An educational program called QuarkNet places high school students and teachers at the frontier of particle physics.
A research team testing a scintillation tile (blue) and a waveshifter fiber (green) for studies with a radiation source.
What are the origins of mass? Can the basic forces of nature be unified? How did the universe begin? How will it evolve? These are some of the big questions that inspire the imagination of humankind.
The equipment that physicists use to probe these questions is big as well—to say the least. The Large Hadron Collider (LHC) is a 27-km ring-shaped particle accelerator that is buried deep underground in Geneva, Switzerland. After years of preparation, the LHC finally went on line this fall (although experiments are currently stalled due to the failure of two semiconductor magnets). In two scientific collaborations—ATLAS and CMS (the Compact Muon Solenoid)—researchers will be looking to discover a whole new world of phenomena in the head-on collisions of protons with extraordinarily high energy.
These experiments will help physicists to learn about the basic forces that have shaped our universe since the beginning of time, and, hopefully, they will shed insight into its fate. These are not simple enterprises, however: The experiments require sophisticated particle detector technology and the conversion of energy from invisible and hard-to-detect particles into electronic signals or visible light energy for detection. Moreover, with each successive generation of particle detectors, new design challenges have emerged, including the need for a faster response, higher efficiency and better radiation robustness.
So how can these complicated endeavors engage the minds and passions of non-scientists? In other words, can big science, photonic technology and outreach come together?
Enter QuarkNet, an educational outreach program in high-energy physics. QuarkNet immerses high school teachers and students in research projects at the forefront of particle physics. The program was first proposed in 1998 by Marjorie Bardeen (Fermilab), R. Michael Barnett (Lawrence Berkeley National Laboratory), O. Keith Baker (Yale) and Randy Ruchti (Notre Dame). It has been funded jointly by the National Science Foundation and the Department of Energy for 10 years.
In QuarkNet, high school teachers are partnered one-on-one with a physicist mentor. In a parallel program, high school students engage in eight-week summer research projects, in which they are mentored by teachers and faculty. Today, roughly 500 teachers and 100 high school students participate in QuarkNet every year; they partner with 150 particle physicist mentors who are affiliated with 52 centers in 25 states and Puerto Rico.
New compact silicon photomultiplier (right) and a conventional photomultiplier tube for size comparison.
Scintillators, waveshifters and photosensors are “bread and butter” detectors in particle and nuclear physics. The QuarkNet Center at Notre Dame hosts teachers and students who work alongside particle physicists to help build and commission photonic detectors for several major experiments, including the upgraded DØ detector at Fermilab and the CMS detector at the LHC. The QuarkNet Center at Notre Dame has been engaged in research to enable the next phase of the CMS detector and future detectors at a possible linear collider or other scientific facility.
To prepare for such opportunities, the Notre Dame QuarkNet team is focusing on developing new photonic technology and materials that detect the presence of ionizing radiation. These are not simply education outreach exercises; they are real research activities focused on unique detector design and the conversion of particle energies into visible light.
The sampling calorimeter is a detector configuration used in the CMS experiments. A high-energy particle whose energy is to be measured penetrates through alternating layers of absorber and active material. The particle interacts with the nuclei and electrons of the atoms of the absorber material to produce a “shower” of lower energy particles. These, in turn, pass through active fluorescent plastic material exciting organic dye molecules in those layers. As the dye molecules de-excite, light is emitted. The amount of visible light produced is proportional to the energy of the incident particle. The Notre Dame QuarkNet Center is focusing on the active material layers for the calorimeter and new solid-state photosensors to detect this light.
One clever technique for light collection uses waveshifting fibers that are embedded in the plastic tiles of active material. The fibers contain an organic dye that absorbs the light produced in the tile and fluoresces at a longer wavelength. They are routed to the face of a photosensor, which detects the light and produces an electronic signal for further processing.
