Impurities detection by optical techniques in KH 2 PO 4 crystals M. Pommie `s a , D. Damiani a , X. Le Borgne a , C. Dujardin b , A. Surmin a , J.C. Birolleau a , F. Pilon a , B. Bertussi a , H. Piombini a, * a CEA-Le Ripault, BP16, 37260 MONTS, France b Universite ´ de Lyon I, Laboratoire de Physico-Chimie des Mate ´riaux Luminescents, Unite ´ Mixte de Recherche CNRS 5620, Ba ˆt. A.Kastler, 10, rue Ampe `re, 69622 Villeurbanne, France Received 21 September 2006; received in revised form 14 March 2007; accepted 16 March 2007 Abstract In this paper we examine how optical techniques can be used for impurities (or defects) detection and identification in KH 2 PO 4 (KDP) components. This is important in so far as some of these defects are responsible for a much weaker than expected Laser-Induced Damage Threshold (LIDT) in these materials, i.e. for a weaker resistance to a laser shot. KDP materials are investigated by photothermal deflec- tion, fluorescence and photoexcitation with the aim of localizing and identifying the laser-induced damage precursors. The rapidly grown KDP crystals are shown to be heterogeneous from the absorption, fluorescence and composition point of view. Impurities concentrations are measured directly by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and tentatively correlated to some opti- cal characteristics and to the LIDT of KDP materials. Ó 2007 Elsevier B.V. All rights reserved. Keywords: KDP heterogeneities; Contamination; Fluorescence; Material characterization 1. Introduction KDP and Deuterated KDP (DKDP) are important materials in high power lasers such as the Laser Me ´gaJoule (LMJ) in France or the National Ignition Facility (NIF) in the USA. They are used to make optical switches or fre- quency converters [1]. One of the issues related to KDP/ DKDP-based components is the relatively low laser power they can tolerate without any damage. Indeed, when they are exposed to very high fluences, scattering centers may be induced in these components reducing their lifetime and increasing the maintenance costs of the huge laser facilities. It is commonly accepted that Laser-Induced Damage (LID) in KDP/DKDP materials is due to the presence of defects in the matrix (the so-called precursor defects). The last models describing the LID or conditioning mech- anisms assume a local absorption (either linear or multiph- otonic) [2,3] but without providing with any direct experimental evidence to support the presence of such pre- cursors. More investigations are then required to link these models to the real defects in KDP. A lot of defects may be present in rapidly grown KDP crystals among which dislo- cations, liquid inclusions, vicinal/growth boundaries... [4–6]. However, in state-of-the-art crystals the critical defects (from the LID point of view) are not identified. Woods et al. observed a weak correlation between the scat- tering centers and the initiation sites of damage [7]. Demos et al. investigated to what extent scattering defects or fluo- rescent clusters are involved in the LID mechanisms but they could not demonstrate any correlation between such defects and the LIDT in KDP [8]. Yet a strong interaction between these fluorescent centers and the laser beam is demonstrated as their fluorescence intensity strongly decreases after laser irradiation at 355 nm [8,9]. Laser irra- diation is also known to alter the electronic state of UV-absorbing defects in KDP samples [10–12]. Other 0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.03.057 * Corresponding author. Tel.: +33 2 47 34 40 00; fax: +33 2 47 34 51 54. E-mail address: herve.piombini@cea.fr (H. Piombini). www.elsevier.com/locate/optcom Optics Communications 275 (2007) 372–378