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New Frontiers in Ultrafast Laser Science

For more than two decades, Margaret Murnane and Henry Kapteyn, who co-lead the Kapteyn/Murnane group, have worked at the forefront of ultrafast laser science. For instance, the group invented the first sub-10 femtosecond mode-locked Ti:sapphire laser that is now a standard fixture in thousands of laboratories around the world, including several here at JILA. Their group also developed technologies to amplify very short pulses to high peak powers. These technologies incorporate cryo-cooling of the amplifier crystal and pulse-shaping techniques to optimize the interaction of light with quantum systems. They have been adopted worldwide for many applications in science and technology.

The group is now developing high peak-power, high average-power ultrafast lasers whose wavelengths span the mid-infrared to the ultraviolet regions of the spectrum. Its most recent innovation is a simple, compact laser with carrier-envelope phase stabilization that is capable of generating millijoule pulse energies at up to 100 kHz repetition rates. This power is unprecedented for a typical laser used in a university setting. The laser’s secret ingredient is a cryo-cooled amplifier crystal. The new laser system is expected to find multiple applications in science and technology.

New laser design, diagnostics, and optics are continually underdevelopment by the group. A key effort is the development of a tabletop x-ray laser.

A New Era for X-rays

Scientists have wanted to create lasers that generate coherent beams of x-rays ever since the invention of the visible laser in 1960. The quest for an x-ray laser first led to the construction of large facilities such as the kilometer-scale x-ray free electron lasers. These facilities reach photon energies of kiloelectron volts. But, they are expensive to use, and it is difficult to obtain research time on them.

Research labs at JILA require an affordable tabletop x-ray laser to peer into the nano world. X-ray wavelengths are well suited for exploring tiny nanoscale objects. X-rays can not only distinguish different elements, but also identify the charge and spin state of their atoms. Ultrafast x-rays can also capture coupled motions of electrons and atomic nuclei inside a molecule. And, thanks to the Kapteyn/Murnane group, JILA now is now using the world’s first tabletop x-ray laser to probe the nano world.

The unique laser is based on high harmonic generation (HHG), which is a coherent version of the well-known Roentgen x-ray tube. However, instead of boiling electrons off a hot filament, in HHG an infrared femtosecond laser plucks an electron from an atom, accelerates the atom coherently away from, and then back to, its parent ion. When the electron recombines with the ion, its energy is coherently upconverted into a more energetic high harmonic photon.

The group has known how to generate high harmonic photons for many years. The critical discovery that led to the tabletop x-ray laser was learning how to generate bright harmonics at the high photon energies needed for applications in imaging and spectroscopy.

To generate bright beams of high-harmonic x-rays, the laser light and x-rays must propagate in phase throughout a medium (such as a noble gas) to ensure that the signals from many atoms add coherently. The problem comes when the high-intensity laser ionizes the gas, which causes the laser light waves to outrun the x-rays waves, causing significant destructive interference.

A collaboration of the Kapteyn/Murnane group with the Baltuska group at the Technical University Vienna solved this problem. By using ultrafast lasers with long wavelengths (~4 µm) in the mid-infrared region of the spectrum, the researchers were able to generate bright x-ray beams whose wavelengths span from the ultraviolet to the soft-ray region. These bright x-rays include a region known as the “water window.” The water window is useful for taking ultrahigh resolution x-ray images of single cells or nanostructures. The bright x-rays also make it possible to image specific elements in magnetic materials.

The success of this work led theorist Andreas Becker to predict that the use of relatively long-wavelngth light (2–4 µm) in HHG could produce bright beams of soft x-rays with all their punch packed into isolated ultrashort bursts. The Kapteyn/Murnane group verified this prediction in 2014 with the creation of soft x-ray bursts with pulse durations measured in tens to hundreds of attoseconds! 

Measuring these pulses was no small feat. First, the attosecond pulse had to be split into two parts. Then a special beam separator had to delay part of the pulse (by a distance of just .5 nm) so the two parts could interfere with each other, creating a short burst of soft x-ray light. Because attosecond soft x-ray bursts can now be readily made in a research laboratory, they will open the door to observing the intricate dance of electrons inside atoms, molecules, liquids, and materials.

