Discover

'Photonic crystals' are periodic optical (nano)structures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons. Photonic crystals occur in nature and in various forms have been studied by science for the last 100 years.

Contents

Introduction

Naturally Occurring Photonic Crystals

History of Photonic Crystals

Fabrication Challenges

Applications

See also

References

External links

Introduction

Photonic crystals are composed of periodic dielectric or metallo-dielectric (nano)structures that affect the propagation of electromagnetic waves (EM) in the same way as the periodic potential in a semiconductor crystal affects the electron motion by defining allowed and forbidden electronic energy bands. Simply put, photonic crystals contain regularly repeating internal regions of high and low dielectric constant. Photons (manifesting in this case, their wave-like nature) propagate through this structure - or not - depending on their "wavelength". Wavelengths of light (stream of photons) that are allowed to travel are known as "modes". Disallowed bands of wavelengths are called photonic band gaps. This gives rise to distinct optical phenomena such as inhibition of spontaneous emission, high-reflecting omni-directional mirrors and low-loss-waveguiding amongst others.
Since the basic physical phenomenon is based on diffraction, the periodicity of the photonic crystal structure has to be of the same length-scale as half the wavelength of the EM waves i.e. ~200 (blue) to 350 (red) nm for photonic crystals operating in the visible part of the spectrum - the repeating regions of high and low dielectric constants have to be of this dimension. This makes the fabrication of optical photonic crystals cumbersome and complex.

Naturally Occurring Photonic Crystals

A prominent example of a photonic crystal is the naturally occurring gemstone opal. Its play of colours is essentially a photonic crystal phenomenon based on Bragg diffraction of light on the crystal's lattice planes.
Another well-known photonic crystal is found on the wings of some butterflies such as those of genus Morpho ^[1].

History of Photonic Crystals

The simplest form of a photonic crystal is a one-dimensional periodic structure, such as a multilayer film (a ''Bragg mirror''); electromagnetic wave propagation in such systems was first studied by Lord Rayleigh in 1887 ^[2], who showed that any such one-dimensional system has a band gap. One dimensional periodic systems continued to be studied extensively, and appeared in applications from reflective coatings where the reflection band corresponds to the photonic band gap and to distributed feedback (DFB) diode lasers where a crystallographic defect is inserted in the photonic band gap to define the laser wavelength.
Two dimensional periodic optical structures, without band gaps, received limited study in the 1970s and 1980s. The possibility of two- and three-dimensionally periodic crystals with corresponding two- and three-dimensional band gaps was not suggested until 100 years after Rayleigh, by Eli Yablonovitch and Sajeev John in 1987 ^{[3] [4]}, and such structures have since seen growing interest by a number of research groups around the world, with potential applications including LEDs, optical fiber, nanoscopic lasers, ultrawhite pigment, radio frequency antennas and reflectors, and photonic integrated circuits. Many research groups have recently succeeded in controlling the pace of light emission using photonic crystals^[5]. In the process, they have verified the then 17-year old prediction of American physicist Eli Yablonovitch that ignited a world-wide rush to build tiny "chips" that control light beams. Researchers say it has many potential uses, not only as a tool for controlling quantum optical systems, but also in efficient miniature lasers for displays and telecommunications, in solar cells, and even in future quantum computers.

Fabrication Challenges

The major challenge for higher dimensional photonic crystals is in fabrication of these structures, with sufficient precision to prevent scattering losses blurring the crystal properties and with processes that can be robustly mass produced. One promising method of fabrication for two-dimensionally periodic photonic crystals is a photonic-crystal fiber, such as a "holey fiber". Using fiber draw techniques developed for communications fiber it meets these two requirements.
For three dimensional photonic crystals various techniques ^[6] have been used including photolithography and etching techniques similar to those used for integrated circuits. To circumvent nanotechnological methods with their complex machinery, alternate approaches have been followed to grow photonic crystals as self-assembled structures from colloidal crystals.

Applications

Photonic crystals are attractive optical materials for controlling and manipulating the flow of light. One dimensional photonic crystals are already in widespread use in the form of thin-film optics with applications ranging from low and high reflection coatings on lenses and mirrors to colour changing paints and inks.
Higher dimensional photonic crystals are of great interest for both fundamental and applied research, and the two dimensional ones are beginning to find commercial applications. The first commercial products involving two-dimensionally periodic photonic crystals are already available in the form of photonic-crystal fibers, which use a nanoscale structure to confine light with radically different characteristics compared to conventional optical fiber for applications in nonlinear devices and guiding exotic wavelengths.
The three-dimensional counterparts are still far from commercialization but offer additional features possibly leading to new device concepts (e.g. optical computers), when some technological aspects such as manufacturability and principal difficulties such as disorder are under control.

See also

★ Photonic-crystal fiber

★ Thin-film optics

★ Superprism

References

1. S. Kinoshita, S. Yoshioka and K. Kawagoe “Mechanisms of structural colour in the Morpho butterfly: cooperation of regularity and irregularity in an iridescent scale” Proc. R. Soc. Lond. B 269, 1417-1421 (2002) http://lib.store.yahoo.net/lib/buginabox/kinoshita.pdf
2. J. W. S. Rayleigh, "On the remarkable phenomenon of crystalline reflexion described by Prof. Stokes." Phil. Mag. 26, 256-265. (1888)
3. E. Yablonovitch "Inhibited Spontaneous Emission in Solid-State Physics and Electronics",
Phys. Rev. Lett., Vol. 58, 2059 (1987) http://www.ee.ucla.edu/faculty/papers/eliy1987PhysRevLett.pdf
4. S. John, "Strong Localization of Photons in Certain Disordered Dielectric Superlattices", Phys. Rev. Lett. 58, 2486 (1987) http://www.physics.utoronto.ca/~john/john/p2486_1.pdf
5. P. Lodahl, A.F. van Driel, I.S. Nikolaev, A. Irman, K. Overgaag, D. Vanmaekelbergh, and W.L. Vos "Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals"
Nature 430, 654 - 657 (2004) http://cops.tnw.utwente.nl/pdf/04/nature02772.pdf
6. Review: S.Johnson (MIT) Lecture 3: Fabrication technologies for 3d photonic crystals, a survey http://ab-initio.mit.edu/photons/tutorial/L3-fab.pdf

External links

★ Photonic Crystal Article in Scientific American by Eli Yablonovitch [1]

★ Yuri A. Vlasov's Collection of Photonic Band Gap Research Links [2]

★ Prof Yablonovitch's Optoelectronics Group at UCLA School of Engineering and Applied Sciences [3].

★ Prof John's page at University of Toronto [4].

★ Prof Vos's group at University of Twente www.photonicbandgaps.com

★ Photonic crystals tutorials by Prof S. Johnson at MIT

★ Photonic crystals tutorials by Prof Cefe Lopez at ICMM

★ Autocloning at Photonic Lattice, Inc .

★ ePIXnet Nanostructuring Platform for Photonic Integration

★ PhOREMOST: Nanophotonics to realise molecular scale tecnology