1 August 1998 Ž . Optics Communications 153 1998 223–225 Active stripe waveguides produced by electron beam lithography in LiF single crystals Rosa Maria Montereali a , Antonella Mancini a , Giancarlo C. Righini b,1 , Stefano Pelli b a ENEA, Dipartimento INN, C.R. Frascati, C.P. 65, 00044 Frascati, Rome, Italy b IROE-CNR, Optoelectronics and Photonics Department, Via Panciatichi 64, 50127 Florence, Italy Received 15 April 1998; accepted 28 May 1998 Abstract Photo-luminescent colored stripes have been fabricated by electron beam lithography in lithium fluoride crystals, and their guiding properties are demonstrated for the first time. An increase of refractive index D n larger than 5 =10 y3 , suitable for light confinement, was achieved by irradiation with a 12 keV electron beam. This single-step process, that induces at the same time the waveguiding structure and the photo-luminescent color centers, offers good perspectives for the development of color-center integrated optical lasers tunable in the visible. q 1998 Elsevier Science B.V. All rights reserved. Keywords: 42.70.H; 42.82; 61.72.J Keywords: Laser materials; Integrated optics; Color centers; Optical waveguides; Lithium fluoride 1. Introduction Ž . Alkali halide crystals containing color centers CCs are well known active media in optically pumped tunable wx solid state lasers 1 . Among such crystals, lithium fluoride is particularly interesting because it can host laser active Ž . defects stable at room temperature RT emitting in the wx visible and in the near infrared 2 . As an example, broad- band RT laser emission in the visible spectral interval was obtained with a single pumping wavelength in a LiF wx q Ž crystal 3 containing F and F centers two electrons 2 3 bound to two and three neighboring anion vacancies, re- . spectively . Low-energy electron-beam bombardment, with limited penetration depth, creates a thin layer containing Ž high concentration of F an electron trapped in an anion . q vacancy , F and F color centers located at the surface of 2 3 the crystal. A sizeable optical gain was measured in LiF crystals and polycrystalline films irradiated by a 3 keV 1 Corresponding author. E-mail: righini@iroe.fi.cnr.it electron beam for both F and F q centers, at the peaks of 2 3 their broad emission bands, namely at 670 and 540 nm, wx respectively 4 . Solid state lasers based on these structures, however, even if compact, do not lend themselves to an easy integra- tion with optical fibers and channel waveguides. A few approaches have been investigated to accomplish such goal. As an example, an evanescent field amplifier was proposed which uses a silica waveguide deposited on top y wx of a LiF:F crystal 5 . In another device, a waveguide 2 coupler was introduced inside the cavity of a LiF:F y color 2 center laser, in order to efficiently extract the laser light wx and make easier the coupling to an optical fiber 6 . This paper presents the results of an experimental investigation which demonstrates that optical waveguides may be fabricated in LiF crystals at the same time that color centers are created. In fact, the electron bombard- ment by a beam of energy in the range of few keV, besides efficiently creating stable optically active lattice defects, induces an increase of the refractive index in the irradiated layer. The depth of such a layer is enough to support at 0030-4018r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0030-4018 98 00299-5