A Tabletop Source of Strong Terahertz Radiation

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Published: Monday, March 09 2009 09:21

By: Ki-Yong Kim

Sandwiched between the traditional optical and microwave regimes, far infrared or terahertz (THz) frequency (1 THz = 10¹² Hz) has recently drawn special attention due to its potential for molecular sensing, biomedical imaging and spectroscopy, security scanners, and plasma diagnostics. These applications provide strong motivation to advance the state of the art in THz source development. In particular, large-scale electron accelerators such as synchrotrons and free electron lasers are currently available to produce THz radiation energy in excess of several microjoule per pulse. However, due to its large cost to build those facilities and thereby limited access, there is a present and growing need to realize such strong THz generation at the tabletop scale. In this effort, we have recently demonstrated a high-energy (>5 microjoule), super-broadband (>75 THz), tabletop THz source via ultrafast photoionization in gases [1].

In this scheme, an ultrafast pulsed laser’s fundamental and second harmonic fields are mixed in a gas of atoms or molecules, causing them to ionize. Microscopically, the laser fields act to suppress the atom’s or molecule’s Coulomb potential barrier, and, via rapid tunneling ionization, bound electrons are freed. The electrons, once liberated, oscillate at the laser frequencies, and also drift away from their parent ions at velocities determined by the laser field amplitudes and the relative phase between the two laser fields. Depending on the relative phase, symmetry can be broken to produce a net directional electron current. As this current occurs on the timescale of photoionization, for sub-picosecond lasers, it can generate electromagnetic radiation at THz frequencies.

This THz generation mechanism turns out to be closely related to the mechanism used to explain high harmonic generation (HHG) in gases, as both processes originate from a common source, that is, a nonlinear electron current. The electrons re-colliding with the parent ions are responsible for HHG, whereas the electrons drifting away from the ions without experiencing re-scattering ions account for THz generation. As demonstrated experimentally [1], the generated THz and third-harmonic are strongly correlated in such a way that changing the relative phase can effectively switch the emission between THz and harmonics. This provides the basis to coherently control electromagnetic radiation in a broad spectral range, from THz to extreme ultraviolet.

Now, the next step is to scale up the laser power to produce even more powerful THz radiation. Using the Maryland’s 30 terawatt (TW) laser, we anticipate producing an unprecedented millijoule level of THz radiation. Such radiation may allow us to observe extreme nonlinear THz phenomena in a university laboratory.

[1]  K. Y. Kim et al., Nature Photon. 2, 605 (2008).

Long-Distance Teleportation Between Atoms

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Published: Friday, January 23 2009 08:54

For the first time, scientists have successfully teleported information between two separate atoms in unconnected enclosures a meter apart – a significant milestone in the global quest for practical quantum information processing.

Teleportation may be nature’s most mysterious form of transport: Quantum information, such as the spin of a particle or the polarization of a photon, is transferred from one place to another, but without traveling through any physical medium. It has previously been achieved between photons over very large distances, between photons and ensembles of atoms, and between two nearby atoms through the intermediary action of a third. None of those, however, provides a feasible means of holding and managing quantum information over long distances. 

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UMD Physicists Play Major Roles in Four of AIP's Top Ten Physics Discoveries of 2008

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Published: Wednesday, January 07 2009 09:28

Editors and science writers at the American Institute of Physics and the American Physical Society selected a list of Top Ten Physics Stories in 2008. The selections were released on December 22, 2008 and included four discoveries in which UMD Physicists had major roles (Large Hadron Collider, Quarks , Ultracold Molecules and Cosmic Rays).

To view the full article, visit: http://www.aip.org/pnu/2008/split/879-1.html

MILAGRO Detects Cosmic Ray Hot Spots

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Published: Monday, November 24 2008 14:51

The University of Maryland-led Milagro collaboration, comprised of scientists from 16 institutions across the United States, has discovered two nearby regions with an unexpected excess of cosmic rays. 

This is the second finding of a source of galactic cosmic rays relatively near Earth announced in the past week. In the November 20 issue of the journal Nature, ATIC an international experiment lead by LSU scientists and conceived by a University of Maryland physicist announced finding an unexpected surplus of cosmic-ray electrons from an unidentified, but relatively close source.

“These two results may be due to the same, or different, astrophysical phenomenon, said Jordan Goodman, a University of Maryland professor of physics and principal investigator for Milagro. However, they both suggest the presence of high-energy particle acceleration in the vicinity of the earth. Our new findings [published in the November 24 issue of Physical Review Letters] point to general locations for the localized excesses of cosmic-ray protons observed with the Milagro observatory.

Cosmic rays are actually charged particles, including protons and electrons, that are accelerated to high energies from sources both outside and inside our galaxy. It’s unknown exactly what these sources are, but scientists theorize they may include supernovae -- massive stars that explode -- quasars or perhaps from other even more exotic, less-understood sources within the universe. Until recently, it was widely held that cosmic-ray particles came toward Earth uniformly from all directions. These new findings are the strongest indications yet that the distribution of cosmic rays is not so uniform.

