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Journal of Physics B Atomic Molecular and Optical Physics

A global crossover saturated-absorption spectroscopy induced by dual-frequency laser

Pengyuan Chang, Duo Pan, Haosen Shang, Tiantian Shi, Jingbiao Chen

Published Jul 28, 2020• 11 citations
Publisher

Abstract

Abstract We report on a global crossover saturated-absorption spectroscopy (SAS) of Cs D 2 transition using a dual-frequency laser field with a frequency difference of less than the Doppler width. With two pairs of dual-frequency laser beams counter-propagating through the vapor cell, the line shape and non-linear features observed in the transmission spectra of the probe beam change with the frequency difference of the dual-frequency laser. The superficially complicated transmission signal includes two sets of common single-frequency SAS and several velocity-selective resonances (VSRs), we call these VSR peaks ‘global crossover SAS’. This phenomenon is explained by VSR effect. The global crossover SAS is formed by the atomic velocity groups resonance with both the initial and modulated laser fields, which are counter-propagated. Moreover, the global crossover SAS dips having the separation of allowed hyperfine transitions move along with the two sets of common single-frequency SAS while the frequency difference of the dual-frequency laser is changed. This indicates a resonance between the velocity shifted pump laser frequency and the velocity shifted probe laser frequency. The results obtained in this system are of great interest for laser spectroscopy. Furthermore, the application of the counter-propagating laser beams in the dual-mode regime increase the number of atoms that effectively interact with the field, hence, this scheme has a potential advantage for laser frequency stabilization.

