Research Highlights

Many physicists use lasers to study quantum mechanics, atomic and molecular physics and nanophysics. While these lasers can be helpful in the research process, there are certain constraints for the researcher. According to JILA Fellow Andreas Becker: "For certain wavelengths of these laser pulses, such as deep ultraviolet, you may not know, or not be able to measure, the temporal profile." The temporal profile of a laser pulse is, however, important for researchers when analyzing data. "A lot of people cannot fully analyze their data, because they don't know the details of the pulse that was used to produce the data," said graduate student Spencer Walker. As a way to research this constraint, the Becker and Jaron-Becker laboratories collaborated to publish a paper in Optics Letters, suggesting a possible solution.

The process of creating spin-polarized electrons has been studied for some time but continues to surprise physicists. These types of electrons have their spin aligned in a specific direction. The probability of creating a spin-polarized electron from an atom tends to be rather small except in some very specific situations. Yet, in a new paper published in Physical Review A, JILA graduate student Spencer Walker, former graduate student Joel Venzke, and former undergraduate student Lucas Kolanz in the Becker Lab theorized a new way towards enhancing this probability through the use of ultrashort laser pulses and an electron’s so-called doorway states. These doorway states are excited states of an electron in an atom that is closest to its lowest energy state, the ground state. 

Follow that electron! JILA researchers have proposed a means of capturing an electron's flight path during ionization, and in doing so, determining the state of the atom at that moment.

The hardest problems are never solved by one person. They are solved by teams; or in the case of science, collaborations. It took a collaboration of 17 researchers, including four JILA fellows and another six JILA affiliates, just a little over five years to achieve robust polarization control over isolated attosecond (one billionth of a billionth of a second) pulses of extreme-ultraviolet light. 

Imagine laser-like x-ray beams that can “see” through materials––all the way into the heart of atoms. Or, envision an exquisitely controlled four-dimensional x-ray microscope that can capture electron motions or watch chemical reactions as they happen. Such exquisite imaging may soon be possible with laser-like x-rays produced on a laboratory optical table. These possibilities have opened up because of new research from the Kapteyn/Murnane group.

The photoelectric effect has been well known since the publication of Albert Einstein’s 1905 paper explaining that quantized particles of light can stimulate the emission of electrons from materials. The nature of this quantum mechanical effect is closely related to the question how much time it might take for an electron to leave a material such as a helium atom.

Mid-infrared (mid-IR) laser light is accomplishing some remarkable things at JILA. This relatively long-wavelength light (2–4 µm), when used to drive a process called high-harmonic generation, can produce bright beams of soft x-rays with all their punch packed into isolated ultrashort bursts. And, all this takes place in a tabletop-size apparatus. The soft x-rays bursts have pulse durations measured in tens to hundreds of attoseconds (10^-18 s).

Many people are familiar with the beautiful harmonies created when two sound waves interfere with each other, producing a periodic and repeating pattern that is music to our ears. In a similar fashion, two interfering x-ray waves may soon make it possible to create the fastest possible strobe light ever made. This strobe light will blink fast enough to allow researchers to study the nuclei of atoms and other incredibly tiny structures. The new strobe light is actually very fast coherent laser-like radiation created by the interference of high-energy x-ray waves.

Former research associate Antonio Picón, research associate Agnieszka Jaron-Becker, and Fellow Andreas Becker have discovered a way to make the hydrogen molecular ion (H₂⁺) fall apart into its constituent atoms without exciting or ionizing the electron. This startling finding was a big surprise for the researchers, who recently figured out how to do something that conventional wisdom said was difficult, if not downright impossible.

Theorists Norio Takemoto (now at the Weizmann Institute of Science) and Fellow Andreas Becker figured that something was amiss when they first analyzed the details of what occurs when an ultrafast laser dislodges an electron from a “simple” molecular ion, H₂⁺. Since H₂⁺ has already lost one of its electrons, its two protons only have one electron left to play with.  How hard would it be to “see” what happened to this electron in a strong laser field? After all, a widely accepted theory said that a strong laser field would make it easier for the lone electron to escape when the ion was stretched apart (as opposed to contracted). 

The dance of electrons as a bromine molecule (Br₂) separates into two atoms is intricate and complex. The process of breaking up takes far longer than expected (~150 vs 85 fs) because the cloud of electrons that bind atoms together in a molecule behaves as if it were still surrounding a molecule until the last possible moment — when the atomic fragments are about twice the normal distance apart (~.55 nm). At this point, there’s simply not enough energy left in the system to hold the molecule together. When the two atoms finally appear as separate entities, it was if someone had snapped a rubber band.

Plucking the two electrons out of helium atoms should allow researchers to study how they interact during a double ionization process — at least in theory. Recently, Fellow Andreas Becker explored whether an ultrashort vacuum ultraviolet (VUV) laser pulse could be used to probe the interactions of helium’s electrons during a double ionization in the presence of an intense infrared (IR) laser field.