Second harmonic generation with giant angular and spectral acceptance

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 walk-off. 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.