The simultaneous excitation of adjacent atomic transitions by two light waves equally detuned from their resonances gives rise to destructive interference of the transitions and causes a narrow dark line, or coherent dip, to appear in both light scattering and absorption. With one of the light fields detuned from resonance, the line is asymmetric and is accompanied by a shifted bright line; the two are represented by a Fano line profile. The spectral separation ∊ of the bright and dark lines is closely related—although not identical—to the net shift that is found in the two-photon-resonant denominator of the susceptibility, to the shift of the normal modes of the atom’s two internal excitations, and to the net difference of the energy separations of dressed and bare atomic levels. We report measurement of the resonance shift ∊ of the 2S1/2–2D3/2 transition of a single cooled and trapped Ba+ ion, and we analyze its relationship to those identified light shifts.
The simultaneous excitation of adjacent atomic transitions by two light waves equally detuned from their resonances gives rise to destructive interference of the transitions and causes a narrow dark line, or coherent dip, to appear in both light scattering and absorption. With one of the light fields detuned from resonance, the line is asymmetric and is accompanied by a shifted bright line; the two are represented by a Fano line profile. The spectral separation ∊ of the bright and dark lines is closely related—although not identical—to the net shift that is found in the two-photon-resonant denominator of the susceptibility, to the shift of the normal modes of the atom’s two internal excitations, and to the net difference of the energy separations of dressed and bare atomic levels. We report measurement of the resonance shift ∊ of the 2S1/2–2D3/2 transition of a single cooled and trapped Ba+ ion, and we analyze its relationship to those identified light shifts.
The fluorescence of a single r.f.-trapped and cooled Ba + ion, induced by the light of two lasers, shows a null at the S 1/2 - D 3/2 resonance of the frequency difference. We have measured the Zeeman effect and power broadening of this "dark line", and also the shift of the associated "bright" line, which is related to the Raman light shift. A 75 kHz wide dark line was observed. Such a resonance might be used to control a 147 THz standard frequency free of systematic shifts in excess of 10 -15 .
High-resolution optical spectroscopy of an individual trapped ion is hampered by lack of sharp lasers. This suggests the use of a second metastable excited ion as an ultrasharp light source. To this end, laser-cool two barium ions to an equilibrium distance of approximately 8 mum on the z (symmetry) axis of the trap and, in this (earth)(Ba(+))(2)-molecule, visually or photoelectrically identify them as A and B by their location. Briefly turn on a 455-nm spectral lamp until one of the ions, say the A ion, is pumped into the metastable D(5/2) level and turns invisible. Focus on the visible, spatially well-resolved B ion and turn off the blue and red illumination lasers for approximately 15 s. Then turn them back on again and check on whether the excitation by chance has been transferred to the B ion and is now in the D(5/2) level and dark while the A ion is bright. The cross section for absorption of the lambda(D(5/2) --> S(1/2)) identical with lambda(0) = 1.76 mum radiation by a stationary ion can be >lambda(0) (2)/2pi. Thus, by pushing the two ions together to approximately lambda(0)/4 by turning on a much stronger trapping field during the excitation exchange period, one might be able to detect excitation transfer in >10% of the attempts. The ions are tuned relative to each other by a 0- to 10-mV/cm variable dc field in the z direction, which displaces them axially and causes them to see different rf fields, which Stark-shifts their frequencies. In this way, a resonant transfer response as sharp as twice the natural width of the D(5/2) level, 11 mHz or a Q approximately 0.4 x 10(17), might be demonstrated.
Ion traps and cw dye lasers allow one to prepare small clouds of ions, or single ions. They can be detected even by absorptive techniques, such that the information is contained in one field mode only, the mode of the irradiating light. The noise can approach the limit. For excitation on weak and narrow lines, random breaks of the scattering on a competing resonance line can reveal the quantized signal absorption. These interruptions are the signature of quantum jumps which characterize the internal dynamics of a single atomic particle under quasi-continuous observation.
Photon absorption spectroscopy is a powerful tool for uncovering the structure of atoms, molecules, and solids. Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures related to the structural and dynamical properties. Recently, Ott et al. [Science 340, 716 (2013)] successfully transferred Fano profile into Lorentzian line shape using an intense infrared laser, after excitation of autoionizing states in helium by attosecond XUV pulse. This is a very important step forward in quantum phase control. However, here we show experimentally that an autoionizing state can have both Fano and Lorentzian behavior naturally, depending on the process involved. This study utilized the inverse process of photon absorption ionization, i.e., electron ion recombination with photon emission, making sure the resonant autoionizing state is not modified or decorated by the laser fields. Our result implies that excitation of the state through different paths---for example, one photon versus multiphoton excitation, or even one step versus multistep excitation---can lead to different Fano profiles for the same resonant state. We also report an experimental determination of the energy shifts in the recombination photon-intensity peaks due to the interference between the resonant and nonresonant processes.
We have measured the spectral position of the absorption minimum in a coherent population trapping resonance in thermal cesium vapor as a function of light intensity. The dependence of position on intensity is found to be almost linear. We have furthermore studied the dependence of this light shift on neon bufier gas pressure and flnd a strong reduction for higher pressures. So the addition of a bufier gas not only reduces the linewidth of the resonance but also a very important systematic efiect for precision measurements. An atomic three-level system with two low-lying and long-lived levels (a so-called ⁄ system) can exhibit a two-photon resonance when the difierence frequency of two driving flelds matches the splitting of the two lower levels (flg. 1), !2 i !1 = !12 (1). The resonance is called \dark since one-photon absorption and resonance ∞uorescence are strongly reduced because of the trapping of the atomic population in a coherent superposition state of the two lower levels that is no longer coupled to the light flelds (coherent population trapping, CPT) (2). The two-photon resonance can be very narrow (3, 4), with linewidths rivaling those of the microwave-driven magnetic dipole transition between the two lower atomic states. The use of dark resonances for the construction of atomic radio frequency standards (5) or applications for precision measurements, for instance in magnetometry (6), has been suggested, and flrst steps in this direction have been taken (7). It is an essential requirement for such precision applications to understand the in∞uence of experimental conditions on the position and shape of the resonance under investigation. Because of the flnite dephasing rate between the two lower states the position of the absorption minimum at the dark resonance can depend on intensity (8,1), and in experiments a shift of the center of the resonance (light shift, AC Stark shift) is readily observed (9), as was discussed, for instance, for an atomic beam (5) or in connection with a CPT maser (10). In this letter we report measurements of the light shift of the two-photon resonance that couples two hyperflne ground-state levels of the cesium atoms in a thermal vapor both with and without an additional bufier gas.
In a recent paper,1 we considered the stability of single-mode operation in two-photon lasers and two-photon optical bistability.2 The models assumed that two photons of equal or nearly equal frequencies span the atomic two-photon transition, with no resonant intermediate levels. The paper neglects possible Stark shifts of the two levels due to virtual transitions to nonresonant levels and back. For one-photon processes, these virtual transitions can be neglected because they have one nonresonant denominator. They cannot in general be neglected for the two-photon case, however, since all transition probabilities have at least one nonresonant denominator. In this paper we give formulas and illustrations for the two-photon two-level absorption and coupling coefficients including Stark shifts. For vanishing shifts, the coefficients have symmetric spectra that look very similar to their one-photon counterparts, but for nonzero shifts, they have asymmetric spectra with exaggerated resonances. These resonances appear to increase the likelihood of instabilities for certain combinations of pump detuning and Stark shifts.