Appl Phys A (2012) 106:257–263 DOI 10.1007/s00339-011-6691-7 INVITED PAPER Nonlinear imaging and THz diagnostic tools in the service of Cultural Heritage G. Filippidis · M. Massaouti · A. Selimis · E.J. Gualda · J.-M. Manceau · S. Tzortzakis Received: 7 July 2011 / Accepted: 7 November 2011 / Published online: 19 November 2011 © Springer-Verlag 2011 Abstract We present the use of novel nonlinear imaging, terahertz time-domain spectroscopy and imaging as power- ful diagnostic tools for studies of works of art. It is shown that nonlinear imaging offers precise in-depth information, while terahertz imaging can reveal hidden objects and un- cover information on highly absorbing organic compounds whose visualization is difficult in other parts of the spec- trum. 1 Introduction The removal of unwanted surface layers consists one of the most delicate interventions in Cultural Heritage (CH) conservation. In particular, both time and exposure to at- mospheric conditions may irreversibly modify these layers, calling thus for their removal. However, either by conven- tional cleaning methods or lately by laser beam application, the conservator interacts directly and irreparably with the cultural material. The removal action gets further compli- cated if the plurality of materials that may be found within a painted artwork (often in ultrathin layers or traces) is taken into account. Therefore, given the demanded high selectiv- ity and special treatment, a detailed in-depth assessment of the painting layers compounds is required. A variety of tech- G. Filippidis ( ) · M. Massaouti · A. Selimis · E.J. Gualda · J.-M. Manceau · S. Tzortzakis Institute of Electronic Structure and Laser, Foundation for Research and Technology Hellas, P.O. Box 1527, 71110, Heraklion, Greece e-mail: filip@iesl.forth.gr S. Tzortzakis e-mail: stzortz@iesl.forth.gr url: http://unis.iesl.forth.gr niques has been employed towards the analysis and charac- terization of materials spanning from X-rays up to the mid- infrared. In the present study, two femtosecond laser-based novel approaches are described: nonlinear imaging and terahertz (THz) time-domain spectroscopy and imaging. The former relying on nonlinear phenomena taking place upon intense laser irradiation, including Multi-Photon Excitation Fluo- rescence (MPEF), Second Harmonic Generation (SHG) and Third Harmonic Generation (THG), while the latter is based on the use of femtosecond laser induced Terahertz radiation. Both of the presented herein techniques are non-invasive and can reliably and accurately reveal in-depth information (e.g. structural modifications and hidden objects below the original surface) of a cultural object layers leading to the mapping of the artwork to be restored. We anticipate that they can serve as monitoring tools to fine-tune the cleaning protocol and safeguard the CH object. 2 Nonlinear imaging: principles and applications MPEF, SHG and THG, are well-established, non-destructive imaging techniques that recently have been used as diagnos- tic tools for art conservation studies, providing very promis- ing results [1, 2]. They have the potential to provide essential information to the art conservation scientists for the assess- ment of the appropriate conservation method that has to be followed for a painted artwork [3]. The use of femtosecond (fs) lasers enables high peak powers for efficient nonlinear excitation, but at low enough energies so that samples are not damaged. The basic prin- ciple underlying MPEF, SHG and THG imaging techniques is that for tightly focused fs laser pulses, the photon den- sity is high enough to induce multiphoton absorption or