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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Chemically induced processes evidencing Raman gain

David R. Grantier, Peter M. Medley, James L. Gole

Published Mar 31, 1995• 3 citations
Publisher

Abstract

The highly selective Na3 + X (Cl,Br,I) reactions have been shown to create a continuous electronic population inversion based on the chemical pumping of Na2. Optical gain through stimulated emission has been demonstrated in regions close to 527, 492, and 460 nm ((alpha) equals 8 X 10-3 cm-1 for an individual rotational level at -527 nm). A device has ben constructed with a focus to increasing amplifier gain length and amplifying medium concentration based on the controlled intersection of supersonically expanded sodium and halogen atom sheaths. The interaction forms an extended reaction-amplification zone centered on-axis in an optical cavity, thus facilitating the conversion of the observed amplifiers to chemical laser oscillators. Initial results with this upscaled device, where the sodium metal expanded in both pure and seeded supersonic expansion is intersected by a bromine atom flow, provide the first example of chemically enhanced Raman scattering. Unique Raman signals are induced by and correlate with emission from the Na D-line components formed in the chemical reaction zone primarily as a result of Na2 + Br yields Na* + NaBr reaction, cannot be readily generated by light scattering due to an external light source, and appear to be enhanced by the environment of the reaction zone itself. The Na D-line emitters interact with cooled sodium dimers in a resonance Raman scattering process, for which computer simulations suggest a scattering linewidth, (Gamma) -4 cm-1. These results suggest an unusually fast resonance Raman scattering process which appears to be chemically enhanced. The results of initial double pass gain measurements suggest that a stimulated Raman process, similar to that associated with optically pumped alkali dimer lasers, has been observed.

DOI
10.1117/12.204961

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Physical Review A•May 12, 2005•0 citations
The highly selective Na3 + X (Cl,Br,I) reactions have been shown to create a continuous electronic population inversion based on the chemical pumping of Na2. Optical gain through stimulated emission has been demonstrated in regions close to 527, 492, and 460 nm ((alpha) equals 8 X 10-3 cm-1 for an individual rotational level at -527 nm). A device has ben constructed with a focus to increasing amplifier gain length and amplifying medium concentration based on the controlled intersection of supersonically expanded sodium and halogen atom sheaths. The interaction forms an extended reaction-amplification zone centered on-axis in an optical cavity, thus facilitating the conversion of the observed amplifiers to chemical laser oscillators. Initial results with this upscaled device, where the sodium metal expanded in both pure and seeded supersonic expansion is intersected by a bromine atom flow, provide the first example of chemically enhanced Raman scattering. Unique Raman signals are induced by and correlate with emission from the Na D-line components formed in the chemical reaction zone primarily as a result of Na2 + Br yields Na* + NaBr reaction, cannot be readily generated by light scattering due to an external light source, and appear to be enhanced by the environment of the reaction zone itself. The Na D-line emitters interact with cooled sodium dimers in a resonance Raman scattering process, for which computer simulations suggest a scattering linewidth, (Gamma) -4 cm-1. These results suggest an unusually fast resonance Raman scattering process which appears to be chemically enhanced. The results of initial double pass gain measurements suggest that a stimulated Raman process, similar to that associated with optically pumped alkali dimer lasers, has been observed.
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.
Energy transfer from selectively formed metastable states of SiO is used to pump sodium-atom laser amplifiers at lambda approximately=569 nm (4d/sup 2/D-3p/sup 2/P), lambda approximately=616 nm (5s/sup 2/S-3p/sup 2/P), and lambda approximately=819 nm (3d/sup 2/D-3p/sup 2/P). The a/sup 3/ Sigma /sup +/ and b/sup 3/ Pi states of SiO are generated in high yield from the Si+N/sub 2/O to SiO+N/sub 2/ reaction. The energy stored in the triplet states is transferred in a highly efficient collisional process to pump sodium atoms to their lowest excited 3d/sup 2/D, 4d/sup 2/D, and 5s/sup 2/S states. Adopting a sequence in which high concentrations of silicon and sodium atoms are mixed and oxidized, a continuous amplification (gain condition) is monitored which suggests the creation of a population inversion among the receptor sodium-atom energy levels and forms the basis for full cavity oscillation on the Na4d/sup 2/D-3p/sup 2/P transition at 569 nm.>
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.
The feasibility of a novel technique for efficient and selective population transfer from a thermally populated level 1 via an intermediate state 2 to level 3 is experimentally demonstrated. It is shown for sodium dimers that the process of on- or near-resonance stimulated Raman scattering with only partially overlapping laser beams is, in particular, useful for the selective population of high vibrational levels of particles in a molecular beam. This is achieved when the interaction with the Stokes laser, coupling levels 2 and 3, begins earlier than the interaction with the pump laser. The phenomenon, which is closely related to the formation of ‘‘trapped states,’’ is quantitatively explained using the basis of eigenstates of molecules strongly coupled to the radiation fields. The similarity and difference to related techniques such as rapid adiabatic passage phenomena in two-level systems, off-resonant stimulated Raman scattering, or stimulated emission pumping is briefly discussed.
The possibility of realizing an efficient gaseous laser-beam-generating medium that utilizes $\ensuremath{\Lambda}$-type coherently phased (i.e., ``dressed'') atoms for the active laser species, but that does not inherently require the use of external laser beams for pumping, is explored. Specifically, it is investigated if multiphoton stimulated hyper-Raman scattering (SHRS) processes driven by fluorescence radiation generated in a continuous electrical discharge present within the vapor-containing cell could produce continuous-wave (cw) optical gain at the $\ensuremath{\Lambda}$-atom resonance frequencies ${\ensuremath{\omega}}_{o}$ and ${\ensuremath{\omega}}_{o}^{\ensuremath{'}}$. It is deduced that such gain could result from nnn-photon $(n\ensuremath{\geqslant}4)$ SHRS processes only if absorption of fluorescence pump light occurs in the first three transitions of the nnn-photon sequence representing the process unit step. Estimates of the amount of optical gain that could be produced in such a system indicate that it should be sufficient to allow multiwatt cw laser operation to occur on one set of $\ensuremath{\Lambda}$ transitions connecting levels in a ``double-$\ensuremath{\Lambda}$'' structure, with the pump light being discharge-produced fluorescence centered about the transitions of the other $\ensuremath{\Lambda}$ pair. However, to initiate operation of such a device would require injection into the laser optical cavity of intense ``starter'' laser pulses at both lasing frequencies. What should be an optimal experimental configuration for determining feasibility of the proposed laser device is described. In the suggested configuration, Cs-atom $6{S}_{1∕2}\text{\ensuremath{-}}6{P}_{1∕2}$ transitions form the double-$\ensuremath{\Lambda}$ structure.