different direction, a new way of generating attosecond pulses has emerged: X-ray free electron lasers (X-FELs). X-FELs accelerate electrons to very high energy, close to the speed of light, and then send the resulting electron bunch through what is called an ‘undulator’ or ‘wiggler’ (a structure of alternating opposing magnets). Forcing the electron bunch to proceed in a wavy motion, they emit X-rays forward at the maxima and minima of the wave motion. The result is
an extremely bright X-ray laser, many
orders of magnitude brighter than HHG sources so that researchers can not only probe electron motion but potentially control it. Already, isolated GW-scale soft X-ray attosecond pulses have been generated at X-FEL facilities.
Democratising attosecond science
Even though huge advances have been made and important insights into fundamental physical processes unlocked, a central problem that has haunted attosecond science since the beginning has been accessibility. The complex, expensive laser set-ups required to conduct attosecond science have remained in the hands of a select few for two decades now.
However, the situation is changing: “People in attosecond science will remain those pushing the frontiers of laser technology,” says Vampa. “But I think attosecond science is becoming progressively more democratised, thanks to better laser sources that are easier to operate and with improved capabilities.”
“When I was a PhD student, I spent 90 per cent of my time aligning the laser and only 10 per cent was spent actually collecting data,” recalls Chini. “These were all homebuilt lasers that were very, very complicated, and were difficult to use.” Fast-forward to today and Chini, Vampa and anyone else working in the field can order high-quality complex ultrafast optical components and femtosecond laser sources from the likes of Edmund Optics, Coherent, Amplitude, Trumpf, Thorlabs and more, simplifying experimental setups and meaning attosecond experiments can be conducted with a push of a button.
“Having this advantage of a laser that just works allows people who are not laser experts to be able to access these very short timescales,” Chini says. “I think that as the field advances, the big discoveries are going to be made by collaborations with chemists, biologists and solid-state physicists, and so being able to transition attosecond science from something that exists only in a few labs worldwide to something that’s more widely available motivates me.”
To this end, recently Chini’s team developed a technique that allows industrial-grade lasers – costing around $100,000, far less expensive than current bespoke set-ups – to perform HHG and generate attosecond pulses. Soon after, a separate team using a similar technique achieved the first demonstration of an industrial-grade laser actually generating and characterising attosecond pulses.
Elsewhere, Vladislav Yakovlev (Max Planck Institute of Quantum Optics, Germany) and his colleagues are developing new, simpler and less expensive techniques for attosecond measurements: “Our labs are focused on investigating, inventing, developing different approaches for attosecond-scale experiments that do not require big expensive vacuum chambers, that do not rely on attosecond light pulses but instead rely on very fast, very nonlinear gating mechanisms,” he says. “We call this attosecond spectroscopy 2.0.”
Going brighter
Efforts such as these are undoubtedly slowly democratising attosecond science performed using HHG, but those interested in multiphoton and nonlinear X-ray physics, or researchers aiming to control electron motion, require brighter sources. And their research remains severely hampered by the paucity of X-FEL facilities.
Only five X-FELs exist worldwide: the European XFEL in Hamburg, Germany; the Linac Coherent Light Source (LCLS) at Stanford University, USA; the SwissFEL in Switzerland; the Spring-8 Angstrom Compact Free Electron Laser (SACLA) in Japan, and the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) in Korea. “It’s virtually impossible to get beam time on them,” says Bjorn Manuel Hegelich, a professor at the University of Texas at Austin, USA. “If you’re a company, forget it. You will not get to use it.”
Hegelich is founder and CEO of Tau Systems, a start-up with ambitions to make X-FELs significantly cheaper and more compact. “We’re replacing the conventional radio frequency copper structure accelerator with a laser-driven accelerator,” he explains. By commercially introducing this technique for laser-driven particle acceleration, the idea is for electrons to surf on three-dimensional plasma waves, thereby accelerating them to ultra-high energies over a short distance. “The accelerator at Stanford is many kilometres long and the one at Hamburg as well; these are big campus-size machines. We get the same electron energy over a distance of about 10 centimetres.”
Simulations reveal that Tau Systems’ X-FELs will be capable of producing pulses in the range of 100 attoseconds. With the first machines touted to be available from around 2026, the team believes that attosecond x-ray science will be within the grasp of not only the wider scientific community but also, finally, industry. “This is where you can really do single-particle imaging of molecules of proteins to understand these chemical and biological processes on a very fundamental level,” says Hegelich. “And if you do that, you can then start thinking about designing drugs, for example, from the molecular level upwards, basically like Lego building blocks.”
