F. R. Cruickshank

Terahertz pulse generation in an organic crystal by optical rectification and resonant excitation of molecular charge transfer

Organic molecular crystals that are extremely efficient at terahertz-pulse generation are in- vestigated. Terahertz pulses produced by optical rectification at 800 nm in (−)2-(α-methylbenzyl-amino)-5-nitropyridine have an order of magnitude higher power than those generated in the commonly used inorganic crystal ZnTe. The organic molecular crystals were also found to generate terahertz pulses when excited on resonance at 400 nm. This may pave the way for studying ultrafast charge-transport dynamics in three dimensions.

The linear and nonlinear optical properties of the organic nonlinear material 4‐nitro‐4’‐methylbenzylidene aniline

The organic crystal 4‐nitro‐4’‐methylbenzylidene aniline (NMBA) was identified as a promising nonlinear material by the powder technique. The material gave a second harmonic intensity 16 times that of urea. Large single crystals of dimensions 5×3×1 cm3 were grown by the temperature lowering of a seeded supersaturated ethyl acetate solution. The principal dielectric axes were defined by orthoscopic examination. The dispersions of the refractive indices were determined to an accuracy of ±0.0015 using the minimum deviation technique and Maker fringe spacings. These dispersion curves were fitted to a Sellmeier equation which allowed the indices to be determined to ±0.0006. The nonlinear d coefficients d11, d33, d31, and d13 were evaluated at 1000, 1064, and 1300 nm using the Maker fringe technique. The coefficient d11 was over 200 times larger than potassium dihydrogen phosphate (KDP) d36. In addition, the nondiagonal coefficient d31 was similar to the phase‐matching coefficient in the organic material 3‐acet...

Surface damage of (-)2-(α-methylbenzylamino)-5-nitropyridine single crystals induced by pulsed laser radiation.

Pulsed laser-induced surface damage experiments were made on the cleaved (001) face of (-)2-(α-methylbenzylamino)-5-nitropyridine crystals at a pulse length of 25 ns FWHM. The highest single-pulse damage threshold value was 24.2 J/cm(2) at 532 nm for light polarized along the crystallographic b axis.

Organic materials for non-linear optics: inter-relationships between molecular properties, crystal structure and optical properties

An account is given of the origins, in the molecular electronic properties and crystal structure, of the large quadratic optical nonlinearities found in some organic crystals. Particular cases, in which powerful second-harmonic generation or large Pockels effects have been found, chosen to illustrate the influence of different aspects of molecular or crystal structure, are introduced. Three of these, MBANP, NMBA and DAN are described in greater detail in the light of recent experimental and theoretical work.

Thermal properties of the nonlinear optical crystal zinc tris (thiourea) sulphate

The heat capacity of crystalline zinc tris (thiourea) sulphate, has been measured in the range from 220 to 500 K by differential scanning calorimetry, and was found to obey the relationship Cp(T)=2.76×10−3 T+0.366 J g−1 K−1. Thermal expansion data have been measured in the range from 150 to 473 K. From these data, the principal thermal expansion coefficients were found to be α1=6.41×10−5 K−1, α2=4.52×10−5 K−1, and α3=−4.32×10−6 K−1. The thermal conductivity tensor of this orthorhombic crystal was calculated from values of the thermal diffusivity in the directions normal to the (100), (010), and (001) crystal planes by the laser flash method. The thermal conductivity coefficients at 295 K are k1=0.27 W m−1 K−1, k2=0.34 W m−1 K−1, and k3=0.54 W m−1 K−1.

Quantitative analysis of hydrogen bonding and atomic thermal motion in the organic non-linear optical material DCNP using X-ray and neutron diffraction

A single-crystal neutron diffraction study of the organic non-linear optical material 3-(1,1-dicyanoethenyl)-1-phenyl-4, 5-dihydro-1H-pyrazole (hereafter DCNP), space group Cc, is presented. The study was conducted in order to relate the structural characteristics of the compound to its physical properties. DCNP exhibits a very large second harmonic generation (SHG) output, an extremely large linear electro-optical effect and photoconductive and pyroelectric properties. The nature of the hydrogen-bonding revealed by the study, in part, accounts for the first two of these phenomena. The neutron study also shows that some rather atypical atomic thermal motion is present in part of the molecule. With the aid of a variable-temperature single-crystal X-ray diffraction study, in conjunction with the neutron study, this thermal motion is attributed to libration and is fully characterized. As a result, suitable corrections to the bond geometry and the anisotropic displacement parameters of DCNP are made. The libration is also shown to enhance the SHG effect. The cell parameters from the variable-temperature X-ray study are also used in order to evaluate the thermal expansivity coefficients of DCNP.

Linear electro‐optic dispersion in (‐)‐2‐(α‐methylbenzylamino)‐5‐nitropyridine single crystals

The variation of the linear electro‐optic effect in (‐)‐2‐(α‐methylbenzylamino)‐5‐nitropyridine with the wavelength of the incident light at room temperature has been measured. The reduced half‐wave voltages have been found to have the values 2.1, 2.8, and 6.0 kV at 488, 514.5, and 632.8 nm respectively and the corresponding values of the linear electro‐optic coefficient have been evaluated. The interpretation of the results in terms of the structures of the molecule and the crystal is discussed. The thermal variation of the birefringence has also been investigated and the coefficient for the temperature variation of the refractive index difference is found to have the value (dΔn/dT)=9.3×10−5 K−1.

Phase-matched Optical Second-harmonic Generation in the Organic Crystal MBA-NP, (−)2-(α-methylbenzylamino)-5-nitropyridine

Abstract Large (5 × 3 × 3 cm3) crystals of the organic nonlinear optical material MBA-NP have been prepared by growth from solution, in an optically and structurally highly perfect state. Oriented cut and polished specimens were examined by the Maker fringe technique at a wavelength of 1·064 µm. Analysis of the data yielded d 22 as 69 × d11 quartz with a corresponding coherence length of 2 µm. A large rotation of the X and Z dielectric axes with wavelength (30° over 200 nm) was observed. Type I phase-matched second-harmonic generation (SHG) was recorded. For the 1·064 µm dielectric axis set and a (010) faced crystal, phase matched SHG was observed for rotation about X to an angle of incidence of 45°. This angle is very sensitive to the correct alignment of dielectric axes and the effect of its misalignment was investigated.

Thermal conductivity of the molecular crystal (–)2–(α–methylbenzylamino)‐5‐nitropyridine

Thermal diffusivities of high quality (‐)2‐(α‐methylbenzylamino)‐5‐nitropyridine (MBANP) single crystals were measured in different directions by the flash diffusivity method. The complete thermal conductivity tensor at temperatures of 295, 323, and 349 K was then obtained for this monoclinic crystal from the thermal diffusivity, specific heat, and density data. For instance, the thermal conductivity coefficients at 295 K are k1=0.27 W/mK, k2=0.29 W/mK, and k3=0.20 W/mK. The peak melting point of MBANP lies at 357.3 K, whereas the corresponding enthalpy of melting ΔHm has a value 67.2 J/g.

The determination of thermal conductivity coefficients of liquids by a thermal lens technique

Abstract The thermal lens technique, already established for gases, is extended to the measurements of liquid-phase thermal conductivity coefficients, the best values obtained at 300±2 K being 0.59, 0.171, 0.116, 0.149 and 0.110 W m −1 K −1 for water, acetone, chloroform, butan-2-one and chlorobenzene respectively. It is also shown that the accuracy of measurement of the curvature of the axial-pulsed-laser beam intensity, with an array of resolution equal to the pinhole diameter, is not sufficient to give the precision of measurement associated with a calibration approach.