Andreas Becker

Creating the “Goldilocks” Zone: Making Special-Shaped Light

Steven Burrows — Thu, 16 Nov 2023 17:54:18 +0000

Creating the “Goldilocks” Zone: Making Special-Shaped Light Steven Burrows Thu, 11/16/2023 - 10:54

Kenna Hughes-Castleberry / JILA Science Communicator

Two different powered polarized lasers combine in the process of High Harmonic Generation. Image credit: Steven Burrows / JILA

In a new study published in Scientific Reports, JILA Fellow and University of Colorado Boulder physics professor Andreas Becker and his team theorized a new method to produce extreme ultraviolet (EUV) and x-ray light with elliptical polarization, a special shape in which the direction of light waves’ oscillation is changing. This method could provide experimentalists with a simple technique to generate such light, which is beneficial for physicists to further understand the interactions between electrons in materials on the quantum level, paving the way for designing better electronic devices such as circuit boards, solar panels, and more.

Many physicists use a process called High-harmonic Generation (HHG) as a source to generate ultrashort EUV and x-ray laser light and use this light to study the ultrafast dynamics of charged particles in different materials. By shooting high-powered laser pulses into a gas of atoms, the researchers can force the atoms to absorb the photons from the laser pulses. This causes the electrons in the atoms to jump to a higher energy level, then fall back to the ground level and emit energy as the atoms radiate in integral multiples of the laser frequency.
JILA graduate student and first author Bejan Ghomashi explained that “these [energies] will be the harmonics. So, if an 800-nanometer light is absorbed, it’s also emitted, along with 400 nanometers, 200 nanometers, etc.”

This process can be conveniently performed within a tabletop laser setup, as pioneered in the laboratories of JILA Fellows Margaret Murnane and Henry Kapteyn. It gives scientists a relatively cost-effective option to learn more about ultrafast electron dynamics.

“More people have access to an idea and can explore it,” Becker added.

Creating Polarization States of Light

Light polarization is a way to describe the direction in which light waves are oscillating. More specifically, polarization describes in which direction the oscillation of the electric field of the light in a laser beam varies over time. For example, the light’s electric field may wiggle along a line, making it linearly polarized. In other cases, the direction of the wiggling electric field may rotate, making the light circularly polarized. Creating light in which the electric field varies along an elliptical shape is a middle-ground between pure linearly and circularly polarized light.

Historically, it has however been challenging to produce elliptically polarized HHG light, but in this new study, Becker and his team explored how to use two linearly cross-polarized lasers at differing frequencies and directions to produce this desired shape. Unlike other, more complex, methods proposed to generate elliptically polarized HHG, an experimental set-up with two cross-polarized laser pulses interacting with an atomic gas is relatively simple.

Sources of elliptically polarized X-ray and EUV light can be useful in helping to study chiral and magnetic materials, as their electrons are sensitive to the direction of applied laser fields. Chiral materials, or materials with a special symmetry, are commonly found in foods and medicines. An example is aspartame sweetener: the left-handed version is sweet, while the right-handed version is not.

Resolving An Odd Puzzle

While previous theories had postulated that it is impossible to create elliptically polarized light using the configuration of two cross-polarized pulses, in 2015, an experimental study produced that exact result. Ghomashi elaborated: “At the time, theoretical physics had no explanation for the ellipticity generated in this experiment and argued it, in fact, should not exist. This was a puzzle to be resolved.”

Intrigued by this discrepancy, Ghomashi, recently graduated JILA Ph.D. student Spencer Walker, and Becker developed a method to analyze the experimental set-up in computer simulations. The results of those simulations produced the same results as found in the 2015 experiment for certain sets of parameters of the two cross-polarized laser pulses.

“You must find what we call the ‘sweet spot’—it is not just one parameter—but you have to tune several parameters simultaneously,” added Ghomashi.

Besides fiddling with the pulse length of the lasers, the researchers also fine-tuned the intensity (or the peak electric fields) of the two laser beams, where one beam was more intense than the other. The result of manipulating these two parameters created a “Goldilocks zone” for producing the rare, elliptically-shaped HHG light.

Walker elaborated that “by reducing the pulse duration, we control the amount of radiation in both [the x and y] directions simultaneously. And if you have emission in both directions at the right energy, you have ellipticity.”

