Since the first formal studies of multi-layer dielectric stacks by Lord Rayleigh in 1887 and subsequent research that lead to the term “photonic crystal” to be coined by Eli Yablonovitch and Sajeev John in 1987, the mathematics that describe the formation of photonic band gaps, low loss waveguiding, and standing wave optical resonances have included terms for the physical dimensions of the structure and the refractive indices of the structure’s materials. As the menu of possible photonic crystal structures has grown to include 3-dimensional “woodpile” stacks, inverse opals, 2-dimensional slabs, guided mode resonant filters, and photonic crystal fiber, the menu of material choices has also expanded to include a cornucopia of possibilities that include silicon, compound semiconductors, dielectrics, and organic (carbon-based) media. It was perhaps inevitable that scientists would begin to manipulate the physical “constants” of these photonic crystal structures (period, thickness, refractive index) to transform photonic crystals into sensors.
In many respects, the photonic crystal is an ideal sensor system. By simply illuminating the structure with a laser, LED, or incandescent lamp, the reflected or transmitted spectrum reveals a great deal about its physical makeup. With the advent of miniature spectrometers, low-power LEDs, and semiconductor lasers, instrumentation for measuring the properties of photonic crystals has become miniature, inexpensive, and rugged. Meanwhile, the ability to inexpensively fabricate photonic crystal structures, despite their nanometer-scale features, has made remarkable advances, which now make them suitable even for sensor applications in which the device will be single-use disposable, as in point-of-care medical diagnostics. As a result, photonic crystal sensors allow high resolution and rapid measurement of structures within microfluidic channels, biomedical tubing, microtiter plates, test tubes, and flasks without the need for electrical contacts, a source of power on the device itself, or any direct physical contact to the detection instrument.
This eBook represents many of the exciting sensing applications that utilize photonic crystal structures. In it, you will find the fundamental operating principles of photonic crystals and a description of the analytical methods that are used to derive their optical properties from Maxwell’s Equations. The text describes methods for creating photonic crystal structures, and in particular stresses designs that enable the structure to interact with gaseous or liquid materials. The ability for photonic crystals to generate high intensity evanescent electric fields on their surfaces allows for chemical sensing using Surface-Enhanced Raman Scattering, while the incorporation of materials into their structure that exhibit optical gain enables the creation of light emitting devices that can be used as sensors. The ability of photonic crystals to form optical standing waves results in “slow light” and associated electric field enhancements that can be used for sensing either through detection of shifts in the resonant wavelength due to biomolecule adsorption or through the enhanced excitation of fluorescent dyes that are used to “tag” biomolecules such as DNA or proteins.
I hope that you will find this text to be a useful guide and introduction to the many exciting ways that photonic crystals are being applied to a variety of problems in sensing. Photonic crystal-based sensing is an exciting multidisciplinary field that involves electromagnetics, optics, nanofabrication, material science, chemistry, biology, and (sometimes) mechanical engineering. It is the goal of the authors to welcome the enthusiasm and ideas of new students with backgrounds in these fields to join with us in the goal of extending photonic crystals into high precision sensing tools that can find applications in research and commercial products.
Brian T. Cunningham
Department of Electrical and Computer Engineering
Department of Bioengineering
University of Illinois at Urbana-Champaign