Generation of terahertz radiation by the optical breakdown induced by a bichromatic laser pulse
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Generation of Terahertz Radiation by the Optical Breakdown Induced by a Bichromatic Laser Pulse R. A. Akhmedzhanov, I. E. Ilyakov, V. A. Mironov, E. V. Suvorov, D. A. Fadeev*, and B. V. Shishkin Institute of Applied Physics, Russian Academy of Sciences, Nizhni Novgorod, 603950 Russia *email: [email protected]nnov.ru Received April 3, 2009
Abstract—Experimental results are presented on generation of terahertz radiation by the air breakdown induced by a highintensity laser pulse having not only a fundamental component, but also a secondhar monic one. A theoretical explanation of the experimental data is proposed, based on a model of field ioniza tion of a gas by a bichromatic laser. PACS numbers: 41.60.m, 52.38.r DOI: 10.1134/S1063776109090027
1. INTRODUCTION The importance of research focused on the devel opment of efficient, inexpensive, and compact sources of terahertz (THz) radiation is due to its unique poten tial for use in diverse applications, such as detection and identification of various semiconductor struc tures, explosive and hazardous chemicals, biological objects; generation of images for purposes of medical, biological, or security screening; environmental mon itoring; etc. [1, 2]. One promising THz generation method is based on the gas breakdown induced by focused femtosecond laser pulses [3–14]. Originally, weak THz emission was detected in experiments on plasma wakefield acceleration of charged particles [3], where THz radiation was considered as a diagnostic tool for estimating the accelerating electric field in a wakefield plasma wave. THz emission from ionized gases was also observed in experiments on laser fila mentation [4–7]. It was demonstrated that THz inten sity is significantly enhanced by applying a longitudi nal or transverse static electric field to the region of gas breakdown [4, 6]. An even higher THz generation effi ciency was achieved in experiments on THz emission from the gas breakdown induced by bichromatic laser pulses having fundamental and secondharmonic components [8–14]. The theoretical results obtained in [7, 10, 14–17] are not sufficient to explain all exper imental data. This paper presents the results of more detailed experimental studies of THz generation by the air breakdown induced by a femtosecond laser beam and its second harmonic. An advanced theoret ical interpretation of the experimental data is pro posed.
2. EXPERIMENTAL Figure 1 schematizes the experimental setup. THz radiation is emitted by the breakdown plasma in the focal region of the beam produced by a femtosecond laser system (SpitfirePro ultrafast amplifier), with an energy of 2.5 mJ/pulse, a pulse duration of 50 fs, a central wavelength of 780 nm, a repetition rate of 1 kHz, and a diameter of 7 mm. The beam is focused in atmospheric air by a parabolic lens. As a frequency doubler, we use a 100 µm thick betabarium borate crystal (BBO) cut for type I phase matching and placed after the parabolic lens. The crystal generates a secondharmonic beam whose polarization is
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