Ultrasensitive measurement method for refractive index difference between two wavelengths

A novel method for the measurement of ultra small refractive index differences between two wavelengths is demonstrated. The method is based on the change of the phase matching conditions of a nonlinear process in two nonlinear crystals separated by a dispersive medium. We demonstrate the performance of our technique by measuring the refractive index difference of air between 775nm and l550nm with an air column of only 33cm.