Abstract : The goal of our work has been to investigate a few well-chosen effects in laser spectroscopy under clean experimental conditions which facilitate a sharp and meaningful contact with theory. The initially-defined task was to study large pulse-area effects in single upward transitions from metastable atoms in a fast, monovelocity Ne* beam. It proved possible as well to investigate multiple-pulse transitions on a cw basis. Ramsey fringe resonance signatures were clearly observed and for the first time in the optical domain, using three pulse excitation. Basically the first two zones of light provide initially a coherence and secondly an atomic population in the excited state. The counter-running fields in the second zone and third zones convert excited state population into a coherence again and finally into a nonlinear (with intensity) change in the excited and ground state populations. In view of the time gap tau in the preparation phase, one could anticipate and did observe narrow Ramsey fringes which vary as cos (delta, omega, tau) in the middle of the single zone excitation linewidth function ( Rabbi pedestal ). The neon beam employed was rather fast (v/c approx. .001) and, being produced by charge transfer had an exceptionally an narrow velocity distribution, delta, approx .0001. These characteristics permitted unambiguous measurements of the fringe shapes and facilitated useful contact with theory. A second extremely successful experiment was the combination of laser excitation of two-photon Doppler-free transitions in the 5S yields nS series in Rubidium with precise Lambdameter interferometer techniques to accurately measure the transition wavelength.
Abstract : The goal of our work has been to investigate a few well-chosen effects in laser spectroscopy under clean experimental conditions which facilitate a sharp and meaningful contact with theory. The initially-defined task was to study large pulse-area effects in single upward transitions from metastable atoms in a fast, monovelocity Ne* beam. It proved possible as well to investigate multiple-pulse transitions on a cw basis. Ramsey fringe resonance signatures were clearly observed and for the first time in the optical domain, using three pulse excitation. Basically the first two zones of light provide initially a coherence and secondly an atomic population in the excited state. The counter-running fields in the second zone and third zones convert excited state population into a coherence again and finally into a nonlinear (with intensity) change in the excited and ground state populations. In view of the time gap tau in the preparation phase, one could anticipate and did observe narrow Ramsey fringes which vary as cos (delta, omega, tau) in the middle of the single zone excitation linewidth function ( Rabbi pedestal ). The neon beam employed was rather fast (v/c approx. .001) and, being produced by charge transfer had an exceptionally an narrow velocity distribution, delta, approx .0001. These characteristics permitted unambiguous measurements of the fringe shapes and facilitated useful contact with theory. A second extremely successful experiment was the combination of laser excitation of two-photon Doppler-free transitions in the 5S yields nS series in Rubidium with precise Lambdameter interferometer techniques to accurately measure the transition wavelength.
Abstract Resonance scattering techniques are very useful for high-resolution atomic spectroscopy. The applicability of these techniques has been much extended, particularly through the rapid development of tunable-laser technology. The use of a narrow-band tunable laser, acting on a collimated atomic beam, gives a direct method enabling, for example, hyperfine structure and isotope shift studies. The intensity of lasers allows stepwise excitations to be performed, and with the two-photon absorption technique, Doppler-free measurements on thermal gases are also possible. By using pulsed lasers, time-resolved measurements yielding radiative life-times and structural information can be performed. The basic resonance scattering methods can be combined with radiofrequency and coherence techniques to yield a resolution, limited only by the uncertainty relation. Optical double resonance and level-crossing techniques, not requiring a narrow-band light source, have been extensively used. Several examples of the application of resonance scattering methods are given.
High-resolution spectroscopy performed on fast-moving absorbers has several desirable features. First, in a collinear geometry, the residual Doppler width of the fast atoms is low due to the velocity bunching taking place during acceleration. Second, the velocity of these cooled atoms can easily be changed very precisely. Third, this makes possible Doppler tuning of a laser into resonance with a particular atomic transition. Doppler shifts tip to ≃80 Å are present, dependent on the beam energy and the atom used. This, in turn, makes it possible to 'create' completely harmonic three-level atoms, using the relativistic transformation between laboratory and atom rest frame and to perform optical pumping in well defined spatial regions. Applications to velocity control, resonant three-level nonlinear field-atom interactions, high-resolution laser-r.f. double-resonance spectroscopy, and lifetime determinations are discussed.
