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Acta Sinica Quantum Optica

Inversionless Gain in Mismatched Doppler-broadened Λ-type Scheme

Yanliang Zhang

Published Jan 1, 2001• 0 citations
OpenAlex

Abstract

Continuous inversionless gain is studied in a Doppler-broadened Λ scheme. The inversionless gain is realizable in Doppler-broadened media for a coupling field wavelength well in excess of probe field wavelength when the laser power is significantly less than the Doppler width, and stronger than that of media for the coupling field wavelength equal to the probe field wavelength. In another scheme for the probe field wavelength well in excess of coupling field wavelength, the inversionless gain doesn't exist. The predictions are explained form the point of electromagneticaly-induced transparency.

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Continuous inversionless gain is studied in a Doppler-broadened Λ scheme. The inversionless gain is realizable in Doppler-broadened media for a coupling field wavelength well in excess of probe field wavelength when the laser power is significantly less than the Doppler width, and stronger than that of media for the coupling field wavelength equal to the probe field wavelength. In another scheme for the probe field wavelength well in excess of coupling field wavelength, the inversionless gain doesn't exist. The predictions are explained form the point of electromagneticaly-induced transparency.
Inversionless gain is predicted in a Doppler-broadened VVV scheme for which the probe laser frequency is approximately twice that of the coupling laser. A comparison is made of inversionless gain achieved via electromagnetically induced transparency in Doppler-broadened matched and mismatched wavelength systems. We show that the presence of Doppler broadening does not preclude inversionless lasing for mismatched wavelengths; importantly, this is shown to hold even for Rabi splittings significantly less than the Doppler width. Our analysis demonstrates that inversionless gain is realizable in Doppler-broadened media, at modest continuous wave laser powers, for a probe frequency well in excess of the coupling field frequency. The presented theory is related to a practical mismatched system in atomic rubidium vapor. Approximately 8% gain is predicted in a 422 nm probe field coherently driven by a 780 nm coupling field, in the presence of a modest level of incoherent excitation.
We present a theoretical study of the effects of mismatching wavelengths for the coupling and probe fields in Doppler-broadened media for the three basic energy level configurations commonly used to realize electromagnetically induced transparency (EIT). Three wavelength regimes are considered: mismatched wavelengths for which the coupling frequency is greater than the probe frequency, matched wavelengths for which the coupling and probe frequencies are equal, and mismatched wavelengths for which the probe frequency is greater than the coupling frequency. The transparency that may be induced in these regimes is compared for the cascade-, \ensuremath{\Lambda}-, and Vee-type systems. We show that in the first mismatched regime $({\ensuremath{\lambda}}_{c}<{\ensuremath{\lambda}}_{p})$ EIT is possible in all schemes and is in fact stronger than in the matched case. It is also demonstrated that for the second mismatched regime $({\ensuremath{\lambda}}_{c}>{\ensuremath{\lambda}}_{p})$ EIT can be realized most readily in the Vee-type configuration in the presence of Doppler broadening. These predictions are explained by considering the absorption as a function of both the probe field detuning and the atomic velocity.
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.
An analysis is made of the effect of Doppler broadening on gain without inversion (GWI) from different aspects in a closed lambda-type three-level system with an incoherent pump. It is shown that, regardless of the driving field being on resonance or not, for the counter- or co-propagating of the probe and driving fields (PDF), GWI does not monotonically decrease or increase with increasing Doppler width. Except for the case of counter-propagating PDF with off-resonance driving field, at a suitable Doppler width one can obtain a gain maximum value much larger than that without Doppler broadening; especially in the situation of the resonant driving field, the co-propagating geometry leads to a larger GWI. In addition, for the counter-propagating geometry, when Doppler width is larger enough, GWI oscillation occurs, and the oscillation amplitude and region increase with increasing Doppler width. These conclusions are very different from that obtained in previous investigation (Lukin et al, Laser Phys. 6 436 (1996)).