Non linear optics is concerned with the interaction of electromagnetic radiation with various media to produce new radiations which is altered in phase, frequency, amplitude, etc, from the incident radiation. The rapid growth of laser technology coupled with telecommunications, industry’s need for sophisticated optical switching devices required for data transmission in this computer age has prompted an enormous interest in non linear optical materials. Organic colorants are best understood as pi electron organic molecules with conjugated donor and functional groups. Non linear optical processes in pi electron organic and polymeric systems have attracted considerable interest because their understanding has led not only to compelling technological promise but also to new phenomenon, new theoretical insights and new materials and devices. The pi electron systems are invariably excited by electromagnetic reactions and this is so with organic colorants which invariably interact with the visible portions of the electromagnetic radiations. Such pi electron excitations occurring on the individual molecules, or polymer chain units are the basic origin of the observed non resonant nonlinear optical coefficients which are often usually large. The coefficients are often broad band and ultra fast. The frequency dependence of these coefficients is determined by many body electron correlations effects. New challenges in non liner optics (NLO) materials are being presented, resulting in new methods of ultra structure synthesis and the discovery of entirely new materials and high performance compositions exhibiting high thermal, mechanical and chemical stability. NLO materials are of widespread interest for optoelectronic applications such as electro-optic wave guiding, frequency modulation or optical information processing. We synthesize and study organic dyes, oligomers and polymers with NLO properties.
NLO properties are characterized by molecular hyperpolarizabilities, the second order terms of which can be measured by EFISH (electric field induced second harmonic generation) experiments. We use a recently developed setup which allows EFISH experiments on solutions of non absorbing as well as of absorbing compounds. The molecular origin of optical nonlinearity is due to the electrical polarization of a molecule as it interacts with electromagnetic radiation. These interactions may change the frequency, phase, polarization or path of incident light. Dyes are predisposed for NLO applications because the mobile electrons that are responsible for the absorption of visible light also bring along the polarizability of the molecules, which is necessary for SHG. When it comes to practical applications of compounds with nonlinear optical properties, a major synthetic challenge is to construct non centrosymmetric molecular systems with suitable processability. Amorphous polymers with covalently attached chromophores can meet this goal.
The origin of second order non linear optical effects in organic molecules is traced to the presence of strong donor acceptor interactions. In 1970, davydov and his co-workers reported a strong second harmonic generation (SHG) in organic molecules having electron donor and acceptor groups connected with benzene ring. This discovery led to an entirely new and useful concept of molecular engineering to synthesize new organic materials for the SHG studies. From 1980 onwards tremendous growth occurred in design and development of organic materials for second order non liner optics.
Non liner optics
It is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the dielectric polarization “P” responds nonlinearly to the electric field ‘E’ of the light. This nonlinearity is typically only observed at very high light intensities such as those provided by pulsed lasers. Nonlinear optics gives rise to a host of optical phenomena.
Dyes used of NLO
Organic dyes appropriate for the polymers include those having
1. At least one hydrosilation reactive carbon -carbon double bond.
2. Absorption maxima between 300-2000nm or more particularly between 300-700nm and extinction coefficients at the absorption maxima greater that about 2X103 L/mol cm
Two or more organic dyes can be used in combination. Preferred moieties for providing hydrosilation reactive carbon -carbon double bonds are pendant alkenyl chains particularly those where the carbon -carbon unsaturated bond is in the terminal position and strained endocyclic bicycloalkenyl groups, because these carbon -carbon double bonds are highly reactive for hydrosilation. Suitable such alkenyl chains are the vinyl, allyl, and 3-butenyl groups appropriate strained endocyclic bicycloalkenyl moieties. Particularly suitable dyes are those including
• And electron donor group
• An electron acceptor group
• A delocalized pi electron system linking these two groups especially where the combination of these groups exhibits and NLO response.
The absorption band of an organic colorant can be tailored by
• Either increasing the pi conjugation length
• Or by substituting donor acceptor groups to a conjugated system
As a result the absorption characteristics of the uv visible spectrum can be shifted and will have either bathochromic (red shift) or hypsochromic shift (blue shift).