Transcript poster gruppo Laser di Aegis

Physics with many positrons
International Fermi School
07-17 July 2009 – Varenna, Italy
The experimental work on a laser for positronium excitation to Rydberg levels in AEGIS
F.Villa, I. Boscolo, F. Castelli, S. Cialdi
Physics Dept. – Università degli Studi di Milano and INFN (Istituto Nazionale di Fisica Nucleare)
Positronium Rydberg excitation in AEGIS
The basics of optical parametric processes
AEGIS:
Our proposal for Ps laser excitation:
Antimatter Experiment: Gravity,
Interferometry, Spectroscopy
Two step incoherent excitation
There’re two different stage in order to generate the pulse:
1. The Optical Parametric Generator (OPG) realize the wavelength
at 1650nm.
1) transition 1  3,
205 nm, the laser
pulse will be
generated by a
dye laser pumped
by a Nd:YAG
laser.
•) Primary scientific goal: the first direct
measurement of the Earth’s local gravitational
acceleration g on antihydrogen, with 1% relative
precision [1].
•) Method for antihydrogen production: resonant
charge - exchange reaction between antiprotons
and positronium excited to Rydberg levels ( ~ n4).
The positronium is generated in a target from a
bunch of positrons.
M. Giammarchi will explain this experiment in his
talk on Friday 17th
2) transition 3  n
(around 20 - 30),
1630-1700 nm, the
laser pulse will be
generated by
using nonlinear
optic crystals and
the same pump of
the first pulse.
Proposed method for laser excitation
2. The radiation is amplified to the requested energy by the Optical
Parametric Amplifier (OPA).
Both process show a similar, well known, theoretical formulation [3].
OPG
OPA
1650 nm
1650 nm
1064 nm
3000 nm
3000 nm
1064 nm
Transition scheme to Rydberg level
k  k p  ks  ki
g
p1  s 1  i1
Theoretical calculation for this second
transition: spectrum of some nm and a
total energy per pulse of around 0.5 mJ [2].
•k is the mismatch between the wave vectors of the pump and
the wavelength generated, it is a loss term in the amplification
Laser
F. Castelli will explain this theory in his talk on
Friday 17th
•The second equation represent the energy conservation in the
process
Proposed method for H formation and g measurement
We present the first results on the
laser for the second transition
H*
p
e+
The Optical Parametric Generator
The experimental apparatus
Requirements: high efficiency production in a down
conversion process of 1650 nm starting from vacuum.
Selected crystal: a PPLN crystal, composed by slices of
Lithium Niobate (that have high nonlinear coefficient)
whose orientation is periodically inverted in order to
compensate the phase mismatch Dk. This process is
called Quasi Phase Matching (QPM) [4].
PUMP
OPA
•The Pump laser is a
Q-switched Nd:YAG
at 1064 nm, with a
duration of about 10
ns, a maximum
energy of 300 mJ
and a repetition rate
of 2 Hz.
Dimensions: The PPLN used has 9 channels with
different periodicity, from 29.50 to 31.50 mm, in order to
matching different QPM conditions.
Scheme of the periodical poling
and of the sizes of each channel
OPG
•Wide width of wavelengths
generation, selecting the
channel and through with
small adjustment of the
temperature
1750
Ps level
n: 12 ... 19
1700
Wavelength (nm)
•The OPG is a
Periodically Poled
Lithium Niobate
(PPLN).
L =29.50mm
L =29.75mm
L =30.00mm
L =30.25mm
L =30.50mm
•The OPA is a
standard KTP crystal
n: 19 ... ionization
1650
Ionization limit
1600
1550
1500
400
410
420
430
440
450
460
470
480
PPLN temperature (K)
Energy at 1650 nm produced by PPLN
50
The Optical Parametric Amplifier
40
Optical crystal axis
Selected crystal: a KTP (KTiOPO4 ) bulk crystal. k ~ 0 by a careful selection of the propagation
direction that compensates the phase mismatch (Phase Matching, PM). The acceptance of the process
is of only a few milliradiants and the amplification is highly dependent on the pulse characteristics.
Laser pulse propagation
Dimensions: The crystal has a cross section of 5 x 5 mm and a length of 1 cm. The crystal has a
higher threshold damage than PPLN.
Energy at 1650 nm (uJ)
Requirement: amplification of the signal up to 0.5 mJ.
30
20
10
0
220
• Measured gain of the signal for different values of the pump
intensity. The maximum gain achieved, around a mean of 4.5,
allow to amplify the 30 mJ signal up to 140 mJ.
7
y = 1+sinh(M0*M1)^ 2
6
Gain
5
m1
Chisq
R
Value
Error
0.0013913 1.1926e-05
0.10928
NA
0.99324
NA
4
•We measured a high
efficiency in signal
conversion (up to about
15% of the pump
•We are measuring a notable shot-by-shot
jitter in the signal amplitude.
240
260
280
300
320
340
360
Pump energy (uJ)
•Wide continuum spectrum that depend
on the pump spectrum and the
imperfection in the periodically poling.
•The crystal can’t reach the
required energy because of its
small cross section and its
relatively small damage threshold.
Future developments
3
•This behavior is due to our pump position
and intensity instability.
2
1
400
500
600
700
800
900
1000
1100
Pump intensity (MW/cm^2)
•The goal of 0.5 mJ per pulse will be
reached using two of this crystals in
sequence.
We are doing further measurement about the characteristics of the amplified beam, in
order to better understand the correlation between its characteristics and those of the
input pump and signal. We are studying:
1) the angular acceptance of the Phase Matching, that seems greater than expected
from simple theory.
2) the statistics of the fluctuation in the OPA gain, in order to optimize the setup
Bibliography
[1] A. Kellerbauer et al, Proposed antimatter gravity measurement with an antihydrogen beam, Nuclear Instrument and Method in Physics Research B, 266 (2008) pag. 351
[2] F. Castelli et al, Efficient positronium laser excitation for antihydrogen production in a magnetic field, Physical Review A 78 (2008) pag. 052512
[3] J. A. Armstrong et al, Interaction between Light Waves in a Nonlinear Dielectric, Physical Review 127 (1962) n. 6, pag. 1918
[4] M. M. Fejer et al, Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances, IEEE Journal of Quantum Electronics, 28 (1992) n. 11, pag. 2631
Another point of interest is the transport line from the laser table to the place where the
Ps will be excited. We are studying:
1) the optimized optical system in order to achieve minimal losses in the transport and
better beam stability. We are comparing different designs whose basic optical
components are dielectric mirrors, prisms and optical fibers.
2) the thermal processes of diffusion for the various configuration, because the Ps
excitation will be realized in a cryogenic environment, at about 1K.