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TEACHING DEMONSTRATION OF SPECTRUM BROADENING IN TWO-PHOTO TRANSITION OF RB

Kengqi WANG, Jinxian GUO

Published Apr 1, 2023• 0 citations
Publisher

Abstract

The measurement of atomic spectrum is an important tool to reveal the microscopic structure and pattern of motion of microscopic particles. Therefore, the teaching of related fields is involved from undergraduate to graduate stage, especially the teaching of spectroscopy. However, nonuniform broadening caused by Doppler effect and uniform broadening caused by atomic collision greatly limit the resolution of the spectral lines. In the development of science and technology, in order to obtain higher resolution, it is necessary to eliminate the broadening of atomic spectral lines. In the related teaching, two-photon transition in the alkali metal atomic system has become an effective way to study the broadening mechanism due to its excellent energy level characteristics. In the current undergraduate teaching, there is a lack of demonstrations of various atomic line broadening mechanisms besides Doppler broadening. In this paper, the rubidium atom 5S1/2→5P3/2→5D5/2 two-photon transition spectral line is employed to measure the half-width change caused by changing the laser power, the temperature of the atomic cell and magnetic field strength. While verifying the two-photon transition, it focuses on demonstrating the mechanism of the atomic spectral line broadening and the measurement method of the half-width of the spectral line. Through this experiment, on the one hand, undergraduate students can grasp the relevant knowledge of atomic spectral line broadening and two-photon transition and understand the connotation of theoretical knowledge more deeply, and on the other hand, they have trained their hands-on ability by building experimental instruments independently, which is particularly important for improving the quality of undergraduate students majoring in physics. The experimental device shown in this paper is simple, easy to build, and suitable for classroom demonstration and experimental teaching.

