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中国激光(英文版)

Competition between Quantum Interference and Stimulated Electronic Raman Scattering Processes via Hybrid Excitation in a Na_2-Na System

Xihua Yang

Published Jan 1, 2002• 0 citations
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Abstract

An experimental observation of the competition between quantum interference and stimulated electronic Raman scattering (SERS) via equal frequency hybrid excitation and collision aided radiative excitation in a Na 2 Na system is reported for the first time. It is found that the SERS for 3P 1/2,3/2 4P are suppressed and the optically pumped stimulated emission (OPSE) for 5S 4P are enhanced dramatically at a particular excitation wavelength where constructive quantum interference between two transitions 3P 1/2 5S and 3P 3/2 5S in atomic sodium takes place. A theoretical analysis based on the dressed atom approach is carried out to explain the experimental results.

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We report on an experimental observation of constructive quantum interference between two atomic transition pathways 3P 1/2 -5S (or 4D) and 3P 3/2 -5S (or 4D) via equal-frequency hybrid excitation and collision-aided radiative excitation in an Na 2 -Na system. Through the special excitation processes, constructive quantum interference is realized for a Λ-type quantum-beat configuration with a widely-spaced doublet (about 17 cm -1 ) in the excited sodium atom driven by one laser. The dependences of quantum interference on the argon buffer gas pressure and sample temperature are also examined.

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Tunable UV coherent radiation and infrared stimulated radiation generated by hybrid pumping in the Na2-Na system

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Laser-based probes of chemically induced Raman pumping

