Abstract

Broadband gain media for laser-diode (LD) pumped mode-locked laser have been widely studied. Åkermanite (Ca₂MgSi₂O₇) is one of the melilite-type crystal which has a two-dimensionally incommensurate structural modulation. Ca_1.96Nd_0.02Na_0.02MgSi₂O₇ is the åkermanite doped with Nd³⁺ ions and co-doped with Na⁺ ions for charge compensation. Due to the restriction of ion sizes, it is expected the Ca²⁺ ions are substituted by the Nd³⁺ ions and the Na⁺ ions. The basic structure is shown in Fig. 1. The coordination number of the Ca-O is basically 8, but the size misfit between the Ca²⁺ ions and the Mg²⁺ ions causes different coordination numbers of 6 and 7[1]. This is considered to be the origin of the structural modulation, but above 358K, the modulation disappears. Thus, åkermanite has the phase transition. The phase with the modulation was called “incommensurate phase (IC-phase)” and the phase without the modulation was called “normal phase (N-phase)”. In the IC-phase, broadening of the luminescence spectrum due to the modulation is expected. In addition, disordered location of Nd³⁺ ions and Na⁺ ions also exists. This makes åkermanite disordered crystal and further spectral broadening is expected. Hence åkermanite is expected to be suitable for ultlashort pulse laser. In this paper, we report the optical properties of Ca_1:96Nd_0:02Na_0:02MgSi₂O₇ and its first laser oscillation by LD pumping. Furthermore, an anomalous behavior is reported, which will be related with the phase transition.

© 2011 Optical Society of America