Velocity Selective Optical Pumping Research Articles

Frequency characteristics of collimated blue light generated by four-wave mixing in cesium vapor.

The frequency evaluation of collimated blue light (CBL) generated by parametric four-wave mixing (FWM) in hot atomic vapor is presented. The cesium (133Cs) atoms are excited from the 6S1/2 ground state to 8S1/2 excited state with the 852 and 795 nm pump lasers, producing an optical field at 4.2 µm through an amplified spontaneous emission via population inversion on the 8S1/2 → 7P3/2 transition and the 456 nm CBL via FWM on the 7P3/2 → 6S1/2 transition. The frequency shift of 456 nm CBL has been measured by velocity-selective optical pumping spectral technique when the frequency of any one of two pumping lasers is tuned. The ratio of the frequency shift of 456 nm CBL to the two-photon frequency detuning of two pumping lasers is 0.8912, implying that the generated 4.2 µm light frequency is also correspondingly shifted.

Room temperature atomic frequency comb storage for light.

We demonstrate coherent storage and retrieval of pulsed light using the atomic frequency comb protocol in a room temperature alkali vapor. We utilize velocity-selective optical pumping to prepare multiple velocity classes in the F=4 hyperfine ground state of cesium. The frequency spacing of the classes is chosen to coincide with the F'=4-F'=5 hyperfine splitting of the 62P3/2 excited state, resulting in a broadband periodic absorbing structure consisting of two usually Doppler-broadened optical transitions. Weak coherent states of duration 2ns are mapped into this atomic frequency comb with pre-programmed recall times of 8ns and 12ns, with multi-temporal mode storage and recall demonstrated. Utilizing two transitions in the comb leads to an additional interference effect upon rephasing that enhances the recall efficiency.

Preparing narrow velocity distributions for quantum memories in room-temperature alkali-metal vapors

Quantum memories are a crucial technology for enabling large-scale quantum networks through synchronisation of probabilistic operations. Such networks impose strict requirements on quantum memory, such as storage time, retrieval efficiency, bandwidth, and scalability. On- and off-resonant ladder protocols on warm atomic vapour platforms are promising candidates, combining efficient high-bandwidth operation with low-noise on-demand retrieval. However, their storage time is severely limited by motion-induced dephasing caused by the broad velocity distribution of atoms comprising the vapour. In this paper, we demonstrate velocity selective optical pumping to overcome this decoherence mechanism. This will increase the achievable memory storage time of vapour memories. This technique can also be used for preparing arbitrarily shaped absorption profiles, for instance, preparing an atomic frequency comb absorption feature.

Inter-ground-state crossover resonances formed in atomic vapor by a dual-frequency laser

We present a study of the saturated-absorption spectroscopy with crossover resonances involving both Cs hyperfine ground-state levels using a dual-frequency laser tuned to the Cs line. The crossover resonances are formed by the atomic velocity groups on resonance with both of the dual-frequency laser beams, which are counter-propagated through the vapor cell. A large increase in Doppler-free atomic absorption is observed for certain frequency differences of the dual-frequency laser. This phenomenon is explained by optical pumping and velocity selective optical pumping, imposed by the dual-frequency laser with frequency differences close to but not exactly equal to the level spacing of the two participating ground-state levels. The results obtained in this system are of great interest for laser spectroscopy.

Doppler narrowing, Zeeman and laser beam-shape effects in Λ-type electromagnetically induced transparency on the 85Rb D2 line in a vapor cell

We study Λ-type Electromagnetically Induced Transparency (EIT) on the Rb D2 transition in a buffer-gas-free thermal vapor cell without anti-relaxation coating. Experimental data show well-resolved features due to velocity-selective optical pumping and one EIT resonance. The Zeeman splitting of the EIT line in magnetic fields up to 12 Gauss is investigated. One Zeeman component is free of the first-order shift and its second-order shift agrees well with theory. The full width at half maximum (FWHM) of this magnetic-field-insensitive EIT resonance is reduced due to Doppler narrowing, scales linearly in Rabi frequency over the range studied, and reaches about 100 kHz at the lowest powers. These observations agree with an analytic model for a Doppler-broadened medium developed in (Javan et al 2002 Phys. Rev. A 66 013805; Lee et al 2003 Appl. Phys. B, Lasers Opt. (Germany) B 76, 33–9; Taichenachev et al 2000 JETP Lett. 72, 119). Numerical simulation using the Lindblad equation reveals that the transverse laser intensity distribution and two Λ-EIT systems must be included to fully account for the measured line width and line shape of the signals. Ground-state decoherence, caused by effects that include residual optical frequency fluctuations, atom-wall and trace-gas collisions, is discussed.

