A near field scanning optical microscopy investigation of photonic structures

Near field scanning optical microscopy (NSOM) is used to study two different photonic structures. The two structures were a photonic crystal sample and a selectively oxidized vertical cavity surface emitting laser (VCSEL) where spectroscopic NSOM is utilized to investigate the transverse mode structure. NSOM is a unique tool for studying these samples as the optical intensity and the spectroscopic signatures of these samples are characterized with sub-wavelength spatial resolution.; The photonic crystal sample is a two-dimensional slab waveguide with a silicon nitride core and silicon dioxide cladding. The photonic lattice comprises of a triangular lattice of 20 rows of air pores, with air pore diameter of 146 nm and lattice constant of 260 nm, etched into the top cladding and the core. Due to the periodic modulation of the dielectric constant in the air pore lattice, a photonic band gap is expected for this lattice in the visible wavelength regime. NSOM was used to characterize the optical intensity distribution within the photonic lattice region at a wavelength of 633 nm. In addition, Bragg reflections were observed from the lattice region and the angular dependence on the angle of incidence of the input beam was recorded and compared to a point scatterer model. The transmission across the lattice was also calculated, in both 2D and 3D, using a program based on a finite difference time domain method and compared to experimental data.; The VCSEL sample is a selectively oxidized 850 nm VCSEL having a square aperture of side length 10 micron. An NSOM coupled to a spectrometer is used to obtain both spatially and spectrally resolved images of the VCSEL's emission. The spatially resolved intensity distributions for eight of the lowest order transverse modes of the VCSEL were obtained, in the near field and far field, at three different currents i.e. around threshold (3.3 mA) and above threshold (at 5 mA and 7.5 mA). The modes are identified as being Hermite-Gaussian and their spatial orientations show how they utilize the available gain of the cavity effectively. Two-dimensional maps of the total integrated intensity of the spectra reveal an inhomogeneous gain distribution. The wavelengths of the transverse modes are observed to increase with increasing current at an average rate of 0.145 nm/mA. Calculations showed that a temperature-induced change in refractive index of the cavity was the dominant contributing factor towards this red shift.