The increasing speed of fibre-optic-based telecommunications has focused attention on high-speed optical processing of digital information1. Complex optical processing requires a high-density, high-speed, low-power optical memory that can be integrated with planar semiconductor technology for buffering of decisions and telecommunication data2. Recently, ring lasers with extremely small size and low operating power have been made3,4,5,6,7, and we demonstrate here a memory element constructed by interconnecting these microscopic lasers. Our device occupies an area of 18 × 40 µm2 on an InP/InGaAsP photonic integrated circuit, and switches within 20 ps with 5.5 fJ optical switching energy. Simulations show that the element has the potential for much smaller dimensions and switching times. Large numbers of such memory elements can be densely integrated and interconnected on a photonic integrated circuit: fast digital optical information processing systems employing large-scale integration should now be viable.

A fast low-power optical memory based on coupled micro-ring lasers.

The production of materials with micrometre- and submicrometre-scale patterns is of importance in a range of applications, such as photonic materials, high-density magnetic data storage devices, microchip reactors and biosensors. One method of preparing such structures is through the assembly of colloidal particles. Micropatterned colloidal assemblies have been produced with lithographically patterned electrodes or micromoulds. Here we describe a different method that combines the well-known photochemical sensitivity of semiconductors with electric-field-induced assembly to create ordered arrays of micrometre-sized colloidal particles with tunable patterns. We show that light affects the assembly processes, and demonstrate how to produce patterns using electrophoretic deposition in the presence of an ultraviolet (UV) illumination motif. The distribution of current across an indium tin oxide (ITO) electrode can be altered by varying the illumination intensity: during the deposition process, this causes colloidal particles to be swept from darkened areas into lighted regions. Illumination also assists in immobilizing the particles on the electrode surface. Although the details of these processes are not well understood, the patterning effects of the UV light are discussed in terms of alterations in the current density that affects particle assembly on an ITO electrode.

Electrophoretic assembly of colloidal crystals with optically tunable micropatterns

We have designed and fabricated waveguides that incorporate two-dimensional (2-D) photonic crystal geometry for lateral confinement of light, and total internal reflection for vertical confinement. Both square and triangular photonic crystal lattices were analyzed. A three-dimensional (3-D) finite-difference time-domain (FDTD) analysis was used to find design parameters of the photonic crystal and to calculate dispersion relations for the guided modes in the waveguide structure. We have developed a new fabrication technique to define these waveguides into silicon-on-insulator material. The waveguides are suspended in air in order to improve confinement in the vertical direction and symmetry properties of the structure. High-resolution fabrication allowed us to include different types of bends and optical cavities within the waveguides.

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Design and fabrication of silicon photonic crystal optical waveguides

Photonic crystal nanobeam cavities are versatile platforms of interest for optical communications, optomechanics, optofluidics, cavity QED, etc. In a previous work [Appl. Phys. Lett. 96, 203102 (2010)], we proposed a deterministic method to achieve ultrahigh Q cavities. This follow-up work provides systematic analysis and verifications of the deterministic design recipe and further extends the discussion to air-mode cavities. We demonstrate designs of dielectric-mode and air-mode cavities with Q > 10⁹, as well as dielectric-mode nanobeam cavities with both ultrahigh-Q (> 10⁷) and ultrahigh on-resonance transmissions (T > 95%).

Deterministic design of wavelength scale, ultra-high Q photonic crystal nanobeam cavities

Electronic devices and their highly integrated components formed from semiconductor crystals contain complex three-dimensional (3D) arrangements of elements and wiring. Photonic crystals, being analogous to semiconductor crystals, are expected to require a 3D structure to form successful optoelectronic devices. Here, we report a novel fabrication technology for a semiconductor 3D photonic crystal by uniting integrated circuit processing technology with micromanipulation. Four- to twenty-layered (five periods) crystals, including one with a controlled defect, for infrared wavelengths of 3-4.5 microm, were integrated at predetermined positions on a chip (structural error <50 nm). Numerical calculations revealed that a transmission peak observed at the upper frequency edge of the bandgap originated from the excitation of a resonant guided mode in the defective layers. Despite their importance, detailed discussions on the defective modes of 3D photonic crystals for such short wavelengths have not been reported before. This technology offers great potential for the production of optical wavelength photonic crystal devices.

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Microassembly of semiconductor three-dimensional photonic crystals.

We form a new type of optical microcavity using 2D photonic crystals embedded in a half wavelength thick waveguide Modes localized to a single defect in the photonic crystal can be theoretically shown to have small mode volumes The flexibility in design of the photonic crystal enables one to tailor the device for vertical emission or for coupling into an in-plane waveguide The added versatility in being able to etch the laser cavity may also help develop low threshold laser sources in material systems in which high index contrast epitaxial mirrors do not exist

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Two-dimensional photonic bandgap defect laser

We have demonstrated optically pumped two-dimensional photonic crystal defect InGaAsP MQW lasers at 1.5 /spl mu/m. Pulsed lasing action at 143 K has been obtained in microcavities as small as 2(/spl lambda//2n)/sup 3/.

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P.D. Dapkus

Papers

Two-dimensional photonic bandgap defect laser

Two-dimensional photonic bandgap defect laser