No abstract

We experimentally demonstrate proof of concept operation of superparamagnetic magnetite nanoparticles and magnetite−TiO2 peapod superstructures for laser intensity modulation and chopping. The frequency of the modulation is shown to be twice that of the driving signal and a function of the size of the particles. Specifically, optical modulation with round nanoparticles of sizes 86, 140, and 190 nm is compared with optical modulation with magnetite−TiO2 peapod superstructures of lengths of around 1 μm. The former gave rise to modulations of up to 2 kHz in frequency, a number comparable to that of commercial optical choppers, the latter up to 100 Hz. We also show that particle shape asymmetry and anisotropy enhance optical modulation and that modulation frequency is inversely proportional to the inertia of the particles used.

An advanced undergraduate laboratory experiment is outlined which uses a dye laser to map out the chromatic dispersion curve of a polymethyl methacrylate (PMMA) optical fibre. Seven different wavelengths across the visible spectrum are employed using five different dyes. The light pulse is split into two pulses, one to a nearby photodetector and the other coupled to the optical fibre cable at the end of which there is another photodetector. The difference in time of arrival at the detectors is used to compute the speed of light in the fibre for a given colour. In addition to a pedagogically simple and intuitive demonstration of the measurement of index of refraction, the use of a long fibre eliminates the need to direct the dangerous UV/visible laser pulse beam across a classroom, as is usually done in similar experiments. Ways to avoid systematic errors and other technical pitfalls—such as ringing oscillations—are presented.