DOI
10.26599/phys.2023.9320219

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The measurement of atomic spectrum is an important tool to reveal the microscopic structure and pattern of motion of microscopic particles. Therefore, the teaching of related fields is involved from undergraduate to graduate stage, especially the teaching of spectroscopy. However, nonuniform broadening caused by Doppler effect and uniform broadening caused by atomic collision greatly limit the resolution of the spectral lines. In the development of science and technology, in order to obtain higher resolution, it is necessary to eliminate the broadening of atomic spectral lines. In the related teaching, two-photon transition in the alkali metal atomic system has become an effective way to study the broadening mechanism due to its excellent energy level characteristics. In the current undergraduate teaching, there is a lack of demonstrations of various atomic line broadening mechanisms besides Doppler broadening. In this paper, the rubidium atom 5S1/2→5P3/2→5D5/2 two-photon transition spectral line is employed to measure the half-width change caused by changing the laser power, the temperature of the atomic cell and magnetic field strength. While verifying the two-photon transition, it focuses on demonstrating the mechanism of the atomic spectral line broadening and the measurement method of the half-width of the spectral line. Through this experiment, on the one hand, undergraduate students can grasp the relevant knowledge of atomic spectral line broadening and two-photon transition and understand the connotation of theoretical knowledge more deeply, and on the other hand, they have trained their hands-on ability by building experimental instruments independently, which is particularly important for improving the quality of undergraduate students majoring in physics. The experimental device shown in this paper is simple, easy to build, and suitable for classroom demonstration and experimental teaching.
We describe an experiment for investigating the 5S1∕2→5D5∕2 two-photon transition in rubidium using a single grating-feedback diode laser operating at 778.1nm (385THz). Continuous tuning of the laser frequency over 4GHz allows for the clear resolution of the Doppler-free spectral features and allows accurate measurement of the hyperfine ground-state splitting. A direct comparison between Doppler-broadened and Doppler-free spectral features is possible because both are distinctly evident in the two-photon spectra. By independently modifying the polarization state of the two laser fields, the impact of electric dipole selection rules on the two-photon transition spectra is investigated. This experiment is a valuable addition to the advanced undergraduate laboratory because it uses much of the same equipment as the single-photon saturated absorption spectroscopy experiment performed on the 5S1∕2→5P3∕2 transition in rubidium (λ=780.24nm) and provides students with an opportunity to investigate characteristics of atomic spectra not evident in the single-photon experiment.
In this paper, we describe two complementary nonlinear spectroscopy methods which both allow one to achieve Doppler-free spectra of atomic gases. First, saturated absorption spectroscopy is used to investigate the structure of the 5S 1/2 → 5P 3/2 transition in rubidium. Using a slightly modified experimental setup, Doppler-free two-photon absorption spectroscopy is then performed on the 5S 1/2 → 5D 5/2 transition in rubidium, leading to accurate measurements of the hyperfine structure of the 5D 5/2 energy level. In addition, electric dipole selection rules of the two-photon transition are investigated, first by modifying the polarization of the excitation laser, and then by measuring two-photon absorption spectra when a magnetic field is applied close to the rubidium vapour. All experiments are performed with the same grating-feedback laser diode, providing an opportunity to compare different high-resolution spectroscopy methods using a single experimental setup. Such experiments may acquaint students with quantum mechanics selection rules, atomic spectra and Zeeman effect.
An ongoing spectroscopic investigation of atomic rubidium utilizes a two-photon, single-laser excitation process. Transitions accessible with our tunable laser include 5P1/2 (F ′ ) ← 5S1/2 (F) and 5P3/2 (F ′ ) ← 5S1/2 (F). The laser is split into a pump and probe beam to allow for Doppler-free measurements of transitions between hyperfine levels. The pump and probe beams are overlapped in a counter-propagating geometry and the laser frequency scans over a transition. Absorption, fluorescence and polarization spectroscopy techniques are applied to this basic experimental setup. The temperature of the vapor cell and the power of the pump and probe beams have been varied to explore line broadening effects and signal-to-noise of each technique. This humble setup will hopefully grow into a more robust experimental arrangement in which double resonance, two-laser excitations are used to explore hyperfine state changing collisions between rubidium atoms and noble gas atoms. Rb-noble gas collisions can transfer population between hyperfine levels, such as 5P3/2 (F ′ = 3) Collision ←− 5P3/2 (F ′ = 2), and the probe beam couples 7S1/2 (F ′′ = 2) ← 5P3/2 (F ′ = 3). Polarization spectroscopy signal depends on the rate of population transfer due to the collision as well as maintaining the orientation created by the pump laser. Fluorescence spectroscopy relies only on transfer of population due to the collision. Comparison of these techniques yields information regarding the change of the magnetic sublevels, mF , during hyperfine state changing collisions.
We describe a one-color, two-photon polarization spectroscopy experiment on the 5S1/2–5D5/2 transition in rubidium, suitable for implementation in an advanced undergraduate instructional laboratory. The proposed laboratory module synergistically combines polarization spectroscopy with established fluorescence detection at a wavelength different from the excitation wavelength. The curriculum provided in this paper allows students to compare experimental results with predicted theoretical lineshapes and to study the intensity dependence of the two-photon transition. The module also provides a springboard that enables instructors to expand the curriculum to incorporate additional measurements and theoretical analyses. These activities include collecting spectra on additional transitions, measuring the temperature dependence of the spectral amplitudes, or using the dispersive polarization spectra to frequency stabilize a laser without modulation.
We experimentally study the high-resolution two-photon spectroscopy of rubidium using a single laser. The 5S1/2 − 5P3/2 − 5D5/2 transition is realized via 778.1 nm laser beams acting on the vapor with a counter-propagating configuration. The intensity modulation method is employed to improve the signal-to-noise ratio of the spectrum by two orders of magnitude compared with that for direct measurement. The effects of the vapor temperature, the frequency-scanning speed of the laser, the polarization combination of the laser beams, and the laser power on the spectra are investigated to obtain the optimal experimental parameters. This research is expected to be beneficial for potential applications since it can be used to realize a compact and high-precision frequency standard for optical communication.
Abstract A cell with a submicron thickness was used to measure the broadening and shift of the D1 and D2 lines of Rb atoms by neon. The resolution of hypefine components is achieved by a combination of two techniques. First, the Rb vapor column thickness in the direction of propagation of the laser radiation was chosen equal to half of its wavelength λ under conditions of resonance with the atomic transition frequency. For rubidium atoms λ/2 ~ 400 nm. At a nanocell thickness L ~ λ/2, in the transmission spectrum A(ν), the spectral lines of atomic transitions are narrowed due to the exclusion of Doppler broadening. Second, further narrowing of the detected signals was achieved by the double differentiation of the transmission spectrum, A''(ν). The transmission spectra of pure rubidium vapor and rubidium vapor with the addition of neon have been measured at different pressures. The measured values of the shift coefficients of the D1 and D2 lines of rubidium in the presence of Ne were –1.1 ± 0.2 MHz/Torr and –2.1 ± 0.2 MHz/Torr, respectively. The broadening coefficients of the D1 and D2 lines coincide and are equal to 10 ± 1 MHz/Torr. Due to its high spectral resolution, this technique allows separate measurements for each individual transition.