Because of this method’s simplicity, the researchers hope that it will be possible for other physicists to reproduce their results in an experimental setup in order to validate their theoretical interpretation.

“It resolves an odd puzzle in the science community,” Becker stated, “which is always important for scientists and researchers.”

As JILA Fellows Margaret Murnane and Henry Kapteyn develop some of the world's most precise table-top laser setups, testing the team’s concept at JILA would also be possible. “The mechanism, so how to change the knobs and why adjusting the parameters achieves the outcome, is very straightforward,” Walker said. “It's just a matter of the details.”

In a new study published in Scientific Reports, JILA Fellow and University of Colorado Boulder physics professor Andreas Becker and his team theorized a new method to produce extreme ultraviolet (EUV) and x-ray light with elliptical polarization, a special shape in which the direction of light waves’ oscillation is changing. This method could provide experimentalists with a simple technique to generate such light, which is beneficial for physicists to further understand the interactions between electrons in materials on the quantum level, paving the way for designing better electronic devices such as circuit boards, solar panels, and more.

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JILA’s Physics Frontiers (PFC) is Awarded a $25 Million Grant by the National Science Foundation (NSF)

Steven Burrows — Tue, 12 Sep 2023 21:31:40 +0000

JILA’s Physics Frontiers (PFC) is Awarded a $25 Million Grant by the National Science Foundation (NSF) Steven Burrows Tue, 09/12/2023 - 15:31

Kenna Hughes-Castleberry / JILA Science Communicator

A compilation of researchers and the research/outreach led by JILA's PFC

This science center brings together 20 researchers across JILA to collaborate to realize precise measurements and cutting-edge manipulations to harness increasingly complex quantum systems. Since its establishment in 2006, the JILA PFC’s dedication to advancing quantum research and educating the next generation of scientists has helped it to stand out as the heart of JILA’s excellence.

Origins and Foundation:

It was JILA Fellow Carl Lineberger who initially conceived the PFC. Arriving at JILA in 1968 as a postdoctoral researcher for JILA Fellow and founder Lewis Branscomb, Lineberger witnessed many changes happen at JILA throughout its decades of science. In the early 1970s, as Branscomb was looking to leave JILA, Lineberger realized that Branscomb’s departure could lead to funding constraints for JILA.

“It was really only Lou and me who knew how to get money for JILA at that time, as we both were the only ones with the strongest links to the Department of Defense (DoD),” Lineberger stated, referring to his own service in the military before arriving at JILA. “We figured that the DoD was the best place to look, as this was during the Vietnam War. The state of Colorado was in severe financial trouble, and they could not help JILA, so we had to get money outside of the university. And I knew all the people in defense and the National Science Foundation who were important for funding where no one else did.”

In the wake of Branscomb’s departure in 1972, Lineberger began thinking about leveraging his network to secure JILA funding.

It wasn’t until the mid-1970s when Lineberger led the effort to draft the first group grant for many quantum researchers within JILA, as JILA’s astrophysicists had already secured funding. This grant began a new era in research at JILA, allowing scientists to push the boundaries of knowledge and explore uncharted territories in physics.

After proposing an extensive collaboration between several JILA scientists, the team submitted their application. Then they waited nervously, as group grants were highly unusual during the 1970s, and the scientists weren’t sure if the NSF would accept it. In fact, the NSF funded this initial group grant and would continue to renew JILA’s funding till the early 2000s when the NSF decided to restructure the group grant altogether.

“It’s tough for the NSF to compare a group grant to an individual scientist’s work,” explained JILA and NIST Fellow Eric Cornell, a Nobel Laureate who served as the PFC Director for over a decade. “You can’t really compare the two applications.”

The NSF decided to institute a new grant type to overcome this challenge, as other institutes also submitted group applications. In 2001, several PFCs were established with the NSF’s new grant structure. However, it wouldn’t be until 2006 that JILA’s group grant was transformed into an official PFC. “It was 50% luck and 50% opportunity,” added Lineberger.

The vision behind the PFC was to bring together researchers from diverse backgrounds to collaborate on projects that transcend traditional disciplinary boundaries. “Carl Lineberger took very seriously the idea that JILA should always be renewing itself,” Cornell added. “What that meant is it shouldn’t always be the same people always running the show.”

To implement this thought, Lineberger transitioned out of the role as the first PFC Director and passed the torch to Cornell, who became the next director in the mid-2000s.

