Inverted crossover resonance within one Zeeman manifold

Abstract

We carry out investigations of inverted crossover resonances in π-driven four-level systems where ∆F can be zero. Through the use of sub-Doppler frequency modulation spectroscopy of the (6s 2 ) 1 S 0 − (6s6p) 3 P 1 transition in 171 Yb the resonance becomes manifest. The centre-frequency is inherently insensitive to first-order Zeeman shifts and equates to the two-level resonance frequency in the absence of a magnetic field. A rate equation model is used to help validate the nature of the resonance. Optical frequency measurements of the F ′ = 1/2 hyperfine line recorded over two months demonstrate a statistical uncertainty of 2 × 10 −11 . The inverted crossover resonance found with the F ′ = 3/2 line is used for 556 nm laser frequency stabilization, which is an alternative means when applied to magneto-optical trapping of 171 Yb.PACS numbers: 42.62.Eh; 42.62.Fi; 32.30.Jc; 37.10.De; 32.10.Fn ‡ To distinguish the two 171 Yb 1 S 0 − 3 P 1 hyperfine lines we will only state the upper F value for the sake of simplicity.

Hyperfine constants and line separations for the S01−P13 intercombination line in neutral ytterbium with sub-Doppler resolution

“…It has been included to show the relative strength of this resonance to the saturated absorption signals. The center of this resonance coincides with the center of the 171 Yb (F = 1/2) line, as shown in [37].…”

“…The spectrum for 171 Yb (F = 1/2) shown in Fig. 2 is not a saturated absorption signal, but is instead an inverted crossover resonance between two Zeeman transitions [37], hence has a central slope that is inverse to the others. It is generated when a bias magnetic field is applied, in this case 0.1 mT.…”

“…The spectral widths of Fig. 2 are not lifetime (184 kHz) or transit time (70 kHz) limited, rather, they are broadened due to the applied modulation [37] and the optical intensity of the probe light. The intensity is ∼70 I sat , where I sat = 1.4 W•m −2 is the saturation intensity for the 1 S 0 − 3 P 1 transition.…”

“…The modulation is sourced from a lock-in amplifier (LIA) (SRS SR830) that receives the PMT signal and outputs the third harmonic dispersive signal. Generation of the 556 nm light is described in [37]. In brief, a fiber laser injection-locks a 50 mW ridge waveguide diode laser for amplification.…”

“…There are several methods we can apply to find the required bias current. One is by examining the strength of the inverted crossover resonances (ICRs) for opposite field settings, as described in [37] (e.g. for 171 Yb, F = 1/2).…”