Appl. Phys. B 67, 151–156 (1998) Applied Physics B Lasers and Optics  Springer-Verlag 1998 Excited state absorption and stimulated emission of Nd 3+ in crystals. Part I: Y 3 Al 5 O 12 , YAlO 3 , and Y 2 O 3 S. Kück, L. Fornasiero, E. Mix, G. Huber Institut für Laser-Physik, Universität Hamburg, Jungiusstraße 9a, 20355 Hamburg, Germany (Fax: +49-40/4123-6281, E-mail: kueck@physnet.uni-hamburg.de) Received: 4 December 1997/Revised version: 8 May 1998 Abstract. The excited state absorption spectra of the 4 F 3/2 ex- cited state of the Nd 3+ ion in Y 3 Al 5 O 12 , YAlO 3 , and Y 2 O 3 were measured in a continuous wave pump- and probe ex- periment in a wide spectral range from 850 nm (780 nm for Y 3 Al 5 O 12 ) to 1500 nm. The cross sections were determined from a comparison with the emission spectra and the simul- taneously measured ground state absorption bleaching. The strongest excited state absorption transitions were found in the 1220–1400 nm spectral region due to transitions to the 2 G 9/2 and 4 G 7/2 levels. The spectral positions of the meas- ured transitions are in good agreement with the theoretically expected transitions calculated from the known Stark-level splittings. PACS: 42.55.P; 42.55.Rz; 42.70.H; 71.55.i; 78.45.+h Neodymium-doped crystals and glasses are in general ef- ﬁcient laser materials. Among the crystals the compo- nents of the Y 2 O 3 −Al 2 O 3 system, i.e. Nd:Y 3 Al 5 O 12 (YAG), Nd:YAlO 3 (YAP), and Nd:Y 2 O 3 (YO) were matters of in- terest since the 60 s because of their excellent physical and optical properties [1–4]. Flashlamp-pumped laser oscilla- tion from these crystals was realized early [5–7]. In the following years, Nd:YO remained an attractive subject of research because of the possibility for testing theoretical models in a simple crystal matrix [8, 9], whereas Nd-doped YAG and YAP became widespread high-gainmaterials. The ﬁrst diode-pumped Nd laser was realized with a Nd:YAG rod in 1968 [10]. Since then laser oscillation in these materials has been achieved on several wavelengths; i.e. around 950 nm ( 4 F 3/2 → 4 I 9/2 ), between 1060 nm and 1100 nm ( 4 F 3/2 → 4 I 11/2 ), and between 1300 nm and 1450 nm ( 4 F 3/2 → 4 I 13/2 ) [11, 12]. The further perfection of the crystal quality and the improvement of the pump-laser diodes lead to high output power, all solid-state laser systems based on Nd:YAG and Nd:YAP. However, only around 1060 nm, i.e. the transition with the highest cross section, are the experimentally deter- mined slope efﬁciencies close to the theoretical ones, which are limited by the quantum yield and the overlap between res- onator and pump mode. In the past, excited state absorption (ESA) competing with the stimulated emission process was assumed to be responsible for the observed discrepancies, especially for the 4 F 3/2 → 4 I 13/2 laser transitions. According to the energy level scheme of the Nd 3+ ion (see Fig. 1) and the common decay times of the various manifolds in the investigated crystals it can be assumed that besides the ground state manifold only the 4 F 3/2 metastable upper laser level is signiﬁcantly populated. Therefore ESA pro- cesses that might adversely inﬂuence the laser performance are the transitions 4 F 3/2 to 2 D 5/2 around 820 nm, 4 F 3/2 to the 2 D 3/2 , 2 K 15/2 , 4 G 11/2 , and 4 G 9/2 levels between 940 nm and 1080 nm, and 4 F 3/2 to 4 G 7/2 around 1350 nm. First measure- ments on ESA losses in Nd-doped materials in these spectral regions were performed in glasses and glass ﬁbers, because the broad inhomogeneous lines increase the possibility of an 0 5000 10000 15000 20000 25000 4 F 3/2 4 F 5/2 + 2 H 9/2 4 F 7/2 + 4 S 3/2 4 F 9/2 4 G 5/2 + 2 G 7/2 4 G 7/2 + 2 K 13/2 + 2 G 9/2 4 G 9/2 + 4 G 11/2 2 K 15/2 + 2 D 3/2 2 P 3/2 2 H 11/2 2 P 1/2 + 4 D 5/2 4 D 1/2 + 4 D 1/2 4 I 15/2 4 I 13/2 4 I 11/2 4 I 9/2 Energy [cm -1 ] Fig. 1. Energy level scheme of the Nd 3+ ion in YAG, Stark levels from [31]