Second harmonic generation with giant angular and spectral acceptance

Abstract - Frequency conversion and, more specifically, second harmonic generation, is a fundamental phenomenon in nonlinear optics, both for applications and fundamental research. The occurring of this last effect in crystals requires that the frequency-doubled propagating beam interferes constructively with the pump itself, a phase-matching (PM) condition which is due to momentum-conservation laws. These lead to wavelength-dependent constraints on the process geometry while the wavevector mismatch is accompanied by chromatic walkoff. Conversely, collinear PM in SHG can be obtained either through optical birefringence, which introduces wavelength and polarization constraints, and quasi-phase-matching, that requires periodic material microstructuring. We present the experimental demonstration of SHG with a spectral acceptance of more than 100 nm angular and acceptance up to ±40◦, with no polarization selectivity or chromatic walk-off. This is achieved in a disordered potassium-based perovskite manifesting giant broadband refraction (GR), which determines a natural and unconditioned PM with no chromatic walk-off. Results open the way to highly efficient versatile and adaptable nonlinear optical devices.