Coherent Dicke Narrowing of Absorption Spectra, Collapse and Revival: Second-Derivative Processing

The coherent Dicke narrowing (CDN) of the absorption spectrum in an Rb, Cs, K, or Na atomic vapor column of thickness L = (2n + 1)λ/2 (when n is an integer) and its collapse (absence of narrowing), which occurs at an atomic vapor column thickness L = nλ (where λ is the wavelength of the laser radiation in resonance with an atomic transition), are experimentally and theoretically studied. The application of second-derivative (SD) processing is shown give additional important information about CDN and its collapse. Optical nanocells with a wedge gap thickness of 30—3000 nm between the windows inner surfaces are used in experiments. At thicknesses λ/2, 1.5λ, and 2.5λ, all atomic transitions, which are even masked by a large Doppler broadening of 1000–1500 MHz, are fully spectrally resolved in SD spectra, and the frequency spacings (30–50 MHz) between the atomic transitions of a hyperfine structure and the relative probabilities of the atomic transitions are correctly displayed. CDN is shown to be applied in practice. Additional narrow velocity selective optical pumping (VSOP) resonances appear in the SD spectra of the resonance fluorescence of the Rb vapors. A developed theoretical model well describes the experimental results.

Theoretical and experimental results of optically pumped sodium atoms

Optical pumping is a process that produces a nonthermal Zeeman sublevel population distribution. It plays an important role in alkali-metal magnetometers for two uses, constructing the sub-Doppler features to lock the laser frequency and providing the spin-polarized atoms to precess around the magnetic field. To comprehensively study the properties of sodium (Na) magnetometer, we present the detailed theoretical analysis of the saturated absorption spectroscopy (SAS) and the magneto-optical resonance signal in sodium D2 line. Accordingly, we have implemented two experimental schemes, the counter propagating pump-probe SAS of thermal Na vapor and the remote sensing Bell-Bloom magnetometry based on the Larmor precession frequency. In the first experiment, sub-Doppler features of D2 consist of two velocity selective optical pumping (VSOP) resonances located at the D2a and D2b transitions and a crossover (CO) resonance located between the VSOP resonances are observed. In the second experiment, an estimated intrinsic magnetometric sensitivity of δBint=286 pT in a 1 Hz bandwidth, is achieved by demodulating the recorded fluorescence signal of the resultant magneto-optical resonance near the 300 kHz Larmor precession frequency. Both kinds of experimental results are in good agreement with our theoretical calculations.

Temperature and excitation power influence on the velocity-selective optical pumping resonances of 133Cs atoms confined in an extremely thin cell

Linear and nonlinear absorption spectra of 133Cs vapor confined in an extremely thin cell were computed via iterations with respect to the resonance radiation intensity. When the incident radiation intensity is low, the transient polarization of the atoms that undergo frequent collisions with the cell walls leads to sub-Doppler features in the absorption spectra. Higher incident radiation intensities result in the appearance of velocity-selective optical pumping resonances. The theory developed agrees quantitatively with the experimental findings.

Plasmonic Enhanced EIT and Velocity Selective Optical Pumping Measurements with Atomic Vapor

In this work, we experimentally observe for the first time nanoscale plasmonic enhanced Electromagnetically Induced Transparency (EIT) and Velocity Selective Optical Pumping (VSOP) effects in miniaturized Integrated Quantum Plasmonic Device (IQPD) for D2 transitions in rubidium (Rb). Our device consists of a vapor cell integrated on top of a prism coated with a thin layer of metal. This configuration is known to allow efficient excitation of Surface Plasmon Resonance (SPR). The evanescent field of the surface plasmon mode interacts with the atomic media in close vicinity to the metal. In spite of the limited interaction length between SPR and Rb atoms, the signature of EIT along with VSOP signals could be clearly observed in our miniaturized IQPD under proper conditions of pump and probe intensities. A gradual decrease in the contrast of the plasmonic enhanced EIT and VSOP signals was observed as the excitation was detuned from the SPR critical angle, due to reduction in electromagnetic field enhancement,...

Velocity selective optical pumping effects on [formula omitted] atoms from various coupling beam polarization configurations

We have investigated velocity selective spectral profile variations of probe beam transmittance at Fg=3→Fe=2,3, and 4 hyperfine manifolds of 85Rb atoms along with coherence effects at the Fg=3→Fe=4 transition with various coupling laser polarization configurations and a fixed probe polarization (σ+). Laser linewidth, atomic velocity distributions, frequency mixing of the coupling and probe laser beams between degenerate magnetic sublevels, and polarization variations of the coupling beam with the probe beam fixed at the Fg=3→Fe=4 transition were used to simulate the line profiles. The calculated transmittance signals are in good agreement with observed signals for each coupling laser polarization configuration.

