We demonstrate here a controllable variation in the Casimir force. Changes in the force of up to 20% at separations of ~100 nm between Au and AgInSbTe (AIST) surfaces were achieved upon crystallization of an amorphous sample of AIST. This material is well known for its structural transformation, which produces a significant change in the optical properties and is exploited in optical data storage systems. The finding paves the way to the control of forces in nanosystems, such as micro-or nanoswitches by stimulating the phase change transition via localized heat sources.Pacs numbers: 78.68.+m, 03.70.+k, 85.85.+j, 12.20.Fv 2 Casimir forces [1][2][3][4][5][6][7][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. 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][10][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 only 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.Here we demonstrate that phase change materials (PCMs) [12][13][14][15][16][17][18][19][20][21], which are renowned to switch reporducibly between an amorphous and a crystalline phase, are very promising candidates to achieve a significant force contrast without a change of composition. These materials are already used in rewriteable optical data storage [13,14,[23][24][25], where the pronounced optical contrast between the amorphous and crystalline 3 state is employed to store information. This storage principle employs a focussed laser beam to locally heat a disk with a thin film of phase change material. Upon a variation of the power and length of the laser pulse the material can be reversibly switched between the amorphous and the crystalline phase many times. Here we will show that the pronounced contrast of optical properties enables a significant change of the Casimi...

Phase change materials (PCMs) can be rapidly and reversibly switched between the amorphous and crystalline state. The structural transformation is accompanied by a significant change of optical and electronic properties rendering PCMs suitable for rewritable optical data storage and non‐volatile electronic memories. The phase transformation is also accompanied by an increase of the Casimir force of 20 to 25% between gold and AIST (Ag5In5Sb60Te30) upon crystallization. Here the focus is on reproducing and understanding the observed change in Casimir force, which is shown to be related to a change of the dielectric function upon crystallization. The dielectric function changes in two separate frequency ranges: the increase of absorption in the visible range is due to resonance bonding, which is unique for the crystalline phase, while free carrier absorption is responsible for changes in the infrared regime. It is shown that free carriers contribute ≈50% to the force contrast, while the other half comes from resonance bonding. This helps to identify PCMs that maximize force contrast. Finally it is shown that if this concept of force control is to be employed in microelectromechanical devices, then protective capping layers of PCMs must be only a few nanometers thick to minimize reduction of the force contrast.

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