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Bulletin of the American Physical Society

Picosecond study of transient optical second harmonic generation (SHG) in K vapor

D. Kim, Christopher S. Mullin, Marla B. Feller, Y.R. Shen

Published May 1, 1993• 0 citations
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Abstract

SHG is forbidden in materials with centrosymmetry such as an atomic vapor, yet it has been observed with appreciable efficiency on atomic resonances in metal vapors with focused nanosecond lasers. We have used a 3 psec tunable laser pulse in a pump-probe geometry to determine how the symmetry is broken in atomic media. We observe a significant instantaneous response with a strong intensity dependence. Enhancement of the SHG of a delayed probe pulse by the pump pulse is weak up to 1 nsec. Other factors such as ionization of K vapor, linewidth and phase matching are also studied.

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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.

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Time-delayed second-harmonic generation in atomic Cs vapor

Alexander I. Lvovsky, Sven R. Hartmann

Time-delayed second harmonic generation has recently been reported in an atomic vapor of Cs at a temperature near 180 degrees Celsius and situated in a magnetic field of the order of a killogauss. Short 10 ps excitation laser pulses at 885 nm were used to excite a macroscopic electric quadrupole density which was then observed to radiate at 442 nm after a delay of a few hundred picoseconds. The experiment is complicated by the presence of several superfluorescence transitions some of which radiate close to the 442 nm wavelength. These emissions are also delayed and our strategy is to set the experimental conditions so that they only become appreciable after the second harmonic emission is complete. In this paper we show how the second harmonic develops by considering the evolution of the radiation pattern of an isolated atomic radiator (not forbidden to radiate at the second harmonic even in the absence of a magnetic field). We estimate the intensity of the superradiant second harmonic emission and obtain agreement with experiment. Excellent agreement is also obtained between measured and calculated delay.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Sep 26, 1997•0 citations
SHG is forbidden in materials with centrosymmetry such as an atomic vapor, yet it has been observed with appreciable efficiency on atomic resonances in metal vapors with focused nanosecond lasers. We have used a 3 psec tunable laser pulse in a pump-probe geometry to determine how the symmetry is broken in atomic media. We observe a significant instantaneous response with a strong intensity dependence. Enhancement of the SHG of a delayed probe pulse by the pump pulse is weak up to 1 nsec. Other factors such as ionization of K vapor, linewidth and phase matching are also studied.
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.>
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.
Second harmonic generation (SHG) in atomic Li vapor is studied when the 2 2 S 1/2 -3 2 S 1/2 two-photon transition is resonantly excited. This is an SHG in a medium with inversion symmetry, with no externally apptied electric or magnetic field, and with no apparent multipole moments associated with the resonant excitation. The SH polarization is found to be parallel to the intensity gradient of the incident laser beam when a cylindrical lens focusing is applied. The mechanism of this highly forbidden SHG is explained by the radial electric field induced by free expansion of photo-electrons around the laser beam axis.
Time-delayed second harmonic generation has recently been reported in an atomic vapor of Cs at a temperature near 180 degrees Celsius and situated in a magnetic field of the order of a killogauss. Short 10 ps excitation laser pulses at 885 nm were used to excite a macroscopic electric quadrupole density which was then observed to radiate at 442 nm after a delay of a few hundred picoseconds. The experiment is complicated by the presence of several superfluorescence transitions some of which radiate close to the 442 nm wavelength. These emissions are also delayed and our strategy is to set the experimental conditions so that they only become appreciable after the second harmonic emission is complete. In this paper we show how the second harmonic develops by considering the evolution of the radiation pattern of an isolated atomic radiator (not forbidden to radiate at the second harmonic even in the absence of a magnetic field). We estimate the intensity of the superradiant second harmonic emission and obtain agreement with experiment. Excellent agreement is also obtained between measured and calculated delay.