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