With further development, the HHG technique may one day lead to the production coherent hard x-rays as well. Thus, today’s coherent tabletop soft x-ray laser could one day become an alternative to the ubiquitous x-ray tubes in medical imaging. The ability to generate ultrafast coherent hard x-rays with a tabletop system will revolutionize scientific investigations in microscopy, magnetism, nanoscience, molecular dynamics, biotechnologies, and nanotechnologies.

Ultrafast Magnetics

Magnetism remains an incompletely understood phenomenon, particularly at very short length and time scales. However, a detailed understanding of nanoscale magnetism is critical for future advances in data storage applications. In new designs, for example, bits may be packed onto hard disks as close together as 20 nm, well within the range where atoms, electrons, and photons interact. Such designs require a detailed understanding of the limits of magnetic switching and storage density. They would also greatly benefit from a comprehensive model of how spins, electrons, photons, and phonons interact, but such a model does not yet exist. Such a model will be challenging to create without more experimental data on nanoscale magnetic behavior.

The Kapteyn/Murnane group is meeting this challenge head on. The researchers first showed that the fastest dynamics in magnetic materials are captured with extreme ultraviolet (XUV) harmonics. This exciting new method can simultaneously resolve individual elements at multiple sites.

In a second seminal experiment, the group probed how rapidly a magnetic state can be destroyed in an alloy of iron and nickel. In the process, atoms of both elements were clearly identified. When the researchers increased the time resolution of the experiment to 10 fs, they were able to show how different elements in the alloy responded to the XUV light on different time scales. The different response times were due to element-specific variations in the rate energy was exchanged with other atoms of the same kind. This exchange interaction energy appears to control ultrafast magnetic dynamics!

The group has also observed (and explained) laser-generated spin currents in multiple layers of magnetic material during a laser-driven ultrafast demagnetization experiment. In the experiment, the researchers excited magnetic multilayers with a laser pulse and probed the magnetization response simultaneously, but separately, in nickel and iron. They discovered that optically induced demagnetization of the nickel layer actually enhanced the magnetization of the buried layer of iron when the two layers were aligned parallel to one another. These findings revealed important new magnetization dynamics in metallic systems consisting of more than one element in which energy exchange interactions occur.

In the future, the group anticipates extending their work to full time-resolved magnetic imaging. They also plan to explore element-specific magnetic behavior with soft x-rays. The latter investigation has been made possible by recent advances in generating bright coherent x-rays from femtosecond lasers. X-rays should make it possible to image buried magnetic structures, interactions between sites in a material, coupled-nano layers, and spin dynamics.

Nanoimaging

The quest to understand the structure, behavior, and function of tiny nanoscale objects is driving the development of new ultrahigh-resolution imaging technologies. The foundation for ultrahigh-resolution imaging was laid by new optical microscopy technologies that combine coherent illumination with computer-assisted image processing. X-ray imaging represents the next important step.

Soft x-ray microscopy complements electron and optical microscopies because of its ability to penetrate thick (opaque) samples and achieve high spatial resolution. It has the added advantage of being able to differentiate among different types of atoms and molecules. Until recently, however, the spatial resolution of x-ray microscopes was limited to approximately 15 nm because of the technical difficulty of making zone plate lenses. And, a laboratory source of coherent x-rays was yet to be realized.

Recently, HHG has made it possible to generate coherent x-ray beams in the laboratory. Because HHG can be combined with new imaging techniques that replace the imaging optics in a microscope with an iterative phase retrieval algorithm, x-ray nano imaging is now possible. In x-ray coherent diffractive imaging, for example, an object is illuminated with a coherent beam of x-ray light. The light scattered from the object is recorded with a CCD detector. An iterative phase-retrieval algorithm then recovers the 3D image from the scattered-light diffraction pattern. For all practical purposes, the computer algorithm replaces the objective lens of the x-ray microscope!

The Kapteyn/Murnane group used bright 13-nm high-harmonic beams to demonstrate a record spatial resolution (for a tabletop x-ray device) of 22 nm. The group was also able to extract three-dimensional information about the sample from a two-dimensional scatter pattern. Recently, the group used their tabletop x-ray technology to image thick samples in three dimensions. 

Pulses of x-rays are also ideal for directly visualizing how the electrons that bind molecules together adjust as a molecule’s structure changes during chemical reactions or conformational changes; how the motion of a biomolecule relates to its function; how fast magnetic materials can switch state in data storage devices; or how light can be used to coherently interact with, and control, the electronic properties of materials.