The realization of miniaturized optofluidic platforms offers potential for achieving more functional and more compact devices. Such integrated systems bring fluid and light together and exploit their microscale interaction for a large variety of applications. The high sensitivity of compact microphotonic devices can generate effective microfluidic sensors, with integration capabilities. By turning the technology around, the exploitation of fluid properties holds the promise of highly flexible, tunable or reconfigurable microphotonic devices. We overview some of the exciting developments so far.

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Integrated optofluidics: A new river of light

We show a scheme for achieving high-speed operation for carrier-injection based silicon electro-optical modulator, which is optimized for small size and high modulation depth. The performance of the device is analyzed theoretically and a 12.5-Gbit/s modulation with high extinction ratio >9dB is demonstrated experimentally using a silicon micro-ring modulator.

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12.5 Gbit/s carrier-injection-based silicon micro-ring silicon modulators

We demonstrate a junction between a silicon strip waveguide and an ultra-compact silicon microring resonator that minimizes spurious light scattering and increases the critical dimensions of the geometry. We show cascaded silicon microring resonators with radii around 1.5 microm and effective mode volumes around 1.0 microm(3) that are critically coupled to a waveguide with coupled Q's up to 9,000. The radius of 1.5 microm is smaller than the operational wavelength, and is close to the theoretical size limit of the silicon microring ring resonator for the same Q. The device is fabricated with a widely-available SEM-based lithography system using a stitch-free design based on a U-shaped waveguide.

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Silicon microring resonators with 1.5-microm radius.

Optical microresonators have recently attracted growing attention in the photonics community1. Their applications range from quantum electrodynamics to sensors and filtering devices for optical telecommunication systems, where they will probably become an essential building block2. Integration of nonlinear and electro–optical properties in resonators represents a very stimulating challenge, as it would incorporate new and more advanced functionality. Lithium niobate is an excellent candidate material, being an established choice for electro–optic and nonlinear optical applications. Here we report on the first realization of optical microring resonators in submicrometre thin films of lithium niobate. High-index-contrast films are produced by an improved crystal-ion-slicing and bonding technique using benzocyclobutene. The rings have radius R = 100 µm, and their transmission spectrum has been tuned using the electro–optic effect. These results open new possibilities for the use of lithium niobate in chip-scale integrated optical devices and nonlinear optical microcavities.

Electro–optically tunable microring resonators in lithium niobate

A new class of planar optics has emerged using subwavelength gratings with a large refractive index contrast, herein referred to as high-contrast gratings (HCGs). This seemingly simple structure lends itself to extraordinary properties, which can be designed top-down based on intuitive guidelines. The HCG is a single layer of high-index material that can be as thin as 15% of one wavelength. It can be designed to reflect or transmit nearly completely and with specific optical phase over a broad spectral range and/or various incident beam angles. We present a simple theory providing an intuitive phase selection rule to explain the extraordinary features. Our analytical results agree well not only with numerical simulations but also experimental data. The HCG has made easy fabrication of surface-normal optical devices possible, including vertical-cavity surface-emitting lasers (VCSELs), tunable VCSELs, and tunable filters. HCGs can be designed to result in high-quality-factor (Q) resonators with surface-normal output, which is promising for wafer-scale lasers and optical sensors. Spatially chirped HCGs are shown to be excellent focusing reflectors and lenses with very high numerical apertures. This field has seen rapid advances in experimental demonstrations and theoretical results. We provide an overview of the underlying new physics and the latest results of devices.

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High-contrast gratings for integrated optoelectronics

The development of ultrahigh-quality-factor Q silicon-on-insulator (SOI) microring resonators based on silicon wire waveguides is presented. An analytical description is derived, illustrating that in addition to low propagation losses the critical coupling condition is essential for optimizing device characteristics. Propagation losses as low as 1.9±0.1 dB/cm in a curved waveguide with a bending radius of 20 µm and a Q factor as high as 139.000±6.000 are demonstrated. These are believed to be the highest values reported for a curved SOI waveguide device and for any directly structured semiconductor microring fabricated without additional melting-induced surface smoothing.

Andreas Vörckel

Papers

Ultrahigh-quality-factor silicon-on-insulator microring resonator