When these high energy cosmic ray particles strike the Earth's atmosphere, a large cascade of secondary particles are created in an extensive “air shower.” The Milagro observatory, located near the Los Alamos National Lab in New Mexico, 'sees' cosmic rays by observing the energetic secondary particles that make it to the surface.

Jordan and his Milagro colleagues used the cosmic-ray observatory to peer into the sky above the northern hemisphere for nearly seven years starting in July 2000. The Milagro observatory is unique in that it monitors the entire sky above the northern hemisphere. Its design and field of view, made it possible for the observatory to record over 200 billion cosmic-ray collisions with the Earth’s atmosphere.

This allowed researchers for the first time to see statistical peaks in the number of cosmic-ray events originating from relatively small regions of the sky. Milagro observed an excess of cosmic ray protons in an area above and to the right of Orion, near the constellation Taurus. The other hot spot is a comma-shaped region in the sky near the constellation Gemini.

“Whatever the source of the protons we observed with Milagro, their path to Earth is deflected by the magnetic field of the Milky Way so that we cannot directly tell exactly where they originate,” said Goodman. “And whether the regions of excess seen by Milagro actually point to a source of cosmic rays, or are the result of some other unknown nearby effect is an important question raised by our observations.”

Even more revelatory observations of cosmic rays and further help solving the mystery of the origin of cosmic rays may come in the form of a new observatory that Jordan and his fellow U.S. Milagro scientists have partnered with colleagues in Mexico to propose to the National Science Foundation. This second-generation experiment named the High Altitude Water Cherenkov experiment (HAWC) would be built at a high-altitude site in Mexico.

The National Science Foundation (NSF) funded construction of the Milagro through the University of Maryland. The observatory’s work was funded by NSF, the US Department of Energy, Los Alamos National Laboratory, and the University of California. For more information on Milagro and HAWC, visit the University of Maryland HAWC website: http://hawc.umd.edu or contact Jordan Goodman (This email address is being protected from spambots. You need JavaScript enabled to view it.) or Brenda Dingus (This email address is being protected from spambots. You need JavaScript enabled to view it.).

"Spooky Action-at-a-Distance" with Individual Atoms

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Published: Monday, September 15 2008 10:53

By: Christopher Monroe

Albert Einstein never liked Quantum Mechanics, with its fuzzy superpositions and confused states of reality. In 1935, he and colleagues Boris Podolsky and Nathan Rosen proposed a thought experiment that they believed would finally show cracks in the new quantum theory. The essentials of their famed proposal can be seen by cconsidering two “quantum coins,” that are prepared in a strange superposition of being both heads-up and both tails-up at the same time. When such coins are brought far apart from each other and then measured, quantum mechanics predicts that the only possible results can be HH and TT – the orientation of the coins always matches in perfect correlation. But when either individual coin is observed, its value is expected to be totally random (H or T). What’s interesting here is that while an individual coin is in an indeterminate state until observed, the observer immediately knows that orientation of the other coin, and this knowledge happens faster than the speed of light can traverse the distance between the coins.

Einstein called this quantum behavior “spooky action-at-a-distance,” and concluded that either quantum mechanics is incomplete, or it is just very weird. We now know, thanks to John Bell in 1964, that if quantum mechanics is indeed incomplete, than any more complete theory must be just as weird, so we might as well stick with quantum mechanics. Bell devised a measure of this weirdness: an inequality involving measured pair-correlations that is violated for situations like the one considered by Einstein, Podolsky and Rosen.

This weird type of quantum state the Einstein introduced is now known as an “entangled state,” and the spooky action-at-a-distance that he bemoaned is now the central resource in the field of Quantum Information Science. Replace the coins by quantum bits that can be in the state 0 and 1 simultaneously, and these qubits can be used for superfast computing applications, or fundamentally secure communication. Qubits are now being investigated in a variety of physical systems, from individual atoms and photons, to superconducting circuits and semiconductor quantum dots.

Recently, a team of researchers from the University of Maryland Department of Physics and Joint Quantum Institute have observed for the first time, quantum entanglement of individual atoms separated by a large distance [Moehring, et al., Nature 449, 68, (2007)]. Two atoms, held in electromagnetic traps one meter apart, were synchronized with a laser pulse, and the resulting emitted light was interfered on a beamsplitter and detected. This detection produced an entangled state of the two atoms, where qubits were stored in the magnetic orientation of each atom. This entanglement (the correlations of the atomic-scale magnets and the randomness of each one individually) was directly verified by measuring the magnetic orientation with a separate laser, resulting in a clear violation of Bell inequalities. This type of quantum linking between atomic qubits may ultimately lead to the fabrication of a large-scale quantum computer, where atomic memories will be able to store exponentially-rich amounts of data and be connected through optical interconnects as demonstrated here. In the nearer term, this is among the most promising roads to a “quantum repeater,” where qubits can be propagated over very large (or even geographic) distances with the use of optical fibers.