The first step to this technique is to produce scintillation light in an organic medium and detect it. The medium could be a solid, liquid, or paint-on material that hardens over time. Next, a fast and efficient waveshifter is utilized to rapidly exchange luminous energy from an active layer and emit light at a longer, more suitable wavelength for light transmission and detection.
The final step is to detect the light and covert it into an electronic signal with a compact solid state photosensor. A new breed of solid-state photomultiplier device is now available that is well matched to the dimensions of optical fibers. Students in the Notre Dame QuarkNet team participated in every step of this process.
After the bench tests, beam tests are expected to follow. Thanks to QuarkNet, students and teachers can be directly engaged in such essential studies and developments, working side-by-side with physicists to advance photonic detector technologies. The QuarkNet program provides students and teachers with more than an educational opportunity; it allows them to be scientists working on the most sophisticated and important experiments taking place in the world today.
Randy Ruchti is a co-founder of QuarkNet and a professor of physics and associate vice president for research at the University of Notre Dame. Mitchell Wayne is a professor of physics and department chair at the University of Notre Dame; he is a co-principal investigator of QuarkNet.
References and Resources
>> CERN—The European Organization for Nuclear Research
The building block of attosecond pulse generation. An electron wave packet is created from a bound electron by tunneling through the potential barrier jointly formed by the ion and laser field. Components of the wave packet move in a classical-like manner, as illustrated by the orange arrow. Since many photons are involved, classical physics is a valuable guide to intuition. When the electron re-collides, it can produce an XUV photon, represented by the red arrow. Note how closely this process is related to photoelectron spectroscopy, but in reverse.
Extraction of single isolated attosecond pulses with a double optical gating.
Finally, the electron can recombine with its parent ion, producing a photon. This is the origin of attosecond extreme ultraviolet (XUV) pulses. The re-collision electron’s amplitude, phase and energy are transferred to the photon through the oscillating dipole created by the interference of the two parts of the wavefunction—the bound electron wavefunction and the re-collision electron wave packet. In other words, the XUV pulse is a replica of the re-collision electron pulse viewed through the transition moment.
The bottom figure on the right clarifies how, at the single atom level, attosecond XUV radiation arises from the dipole oscillation induced by the interference between the re-collision electron wave packet and the remaining bound state population. The analogy with an interferometer is obvious. Tunneling is the beam splitter. The electron motion in response to the laser field delays one arm of the interferometer—precisely controllable with light. The interference (or time-dependent dipole moment) is read in the emitted attosecond pulse. The amplitude, energy and phase of the re-collision electron are transferred to light through the dipole moment (the transition moment).
Attosecond pulse trains and gating
We have concentrated on the response of a single atom. However, an experiment is a multi-atom measurement aided by phase matching. In a gas jet, cell or hollow fiber, each atom emits identically with all others, synchronized by the fundamental beam. Provided that the fundamental and the XUV propagate with the same phase velocity, a macroscopic signal is produced. Phase matching is as important in attosecond pulse generation as in second harmonic or optical parametric generation.
When atoms are driven by a many-cycle laser pulse, one attosecond XUV pulse is generated every half of a laser period. As the laser approaches a single optical cycle, the cycle-to-cycle laser amplitude variation becomes significant. As a result, the spectrum of the attosecond pulses generated near the peak of the laser pulse envelope extends to a shorter XUV wavelength range compared to the adjacent attosecond pulses when the CEP of the pump laser is set to zero. A single isolated pulse as short as 80 as has been obtained by selecting the cut-off region of the XUV spectrum with a high-pass filter, using less than 4fs pump lasers.
At lower XUV frequencies (the so-called plateau region), single isolated attosecond pulses can be extracted by a scheme called “polarization gating.” This approach uses a laser field with a time-dependent ellipticity. XUV attosecond pulses can only be efficiently generated during the brief interval dtG, when the field is near linearly polarized. So far, the shortest single isolated XUV pulses in the plateau region, 130 as, have been produced this way.
