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The Topolariton, a New Half-Matter, Half-Light Particle

Caltech scientist theorizes a new quasiparticle with unique characteristics

A new type of "quasiparticle" theorized by Caltech's Gil Refael, a professor of theoretical physics and condensed matter theory, could help improve the efficiency of a wide range of photonic devices—technologies, such as optical amplifiers, solar photovoltaic cells, and even barcode scanners, which create, manipulate, or detect light.

Electrons traveling through the semiconductors used in modern computers lose energy via heat because of resistance. This is not the case with light signals, but there can be other causes of signal loss in light transmission, such as unwanted reflection and scattering of photons, or light particles. Refael says that a type of quasiparticle called the "topolariton" could reduce such signal degradation and enhance the stability of the photons as they move along the edges of semiconductors. He described topolaritons and their properties in a paper published in the July 2015 issue of the journal Physical Review X.

Refael's work at Caltech concentrates on quantum aspects of matter, including quantum entanglement (in which quantum particles share behaviors regardless of distance), quantum computing, and the emergence and control of new quantum states.

Quasiparticles such as the topolariton are entities that exhibit some, but not all, of the characteristics of elementary particles like the electrons and quarks that make up atoms. They are classified as emergent phenomena—those that arise from the dynamical behavior of a system—and they exist only inside solid materials. Some examples of quasiparticles include phonons ("packets" of atoms or molecules vibrating collectively), solitons (solitary wave packets or pulses that retain their shape while moving at a constant velocity), and excitons (formed when an electron binds with an electron hole, or the void left when an electron departs a valence orbital). The interaction of photons with excitons can form another quasiparticle called the polariton.

During a workshop at the National Science Foundation (NSF)-sponsored Institute for Quantum Information (now the Institute for Quantum Information and Matter) in 2010, Refael and two colleagues came up with the idea for the topolariton, a type of polariton with the ability to flow in one direction along the edges of semiconducting quantum wells embedded in optical cavities. The quantum well and the cavity confine electrons and photons to motion within a single plane. "We suggested that you could take a simple semiconductor and a regular quantum well, and give rise to a special excitation, which is a hybrid of a photon and an electron-hole pair," that is, of a polariton. "Shining light at the frequency can kick an electron out of the balance and initiate a polariton that travels exclusively on the edge of the system. The light–matter interaction, in this case, produces so-called topological quantum states that are not there in each of the components."

Because topolaritons are part matter and part light, they could be guided and controlled with reflectors or with photonic band-gaps—regions in the optical medium through which photons cannot travel. Furthermore, the direction of a topolariton's motion could be reversed by the application of a magnetic field. "This would be like a one-way filter for light, providing a directional communication with minimum losses of energy," he says.

Photonic devices are currently in widespread use, and are expected to eventually replace traditional semiconductors in many applications. They are more efficient and accurate, work better over long distances, and save energy. They are also less prone to interference from outside influences such as electromagnetic fields. Exploiting toploaritons into such devices should lead to further improvements in their performance.

Evolving from theory to practical applications, however, may be a rather long process, Refael admits. "We'll need to create some new interfaces between the photonic world and the electronic world," he says. "One challenge is making one-way photon wave-guides for visible light. The topolaritons provide a route to such devices using standard semi-conductor technology, and can also act as an intermediary between photonic and electron-based devices—a necessary step for any optoelectronic device."

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Summer Interns Return with a World of Experiences

Caltech undergraduate students returned to campus this week, many after spending the summer working at companies in biotechnology, technology, and finance, among other fields. These students have had the opportunity to learn firsthand about the career opportunities and paths that may be available to them after graduation. They also had the chance to put Caltech's rigorous academic and problem-solving training to the test.

In the summer of 2015, nearly a third of returning sophomores, juniors, and seniors were placed in an internship position through Caltech's Summer Undergraduate Internship Program (SUIP). The program, run through the Institute's Career Development Center (CDC), helps connect current undergraduate students with a wide range of companies and businesses that can provide practical skills and work experiences that give the students an edge in the future job market.

Many undergraduates find paid summer internships through the CDC, says Lauren Stolper, the director of fellowships, advising, study abroad, and the CDC. The center organizes fall and winter career fairs and offers workshops related to finding internships; provides individual advising on internship options and conducting a job hunt for an internship; organizes interviews for students through its on-campus recruiting program; and provides web-based internship listings and company information through Techerlink, its online job-posting system.

