Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.
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Technology optical circuits and in nanocavity laser designs. 1, 2, 30 The application of the photonic crystal slab to nano-cavity laser technology uses the slab as a Fabry- Pérot laser cavity 30 (Figure 2). A semiconductor heterojunction light source is placed within the photonic crystal slab. The light generated by the semiconductor source is confined in the plane of the slab by setting the frequency of the generated light within a stop band of the periodic patterning of the dielectric slab. The two planar surfaces of the slab then act as the partially reflective surfaces for the horizontally stop band-confined light. Light that does not satisfy the incident angle conditions with the slab surfaces to be internally reflected back into the slab is partially passed from the slab through the surfaces. The laser light then is projected vertically normal to the slab surfaces. These types of geometries and their variants have been used in the design of so-called zero- threshold lasers. In these devices, the idea is to make lasers that lase at very low input energy thresholds. 30 Photonic crystals also have applications in the design of optical circuits of interconnecting waveguides, and information can be downloaded from such circuitry by introducing an optical multiplexer configuration within the photonic crystal slab. 1,2, 11-14 The multiplexer configuration 14 is shown in Figure 1d. A waveguide- carrying signal is resonantly coupled to an off-channel cavity site. At the resonant frequency of interaction between the channel and site, energy traveling within the waveguide will transfer to the cavity and download through the surface of the photonic crystal slab. Another important application of photonic crystal cavity technology is in the modification of atomic decay rates. The decay rates of excited atoms, from Fermi's golden rule, are proportional to the density of states in space of the photon radiated by the excited atom. Photonic crystals allow the modification of these decay rates through the changes they cause in the photon density of states. In electronic semiconductors, the density of electron states is known to be modified by the band structure. In stop bands there are no electronic states, while states at the top and bottom of pass bands are known to be enhanced. This also is true for the photonic density of states in photonic crystals. The decrease and/or increase in the density of photonic states within photonic crystal devices can be used to slow or increase the rates of decay of excited atoms located within them. 1,2 Such applications are important to some technologies available for quantum computing. 31-33 Metamaterials: A Homogeneous Medium With Designer Optical Properties Metamaterials are another type of artificially designed material, involving the introduction of nanoscale features and/or wires within a naturally occurring optical medium. 3-6 Unlike photonic crystals, metamaterials are formulated to interact with electromagnetic waves as homogeneous materials. Naturally occurring optical media are composed of atoms and molecules, but the wavelength of light interacting with a particular medium is much greater than the individual atomic separations. This allows the interaction of light with the medium to be described by a dielectric constant, which accounts for changes in the speed of light and refractive effects. In metamaterials, the wavelength of light is to be greater than its nanoscale features (i.e., typically at microwave to visible frequencies). In this limit, ray optics applies. The refractive effects of metamaterials are different from the diffractive effects found in photonic crystals, which are an analogy of X-ray Bragg diffraction in naturally occurring media. Metamaterials were first proposed because naturally occurring substances do not exhibit, simultaneously at a single frequency, a negative permeability and a negative permittivity. 3-6 As only substances with a simultaneous negative permeability and permittivity will display a negative refractive index, 5,6 a consequence of this is that naturally occurring media cannot display a negative refractive index. 5,6 The nanoscale features engineered into metamaterials are specifically designed to allow its permeability and permittivity to be simultaneously negative at certain frequencies. To understand negative refractive index, consider Snell's law for the refraction of light through a planar interface between two different media (n 1 sin Ɵ i1 = n 2 sin Ɵ r2 ). This relates the incident and refractive angles of light to the index of refraction of the two media. The index of refraction of either or both media now can be negative. 2, 5, 6 This provides a much greater variety of refractive angle solutions and opens up a greater range of optical effects. Electronic Military & Defense Annual Resource, 6th Edition 26 Figure 2: Photonic crystal slab with a light source introduced into the center of the patterning. Shown are the top of the slab (above) with a triangle lattice patterning and a side view of the slab (below).