Theory of multiphoton and tunnel ionization in a bichromatic field
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Theory of Multiphoton and Tunnel Ionization in a Bichromatic Field D. S. Bagulova,* and I. A. Kotelnikovb,** a
Novosibirsk State University, Novosibirsk, 630090 Russua Budger Institute of Nuclear Physics, Russian Academy of Sciences, Siberian Branch, Novosibirsk, 630090 Russia *email: bagulov[email protected] **email: [email protected]
b
Received June 7, 2012
Abstract—The imaginarytime method [6, 7] is used to calculate the multiphoton and tunnel ionization probabilities for atoms in a laser radiation field part of which is converted into the second harmonic. We assume that the first harmonic has a linear or elliptical polarization and the second harmonic is polarized lin early, with its polarization vector making an arbitrary angle with that of the first harmonic. The mean momen tum of the photoelectrons knocked out from atoms is shown to depend on the phase shift between the first and second harmonics and their mutual polarization and to be identically equal to zero for a monochromatic field. An important difference between the case of elliptical polarization and the case of linear polarization of both harmonics is the absence of conditions under which the conditions for dominance of one of the two gen eration mechanisms considered here can be identified during the generation of terahertz radiation from the region of optical breakdown in a gas. DOI: 10.1134/S1063776113010196
1. INTRODUCTION Multiphoton ionization is the main mechanism of optical breakdown at the focus of a femtosecond laser. The plasma produced in the focal spot is a source of terahertz emission whose power can increase by hun dreds of times when a laser pulse is passed through a nonlinear β barium borate (BBO) crystal placed in front of the focal spot [1]. This nonlinear transducer converts part of the laser radiation power into a lin early polarized second harmonic. Two amplification mechanisms of terahertz emis sion from the focal spot are known. The first mechanism is based on the obvious assumption that multiphoton ionization at double fre quency requires the simultaneous absorption of half as many photons. Since the probability of such ioniza tion decreases with increasing necessary number of photons, the ionization from the second harmonic can exceed that from the first one even if the amplitude of the latter is considerably larger. The other terahertz emission amplification mecha nism is based on the appearance of an initial momen tum for the photoelectrons knocked out by laser radi ation from atoms, which gives rise to a transient pho tocurrent [2, 3]. The transient photocurrent arises only in the presence of the second harmonic and only in the presence of a phase shift between the harmonics that is not a multiple of π if both harmonics have a lin ear polarization [4]. An attempt to construct a quantum model for the generation of terahertz emission from the focus of a femtosecond laser describing both mechanisms has
recently been made in [5] (see also the references therein). Using the imagi
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