Furthering in this spirit, Cornell just recently handed over the torch to current co-directors Ana Maria Rey and Andreas Becker. The co-directors, together with, JILA Fellows Eric Cornell, Cindy Regal, Jun Ye, and Heather Lewandowski form the executive committee that will lead and manage the Center for the next six years.

The Structure of the PFC:

While the PFC includes about 20 JILA researchers, it is led by a much smaller executive committee. “We sometimes call it an oligarchy,” stated Cornell. “As the executive committee decides things by consensus, the Director is not especially important. However, the NSF does need a point of contact for the grant, so the Director does play a role in government relations.”

Interdisciplinary Collaboration:

One of the distinguishing features of the PFC is its commitment to fostering interdisciplinary collaboration. By bringing together physicists, chemists, biologists, and other scientific experts, the PFC enables a unique environment for innovation and cross-pollination of ideas. The center encourages researchers to step outside their comfort zones and tackle complex scientific challenges from multiple perspectives, leading to breakthrough discoveries that would be difficult to achieve in isolation.

“The JILA PFC, in my point of view, is the spinal cord of JILA,” explained Rey. “The reason is that the Center serves as a connecting tissue among JILA investigators with different but complementary research interests. We all understand the added value of the Center and are excited about the scientific barriers we can overcome as a team. We are willing to take risks and commit to very challenging problems that have long-term horizons which are only possible by the joint and synergistic capabilities of the investigators.”

Milestones and Breakthroughs:

Over the years, the PFC and JILA’s group grant before it, have embarked on numerous research projects that have pushed the boundaries of physics. From exploring the properties of ultracold molecules to developing advanced precision measurement techniques, the PFC has consistently been at the forefront of pioneering research. Researchers at the center have significantly contributed to areas such as quantum information science, atomic and molecular physics, quantum optics, ultrafast science, and condensed matter physics.

The PFC has achieved several significant milestones and breakthroughs throughout its history. In ultracold physics, JILA Fellows, including Cornell and Carl Wieman, won the Nobel Prize in Physics in 2001 for creating the first Bose-Einstein Condensate—a remarkable state of matter with extraordinary properties. This groundbreaking achievement opened up new avenues for exploring quantum phenomena and laid the foundation for subsequent research in ultracold physics.

Another notable milestone came in 2008 when the PFC researchers developed an atomic clock that was accurate to within one second every 300 million years. This achievement revolutionized timekeeping technology and led to advancements in global positioning systems (GPS), telecommunication networks, and fundamental tests of the laws of physics.

That same year PFC investigators Deborah Jin, Jun Ye, and John Bohn, with input from David Nesbitt, prepared the first high-space-density KRb molecular gas, by combining trapping and cooling methods with frequency comb spectroscopy. This development set the stage for impressive investigations on quantum chemistry and many-body physics which are currently generating even richer and faster worldwide developments.

The PFC has also made significant strides in quantum information science. In 2017, JILA scientists successfully created a long-lived quantum memory for photons, a crucial step towards developing quantum computers and secure quantum communication networks. These advancements have the potential to revolutionize computing and information processing, opening up a new era of technology.

Furthermore, the PFC helped to push forward many new ideas in the development of ultrafast lasers, a technology used collaboratively in many PFC labs. Most recently, the path towards polarization control of ultrashort laser light pulses over a broad wavelength regime, led by PFC investigators Margaret Murnane and Henry Kapteyn, was supported using PFC funds.

“As the years passed,” Cornell explained, “the amount of money given by the NSF for the PFC got smaller and smaller due to inflation.” However, the slack in funding was taken up by individual grants for each scientist. While this group grant once was a more significant source of JILA’s funding, it has now become less so as other organizations, such as the Department of Energy, fund JILA.

“While the money is useful, the PFC has become greater than the sum of its parts,” Cornell stated. “It’s much more of a way to keep us thinking about research collaborations and to wish each other well in our projects. It’s about making it a place that good students want to come to and good staff wants to stay at.”

For Rey and Becker, the feeling is similar. “We are nevertheless excited and proud to report that in this re-competition, in contrast to prior ones, NSF provides an increase of the JILA PFC budget,” said Rey. “This is exciting and will allow us to attract an even larger poll of fantastic and productive students and postdocs and undertake broader outreach activities that will benefit our community.”

