How small can a microring resonator be and yet be polarization independent?

There has been a recent trend to reduce the size of photonic waveguide devices to enable high-density integration in silicon photonic integrated circuits. However, this miniaturization tends to result in increased polarization dependency. Particularly challenging is designing devices based on ring waveguides with small radii, which exacerbates the polarization sensitivity. For these microring resonators, a legitimate question is then: Is it possible to simultaneously maintain the conditions of single-mode and structural polarization independence while shrinking the size of both the bend radius and the waveguide cross section, and, if so, how small can the ring resonator be? We demonstrate theoretically the feasibility of achieving this via deeply etched submicrometer silicon-on-insulator rib waveguides, and we show that, for a given cladding and core thickness, the radius of a polarization independent microring resonator can be as small as 3 microm, being limited chiefly by the residual birefringence of the resonator cavity and the bend losses.

The effects of periodic and quasi-periodic orders on the photonic bandgap structures of microring coupled-resonator optical waveguides.

We present a coupling matrix formalism to investigate the effects of periodic and quasi-periodic orders on the photonic bandgap (PBG) structures of coupled-resonator optical waveguides (CROWs) based on microring resonators. For the periodic order case, size-tuned defects are introduced at periodic locations among the regular rings, which are size-untuned, to form a periodic ordered CROW system. The periodic coupled defects result in multiple localization states that lead to the formation of mini-defect bands and mini-PBGs within the PBG of a defect-free CROW. The position and number of such mini-defect bands depend on the size tuning of the defects. For the quasi-periodic order case, the arrangement of the defects and the regular rings in the ring cascade is an intermediate between periodic order and randomness, thus forming a quasi-periodic ordered CROW system. The effects of quasi-periodicity on the PBG structures are illustrated using the Fibonacci sequences, which result in a single high-Q localized state to appear that gradually transits to a mini-band within a wide photonic stop band as the number of lattice cells increases.

Boxlike filter response based on complementary photonic bandgaps in two-dimensional microresonator arrays.

We propose that a boxlike filter response can be obtained by utilizing complementary photonic bandgap properties of the column and row configurations in two-dimensional microresonator arrays. The filters are fabricated using deep-UV lithography in silicon-on-insulator technology. The observed sidelobes reduction is approximately 10 dB, and the usable bandwidth can be as high as 500-750 GHz.

Demonstration of defect modes in coupled microresonator arrays fabricated in silicon-on-insulator technology.

We show experimentally the existence of defect modes in mutually coupled microring resonator arrays fabricated in silicon-on-insulator technology. The movements of donor-like and acceptor-like modes are demonstrated for various defect lengths, in good agreement with earlier theoretical prediction.

Optical buffer with higher delay-bandwidth product in a two-ring system.

We propose a new scheme for buffering optical signals in two-ring resonator system that has a larger delay-bandwidth product than those achievable in single-ring and two-ring configurations of the optical analog of electromagnetically-induced transparency (EIT).

First-step nucleation growth dependence of InAs/InGaAs/InP quantum dot formation in two-step growth.

First-step nucleation growth has an important impact on the two-step growth of high-quality mid-infrared emissive InAs/InGaAs/InP quantum dots (QDs). It has been found that an optimized growth rate for first-step nucleation is critical for forming QDs with narrow size distribution, high dot density and high crystal quality. High growth temperature has an advantage in removing defects in the QDs formed, but the dot density will be reduced. Contrasting behavior in forming InAs QDs using metal-organic vapor phase epitaxy (MOVPE) by varying the input flux ratio of group-V versus group-III source (V/III ratio) in the first-step nucleation growth has been observed and investigated. High-density, 2.5 × 10(10) cm(-2), InAs QDs emitting at>2.15 µm have been formed with narrow size distribution, ∼1 nm standard deviation, by reducing the V/III ratio to zero in first-step nucleation growth.

Argon-plasma-induced InAs/InGaAs/InP quantum dot intermixing.

We report the first study of argon (Ar)-plasma-enhanced intermixing of InAs/InGaAs/InP self-assembled quantum dots (QDs) in an inductively coupled plasma reactive ion etch system. The Ar-plasma exposure creates point defects, which propagate into the QD structure and enhance the intermixing between the QDs and their barrier layers, hence tuning the energy bandgap of the QDs. By optimizing the plasma exposure time and the annealing temperature, we observe (i) a blueshift of 160 nm and an increase in the photoluminescence (PL) intensity of the QD samples immediately after Ar-plasma exposure for 90 s, and (ii) a further increase in the blueshift of 330 nm, accompanied by 2.5-times increase in the PL intensity and 37 nm narrowing in the PL linewidth after subsequent rapid thermal annealing at 720 °C. The ability to generate a large blueshift without degrading the material quality shows that Ar-plasma exposure is an efficient post-growth technique for tuning the energy bandgap of QD structures.

High-index-contrast waveguides and devices.

We present a theoretical and experimental study of high-index-contrast waveguides and basic (passive) devices built from them. Several new results are reported, but to be more comprehensive we also review some of our previous results. We focus on a ridge waveguide, whose strong lateral confinement gives it unique properties fundamentally different from the conventional weakly guiding rib waveguides. The ridge waveguides have distinct characteristics in the single-mode and the multimode regimes. The salient features of the single-mode waveguides are their subwavelength width, strong birefringence, relatively high propagation loss, and high sensitivity to wavelength as well as waveguide width, all of which may limit device performance yet provide new opportunities for novel device applications. On the other hand, wider multimode waveguides are low loss and robust. In addition, they have a critical width where the birefringence is minimal or zero, giving rise to the possibility of realizing intrinsically polarization-independent devices. They can be made effectively single mode by employing differential leakage loss (with an appropriate etch depth) or lateral mode filtering (with a taper waveguide). Together these waveguides provide the photonic wire for interconnections and the backbone to build a broad range of compact devices. We discuss basic single-mode devices (based on directional couplers) and multimode devices (multimode interferometers) and indicate their underlying relationship.

Polarization-independent vertical coupler for photonics integration.

We present the first systematic design approach for compact polarization-independent mode-size transformers based on tapered resonant vertical couplers with transfer efficiencies greater than 90%. Resonant coupling occurs at a critical taper width having equal TE and TM effective index. Being polarization-independent broadens the usefulness of vertical coupler as a building block for minimizing insertion loss in photonic integrated circuits.

Transformation between directional couplers and multi-mode interferometers based on ridge waveguides.

We present a unique comparison of ridge-type directional couplers (DC) and multi-mode interferometers (MMI) in terms of their transformational relationship. The two devices are intimately related as the MMI is derived from the DC. We show for the first time the continuous evolution from the two-mode coupling characteristic of DC to the multimode mixing and interference characteristic of MMI, as the DC is structurally transformed into the MMI. We also show that DC can be designed to have the MMI features of compactness and polarization-insensitivity, two traits that reflect their shared lineage. However, the design of DC requires careful control of a large set of design parameters, while the MMI design is more robust and involves fewer design variables.

Theoretical approach to a polarization-insensitive single-mode microring resonator.

This paper addresses the polarization sensitivity issue of micro-ring resonators by proposing a novel design of an MMI-coupled resonator with substantially reduced or zero polarization sensitivity, while maintaining singlemode and low-loss conditions. The design is based on polarization-independent, single-mode waveguide obtained by using a judicious combination of critical ridge width and etch depth. The design is limited to relatively large resonators having small FSR (free spectral range). For the first time, it gives designer a handle on the intrinsic polarization-dependent characteristics of waveguide microresonators.