In this series of papers we report on the generation and application of multiple pulse phase coherent sequences in optical spectroscopy.In this paper the effects of intense pulse trains on systems with only two resonant energy levels are analyzed, with particular attention to the effects of extreme inhomogeneous broadening and population depletion to nonresonant levels.It is shown that these effects, which are present in virtually all optical systems, make the simple gyroscopic model of optical coherent transients invalid.Exact calculations show, e.g., that a two-pulse photon echo is not maximized by a 1:2 length ratio for the pulses; that the maximum excited state population is not created by a 180" pulse; and that three equal pulses are almost as effective as a 1:2:1 ratio for producing three pulse echoes.The role of pulse phase is extensively analyzed.Pulse sequences are proposed and experimentally demonstrated which permit optical phase sensitive detection and measurement of ground state relaxation parameters.The experimental results are based on an extension of the acousto-optic modulation and fluoresence detection techniques of Zewail and Orlowski [Zewail et al., Chem.Phys.Lett.48, 256 (1977); Orlowski et al., ibid.54, 197 (1978)).The relative merits offluorescence and transverse polarization detection are discussed, and fluorescence detection is shown to be more generally useful for these new sequences.Finally, composite pulse trains are shown to be capable of substantially increasing the signal available from highly inhomogenously broadened transitions.In paper II we extend the treatment to multilevel systems with some emphasis on solid state applications.
In this series of papers we report on the generation and application of multiple pulse phase coherent sequences in optical spectroscopy. In this paper the effects of intense pulse trains on systems with only two resonant energy levels are analyzed, with particular attention to the effects of extreme inhomogeneous broadening and population depletion to nonresonant levels. It is shown that these effects, which are present in virtually all optical systems, make the simple gyroscopic model of optical coherent transients invalid. Exact calculations show, e.g., that a two-pulse photon echo is not maximized by a 1:2 length ratio for the pulses; that the maximum excited state population is not created by a 180° pulse; and that three equal pulses are almost as effective as a 1:2:1 ratio for producing three pulse echoes. The role of pulse phase is extensively analyzed. Pulse sequences are proposed and experimentally demonstrated which permit optical phase sensitive detection and measurement of ground state relaxation parameters. The experimental results are based on an extension of the acousto-optic modulation and fluoresence detection techniques of Zewail and Orlowski [Zewail et al., Chem. Phys. Lett. 48, 256 (1977); Orlowski et al., ibid. 54, 197 (1978)]. The relative merits of fluorescence and transverse polarization detection are discussed, and fluorescence detection is shown to be more generally useful for these new sequences. Finally, composite pulse trains are shown to be capable of substantially increasing the signal available from highly inhomogenously broadened transitions. In paper II we extend the treatment to multilevel systems with some emphasis on solid state applications.
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.
In conventional saturated absorption experiments utilizing I 2 absorption cells, the acccuracy in determining the line center is ultimately limited by : unresolved recoil structure /1/ ; residual transit-time broadening ; second-order Doppler broadening and shift ; and collisions (nonlinear pres sure broadening and shift). These limitations can be greatly reduced by applying the method of separated field excitation /2/ in a supersonic I 2 beam. The experiments were carried out using a high-resolution I 2 -stabilized Ar'-laser spectrometer at 514.5 nm and a collimated supersonic beam. The spectral width of the Ar'-laser was estimated to be in the range of 10 kHz. From the high density and the narrow velocity distribution of the super sonic beam we expect to achieve good signal-to-noise ratio, reduced second order Doppler broadening and improved knowledge of the second order Doppler shift. The experimental setup is shown in Fig. 1. The I 2 molecules of the supersonic be am are excited sequentially by two pairs of travelling waves propagating parallel but in opposite directions. A coherence between the initial and the excited states is built up in the first interaction zone /4/. It then precesses freely through the field-free region. In the second zone, two of the initial state and the excited state are created in the Vz -space, depending on the phase difference between the coherence and the exciting field. The two gratings are slightly shifted relatively to each other due to the recoil splitting. The distance a bet ween the first two interaction zones is limited to about 3 mm to avoid excessive decay of the upper state of the selected absorption line. During the free flight between the second and third interaction zones, separated by a distance d = 35 mm, the upper state population decays completely into many ro-vibrational levels of the electronic ground state, leaving the population grating of the initial state practically unchanged. Correspon dingly, only the initial state population grating will contribute to the Ramsey signal generated after the final two interactions and only the recoil component associated with the initial state will contribute to the Ramsey fringes /1,3,4/.