New optical and optoelectronic materials
During the last twenty years Science and Technology have shown a feverish rush about small systems (micro and nano systems), where the physico-chemical properties of matter suffer dramatic changes as compared to those of the macroscopic bulk materials. The first revolution in the nanostructures field came about when quantum effects where shown in semiconductor systems where the spatial confinement of the electrons gives rise to the appearance of astonishing effects that range from the pioneering quantum well effect (1) to the now days single electron transistor (2). Between both phenomena a large variety of new devices based on quantum effects have completely changed the semiconductor science and technology landscape. Size effects are also of paramount importance in redox reactions where the concept of acid or basic complex may depend on the size of the reactor that encloses the chemical. Reactions within the nanometric cages of zeolites can be taken as an example ofthose phenomena (3). Quantum effects induced by sample size have also dramatically modified superconductivity and magnetic properties of materials. As an example, the inhibition of the superconductivity induced by size effects, can be given (4). All these have led to a control of the chemical and also electrical properties of the matter.
The control the optical properties of matter has been one of the principal goals in recent years’ research. What this finally aims at is the use of the photon as an information carrier as the electron is used in semiconductor technology. Photons have several advantages as compared with electrons. Transmission is faster and less costly partly due to the absence of charge and mass. Photons are thereby made more efficient than electrons in the transport of information. In this sense, the invention of the optical fibre has strongly boosted the communication technology. However, the true breakthrough in the control of the optical properties of matter was the introduction of the concept of photonic band gap (PBG) crystal, photonic crystal (PX), for short, by Yablonovitch (5) and John (6) a decade ago.
A PX can be defined as a material in which a periodic modulation of the refractive index is attained. These materials were called PXs owing to the similarities in the behaviour of photons therein and electrons in solids. (7) An electronic crystal can be viewed as a periodic distribution of basic units such as atoms or molecules. As a direct consequence of the periodicity of the electronic potential the energy levels are eventually restricted to certain allowed energy bands separated by energy gaps. In the same way PXs present bands and gaps for photon states. This analogy has opened up a new field in materials science and technology. Alluring new optical properties with revolutionary implications in technology have been predicted. Just to name a few of the most impressive applications, lossless frequency selective mirrors or quasi zero threshold lasers may serve. In the first prototype of PX, a piece of a dielectric material was microstructured by drilling holes periodically distributed along the three directions of space. A strongly modulated dielectric constant is thereby produced in the resulting composite (material-air) and the system becomes opaque for certain frequency bands.
The electromagnetic (EM) wave equation can be generalised to any other classical waves, like elastic or sound waves for instance. For these waves the length scales can range from a few millimetres to several kilometres. Therefore the frequency region for gap appearance would cover from ultrasounds up to typical wavelengths of earthquakes. In general, this sort of system can be called sound crystals or elastic wave crystals. Recently, it was shown that certain modern art objects (minimalism sculptures) can show properties of a crystal of sound, in a manner similar to photonic crystals. (8)
1) T. Ando, A.B Fowler and P. Stern. 1982. Rev. Mod. Phys. 54, 437.
2) D.L. Klein, R. Roth, A.K.L. Lim, A.P. Alivisatos, and P. L. McEulen, 1997. Nature 390, 259.
3) A. Corma. 1995. Chem. Rev. 95, 559.Fig. 3
4) A.K. Geim, I.V. Grigorieva, S.V. Dubonos, J.G.S. Lok, J.C. Maan, A.E. Filippov, and F.M. Peeters. 1997. Nature 390, 259.
5) E. Yablonovitch. 1987. Phys. Rev. Lett. 58, 2059.
6) S. John. 1987. Phys. Rev. Lett. 58, 2486.
7) J.D. Joannopoulos, R.D. Meade, and J.N. Winn. 1995. "Photonic Crystals", Princeton University Press.
8) R. Martínez-Sala, J. Sancho, J.V. Sánchez, V. Gómez, J. Llinares and F. Meseguer. 1995. Nature 378, 241; also J. V. Sánchez-Pérez, D. Caballero, R. Martínez Sala, C. Rubio, J. Sánchez-Dehesa, F.Meseguer, J. Llinares and F. Gálvez. 1998. Phys. Rev. Lett. 80, 5325.