DOI
10.1088/1361-6455/aba9e3

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Abstract We report on a global crossover saturated-absorption spectroscopy (SAS) of Cs D 2 transition using a dual-frequency laser field with a frequency difference of less than the Doppler width. With two pairs of dual-frequency laser beams counter-propagating through the vapor cell, the line shape and non-linear features observed in the transmission spectra of the probe beam change with the frequency difference of the dual-frequency laser. The superficially complicated transmission signal includes two sets of common single-frequency SAS and several velocity-selective resonances (VSRs), we call these VSR peaks ‘global crossover SAS’. This phenomenon is explained by VSR effect. The global crossover SAS is formed by the atomic velocity groups resonance with both the initial and modulated laser fields, which are counter-propagated. Moreover, the global crossover SAS dips having the separation of allowed hyperfine transitions move along with the two sets of common single-frequency SAS while the frequency difference of the dual-frequency laser is changed. This indicates a resonance between the velocity shifted pump laser frequency and the velocity shifted probe laser frequency. The results obtained in this system are of great interest for laser spectroscopy. Furthermore, the application of the counter-propagating laser beams in the dual-mode regime increase the number of atoms that effectively interact with the field, hence, this scheme has a potential advantage for laser frequency stabilization.
We report on Doppler-free laser spectroscopy in a Cs vapor cell using a dual-frequency laser system tuned on the Cs D1 line. Using counter-propagating beams with crossed linear polarizations, an original sign reversal of the usual saturated absorption dip and large increase in Doppler-free atomic absorption is observed. This phenomenon is explained by coherent population trapping (CPT) effects. The impact of laser intensity and light polarization on absorption profiles is reported in both single-frequency and dual-frequency regimes. In the latter, frequency stabilization of two diode lasers was performed, yielding a beat note fractional frequency stability at the level of 3×10−12 at 1 s averaging time. These performances are about an order of magnitude better than those obtained using a conventional single-frequency saturated absorption scheme.
Abstract An experiment on the saturated absorption spectroscopy (SAS) of 87Rb-D2 line is performed using the Laguerre–Gaussian (LG) beam as structured light. The theoretical simulation of which is done by considering a four-level atom-laser coupled system. The theoretically simulated spectra fairly match with the experimentally observed spectra. The line-width narrowing behavior of both the observed hyperfine and crossover resonances in the Doppler-broadened probe absorption spectra is attributed by the spatially dependent Rabi frequency of the pump LG beam irrespective of the characteristics of the probe beam. The results also illustrated that the narrowing behavior of the line shapes of the hyperfine structure using the LG beam is universal in nature, i.e. irrespective of hyperfine or crossover transitions. This structured light-induced narrowing of line-width of the sub-Doppler hyperfine and crossover resonances can be utilized for high-precision laser frequency locking at a particular hyperfine or crossover transition. Moreover, a significant narrowing of hyperfine and crossover resonances with the variation of the orbital angular momentum (OAM) number of the LG beam shows that the generated SAS signal can be manipulated by using the OAM of the structured light as a control knob.
The utilization of atomic or molecular spectroscopy for frequency locking of single-frequency laser to improve laser frequency stability plays an important role in the experimental investigation of optically pumped atomic magnetometers, atomic clocks, laser cooling and trapping of atoms, etc. We have experimentally demonstrated a technique for frequency stabilization of a single-frequency laser employing the bright state spectroscopy (BSS) with a rubidium atomic vapor cell. By utilizing the counter-propagating dual-frequency 795 nm laser beams with mutually orthogonal linear polarization and a frequency difference of 6.834 GHz, which is equal to the hyperfine splitting of rubidium-87 ground state 5S1/2, an absorption-enhanced signal with narrow linewidth at the center of Doppler-broadened transmission spectroscopy is observed when continuous scanning the laser frequency over rubidium-87 D1 transition. This is the so-called BSS. Amplitude of the absorption-enhanced signal in the BSS is much larger compared with the conventional saturation absorption spectroscopy (SAS). The relationship between linewidth and amplitude of the BSS signal and laser beam intensity has been investigated. This high-contrast absorption-enhanced BSS signal has been employed for the laser frequency stabilization. The experimental results show that the frequency stability is 4.4×10−11 with an integration time of 40 s, near one order of magnitude better than that for using the SAS.
We have shown earlier that hyperfine spectroscopy in a vapor cell using co-propagating pump-probe beams has many advantages over the usual technique of saturated-absorption spectroscopy using counter-propagating beams. The main advantages are the absence of crossover resonances, the appearance of the signal on a flat (Doppler-free) background, and the higher signal-to-noise ratio of the primary peaks. Interaction with non-zero-velocity atoms causes additional peaks, but only one of them appears within the primary spectrum. We first illustrate the advantages of this technique for high-resolution spectroscopy by studying the D2D_2D2​ line of Rb. We then use an acousto-optic modulator (AOM) for frequency calibration to make precise hyperfine-interval measurements in the first excited P3/2P_{3/2}P3/2​ state of 85,87^{85,87}85,87Rb and 133^{133}133Cs.
Precise laser stabilization is a fundamental aspect of modern metrology, directly influencing the accuracy of fundamental measurements in fields such as high-precision metrology, atomic sensor applications, and quantum technologies. Saturation absorption spectroscopy (SAS) is one of the most effective techniques for achieving high spectral stability, as it enables laser frequencies to be locked to atomic resonances. This technique exploits the nonlinear interaction between a weaker probe beam and a stronger pump beam within a rubidium (Rb) atomic vapor cell. By eliminating Doppler broadening, SAS allows well-defined hyperfine transitions to be resolved, providing a reliable optical frequency reference. This study aims to optimize laser frequency stabilization using the SAS technique by investigating the effects of laser light polarization, atomic vapor cell temperature, and pump-probe power adjustments. Polarization-dependent selection rules influence transition probabilities, thereby affecting spectral contrast and frequency stability. Additionally, atomic density variations, which depend on the vapor cell temperature, influence signal strength and linewidth, necessitating precise temperature control for optimal performance. Furthermore, the pump power must be sufficient to induce saturation without excessive power broadening, while the probe power must remain low enough to prevent spectral line distortion. By optimizing polarization, temperature, and power parameters, the stabilization of the Rb atomic laser can be enhanced, leading to improved reproducibility and consistency in high-precision measurements. SAS-based laser stabilization is particularly crucial for atomic sensor applications and quantum technologies, where highly stable, narrow-linewidth lasers are essential for precision measurements. Applications such as atomic magnetometers, gyroscopes, and inertial navigation systems depend on precisely stabilized lasers for optimal performance. By enhancing the reliability of laser frequency stabilization, this work contributes to the advancement of next-generation atomic sensors with improved sensitivity and stability.
Vapor-cell spectroscopy is an essential technique in many fields and is particularly relevant for quantum technologies. Most atom- and ion-trapping experiments rely on simultaneous spectroscopy of two atomic transitions, employing a separate apparatus for each transition. We demonstrate simultaneous spectroscopy on two atomic transitions, within a single apparatus, using spatially overlapped beams from two independent lasers. This method enables compact setups and offers superior spectroscopic performance, leading to sharper spectroscopy peaks, stronger absorption signals, and superior laser stability. Doppler-free locking features become accessible over a frequency range several hundred megahertz wider than for standard saturated absorption spectroscopy. Exploring the full parameter space associated with dual-frequency spectroscopy reveals a latticelike structure of sharp resonance features, which enhances experimental versatility by allowing laser frequency stabilization within a wide manifold of locations in two-dimensional frequency space. We present data for 133Cs^{133}\mathrm{Cs}133Cs and 85Rb^{85}\mathrm{Rb}85Rb and compare our results to a theoretical model. Employing the technique for frequency stabilization close to the D2{D}_{2}D2​ line of 85Rb^{85}\mathrm{Rb}85Rb results in an improvement in frequency stability by a factor typically between 2 and 3 for averaging times of up to 1 s. The technique will advance portable quantum technologies and facilitate high-precision measurements.