Desmond R. Lim

High-quality Ge epilayers on Si with low threading-dislocation densities

High-quality Ge epilayers on Si with low threading-dislocation densities were achieved by a two-step ultrahigh vacuum/chemical-vapor-deposition process followed by cyclic thermal annealing. On large Si wafers, Ge on Si with threading-dislocation density of 2.3×107u2002cm−2 was obtained. Combining selective area growth with cyclic thermal annealing produced an average threading-dislocation density of 2.3×106u2002cm−2.We also demonstrated small mesas of Ge on Si with no threading dislocations. The process described in this letter for making high-quality Ge on Si is uncomplicated and can be easily integrated with standard Si processes.

Fabrication of ultralow-loss Si/SiO 2 waveguides by roughness reduction

We demonstrate 0.8-dB/cm transmission loss for a single-mode, strip Si/SiO(2) waveguide with submicrometer cross-sectional dimensions. We compare the conventional waveguide-fabrication method with two smoothing technologies that we have developed, oxidation smoothing and anisotropic etching. We observe significant reduction of sidewall roughness with our smoothing technologies, which directly results in reduced scattering losses. The rapid increase in the scattering losses as the waveguide dimension is miniaturized, as seen in conventionally fabricated waveguides, is effectively suppressed in the waveguides made with our smoothing technologies. In the oxidation smoothing case, the loss is reduced from 32 dB/cm for the conventional fabrication method to 0.8 dB/cm for the single-mode waveguide width of 0.5 microm . This is to our knowledge the smallest reported loss for a high-index-difference system such as a Si/SiO(2) strip waveguide.

Effect of size and roughness on light transmission in a Si/SiO2 waveguide: Experiments and model

In this letter, we experimentally evaluate the effect of miniaturization and surface roughness on transmission losses within a Si/SiO2 waveguide system, and explain the results using a theoretical model. Micrometer/nanometer-sized waveguides are imperative for its potential use in dense integrated optics and optical interconnection for silicon integrated circuits. A theoretical model was employed to predict the relationship between the transmission losses of the dielectric silicon waveguide and its width. This model accurately predicts that loss increases as waveguide width decreases. Furthermore, we show that a major source of loss comes from sidewall roughness. We have constructed a complete contour map showing the interdependence of sidewall roughness and transmission loss, to assist users in their design of an optimal waveguide fabrication process that minimizes loss. Additionally, users can find an effective path to reduce the scattering loss from sidewall roughness. Using this map, we confirm that n...

SiO2/TiO2 omnidirectional reflector and microcavity resonator via the sol-gel method

Thin films of SiO2 and TiO2 were used to fabricate one-dimensional photonic crystal devices using the sol-gel method: an omnidirectional reflector and microcavity resonator. The reflector consisted of six SiO2/TiO2 bilayers, designed with a stopband in the near infrared. Reflectivity over an incident angle range of 0°–80° showed an omnidirectional band of 70 nm, which agrees with theoretical predictions for this materials system. The microcavity resonator consisted of a TiO2 Fabry–Perot cavity sandwiched between two SiO2/TiO2 mirrors of three bilayers each. We have fabricated a microcavity with resonance at λ=1500 nm and achieved a quality factor of Q=35. We measured a resonance frequency modulation with a change in incident angle of light and defect layer thickness.

Pedestal antiresonant reflecting waveguides for robust coupling to microsphere resonators and for microphotonic circuits

Strip-line pedestal antiresonant reflecting waveguides are high-confinement, silica integrated optical waveguides in which the optical modes are completely isolated from the substrate by thin high-index layers. These waveguides are particularly well suited for whispering-gallery mode excitation in high-Q microspheres. They can also be used in microphotonic circuits, such as for microring resonators. The theory and design of these structures are highlighted. Experiments that show high coupling efficiency to microspheres are also demonstrated.

Mode transformer for miniaturized optical circuits.

