Cathodoluminescence study of extended defects in hydrothermal ZnO crystals

Understanding the luminescence of ZnO is very important for some applications. In spite of the many studies carried out, there are still some points concerning the origin of some of the luminescence emissions in ZnO crystals that require additional study; in particular, the role of extended defects remains to be a matter of controversy. We present here a cathodoluminescence analysis of the defects generated by Vickers indentation in hydrothermal HTT crystals. Special emphasis was paid to the luminescence band peaking around 3.3 eV. The origin of this band is a matter of controversy, since it has been related to different causes, extended defects being one of the candidates for this emission. The CL images were acquired around crystal defects. It is observed that the 3.3 eV emission is enhanced around the crystal defects; though it is also observed, but weaker, out of the defect regions, which suggests that there exist two luminescence emissions peaking very close to 3.3 eV. The two emissions, one related to structural defects and the other to the LO phonon replica of the free excitonic band, appear very close each other and their relative intensity should determine the shape of the spectrum.     

Pulse duration dependence of laser damage mechanisms

A number of mechanisms for internal laser damage in transparent dielectrics are examined for pulse duration dependence and the results are compared with experimental measurements when possible. The differences in dependence on pulse duration and other variables from one mechanism to another suggest that different mechanisms may initiate damage in different parameter ranges. Experimental identification of these mechanisms will be aided by measuring the pulse duration dependence of the damage threshold.Symbols bulk absorption coefficient - thermal volume expansion coefficient - C heat capacity per unit volume - c velocity of light in vacuum - D thermal diffusivity - E instantaneous electric field strength - E ₀ electric field amplitude - e charge on an electron - e spectral emissivity at the laser wavelength - _c band gap energy - _d energy density (energy per unit cross-sectional area) - _e electron energy - k volume compressibility - k wave vector - L sample length - L _f focusing length - wavelength in vacuum - M multiplication factor to correct particle cross-section for Mie effect - m ^* electron effective mass - N conduction electron number density - N _b value ofN required for breakdown - n number of photons absorbed inn-photon ionisation - n refractive index - n ₀ original refractive index - n ₂ coefficient for quadratic term in expression for refractive index - n change in refractive index - P _d power density (power per unit cross-sectional area) - R inclusion radius - R _s sample radius - r distance from centre of inclusion - r _b beam radius - S breaking strength, damaging stress - T temperature - t time - t time required to accelerate an electron to the energy of the conduction band - t pulse duration - time required for acoustic wave to propagate across the beam - _e conduction electron lifetime - U velocity - angular velocity (frequency times 2)     

Cumulant functions in optical coherence theory

Cumulant functions are introduced to describe the statistical state of a radiation field. These functions are simply related to the optical coherence functions but have some interesting features. It is shown that if the cumulant functions of all orders greater than some numberN ₀ vanish then they also vanish for all orders greater than 2. Thermal field is the only field having this property. This property holds whether the field is described by a classical stochastic process or by a quantum density operator. Further the particular operator ordering used in defining these cumulant functions for the quantized field affects only the second order cumulant function. To describe the statistical state of a vector field such as partially polarized or unpolarized radiation, one would need to introduce cumulant tensors.     

Search for efficient,near UV lasing dyes. III. Monocyclic and miscellaneous dyes

A potential new class of “monocyclic” lasing dyes is discussed and four families out of nine within the class are examined. The 2,6-diaminopyridines in acid are found to lase under flashlamp excitation but show low slope efficiencies and lower stabilities than the “bicyclic” dyes described previously. Other, miscellaneous dyes show oxygen quenching of laser action rather than enhancement and the best of these produces short pulses under long pulse excitation. This study was supported under contract SANL 284 from the Lawrence Livermore Laboratory.     

Gas phase activation energy for unimolecular dissociation of biomolecular ions determined by focused RAdiation for gaseous multiphoton ENergy transfer (FRAGMENT)

We present a novel approach for the determination of activation energy for the unimolecular dissociation of a large (>50 atoms) ion, based on measurement of the unimolecular dissociation rate constant as a function of continuous-wave CO(2) laser intensity. Following a short ( approximately 1 s) induction period, CO(2) laser irradiation produces an essentially blackbody internal energy distribution, whose 'temperature' varies inversely with laser intensity. The only currently available method for measuring such activation energies is blackbody infrared radiative dissociation (BIRD). Compared with BIRD, FRAGMENT: (a) eliminates the need to heat the surrounding ion trap and vacuum chamber to each of several temperatures (each requiring hours for temperature equilibration); (b) offers a three-fold wider range of effective blackbody temperature; and (c) extends the range of applications to include initially cold ions (e.g., gas-phase H/D exchange). Our FRAGMENT-determined activation energy for dissociation of protonated bradykinin, 1.2 +/- 0.1 eV, agrees within experimental error to the value, 1.3 +/- 0.1 eV, previously reported by Williams et al. from BIRD experiments. Copyright 1999 John Wiley & Sons, Ltd.