The PFC’s Influence on the JILA Community

When examining how the PFC has impacted JILA’s community and culture, JILA’s Chief Operations Officer Beth Kroger agreed with Cornell. “The NSF PFC funding enables JILA to provide critical infrastructure in support of the transformational research done at JILA,” she stated. “A key component of JILA’s infrastructure is the JILA Shops which include Scientific Instrument Design/Fabrication, Electronics Design/Fabrication and Computing, as well user facilities such as our Metrology Lab, Clean Room, and student workshops. The JILA Shops are instrumental in advancing research and providing mentoring and hands-on applied learning for scientists-in-training. This is just one example of the impact of the PFC.”

Educating Future Leaders

The PFC's contributions to the field of physics extend beyond groundbreaking discoveries. It has nurtured generations of scientists, providing an environment fostering creativity, collaboration, and scientific excellence. The center has trained numerous graduate students and postdoctoral researchers, equipping them with the knowledge and skills to make a lasting impact in their respective fields.

“During the next PFC grant period we plan to initiate new training and mentoring programs at JILA which should further help our graduate students and postdocs in preparing them for their future careers in academia and industry”, said Becker.

Furthermore, a key part of the PFC has been its outreach program, PISEC, or “Partnerships for Informal Science Education in the Community”. A semester-long afterschool program where CU volunteers work with K-12 students on inquiry-based physics experiments. It is mainly targeted to students from underrepresented groups in STEM: primarily Hispanic/Latinx with low income. The goal is to cultivate in the students involved an interest in science, and facilitate pathways into STEM degrees.

PISEC is a very important part of the JILA-PFC. Jessica Hoehn is the current full-time PFC director for public engagement. She in collaboration with executive member Heather Lewandowski and Eric Cornell are envisioning exciting new directions in which the PISEC can further expand and become even better during this funding period.

Thanks to the $25 million grant awarded to JILA’s PFC, its vision and ongoing projects can continue to push the boundaries of quantum science and influence JILA’s culture, community, students, and postdoctoral researchers.

The JILA Physics Frontiers Center (PFC), an NSF-funded science center within JILA (a world-leading physics research institute), has recently been awarded a $25 million grant after a re-competition process.

This science center brings together 20 researchers across JILA to collaborate to realize precise measurements and cutting-edge manipulations to harness increasingly complex quantum systems. Since its establishment in 2006, the JILA PFC’s dedication to advancing quantum research and educating the next generation of scientists has helped it to stand out as the heart of JILA’s excellence.

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JILA Fellow Andreas Becker is awarded an Optica Fellowship

Steven Burrows — Wed, 09 Nov 2022 18:58:41 +0000

JILA Fellow Andreas Becker is awarded an Optica Fellowship Steven Burrows Wed, 11/09/2022 - 11:58

Kenna Hughes-Castleberry / JILA Science Communicator

JILA Fellow and ��Ҵ�ý�ƽ�� Distinguished Professor Andreas Becker

JILA Fellow and University of Colorado Boulder Distinguished Professor Andreas Becker has been awarded a 2023 fellowship to Optica (formerly the Optical Society of America). Becker's work at JILA focuses on the analysis and simulation of ultrafast phenomena in atoms, molecules, and clusters, in particular attosecond electron dynamics, coherent control, and molecular imaging. Using special laser frequencies, Becker and his team are able to study the dynamics of these atoms and molecules in different time scales.

Becker was one of 109 scientists selected as fellows from 24 different countries. “I am pleased to welcome the new Optica Fellows,” said Satoshi Kawata 2022 Optica President in a recent announcement from Optica. “These members join a distinguished group of leaders who are helping to advance the field optics and photonics. Congratulations to the 2023 Class.” Becker was cited "For outstanding contributions to our understanding of intense laser-atom interactions" for his new fellowship. Congratulations!

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JILA Fellow Andreas Becker is awarded CU Distinguished Faculty Title

Steven Burrows — Fri, 05 Nov 2021 20:12:35 +0000

JILA Fellow Andreas Becker is awarded CU Distinguished Faculty Title Steven Burrows Fri, 11/05/2021 - 14:12

Kenna Hughes-Castleberry / JILA Science Communicator

JILA Fellow Andreas Becker is one of the 11 University of Colorado Boulder faculty to be awarded a 2021 Distinguished Professor title. CU Distinguished Professors are tenured faculty members who give outstanding work in research or creative work and have a reputation of excellence in promoting learning and student engagement in the research process as well as dedicated to the profession, the university, and its affiliates. This year’s honorees will be formally recognized during a board meeting in spring 2022. Only 129 professors at ��Ҵ�ý�ƽ�� have been given this title since the title’s inception in 1977.