A novel mode transformer was fabricated that transforms a modal area by a factor of 100. Using the mode transformer improves the efficiency of mode transformation by an order of magnitude compared with that when no mode transformer is used. With this mode transformer, input-output coupling of miniaturized, on-chip integrated optical circuits to external optical fibers is achieved with low loss. The mode transformers design, fabricated in silicon, is scalable to virtually any waveguide size, facilitating continuous miniaturization in silicon optoelectronics.

Microsphere resonator mode characterization by pedestal anti-resonant reflecting waveguide coupler

The stripline pedestal anti-resonant reflecting optical waveguide (SPARROW) is an efficient and robust coupling device for silica microsphere whispering-gallery-mode excitation. The concept incorporates alternating layers of Si and SiO/sub 2/, designed to isolate the mode of the sphere and the waveguide from the dielectric substrate. Experimental characterizations of this coupling technique are presented, including displacement measurements, and whispering-gallery-mode intensity mapping. Power extraction efficiencies of over 98% are reported.

On-chip interconnection beyond semiconductor roadmap: silicon microphotonics

The present paper describes an emerging field, Si microphotonics, and its application to on-chip interconnection beyond semiconductor roadmap. Current Si-LSIs have been facing three fundamental limits associated with metal interconnection; i.e., slow clock speed, multilayer interconnection for high density interconnects, and high power consumption. These limits are induced by slow signal messengers, electrons. There is no solution beyond the Cu and low k technology but optical interconnection. To implement optical clock distribution on a chip, one challenge is sharp bending of waveguides. High-index contrast optics has shown their significant potentials. Right angle bends have been proto-typed whose area is less than 1 tm2. Ge directly grown on Si wafers shows an excellent characteristics as photodetectors for 1.3 and 1 .55 tm. A high-density interconnection needs wavelength division multiplexing (WDM). Ultrasmall multiplexer/demultiplexer (DEMUX/DEMUX) has been achieved on a chip based on micro-ring resonators (1O tm). Minimization ofpower consumption is of importance when light sources are implemented on a chip. Microcavity resonators based on photonic crystal concepts should be a unique solution.

Planar integrated wavelength-drop device based on pedestal antiresonant reflecting waveguides and high-Q silica microspheres.

Whispering-gallery modes in silica microspheres can be accessed very efficiently with the recently introduced stripline pedestal antiresonant reflecting optical waveguide (SPARROW) structure. This integrated-optics coupling technique creates novel application opportunities for the high-Q spherical cavities. We report the demonstration of a narrow-band wavelength-drop configuration utilizing SPARROW waveguides and a silica microsphere.

Micron-sized channel-dropping filters using silicon waveguide devices

High density integrated optics on the scale of VLSI is of interest as it allows complicated optical interconnect circuitry to be mass produced. In this paper we present micron-sized high Q resonant cavity structures based on silicon on insulator devices. These resonant cavities may be used in channel dropping filters and modulators. Because of their small size, they have high packing densities on the order of one million devices per square centimeter. This technology has the added advantage in that it can utilize the embedded VLSI electronics manufacturing capacity. In previous work, we studied silicon on oxide photonic band gap (PBG) devices and demonstrated devices with a 400 nm stop band and with a defect which had a Q of 265 centered at a wavelength of 1560 nm. In addition, we fabricated 3 to 5 micrometer radii micro-rings with Qs of approximately 250 and free spectral widths of over 20 nm. In this work, we report results on micro-racetracks, which are oval shaped resonators, with resonances that are approximately 16 nm apart and Qs of about 1000. These racetracks incorporate a vertical coupling technology in which the bus waveguides and the ring are on separate planes. This vertical coupling scheme allows for independent control of the Q of the ring via the distance between the ring and the bus. We demonstrate higher order multi-resonator filters with similar Q and free spectral range to the single resonator filters. The individual resonators in each filter have slightly different resonant frequencies from each other resulting in multi-peaked resonances and lower drop efficiencies. Finally, we show that it is possible to thermally tune the resonances by 1 nm leading to a 10:1 contrast ratio.