Stop-band

Films of pure CNLCs have a unique transmission behavior as CP light with the same sense of circular polarization as the CNLC is filtered out by reflection, while CP light of the opposite handedness as the CNLC film is transmitted. This selective optical transmission characteristic is referred to as a one-dimensional photonic stop-band or a selective reflection band. The stop-band is centered at a certain wavelength Ac, which is dependent on the pitch length p and the average refractive index n of the CNLC ... 

VI Kopp, B Fan, HKM Vithana, and AZ Genack, Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals, Opt. Lett., 23 1707-1709, 1998. 

Colloidal crysfals can be viewed as the mesoscopic counterpart of atomic or molecular crystals. They have been used to explore diverse phenomena such as crystal growth [52-54] and glass transition [55,56], and have many interesting applications for sensors [57], in catalysis [58,59], advanced coatings [60], and for optical/electro-optical devices for information processing and storage [61,62]. In particular, their unusual optical properties, namely the diffraction of visible light and the existence of a photonic stop band, make them ideal candidates for the development of photonic materials [61,63-66]. They may lead to the fabrication... 

The first filter in the chapter is one of the most popular. The schematic of the fourth-order Butterworth response low pass filter is shown in Fig. 3.1. The frequency response of the filter to an AC sweep is shown in Fig. 3.2. Note the flat response in the pass band and the stop band frequency of 100 kHz. 

The filter bank used in the MPEG/Audio coding system will be used as an example. A 511 tap prototype filter is used. Figure 2.7 shows the prototype filter (window function). It has been optimized for a very steep filter response and a stop band attenuation of better than 96 dB. Figure 2.8 shows the frequency response of the filter bank. In addition to the attenuation requirements it was designed as a reasonable tradeoff between time behavior and frequency localization [Dehery, 1991],... 

A designer transfer function can be crafted to obtain a specific type of smoothing, one that is, for example, maximally flat over a range of frequencies or with a steep transition from the passband to the stop band. Some common designer transfer... 

Figure 1.3. Sketch of the polariton dispersion for a given direction K (notice the scale change to cover the entire Brillouin zone). The broken straight lines indicate the dispersion of the electromagnetic waves in the crystal far from the excitonic b transition. In the stopping band (hatched), only excitonic states with large wave vectors may be created, and the crystal reflection is "quasi-metallic .

Satoh, S., Kajii, H., Kawagishi, Y. et al., Tunable optical stop band utilizing thermochromism of synthetic opal infiltrated with conducting polymer, Jpn. J. 

Ozaki, M., Shimoda, Y, Kasano, M. et al.. Electric field tuning of the stop band in a liquid-crystal-infiltrated polymer inverse opal, Adv. Mater., 14, 514, 2002. 

Ramanov, S.G., Eokin, A.V., and La Rue, R.M., Stop-band structure in complementary three-dimensional opal-based crystals, J. Phys. Condens. Matter, 11, 3593, 1999. 

Fig. 4(a) presents the reflection spectra from the (111) composite surface at normal light incidence. The spectra were measured at two temperature values below the phase transition temperature (the semiconductor phase) and above it (the metallic phase). The observable peaks are due to the Bragg diffraction of electromagnetic waves by the periodic stmeture of the samples, characterizing the stop band in the [111] direction. 

Over the last several decades photonic band-gap materials attracted considerable interest due to the possibility of inhibition of the spontaneous emission and light propagation [1-3]. Mesoporous structures like three-dimensional artificial opals and two-dimensional PAA are considered as photonic band gap materials, demonstrating the photonic stop-band in transmission and reflection spectra [4,5] and anisotropy of photonic density of states (DOS) on scattering indicatrices [6]. An influence of photonic band-gap materials on photoluminescence and spontaneous emission rate of the embedded inclusions have been reported and discussed [7-9]. 

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