According to a CU Connections article, Becker is one of the best and most versatile physics professors at the University of Colorado. He is highly regarded and has been named a “Favorite Professor” by the Physics Honor Society four times. Becker has been both a leader in the Department of Physics and at JILA, and is a highly successful graduate student mentor over the last decade.

You can read more about the Distinguished Professor Title at the link here.

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Reconstructing Laser Pulses

Steven Burrows — Mon, 19 Jul 2021 18:53:13 +0000

Reconstructing Laser Pulses Steven Burrows Mon, 07/19/2021 - 12:53

Kenna Hughes-Castleberry / JILA Science Communicator

Representation of the temporal profile of a laser used in the Becker Lab. Image credit: Steven Burrows

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.

With a Little Help From Chemistry

When looking for a possible solution to solving the temporal profile problems at certain wavelengths, Spencer Walker and former graduate student Ran Brynn Reiff turned to simulations, statistics, and other research areas. Becker explained: "it was so essential to have Brynn Reiff there. He was simulating these kinds of pulses for his whole Ph.D., and from his work we could simulate how these pulses look without using experiment." Key in their studies was to use the ionization signal produced by duplicates of the unknown simulated pulse, which were delayed in time relative to each other–a method called autocorrelation. The process of ionization happens when an atom or molecule acquires a positive charge by losing electrons when interacting with the laser pulses.

In their calculations, the team compared the ionization signal from the unknown pulse to those calculated from theoretical temporal profiles using Gaussian functions. Gaussian functions have many applications, for example, in statistics they are used as the density function of the normal distribution of data. First author Spencer Walker applied these Gaussian functions in a particular way to the simulations in order to suggest theoretical temporal profiles of laser pulses. "I really just got the idea from chemistry," Walker stated. "Chemists represent the quantum mechanical electronic wave function using Gaussian functions. We took the electric field of the laser and decided to expand it using Gaussian functions as well. And since for the ionization probability we had analytic formulas when using Gaussian functions, we were able to do calculations much faster than if we had to do the entire problem numerically." From the Gaussian functions and the ionization signal via the electric field, the team was able to reconstruct the temporal profiles of the unknown pulses.

The Next Steps:

After the team identified a new way to reconstruct temporal profiles of unknown laser pulses, the next steps would be to turn to their experimental colleagues. "We're looking forward to someone testing our theory with an experiment," Becker added. Time will reveal if their theory will help make other research easier. "Basically, just knowing what the pulse actually looks like is very important for a lot of applications," Reiff explained. "It's important for all sorts of very precise sensing and probing applications, where if you know exactly what your pulse looks like, then you can shine your laser on something that you don't know, and get a lot of information on the unknown object from knowing about your laser pulse. The more you know about your pulse, the more control you have over the pulse, the more you can do with it." As the JILA theorists continue looking into methods to measure temporal profiles, their research will have a positive impact on other research using lasers to study physics and other subjects.

This research was supported by a MURI grant and the NSF.

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The Atomic Trampoline

Steven Burrows — Fri, 02 Jul 2021 18:55:20 +0000

The Atomic Trampoline Steven Burrows Fri, 07/02/2021 - 12:55

Kenna Hughes-Castleberry / JILA Science Communicator

A model of the counterrotating electrons taking doorway states. Image credit: Steven Burrows / JILA

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.

In their work, the Becker Lab did not directly study spin-polarized electrons but investigated an important step towards producing such electrons via these doorway states. Atoms have a set of electrons that rotate in one direction around the atom, while another set of electrons rotate in the opposite direction: the co-rotating and counter-rotating electrons. The Becker Lab used a circularly polarized laser pulse, in which the electric field rotates in space, to initiate ionization in these electrons, a process that causes them to leave the atom. And, they found that doorway states can specifically enhance the probability to ionize electrons, that rotate opposite to the rotation direction of the electric field of the laser

The ionization process is different for the counter-rotating electrons as compared with the co-rotating electrons. According to JILA fellow Andreas Becker: "The counter-rotating electrons can use excited states as a trampoline and can jump out with a higher probability. For certain wavelengths of the laser these doorway states can only be accessed by the counter-rotating electrons, but not by the co-rotating electrons. So, the trampoline can only be used by the counter-rotating electrons."

