Yazhao Liu

Fully embedded board-level guided-wave optoelectronic interconnects

A fully embedded board-level guided-wave optical interconnection is presented to solve the packaging compatibility problem. All elements involved in providing high-speed optical communications within one board are demonstrated. Experimental results on a 12-channel linear array of thin-film polyimide waveguides, vertical-cavity surface-emitting lasers (VCSELs) (42 /spl mu/m), and silicon MSM photodetectors (10 /spl mu/m) suitable for a fully embedded implementation are provided. Two types of waveguide couplers, titled gratings and 45/spl deg/ total internal reflection mirrors, are fabricated within the polyimide waveguides. Thirty-five to near 100% coupling efficiencies are experimentally confirmed. By doing so, all the real estate of the PC board surface are occupied by electronics, and therefore one only observes the performance enhancement due to the employment of optical interconnection but does not worry about the interface problem between electronic and optoelectronic components unlike conventional approaches. A high speed 1-48 optical clock signal distribution network for Cray T-90 super computer is demonstrated. A waveguide propagation loss of 0.21 dB/cm at 850 nm was experimentally confirmed for the 1-48 clock signal distribution and for point-to-point interconnects. The feasibility of using polyimide as the interlayer dielectric material to form hybrid three-dimensional interconnects is also demonstrated. Finally, a waveguide bus architecture is presented, which provides a realistic bidirectional broadcasting transmission of optical signals. Such a structure is equivalent to such IEEE standard bus protocols as VME bus and FutureBus.

Photonic crystal-based all-optical on-chip sensor

In this paper we demonstrate a sensor based on a two-dimensional photonic crystal cavity structure. Design, theoretical simulations, fabrication and experiments are shown to illustrate the working principle of this device. Sensitivity of our sensor is determined by observing the shift of resonant wavelength of the photonic crystal cavity as a function of the refractive index variation of the analyte. By experimentally infiltrating solutions of water and ethanol through an elastomeric micro-fluidic channel, we have confirmed that our all-optical sensor achieves a sensitivity of 460 nm/RIU.

High resolution three-port filter in two dimensional photonic crystal slabs

An in-plane, three-port filter consisting of input/output waveguides and two point-defect cavities in a 2D PC slab is designed and fabricated, where a new feedback method is introduced, and its transmission properties are measured. The measured minimum output wavelength spacing between two channels is 1.5 nm, which is realized by slightly adjusting the size of the resonant cavities. The measured resonant wavelengths of two cavities agree well with the calculated ones and the quality factors of the cavities are almost the same. It is believed that this kind of filter may be useful in optical integrated circuits with high density.

Multichannel filters with shape designing in two-dimensional photonic crystal slabs

We investigated the influence of the irregular shape of air holes on the optical characteristics of channel-drop filters built in a two-dimensional photonic crystal slab. Three differently shaped structures are tested by experiments and simulations to demonstrate the shape-tuning capability of the optical devices. A four-channel filter based on this shape design is presented. The photonic crystal consists of a triangular array of air holes fabricated by focused ion beam lithographic technology on a silicon slab with thickness of 260 nm. The filter contains a heterostructure with different lattice constants of 420 and 430 nm. In each channel, the filtering functionality is guaranteed by the indirect resonant coupling between waveguide and cavity. The elliptical air holes that surround the cavity have different parameters, and this leads to a fine tuning of the cavity’s resonant frequencies. The performance of the multichannel filter is found to be very encouraging.

Ray trace visualization of negative refraction of light in two-dimensional air-bridged silicon photonic crystal slabs at 1.55 µm

We demonstrate design, fabrication, and ray trace observation of negative refraction of near-infrared light in a two-dimensional square lattice of air holes etched into an air-bridged silicon slab. Special surface morphologies are designed to reduce the impedance mismatch when light refracts from a homogeneous silicon slab into the photonic crystal slab. We clearly observed negative refraction of infrared light for TE-like modes in a broad wavelength range by using scanning near-field optical microscopy technology. The experimental results are in good agreement with finite-difference time-domain simulations. The results indicate the designed photonic crystal structure can serve as polarization beam splitter.

Multichannel filters via Γ-M and Γ-K waveguide coupling in two-dimensional triangular-lattice photonic crystal slabs

We demonstrate the design, fabrication, and characterization of a multichannel filter in a two-dimensional triangular-lattice photonic crystal slab. The output signal channel, which directs in the Γ-M crystalline direction, is orthogonal to the input signal channel that directs in the Γ-K crystalline direction. In each channel, the filtering function is guaranteed by the indirect resonant coupling between the waveguide and the cavity. Four resonant cavities with different sizes provide the mechanism of finely tuning the output wavelength. Our experimental results are in good agreement with simulations.

Γ–M waveguides in two-dimensionaltriangular-lattice photonic crystal slabs

We propose a line defect waveguide structure along the Γ-? direction in two-dimensional triangular lattice silicon photonic crystal slabs. The modal dispersion relation and the transmission spectra of this waveguide are studied. The results show that by perturbing the width of the line defect and the diameter of the air holes adjacent to the waveguide core, one can control the width of the single mode transmission window and make it far broader than the original one. The proposed Γ-? waveguide will help to build a more flexible network of interconnection channel of light in two-dimensional photonic crystal slabs.

All-optical on-chip sensor for high refractive index sensing in photonic crystals

In this paper we demonstrate an optical sensor designed to detect material infiltrations with relatively high indices, based on a two-dimensional photonic crystal cavity structure. The locations and sizes of the holes surrounding a L3 cavity were modified to increase the Q factor to a value of 1500 with a high refractive index infilling of . With precise design and simulation, we overcome the difficulty of low index contrast, and observe a very clear wavelength shift of 10.4 nm in simulation and 12.4 nm in experiment between water and oil samples at resonance.

All-optical on-chip sensor for high refractive index sensing

A highly sensitive sensor design based on two-dimensional photonic crystal cavity is demonstrated. The geometric structure of the cavity is modified to gain a high quality factor, which enables a sensitive refractive index sensing. A group of slots with optimized parameters is created in the cavity. The existence of the slots enhances the light–matter interactions between confined photons and analytes. The interactions result in large wavelength shifts in the transmission spectra and are denoted by high sensitivities. Experiments show that a change in refractive index of Δn ∼ 0.12 between water and oil sample 1 causes a spectral shift of 23.5 nm, and the spectral shift between two oil samples is 5.1 nm for Δn ∼ 0.039. These results are in good agreement with simulations, which are 21.3 and 7.39 nm for the same index changes.

Optical Quality Improvement of Si Photonic Devices Fabricated by Focused-Ion-Beam Milling

Focused-ion-beam directly milling strategy was used to fabricate photonic resonators on crystalline silicon-on-insulator substrate. In order to reduce damages such as implanting ions, amorphous layers and re-deposition process which are induced by the ions, a sacrificed silica layer was used as an etching mask and a silicon thermal oxidation process was performed. The transmission spectra of both photonic crystal cavities and micro-ring resonators were measured. The resulting data demonstrate that the Q factors are significantly improved after the oxidation treatment.