Observation of paramagnetic point defects in BBO (b-BaB 2 O 4 ) crystals Wei Hong a , L.E. Halliburton a, * , D. Perlov b , K.T. Stevens c , R.K. Route d , R.S. Feigelson d a Department of Physics, West Virginia University, P.O. Box 6315, Morgantown, WV 26506, USA b Coherent Crystal Associates, East Hanover, NJ 07936, USA c Northrop Grumman Space Technology, Synoptics, Charlotte, NC 28273, USA d Center for Materials Research, Stanford University, Stanford, CA 94305, USA Received 24 July 2003; accepted 11 August 2003 Available online 18 March 2004 Abstract The electron paramagnetic resonance (EPR) technique has been used to investigate point defects in single crystals of BBO (b- BaB 2 O 4 ). This borate compound is a widely used nonlinear optical material, especially in the ultraviolet region, and point defects are expected to play a role in its device performance. There were no observable EPR spectra in our as-grown BBO crystals. However, an irradiation at room temperature with X-rays produced two stable EPR signals. One of these spectra, containing ten hyperfine lines when the magnetic field is parallel to the crystal’s c axis, is assigned to a hole trapped on an oxygen ion adjacent to a barium vacancy. The ten lines are due to interactions with the two neighboring 11 B nuclei. The other radiation-induced spectrum consists of a single line with an effective g value of 2.017 when the magnetic field is along the c axis. We tentatively assign this spectrum to Ni þ ions substituting for Ba 2þ ions. In separate experiments, it was found that heating an as-grown BBO crystal to 760 °C for several hours produced an EPR spectrum due to Cu 2þ ions. Subsequent exposure at room temperature to X-rays reduces the intensity of the Cu 2þ spectrum. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.72.Ji; 76.30.Mi; 42.70.Mp Keywords: Electron paramagnetic resonance; EPR; Nonlinear optical materials; BBO crystals; Point defects; Electron traps; Hole traps 1. Introduction Electron paramagnetic resonance (EPR) is a high- resolution experimental technique often used to identify point defects in wide-bandgap optical materials [1,2]. The sensitivity of the technique can be extraordinarily high, allowing defect concentrations of approximately 5 · 10 12 cm 3 to be detected under favorable conditions (i.e., a spectrum measured at 10 K or below and con- sisting of a single line with a width of one gauss or less). Most EPR spectra have either broader lines and/or multiple lines due to hyperfine splittings and crystal field structure. Even in these more typical situations, the EPR technique can be used to routinely monitor defect con- centrations near or below 10–20 ppb (parts per billion). Although single crystals of beta barium borate (b- BaB 2 O 4 , better known as BBO) are widely used in nonlinear optical devices [3,4], it is only recently that the EPR technique has been applied to this important material. The paramagnetic electron and hole centers formed at 77 K in BBO by ionizing radiation have been described by Hong et al. [5]. These investigators ob- served an electron trapped at an oxygen vacancy, a self- trapped hole, and a series of holes trapped by neigh- boring perturbations. Although these defects were only stable at or near 77 K, they are expected to play a major role in the transient optical absorption produced in BBO by high-power ultraviolet laser pulses. Also, Bravo et al. [6] have used EPR to characterize spectra due to Nd 3þ ions in doped BBO crystals. In another EPR study, * Corresponding author. E-mail address: larry.halliburton@mail.wvu.edu (L.E. Hallibur- ton). 0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2003.08.012 Optical Materials 26 (2004) 437–441 www.elsevier.com/locate/optmat