Jumping with Computer Codes

To study this further, the team used computer codes to determine what was happening. In describing the process, Spencer Walker explained: "We used two codes we wrote: a grid code and a basis code. For the basis code, we diagonalize the finite-difference Hamiltonian for argon and neon, the two atoms we studied, and then we write down the dipole interaction and wave function in energy basis for time propagation." In quantum mechanics, the Hamiltonian is an operator describing the interactions of the electrons in the atom and with the laser field, here being the counter- and the co-rotating electrons. From their work, the Becker Lab could show that since counter-rotating electrons are the only ones able to access the doorway states, their ionization probability can be 10 times greater than that of their co-rotating electron counterparts. This large difference was surprising to the whole team. "You come in with a theoretical prediction, so you have a picture in your mind." Becker said, "And all four of us working on this thought that this specificity of doorway states was a really cool effect, but we did not know that it could be so significant. This is one of the few times I've had in my career where you've had the idea before and the effect comes out so large. We were expecting maybe a factor or two, but now it's a factor of 10 or so."

Since the enhancement in ionizing a specific set of electrons---here those counter-rotating with respect to the laser field---is an important step towards generating spin-polarized electrons, both Walker and Becker believe that their proposed method would make this process more efficient. "In these laser pulses that we are studying theoretically, you create electrons in very short pulses,” Becker clarified. “These short pulses can allow you to do pump-probe experiments. And this may be opening an alternative pathway toward probing magnetic materials on ultrafast timescales." Becker and Walker both look forward to other physicists expanding on their work to produce spin-polarized electrons.

This work was supported primarily by the U.S. Department of Energy

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.

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Measuring Spinning Donuts

Steven Burrows — Wed, 04 Nov 2020 17:36:51 +0000

Measuring Spinning Donuts Steven Burrows Wed, 11/04/2020 - 10:36

Rebecca Jacobson / NIST Public Outreach Coordinator

During ionization, electrons leave an atom on varying flight paths. By capturing those flight paths, the Becker Group at JILA can determine the state of the atom at that moment. Image credit: Steven Burrows / JILA

Atoms are busy objects. Electrons whiz around the nucleus of the atom in attoseconds—quintillionths of a second. Those electrons can be orbiting farther out from the nucleus in an excited state, close to the nucleus in the lowest energy level called a ground state, or in a superposition—in two or more energy levels at once.

During ionization, some of those electrons will fly away from the atom. The direction and path those electrons take can tell scientists a lot about the state the atom was in before—ground state, excited state, or superposition.

“We want tools that can identify if it’s this particular state or that particular state. That’s important in the ionization process,” said Joel Venzke, a graduate student at JILA. “[We] would like to know exactly what that superposition is, and what is the relative phase between these two states…we want to follow this motion and be able to write down the wave function at a certain point in time.”

Venzke has developed a set of tools to do this using the time-dependent Schrödinger equation and ultrashort laser pulses to capture the path of the electron and the state of the atom during ionization. The team’s results were published in Nature Scientific Reports.

Asymmetry in motion

The way an electron leaves an atom during ionization can tell scientists the state of the atom at the time. JILA’s researchers have used two key tools to track the electron’s escape: attosecond (10^-18 second) laser pulses and numerical solutions to the Schrödinger equation—a differential equation which describes the evolution of the wave function of a quantum system in time. In this scheme, the electron is stripped from the atom by the attosecond laser pulse, and the rapid laser pulses take snapshot images of the atomic state and the ionization process, by capturing the directions in which the electrons fly.

“If it was in the ground state, the electron wave function is essentially distributed in this ball [around the nucleus]. If it’s in its excited state, it’s in a donut. If it’s in the superposition state, it is moving around,” said JILA Fellow Andreas Becker.

“We're interested in how does that shape break down from being either this perfect donut or perfect ball into this kind of asymmetric shape [of the ionized electrons],” Venzke added.

Using the Schrödinger equation, scientists can determine the electrons’ “flight paths” from the symmetrical or asymmetrical shapes that illustrate the directions in which the electrons are leaving the atom. Venzke, with some guidance from JILA Fellows Becker and Agnieszka Jaron-Becker, took the lead and developed generalized asymmetry parameters (GAPs) to quantify these uneven, asymmetrical distributions—which tells them valuable new information about how those electrons left the atom.

