Second harmonic generation of cw laser radiation at 244 nm by temperature tuned noncritical phase-matching in lithium tetraborate

We report for the first time to our knowledge on noncritical temperature tuned second harmonic generation (SHG) in Li/sub 2/B/sub 4/O/sub 7/ (LB4) with the 488 nm line ofa cw Ar/sup +/ laser. LB4 is a negative uniaxial crystal belonging to the point group 4mm which means that its effective nonlinearity for type-I interaction is maximized in the noncritical (/spl theta/=90/spl deg/) limit. The spectral linewidth of 6 GHz for the 488-nm Ar/sup +/-laser line should allow in principle the use of LB4 crystals as long as 10 cm. We used for the present experiment LB4 samples of length l=1 cm and 5 cm cut at /spl theta/=90/spl deg/ with apertures of 3/spl times/5 mm and 6/spl times/10 mm, respectively. The Ar/sup +/ laser provided 10 W of maximum cw power (measured after the focusing lens and in front of the LB4 sample) in the multimode regime. After frequency doubling the radiation at the fundamental was removed using a 45/spl deg/ dielectric reflective mirror and a 60/spl deg/ fused silica prism. Absolute measurements of the second harmonic power at 244 nm were performed by a calibrated solar blind photomultiplier and a lock-in amplifier. In each case the incident power on the photomultiplier was attenuated by a 2-mm thick UG5 filter and one or two reflecting neutral density filters to levels within the linear response region of the photomultiplier.