Defect modes

One period of such a structure can be considered as a finite periodic grating with a defect layer in its middle and that is why a very narrow resonance (defect mode) appears inside of the band gap. 

The extensive studies of Snyder et al. (10) on the infrared spectra of hydrocarbons have also identified several CH2 wagging bands, which are nearly constant in frequency in the spectra of compounds with various chain lengths, that appear to be related to certain conformations, and are hence known as "defect modes. Changes in the relative intensity of these bands have been used to estimate the conformational disorder of the tails of micellar SDS (11), and the changes in packing of solid hydrocarbons as a function of temperature (12). 

Considerable experimental evidence (1-6,11) suggests that the methylene chains inside of a spherical micelle are almost as disordered as in the bulk liquid state (i.e. they contain a significant proportion of gauche conformers). The FTTR spectra of micellar SDS support this assertion, exhibiting CH2 stretching and scissoring band frequencies which are comparable to those found in the spectra of liquid hydrocarbons (1-6,11). A recent quantitative analysis of the CH2 defect modes of SDS has shown that the disorder of me methylene tails is similar to that found in liquid tridecane (11). 

Changes in the fingerprint region of the spectra can also be related to surfactant penetration. The frequency and shape of the 8, CH2 band are very sensitive to intrachain interactions The CH2 wagging defect modes and the C-0 bands of the surfactant appear in this region as well. 

Fig. 2. Internal reflection spectra for HF-treated Si(l 11) surfaces, (a) Surface treated with buffered HF, pH 5.0 (b) subsequently rinsed in water (b/a) difference spectrum showing a decrease in intensity of dihydride defect modes (D, and Dg ) utd an increase in the intensity of the monohydride mode (M) [20],...

Zoval JV, Biemacki P, Penner RM (1996) Implementation of electrochemically synthesized silver nanocrystallites for the preferential SERS enhancement of defect modes on thermally etched Graphite surfaces. Anal Chem 68 1585... 

Figure 12. (F—E) Characteristic FTIR spectrum of 110 chain folds in solution-grown single crystals of n-alkane Ci98H3g8 (spectrum of extended-chain crystals subtracted from that of once-folded crystals) (PESC) solution-grown single crystals of polyethylene (bulk PE) melt-crystallized linear polyethylene. All bands are CH2 wagging defect modes except the CH3 band at 1378 cm-1. Spectra recorded at 110 K (from ref 68 by permission of Elsevier Science Publ.)...

Figure 1. Mode density spectrum with a PBG. The atomic frequency u>at is inside a PBG, near the cutoff frequency u>o and defect mode frequency cOd-...

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]... 

Raman experiments on sllicalite and TS-1 with excitation wavelengths of 1064 nm (non resonant) and 244 nm (resonant) show that (i) the main features associated with Ti insertion in the lattice are vibrations at 1125 and 960 cm" the former being drastically enhanced by UV-resonance, while the latter is not (ii) a mode is observed at 978 cm" on defective silicalites and TS-1, which we attribute to the Si-0 stretching in silanols. The proximity of the 960 and 978 cm" modes has prompted us to re-examine IR spectroscopy in the same region in order to distinguish the 960 cm band from defect modes. 

In addition to band-edge lasing, defect mode lasing is possible [97]. A defect in the photonic structure creates fine bands of allowed transmissions inside the photonic band gap. A defect may result from an isotropic or anisotropic layer between adjacent cholesteric layers, but also from phase shifts [106], particle stabilized defects [107], deformation of the cholesteric helix [108] or local polymerization [109]. The threshold of such defect modes are low and can be as low as a few nanojoules per pulse [110]. 

Haase W, Podgomov F, Matsuhisa Y, Ozaki M (2008) Lasing in dye-doped chiral liquid crystals influence of defect modes. In Wehrspohn RB, Kitzerow H-S, Busch K (eds) Nanophotonic materials photonic crystals, plasmonics and metamaterials. Wiley-VCH, Weinheim, p 239... 

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