Appl. Phys. B 57,431-434 (1993) Applied Photo- physics Physics B ,n,, ,.,,,., Chemlsb'y © Springer-Verlag1993 Noncollinear Ultraviolet Generation in a Lithium Borate Crystal G. C. Bhar, P. K. Datta, A. M. Rudra Burdwan University, Physics Department, Laser Laboratory, Burdwan-713104 India (Tel.: +91-342/2371, Extn. 545) Received 10 May 1993/Accepted 3 August 1993 Abstract. Nd:YAG and a Nd:YAG-pumped dye laser are used to generate tunable deep ultraviolet radiation down to 240 nm in a lithium triborate crystal (LBO) by noncol- linear sum-frequency mixing. All longer wavelengths can be generated by a combination of harmonic generation of the dye laser and sum-frequency mixing, 240 nm being the near noncritical limit. A set of versatile Sellmeier disper- sion equations is derived to satisfactorily predict phase- matching in LBO. PACS : 42.70, 42.65 A sound theoretical understanding of the optical non- linearity of crystals have led Chen's group in China [1, 2] to the development of the two most-'wonder' materials beta barium borate (BBO) and lithium triborate (LBO) during the last decade. Both crystals have been reported to possess several attractive properties for nonlinear fre- quency conversion [3-6]. As an UV generating nonlinear optical material LBO has at least two important advan- tages over BBO. The transmission cut-off of LBO in the UV region is at a much shorter wavelength than that for BBO. Secondly, the damage threshold of LBO is much higher than that of BBO. The comparatively low birefrin- gence of LBO offers low angular sensitivity. Due to its high temperature-sensitivity, temperature-tuned noncri- tical phase-matching has been realized [7]. Due to low walk-off and non-deliquescency, a higher efficiency may be achieved by using larger LBO crystals. The only disadvan- tage of LBO compared to BBO is its comparatively low effective nonlinear coefficient. Deep ultraviolet radiation up to its transmission cut-off has been generated in BBO [8]. In LBO, the Wallenstein group [9] has reported UV generation upto 188 nm with an efficiency of 0.2% to 2.0% by sum-frequency mixing of higher harmonics of the Nd: YAG and 1.6 I~m radiation and has also predicted the possibility of generating UV radiation until its transmis- sion cut-off at 160 nm. High power, tunable, deep UV sources are needed for medical and biological applications. Though the importance of LBO is in the generation of UV radiation in and around 200 nm, non-phasematchability does not permit the generation of deep UV radiation using the commonly available pump source as the Nd:YAG laser (1064 nm) and its second- or third-harmonic-pumped dye lasers. In the present paper efficient and tunable from 240 nm to longer wavelengths coherent ultraviolet radiation generated in an LBO crystal using type-I noncollinear sum-frequency mixing of the Nd:YAG laser radiation and Nd:YAG-pumped dye laser at room temperature is de- scribed. BBO crystal is used to generate the second har- monic of a DCM dye laser. For a small noncollinear angle, there is no noticeable reduction in interaction volume but the generated beam is separated out automatically from the input beams in this technique, thereby not requiring any filter. A Q-switched Nd:YAG laser (Spectra-Physics, DCR- 11) having filled-in-beam optics and its second-harmonic- pumped DCM dye laser (Spectra-Physics, PDL-2) serve as pump sources in the experiment. Our dye laser is tunable from 620 nm to 670 nm, though still longer wavelengths can be used. In the first step the second harmonic of the dye laser radiation is generated in a type-I, 22.8 ° cut BBO crystal by type-I collinear SHG phase-matching. The sec- ond harmonic of the dye laser in the range 310 nm to 335 nm is again frequency mixed with the unconverted Nd:YAG laser beam at 1064 nm in the LBO crystal to generate tunable UV radiation in the deep ultraviolet from 240 nm to 255 nm. The schematic experimental setup is shown in Fig. 1. The vertically polarized dye laser beam is weakly focused in the nonlinear crystal. A 90 ° polarization rotator (642 nm) is placed after the lens so that the dye-laser polariza- tion becomes horizontal. The generated dye-laser beam is frequency-doubled by the BBO crystal. It is rotated in the vertical plane for type-I phase-matching, so that the gener- ated second-harmonic beam becomes vertically polarized. By tuning the dye laser from 620 nm to 670 nm, output corresponding to UV radiation from 310 nm to 335 nm is generated. A filter having a transmission of 80% in this UV region is used to separate the second-harmonic output from the dye fundamental. A maximum conversion effi- ciency of 19~o has been obtained corresponding to a dye-