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Physical Review A

Interaction of high-power laser pulses with atomic media. II. Optical second-harmonic generation

Kenzo Miyazaki, Takuzo Sato, Hiroshi Kashiwagi

Published Mar 1, 1981• 43 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.23.1358

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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.>

Nonlinear Optics•Dec 17, 2002•0 citations
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.
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.
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.
We have observed optical second harmonic generation (SHG) in atomic sodium when the fundamental is tuned in the vicinity of the two-photon allowed transition 3s-4d. This result is obtained with the fundamental in a single loosely focused laser beam in the absence of any external fields. Although doubling in atomic media using noncollinear inputs1 or external fields2 has been reported by several groups, there remain only two previously reported doubling processes involving collinear beams and no fields.3 Both of these previous experiments reported both weak conversions and narrowly tunable outputs.
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.
We have observed second-harmonic generation in atomic hydrogen in the Lyman-α region, with and without an applied dc electric field, Ea. Experiments with a continuous atomic beam were carried out by using a pulsed dye laser tuned to 243 nm for excitation of the n = 2-1 transition. The laser beam was focused 0.4 mm below the H-source nozzle, where the H-atom density was estimated to be 5 × 1013 cm-3. Measurements of second-harmonic generation (SHG) at ~122 nm were made at various electric fields up to ~27 kV/cm. It was shown that the SH signal produced at E a = 0 occurs because of a charge-separation field of ~800 V/cm that is caused by three-photon ionization. Measurements of the SHG and ion current were found to vary as I2 L and I3 L , respectively. At high fields (E a > 10 kV/cm) the SH signal at the center of the Stark-split components decreased somewhat from the peak value, but the ion current (corresponding to photon absorption) approached zero. This observation is a clear demonstration of a similar effect, predicted by Harris et al.1 of efficient nonlinear conversion with induced transparency, and it suggests the possibility of obtaining intense Lyman-a radiation.
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.>