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Single-frequency cw second harmonic generation in sodium vapor

Bruce Sinclair, Malcolm H. Dunn

Published Jun 9, 1986• 0 citations
OpenAlex

Abstract

We investigated the process of magnetic field- induced optical second harmonic generation (SHG) in sodium vapor with a cw single-frequency dye laser tuned in the vicinity of the resonantly enhancing 3S-4D two-photon transition at 578.7 nm.1 All previous investigators have used pulsed dye lasers with relatively large linewidths.2 By exploiting the narrow linewidth capability of the cw dye laser (0.5 MHz) we have been able to investigate in detail for the first time properties of the SHG process which change across the line profile. We modeled the process on a theory developed from that reported by Uchlki et al.2 The two-photon absorption creates coherent quadrupole moments which rotate in the magnetic field due to Zeeman splitting to allow generation of the second harmonic.

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Journal of the Optical Society of America B•Oct 1, 1997•8 citations
We investigated the process of magnetic field- induced optical second harmonic generation (SHG) in sodium vapor with a cw single-frequency dye laser tuned in the vicinity of the resonantly enhancing 3S-4D two-photon transition at 578.7 nm.1 All previous investigators have used pulsed dye lasers with relatively large linewidths.2 By exploiting the narrow linewidth capability of the cw dye laser (0.5 MHz) we have been able to investigate in detail for the first time properties of the SHG process which change across the line profile. We modeled the process on a theory developed from that reported by Uchlki et al.2 The two-photon absorption creates coherent quadrupole moments which rotate in the magnetic field due to Zeeman splitting to allow generation of the second harmonic.
A high-resolution study of magnetic-field-induced second-harmonic generation in sodium vapor has been carried out with use of a single-frequency continuous-wave dye laser. Resonant enhancement was obtained by tuning the laser to the frequency of the 3S\ensuremath{\rightarrow}4D two-photon transition of the sodium atom. Experimental and theoretical results are presented showing the variation of the power and polarization properties of the second-harmonic radiation as the laser is tuned across the two-photon resonance, and as the transverse magnetic field is varied.
We analyze second harmonic generation in sodium vapor in a transverse magnetic field induced by the two-photon resonant excitation of the 3S-3D transition. The phenomenon is explained by the rotation of the two-photon induced quadrupole moment in the magnetic field. The analysis explains well the saturation of the intensity and rotation of the polarization of the second harmonic radiation in the higher magnetic field ≳0.5{\gtrsim}0.5≳0.5 kG. The collisional decay rate of the coherence of the E1 forbidden transition has been measured as an application of this SHG.
An experimental and theoretical study of magnetic-field-induced sum-frequency mixing in sodium vapor has been carried out using two single-frequency, continuous-wave dye lasers as the sources of the fundamental radiation. In addition to the usual two-photon resonant enhancement of the three-wave-mixing nonlinear-optical process, the use of near- or on-resonant single-photon transitions for further resonant enhancement was investigated. The macroscopic phase-matching behavior of the sum-frequency mixing was examined with the intermediate states off resonance. Experimental and theoretical results are presented to illustrate the differences that occur between the coherent quadrupole emission and the incoherent cascade fluorescence when the intermediate states in the two-photon absorption are resonant. Control over the single-photon resonant enhancement and the sum-frequency-mixing phase matching was achieved to increase the power conversion efficiency to 1.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$.
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 free atoms is one of the unsolved problems in nonlinear optics. The most probable mechanism is due to Stark mixing between nearby excited states through an electric field caused by charge separation through the process of three-photon ionization. To test the mechanism, atomic hydrogen appears to be the best candidate because of its well-known electronic structure and large dipole moments of its excited states. In this work we report on the observation of SHG in atomic hydrogen in the Lyman-α region. Pulsed dye laser radiation was doubled and tuned to 243 nm for excitation of the 2s ← 1s transition at 121.6 nm by a two-photon resonance process. A cw atomic beam was used with hydrogen atoms generated in a dc discharge of H2 gas. The laser beam was focused ~1 mm below the nozzle where the hydrogen atom density was estimated to be 1013 cm−3. The second harmonic radiation was strongly enhanced by adding an external dc field of up to 10 kV/cm. Stark splitting was clearly observed at higher fields.
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.