Ultra-thin Si-padded Si3N4 waveguides for low-loss photonics.

We proposed an ultra-thin Si-padded Si3N4 waveguide consisting of a very thin Si slab underneath a Si3N4 strip separated by a SiO2 layer. The Si slab and the Si3N4 strip form a hybrid waveguide mode, where the mode field is confined in these two structures. The measured waveguide propagation loss is 0.055 dB/cm, and the bending loss is 0.09 dB per 90° bend, depending on the bending radius. Because part of the waveguide mode is distributed in the Si slab, this waveguide structure has potential for implementing low-loss and high-speed photonic integrated circuits.

Fringe-field carrier-depletion modulators with high modulation efficiency and low free carrier absorption.

A high-speed carrier-depletion silicon modulator based on a fringe field pn junction design is presented. Due to the strong fringe field, the size of heavily doped regions can be reduced and away from the waveguide core, whereas large modulation efficiency is still accomplishable. The VπL is 1.8 V-cm and the phase shifter loss is 1.3 dB/mm. The figure of merit (FOM), defined by the product of VπL and phase shifter loss, is estimated to be 23.4 dB-V. The modulation speed and depth are 11.8 GHz and 8.1 dB, respectively, which is mainly limited by the mobility of poly-Si.

Enhancing detection sensitivity of metallic nanostructures by resonant coupling mode and spectral integration analysis.

We report a simple method to efficiently improve the detection limit of surface plasmon resonance in periodic metallic nanostructures by using small angle illumination and spectral integration analysis. The large-area gold nanoslit arrays were fabricated by thermal-annealing template-stripping method with a slit width of 60 nm and period of 500 nm. The small angle illumination induced a resonant coupling between surface plasmon mode and substrate mode. It increased ~2.24 times intensity sensitivity at 5.5° incident angle. The small-angle illumination also resulted in multiple resonant peaks. The spectral integration method integrated all changes near the resonant peaks and increased the signal to noise ratio about 5 times as compared to single-wavelength intensity analysis. Combining both small angle and spectral integration, the detection limit was increased to one order of magnitude. The improvement of the detection limit for antigen-antibody interactions was demonstrated.

Application of strong transverse magneto-optical Kerr effect on high sensitive surface plasmon grating sensors.

A high sensitive sensor is demonstrated by exploiting strong transverse magneto-optical Kerr effect on a ferromagnetic surface plasmon grating. The surface plasmon grating, made of a hybridized Au/Fe/Au layer, exhibits a very dispersive Kerr parameter variation near the surface plasmon polariton (SPP) wavelength via coherent scattering of the SPP on the grating structure. Interrogating this Kerr parameter can be utilized for detecting chemical or biological objects in a fluid medium. The experiment results show the minimal detectable mass concentration of sodium chloride in a saline solution is 4.27 × 10(-3) %, corresponding to a refractive index change of 7.60 × 10(-6) RIU. For an avidin-biotin interaction experiment, the sensitivity of avidin detection in PBS solution is 1.97 nM, which is limited by the index fluctuation of flowing media during measurement.

A critically coupled Germanium photodetector under vertical illumination.

We propose and study a practical design of a Germanium photodetector implemented on a Silicon-on-insulator substrate to reach the critical coupling regime under vertical illumination at 1310 nm wavelength. With appropriate optimization procedures, a high efficiency bandwidth product larger than 50 GHz and a large 3dB spectral full width around 30 nm can be obtained given realistic material parameters and fabrication constraints. Our device is fully compatible to the state-of-art CMOS process technology, and may serve as a high performance, low cost solution for the optical receiver in Silicon photonics based optical interconnects.

Study of disklike microdroplet cavities directly coupled to liquid-core waveguides.

A design of microfluidic devices is presented to integrate single-mode, liquid-core waveguides with microfluidic channels that generate and deliver disklike emulsion microdroplet cavities doped with an organic dye. The microcavity modes can be directly coupled to the liquid waveguide. Cavity-enhanced spontaneous emission was observed at the waveguide with low pump pulse energy.

Optical phase modulators using deformable waveguides actuated by micro-electro-mechanical systems.

