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arXiv:1904.08529 · New Journal of Physics

Power narrowing: counteracting Doppler broadening in two-color transitions

Ran Finkelstein, Ohr Lahad, Ohad Michel, Omri Davidson, Eilon Poem, Ofer Firstenberg

Published Sep 19, 2019• 19 citations
arXivHTMLPDF

Abstract

Abstract Doppler broadening in thermal ensembles degrades the absorption cross-section and the coherence time of collective excitations. In two photon transitions, it is common to assume that this problem becomes worse with larger wavelength mismatch. Here we identify an opposite mechanism, where such wavelength mismatch leads to cancellation of Doppler broadening via the counteracting effects of velocity-dependent light-shifts and Doppler shifts. We show that this effect is general, common to both absorption and transparency resonances, and favorably scales with wavelength mismatch. We experimentally confirm the enhancement of transitions for different low-lying orbitals in rubidium atoms and use calculations to extrapolate to high-lying Rydberg orbitals. These calculations predict a dramatic enhancement of up to 20-fold increase in absorption, even in the presence of large homogeneous broadening. More general configurations, where an auxiliary dressing field is used to counteract Doppler broadening, are also discussed and experimentally demonstrated. The mechanism we study can be applied as well for rephasing of spin waves and increasing the coherence time of quantum memories.

DOI
10.1088/1367-2630/ab4624

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Quantum Electronics and Laser Science Conference•May 18, 1997•0 citations
Abstract Doppler broadening in thermal ensembles degrades the absorption cross-section and the coherence time of collective excitations. In two photon transitions, it is common to assume that this problem becomes worse with larger wavelength mismatch. Here we identify an opposite mechanism, where such wavelength mismatch leads to cancellation of Doppler broadening via the counteracting effects of velocity-dependent light-shifts and Doppler shifts. We show that this effect is general, common to both absorption and transparency resonances, and favorably scales with wavelength mismatch. We experimentally confirm the enhancement of transitions for different low-lying orbitals in rubidium atoms and use calculations to extrapolate to high-lying Rydberg orbitals. These calculations predict a dramatic enhancement of up to 20-fold increase in absorption, even in the presence of large homogeneous broadening. More general configurations, where an auxiliary dressing field is used to counteract Doppler broadening, are also discussed and experimentally demonstrated. The mechanism we study can be applied as well for rephasing of spin waves and increasing the coherence time of quantum memories.
The Doppler effect is one of the dominant broadening mechanisms in thermal vapor spectroscopy. For two-photon transitions one would naively expect the Doppler effect to cause a residual broadening, proportional to the wave-vector difference. In coherent population trapping (CPT), which is a two-photon narrow-band phenomenon, such broadening was not observed experimentally. This has been commonly attributed to frequent velocity-changing collisions, known to narrow Doppler-broadened one-photon absorption lines (Dicke narrowing). Here we show theoretically that such a narrowing mechanism indeed exists for CPT resonances. The narrowing factor is the ratio between the atom's mean free path and the wavelength associated with the wave-vector difference of the two radiation fields. A possible experiment to verify the theory is suggested.
Abstract For multi-level systems in hot atomic vapors the interplay between the Doppler shift due to atomic motion and the wavenumber mismatch between driving laser fields strongly influences transmission and absorption properties of the atomic medium. In a three-level atomic ladder-system, Doppler broadening limits the visibility of electromagnetically-induced transparency (EIT) when the probe and control fields are co-propagating, while EIT is recovered under the opposite condition of counter-propagating geometry and k p < k c , with k p and k c being the wavenumbers of the probe and control fields, respectively. This effect has been studied and experimentally demonstrated as an efficient mechanism to realize non-reciprocal probe light transmission, which may enable applications as magnetic-field free optical isolators. Here, we describe the basics of this effect and discuss a simple picture for the underlying mechanism. We illustrate how the non-reciprocity scales with wavelength mismatch and show how to experimentally demonstrate the effect in a simple Rydberg-EIT system using thermal Rubidium atoms.
For pt.I see ibid., vol.8, p.1795 (1975) where the theory of Doppler-free two-photon transitions was discussed for the case of one single laser frequency. Here, the effects of two different frequencies are considered. When the residual Doppler width of the two-photon transition is less than the natural linewidth, the previous results are essentially regained. For large residual Doppler shifts, the absorption profile is a power-broadened Lorentzian if the frequency of the probe field is larger than the frequency of the strong field. In the opposite case, the absorption peak splits because of the AC Stark effect. The high-intensity distortions are minimized when equal frequencies can be used. Power-dependent shifts do not ordinarily occur with a resonantly tuned intermediate level. Some modifications of the spectra are introduced by phase-disturbing collisions.
The line shape of a Doppler-broadened transition is dramatically altered by the presence of a laser field resonating with a second Doppler-broadened transition sharing a common level: Two narrow resonances of different widths appear superimposed upon the broad background signal at frequencies symmetrically located about the corresponding line center. The effect has already found application in a number of seemingly different though intimately related studies, including high-resolution hfs and isotope-shift determinations. The theory of the effect is developed with reference to these applications. The treatment is formulated in terms of transition rates induced by two classical fields resonantly interacting with a pair of coupled Doppler-broadened transitions of arbitrary frequencies. The perturbation approach adopted is valid for one field fully saturating its transition; the resulting line-shape expression exhibits important power-broadening effects. This approach is equivalent to the familiar density-matrix formulation, which is also presented. Various features of the resulting expression are discussed in detail as they apply to two precision spectroscopic applications, mode crossing and spontaneous emission line narrowing. The connection with previous work is also discussed.
The power broadening of a coupling laser can be converted into two-photon detuning by electromagnetically induced transparency (EIT), resulting in a residual Doppler effect. The residual Doppler effect in a ladder-type EIT in a room-temperature atom ensemble is further amplified through a wavelength mismatch effect between the probe and coupling laser beams, which reduces the atomic coupling of light or microwaves. We measured the Rydberg spectra of the electric dipole (E1) and electric quadrupole (E2) microwave transitions, demonstrating that the reduction in the Rydberg EIT signal can be recovered through far-off-resonance E2 microwave transition dressing and achieving an 8-dB enhancement in the Rydberg EIT signal. The frequency-dependent dressing of the E2 transition enables the shift of the dressed Rydberg states to be tuned, thereby providing a scalable approach to optimize the interaction between the Rydberg state and microwave field.
Recently it was predicted, and shown experimentally, that a cw coherently induced transparency feature could be observed in a Doppler-broadened cascade medium even when the probe and coupling fields had a wavelength mismatch of several hundred nanometers.1 This observation is significant since it removes the requirement for finding wavelength-matched systems to carry out cw atomic coherence experiments within an inhomogeneously broadened medium.2,3 The explanation of this transparency lies with the fact that both E.I.T. and Autler-Townes splitting4 are velocity group dependent. Each velocity group experiences its own Doppler-shift and hence detuning from the probe and coupling field transitions. Therefore, there exists a variation in the frequency position of maximum E.I.T. and of the Autler-Townes split components for each velocity groups. It is the integration of both of these effects over all of the velocity groups that produces the overall probe absorption.