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Sum frequency mixing and quantum interference in three-level atoms

Richard R. Moseley

Published Jul 1, 1995• 2 citations
OpenAlex

Abstract

This work contains theoretical and experimental studies of the properties of three-level atoms subjected to two single-frequency, continuous-wave laser sources with special emphasis on magnetic-field-induced sum frequency mixing (SFM) and electromagnetically-induced transparency (EIT). In sodium vapour, two resonant-enhancement routes for SFM were experimentally studied. In one route (3S-3P-3D) the output is generated on a quadrupole-allowed transition and on the other (3S-3P-4P), a dipole-allowed transition. Phase matching conditions are studied in detail. On the second route, the two contributions to the phase mismatch can combine to heavily distort the spectroscopic lineshapes observed. Doubly-resonant SFM is considered and two paths for excitation of the output coherence are identified. Control of their relative strength, via the input laser strengths, is predicted, as well as interference between them. These are both experimentally observed on the 3S1/2-3P1/2-3D3/2 transition scheme in sodium. Theory is presented to show how the use of a strong upper laser in resonant SFM can, by EIT, greatly reduce the absorption on the lower transition without similarly reducing the nonlinear process. A significant increase in conversion efficiency is predicted. Theory is also presented on the inclusion of the local field correction, which is relevant for high vapour densities, in the Bloch equations for the three-level atom. Nonlinear coherence cross-coupling terms arise and the generalised equations are then used for two case studies; one concerning EIT, the other SFM. EIT is experimentally studied using continuous-wave lasers in rubidium vapour. Focusing and de-focusing were observed on the probe laser beam as it tunes through the transparency window at close, but separate, detunings. This is attributed to the radial intensity profile of the coupling laser which imposes a spatial refractive index modulation on the medium at the probe laser wavelength.

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Aug 5, 1994•0 citations
This work contains theoretical and experimental studies of the properties of three-level atoms subjected to two single-frequency, continuous-wave laser sources with special emphasis on magnetic-field-induced sum frequency mixing (SFM) and electromagnetically-induced transparency (EIT). In sodium vapour, two resonant-enhancement routes for SFM were experimentally studied. In one route (3S-3P-3D) the output is generated on a quadrupole-allowed transition and on the other (3S-3P-4P), a dipole-allowed transition. Phase matching conditions are studied in detail. On the second route, the two contributions to the phase mismatch can combine to heavily distort the spectroscopic lineshapes observed. Doubly-resonant SFM is considered and two paths for excitation of the output coherence are identified. Control of their relative strength, via the input laser strengths, is predicted, as well as interference between them. These are both experimentally observed on the 3S1/2-3P1/2-3D3/2 transition scheme in sodium. Theory is presented to show how the use of a strong upper laser in resonant SFM can, by EIT, greatly reduce the absorption on the lower transition without similarly reducing the nonlinear process. A significant increase in conversion efficiency is predicted. Theory is also presented on the inclusion of the local field correction, which is relevant for high vapour densities, in the Bloch equations for the three-level atom. Nonlinear coherence cross-coupling terms arise and the generalised equations are then used for two case studies; one concerning EIT, the other SFM. EIT is experimentally studied using continuous-wave lasers in rubidium vapour. Focusing and de-focusing were observed on the probe laser beam as it tunes through the transparency window at close, but separate, detunings. This is attributed to the radial intensity profile of the coupling laser which imposes a spatial refractive index modulation on the medium at the probe laser wavelength.
We have examined the effect of various phase-matching conditions on sum frequency generation in vapors. Using sodium vapor two resonant-enhancement routes for sum-frequency mixing (SFM) via a second-order magnetic-field-induced nonlinearity were explored. On one route the output is resonant with a quadrupole-allowed transition and the other a dipole-allowed resonance. We will show that the first route is preferential for SFM as phase matching on this route is easier to control and we present results detailing the important parameters and their effect in focused and unfocused cases. A second-order nonlinearity of 0.5--1 ${\mathrm{pmV}}^{\mathrm{\ensuremath{-}}1}$ is observed under optimum conditions. In the second route, variations in phase matching as one tunes across the transition may lead to a significant distortion of the spectroscopic line shapes. We show how the combination of different refractive-index sources can account for the different features seen.
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 present theory and observation to show that in resonant sum-frequency mixing the two pathways to the excitation of the coherence driving the output may be controlled in relative strength, and combine in antiphase on resonance. These two pathways are, first, a two-step route resulting in the creation of a real intermediate population, and second, a direct two-photon excitation resonantly enhanced by, but not populating, the intermediate level. The former route is shown to be raised over the latter by use of a stronger lower laser field. Experimental observation uses the 3S-3P1/2{\mathit{P}}_{1/2}P1/2​-3D level scheme in sodium vapor and single-frequency continuous-wave lasers for steady-state conditions and high resolution. A cancellation ratio of 94% is reported. Further, noninterfering incoherent fluorescence and an anti-Stokes Raman process are observed and contrasted.
The sum-frequency generation process using strong-field coupling and quantum interference is described systematically using atomic hydrogen both in theory and experiment. It is shown that strong coupling at the 3p-2s transition creates destructive interference to reduce the linear susceptibility at the resonant 3p\ensuremath{\leftarrow}1s transition. Simultaneously, two-photon coupling of the 2s and 1s states produces constructive interference to resonantly enhance the nonlinear susceptibility for sum-frequency generation at the 3p\ensuremath{\rightarrow}1s (Lyman-\ensuremath{\beta}) transition. Experimental results clearly demonstrate the theoretical predictions. When the Rabi frequency for the 3p-2s coupling exceeds the Doppler tail of the medium, the photoion signals on resonance are suppressed significantly because of induced transparency. The generated Lyman-\ensuremath{\beta} radiation grows rapidly by a factor of 250 at an atom density of 1015{10}^{15}1015 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and interaction length of 3.5 cm, resulting in a conversion efficiency of 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$.
We report on an investigation of electromagnetically induced transparency (EIT) in sodium vapour. In our experiment, sodium atoms are excited on the D1-line with laser radiation containing two components with a frequency difference close to that of the two hyperfine ground states of sodium (1.772 GHz). Such an excitation leads to coherent trapping of atomic population in “dark” superpositional states, which dramatically reduces the absorption of light. A frequency transparency window is measured to have a subnatural width, which is a clear indication of coherent population trapping. Dependence of EIT on laser frequencies and intensities, on the magnetic field strength as well as on the temperature of the sodium vapour is studied.
Sum-frequency generation with electromagnetically induced transparency is investigated experimentally as the generation of coherent Lyman-(Beta) radiation (103 nm) in atomic hydrogen. We demonstrate that with the application of a strong laser field to couple the 2 s and 3 p levels, the resonant Lyman-(Beta) transition becomes transparent, and the generated wave continues to grow without suffering resonant absorption. At a density-length product NL of 3 X 1015 cm-2, a conversion efficiency of 2 X 10-4 is obtained for the coherent 103 nm radiation.