“The asymmetry tells us something about which of the possible pathways is dominant, or if they are interfering,” Becker said. “It tells us how the electron was actually removed from the atom and which pathway did it take, or was there more than one pathway involved in it.”

Studying and quantifying atoms in their excited or superposition states is really important in physics, Venzke said. And attosecond laser pulses are useful to capture and follow the motion of the electrons in these states. With this insight, scientists can better understand what happens during interactions between atoms—perhaps even capturing videos of those interactions—or whether electrons are exchanging energy and what energy levels they are occupying.

More importantly, if scientists can see what happens in these interactions, there’s the possibility that they can manipulate those interactions—although that’s still some way off, Becker added.

“It’s a small piece of the puzzle,” Becker said. “First, we have to understand. Then, once we understand, we can try to control it…there are many steps in between. But it’s part of the process.”

This study was published in Nature Scientific Reports on September 30, 2020. It was supported by a National Science Foundation Physics Frontier Center grant and the U.S. Department of Energy, Division of Chemical Sciences, Atomic, Molecular and Optical Sciences Program.

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.

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Three JILA Fellows named 2018 APS Fellows

Steven Burrows — Wed, 03 Oct 2018 19:53:29 +0000

Three JILA Fellows named 2018 APS Fellows Steven Burrows Wed, 10/03/2018 - 13:53

Catherine Klauss / JILA Science Communicator

Andreas Becker was nominated by the APS Division of Atomic, Molecular & Optical physics for his contributions to the understanding of the behavior of atoms and molecules in intense light fields, including seminal theoretical studies of attosecond dynamics, photoionization, complex electron dynamics in simple systems such as H2, and a better understanding of high-harmonic generation.

Heather J. Lewandowski was nominated by the APS Forum on Education for her pioneering and comprehensive research on, and leading development of resources for, teaching and learning in advanced physics instructional lab courses.

James K. Thompson was nominated by the APS Topical Precision Measurements & Fundamental Constants for his development of precision measurement techniques, in particular for atomic mass and for measurements with atomic ensembles beyond the standard quantum limit.

Each year, no more than one half of one percent of the Society’s membership (excluding student members) is recognized by their peers for election to the status of Fellow of the American Physical Society. For 2018, APS selected only 155 Fellows. Of these new fellows, 23 percent are women, which is a 77 percent increase in the fraction of women in the 2017 Fellows class.

The APS Fellowship Program was created to recognize members who may have made advances in physics through original research and publication, or made significant innovative contributions in the application of physics to science and technology. They may also have made significant contributions to the teaching of physics or service and participation in the activities of the Society.

Since 1965, JILA has seen 23 of its own Fellows named APS Fellows.

Three JILA Fellows have been named 2018 Fellows of the American Physical Society. The three new Fellows—Andreas Becker, Heather J. Lewandowski, and James K. Thompson—were nominated from varying divisions of APS. Andreas Becker was nominated by the APS Division of Atomic, Molecular & Optical physics for his contributions to the understanding of the behavior of atoms and molecules in intense light fields, including seminal theoretical studies of attosecond dynamics, photoionization, complex electron dynamics in simple systems such as H2, and a better understanding of high-harmonic generation.

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An Ultrafast Photoelectric Adventure

Steven Burrows — Mon, 02 Mar 2015 20:28:19 +0000

An Ultrafast Photoelectric Adventure Steven Burrows Mon, 03/02/2015 - 13:28

Julie Phillips / Science Communicator

During the photoelectric effect in a helium atom, a nonresonant electron leaves the atom much faster than an electron first pushed into resonance by an attosecond photon and then all the way out of the atom by a second photon. Theoretical work shows that the time delay can be captured by an ultrafast streaking camera, in which the variation of a photoelectron's energy with time is measured by a combination of two different laser pulses. Image credit: Steven Burrows / JILA

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. The exciting news at JILA is that the Ultrafast AMO Theory Group has come up with a clever way that may help to answer this question by observing a photoelectron on its way out of, but still inside, an atom.

