The temperature dependence of the luminescence intensity in nanocrystalline semiconductors, amorphous semiconductors, and chalcogenides has been reported to be of the Berthelot type exp(T/T B ), where T B is some characteristic temperature. A similar behavior has been reported for transport properties in certain semiconductors and in porous silicon. We propose a simple microscopic model for the origin of the Berthelot term. We assume that luminescence arises from a competition between radiative and hopping processes. The hopping process is modeled by assuming that the carrier tunnels through a static barrier. Optimizing this tunneling in a fashion similar to Mott's treatment of variable range hopping leads to the Berthelot-type behavior. The class of barriers for which our result holds is large. We examine alternative proposals and find them wanting. Our model predicts that acceptable values of the barrier width ͑1 nm͒ yields Berthelot temperatures T B in the range 30-300 K. The experimentally reported T B in diverse systems ranging from nanocrystalline semiconductors to amorphous chalcogenides fall in our predicted range. Thus we demonstrate that the Berthelot temperature dependence has a definite and reasonable physical basis.
The cavitation effects given by a high-frequency pulsed ultrasound field are studied with and without the stimulation of a low-frequency field. Sonoluminescence intensity and subharmonic one-half intensity of the high-frequency field are measured. The stimulation gives a sharp rise of both subharmonic and sonoluminescence intensities.
Mechanisms of the overlapping of gaps due to a refractive index difference minimum and Anderson localization for photonic band gap (PBG) have been used and they give a refractive index contrast difference of less than two percent for X-, L-, and W-points of the Brillouin zone for the pseudogap. Another physical process for the existence of PBG is the use of scattering strength (ε r ≥ 1) for fcc lattice structure. We have found refractive index contrast in the range 2.41–14.21 for the existence of the complete photonic band gap for bound photons (ε r ≥ 1). The lowest limit to yield a gap is 2.41 for valence photons (ε r = 1) at volume filling fraction 85.5% for spherical air atoms and at 14.5% for dielectric spheres. This work is reported for the first time and it is useful for maintaining connectivity and for easier fabrication of photonic crystals.
We have theoretically studied the occurrence of photonic band gap. Calculated values of relative width and lattice size for nematic liquid crystal ZLI-1132 and smectic ferroelectric liquid crystal (R)-4 -(1-methoxycarbonyl-ethoxy)-phenyl-4-[4-(noctyloxy)phenyl]benzoate (1MC1EPOPB) synthetic opal (SiO 2 ) infiltrated with liquid crystal as a function of temperature using a model of strong localization for the occurrence of pseudogap are reported for the W-point in the Brillouin zone. A new expression for refractive index calculation for synthetic opal infiltrated with liquid crystal is proposed. Central wavelength is also calculated and compared with the observed ones. The structure parameter and opal size are also given. This work is important for temperature tuning and anisotropy of photonic crystal.
The equivalent temperature of sonoluminescence is analyzed by focused steady and pulsed ultrasonic irradiation at 0.7 MHz for different monoatomic and diatomic gases saturating the distilled water. Transducer voltage, pulse time, total excitation time, and pulse duty ratio have been varied. The intensities of sonoluminescence in a yellow, a blue, and an ultraviolet wavelength range are measured using corresponding filters and three photomultiplier tubes viewing a cell of 160 cm3 in volume. The data are taken near the threshold and well inside the region of developed cavitation. The equivalent temperature decreases by decreasing the inverse pulse duty ratio from the threshold value down to the steady irradiation; it increases by increasing the sound pressure amplitude and decreases by increasing the pulse time and the excitation time. The equivalent temperature increases by increasing the atomic weight of the monoatomic gas saturating the water when the cavitation zone is in a nonsaturated condition, i.e., near the incipient threshold of the pulsed irradiation. The equivalent temperature of the sonoluminescence is analyzed in the initial and final stages of development of the cavitation zone, which correspond to bubble densities different by many orders of magnitude.
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