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

Optical properties of single axes Li2B4O7 (LB4) crystal are defined at room temperature in the spectral range 0.03˗0.5 THz. Dispersion of the refractive index components are approximated in the form of Sellmeier equations. Dispersion properties were used to determine phase-matching conditions for THz wave generation by collinear difference frequency generation processes. To the damage threshold under the pump by train of hundreds of 60 fs pulses of Ti:sapphire laser operating at 950 nm is found to be 250 TW/cm2, as well as the coherence length and efficiency of all possible types of three wave interactions are defined

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

Statistical theory of helical twisting in nematic liquid crystals doped with chiral nanoparticles

A molecular field theory of the cholesteric ordering in nematic nanocomposites doped with chiral nanoparticles was developed taking into consideration chiral dispersion interaction between rod-like nanoparticles. It was shown that the inverse pitch of the cholesteric helical structure is proportional to the anisotropy of the effective polarizability and the anisotropy of the effective gyration tensor of a nanoparticle in the nematic host. The theory enables one to predict the helical sense inversion induced by a change of the low-frequency dielectric susceptibility of the nematic host phase. The components of the high-frequency effective polarizability and the effective optical activity of a gold rod-like nanoparticle in a particular nematic solvent were calculated numerically

Effect of rod-like nanoparticles on the dielectric susceptibility of nematic nano-composites : a molecular theory

The effect of rod-like nanoparticles on the high-frequency dielectric susceptibility of the nematic nano-composites has been investigated in the framework of a molecular theory. Analytical expressions for the components of the effective polarizability of a rod-like nanoparticle in the nematic host have been obtained and used in the calculations of the dielectric susceptibility of the composites as functions of the nanoparticle volume fraction. Numerical calculations of the susceptibility have been undertaken using the nematic liquid crystal 5CB as a host doped with either gold or silver particles for different values of the concentration of nanoparticles. It has been shown that the rod-like nanoparticles have a much stronger effect on the components of the dielectric susceptibility of the nano-composites including, in particular, the one with gold nanoparticles in the vicinity of the plasmon resonance. The main conclusion is that at sufficiently large concentration of nanoparticles, the anisotropy of the dielectric susceptibility of the nano-composites may even change the sign with an increasing concentration which may be important for various applications

Molecular-theory of high frequency dielectric susceptibility of nematic and cholesteric nano-composites

A molecular-statistical theory of the high frequency dielectric susceptibility of the nematic nanocomposites has been developed and approximate analytical expressions for the susceptibility have been obtained in terms of the effective polarizability of a nanoparticle in the nematic host, volume fraction of the nanoparticles and the susceptibility of the pure nematic phase. A simple expression for the split of the plasmon resonance of the nanoparticles in the nematic host has been obtained and it has been shown that in the resonance frequency range the high frequency dielectric anisotropy of the nanocomposite may be significantly larger than that of the pure nematic host. As a result all dielectric and optical properties of the nanocomposite related to the anisotropy are significantly enhanced which may be important for emerging applications. The components of the dielectric susceptibility have been calculated numerically for particular nematic nanocomposites with gold and silver nanoparicles as functions of the nanoparticle volume fraction and frequency. The splitting of the plasmon resonance has been observed together with the significant dependence on the nanoparticle volume fraction and the parameters of the nematic host phase

Millimetre-wave range optical properties of BIBO

We present the thorough studies of optical properties of BiB3O6 (BIBO) crystal in the millimeter-wave (subterahertz) range. We observe a large birefringence Δn = nZ −nX = 1.5 and the values of absorption coefficients of all three axes to be less than 0.5 cm−1 at the frequency of 0.3 THz. The difference from visible range in angle ϕ between the dielectric axis z and crystallophysical axis X is found to be more than 6°. The simulated phase-matching curves in the xz plane of the crystal show the optimal value of the angle θ to be around 25.5°±1° for an efficient millimeter-wave generation under the pump of 1064 nm laser radiation

Potential of sub-THz-wave generation in Li2B4O7 nonlinear crystal at room and cryogenic temperatures

Due to their high optical damage threshold, borate crystals can be used for the efficient nonlinear down-conversion of terawatt laser radiation into the terahertz (THz) frequency range of the electromagnetic spectrum. In this work, we carried out a thorough study of the terahertz optical properties of the lithium tetraborate crystal (Li2B4O7; LB4) at 295 and 77 K. Approximating the terahertz refractive index in the form of Sellmeier’s equations, we assessed the possibility of converting the radiation of widespread high-power laser sources with wavelengths of 1064 and 800 nm, as well as their second and third harmonics, into the THz range. It was found that four out of eight types of three-wave mixing processes are possible. The conditions for collinear phase matching were fulfilled only for the o - e -o type of interaction, while cooling the crystal to 77 K did not practically affect the phase-matching curves. However, a noticeable increase of birefringence in the THz range with cooling (from 0.12 to 0.16) led to an increase in the coherence length for o-o-e and e-e-e types of interaction, which are potentially attractive for the down-conversion of ultrashort laser pulses