The effect of electromagnetically induced transparency in a potassium nanocell

The effect of electromagnetically induced transparency (EIT) has been experimentally implemented for the first time for the (4S 1/2–4P 1/2–4S 1/2) Λ-system of potassium atom levels in a nanocell with a 770-nm-thick column of atomic vapor. It is shown that, at such a small thickness of the vapor column, the EIT resonance can be observed only when the coupling-laser frequency is in exact resonance with the frequency of the corresponding atomic transition. The EIT resonance disappears even if the coupling-laser frequency differs slightly (by ~50 MHz) from that of the corresponding atomic transition, which is due to the high thermal velocity of K atoms. The EIT resonance and related velocity selective optical pumping resonances caused by optical pumping (formed by the coupling) can be simultaneously recorded because of the small (~462 MHz) hyperfine splitting of the lower 4S 1/2 level.

Electromagnetically induced transparency of 87Rb in a buffer gas cell with magnetic field

We have studied the phenomenon of electromagnetically induced transparency (EIT) of 87Rb vapor at room temperature in a magnetic field with an arbitrary angle to the laser propagation direction. Rather than exposing atoms to a parallelled or transverse magnetic field as usual, in our work, we apply a magnetic field (up to 45 Gauss) with an arbitrary angle to the laser propagation direction and the spectra become much more complex. More EIT dips are observed due to the Zeeman splitting on the D2 line of 87Rb in a -type configuration. With a 5 Torr N2 buffer gas in the thermal 2 cm vapor cell, the state has a very short effective lifetime, corresponding to a large energy broadening, which removes the velocity selective optical pumping effect almost completely and keeps the high resolution EIT spectrum for the energy splitting of 87Rb in magnetic fields. The shifting of the EIT resonances with the strength of the applied magnetic field coincides well with the theory based on a full matrix Hamiltonian combined with a spectral decomposition method. Our work can be extended to measure the magnetic field vector in space. The effects of the detuning of the probe and coupling beams on the spectral lines are also investigated.

Theoretical simulation of 87Rb absorption spectrum in a thermal cell**Project supported by the National Basic Research Program of China (Grant No. 2013CB922003) and the National Natural Science Foundation of China (Grant Nos. 91421305, 91121005, and 11174329).

In this paper, we present a theoretical simulation of 87Rb absorption spectrum in a thermal cm-cell which is adaptive to the experimental observation. In experiment, the coupling and probe beams are configured to copropagate but perpendicular polarized, making up to five velocity selective optical pumping (VSOP) absorption dips able to be identified. A Λ-type electromagnetically induced transparency (EIT) is also observed for each group of velocity-selected atoms. The spectrum by only sweeping the probe beam can be decomposed into a combination of Doppler-broadened background and three VSOP dips for each group of velocity-selected atoms, accompanied by an EIT peak. This proposed theoretical model can be used to simulate the spectrum adaptive to the experimental observation by the non-linear least-square fit method. The fit for the high quality of experimental observation can determine valuable transition parameters such as decaying rates and coupling beam power accurately.

Collapse and revival of a Dicke-type coherent narrowing in potassium vapor confined in a nanometric thin cell

A nanometer-thin cell (in the direction of laser beam propagation) has been elaborated with the thickness of the atomic vapor column varying smoothly in the range of . The cell allows one to study the behavior of the resonance absorption over the D1 line of potassium atoms by varying the laser intensity and the cell thickness (from to with the step where is the resonant wavelength of the laser). It is shown that despite the huge Doppler broadening ( at the cell temperature ), at low laser intensities a narrowing of the resonance absorption spectrum is observed for ( at FWHM) and , whereas for and the spectrum broadens. At moderate laser intensities narrowband velocity selective optical pumping (VSOP) resonances appear at and with the linewidth close to the natural one. A comparison with saturated absorption spectra obtained in a 1.4 cm-sized K cell is presented. The developed theoretical model well describes the experiment.

Investigation of high-contrast velocity selective optical pumping resonance at the cycling transition of Cs using fluorescence technique

A high contrast (∼48%) Velocity Selective Optical Pumping (VSOP) resonance at the closed transition Fg=4→Fe=5 of Cs-D2 line is obtained in the fluorescence signal under co-propagating pump-probe configuration. We use a 5.2μm cell operating at reduced temperature (∼55°C) and the intensity of the pump-laser is kept lower than that of the probe-laser. The observed sharp narrow structure is suitable for side-arms frequency-locking of the cooling- (i.e. probe-) laser in a cold atom experiment, with possibility for “−Γ” to “−4Γ” red-detuning and “+Γ” to “+10Γ” blue-detuning using the standard properties of the commercially available electronics. We have developed a theoretical model corresponding to the thin cell, incorporating the atomic time-of-flight dependent optical pumping decay rate to describe the dimensional anisotropy of the thin cell. The model shows good qualitative agreement with the observation and simulates as well the cases of cells with smaller thickness. It also describes correctly the temperature dependence of the line broadening and shows the potential for further optimization and red-shift detuning above “−4Γ”. It may be of interest for further development of miniaturized modules, like the recently developed portable small magneto-optical traps.

