Nonlinear Optics
Phase Conjugation
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Z-Scan Trace

 
 

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Experimental measurement of optical nonlinearity of materials is of great interest because it not only helps us to understand the mechanism and origin of nonlinearity but also enables us in its implementation in appropriate areas of optical technology. Measurement of nonlinear optical (NLO) parameters of different materials acquires some importance in this context. Large nonresonant optical susceptibility, ultra fast response, thermal and chemical stability, ease of molecular structural engineering etc. are a few among the desired properties for good NLO materials. In ISP laboratories we are mainly studying Porphyrins derivatives like Phthalocyanines and Naphthalaocyanines, which have high pi-electron density with extensive delocalization. Their structure can easily be tailored by introducing metal ions at the center and different substituents at peripheral positions without compromising their other properties like thermal and chemical stability. These materials exhibit intensity dependant absorption and refraction on excitation by intense laser pulses.

Z-Scan Measurements

The Z-scan method is an easy and relatively simple way for measuring nonlinear absorption coefficients as well as nonlinear refractive indices for a wide variety of optically interesting materials. We have measured the nonlinear absorption coefficient of many rare earth metallo phthalocyanines using open aperture Z-scan technique.
 

Z-Scan Experimental Set-up

In our Z-scan set-up, the transmittance of the sample is measured, using a large area photodetector, as the sample is moved along the propagation
direction (z) of a focussed Gaussian laser beam. The measurement is done using a 532 nm Gaussian laser beam of a frequency doubled Nd:YAG laser. A portion of the laser beam is sampled out to monitor the input energy. Laser energy is measured and averaged using a commercial energy meter and a ratio meter respectively. From open aperture Z-scan experimental data, nonlinear absorption coefficient is calculated using two-photon absorption(TPA) model. It is obtained by fitting the experimental data to the normalized transmittance equation. In porphyrins like phthalocyanines, nonlinear absorption at 532 nm, is not due to pure TPA but due to sequential two photon absorption (STPA). It is sometimes referred as excited state absorption (ESA) or reverse saturable absorption (RSA). In this context, the concept of nonlinear absorption coefficient needs be modified to take into account the two step excitation process occurring in STPA and hence may be called effective nonlinear absorption coefficient.. 
  

Z-Scan Graph

Optical Limiting

We have extensively studied the optical limiting response of many metallo phthalocyanines. There are three basic requirements for a material to be good optical limiter viz.(1) An excited state absorption cross-section that exceeds the ground state absorption cross-section,(2) fast response and (3) high damage threshold. At 532 nm the excited state absorption cross-section of metallo phthalocyanines are higher than the ground state absorption cross-section. Magnitude of effective nonlinear absorption coefficient is an indication of the usefulness of a sample to act as good optical limiter. Samples with higher value of effective nonlinear absorption coefficient are obviously better optical limiters. Our investigations revealed that many rare earth phthalocyanines can be used to attenuate the 532 nm high energy laser pulses.
 

Optical Limiting

Four Wave Mixing

Organic compounds that are asymmetrically polarizable e.g. through conjugated pi-electron systems, have been shown to produce large NLO responses. We are interested in both organic and organometallic materials for our four wave mixing studies. Initial work has focused on metallo phthalocyanines and naphthalocyanines to study the nonlinear response in solutions.
 

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