The electromagnetic spectrum. The scarcity of
intense broadband sources of radiation in the 1012
hertz (terahertz) frequency range leaves us blind to a wide range
of interesting phenomena.
Filling the Terahertz Gap
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The earlier demonstration
of femtoslicing at the ALS as a source of ultrafast x rays showed
that we can manipulate the distribution of an electron beam using
a short-pulse laser on a time scale of several hundred femtoseconds.
One of the by-products of this technique is that femtoslicing can
create very short "holes" in the time distribution of the
electron bunch. While short electron bunches can radiate coherently
(i.e., photons are emitted in phase), the researchers found that these
"holes" in the electron bunch can radiate coherently as
well, and that this technique could be extended to create a novel
source of terahertz radiation. For example, by shaping the slicing
laser pulse, we can tailor the shape of the hole that is "sliced"
in the bunch and thus shape the electric field of the coherent terahertz
pulse. This would make the terahertz radiation tunable.
Femtoslicing works by modulating the energy of electrons in the
bunch using a high-power laser pulse co-propagating with the electrons
in a wiggler field. For example, the interaction of a 75-femtosecond
laser pulse with an electron bunch results in the formation of "wings"
in the bunch energy distribution. The projection of the distribution
on the time axis represents the relative variation in the peak bunch
current. As the bunch passes through the accelerator, the high-
and low-energy "wings" of the bunch energy distribution
slip backward and forward along the bunch, creating a "hole"
in the center of the bunch that emits terahertz radiation. As the
bunch continues around the storage ring, the "hole" quickly
spreads and fills with electrons. Because of the short laser pulses
that are used to slice the electron beam, the emission spectrum
initially extends up to terahertz frequencies but shifts to lower
frequencies as the hole spreads.
The effect of a co-propagating laser pulse on
an electron bunch. The electron energy distribution (blue band)
shows that electrons gaining energy (deltaE > 0) gather toward
the back of the bunch (because they follow a longer path), while
those that lose energy (deltaE < 0) gather toward the front
(because they follow a shorter path). The relative peak bunch
current (superimposed white curve) shows a "hole" in
the electron bunch that emits coherent terahertz radiation (frequency
spectrum shown in inset). As the bunch travels around the storage
ring, the "hole" quickly spreads and fills with electrons.
Click on the image above to view a movie of the process.
To observe these effects, the researchers used a liquid-helium-cooled
bolometer sensitive to terahertz wavelengths and recorded bursts
of coherent signal coincident with the slicing, which occurred at
a 1-kHz repetition rate. They also measured the spectrum of the
coherent radiation at Beamline
5.3.1, immediately following the slicing, and at Beamline
1.4, three-fourths of the way around the ring from the slicing.
Spectral measurements are difficult at the ALS because the vacuum
chamber design has very poor transmission of long-wavelength radiation.
Because the coherent signal is very sensitive to the width and depth
of the hole, it is currently used as the primary diagnostic signal
for optimizing slicing efficiency.
Left: Bolometer signal observed at Beamline 5.3.1
and Beamline 1.4 with slicing on. Right: The spectral measurement
of the coherent radiation at the two beamlines.
Given the ability to slice holes in the electron bunch, we can
now consider tailoring the terahertz signal to the needs of a particular
experiment. Laser technology allows us to shape the temporal profile
of the laser pulse and modulate the electron bunch with a multitude
of patterns. For example, if we slice the beam with a train of laser
pulses, we can apply a periodic modulation on the electron bunch,
generating a narrow-band terahertz signal. This signal would be
tunable in frequency by varying the laser pulse spacing.
Research conducted by J.M. Byrd, D.S. Robin, F. Sannibale, A.A.
Zholents, and M.S. Zolotorev (Accelerator and Fusion Research Division,
Berkeley Lab); Z. Hao and M.C. Martin, (ALS); and R.W. Schoenlein
(Materials Sciences Division, Berkeley Lab).
Research funding: U.S. Department of Energy, Office of High Energy
Physics and Office of Basic Energy Sciences (BES). Operation of
the ALS is supported by BES.
Publication about this research: J.M. Byrd, Z. Hao, M.C. Martin,
D.S. Robin, F. Sannibale, R.W. Schoenlein, A.A. Zholents, and M.S.
Zolotorev, "Tailored terahertz pulses from a laser-modulated
electron beam," Phys. Rev. Lett. 96,
164801 (2006). |