Positronium Rydberg excitation in AEGIS

1. g H* Laser p e+ Optical crystal axis Laser pulse propagation 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 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 • There’re 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 Nd:YAG 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 • kis 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 • ThePumplaser 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. • 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 ~ 0by 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 can’t 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