Kevin Welford

Monolayer behaviour and Langmuir–Blodgett film properties of some amphiphilic phthalocyanines: factors influencing molecular organisation within the film assembly

The monolayer and LB film-forming properties of 20 structurally related amphiphilic octa-substituted phthalocyanine derivatives have been assessed. The molecular packing within examples of the films has been probed by visible-region spectroscopy and low-angle X-ray diffraction methods. Among derivatives where the aliphatic substituents are attached by ether linkages, there is a variation in the behaviour according to the length of the chains and whether or not the chains are branched. Films are not highly ordered but may contain domains of ordered structure, giving rise to a red-shifted absorption band in the visible spectrum. Analogues where the chains are attached by carbon–carbon bonds show superior monolayer behaviour and are excellent materials for deposition as LB films. Furthermore, there is good evidence from the spectra that change in the length of the alkyl chains provides a means of controlling the type of molecular packing within the films.

Octaalkyl- and Octaalkoxy-2,3-naphthalocyanines

The synthesis and visible near-IR region absorption spectra of an 1,6,10,15,19,24,28,33-octaalkylnaphthalocyanine and examples of 3,4,12,13,21,22,30,31-octaalkyl and 3,4,12,13,21,22,30,31-octaalkoxy-2,3-naphthalocyanines are reported.

Nonlinear Absorption in Organic Dyes

The non-linear optical properties of chloro-aluminium phthalocyanine and 1,3,3,1′,3′,3′- hexamethylindotricarbocyanine have been studied as model systems using z-scan and time resolved experimental techniques. Excited state lifetimes and absorption cross sections have been measured as well as the spectral response of the induced absorption. Excited state lifetimes are critical in determining the performance of a reverse saturable absorber. Even though a first excited state absorption cross section may be large compared with the ground state absorption cross section the lifetimes and transition probabilities of higher lying states can lead to constraints upon how effective organic dyes are as optical limiting materials. Both novel saturation of the induced absorption at high laser fluences and intermolecular quantum beats in 1,3,3, 1′,3′,3′-hexamethylindotricarbocyanine are demonstrated experimentally and accounted for theoretically. Quantum chemical calculations have been developed to propose a probable inter-molecular mechanism for the observed quantum beats.

Development of Solid State Non-Linear Absorbers - Dyes in Polymer Hosts

This paper describes the development of high quality solid state optical components which have been made by the incorporation of the reverse saturable dye chloro aluminium phthalocyanine (CAP) into polymeric host material. In particular, the effect of using different host materials with CAP is investigated in terms of key parameters such as solubility, integrity of the dye molecules in the polymer host, hardness of the resulting polymer, ease of polishing to the production of a good optical finish, induced stress, and non-linear absorption performance. Results are presented highlighting the above considerations, and problems encountered in material preparation will be described together with details on how, by careful choice of guest/host combination, good optical quality samples can be produced.

Molecular beam deposited bistable interference filters

Ultrahigh vacuum and molecular beam deposition techniques have been used to fabricate ZnSe interference filters. These demonstrate optical bistability with critical switching power being measured at five wavelengths between 521 and 676 nm. These filters demonstrate considerably improved long term operation stability over that offered by samples deposited using conventional thermal evaporation. Both dispersive and absorptive bistability could be demonstrated in the same sample.

Wavelength Dependence of the Non-Linear Transmission of Hitci Using the Z-Scan Technique

The excited state properties of dyes become important under intense illumination of a short laser pulse. The pulse induces a significant population in an excited state which will have different absorptive properties, this causes a change in absorption of the incident pulse within its duration. An increase in absorption arises from molecules in an excited state having an absorption greater than the corresponding ground state. The z-scan technique has been used to measure the induced absorption of a carbocyanine dye 1,1′,3,3,3′,3′- Hexamethylindtricarbocyanine Iodide (HITCI) for laser pulses of different wavelengths. An optical parametric oscillator pumped by the third harmonic of a Q-switched Nd:YAG laser was used. Z-scans were taken at a number of different wavelengths between 460 and 630 nm. Data is presented which shows the spectral region where induced absorption occurs. An eight level rate equation model is used to predict the non-linear behaviour and to fit to the experimental data, good correlation is found between the model and data.