Nhng tin b trong Quang hc, Quang ph vÃ ng dng VI ISSN 1859 - 4271

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Advances in Optics, Photonics, Spectroscopy & Applications VI ISSN 1859 - 4271anti-stokes Raman scattering, stimulated Compton scattering, etc.), nonlinear refractive index(self-<strong>ph</strong>ase modulation, self focusing, beam breakup, etc.), multi<strong>ph</strong>oton absorption, and coheren<s<strong>trong</strong>>tin</s<strong>trong</strong>>teractions (self-induced transparency, <strong>ph</strong>oton echo, etc.). In this paper we first briefly reviewthe previous research related to the nonlinear optics which one of the authors (YK) hasexperienced, and report up-dated works of nonlinear optics which may lead to breakdown ofvacuum by the nonlinearity of vacuum.II. HIGH RESOLUTION TWO-PHOTON ABSORPTION SPECTROSCOPY OFRYDBERG STATESNonlinear optics is an important approach to high resolution spectroscopy. One of the factorsthat limit the resolution is the Doppler broadening. During the research on high resolutionBrillouin scattering spectroscopy of rare gas liquids and solids [5], one of the authors (YK) havecome up with the idea of elimina<s<strong>trong</strong>>tin</s<strong>trong</strong>>g the Doppler broadening in two-<strong>ph</strong>oton absorption by usingcounter propaga<s<strong>trong</strong>>tin</s<strong>trong</strong>>g laser beams. The Doppler shifts of an atom by the counter propaga<s<strong>trong</strong>>tin</s<strong>trong</strong>>g laserbeams cancel each other, thus enabling two <strong>ph</strong>oton absorption spectrum which is free from theDoppler broadening.Fig. 2. Energy level diagram of the 5s and nd states of Rb 85 and Rb 87 atoms [6]We have applied this method to high resolution spectroscopy of highly excited states(Rydberg states) of Rb atoms [6]. By using a frequency tunable dye laser as an excitation source,the two-<strong>ph</strong>oton absorption spectra of 5s-nd transitions of Rb atoms were taken for the excitedstates with the principal quantum numbers of n = 11 to 32. The ground state of the Rb atom (5sstate) has hyperfine split<s<strong>trong</strong>>tin</s<strong>trong</strong>>g arising from the coupling of the electron spin with the nuclear spin.The hyperfine split<s<strong>trong</strong>>tin</s<strong>trong</strong>>g is different for the two isotopes Rb 85 and Rb 87 which have nuclear spinsof I=5/2 and I=3/2, respectively (Fig. 2). The isotope shift of the ground state energy (δ in Fig. 2)reflects the interaction of the nucleus with the electron.The excited nd states also have fine structures (Δ and Δ’ in Fig. 2). Since the binding energiesof the highly excited states are very weak, the electron clouds spread far from the nucleus,approaching to ~μm from the nucleus. Under this condition, the energies of the electronic statesmay be broadened even by small perturbations to the electrons. Also it was pointed out that thereis a possibility that the fine structure split<s<strong>trong</strong>>tin</s<strong>trong</strong>>g is inverted at highly excited states. In spite of20
Những tiến bộ <strong>trong</strong> <strong>Quang</strong> học, <strong>Quang</strong> <strong>ph</strong>ổ và Ứng dụng VI ISSN 1859 - 4271various interes<s<strong>trong</strong>>tin</s<strong>trong</strong>>g aspects, the Rydberg states were not well studied since the experimentrequires frequency tunable short wavelength light sources and the hyperfine and fine structuresare not resolved in the standard one-<strong>ph</strong>oton absorption spectroscopy.Fig. 3. The 2-<strong>ph</strong>oton absorption spectra of Rb for the 5s-12d and the 5s-25d transitions [6]With the Doppler-free two-<strong>ph</strong>oton absorption spectroscopy, we can study the highly excitedstates with high resolution using the tunable laser in the visible region. Figure 3 shows the two<strong>ph</strong>otonabsorption spectra of the 5s-12d transition and the 5s-25d transition. Here the signals aredue to the fluorescence in the visible region from excited p states to the 5s ground state arisingfrom the d-p-s spontaneous decay processes. This experiment has shown that the Rydberg statesare not broadened as seen in the 5s-25d spectrum in spite of the very small binding energy. Theisotope shift and the fine structure split<s<strong>trong</strong>>tin</s<strong>trong</strong>>g with its dependence on the principal quantum numberhave been determined. This example shows that the nonlinear optics can play an important rolein high resolution spectroscopy.III. SELF FOCUSING IN HIGH POWER LASERSAt high laser intensity, the refractive index n of an optical component increases with the laserintensity I as n=n 0 + n 2 I, where n 2 is referred to as nonlinear refractive index. In high powerlasers, self focusing of a laser beam occurs during propagation due to the nonlinear <strong>ph</strong>ase shift ofthe laser beam in optical components. Due to self focusing, the laser beam breaks up into severalhigh intensity parts, leading to permanent damages to optical components. The layout of a highpower glass laser G-MII at the Institute of Laser Engineering, Osaka University is shown in Fig.4 [7], as an example of the high power glass lasers used for laser fusion research. This lasersystem of 2 beams with 20-cm diameter output apertures consists mainly of an oscillator (OSC),rod amplifiers (RA) of 25 and 50 mm apertures, disk amplifiers (DA) of 100 and 200 mmapertures, spatial filters (SF) for image relaying and filtering out high spatial frequencycomponents of the laser beam, optical shutters (OS), and Faraday rotators (FR) to isolate theamplifier chain from the target reflection.The figure of merit of the laser glasses is evaluated by the ratio of the stimulated emissioncross section σ and the nonlinear refractive index n 2 ; σ/n 2 . Due to higher gain, the higher value ofσ enables using less length of the laser glasses, thus reducing the nonlinear <strong>ph</strong>ase shift during21
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