We report synthesis and photoluminescence optimization of luminescent Si nanocrystals in silicon rich oxide films using a CO2 laser beam. Laser annealing allows for a very localized heat deposition. This results in appreciable temperature rise in an area that is equivalent to only a few spot sizes. This could be important in CMOS back-end compatible processing where high temperatures on the entire wafer scale might not be acceptable. Furthermore, temperature optimization studies in furnace annealing are time consuming because the furnaces have to be programmed to each individual temperature and the stabilization takes long times. In CO2 laser annealing, the entire temperature range for nanocrystal formation is available along the radial and axial directions of the laser spot - thereby allowing temperature optimization in a single short experiment. Presence of crystalline nanoparticles is ascertained using structural analysis techniques like transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). We also report luminescence optimization with respect to laser power and annealing time. It is observed that laser annealing in an air ambient results in two peaks in the luminescence spectrum - on in the visible at 570 nm and one in the near infra read at 800 nm. Origin of luminescence in these two peaks is probed by hydrogen passivation and time resolved measurements.
In the second part of the paper, we focus on continuous wave characterization of photoluminescence from Si nanocrystals embedded in microdisk resonators. There have been numerous reports on observation of continuous-wave and transient gain in planar optical waveguides with Si nanocrystal active layer. However, there are relatively very few investigations focusing on photoluminescence emission from Si nanocrystals (quantum dots) embedded in on-chip optical microcavities. Microcavities spectrally filter the luminescence from the quantum dots and, depending on the (Q/V) ratio, can significantly alter the spontaneous photoemission from the quantum confined excitons. In our work, planar microcavities are patterned on the emitter layer by high resolution electron beam lithography and a combination of dry and wet chemical etching. Fabrication procedure is optimized to maximize the ratio of the quality factor and the mode volume. Continuous-wave photoluminescence measurements are performed by top-pumping the resonators with a 488 nm line of an argon ion laser. We study the photoemission from the microdisks for the polarization dependence, and quality factors. Contributions of various mechanisms leading to the observed loss are estimated. We believe that our studies will help gain further insight into photoemission physics of the group-IV nanostructures.
Fiber tapers have found a wide range of important applications in communication and sensing, including narrow-band filters, mode-matching between waveguides, evanescent mode-coupling and fused couplers. Applying these taper-based technologies to air-core photonic bandgap fibers (PBFs) is very appealing because it would enable creating these same components directly in air-core fibers. Although there have been several studies of tapers in solid-core microstructured fibers, the transmission properties of tapered air-core photonic-bandgap fibers have not yet been studied. In this work, we report on the fabrication and testing of tapered air-core photonic-bandgap fibers. Our motivation in this work was to study the basic transmission properties of PBF bitapers in the bandgap region, and in particular, to see how the overall transmission was impacted by the taper, e.g., whether the taper induced resonant coupling to one or more cladding modes. Our experimental results indicate that air-core PBFs are highly sensitive to tapering, and unlike conventional single-mode telecommunication fibers, even a small tapering ratio results in significant modal interference in the transmission spectrum. Furthermore, we found out that the mechanical silica support surrounding the holey region of the PBF contributes as a lossy Fabry-Perot resonator to the observed transmission properties.
We report the development of a novel low energy optical switch that consists of a silica microsphere optical resonator coated with a layer of silicon nanocrystals. A 150 μm-diameter silica microsphere was coated with a 140 nm thick layer of silicon rich silicon oxide (SRSO) by PECVD. The microsphere/SRSO was annealed in argon at 1100C to facilitate nanocrystal growth. The optical properties of the microsphere were characterized by evanescently coupling 1450 μm tunable laser light through a tapered optical fiber into the whispering gallery mode resonances of the microsphere. A quality factor of 3×105 was measured at this wavelength. Light from an Ar+ laser at 488 nm was introduced into the tapered fiber and was used to excite the nanocrystals near the whispering gallery modes (WGM) of the sphere. WGM resonance wavelengths shifts of 5 pm at an operating wavelength of 1450 nm were observed when the Ar+ light was coupled into the tapered fiber. Powers as low as 3 μW were sufficient to shift the resonance by a half a linewidth and cause full switching of the 1450 nm signal with a fast rise time (which was limited by the time width of the laser pulse). The speed of the switch is limited by the fall time, which has a time constant of 30 ms.