Wave-front phase-modulation control and focusing of second-harmonic light generated in transparent nonlinear random structures

摘要

We theoretically investigate how phase-only spatial light modulation can enable controlling and focusing thensecond-harmonic light generated in transparent nonlinear random structures. The studied structures are composednof domains with random sizes and antiparallel polarization, which accurately model widely used ferroelectricncrystals such as strontium barium niobate. Using a first-principles Green-function formalism, we account for theneffect that spatial light modulation of the fundamental beam introduces into the second-order nonlinear frequencynconversion occurring in the considered class of structures. This approach provides a complete description of thenphysical origin of the second-harmonic light generation in the system, as well as the optimization of the lightnintensity in any arbitrary direction. Our numerical results show how the second-harmonic light is influenced bynboth the disorder in the structure and the boundaries of the crystal. Particularly, we find that the net result fromnthe interplay between disorder and boundary effects is strongly dependent on the dimensions of the crystal andnthe observation direction. Remarkably, our calculations also show that although in general the maximum possiblenenhancement of the second-order light is the same as the one corresponding to linear light scattering in turbidnmedia, in the Cerenkov phase matching direction the enhancement can exceed the linear limit. The theoreticalnanalysis presented in this work expands the current understanding of light control in complex media and couldncontribute to the development of a new class of imaging and focusing techniques based on nonlinear frequencynmixing in random optical materials.