poster gruppo Laser di Aegis Varenna

1

• 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

Our proposal for Ps laser excitation Two step
incoherent excitation

• AEGIS
• Antimatter Experiment Gravity,
• Interferometry, Spectroscopy
• ) Primary scientific goal the first direct
  measurement of the Earths 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

• Therere two different stage in order to generate
  the pulse
• The Optical Parametric Generator (OPG) realize
  the wavelength at 1650nm.
• The radiation is amplified to the requested
  energy by the Optical Parametric Amplifier (OPA).
  
• Both process show a similar, well known,
  theoretical formulation 3.

• transition 1 ? 3, 205 nm, the laser pulse will
  be generated by a dye laser pumped by a NdYAG
  laser.
• 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.

OPG
OPA
1650 nm
1650 nm
1064 nm
3000 nm
3000 nm
1064 nm
Proposed method for laser excitation
Transition scheme to Rydberg level
Theoretical calculation for this second
transition spectrum of some nm and a total
energy per pulse of around 0.5 mJ 2. F.
Castelli will explain this theory in his talk on
Friday 17th

• ?k is the mismatch between the wave vectors of
  the pump and the wavelength generated, it is a
  loss term in the amplification
• The second equation represent the energy
  conservation in the process

We present the first results on the laser for the
second transition
Proposed method for H formation and g measurement
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. Dimensions The PPLN
used has 9 channels with different periodicity,
from 29.50 to 31.50 mm, in order to matching
different QPM conditions.
PUMP
OPA

• The Pump laser is a Q-switched NdYAG at
  1064 nm, with a duration of about 10 ns, a
  maximum energy of 300 mJ and a repetition rate of
  2 Hz.
• The OPG is a Periodically Poled Lithium Niobate
  (PPLN).
• The OPA is a standard KTP crystal

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

• We measured a high efficiency in signal
  conversion (up to about 15 of the pump

The Optical Parametric Amplifier
Requirement amplification of the signal up to
0.5 mJ. 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. 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.

• 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.

• The crystal cant reach the required energy
  because of its small cross section and its
  relatively small damage threshold.

• Wide continuum spectrum that depend on the pump
  spectrum and the imperfection in the periodically
  poling.

Future developments

• We are measuring a notable shot-by-shot jitter in
  the signal amplitude.
• This behavior is due to our pump position and
  intensity instability.
• 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
• the angular acceptance of the Phase Matching,
  that seems greater than expected from simple
  theory.
• the statistics of the fluctuation in the OPA
  gain, in order to optimize the setup
• Another point of interest is the transport line
  from the laser table to the place where the Ps
  will be excited. We are studying
• 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.
• the thermal processes of diffusion for the
  various configuration, because the Ps excitation
  will be realized in a cryogenic environment, at
  about 1K.

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