An optical phase modulator is presented by using micro-electro-mechanical systems to actuate deformable silicon waveguides. Via mechanically stretching the waveguide length, the optical path is extended, resulting in a phase shift. The experimental results show that a phase shift of near 0.4π is achieved at 200 V for both TE- and TM-polarized waves by cascading six phase modulation units, agreeing well with the theoretical prediction. The power consumption is estimated to be smaller than 0.2 mW at 200 V, mainly resulting from the leakage current.

Study of cross-phase modulation and free-carrier dispersion in silicon photonic wires for Mamyshev signal regenerators.

A numerical study on Mamyshev signal regeneration realized on silicon photonic wires is reported. Unlike fiber-optics Mamyshev regenerators employing cross-phase modulation, silicon photonic wires have to include two-photon absorption and the two-photon-absorption-induced free-carrier effect. By well adjusting time delay between the co-propagating signal and clock pulses, both cross-phase modulation and free-carrier dispersion could induce nonlinear wavelength shift, which is essential for signal recovery in the Mamyshev regeneration scheme. A simulation result shows the quality factor of signal eye diagram improved by more than 4 dB for Return-to-Zero signals with pulse width 10 ps, peak power 6.5 W, and operation speed 10 Gbit/s through a 1-cm silicon photonic wire.

Tunable coupling regimes of silicon microdisk resonators using MEMS actuators.

Tunable coupling regimes of a silicon microdisk resonator controlled by MEMS (microelectromechanical system) actuation are demonstrated for the first time. By varying the gap spacing between the waveguide and the disk, this microresonator can dynamically operate in either under-, ciritcal or over-coupling regime. The waveguide transmittance is suppressed by 30 dB in critical coupling, and the quality factor of the microdisk is measured to be as high as 10(5). Additionally, the microdisk presents tunable group delay from 27 ps to 65 ps, and tunable group velocity dispersion from 185 ps/nm to 1200 ps/nm. Waveguides, microdisks and actuators are all integrated on a silicon-on-insulator (SOI) substrate. This compact device exhibits the promise to construct resonator-based reconfigurable photonic integrated circuits.

A self-assembled microbonded germanium/silicon heterojunction photodiode for 25 Gb/s high-speed optical interconnects.

A novel technique using surface tension to locally bond germanium (Ge) on silicon (Si) is presented for fabricating high performance Ge/Si photodiodes. Surface tension is a cohesive force among liquid molecules that tends to bring contiguous objects in contact to maintain a minimum surface energy. We take advantage of this phenomenon to fabricate a heterojunction optoelectronic device where the lattice constants of joined semiconductors are different. A high-speed Ge/Si heterojunction waveguide photodiode is presented by microbonding a beam-shaped Ge, first grown by rapid-melt-growth (RMG) method, on top of a Si waveguide via surface tension. Excellent device performances such as an operating bandwidth of 17 GHz and a responsivity of 0.66 and 0.70 A/W at the reverse bias of -4 and -6 V, respectively, are demonstrated. This technique can be simply implemented via modern complementary metal-oxide-semiconductor (CMOS) fabrication technologies for integrating Ge on Si devices.

Manipulation of micro-particles through optical interference patterns generated by integrated photonic devices.

Micro-particle transport and switch governed by guided-wave optical interference are presented. The optical interference, occurring in a directional coupler and a multi-mode interferometer made by inverted rib waveguides, results in a specific evanescent field dependent on wavelength. Through a detailed theoretical analysis, the field of induced optical force shows a correlative pattern associated with the evanescent field. Experimental results demonstrate that 10 µm polystyrene beads are propelled with a trajectory subject to the interference pattern accordingly. By launching different wavelengths, the polystyrene beads can be delivered to different output waveguide ports. Massive micro-particle manipulation is applicable.

Variable bandwidth of dynamic add-drop filters based on coupling-controlled microdisk resonators.

Dynamic add-drop filters with variable bandwidth are demonstrated on microelectromechanical-system (MEMS)-actuated microdisk resonators for what is believed to be the first time. The tuning mechanism is based on a variable power coupling ratio that is controlled by varying the gap spacing between the waveguide and the microdisk through MEMS actuators. The results show wavelength switching with an extinction ratio of 20 dB and a tunable bandwidth ranging from 12 to 27 GHz. These dual functions were realized using the device with the operation voltage varying between 0 and 35 V.