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Scientific Report 2007-2009
Condensed matter physics and biophysics
C46. Development of coherent terahertz radiation sources from third
generation synchrotron machines and Free Electron Lasers
The last decade has witnessed a huge amount of experimental
effort in order to fill the so-called Terahertz
(THz) gap. This range of the electromagnetic spectrum,
which is roughly located between the infrared and the
microwave region (0.1-20 THz), has been indeed scarcely
investigated so far mainly because of the lack of intense
and stable THz sources. Scientific problems which could
be addressed by THz spectroscopy and imaging include
the ps and sub-ps scale dynamics of collective modes in
superconductors and in exotic electronic materials, the
ps-scale rearrangement dynamics in the secondary structure
of proteins and biological macromolecules, early
cancer diagnosis, and security applications.
Energy (µJ)
0.0
-0.5
-1.0
-0.04
No filter
Filter 1.5 THz
0.00
Time (s)
0.04
σ t
= 500 fs
Q= 500 pC
0.08
0
-5
-10
Energy (µJ)
Intensity over background
10 5
10 4
10 3
10 2
10 1
10 0
20
Frequency (THz)
1 2
900 MeV
13% filling
40
60
Frequency (cm -1 )
0.53 mA/bunch
0.96
1.07
1.28
1.66
2.15
2.82
Figure 1: THz spectra for different currents stored in the
ELETTRA ring and for an electronic energy of 900 MeV.
Short (sub-millimeter) electron bunches in storage
rings and in Free Electron Laser (FEL) machines emit
Coherent Radiation up to wavelengths of the order of
the bunch length, i.e. in the THz range. As the coherent
amplification scales with N[1 + N ∗ f(ω)], with f(ω)
being the Fourier Transform of the charge density in the
longitudinal bunch profile and N the number of electron
in the bunch, the brilliance gain with respect to most
existing THz sources can be huge.
On this ground, we have recently investigated the
possibility to produce coherent THz radiation from our
beamline SISSI (Synchrotron Source for Spectroscopy
and Imaging) at the third generaton machine ELETTRA
(Trieste) [1]. Short bunches can be created by taking
80
100
Figure 2: THz signal in µJ/pulse as measured by a pyroelectric
detector at SPARC for a stored charge of 500 pC and for
a bunch length of 500 fs. The red curve corresponds to the
signal at 1.5 THz with a bandwidth of nearly 0.3 THz. The
blue curve corresponds to the signal up to 3 THz.
into account the strong bunch length energy dependence
(σ ≈ E 3/2 ), i.e. the bunch length can be reduced by lowering
the machine energy [2]. In this way, strong pulse
of THz radiation up to 1 THz have been obtained (see
Fig.1). This radiation has been already used for linear
spectroscopical applications [3].
Despite all efforts spent on THz research, the possibility
of performing pump-probe time-resolved THz and
infrared (IR) spectroscopy is instead basically unexplored.
The difficulty lies in achieving an energy/pulse
>10 µJ, which corresponds roughly to the electric field
of 1 MV/cm at which the THz pulse becomes useful as
a pump-beam. This task can be accomplished in FEL
machines through ultra-short highly-loaded bunches.
In this context, we are developing a new source which
extracts pulsed THz and IR radiation from the SPARC
FEL in Frascati (Italy). Preliminary measurements
show that sub-ps radiation pulses can be obtained in the
10 µJ/pulse scale, a value which ranks SPARC among
the best FEL THz sources worldwide (see Fig.2) [4].
References
1. M. Ortolani, et al., Phys. Rev. B 77, 100507 (2008)
2. C. Mirri, et al., Phys. Rev. B 78, 155132 (2008)
3. S. Lupi, et al., Phys. Rev. B 77, 054510 (2008)
Authors
P. Calvani, A. Nucara, D. Nicoletti, O. Limaj, S. Lupi
Sapienza Università di Roma 99 Dipartimento di Fisica