Detailed study on optical properties of Li2B4O7 for down conversion to millimeter waves

For the first time, we measured the laser damage threshold of LB4 nonlinear crystal (≥ 10 TW/cm2) for an fs- (60 fs, 950 nm) pulses. Refractive index components for a THz range 0.15-1.6 THz were measured and carefully reapproximated in terms of Sellmeier dispersion equations. Using the equations and the measured optical breakdown threshold, the efficiency of THz radiation generation is estimated for all possible types of three wave interactions. For e–e → e type of interaction deff = (2d15 + d31)cos2sin + d33sin3, o–o → e type deff = d31sin, and for o–e → o type deff = d15sin. The angle at which the phase matching conditions are achieved for THz radiation generation at frequencies < 1.5 THz is ≤ 10. Small values of the phase matching angles and, accordingly, small values of effective nonlinear coefficients explain the lack of experimental studies on the THz generation. The extremely high optical breakdown threshold and high atmospheric transmission in the longwavelength part of the THz range overcompensate low efficient nonlinear coefficients of LB4. Which makes it prospective for long trace high sensitivity probing of the atmosphere composition and parameters