Blue-light-induced infrared absorption in KNbO3

Owing to its high nonlinearity and low passive losses, KNbO3 has great potential as a medium for efficient frequency doubling and parametric oscillation." 2 For example, KNbO3 has been used successfully for the production of cw blue light by frequency doubling of a dye laser,3 doubling of a semiconductor diode laser,4 and nonlinear mixing of diode and Nd:YAG lasers.5 In recent work we observed 70% conversion efficiency for external cavity doubling of Ti:A1203 light near 860 nm and secondharmonic output power in excess of 0.6 W for 1.35 W of pump.6'7 We have also achieved parametric oscillation thresholds as low as 1 mW in an optical parametric oscillator (OPO) with KNbO3. 5 In principle even better performance should be possible-the measured crystal nonlinearity and passive intracavity losses of our current experimental setup imply a 90% conversion efficiency and oscillation threshold close to 200 /.LW. However, we have been prevented from achieving these values in practice by blue-light-induced infrared absorption (BLIIRA) in the crystal. ' Phenomenologically, we define BLIIRA as the difference between the passive infrared absorption of a crystal and the infrared absorption that it exhibits when it is simultaneously illuminated by blue light. In the case of frequency doubling, the blue light is just the generated second-harmonic radiation, whereas in the process of parametric downconversion, BLIIRA is caused by the blue OPO pump. We have encountered BLIIRA in KNbO3 for infrared wavelengths of 850-885 nm when the blue light is in the wavelength range 425-440 nm. To quantify the detrimental effects of light-induced losses on the efficiency e of external-cavity frequency doubling, we can write e as