Nonlinear crystals convert laser light of one wavelength to another. Usually, the nonlinear crystal also converts efficiently other wavelengths that are close enough to the designed wavelength. When the input laser has multiple wavelengths and we are interested only in the specific designed wavelength, this wide response must be narrowed so that the unwanted wavelengths would be filtered out.
However, in order to achieve a narrow conversion response, a long crystal is needed, since the width of the efficiency response (with respect to wavelength) is usually inversely proportional to the crystal’s length. Unfortunately, longer crystals are more expensive, consume a larger physical size and moreover, the maximum length of most crystals is limited to a few centimeters at most.
In our research, we show that by appropriate modulation of the nonlinear crystal, the width of the converter can be made as narrow as we like, irrespective of the crystal’s length. This is achieved by modulating the nonlinear coefficient with a “super-oscillating” function. Super oscillating functions are band-limited functions that oscillate locally much faster than their highest Fourier component. This work therefore brings the concept of super oscillations to the nonlinear optics regime for the first time. This high resolution nonlinear filter can have applications in optical communication, spectroscopy, quantum information, and more.
Roei Remez and Ady Arie, “Super-narrow frequency converter”, Optica2, 472-475 (2015),