Through the formal establishment of SUIP two years ago—thanks, in part, to the initiative of Craig SanPietro (BS '68, engineering; MS '69, mechanical engineering) and with seed money provided by him and three of his alumni friends and former Dabney House roommates, Peter Cross (BS '68, engineering), Eric Garen (BS '68, engineering), and Charles Zeller (BS '68, engineering)—the CDC has been able to dedicate even more time and attention to helping undergraduates secure these important positions, Stolper says.

"Through internships, students have the opportunity to learn more about the practical applications of their knowledge by contributing to ongoing projects under the guidance of professionals," says Aneesha Akram, a career counselor for internship development/advising, who oversees SUIP.

"Completing summer internships help undergraduates become competitive candidates for full-time positions," says Akram. "When it comes to recruiting for full-time positions, companies seek out candidates with previous internship experience. We have found that many large companies extend return offers and full-time conversions to students who previously interned with them."

The infographic at the above right provides a snapshot of Caltech undergraduate internships over this past summer. Students seeking internships for next summer can contact Akram or look at the CDC website for more information.

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NSF Supports Caltech-Led Global Project to Study Cosmic Flashes

GROWTH network aims to keep astronomers and telescopes unbeaten by sunrise

An international project led by Caltech astrophysicist Mansi M. Kasliwal has been selected to receive $4.5 million over five years by the National Science Foundation through its Partnership for International Research and Education (PIRE) program. The project aims to improve our understanding of cosmic transients—extremely bright flashes of light that suddenly appear in the night sky, shining like new stars, a million to a billion times brighter than the sun, and then quickly fade away.

The project, dubbed Global Relay of Observatories Watching Transients Happen (GROWTH), is a collaboration among six universities in the United States and six abroad. It formally establishes a network of telescopes at longitudes throughout North America, Asia, and Europe (see illustration at right) to extend the hours of night-time observing, enabling researchers to continue monitoring cosmic transients that might otherwise disappear before the next night in a single location.

"There are many questions in astrophysics that depend on these hour timescales, where all the action happens in that first night," says Kasliwal, a new assistant professor of astronomy at Caltech. "So we aim to stay unbeaten by sunrise."

This is the fifth round of PIRE funding since the program started in 2005 with the goal of supporting innovative, high-quality projects in which advances in research and education could not occur without international collaboration.

"In astronomy, where international collaboration is the norm, the PIRE award provides the resources for students and postdocs to interact closely with international partners through extended visits and internships. Such experiences will be invaluable in their future scientific careers," says Tom Prince, an investigator on the GROWTH project, professor of physics and director of the W.M. Keck Institute for Space Studies at Caltech, and senior research scientist at JPL.

Many GROWTH observations will begin with a transient candidate discovered in the data stream generated every night by the Palomar Transient Factory (PTF), a fully automated, wide-field survey systematically searching for such flashes using a camera mounted on the 48-inch Samuel Oschin Telescope at Palomar Observatory.

"GROWTH is extremely timely. By 2017, we will have commissioned the Zwicky Transient Facility, or ZTF, at Palomar, which will be an order of magnitude more sensitive than PTF. GROWTH follow-up of these transients should result in spectacular science," says Shri Kulkarni, an investigator on the GROWTH project, principal investigator on PTF and ZTF, the John D. and Catherine T. MacArthur Professor of Astronomy and Planetary Science, and director of the Caltech Optical Observatories.

If a transient candidate looks promising, Kasliwal might trigger larger optical telescopes like the Gemini North telescope or the W. M. Keck Observatory, both in Hawaii, gaining three additional hours of darkness. After collecting data there, the team might contact members of the network in Japan or Taiwan, then India, Israel, Sweden, and Germany. "We just go around the globe and keep passing the baton so that the sky remains dark," says Kasliwal.

While supernovae—explosions related to the collapse of massive stars—typically fade over months, some cosmic transients, especially rarer events such as the merger of two extremely dense stellar remnants called neutron stars, or the merger of a neutron star and a black hole, are believed to disappear in a matter of hours—a day at most. Some of these more exotic transients are thought to be the source of heavy elements, such as gold and platinum. But no one has seen, in action, the process that creates them. "None of the explosions that we've found so far has been extreme enough to actually synthesize enough heavy elements," says Kasliwal. "As you try to find these rarer and rarer events, you need to be able to respond quickly because the flash of light they produce is even more short-lived, and that's where GROWTH becomes necessary."

