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Atomic and Molecular Physics
Published in Walter Fox Smith, Experimental Physics, 2020
In this experiment you will use laser light to make high-resolution absorption measurements on a gas composed of the two naturally occurring isotopes of rubidium, 85Rb (natural abundance 72%) and 87Rb (natural abundance 28%). You will learn the technique of “Doppler-free saturated absorption spectroscopy,” which can reveal energy differences smaller than the natural Doppler broadening of spectral lines, allowing, in the case of Rb, the hyperfine structure of energy levels to be observed. You will apply the Doppler-free technique to the 5S1/2 to 5P3/2 transition at 780 nm in the infrared. Saturated absorption spectroscopy, first developed in the 1970s, and for which the 1981 Nobel Prize was awarded, is now a common high-resolution spectroscopic tool in a variety of laboratory settings, including apparatuses used to produce “optical molasses” and Bose-Einstein condensates. These techniques have led to many important applications, for example, improving the accuracy of atomic clocks, which has led to dramatic improvements in the accuracy of global positioning systems.
Absorption Spectroscopy and Its Implementation
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
Because of the Doppler limit encountered in discharge (required to generate H-atoms) experiments, one would be hard-pressed to resolve the two groups of transitions (1−3 and 4−7) as two peaks. Exploiting saturated absorption spectroscopy, nearly all transitions are visible and resolved in the spectrum, safe for the low-intensity components 1 and 2, which are below the noise limit (see Hänsch et al. 1972). The composite of the laser linewidth of δνL ≅ 30 MHz utilized in Hänsch’s experiments and the natural width of the individual fine-structure transitions δνn ≤ 100 MHz, associated with the respective Aif, would be less than the observed Lamb-dip widths of δνs ≈ 250 – 300 MHz (power and pressure broadening in said experiments were nearly insignificant). A significant additional width contribution stems from the overall short lifetimes of the 2P states (inverse of the sum of all Aif commencing in those levels) involved in the transitions; furthermore, the unresolved hyperfine components with Δνhfs ≤ 60 MHz increase the overall width. The latter point is an interesting one since it exposes the dilemma: even if the laser linewidth were sufficient to resolve much of the hyperfine structure in the transitions, intrinsic quantum characteristics may cause broadening larger than the hfs-splitting. However, it is worth noting that this is a peculiar feature of the hydrogen atom; resolved hfs-splitting of transitions is routinely demonstrated for the alkaline atoms, which even have become demonstration experiments for undergraduate students (see Jacques et al. 2009).
Precision measurement of quasi-bound resonances in H2 and the H + H scattering length
Published in Molecular Physics, 2022
K.-F. Lai, E. J. Salumbides, M. Beyer, W. Ubachs
The frequency calibration of the spectroscopy laser, probing the excitation of quasi-bound resonances to the F0-outer well state, is crucial because it determines the accuracy at which the energy of those resonances are determined. This laser is a travelling-wave Pulsed-Dye-Amplifier (PDA) amplifying the output of a continuous-wave (CW) ring-dye-laser, upon-frequency doubling delivering a pulsed output with a frequency bandwidth of MHz [55]. The absolute frequency calibration relies on saturated absorption spectroscopy of hyperfine-resolved using the CW-output of the ring laser as well as a wavemeter (Toptica High-Finesse WSU-30) [56]. Effects of frequency chirp in the pulsed output of the PDA is analysed and corrected for, following established methods [55]. Excitation of the two-photon transitions is established in a Doppler-free geometry with counter-propagating beams aligned in a Sagnac interferometric scheme [57].
Resonant semiconductor laser absorption of atomic Rb in a pulsed Nd:YAG laser-induced plasma
Published in Journal of Modern Optics, 2019
Xiaoxu Zhang, Huiqi Zheng, Lili Ge, Hua Zhao, Qiongying Ren, Xingqiao Ma
For absorption signal detection, the probe beam from VCESL propagated through the area of LIP. To achieve an accurate irradiation on the focus, we made the pulsed pump beam and the cw probe beam aligned by means of a dichroic mirror which is highly reflective at 795 nm and almost transmissive at 1064 nm. The dichroic mirror is designed for use at a 45° angle of incidence. A narrow bandpass filter centred at 795 nm was placed before the photodetector (New Focus 1621, PD2 in Figure 2) to improve the signal-to-noise ratio (SNR) of resonant absorption. Another key point for SNR is to isolate the photodetector from the strong electromagnetic interference (EMI) noise accompanied by Q-switching of the pulsed pump laser. The five-layer µ-metal magnetic shielding cylinder (Central Iron & Steel Research Institute, CISRI) greatly attenuated the EMI in its internal space. A reference beam from VCESL passed through the Rb vapour cell to obtain a saturated absorption spectroscopy, and a proportional–integral–derivative (PID) control system (not shown in Figure 2) locked the VCSEL frequency to Rb atomic transition |52S1/2, F = 2 → |52P1/2, F′= 1 (377.1044 THz, 794.9853 nm).
Laser cooling of rubidium atoms in a 2D optical lattice
Published in Journal of Modern Optics, 2018
Chunhua Wei, Carlos C. N. Kuhn
The experiment utilizes external cavity diode lasers (ECDL) locked to an atomic transition using saturated absorption spectroscopy (SAS). The trapping laser (red detuned from the of the D transition hyperfine level) is amplified () then sent through an acoustic-optic modulator using a double-pass setup (DP AOM). The laser mode is then cleaned through of polarization maintaining single-mode (PMSM) fibre, resulting in about of trapping light sent to the glass cell. Immediately after the locking loop, a small amount of light is removed for use as a probe beam in an absorption image system and is capable of being tuned independently of the trapping light using an AOM double-pass set up. The image beam, in the science table, is located horizontally and orthogonal to the horizontal lattice beam. The repump laser () is passed through an AOM, with the first-order diffracted light fibre coupled to the glass cell resulting in a total power of at the science cell.