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Nonlinear crystal does supernarrow-linewidth frequency convers

Many applications in optical communications and scientific research require very narrow spectral linewidths. What limits the width of a frequency converter? The spectral width of a nonlinear converter is usually thought to be inversely proportional to the length of the nonlinear crystal.
Roei Remez and Ady Arie from Tel Aviv University (Israel) bring a new introduced concept of super-oscillations into the nonlinear optics regime.
Super-oscillation refers to the phenomenon of a band-limited function that oscillates faster than its highest Fourier component. Super-oscillation can be activated in a nonlinear crystal by modulating its quadratic nonlinear coefficient in a periodic manner to achieve quasiphase-matched interaction.

We implemented this concept experimentally by modulating the nonlinear coefficient of a KTP crystal using the electric field poling method and demonstrated super-oscillation in the converter frequency response—as a function of the pump wavelength and as a function of the crystal’s temperature. While here we utilized the concept of super-oscillation to design a frequency doubler with an isolated peak in the frequency conversion spectrum, the method can be further extended using different super-oscillating functions.
The super-oscillating crystal approach may be useful for nonlinear processing and nonlinear filtering of optical signals in optical communications, say the researchers, as well as for spectroscopy and quantum-information applications. Other types of nonlinear interactions that require phase-matching could also benefit, such as nondegenerate four-wave mixing. The crystal could even enhance coupling between a free-space beam and either a waveguide mode or a surface-plasmon-polariton wave.
1. R. Remez and A. Arie, Optica, 2, 5, 472 (2015); http://dx.doi.org/10.1364/OPTICA.2.000472.