Post-recombination effects in confined gases photoionized at megahertz repetition rates.

Recombination-driven acoustic pulses and heating in a photoionized gas transiently alter its refractive index. Slow thermal dissipation can cause substantial heat accumulation and impair the performance and stability of gas-based laser systems operating at strong-field intensities and megahertz repetition rates. Here we study this effect by probing the pulse-by-pulse buildup of refractive index changes in gases spatially confined inside a capillary. A high-power repetition-rate-tunable femtosecond laser photoionizes the gas at its free-space focus, while a transverse-propagating probe laser interferometrically monitors the resulting time-dependent changes in refractive index. The system allows convenient exploration of the nonlinear regimes used to temporally compress pulses with durations in the ∼30 to ∼300 fs range. We observe thermal gas-density depressions, milliseconds in duration, that saturate to a level that depends on the peak intensity and repetition rate of the pulses, in good agreement with numerical modelling. The dynamics are independently confirmed by measuring the mean speed-of-sound across the capillary core, allowing us to infer that the temperature in the gas can exceed 1000 K. Finally, we explore several strategies for mitigating these effects and improving the stability of gas-based high-power laser systems at high repetition rates.

Efficient single-cycle pulse compression of an ytterbium fiber laser at 10†MHz repetition rate.

Over the past years, ultrafast lasers with average powers in the 100 W range have become a mature technology, with a multitude of applications in science and technology. Nonlinear temporal compression of these lasers to few- or even single-cycle duration is often essential, yet still hard to achieve, in particular at high repetition rates. Here we report a two-stage system for compressing pulses from a 1030 nm ytterbium fiber laser to single-cycle durations with 5 µJ output pulse energy at 9.6 MHz repetition rate. In the first stage, the laser pulses are compressed from 340 to 25 fs by spectral broadening in a krypton-filled single-ring photonic crystal fiber (SR-PCF), subsequent phase compensation being achieved with chirped mirrors. In the second stage, the pulses are further compressed to single-cycle duration by soliton-effect self-compression in a neon-filled SR-PCF. We estimate a pulse duration of ∼3.4 fs at the fiber output by numerically back-propagating the measured pulses. Finally, we directly measured a pulse duration of 3.8 fs (1.25 optical cycles) after compensating (using chirped mirrors) the dispersion introduced by the optical elements after the fiber, more than 50% of the total pulse energy being in the main peak. The system can produce compressed pulses with peak powers >0.6 GW and a total transmission exceeding 66%.

Spatio-temporal measurement of ionization-induced modal index changes in gas-filled PCF by prism-assisted side-coupling.

We report the use of prism-assisted side-coupling to investigate the spatio-temporal dynamics of photoionization in an Ar-filled hollow-core photonic crystal fiber. By launching four different LP core modes we are able to probe temporal and spatial changes in the modal refractive index on timescales from a few hundred picoseconds to several hundred microseconds after the ionization event. We experimentally analyze the underlying gas density waves and find good agreement with quantitative and qualitative hydrodynamic predictions. Moreover, we observe periodic modulations in the MHz-range lasting for a few microseconds, indicating nanometer-scale vibrations of the fiber structure, driven by gas density waves.

Optomechanical nonlinearity in dual-nanoweb structure suspended inside capillary fiber.

A novel kind of nanostructured optical fiber, displaying an extremely high and optically broadband optomechanical nonlinearity, is presented. It comprises two closely spaced ultrathin glass membranes (webs) suspended in air and attached to the inner walls of a glass fiber capillary. Light guided in this dual-web structure can exert attractive or repulsive pressure on the webs, causing them to be pushed together or pulled apart. The elastic deflection of the webs is, in turn, coupled to the electromagnetic field distribution and results in a change in the effective refractive index within the fiber. Employing a pump-probe technique in an interferometric setup, optomechanically induced refractive index changes more than 10^{4} times larger than the Kerr effect are detected. Theoretical estimates of the optomechanical nonlinearity agree well with the experimental results. The dual-web fiber combines the sensitivity of a microoptomechanical device with the versatility of an optical fiber and could trigger new developments in the fields of nonlinear optics, optical metrology, and sensing.

Surgical management of patients on warfarin sodium.

PURPOSE: Management of anticoagulated patients has changed significantly over the past 10 years. The change occurred after the introduction of the international normalized ratio (INR) in 1983. This method of reporting prothrombin time for anticoagulated patients has resulted in a decrease in the level at which hematologists and cardiologists keep their patients anticoagulated. Currently, patients are anticoagulated less for the successful prevention of thromboemboli. Recent recommendations are to keep patients anticoagulated to an INR no greater than 3.5. It has been proposed that the extraction of teeth can be performed with INRs of 4 or less. Therefore, the current trend is to maintain patients on their anticoagulation regimens without altering their warfarin dosages. CONCLUSION: With proper local measures, teeth can be extracted safely and the development of thromboemboli in high-risk patients can still be prevented. However, with procedures having a high risk of bleeding, warfarin dosage may need to be modified.

24-hour shear bond strength of metal orthodontic brackets bonded to porcelain using various adhesion promoters.

