COHERENT emission of light
PIONNERING EXPERIMENT dipole close to the interface may also excite a surface wave whose wavevector k sw( ω) is given by the dispersion relation at each frequency ω( see Fig. 3 b). The surface wave propagates over a typical decay length which is on the order of 100 µ m before being absorbed. When ruling a grating on the interface, the different lines of the grating are coherently illuminated by the same surface wave which is thus diffracted in a well-defined direction given by the usual grating law( see Fig. 3 c). This is the mechanism that produces directional emission. In summary, the grating does not generate spatial coherence but reveals the existing spatial coherence of the field due to the excitation of a surface wave.
Applications and further developments
The experiment has proved that a source consisting of incoherent emitters can be turned into a directional source. It led to the understanding that Kirchhoff ' s law validity is broader than initially thought so that the tool box of nanophotonics can be used to tailor thermal emission. This idea has been further developed to explore a large variety of thermal sources [ 2 ]. More recently, this idea has been applied to analyse and control visible luminescence by semiconductors and quantum dots [ 12, 13 ]. The basic mechanism is the same: the incoherent emitters must be coupled to a mode guided by a surface or by a planar guide whose radiative losses can be engineered to control the emission polarization and angular pattern. These features enable the design of light-emitting metasurfaces with a thickness of a few hundred nanometers. This could mark the advent of a new generation of light sources that do not require the use of bulky lenses and polarizers [ 14 ].
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