G. S. Simpson

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...

The growth and perfection of organic non-linear optical crystals: N-methyl urea (NMU) from methanol solution. III. The growth of large single crystals for optical examination

Using growth conditions defined from previous studies of the kinetics of growth of this material, large crystals of N-methyl urea (NMU) have been prepared by seeded solution growth using the temperature lowering technique. The principal constraints to good growth are that crystals should be prepared using seeds cut from good quality crystals previously prepared by this technique and that they should be developed under conditions of low supersaturation (σ < 0.5−3%). Efficient stirring conditions should be used with flow reversal to minimise the effect of volume diffusion and macrostep formation which in turn leads to the inclusion of solvent in some sectors. The role of seed orientation in the definition of the development process is discussed. Under well-defined conditions of seed orientation and development crystals of dimensions 5 × 4 × 4 cm3 have been produced of high optical and structural perfection and with efficient non-linear optical performance. This material shows a figure of merit performance which improves on urea. For the doubling of light of wavelength 871.6 nm; d14 ≈ 2pm V−1, cf. urea d14 = 1.7pm V−1.

The growth, perfection and properties of organic non-linear optical materials I. 4-nitro-4′-methyl benzylidene aniline

Single crystal specimens of the organic non-linear optical material 4-nitro-4′-methyl benzylidene aniline (NMBA) have been grown from the melt (Bridgman technique) and from solution (slow cooling technique). Crystals produced by the former technique were strained, principally due to the instability of the material in the melt and the tendency of the decomposition products to accumulate in the solid during growth. Crystals grown from solution were of much higher perfection exhibiting the forms: (010), {111}, {011} (all dominant) and (102), (100) (less dominant) predicted by theoretical calculations of the equilibrium form. The rhombic shaped crystals which occasionally contained striations parallel to the (102) plane were grown to sizes of 5x3x0.5 cm3 (maximum dimensions). Large volumes of the crystals were optically clear and unstrained. From these could be prepared cut and polished specimens (0.5×0.3×0.2 cm3) for structural and optical assessment.

The growth of large single crystals of the organic nonlinear optical material 2-(α-methylbenzylamino)-5-nitropyridine

The crystal growth behaviour of the important organic nonlinear optical material 2-(a-methylbenzylamino)-5-nitropyridine (MBANP) (space group P21) has been investigated and techniques have been developed for the preparation of large (up to 7 cm x 4 cm x 3 cm) high-quality single crystals suitable for detailed optical, dielectric, ultrasonic and piezoelectric studies and for device fabrication. Observation of the crystal growth behaviour and measurement of growth kinetics revealed a strong and anomalous dependence of growth rate on supersaturation. At relative supersaturations less than 0.08, slow growing irregular faces were formed. Once the crystal was completely bounded by such faces, little or no further growth occurred. At supersaturations below 0.24 no growth was observed in either <010> direction. Above this, rapid, but highly imperfect, growth occurred in one <010> direction while the opposite end of the crystal exhibited zero growth rate at all achievable supersaturations. At relative supersaturations greater than 0.4 considerable instability in the growth along [010] was observed. The factors potentially responsible for this behaviour are discussed. A novel technique is described for the preparation of seed crystals elongated parallel to <010>. The use of such seeds allows the growth of large crystals of MBANP at supersaturations less than 0.24, thereby avoiding the highly imperfect and unstable growth at higher supersaturations and yielding crystals of very high quality. The nature, density and distribution of growth induced defects in large crystals of MBANP have been investigated using X-ray diffraction topography. In addition to revealing growth dislocations, growth sector boundaries, inclusions and striations, these studies provide a detailed insight into the growth history of the crystals.

X-ray topographic study of flux grown KTP crystals

Abstract The structural quality of crystals of potassium titanyl phosphate (KTiOPO 4 : KTP) prepared by top-seeded solution growth from K 6 P 4 O 13 flux, has been investigated using synchrotron X-ray topography. A very high level of structural perfection was observed, with average dislocation densities less than 10 cm −2 and substantial volumes being dislocation-free. A major factor in this respect is the low level of dislocation formation at the capping region of the crystal. Initial observations relating to the characterisation of dislocations and other defects are discussed together with aspects of the crystal growth history revealed by X-ray topography.

