E*PCOS2010 Casimir forces and phase-change materials P. J. van Zwol 1 , G. Palasantzas 1 , O. Shpak 1 , B.J. Kooi 1 , G. Torricelli 2 , C. Binns 2 , V. B. Svetovoy 3 , M. Wuttig 4 1 Materials innovation institute M2i and Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands 2 Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, United Kingdom 3 MESA+ Institute for Nanotechnology, University of Twente, PO 217, 7500 AE Enschede, The Netherlands 4 I. Physikalisches Institut (IA), RWTH Aachen University, 52056 Aachen, Germany ABSTRACT Phase-change materials are well known for their structural transformation producing a significant change in the optical properties and this is exploited in optical data storage systems. Optical properties are also the defining factor for the so-called Casimir force, that is related to the ground-state of the electromagnetic field in vacuum. We demonstrate that for a single material system a significant variation in the Casimir force can be achieved. Changes in the force of up to 20% at separations of ~100 nm between Au and AgInSbTe (AIST) surfaces were found (in both experiment and theory) by changing the AIST from the amorphous to the crystalline phase. The present finding paves the way to force control in nano-systems, such as micro- or nano-switches. However, then the effect of a capping layer on the phase-change film has to be included. We show here that SiO 2 capping layers significantly reduce the Casimir force contrast between the amorphous and crystalline phase and therefore they should be applied as thin as possible. Key words: Casimir force, AgInSbTe, amorphous films, crystalline films, ellipsometry, atomic force microscopy 1. INTRODUCTION Casimir forces [1-8] arise between two surfaces due to the quantum zero-point energy of the electromagnetic field. The surfaces restrict the allowed wavelengths and thus the number of field modes within the cavity, which locally depresses the zero-point energy of the electromagnetic field (see Fig.1). The reduction depends on the separation between the plates; thus there is a force between them, which for normal materials is always attractive [1]. The zero point energy manifests itself as quantum fluctuations, which in the small separation limit give rise to the familiar van- der-Waals force. The original calculation of the Casimir force assumed two parallel plates with an infinite conductivity [1]. This was later modified to include the dielectric properties of real materials and the intervening medium [2,3], providing the first glimpse of possible methods to control the magnitude and even the direction of the force. This finding has motivated our attempts to manipulate the dielectric properties of a material and hence generate force contrast [9-11]. A particularly exciting possibility is to produce a ‘switchable’ force by employing materials whose optical properties can be changed in situ in response to a simple stimulus [9,10]. So far the only significant contrast that has been demonstrated is between different materials [11]. To obtain a large Casimir force contrast for a single material requires a large modification of its dielectric response, which has not been achieved in materials used up to now. Figure 1. Impression of the Casimir force as a consequence of vacuum fluctuations of the electromagnetic field. Outside the cavity there is a ‘fluctuating sea’ of virtual photons or fields, inside only certain modes of these fields can exist due to the imposed boundary (where the field must be zero). The ‘pressure’ of the ‘sea’ outside is bigger than the pressure due to the fields inside and it will result in an attractive force between the opposite walls of the cavity.