Optical spin orientation in ruby by Zeeman-selective U-band absorption

Y. Takagi, Ryuichiro Minamihara, T. Makino

G. F. Imbusch, Stanley Geschwind

Theodore H. Maiman

Thomas Godin, R. Moncorgé, J.L. Doualan +4 more

The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with Cr3+ and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the Cr3+ ion in its excited and ground states, Δαp. The Cr3+ optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to Δαp has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths ∼3 orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of Δαp in Cr3+-doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in Cr:Al2O3 (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its E2 and T24 emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before. On the other hand, Cr:Al2O3 (ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the E2–T24 energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative Δαp value, which is contrary to all refractive index variation (including Z-scan) measurements.

Di Chen

Ruby exhibits the Zeeman effect in the energy levels responsible for the absorption of the R1line. When a ruby laser is directed to pass through a section of unexcited ruby rod, the intensity of the transmitted beam is strongly dependent upon the magnetic field applied to the absorber. This effect was used to modulate the amplitude of a pulsed ruby laser in the microwave frequency region. The experimental modulator consisted of a 0.2- inch long ruby absorber located at one end of anS-band TE111mode cylindrical cavity pumped by a 3.2-Gc/s microwave source. A dc bias magnetic field was also applied to the absorber. When both the laser and the absorber rods were cooled to 80°K, the modulated beam attained maximum modulation index with a bias field of 2.5 kOe. This and other experimental results agree well with calculations made from theoretical analysis of this modulation technique, and indicate that the various known relaxation mechanisms in the spin system of ruby do not impose a frequency limit to this method of modulation.

Theodore H. Maiman