Sponsored: Opening up spectral measurement for multiple applications
Back in the early 2000s, engineers Ruud Bouten, Peter Franssen and Marcel Janssen were working at Phillips, focusing on the development and fabrication of mobile displays. It was here that they spotted a gap in the market for fast and accurate colour and light measurement systems that would best suit production lines for consumer electronics products, having found that the available equipment at the time could be challenging to operate in an industry environment – not to mention cost-inefficient.
The three founded Admesy in 2006, using the expertise built up at Phillips to develop the kind of specialist test equipment they felt would be best suited to this market. The past 16 years has seen the company grow to become a global operation, with headquarters in Ittervoort, the Netherlands, and offices as far afield as Korea, China and Japan. They have also seen Admesy develop working relationships with some of the leading consumer electronics manufacturers. In 2012 Steven Goetstouwers joined as CEO.
In terms of technological developments during this time, the company initially developed a suite of high-speed, high-precision colorimeters specifically for the niche requirements of display and lighting applications.
As an example of such niche requirements, consumer device displays from the same brand must look exactly the same to the end customer, with no differences in appearance or colour. Colour point accuracy is therefore essential. When it comes to the testing of these consumer devices this must also be very fast, because devices can be produced in the tens of millions. Admesy later expanded into spectroradiometers, light meters and 2D imaging devices.
Expanding expertise
The company’s latest innovation is the Neo, a versatile platform that stems from the firm’s almost two decades of work in measurement devices for consumer electronics, but which can create spectral measurement solutions for a much wider array of applications. These include analytical, transmission or absorbance testing, fluorescence and Raman spectroscopy, solid-state lighting such as LED testing, or other demanding applications such as thin film coating.
Steven Goetstouwers, CEO at Admesy
Neo is based on the trusted Rhea series of spectroradiometers, already a high-end measurement device with very high optical performance in terms of linearity signal-to-noise ratio, wavelength accuracy, and absolute accuracy. CEO, Steven Goetstouwers, explains: “With the Neo, we wanted to make it more accessible so that more or less everybody has access to high end spectral measurements. A professor can use it in a university, or someone in a lab working on research and development can use it. And, of course, it can be used in production.”
Neo uses a high-end cooled CCD detector for low noise and high dynamic range, and can virtually cover any wavelength range in the 250-1,100 nm range. There are two standard versions available: broadband and visible. The broadband has a dispersion range of 850 nm while the visible has a dispersion of 480 nm. Custom wavelength configurations are also possible. The series is available with a number of accessories, including lenses, cosine correctors, cuvette systems and spheres. These can either be directly coupled or via a fibre. A unique robust coupler assures repeatable precise connection and correct measurement results are guaranteed by the matching calibrations for a specific setup stored in the device itself.
Goetstouwers continues: “Our aim was to make a really versatile unit that can be used for measurement in many different kinds of applications. Its wavelength accuracy is even further improved and the luminance accuracy is very high, which, in combination, is very important for measuring colour, which is directly derived from the existing market. It can be used for measurements in different kinds of applications, some of which are already happening commercially, with potential for many others in the near future.”
Neo is even suitable for high-volume production and reliable even in harsh environments. It features simple hardware and software integration and can be customised according to requirements. Just some applications include analytical testing, such as general, Raman or fluorescence spectroscopy – which require high accuracy, good linearity, low noise and repeatability as well as luminance accuracy in a measuring device. It can be used to test solid-state lighting products or for testing LEDs for niche applications such as heart-rate monitoring, and even for spectral measurements in OEM solutions.
Application example
An example of the latter that is already in commercial use is in the development of ion beam sputtering (IBS) deposition systems, following a partnership with Cutting Edge Coatings (CEC). CEC’s Navigator IBS deposition system is designed to achieve high-quality coating processes spanning optical wavelengths from deep ultraviolet and visible to infrared. The system can produce coatings for very high-performance laser components and various optical filters, using a spectral thin film thickness control system that relies on spectral transmission measurements to monitor and control the process in real time.
Ensuring deposition of the correct thickness of each layer is essential in terms of the accuracy of the final filter. CEC uses atomic layers, which can be time consuming and labour intensive, so the company needed a high accuracy broadband optical monitoring system to help meet this challenge. The company partnered with Admesy and adopted the Neo platform, which was ideally placed to help the filters meet tighter tolerances thanks to the well thought-out optical and mechanical construction.
Further information: www.admesy.com/products/neo
