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Forward four-wave mixing via recoil-induced resonances in cold cesium atoms

Álvaro Mitchell Galvão de Melo

Published Dec 17, 2019• 0 citations
OpenAlex

Abstract

Recoil-induced resonances (RIR) can be detected in experiments involving light-atom interaction where atomic external degrees of freedom play an important role. In pump-probe experiments the recoil of atoms due to absorption and emission of radiation is responsible for the amplification and attenuation of the probe beam under certain circumstances, resulting in a disperse and narrow (of the order of 10 kHz) feature in the spectrum of the probe beam. Backward four-wave mixing (BFWM) signals, via recoil-induced resonances, using cold atoms as nonlinear medium were also extensively studied during the past years. In this work it will be presented a systematic study of forward four-wave mixing (FFWM) via RIR. It will be also presented a simple theoretical model that allowed us to predict the lineshapes of the observed signals and to better understand the phenomenon studied. The model is also compared with results that are already known in order to verify its validity and predict the possibility of measure the velocity distribution of the atomic cloud in a future experiment.

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Recoil-induced resonances (RIR) can be detected in experiments involving light-atom interaction where atomic external degrees of freedom play an important role. In pump-probe experiments the recoil of atoms due to absorption and emission of radiation is responsible for the amplification and attenuation of the probe beam under certain circumstances, resulting in a disperse and narrow (of the order of 10 kHz) feature in the spectrum of the probe beam. Backward four-wave mixing (BFWM) signals, via recoil-induced resonances, using cold atoms as nonlinear medium were also extensively studied during the past years. In this work it will be presented a systematic study of forward four-wave mixing (FFWM) via RIR. It will be also presented a simple theoretical model that allowed us to predict the lineshapes of the observed signals and to better understand the phenomenon studied. The model is also compared with results that are already known in order to verify its validity and predict the possibility of measure the velocity distribution of the atomic cloud in a future experiment.
In this work, we perform a detailed theoretical and experimental investigation of an atomic memory based on recoil-induced resonance in cold cesium atoms. We consider the interaction of nearly degenerate pump and probe beams with an ensemble of two-level atoms. A full theoretical density matrix calculation in the extended Hilbert space of the internal and external atomic degrees of freedom allows us to obtain, from first principles, the transient and stationary responses determining the probe transmission and the forward four-wave-mixing spectra. These two signals are generated together at the same order of perturbation with respect to the intensities of pump and probe beams. Moreover, we have investigated the storage of optical information on the spatial modes of light beams in the atomic external degrees of freedom, which provided a simple interpretation for the previously reported nonvolatile character of this memory. The retrieved signals after storage reveal the equivalent role of probe transmission and four-wave mixing, as the two signals have similar amplitudes. Probe transmission and forward four-wave-mixing spectra were then experimentally measured for both continuous excitation and after storage. The experimental observations are in good agreement with the developed theory, and they open another pathway for the reversible exchange of optical information with atomic systems.
We present experimental observation of recoil-induced resonance (RIR) in an operating cesium magneto-optical trap (MOT) by both pump–probe absorption and four-wave mixing spectra simultaneously. We investigate the dependence of amplitudes of these two spectra on pump beam intensity and frequency. The measurement results agree well with the recoil-induced theory with modifications of Raman transition effect and atomic number. The systematical study on RIR spectra is meaningful for the diagnostic measurement of cold atoms in an operating MOT.
In this PhD work, two distinct phenomena are considered, which are both related to non-linear interactions between light and atoms. The first part of the thesis is dedicated to four wave mixing based on the internal degrees of freedom of room temperature helium atoms and uses it for amplification processes and generation of squeezed light. The second studied process is based on external degrees of freedom of cold cesium atoms and used for light storage and phase conjugate field generation through multi-wave mixing. I experimentally observed and characterized phase sensitive amplification via four-wave mixing in metastable helium at room temperature. I have obtained about 9 dB of maximum gain with a bandwidth of about 300 kHz. The obtained phase transfer functions showed a strong phase squeezing, indicating that the phenomenon was almost free of unwanted processes. In the second part, I explain how recoil induced resonances, which are due to the transfer of momentum between a photon and an atom, can be used to store light. I also explain how this phenomenon can lead to generation of a phase conjugate field, and why the existing theory fails to model the dip, which appears in the phase conjugate generation spectrum when the field power is increased. I extend the model to the fifth order so that it can reproduce this new feature and demonstrate that it depends on the decay rate of the coherence, which is excited between atomic levels of different momenta. I then show that a simpler model, which is based on three levels defined by internal and external degrees of freedom of the atom, can explain the observed phenomenon.
It is shown that, as a result of the atomic recoil associated with the absorption and emission of radiation, resonances can appear in nonlinear spectroscopy. The theory is illustrated by calculating the line shapes associated with nearly degenerate four-wave mixing and pump-probe spectroscopy when laser fields interact with an ensemble of two-level atoms. The recoil-induced resonances should be observable for atoms cooled below the Doppler limit of laser cooling, and the line shapes may provide a means for measuring the velocity distribution of these cooled atoms.
In this paper we report on the first purely ``pump-probe'' nonlinear laser spectroscopy results in a slow atomic beam. We have observed Raman, Rayleigh, and recoil-induced resonances (RIR) in a continuous beam of slow and cold cesium atoms extracted from a two-dimensional (2D) magneto-optical-trap (MOT) with the moving molasses technique. The RIR enabled us to measure the velocity distribution, therefore the average speed (0.6--4 m/s) and temperature (50--500 \ensuremath{\mu}K) of the atomic beam. Compared to time of flight, this technique has the advantage of being local, more sensitive in the low-velocity regime (v<1m/s),(v<1 \mathrm{m}/\mathrm{s}),(v<1m/s), and it gives access to transverse velocities and temperatures. Moreover, it may be extended to measure atomic velocities in the 2D MOT source of the atomic beam.
Abstract We report on the observation of efficient light storage based on recoil-induced resonances and the associated generation of forward four-wave mixing signal. We use a strong coupling beam red-detuned from the closed transition 6S1/2,F=4→6P3/2,F′=56S_{1/2}, F=4\rightarrow6P_{3/2}, F^{\prime}=56S1/2​,F=4→6P3/2​,F′=5 of cold cesium atom and a two-mode probe beam, whose frequency can be scanned around the coupling beam frequency. First, we demonstrate the coherent phase transfer from one mode component of the probe beam to the corresponding forward generated four-wave mixing beam. We then explore the coherent nature of the stored optical information to retrieve the two-mode probe energy with an efficiency 3.5 fold higher as compared with the total retrieved energy efficiency associated with the case where each mode component of the probe acts separately. Finally, we discuss a possible application of our experimental scheme and observed effects to investigate squeezing and quantum photon correlations.