David R. Grantier, James L. Gole

The highly selective Na3 + Br reaction creates a continuous electronic population inversion on Na2 transitions which demonstrates optical gain through stimulated emission in regions close to 527, 492, and 460 nm ((alpha) equals 8 X 10-3 cm-1 for an individual rotational level at approximately 527 nm). With a focus to increasing amplifier gain length and amplifying medium concentration (conversion to oscillator), we extend these initial studies employing the intersection of sodium and bromine flows formed from slit source expansions to create an extended reaction-amplification zone. Initial results with this extended reaction zone provide the first example of chemically induced Raman pumping. Unique Raman signals are induced by and correlate with emission from the NaD-line components (D1, D2) formed in the chemical reaction zone primarily as a result of the Na2 + Br yields Na* (3P) + NaBr reaction. The NaD-line emitters interact with cooled sodium dimers in a resonance Raman scattering process, for which computer simulation of the experimentally observed spectra predict an unusually large scattering linewidth close to 4 cm-1. We describe initial laser based studies on a pure sodium expansion to probe the mechanism of the unusually strong Raman scattering and to understand the origin of the large scattering linewidth. These experiments demonstrate that the Raman process can be induced only upon excitation of the NaD-line components, the Raman scattering being insufficient or nonexistent for wavelength shifts (Delta) (lambda) > 1 nm from the D lines. The scattering linewidth for the reactive process is not readily correlated with the line-broadening of the NaD-line and may result from the nature of the Na- Na2 interaction in the reactive environment. Initial double pass measurements indicate the possibility of optical gain associated with the Raman process.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Mar 29, 1996•2 citations
An experimental observation of the competition between quantum interference and stimulated electronic Raman scattering (SERS) via equal frequency hybrid excitation and collision aided radiative excitation in a Na 2 Na system is reported for the first time. It is found that the SERS for 3P 1/2,3/2 4P are suppressed and the optically pumped stimulated emission (OPSE) for 5S 4P are enhanced dramatically at a particular excitation wavelength where constructive quantum interference between two transitions 3P 1/2 5S and 3P 3/2 5S in atomic sodium takes place. A theoretical analysis based on the dressed atom approach is carried out to explain the experimental results.
We report on an experimental observation of constructive quantum interference between two atomic transition pathways 3P 1/2 -5S (or 4D) and 3P 3/2 -5S (or 4D) via equal-frequency hybrid excitation and collision-aided radiative excitation in an Na 2 -Na system. Through the special excitation processes, constructive quantum interference is realized for a Λ-type quantum-beat configuration with a widely-spaced doublet (about 17 cm -1 ) in the excited sodium atom driven by one laser. The dependences of quantum interference on the argon buffer gas pressure and sample temperature are also examined.
For the first time by means of two-photon hybrid excitation in the Na2-Na system, which is distinguished from the usual two-photon excitation in atomic vapor, tunable UV coherent radiation (between 337.6 and 339.1 nm and between 358.0 and 359.6 nm) and tunable infrared stimulated radiation (3.38-3.46 µm) were generated. By optically pumping with a pulsed dye laser scanning from 614.5 to 617.4 nm for sodium vapor in a heat-pipe oven, Na2 was excited by the first photon absorption from the X ′ Σ g + to the A ′ Σ u + state. The energy was transferred to Na by Na2*-Na collision to populate the levels 3p1/2,3/2 in Na, which was followed by another photon excitation from the same beam up to near the 5S level, resulting in SERS (3P1/2-4P and 3P3/2-4P) and OPSE(5S → 4P and 4P → 4S) radiation. Assuming the detunings of pumping frequency ω1, from ω(5S,3P3/2) and ω(5s,3p1/2) to be Δ1 and Δ2, respectively, the three UV coherent radiation components were supposed to be generated by the following mixing processes: ωUV = 2ω1 − ω(5S − 4P); ω U V ′ = 2 ω 1 − [ ω ( 5 S → 4 P ) + Δ 1 ] ; ω U V ′ ′ = 2 ω 1 − [ ω ( 5 S → 4 P ) + Δ 2 ] , and also ω U V ′ ′ ′ = 2 ω 1 − ω ( 4 P → 4 S ) . The SERS and the UV signals were tuned as the pumping wavelength was tuned. The measured wavelengths agreed well with the calculated values.
Laser excitation and ionization processes in dense (1–10 Torr) sodium vapor have been studied for laser wavelengths near the two-photon allowed nd 2D(n=3, 4, 5) and ns 2S(n=4, 5, 6) states. In particular, the 1 3Σ+g→1 3Σ+u excimer emission in Na2, predicted in 1980 by Konowalow and Julienne and observed recently by Dinev et al, was studied here in greater detail. Strong excimer emission (∼830 nm) was observed for two-photon pumping to both sides of the unresolved 4D states, and weak excimer emission was seen when pumping near the 5D levels. The excimer emission exhibits a complicated pump laser profile with a pronounced ‘‘dip’’ at the 4d 2D two-photon resonance. Similarly, [2+1] photon ionization via the 3d 2D and 4d 2D states shows a dramatic decrease as the sodium density increases. These results can be attributed either to depleted 3d 2D or 4d 2D population due to stimulated electronic Raman scattering (SERS) or to the interference effects recently reported by Malcuit et al. and Krasnikov et al. and treated theoretically by Manykin and Afanas’ev and by Agarwal. It is argued that both mechanisms are operative. Strong ionization and SERS signals were observed at the hybrid resonances corresponding to 3p 2P3/2, 1/2 →4d 2D transitions; however, no excimer lasing at 830 nm was detected. No excimer emission was detected upon two-photon pumping near or at the 3d 2D or ns 2S(n=4, 5, 6) states. Based on these and other observations, the 1 3Σ+g→1 3Σ+u excimer emission is attributed to a molecular Raman process involving stimulated emission or six-wave mixing via a pathway of the type 1 3Σ+u→2hν 3Δu→ j 3Πg→(k 3Πu, l 3Σ +u)→1 3Σ+g→1 3Σ +u .
We have studied theoretically constructive interference via collision-aided radiative excitation in an open four-level system using a density matrix approach. The four-level system consists of a Λ-type three-level quantum-beat configuration driven by one laser field and a fourth level coupled by a vacuum mode. It is shown that through the incoherent process (collision), coherence between widely-separated doublets and subsequent constructive interference can be realized. We analyse the effects of the collision-induced coherent and incoherent decay rates, laser intensity and energy separation of the doublets on the interference. Meanwhile, the constructive interference between the two transition pathways 3P 1/2 -4D and 3P 3/2 -4D via equal-frequency hybrid excitation and collision-aided radiative excitation in an Na 2 -Na system is experimentally observed. A good agreement between the theoretical and experimental results is obtained.
The highly selective Na3 + Br reaction creates a continuous electronic population inversion on Na2 transitions which demonstrates optical gain through stimulated emission in regions close to 527, 492, and 460 nm ((alpha) equals 8 X 10-3 cm-1 for an individual rotational level at approximately 527 nm). With a focus to increasing amplifier gain length and amplifying medium concentration (conversion to oscillator), we extend these initial studies employing the intersection of sodium and bromine flows formed from slit source expansions to create an extended reaction-amplification zone. Initial results with this extended reaction zone provide the first example of chemically induced Raman pumping. Unique Raman signals are induced by and correlate with emission from the NaD-line components (D1, D2) formed in the chemical reaction zone primarily as a result of the Na2 + Br yields Na* (3P) + NaBr reaction. The NaD-line emitters interact with cooled sodium dimers in a resonance Raman scattering process, for which computer simulation of the experimentally observed spectra predict an unusually large scattering linewidth close to 4 cm-1. We describe initial laser based studies on a pure sodium expansion to probe the mechanism of the unusually strong Raman scattering and to understand the origin of the large scattering linewidth. These experiments demonstrate that the Raman process can be induced only upon excitation of the NaD-line components, the Raman scattering being insufficient or nonexistent for wavelength shifts (Delta) (lambda) > 1 nm from the D lines. The scattering linewidth for the reactive process is not readily correlated with the line-broadening of the NaD-line and may result from the nature of the Na- Na2 interaction in the reactive environment. Initial double pass measurements indicate the possibility of optical gain associated with the Raman process.