Clinically, orthodontists are often faced with the difficulties of bonding to porcelain crowns, veneers, or bridges. Traditional methods of dealing with this are often time-consuming and generally esthetically unacceptable to patients. Current interest involves the use of organosilane primers with or without NTG-GMA and BPDM resins to aid in chemically bonding porcelain with traditional orthodontic adhesives. This study compared the bond strength of three types of adhesion promoters: Ormco Porcelain Primer (OR) (Ormco Corp., Glendora, Calif.), All-Bond2 (AB2) (Bisco Dental Products, Inc., Itasca, Ill.), and Scotchprime Ceramic Primer (SP) (3M, St. Paul, Minn.) with two orthodontic adhesives: Phase II (Reliance, Inc., Itasca, Ill.) and Rely-a-bond (Reliance, Inc., Itasca, Ill.). Eighty ceramometal samples were fabricated and hydrated for 1 week in distilled water before bonding. Next, the samples were etched with 2.5 % HF acid for 90 seconds, rinsed and upper central incisor metal orthodontic brackets were bonded with the various primers and adhesives. After 24 hours they were sheared off with an MTS machine at at rate of 0.5 mm/min, and the bond strength was measured. The mean shear bond strengths (MPa) with Phase II were as follows; Control 0.44 (sigma = 0.22), AB2 8.40 (sigma = 3.61), OR 13.31 (sigma = 5.79), SP 13.53 (sigma = 3.34). With Rely-a-bond, the shear bond strengths were Control 0.41 (sigma = 0.67), AB2 4.34 (sigma = 1.88), OR 9.73 (sigma = 4.58), and SP 12.40 (sigma = 3.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Shear/peel bond strength of repositioned ceramic brackets.

Improper orthodontic bracket position may necessitate bracket removal and rebonding to establish correct bracket position. This procedure is necessary to use efficient orthodontic mechanics. The purpose of the study was to investigate (1) the amount of bonding resin remaining on single crystal bracket bases following electrothermal debonding, and (2) the bond strength of rebonded single crystal ceramic brackets under different treatment conditions. The bases of debonded, single crystal ceramic brackets (n = 100) were inspected for resin, classified with an adaptation of the adhesive remnant index (ARI), and evenly assigned to four experimental groups (n = 25). Groups were (1) silane coupling agent, (2) heat plus silane coupling agent, (3) hydrofluoric acid plus silane coupling agent, and (4) heat plus hydrofluoric acid plus silane coupling agent. An additional group of brackets not previously bonded was used as the control (n = 25). The brackets were bonded to 125 fresh bovine teeth. A force was applied 1 mm from the bracket-resin interface by a testing machine. The force measured in this experiment was shear/peel and the ratio of shear to peel was 0.53. The AI index showed 79% of the brackets had no resin on their bases. The shear/peel bond strnegth was significantly greater for the control group than all other groups (P < 0.01). Treatment of electrothermally debonded ceramic brackets with silane or heat plus silane resulted in bond strength greater than 9 MPa. The use of hydrofluoric acid significantly reduced the bond strength below 2 MPa.

Blood lead levels in children from lower socioeconomic communities in Denver, Colorado.

We measured blood lead levels and inquired about environmental exposure pathways in 443 children aged 6 mo to 6 y in four lower socioeconomic Denver, Colorado, neighborhoods. Two neighborhoods were adjacent to a cadmium refinery. Eight percent of the children had blood lead levels > or = 10 micrograms/dl and 45% were below the detection limit of 4 micrograms/dl. Statistical analyses specific to the problem of below-detection values (i.e., dichotomization of variables, the probability plot method of estimation, and the bootstrap estimate of the standard error) were conducted to detect neighborhood differences. A child who lived in Globeville, the neighborhood surrounding the refinery, had a slightly higher probability of having a blood lead level > or = 5 micrograms/dl. Yet, the results as a whole documented the striking decline in blood lead levels in urban children after the deleading of gasoline.

Non-analytic problems in detecting arsenic and cadmium in children living near a cadmium refinery in Denver, Colorado.

The aim of the present study was to determine urinary arsenic (N = 322) and cadmium (N = 366) levels in children aged six months to six years who live near a working cadmium refinery and to compare their values with those of children from comparison neighborhoods. A questionnaire designed to identify exposure pathways was administered to the parents. There were unexpected problems in the study. Eighty-four percent of the arsenic samples were below the detection limit of 10 micrograms/l and summary statistics could not be calculated. Urinary arsenic and cadmium values could not be standardized for volume and concentration of urine because a large proportion of the samples had very low creatinine values. The original round of cadmium testing was afflicted with contamination problems, possibly due to the mishandling of pediatric urine bags by the parents during the collection procedure. A retest for cadmium levels under clinical conditions showed lower cadmium levels, all but two were below the detection limit. While biological monitoring of exposure to metals can be undertaken indirectly by measuring the concentration of the metals in urine, the analyses in this study were complicated by (1) not using more sensitive analytical tests for arsenic and cadmium determination, (2) not being able to standardize children's urinary values with creatinine, and (3) allowing in-home urine collection.