Work plan for MS project is to synthesize graphene, graphene oxide and study optical nonlinearities in
them using experimental techniques such as Z-scan and spatial self-phase modulation (SSPM).
1. Z-Scan: Z-scan technique is used to measure third order nonlinear optical properties of a nonlinear material such as nonlinear refractive index, nonlinear absorption, carrier dynamics etc.
In this method, sample under investigation is moved along the direction of focused Gaussian laser beam. The intensity of light beam changes with sample position, and
the power transmitted through the sample is measured in two regimes, namely open aperture (OA) and closed aperture (CA). OA Z-scan is sensitive to nonlinear absorption,
while CA Z-scan is sensitive to nonlinear absorption as well as
nonlinear refractive index.
In our experiment, we perform Z-scan with femtosecond pulsed laser. Experimental set-up for Z-scan is
completed and measurement is performed with CS2 for reference, and also with few topological insulator samples.
We are yet to perform Z-scan with graphene and graphene oxide sample.
Figure 1: Simplified schematic Z-scan set up (Left) and obtained Z-scan curve for CS2 with femtosecond pulsed laser beam at wavelength 780nm and 650mW power (Right).
2. SSPM: SSPM is another experimental technique to measure third order nonlinearities (in liquid medium). When a Gaussian beam propagates through
a medium, a refractive index distribution arise due to nonuniform intensity distribution of beam along its radial direction. This causes radial phase shift, which
ultimately results into interference pattern in far field evidenced as multiple concentric bright and dark rings.
We performed SSPM in liquid suspension of graphene and graphene oxide. For this purpose, graphene was prepared by electrochemical exfoliation of graphite sheet, and graphene oxide was prepared using
modified Hummer's method. Nonlinear refractive index in graphene and graphene oxide was estimated from the obtained data.
There are two possible origin of SSPM in liquid suspension of graphene. Coherent electronic effect attributes SSPM to electronic polarization
in graphene layers and corresponding re-orientation of suspended graphene sheets under the exposure of laser beam. However, laser induced thermal effect cannot be ignored as
another major contributor to SSPM. In our work, we are trying to distinguish these two effects experimentally. Observations made till yet indicate posible dominance of thermal effect
over electronic origin in SSPM in graphene suspension.
Figure 2: Schematic set up for SSPM experiment and obtained diffraction pattern with graphene suspension.
Brief summery of SSPM work (performed till now) can be found here .
Recently, a poster related to this work was presented at CoOpt-2017.
