On the coherence of photons related with various spectral lines

Kengqi WANG, Jinxian GUO

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.

Robert Hanbury Brown, R. Q. Twiss

G. M. Barykinsky, Vladimir V. Lebedev, V M Plyasulya

An investigation was made of the spectral properties of four-photon parametric oscillation when the frequency of the generated field is tunable near an atomic resonance line. An ω4 = ω1–ω2+ω3 generation process involving coherent anti-Stokes scattering in thallium vapor was achieved experimentally. Biharmonic pumping was provided by resonant excitation of a 6P1/2–6P3/2 two-photon transition at the frequency ω1–ω2. Measurements were made of the oscillation power when the frequency ω3 was tunable near the frequency of a 6P3/2–7D3/2 transition at various thallium vapor pressures. It was found that the spectral properties of resonant parametric oscillation depend strongly on the concentration of active-mixture atoms and also on various saturation effects.

Edward M. Purcell

Edward M. Purcell

Tadao Shimizu

Various characteristics of molecules have been clarified by spectroscopic methods in which the interactions of molecules with the radiation field were investigated. A stationary response of a molecule excited by a radiation field appears as a spectral line. Information on molecules may be obtained from position (resonance frequency), intensity, and shape and width of the spectral line. So far the interaction of molecules with the radiation field has been studied primarily in a “frequency domain.” Although molecular spectroscopy is one of the research fields in modern science which has the longest history, a variety of important aspects have been demonstrated very recently in this field as radiation sources have been improved. When the radiation field is “weak,” the photon nature of the radiation field is dominant in the interaction with a molecule, such as in the cases of emission, absorption, and scattering of photons. When the radiation field is “strong,” in the sense that many photons exist in a proper mode of the radiation field, the wave nature of the field plays an important role in the interaction. In this case the radiation may be described in the form of the classical field, say, a sinusoidal function of the time and spatial coordinates, coherently interacting with the molecule, which is described quantum mechanically. Here nonlinear properties would appear in the interactions: saturation in absorption, induced scattering, multiphoton processes, coherent transients, etc.