J. Liebertz

Exceptional large nonlinear optical coefficients in the monoclinic bismuth borate BiB3O6 (BIBO)

Abstract The monoclinic BiB3O6 (abbreviation BIBO hereafter) (space group C2 [1] ) has been grown from stoichiometric melts in single crystals of dimensions up to 5×4×2 centimeters in optical quality. We have determined all independent nonlinear optical (nlo) coefficients of second harmonic generation dijkSHG for the fundamental λ0=1079.5 nm. The complete determination of a dijkSHG tensor in crystals of monoclinic or lower symmetry has not been published to our knowlegde yet. Linear optical investigations indicate a wide range of wavelengths for phase matchable processes. The highest deff we found along the phase matched direction of second harmonic generation at 1079.5 nm is 3.2 pm/V. This value is larger than that of many other substances, such as KTiOPO4 (KTP) or β−BaB2O4 (BBO), being widely used in applications.

Second-harmonic generation of light at 245 nm in a lithium tetraborate whispering gallery resonator

A millimeter-sized, monolithic whispering gallery resonator made of a lithium tetraborate, Li2B4O7, crystal was employed for doubly resonant second-harmonic generation with a continuous-wave laser source at 490 nm. An intrinsic quality factor of 2×10(8) was observed at the pump wavelength. A conversion efficiency of 2.2% was attained with 5.9 mW of mode-matched pump power. In the lithium tetraborate resonator, it is feasible to achieve phase-matching of second-harmonic generation for pump wavelengths between 486 and 506 nm.

Electro-optic and electrostrictive properties of the low-temperature phase of barium borate BaB2O4*

All components of the linear electro-optic tensor [rσijk] (unclamped) of low-temperature BaB2O4 and the liner electrostrictive tensor [dijk] were determined (T= 293 K; λ= 633 nm). In its electro-optic and electrostrictive behaviour low-temperature BaB2O4 resembles the tetraborates Li2B4O7 and PbB4O7. The maximum value of the longitudinal and transversal electro-optic effects is only one fourth of that in KDP. This reduces the importance of BaB2O4 for electro-optic applications.

Crystal growth and characterization of LiPbPO4

Crystals of LiPbPO 4 were grown by the Czochralski method and with a top-seeded solution growth (TSSG) technique. The main difficulty in the crystal growth of LiPbPO 4 arises from the incongruent melting behaviour of the compound. The incongruent melting temperature is T p = 900°C and the liquid temperature of a corresponding stoichiometric composition LiPbPO 4 is T l = 920°C. Precipitation of a second crystalline phase Li 3 PO 4 could be observed at about 860°C. A small amount of Bi 2 O 3 was added to the melt in order to prevent the precipitation of Li 3 PO 4 .

Whispering gallery resonator from lithium tetraborate for nonlinear optics

For second-order nonlinear-optical processes in the ultraviolet, appropriate materials with a sufficiently large band-gap typically exhibit smaller nonlinear coefficients than materials with comparably smaller band-gap. Whispering gallery resonators, with their outstanding quality factors, provide field enhancement and can compensate for these small coefficients. We report on the successful fabrication of a whispering gallery resonator made of lithium tetraborate, a suitable material for ultraviolet applications with a small nonlinear coefficient of d31 = 0:073 pm/V. Quality factors of the order of 108 are observed from the ultraviolet to the near-infrared spectrum. The inferred absorption coefficients of lithium tetraborate are below 0.2 m-1 in the visible and near-infrared. Continuous-wave second harmonic generation from 490 nm light to 245 nm is observed with conversion efficiencies up to 2.2 %.