John K. McIver

The role of thermal conductivity in the pulsed laser damage sensitivity of optical thin films

Abstract Pulsed-laser-induced damage of optical thin films is, in general, initiated by the absorption of laser radiation by imperfections in the films or at interfaces between film layers and/or the substrate. A heat flow analysis of this process stresses the importance that the thermal conductivity of both the thin film host and the substrate play in establishing the laser-induced damage threshold. Unfortunately, recent work which will be reviewed in this presentation indicates that the thermal conductivity of thin films can be several orders of magnitude lower than that of a corresponding material in bulk form as a consequence of the film structure which results principally from the deposition process. The importance of thermal conductivity will be related to parameters such as absorption mechanisms, film materials, composition and other variables. Its implication for the ultimate optical strength of materials and the direction in which thin film research and processing should proceed will be highlighted.

Depolarizing channel as a completely positive map with memory

The prevailing description for dissipative quantum dynamics is given by the Lindblad form of a Markovian master equation, used under the assumption that memory effects are negligible. However, in certain physical situations, the master equation is essentially of a non-Markovian nature. This paper examines master equations that possess a memory kernel, leading to a replacement of white noise by colored noise. The conditions under which this leads to a completely positive, trace-preserving map are discussed for an exponential memory kernel. A physical model that possesses such an exponential memory kernel is presented. This model contains a classical, fluctuating environment based on random telegraph signal stochastic variables.

Quantum Markov channels for qubits

We examine stochastic maps in the context of quantum optics. Making use of the master equation, the damping basis, and the Bloch picture we calculate a nonunital, completely positive, trace-preserving map with unequal damping eigenvalues. This results in what we call the squeezed vacuum channel. A geometrical picture of the effect of stochastic noise on the set of pure state qubit density operators is provided. Finally, we study the capacity of the squeezed vacuum channel to transmit quantum information and to distribute Einstein-Podolsky-Rosen states.

The Role Of Thermal Conductivity In The Pulsed Laser Damage Sensitivity Of Optical Thin Films

Pulsed laser induced damage of optical thin films is, in general, initiated by the absorption of laser radiation by imperfections in the films or at interfaces between film layers and/or the substrate. A heat flow analysis of this process stresses the importance that the thermal conductivity of both the thin film host and that of the substrate play in establishing the laser-induced damage threshold. Unfortunately, recent work which will be reviewed in this presentation, indicates that the thermal conductivity of thin films can be several orders of magnitude lower than that of a corresponding material in bulk form, as a consequence of the film structure resulting principally from the deposition process. The importance of thermal conductivity will be compared to parameters such as absorption mechanisms, film materials, composition, and other variables. Its implication for the ultimate optical strength of materials and the direction in which thin film research and processing should proceed will be highlighted.

Pulsed laser damage in thin film coatings: Fluorides and oxides

Abstract The absorbing-inclusion model of pulsed-laser-induced damage in thin films is reviewed and generalized. A solution is derived which can accommodate arbitrary absorption functions and pulse shapes. It is shown that this model applies to regions which were previously excluded. This occurs when the wavelength dependence of Mie absorption by a small inclusion in a thin film is taken into account. When this is done an additional wavelength dependence of the imaginary index of refraction is observed. That is, the localized absorbing region exhibits an intrinsic absorption wavelength dependence. The model reveals that fluoride film damage is thermally dominated and dependent on the thermal properties of the film materials with absorption that appears generally to be material independent but wavelength dependent. Furthermore, it also shows that thermal diffusion plays an important part in oxide film damage; however, there is apparently some material dependence not accounted for in the present model. It is postulated that this material dependence enters through the absorption process. As might be expected, absorption in oxide films is also shown to be wavelength dependent.

The effect of pump bandwidth, lens focal length and lens focal point location on Stimulated Brillouin Scattering threshold and reflectivity

Abstract The dependence of the Stimulated Brillouin Scattering (SBS) threshold on the focal length of the lens has been measured for two extreme limits of the pump bandwidth. For a single mode pump with a long coherence length the threshold is found to be independent of the focal length while for a broadband pump the threshold is found to vary quadratically with the focal length. The measurements also show that the location of the focal point in the SBS cell can seriously affect the reflectivity when the pump energy is small.

