Band structure, photonic crystal

SILAR has been used for the synthesis of CdS/ZnS coatings for CdSe quantum dots. The precursor solutions were prepared by dissolving CdO, ZnO, and S in oleic acid and octadecane. The final coating consisted of three layers of CdS and three additional layers of ZnS. The photonic band structure of the photonic crystal had a modifying influence on the photoluminescence of the embedded quantum dots.90... 

Bulk silicon is a semiconductor with an indirect band structure, as schematically shown in Fig. 7.12 c. The top of the VB is located at the center of the Brillouin zone, while the CB has six minima at the equivalent (100) directions. The only allowed optical transition is a vertical transition of a photon with a subsequent electron-phonon scattering process which is needed to conserve the crystal momentum, as indicated by arrows in Fig. 7.12 c. The relevant phonon modes include transverse optical phonons (TO 56 meV), longitudinal optical phonons (LO 53.5 meV) and transverse acoustic phonons (TA 18.7 meV). At very low temperature a splitting (2.5 meV) of the main free exciton line in TO and LO replicas can be observed [Kol5]. 

The manufacture of a photonic crystal requires extreme process control because a deviation from perfect periodicity in the order of a few percent of the wavelength worsens the optical performance. Macroporous silicon is a potential candidate for the realization of such structures because of its photolithographic patterning. The precision of the macroporous structures is reflected in the transmission measurements along the T-M and T-K directions, which exhibit a photonic band-gap centered at 5 pm, as shown in Fig. 10.16. For measurement the macroporous... 

Fig. 10.16 In the center part, the calculated photonic band structure (TE-pol.) of a two-dimensional photonic crystal of lattice constant 2.3 pm is plotted, while the inset shows the...

Wave propagation in periodic structures can be effieiently modeled using the eoncept of Bloeh (or Floquet-Bloch) modes . This approach is also applicable for the ealeulation of band diagrams of 1 -D and 2-D photonic crystals . Contrary to classical methods like the plane-wave expansion , the material dispersion ean be fully taken into aeeount without any additional effort. For brevity we present here only the basie prineiples of the method. 

Three-dimensional (3D) structuring of materials allows miniaturization of photonic devices, micro-(nano-)electromechanical systems (MEMS and NEMS), micro-total analysis systems (yu,-TAS), and other systems functioning on the micro- and nanoscale. Miniature photonic structures enable practical implementation of near-held manipulation, plasmonics, and photonic band-gap (PEG) materials, also known as photonic crystals (PhC) [1,2]. In micromechanics, fast response times are possible due to the small dimensions of moving parts. Femtoliter-level sensitivity of /x-TAS devices has been achieved due to minute volumes and cross-sections of channels and reaction chambers, in combination with high resolution and sensitivity of optical con-focal microscopy. Progress in all these areas relies on the 3D structuring of bulk and thin-fllm dielectrics, metals, and organic photosensitive materials. 

Once the building blocks for the photonic crystal have been made (or purchased), they must be placed in an ordered arrangement. Different crystal structures will yield varying degrees of snccess in obtaining a photonic band structure. A case... 

We consider a two-level atom with excited and ground states e) and g) when in a photonic crystal coupled to the field of a discrete (or defect) mode and to the photonic band structure in the vacuum state. The hamiltonian of the system in the rotating-wave approximation assumes the form [Kofman 1994]... 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

The FEs produced at low temperature by illumination with photons in the vicinity or above Eg have finite lifetimes that depend on temperature (see [34] for silicon), their binding energies, and on the band structure of the semiconductor (the lifetime is larger in semiconductors with indirect gap than direct gap). During their lifetime, they can diffuse in the crystal and be trapped by impurities and defect to become bound excitons (BEs) with energies slightly different from that of the FE. 

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