A method called “double optical gating” allows single isolated attosecond pulses to be generated with greater-than-10-fs pump lasers. A second harmonic field is added to the fundamental field to break the symmetry. Ionization only occurs once per cycle and therefore the spacing between the adjacent attosecond pulses to one optical cycle. Then a polarization gate width, δtG, equal to one optical cycle is sufficient to select one isolated XUV pulse.
Measuring attosecond optical pulses
Having produced attosecond optical (and, from the perspective of the ion, electron) pulses, we must confirm their duration. Conventional methods of measuring optical pulses rely upon low-order nonlinear optics. Extending their methods to attosecond pulses has proven difficult.
Therefore, it is natural to look toward extreme nonlinear optics, the method that produced attosecond pulses in the first place. Here, we have an extremely valuable resource. Every attosecond pulse is perfectly synchronized to the time-dependent field of an infrared pulse (as an optical pulse can be synchronized with THz radiation). This is one of the major tools of attosecond science.
Photo-ionization of a simple atom produces a photo-electron replica of the attosecond pulse. If the replica is created in the presence of the phase infrared field, it will be deflected or accelerated by a rapidly varying field. Its final momentum is determined by the spectrum of the attosecond pulse and the moment of birth of a photoelectron into the field.
Thus, the momentum distribution of the photoelectrons contains all the information needed to determine the pulse duration. The concept is very similar to a conventional streak camera. The attosecond streak camera can be generalized to FROG- or SPIDER-like methods. Photoelectron replicas created by atomic ionization in the presence of an infrared field can measure the duration of isolated attosecond pulses and the individual pulses of attosecond trains.
We emphasized earlier that an attosecond pulse was a replica of a pre-existing re-collision electron pulse (as seen through the transition moment). Why, you might ask, is a new photoelectron replica needed for measurement when we already have a pre-existing one? In fact, it is not necessary. Attosecond pulses can be measured in the medium in which they are created.
This is a very un-laser-like idea—generation and measurement are entwined. It opens the exciting prospect that attosecond metrology can be generalized to other high-order nonlinear optical processes such as inelastic scattering. Never before has it seemed possible to find systematic methods of measuring the dynamics of collision-induced processes. To us, both optical scientists, it seems inevitable that optical techniques will be increasingly transferred to collision physics.
In conventional ultrafast technology, measuring dynamics is very similar to measuring optical pulses. If the pulse is unknown, then one chooses a known phenomenon (for example, frequency doubling) to characterize it. If the pulse is known, one can choose an unknown phenomenon and study its response. It is no different in attosecond science.
It is useful to note a few achievements of attosecond science, some of which rely on the attosecond streak camera:
• Auger recombination has been time-resolved in krypton.
• The time-dependent field of a laser been has been traced.
• Electron photoemission dynamics has been measured in tungsten.
Electron tunneling from molecular O2. The wave packet that tunnels is a filter version of the momentum wavefunction of the bound orbital. As it propagates, it expands. Re-collision drives this almost plane electron wave against the ion core from which it left only about 1 fs ago. There, it diffracts.
Tunneling, which is confined to the field crests, provides another measurement method.
• The dynamics of a strongly driven two-surface system has strobed by tunneling, revealing an unexpected spatial structure of the dynamics.
• Hole dynamics in xenon have been time-resolved.
Already the attosecond streak camera measurements are frequently applied to collision problems. For example, the dynamics of double ionization have been measured in a variety of rare gas atoms and for aligned N2 for different molecular alignments.
Imaging molecular structures and dynamics
So far, we have seen that attosecond technology relies on advanced laser science but has a unique flavor. Perhaps its greatest uniqueness comes from the interplay between collision physics and optical physics. Collision physics has a long tradition of measuring molecular structure. The re-collision electron can be used in a similar fashion, giving optics access to the electron wavelength (1-5 Å). Every step of the strong field ionization processes has imaging potential. Together they provide an array of methods to observe both the electrons and nuclei in molecules.
