Determination of refractive indices of glass materials containing semiconductor nanocrystal structures via interferometric methods

DETERMINATION OF REFRACTIVE INDICES OF GLASS MATERIALSCONTAINING SEMICONDUCTOR NANOCRYSTAL STRUCTURESVIA INTERFEROMETRIC METHODSEKREM SINIRABSTRACTTwo kinds of glasses doped with CdSe or CdTe semiconducting materials were cut todimensions of 15x15x1.5 mm with a diamond saw, and these samples were melted at1000 0 C for approximately 10 minutes and quenched quickly to the roomtemperature. The samples were heat treated at temperatures between 550 0 C and675 0 C and nanometer size semiconductor structures (quantum dots) were obtainedinside the specimens. After heat treatment, sandpapering and polishing were appliedto the specimens, and parallel plate glasses with thicknesses between 0.4 mm and 1.0mm were produced. Using a computer controlled optical set up, the optical absorptionspectra of the samples were obtained. Average radii of the quantum dots wereobtained using the energy values of the first exciton peaks of the optical absorptionspectra, and it was found that the average radii values lie within the range between1.89 nm and 2.75 nm for CdSe, and between 2.91 nm and 3.33 nm for CdTe.These semiconductor doped glasses in the parallel plate form were placed on amotorised rotatable table within the sample arm of a Michelson interferometer androtated through angles measured by the controller unit of the rotary stage. Therefractive indices of the glasses were determined via a transcendental model of thechange in the optical path length of the light used in the interferometric setup.The light beam suffers increasing refraction on rotation of the sample from normalincidence. This causes the optical path of light for air to decrease, but that in the glasssample to increase. This change in the optical path length produces a shift in theinterefence fringes. This shift was captured in real-time on a computer and the numberof fringes that shifted was counted. The angle of rotation , the thickness of the sampleand the fringe shift were entered into a transcendental model, and a series ofwavelength values for the laser light that corresponds to the ?guessed? refractiveindex values of the sample were obtained. The guessed refractive index value whichgives us the wavelength of the He-Ne laser light used in the experiment is the index ofrefraction of the glass sample. Experiments conducted by a fused quartz prism ofknown refractive index of 1.457 gave a measured value of 1.458 ± 0.002 which showsthat the measurement method is accurate to 3 decimal places.When the obtained values of refractive indices were compared against the averageradii of the quantum dots, it was found that the increase in the radius of a quantum dotcauses an increase in the refractive index.Keywords: Refractive index, quantum dots, Michelson interferometer, optical pathlength.