All materials are characterized by a work-function, the energy necessary to remove an electron from the Fermi level into vacuum, which is a fundamental property critical for many applications.[1, 2] Electronic devices, such as organic transistors and solar cells, are designed with sets of metal electrodes carefully chosen according to their intrinsic work-function.[3–7] The work-function can be further adjusted by chemical modification of the interfaces to optimize the performance of such devices.[7–11] Here we demonstrate that the work-function can also be modified by engineering the electromagnetic environment of the material. To do this, we create hybrid light-matter states that strongly change the electronic energy levels of the system as explained next.

Through the rapid exchange of photons, matter can enter into the so-called strong coupling regime with the surrounding electromagnetic field. This leads to the formation of two new eigenstates,| P−〉 and| P+〉, separated by the Rabi splitting energy as illustrated in Figure 1A and B. To achieve this regime, the material is typically placed in a photonic structure or an optical cavity that is resonant with one of its electronic transitions. A prerequisite for strong coupling is that the photon exchange must be faster than any dissipation process, which is facilitated by shaping the electromagnetic environment and confining the field. Strong coupling has been extensively studied with atoms, semi-conductors and quantum wells as it offers much potential in areas such as Bose-Einstein type condensation of polaritons, spintronics, lasing and quantum information processing.[12–18] Nevertheless the potential of strong coupling is not limited to such physics applications. One of the fascinating features of strong coupling for material and molecular science is its collective nature.[19] In a strongly coupled molecular material, the Rabi-splitting is determined by the square root of the molecular concentration within the optical mode, and molecules microns apart will emit coherently if they are strongly coupled to the same mode.[20] Another important feature is the fact that strong coupling occurs even in the absence of light: according to quantum mechanics, the Rabi-splitting energy h