Even being able to observe supernovae within a few hours of the blast yields important information that is lost if observations are completed later. That is because after the initial burst flash-ionizes everything, a shock wave follows, sweeping up all the surrounding material. And with that material goes a lot of data about the progenitor, the star that exploded.

"If you respond quickly enough to a young supernova, you can get direct clues about the progenitor system," explains Kasliwal. "With our network, the most common events that we will follow up are very young supernovae that are within a few hours of explosion. For the first time, we're seeing certain ionization lines, which tell us what sort of star it was that just died and gave rise to this particular supernova."

Beyond rapid response, the GROWTH network is also able to use ground- and space-based telescopes to observe cosmic transients at X-ray, ultraviolet, infrared, optical, and radio wavelengths. "Within minutes to hours, we will be able to get a panchromatic picture," says Kasliwal.

That will be key for the network's coordination with Advanced LIGO (aLIGO). That project, led by Caltech and MIT, is searching for gravitational waves—ripples in the fabric of space and time that are predicted to accompany violent events in the universe such as the merger of a neutron star with a black hole. PTF will receive notifications of events that aLIGO identifies as possible sources of gravitational waves. But aLIGO can only narrow the source down to a swath of the sky that is hundreds of square degrees (the full moon on the sky covers about half a degree). PTF will image that swath of sky and pinpoint new transients within it, most likely identifying tens of thousands of events. Then, using new software, the researchers will be able to narrow those down to just a few promising candidates, which will then be fully investigated by the GROWTH network.

"Then we will have data to actually inform us that this one is just a blip, this one is a supernova that has nothing to do with an Advanced LIGO event, and this one is the real counterpart," says Kasliwal. "You need the full picture to be able to get to the heart of the physics."

In addition to its work with transients, the GROWTH network will also help detect and characterize small near-Earth asteroids—those with a diameter smaller than about 140 meters, which can still do significant damage.

"I am very excited about GROWTH's capability for following up detections of small near-Earth asteroids," says Prince. "Determination of the orbits of such objects is critical, and the GROWTH network of telescopes will allow us to make such determinations much more effectively."

Additional participants in the consortium include Lin Yan, a staff scientist at Caltech; Bryan Penprase of Pomona College; Robert Quimby of San Diego State University; Przemek Wozniak of Los Alamos National Laboratory; Stuart Vogel of University of Maryland College Park; David Kaplan of University of Wisconsin–Milwaukee; Nobuyuki Kawai of the Tokyo Institute of Technology; Chow-Choong Ngeow of National Central University in Taiwan; G. C. Anupama of Indian Institute of Astrophysics in Bengalore, India; Varun Bhalerao of The Inter-University Centre for Astronomy and Astrophysics in Pune, India; Eran Ofek of Weizmann Institute of Science in Israel; Ariel Goobar of Stockholm University in Sweden; and Marek Kowalski of Humboldt University in Germany. 

Kimm Fesenmaier
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Global Project to Study Cosmic Flashes
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Friday, October 23, 2015
Winnett Lounge – Winnett Student Center

Flat Space, Deep Learning: A Workshop by Eric Mazur

Wednesday, October 21, 2015
Beckman Institute Auditorium – Beckman Institute

The Teaching and Learning Project, a National Photographic Essay on Higher Education Featuring Caltech

Tuesday, October 20, 2015 to Wednesday, October 21, 2015
Center for Student Services 360 (Workshop Space) – Center for Student Services

Guest Consultations on Teaching, with Chris Duffy

Tuesday, October 20, 2015
Dabney Hall, Lounge – Dabney Hall

Bringing Joy into Your Teaching: A Workshop by Chris Duffy

Monday, October 19, 2015
Guggenheim 101 (Lees-Kubota Lecture Hall) – Guggenheim Aeronautical Laboratory

The Future of Teaching and Learning at Caltech: An Innovation Showcase

Monday, October 19, 2015 to Friday, October 23, 2015

TeachWeek Caltech

Wednesday, October 21, 2015
Keck Center

Engaging Students Beyond Their Field