D. Pugh

Calculation of the second-order electronic polarizabilities of some organic molecules. Part 1

A version of the CNDO/S method has been re-parametrized by correlating computed and measured dipole moments and transition wavelengths for a range of organic conjugated molecules. The method has then been used to calculate the hyperpolarizability tensor, βijk, in the form related to second-harmonic generation, for an extended set of molecules of similar type. A good correlation between theory and experiment has been found. The convergence of the excited-state perturbation expansion used in the calculation of the tensor has been extensively investigated.

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.

Calculations of the electronic spectra and hyperpolarisabilities of selected dyes and pigments

A method has been developed for the calculation of the spectra and hyperpolarisability tensor for the linear electro-optic effect at the CNDO/S level of approximation using a sum-over-states procedure with singly excited states. A wide range of dyes and pigments have been studied to assess their potential as non-linear optical materials using both static-field and frequency-dependent values. Conjugated systems containing strong donors and acceptors positioned at either end of the conjugation path show the largest intrinsic effects. Substantial resonance enhancement effects occur for molecules such as azobenzenes and azoheterocycles which absorb in the red or infrared region of the spectrum.

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...

Pulsed source thermal lens. Part 1.—Theoretical analysis

The principle of operation of a pulsed source thermal lens is described and a practical system, based on a pulsed, CO2, line-tunable laser, is outlined. The time variation of the thermal lens is probed by a continuous He/Ne laser monitored on a photomultiplier tube behind a pinhole mounted on the beam axis in far field. This photomultiplier signal is fully analysed theoretically as a function of time and the physical properties of the sample gas. It is shown that this analysis enables calculation of the magnitude of the temperature at any point in the cell as a function of time. The risetime of the signal is related to the vibrational–translational relaxation time of the sample and the lifetime of the signal decay is related quantitatively to the thermal conductivity coefficient.

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.

Pulsed-source thermal lens. Part 2.—Experimental tests of the theory

The theory of the thermal lens presented in Part 1 is tested by using the decay to calculate the thermal-conductivity coefficients of Ne, Ar, Kr and Xe (48.2, 18.48, 10.05 and 6.02 W m–1 K–1, respectively) in terms of one adjustable parameter, the pulsed laser beam radius, which is found to be close to the measured value for the c.w. beam. These values agree closely with the best values currently available. The treatment of the rise of the thermal-lens signal is tested by comparison of observed with a priori calculated variation of maximum probe-laser modulation as a function of added Ar pressure. The system is shown to be useful for TEM00 laser beam mode diameter measurement and calculation of the temporal variation of the spatial distribution of temperature in a laser-irradiated cell.

Transverse and longitudinal surface plasmon resonances of a hexagonal array of gold nanorods embedded in an alumina matrix

The specular reflectance from a hexagonal array of gold nanorods embedded in an alumina matrix supported on an aluminum substrate is reported. The rods were grown by electrodeposition of gold in an alumina template and were oriented with their long axis perpendicular to the film surface. Optical reflectance measurements performed with an incident light beam of S polarization only exhibited the transverse surface plasmon resonance whereas the measurements obtained with P polarization exhibited both transverse and longitudinal resonances. A model for the reflectance from a thin anisotropic film was developed and shown to be in agreement with the experimental data.