Programmable MZI based on a silicon photonic MEMS-tunable delay line.

We report on a scalable and programmable integrated Mach-Zehnder interferometer (MZI) with a tunable free spectral range (FSR) and extinction ratio (ER). For the tunable path of the MZI, we designed and utilized a tunable delay line having high flexibility based on silicon photonic microelectromechanical systems (MEMS). By utilizing MEMS, the length of the delay line can be geometrically modified. In this way, there is no optical loss penalty other than the waveguide propagation loss as the number of tunable steps increases. Therefore, our device is more scalable in terms of optical loss than the previous approaches based on cascaded MZIs. In addition, the tuning energy required to reconfigure the length is only 8.46 pJ.

Ultra sub-wavelength surface plasmon confinement using air-gap, sub-wavelength ring resonator arrays.

Arrays of sub-wavelength, sub-10 nm air-gap plasmonic ring resonators are fabricated using nanoimprinting. In near infra-red (NIR) range, the resonator supports a single dipole mode which is excited and identified via simple normal illumination and explored through transmission measurements. By controlling both lateral and vertical confinement via a metal edge, the mode volume is successfully reduced down to 1.3 × 10(-5) λ0(3). The advantage of such mode confinement is demonstrated by applying the resonators biosensing. Using bovine serum albumin (BSA) molecules, a dramatic enhancement of surface sensitivity up to 69 nm/nm is achieved as the modal height approaches the thickness of the adsorbed molecule layers.

Disorder and broad-angle iridescence from Morpho-inspired structures.

The ordered, lamellae-structured ridges on the wing scales of Morpho butterflies give rise to their striking blue iridescence by multilayer interference and grating diffraction. At the same time, the random offsets among the ridges broaden the directional multilayer reflection peaks and the grating diffraction peaks that the color appears the same at various viewing angles, contrary to the very definition of iridescence. While the overall process is well understood, there has been little investigation into confirming the roles of each factor due to the difficulty of controllably reproducing such complex structures. Here we use a combination of self-assembly, selective etching, and directional deposition to fabricate Morpho-inspired structure with controlled random offsets. We find that while random offsets are necessary, it alone is not sufficient to produce the broad-angle reflection of Morpho butterflies. We identify diffraction as a critical factor for the bright, anisotropic broadening of the reflection peak of Morpho butterflies to a solid angle of 0.23 sr, and suggest random macroscopic surface curvature as a practical alternative, with an isotropic broad reflection peak whose solid angle can reach 0.11 sr at an incident angle of 60°.

Label-free optical biosensing using a horizontal air-slot SiNx microdisk resonator.

We demonstrate label-free optical biosensing using a horizontal air-slotted silicon-rich silicon nitride (SiNx) microdisk resonator. Due to the strong confinement of light in the air-slot, a large resonance shift of 6.2 nm is observed upon reaction of the biotin-functionalized disk with a streptavidin solution with concentration of 2.5 µg/ml. Assuming a linear relationship between resonance shift and streptavidin concentration, we estimate the sensitivity to be 2.5 ± 0.2 nm/(µg/ml). Comparing this value with surface sensitivity of 5.7 nm/nm calculated using FDTD simulations, a detection limit of 30 ± 2 ng/ml, is extrapolated.

A silicon nitride microdisk resonator with a40-nm-thin horizontal air slot.

We design and fabricate pedestal-type, 15 microm diameter silicon nitride microdisk resonators on Si chip with horizontal air-slot using selective wet etching between Si, SiO2, and SiNx. As the slot structure is determined by deposition process, air slots that are as thin as 40 nm and as deep as 5 microm with ultra-smooth slot surfaces can easily be fabricated with photolithography only. Fundamental TM-like slot mode in which the E-field is greatly enhanced within slot was observed with an intrinsic Q factor of approximately 34,000 (lambdares=1523.7 nm) and energy overlap in slot region of 21.6%.

On-chip, planar integration of Er doped silicon-rich silicon nitride microdisk with SU-8 waveguide with sub-micron gap control.

On-chip, planar integration of Er-doped Silicon-rich silicon nitride microdisks with SU-8 waveguide and polymer cladding is achieved. The lack of high temperature or etching processes allows back-end integration without any optical damage to the microcavity resonator. The maximum measured Q-factor at 1475.5 nm was 13,000, corresponding to calculated intrinsic resonator Q-factor of 25,000 that is limited by process-related roughness.