Ultra-narrow spectroscopic cells in atomic spectroscopy: reflection, transmission, fluorescence, and nonadiabatic transitions at the walls

Ultra-narrow cells with the thicknesses in the range from several wavelengths to the small fractions of the wavelength brought a number of new opportunities for atomic spectroscopy. Depending on the cell thickness, spectral lines recorded in ultra-narrow cells are either Doppler-free or Doppler-broadened. With careful selection of the cell thickness hyperfine structure may be easily resolved without resorting on the multibeam nonlinear optical techniques. Moreover, frequent collisions with the walls leads to the important modifications of velocity selective optical pumping resonances. Finally, ultra-narrow cells provide with the unique opportunity to study collisions of the excited atoms with the solid surfaces. In this contribution several examples of the use of the ultra-narrow spectroscopic cells filled with the alkali atomic vapour is presented. First, we discuss general aspects of the transient polarisation that defines all peculiarities of an ultra-narrow cell as a spectroscopic tool. Second, we demonstrate the resolution of the magnetic sublevels in the transition from Zeeman to Paschen-Back regime in the Cs hyperfine structure. Third, new aspects of velocity selective optical pumping resonances in reflection and transmission of resonant radiation by the 6 wavelengths thick cell filled with Cs are discussed. Forth, the experimental evidences of the nonadiabatic transitions between excited states of Rb atoms in the course of collisions with the sapphire surface are presented.

Active Faraday optical frequency standard.

We propose the mechanism of an active Faraday optical clock, and experimentally demonstrate an active Faraday optical frequency standard based on narrow bandwidth Faraday atomic filter by the method of velocity-selective optical pumping of cesium vapor. The center frequency of the active Faraday optical frequency standard is determined by the cesium 6 (2)S(1/2) F=4 to 6 (2)P(3/2) F'=4 and 5 crossover transition line. The optical heterodyne beat between two similar independent setups shows that the frequency linewidth reaches 281(23) Hz, which is 1.9×10(4) times smaller than the natural linewidth of the cesium 852-nm transition line. The maximum emitted light power reaches 75 μW. The active Faraday optical frequency standard reported here has advantages of narrow linewidth and reduced cavity pulling, which can readily be extended to other atomic transition lines of alkali and alkaline-earth metal atoms trapped in optical lattices at magic wavelengths, making it useful for new generation of optical atomic clocks.

Observation and theoretical simulation of electromagnetically induced transparency and enhanced velocity selective optical pumping in cesium vapour in a micrometric thickness optical cell

Observation of electromagnetically induced transparency (EIT) and enhanced velocity selective optical pumping (VSOP) signals in a micrometric cell with cesium is reported. The line shape and non-linear features observed in the case of fluorescence in the direction parallel to the cell windows and the transmission spectra observed along the propagation direction of the probe beam show considerable differences in the spectral profile. A theoretical model based on five level optical Bloch equations is used to simulate the spectra. The Doppler convolution includes all possible orientations of atomic velocities with respect to the laser beam direction. Atoms moving nearly parallel to the windows and perpendicular to the collinear pump and probe beams have much lower Doppler shift and hence produce considerable narrowing of the Doppler background in the fluorescence spectra. The coherence decay rate is also low for such atoms as they do not meet with the cell walls. The simulated curves reproduce the observed sharp EIT peaks and enhanced broad VSOP signals for the closed probe transition in the fluorescence and absorption spectra. The observed effect of detuning of the pump frequency on the non-linear features is also reproduced by the simulation.

Light shift manipulation and suppression in a double pass optical-magnetic double resonance system

A double pass optical-magnetic double resonance system is proposed and applied for observing and manipulating the light shift in metastable . By adjusting the degree of overlap in space between the incident and retro-reflected laser beams, the manipulation on light shift is realized and a flat range for light shift insensitive to optical frequency detuning can be formed. A simple hole-burning model based on velocity-selective optical pumping, including the velocity diffusion effect caused by velocity-changing collisions, is adopted to qualitatively clarify the light shift observed in the double pass system.

Velocity Selective Polarization Spectroscopy for Modulation-Free Dispersion Signals at Detuned Frequencies

We present a simple technique to obtain modulation-free locking signals at the detuned frequency from an atom transition between hyperfine structures. Polarization spectroscopy allows us to obtain the dispersion signals that are suitable for frequency locking. Velocity selective optical pumping using another laser beam also allows us to obtain signals at the detuned frequency and to shift crossover signal away. By combining these two techniques, we were able to obtain the velocity selective birefringence signal only at the principle transition in an Rb vapor cell and compare the birefringence signal with the theoretical spectrum predicted by using Nakayama’s model.