We collected the backscattered light by the microscope objective and passed it through a notch filter (NF785-33, Thorlabs) to reduce the Rayleigh scattered laser line. Further, to increase the signal-to-noise ratio, we have mounted a 150 m diameter pinhole in the focal point of the telescope. The filtered light is coupled into our spectrometer (Model 207, McPherson) through a 150 m wide entrance slit where a 600 grooves/mm holographic grating disperses the light . The Raman spectrum was then