Structural information is impressed on the electron right from the beginning—tunneling. The figure above illustrates how tunneling from the molecule carries information on the orbital. An electron wavefunction interacts with the spatial filter of the tunnel in much the same way that a mode of an optical beam interacts with a spatial filter a node on. Not only symmetry (the position of nodes), but more detailed information about a mode (orbital) is transferred through the filter (tunnel) as we scan the beam across the filter (rotate the molecule).
The analogy with scanning tunneling microscopy (STM), used for surface science, is also very good. In a laser STM for molecules, rotating the molecule is the analogue of scanning the tip in a conventional STM.
Laser-induced electron diffraction
If the tunneling electron emerges from a single orbital, it is perfectly coherent. When it recollides, it diffracts from its parent ion. Although there are a range of collision energies and the collision occurs in the presence of the laser electric field, this diffraction pattern can be read. It gives structural information about the ion from which it originated.
In optics, interferometry allows us to fully characterize the interfering waves. This should be equally true for the electron interferometer created by the laser field. The attosecond or high harmonic pulse, produced during re-collision, encodes the interference. Recording the spectrum as a function of molecular alignment, one obtains all the information needed to reconstruct the orbital.
Laser-induced electron holography
Gabor, who discovered holography, initially dreamed of using electrons, not light. As you have seen, there are many ways that interference between a reference wave and a scattered wave can arise in strong field and attosecond technology. For example, holographic information is present in all re-collision experiments because parts of the ionizing electron wave packet escape directly to the detector, while parts re-scatter and can gain the same momentum.
Holographic information is also present when two identical attosecond XUV pulses create two photoelectron replica wave packets in the presence of the phased infrared field. All that is needed is for one replica to escape directly to the detector while the infrared field forces others to re-collide and elastically scatter. The interference between the two wave packets contains holographic information. It seems clear that Gabor’s original dream is alive and well in attosecond technology.
We have discussed how sub-cycle science was developed from studies of the highly nonlinear interaction between light and matter. In fact, it is the high nonlinearity that allows the laser cycle to be sub-divided. Re-collision is one form of highly nonlinear interaction. With re-collision and using atoms or molecules, it is probably possible to produce pulses with bandwidth to reaching pulse duration of about 25 as—one atomic unit of time. For many applications, a transform-limited pulse is not needed. Transform-limited measurement is possible as long as the chirp is known.
Re-collision is not the only highly nonlinear process to exploit. Many other such interactions await us, so there is no obvious lower limit to the pulse duration since there is no obvious limit to high-order nonlinear light-matter interactions. In addition, there is no fundamental reason why time is a unique variable. Almost certainly, related ideas can be extended to space—that is, to nano- as well as atto-optics.
In fact, molecular structure can already be measured optically, so one form of sub-nanometer optics is demonstrated. Furthermore, many of the attosecond science ideas should translate into other media—anyplace where highly nonlinear interactions are possible. In other words, there is a broad vista for extreme nonlinear optics, just as there was a broad vista for low-order nonlinear optics in 1960.
Finally, there are still other routes to attosecond science. We have discussed how, aside from the transition moment, attosecond optical pulses are replicas of re-collision electron pulses. Linacs and other electron accelerators also create intense electron beams. These electron pulses can be compressed to the attosecond time scale. It seems inevitable that attosecond free electron lasers will ultimately be constructed, introducing a complementary technology. Already there is progress. Optical and synchrotron technology are becoming entwined, further extending the reach of both.
The authors jointly acknowledge U.S. Army Research Office support under grant number W911NF-07-1-0475.
P.B. Corkum is with the Joint Laboratory for Attosecond Science at the University of Ottawa/National Research Council, Ottawa, Canada. Zenghu Chang, the chair of the newly established “Optical Attoscience Technical Group” of OSA, is with the department of physics, Kansas State University, Manhattan, Kan., U.S.A.
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