Photonic crystals are materials patterned with a periodicity in dielectric constant, which can create a range of 'forbidden' frequencies called a photonic bandgap. Photons with energies lying in the bandgap cannot propagate through the medium. This provides the opportunity to shape and mould the flow of light for photonic information technology.

Photonic crystals: putting a new twist on light

We describe devices in which optics and fluidics are used synergistically to synthesize novel functionalities. Fluidic replacement or modification leads to reconfigurable optical systems, whereas the implementation of optics through the microfluidic toolkit gives highly compact and integrated devices. We categorize optofluidics according to three broad categories of interactions: fluid–solid interfaces, purely fluidic interfaces and colloidal suspensions. We describe examples of optofluidic devices in each category.

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Developing optofluidic technology through the fusion of microfluidics and optics

The various arguments for introducing optical interconnections to silicon CMOS chips are summarized, and the challenges for optical, optoelectronic, and integration technologies are discussed. Optics could solve many physical problems of interconnects, including precise clock distribution, system synchronization (allowing larger synchronous zones, both on-chip and between chips), bandwidth and density of long interconnections, and reduction of power dissipation. Optics may relieve a broad range of design problems, such as crosstalk, voltage isolation, wave reflection, impedence matching, and pin inductance. It may allow continued scaling of existing architectures and enable novel highly interconnected or high-bandwidth architectures. No physical breakthrough is required to implement dense optical interconnects to silicon chips, though substantial technological work remains. Cost is a significant barrier to practical introduction, though revolutionary approaches exist that might achieve economies of scale. An Appendix analyzes scaling of on-chop global electrical interconnects, including line inductance and the skin effect, both of which impose significant additional constraints on future interconnects.

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Rationale and challenges for optical interconnects to electronic chips

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Wave propagation and group velocity

The photonic band structure in a two-dimensional dielectric array is investigated using the coherent microwave transient spectroscopy (COMITS) technique. The array consists of alumina-ceramic rods arranged in a regular square lattice. The dispersion relation for electromagnetic waves in this photonic crystal is determined directly using the phase sensitivity of COMITS. The experimental results are compared to theoretical predictions obtained using the plane-wave expansion technique. Configurations with the electric field parallel and perpendicular to the axis of the rods are investigated.

Measurement of photonic band structure in a two-dimensional periodic dielectric array.

Experiments are described on the prism-coupled excitation of surface electromagnetic waves in one-dimensional photonic band-gap arrays. The low loss of photonic band-gap materials leads to narrow angular reflectivity resonances and high surface fields. These attributes, coupled with the ability to engineer the optical properties of photonic band-gap arrays, suggest these materials as powerful replacements for metal films in many applications that make use of surface-plasmon resonance.

Surface electromagnetic wave excitation on one-dimensional photonic band-gap arrays

The design and operating principles of a new type of surface electromagnetic wave sensor are described and demonstrated. The method of operation of this device is similar to surface plasmon sensors except that the surface-active material—a metal film in the case of surface plasmon sensors—is replaced by a one-dimensional photonic band-gap array. The advantages of using a photonic band-gap material instead of a metal film include enhanced sensitivity, physical, and chemical robustness, and the ability to engineer the optical response of the surface active layer to create a device that operates at any optical wavelength. Experimental results are presented that illustrate sensing action by measuring the shift in the surface mode coupling angle with surface loading.

Surface plasmon-like sensor based on surface electromagnetic waves in a photonic band-gap material

The first observation to the authors’ knowledge of electromagnetic surface waves in a two-dimensional dielectric crystal is reported. By using the coherent microwave transient spectroscopy technique, surface waves are shown to exist at frequencies within the photonic band gap for certain lattice terminations. Energy at gigahertz frequencies is coupled into the surface mode using a prism coupling technique. The experimental results are in excellent agreement with theoretical predictions.

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Observation of surface photons on periodic dielectric arrays.

Two different attenuated total-internal reflection prism configurations are used to explore the excitation of surface electromagnetic waves on one-dimensional (1-D) photonic bandgap (PBG) arrays. The effect of surface termination of the photonic crystal is shown to have a significant effect on the dispersion of the surface modes excited at that interface. The results show that it is possible to engineer the position of the surface mode within the forbidden bandgap. Modes that are located close to the center of the bandgap are shown to be more localized, leading to significantly higher surface electromagnetic fields than modes located near the band edge. The existence of surface modes can have an effect on many of the proposed applications for PBG materials. The modes are also of interest in their own right for use in applications such as sensors and modulators.

William M. Robertson

Papers

Measurement of photonic band structure in a two-dimensional periodic dielectric array.

Surface electromagnetic wave excitation on one-dimensional photonic band-gap arrays

Surface plasmon-like sensor based on surface electromagnetic waves in a photonic band-gap material

Observation of surface photons on periodic dielectric arrays.

Experimental measurement of the effect of termination on surface electromagnetic waves in one-dimensional photonic bandgap arrays