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.

The Linear Optical Properties of the Organic Molecular Crystal (+)2-(δ-Methylbenzylamino)-5-Nitropyridine (MBA-NP)

Abstract The linear optical properties of the organic molecular crystal (+)2-(δ-methylbenzylamino)-5-nitropyridine (MBA-NP) are investigated. The specific rotation associated with this chiral molecule has been determined as + 239.11°. The effective transparency range for nonlinear optical applications was found to be 465–1500 nm, with a 10 nm dichroism in the high frequency absorption edge position. MBA-NP displays angular dispersion to a larger extent than any other known organic nonlinear material, a 200 nm change in the wavelength of the incident light causing a 40° rotation of the dielectric axes. The linear refractive indices of MBA-NP have been determined using the minimum deviation technique, Maker fringe spacings and phase-matching directions. The principal refractive index dispersion curves are fitted with a Sellmeier equation and the coefficients of this equation are presented for the first time.

Growth, perfection and properties of the organic non-linear optical crystal, 4-aminobenzophenone (ABP)

4-Aminobenzophenone (ABP) has been purified and grown as a single crystal using both melt and solution growth techniques. Solution growth fromN,N-dimethylformamide (DMF) yielded the most perfect crystals in sizes up to 25×20×40 mm. Due to the development with time of a pink impurity in DMF solutions all ABP crystals grown from DMF solution incorporated this impurity in the {001} sectors. This could be minimised by adjusting the growth conditions to reduce the development of these sectors. Investigation showed that residual quantities of this impurity in the crystals had little influence on the crystal properties. The growth mechanism is discussed from the viewpoints of the structures of the molecule and the crystal and their relationship to the external crystal morphology. The perfection of the crystal was defined by microscopy and by section and projection X-ray topography. The relationship between the perfection of the crystal and its linear and non-linear optical performance was investigated.

The birefringence of the optically nonlinear crystal 4‐amino benzophenone

A precise method is presented for measuring the refractive index of an anisotropic prism with light polarized orthogonal to the prism axis which is also a dielectric axis. Prisms have been cut from crystals of the optically nonlinear 4‐amino benzophenone with exceptionally low defect concentration as identified by synchrotron white light topography. The linear refractive indices of these have been measured and fitted to four‐parameter Sellmeier equations to an accuracy of 7×10−6–3×10−3. The values are accurately verified by observation of the external angle of incidence to a (001) plane required to meet the conditions of type‐I phase matching with a fundamental wavelength of 1064 nm. The phase matching is of Hobden class six for this wavelength. In common with similar organic molecular materials, the molecular charge transfer axis defines the most polarizable direction in the crystal, with the C=O direction being much less significant.A precise method is presented for measuring the refractive index of an anisotropic prism with light polarized orthogonal to the prism axis which is also a dielectric axis. Prisms have been cut from crystals of the optically nonlinear 4‐amino benzophenone with exceptionally low defect concentration as identified by synchrotron white light topography. The linear refractive indices of these have been measured and fitted to four‐parameter Sellmeier equations to an accuracy of 7×10−6–3×10−3. The values are accurately verified by observation of the external angle of incidence to a (001) plane required to meet the conditions of type‐I phase matching with a fundamental wavelength of 1064 nm. The phase matching is of Hobden class six for this wavelength. In common with similar organic molecular materials, the molecular charge transfer axis defines the most polarizable direction in the crystal, with the C=O direction being much less significant.

The birefringence of the optically nonlinear crystal N‐methyl urea

The linear refractive indices of N‐methyl urea have been measured and fitted to a three parameter Sellmeier equation with an accuracy of ±6×10−4–±2×10−4. The values are accurately verified by observation of the external angle of incidence to an (011) plane, required to observe type II phase matching with a fundamental wavelength of 871.6 nm. The phase matching is established as Hobden class nine for this wavelength. The refractive index values are consistent with the C=O direction being considered as the most polar axis.