Investigation of the state-to-state rotational relaxation rate constants for carbon monoxide (CO) using infrared double resonance

State-to-state rotational relaxation of carbon monoxide (CO) has been studied using an ir double resonance technique. Individual rotational lines of the (2-0) vibrational overtone band were pumped by a pulsed tunable ir laser and the subsequent evolution of the v=2 rotational population distribution was monitored by the absorption of a tunable cw ir laser via the (3-2) band transitions. Both the excitation and probe lasers were linearly polarized, with linewidths that were narrower than the CO Doppler width. Consequently, alignment and velocity relaxation effects were observed in these measurements. A data set consisting of 54 time-dependent rotational state population profiles was acquired. The full CO–CO rotational relaxation matrix, which consists of state-to-state rate constants for rotational levels up to J=29, was deduced from computer simulations of the data. Scaling and fitting laws were used to provide parametric representations of the rate constants. The three most common models, modified exponential gap, statistical polynomial-exponential gap (SPEG), and energy corrected sudden with exponential-power gap (ECS-EP) were investigated. We concluded that the SPEG law best reproduced the CO–CO rotational energy transfer data. A propensity to preserve the CO parity in rotational energy transfer was observed for collisions where the amount of energy transferred was small. Hence even ΔJ processes were favored for transitions between levels with low J values. For near-single collisions events a correlation was noted between the amount of rotational energy transferred and the degree of velocity distribution relaxation. This correlation yielded insights regarding the energy transfer dynamics.State-to-state rotational relaxation of carbon monoxide (CO) has been studied using an ir double resonance technique. Individual rotational lines of the (2-0) vibrational overtone band were pumped by a pulsed tunable ir laser and the subsequent evolution of the v=2 rotational population distribution was monitored by the absorption of a tunable cw ir laser via the (3-2) band transitions. Both the excitation and probe lasers were linearly polarized, with linewidths that were narrower than the CO Doppler width. Consequently, alignment and velocity relaxation effects were observed in these measurements. A data set consisting of 54 time-dependent rotational state population profiles was acquired. The full CO–CO rotational relaxation matrix, which consists of state-to-state rate constants for rotational levels up to J=29, was deduced from computer simulations of the data. Scaling and fitting laws were used to provide parametric representations of the rate constants. The three most common models, modified expone...

Femtosecond pulse damage and predamage behavior of dielectric thin films

The damage behavior of five different oxide dielectric thin films (Ta2O5, TiO2, Al2O3, HfO2, and SiO2) has been investigated with ultrashort laser pulses with durations from 25 fs to 1 ps. At all pulse durations the damage threshold is well defined and scales with the bandgap energy of the material. The damage behavior can be described with a phenomenological model taking into account multi-photon excitation, impact ionization, and electron relaxation. The temporal evolution of the dielectric constant of the film following the excitation with pulses below the damage threshold has been measured with time-resolved pump-probe spectroscopy. The complex dielectric constant was retrieved from transient reflection and transmission data.

Infinite‐dimensional Estabrook–Wahlquist prolongations for the sine‐Gordon equation

We are looking for the universal covering algebra for all symmetries of a given partial differential equation (PDE), using the sine‐Gordon equation as a typical example for a nonevolution equation. For nonevolution equations, Estabrook–Wahlquist prolongation structures for nonlocal symmetries depend on the choice of a specific subideal of the contact module to define the PDE. For each inequivalent such choice we determine the most general solution of the prolongation equations, as subalgebras of the (infinite‐dimensional) algebra of all vector fields over the space of nonlocal variables associated with the PDE, in the style of Vinogradov covering spaces. We show explicitly how previously known prolongation structures, known to lie within the Kac–Moody algebra, A(1)1, are special cases of these general solutions, although we are unable to identify the most general solutions with previously studied algebras. We show the existence of gauge transformations between prolongation structures, viewed as determinin...

Frequency tuning of a CW atomic iodine laser via the Zeeman effect

A continuously operating, C/sub 3/F/sub 7/I photolytic 1.315- mu m atomic iodine laser has been used to make the first precise observations of frequency tuning of an atomic iodine laser by means of the Zeeman effect. Application of a uniform magnetic field to the gain region of the photolytic iodine laser causes the laser to operate at different frequencies as a function of the strength of the applied field and the polarization of the laser. With the light polarized perpendicular to the applied magnetic field by means of Brewster output windows, the laser could be tuned to frequencies near the 3-4, 2-2, and 3-3 zero-field transitions of the hyperfine spectrum of atomic iodine. With the light polarized parallel to the applied magnetic field the laser could be tuned to two frequencies bracketing the 3-4 zero-field transition and one frequency near the 2-2 transition. Measurements show close agreement between the observed frequency behavior and theoretical models. >