Open channels are peculiar incidents modes of the optical field that are transmitted by a highly diffusing medium with high efficiency (100 % in the ideal case). The existence of these modes has been predicted by Dorokhov et al. [1] more than 25 years ago. Although many experiments were made to prepare these modes [2,3], the maxima of transmission obtained in experiments, are ever much lower than 100 %. To test the theory, we have considered here another Dorokhov's prediction: the average transmission of the modes that sustain the transmitted field must be TT=66%, where TT is defined by N2 TT= N1 T (where T is the averaged transmission of energy, and N1 and N2 the number of incident and transmitted modes i.e. the size of the incident and transmitted mode basis). Fig.1 (a) Holographic experimental setup. (b) Residual correlation of the transmitted field. The horizonal axis is SQ'. The sample transverse size is L=70 µm red, 50 µm green, 35 µm blue.... The horizontal grey line is the residual correlation expected for N1=N2. To verify this prediction, we have measured N2 on a slab sample of ZNO powder (transverse size L<=70 µm, thickness 20 µm) [4], by assuming that N1 is given by the sample geometry. The experimental setup is seen in Fig. 1(a). The incident fields are random (there are scrambled by a diffusing liquid DL) and digital holography is used to measure the complex transmitted field E(x,y) for the two polarisations. N2 is measured by analysing the residual correlations of the transmitted field measured for uncorrelated incident fields. Since the average transmission is T=1/25, we expect N2=N1/16. To get this optimal result, the transverse size of sample L must be infinite and the defocusing SQ' of the microscope objective MO2 must be zero (where S is the sample outgoing surface and Q' the plane imaged by MO2). Figure 1 (b) shows the residual correlation (which is proportional to 1/N2) as a function these two parameters. We get N2=N1/12 without extrapolation, and N2=N1/16 with extrapolation to L infinite. Here, the open channel effect is huge (factor 16) and the aggreement with the Dorokhov's prediction is excellent. We acknowledge Labex Numev (convention ANR-10-LABX-20) for funding. References 1. O.N. Dorokhov. On the coexistence of localized and extended electronic states in the metallic phase. Solid state communications, 51(6): 381-384, (1984).

This article deals with the fundamental problem of light-matter interaction in the quantum theory. Although it is described through the vector potential in quantum electrodynamics, it is believed by some that a hamiltonian involving only the electric and the magnetic fields is preferable. In the literature this hamiltonian is known as the Power-Zienau-Woolley hamiltonian. We question its validity and show that it is not equivalent to the minimal-coupling hamiltonian. In this article, we show that these two hamiltonians are not connected through a gauge transformation. We find that the gauge is not fixed in the Power-Zienau-Woolley hamiltonian. The interaction term is written in one gauge whereas the rest of the hamiltonian is written in another gauge. The Power-Zienau-Woolley hamiltonian and the minimal-coupling one are related through a unitary transformation that does not fulfill the gauge fixing constraints. Consequently, they predict different physical results. In this letter, we provide the correct quantum theory in the multipolar gauge with a hamiltonian involving only the physical fields.

Open channels are peculiar incidents modes that are transmitted by a diffusing medium with high efficiency (up to 100%). Digital holography is used to analyze the correlations of the transmitted field, and to test open channel theory. Quantitative agreement is obtained.

We study a device that consist of quantum resonators coupled to a mesoscopic photonic structure, such as a metasurface or a 2D metamaterial. For metasurfaces, we use surface Bloch modes in order to reach various coupling regimes between the metasurface and a quantum emitter, modelized semi-classically by an oscillator. Using multiple scattering theory and complex plane techniques, we show that the coupling can be characterized by means of a pole-and-zero structure. The regime of strong coupling is shown to be reached when the pole-and- zero pair is broken. For 2D metamaterial, we show the possibility of controlling optically the opening or closing of a gap. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Metamaterials made of periodic collections of dielectric nanorods are considered theoretically. When quantum resonators are embedded within the nanorods, one obtains a quantum metamaterial, whose electromagnetic properties depend upon the state of the quantum resonators. The theoretical model predicts that when the resonators are pumped and reach the inversion regime, the quantum metamaterial exhibits an all-optical switchable conduction band. The phenomenon can be described by considering the pole stucture of the scattering matrix of the metamaterial.