The theorists show how a combination of attosecond (10-18 s) and femtosecond (10-15 s) laser pulses could be used in the laboratory to follow the electrons inside a helium atom on an ultrafast time scale. Such an experiment would open the door to observations of the behavior of electrons inside different atoms and molecules during the photoelectric effect. This seminal work appeared in an article published online December 24, 2014, in Physical Review Letters.

The researchers responsible for proposing the use of ultrafast laser pulses to really see something happening inside an atom include recently minted JILA Ph.D.s Jing Su and Hongcheng Ni as well as Fellows Agnieszka Jaron-Becker and Andreas Becker.

Their secret was to use a streaking camera, in which the variation of the photoelectron’s energy with time is measured by the combination of two different laser pulses. First, one or two photons from the attosecond laser pulse kick the electron out of its ground state inside the atom. Then, the photoelectron interacts and oscillates in the electric field of the second longer femtosecond laser pulse. The femtosecond laser field changes the energy of the electron depending on the time for the photoelectric effect to happen. This allows the researchers to probe the electron’s behavior inside the atom (or molecule) during the photoelectric effect.

For example, if an attosecond photon kicks the electron first into resonance with one of the higher energy states in the atom, then the electron hangs out a while in the excited state before a second photon finally pushes the electron all the way out of the atom. The femtosecond laser pulse in the streaking camera measures the time the electron takes to move through and leave the atom.

In contrast, if the photon of the attosecond laser doesn’t kick a helium electron into resonance, the electron is immediately pushed out of the atom. The femtosecond pulse instantaneously captures this behavior. In this way, the streaking camera may make it possible for researchers to observe and follow the kicking of an electron into a resonance state in real time. They may also be able to answer the question of how much it prolongs the photoelectric process inside the atom.

Jaron-Becker, Becker, and their colleagues have opened the door to watching the inner workings of an atom during the photoelectric absorption and emission process. Now they have to wait and see if experimentalists can actually accomplish this feat in the laboratory. In the meantime, the Ultrafast AMO Theory Group is beginning work on using a streaking camera to peer inside other atoms and molecules. Stay tuned.

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The Long and the Short of Soft X-rays

Steven Burrows — Tue, 27 May 2014 19:35:20 +0000

The Long and the Short of Soft X-rays Steven Burrows Tue, 05/27/2014 - 13:35

Julie Phillips / Science Communicator

Long-wavelength mid-infrared light interacting with argon atoms in a process known as high-harmonic generation leads to the production of isolated bursts of ultrashort-pulse soft x-rays, which are tens to hundreds of attoseconds (10^-18 s) long. Credit: Steve Burrows, JILA

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).

Until now, attosecond pulses were limited to the extreme ultraviolet (XUV) region of the spectrum. However, these XUV attosecond pulses don’t penetrate most materials, liquids, and complex molecular systems. In contrast, soft x-ray attosecond pulses can penetrate many materials and liquids. Hence they promise to expand the field of attosecond science. 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.

A report of the generation of the first attosecond soft x-ray bursts appeared online in theProceedings of the National Academy of Sciences, USA, May 21, 2014. The international team of researchers responsible for this feat included Ming-Chang Chen (National Tsing Hua University), graduate students Chris Mancuso, Ben Galloway, and Dimitar Popmintchev, research associates Carlos Hernández-Garcia and Franklin Dollar, as well as Pei-Chi Huang (National Tsing Hua University), Barry Walker (University of Delaware), Luis Plaja (Universidad de Salamanca), Fellows Andreas Becker, Margaret Murnane, and Henry Kapteyn, and senior research associates Agnieszka Jaron-Becker and Tenio Popmintchev.

Theorists and experimentalists working together hand in hand were able to learn to create bursts of attosecond soft x-rays via high harmonic generation, which has been used extensively and effectively by the Kapteyn/Murnane group. In high harmonic generation, x-rays are produced when electrons are first plucked from argon atoms by a mid-IR laser and then smashed back into their parent ions when the oscillating field of the laser reverses (like the motion of a boomerang). The atoms then naturally emit their excess energy as isolated bursts of soft x-rays. (These harmonics of laser light are like the audible overtones that you can hear when a piano key or guitar string is struck hard.)

The fact that the high harmonics emerged as isolated attosecond bursts of soft x-rays was a beautiful confirmation of theory work that suggested that making attosecond pulses in the soft x-ray region might be possible. But, until the experimental physicists could actually measure the pulses, this theory could not be tested. However, 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.

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