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Physical Review Letters

Spontaneous-Field-Induced Optical Second-Harmonic Generation in Atomic Vapors

Kenzo Miyazaki, Takuzo Sato, Hiroshi Kashiwagi

Published Oct 15, 1979• 45 citations
Publisher

Abstract

It is shown that optical second-harmonic generation is possible in dispersive atomic vapors since a static electric field is spontaneously created by the fundamental laser beam. When multiphoton ionization of atoms occurs simultaneously in the interaction, the spontaneous field and second-harmonic generation are significantly enhanced. Our experimental results for atomic sodium vapor irradiated by picosecond Nd-doped yttrium-aluminum-garnet laser pulses agree well with the theoretical predictions.

DOI
10.1103/physrevlett.43.1154

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It is well-known that the second-order nonlinear effects, such as second harmonic generation (SHG), difference frequency generation (DFG), and sum frequency generation (SFG), in atomic vapors are theoretically forbidden by symmetry on both a microscopic level because the individual atoms in the vapor do not possess a dipole, and on a macroscopic level because the atoms are isotropically distributed. However, SHG is vapor was first observed by Mossberg et al. (1978) in Tl and then in many other atomic vapors and some noble gases by other groups. Several explanations for this phenomenon have been presented. A model proposed by Mossberg et al. and later modified by Bethune (1981), attributes it to a DC field, which is created by laser-induced multi-photon ionization and spatial separation of free photoelectrons from the heavier ions either by the pondermotive potential of the laser or by the kinetic energy of photoelectrons and makes SHG allowed. All experiments reported so far have been carried out by using either nanosecond or picosecond laser pulses. The authors report the first femtosecond SHG in vapor. The experimental study is done in potassium vapor using femtosecond pulses from a mode-locked Ti:sapphire laser.>

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European Quantum Electronics Conference (EQEC)•Aug 24, 2005•0 citations
It is shown that optical second-harmonic generation is possible in dispersive atomic vapors since a static electric field is spontaneously created by the fundamental laser beam. When multiphoton ionization of atoms occurs simultaneously in the interaction, the spontaneous field and second-harmonic generation are significantly enhanced. Our experimental results for atomic sodium vapor irradiated by picosecond Nd-doped yttrium-aluminum-garnet laser pulses agree well with the theoretical predictions.
It is theoretically and experimentally demonstrated that optical second-harmonic generation (SHG) is possible to be observed in atomic media irradiated with an intense laser pulse. The SHG is induced by a static electric field spontaneously generated in the interaction, and any special condition is not required for the SHG. The experiment to observe the SHG has been made using atomic sodium vapor and a mode-locked Nd-doped yttrium aluminum garnet laser system (pulse duration, 28 psec; output energy \ensuremath{\lesssim}50 mJ). The second harmonic (SH) polarization, spatial beam pattern, and dependences of SH signal on the sodium density and on the fundamental intensity have been examined. The experimental results agree well with the theoretical predictions and numerical results.
Picosecond laser pulses were used to study the highly forbidden resonant second-harmonic generation (SHG) in potassium vapor. The input intensity dependence, vapor density dependence, buffer-gas pressure dependence, and spatial profile of the SHG were measured. A pump–probe experiment was conducted to probe the time dependence of the SHG signal. The experimental results can be understood from an ionization-initiated dc-field-induced SHG model. A theory of a dc-field-induced SHG model is developed that takes into account the time development of the dc electric field in detail. This temporal buildup of the dc field along with transient coherent excitation between two-photon-allowed transitions can explain the experimental results quantitatively, including the previous vapor SHG results with nanosecond laser pulses.
It is well-known that the second-order nonlinear effects, such as second harmonic generation (SHG), difference frequency generation (DFG), and sum frequency generation (SFG), in atomic vapors are theoretically forbidden by symmetry on both a microscopic level because the individual atoms in the vapor do not possess a dipole, and on a macroscopic level because the atoms are isotropically distributed. However, SHG is vapor was first observed by Mossberg et al. (1978) in Tl and then in many other atomic vapors and some noble gases by other groups. Several explanations for this phenomenon have been presented. A model proposed by Mossberg et al. and later modified by Bethune (1981), attributes it to a DC field, which is created by laser-induced multi-photon ionization and spatial separation of free photoelectrons from the heavier ions either by the pondermotive potential of the laser or by the kinetic energy of photoelectrons and makes SHG allowed. All experiments reported so far have been carried out by using either nanosecond or picosecond laser pulses. The authors report the first femtosecond SHG in vapor. The experimental study is done in potassium vapor using femtosecond pulses from a mode-locked Ti:sapphire laser.>