Mark Russell Dickinson

Thermal effects of the Er:YAG laser on a simulated dental pulp: a quantitative evaluation of the effects of a water spray

OBJECTIVES To quantify the temperature increments in a simulated dental pulp following irradiation with an Er:YAG laser, and to compare those increments when the laser is applied with and without water spray. METHODS Two cavities were prepared on either the buccal or lingual aspect of sound extracted teeth using the laser. One cavity was prepared with water spray, the other without and the order of preparation randomised. Identical preparation parameters were used for both cavities. Temperature increments were measured in the pulp chamber using a calibrated thermocouple and a novel pulp simulant. RESULTS Maximum increments were 4.0 degrees C (water) and 24.7 degrees C (no water). Water was shown to be highly significant in reducing the overall temperature increments in all cases (p<0.001; paired t-test). None of the samples prepared up to a maximum of 135 J cumulative energy prepared with water spray exceeded that threshold at which pulpal damage can be considered to occur. Only 25% of those prepared without water spray remained below this threshold. DISCUSSION Extrapolation of the figures suggests probably tolerable limits of continuous laser irradiation with water in excess to 160 J. With the incorporation of small breaks in the continuity of laser irradiation that occur in the in vivo situation, the cumulative energy dose tolerated by the pulp should far exceed these figures. CONCLUSIONS The Er:YAG laser must be used in conjunction with water during cavity preparation. As such it should be considered as an effective tool for clinical use based on predicted pulpal responses to thermal stimuli.

Laser manipulation in liquid crystals: an approach to microfluidics and micromachines

Laser trapping of particles in three dimensions can occur as a result of the refraction of strongly focused light through micrometre-sized particles. The use of this effect to produce laser tweezers is extremely common in fields such as biology, but it is only relatively recently that the technique has been applied to liquid crystals (LCs). The possibilities are exciting: droplets of LCs can be trapped, moved and rotated in an isotropic fluid medium, or both particles and defects can be trapped and manipulated within a liquid crystalline medium. This paper considers both the possibilities. The mechanism of transfer of optical angular momentum from circularly polarized light to small droplets of nematic LCs is described. Further, it is shown that droplets of chiral LCs can be made to rotate when illuminated with linearly polarized light and possible mechanisms are discussed. The trapping and manipulation of micrometre-sized particles in an aligned LC medium is used to provide a measure of local shear viscosity coefficients and a unique test of theory at low Ericksen number in LCs.

The first galaxies: structure and stellar populations

The Hubble deep fields (HDFs) continue to be a valuable resource for studying the distant Universe, particularly at z > 2 where their comoving volume becomes large enough to encompass several hundred L* galaxies or their progenitors. Here, I present recent results from a near-infrared (NIR) imaging survey of the HDFnorth with the Near Infrared Camera and Multi–Object Spectrograph (NICMOS), which provides structural and photometric information in the optical rest frame (λλ24000–5500 Å) for hundreds of ‘ordinary’ galaxies at 2 < z < 3, and which offers the means to search for still–more–distant objects at z ≫ 5. Lyman–break galaxies (LBGs) at 2 < z < 3 are compact and often irregular in the NICMOS images; ordinary Hubble sequence spirals and ellipticals seem to be largely absent at these redshifts, and apparently reached maturity at 1 < z < 2. The LBGs have ultraviolet (UV)-optical spectral energy distributions like those of local starburst galaxies. Population synthesis models suggest typical ages of a few x 108 years and moderate UV extinction (ca .1.2 mag at 1700 Å), but the constraints are fairly weak and there may be considerable variety. Considering an NIR selected galaxy sample, there is little evidence for a significant number of galaxies at z ∽ 3 that have been missed by UV–based Lyman–break selection. Using the well–characterized z ∽ 3 galaxy population as a point of reference, I consider LBG candidates at 4.5 < z < 9, as well as one remarkable object that might (or might not) be an LBG at z > 12. The space density of UV–bright galaxies in the HDF appears to thin out toward larger redshifts, although surface–brightness selection effects may play an important role.

Laser-tissue interaction with a continuous wave 3-μm fibre laser: Preliminary studies with soft tissue

Lasers operating at wavelengths in the mid‐infrared region have become increasingly popular for applications in areas of surgery and medicine. Advances in fibre laser technology have introduced a highly efficient, compact, diode‐pumped source operating at around the 3‐μm wavelength. This study examines the effects of this recently developed laser on soft biological tissue.

Erbium-YAG and holmium-YAG laser ablation of bone

Results are presented for the latent heat of ablation of bone using an erbium-YAG laser operating at 2.9Μm, and a holmium-YAG laser operating at 2.1Μm. The values are 8.2±1.0 kJ cm−3 and 18±2.0 kJ cm−3, respectively. Secondary damage to surrounding tissue is found to extend approximately 5Μm with the erbium laser and is greatly increased to 80Μm with significant charring in the case of holmium. These secondary damage zones are much smaller than those produced by the CO2 laser.

Mechanisms of optical angular momentum transfer to nematic liquid crystalline droplets.

A detailed study is presented that evaluates the relative importance of wave plate behavior, scattering processes and absorption phenomena in transferring optical torque from circularly polarized light to optically trapped nematic droplets. A wide range of parameters is considered: droplet diameters between 1 and 15 μm, birefringence values from 0.15 to 0.26 and trapping beam powers from 50 mW to 400 mW. Wave plate behavior is verified through the dependence of torque on droplet diameter and material birefringence. The dependence of the magnitude of the torque on material birefringence confirms the additional importance of the scattering mechanism. Absorption processes are found to be negligible.

Osseointegration of titanium metal implants in erbium-YAG laser-prepared bone.

Titanium screws were implanted in rat calvarial defects of identical size using either a laser or bur. The aims of this study were to determine whether the screws were able to osseointegrate in a laser-prepared bone defect and to compare the pattern of bone healing around these screws. The optimal laser settings to produce a 0.7-mm-diameter hole in the rat calvaria were determined. A 0.7-mm-diameter hole was prepared on the left calvaria with the erbium-YAG laser to receive a 1-mm-diameter self-threading titanium screw. Each animal also received a 0.7-mm-diameter hole prepared on the right calvaria with a conventional metal bur, and a 1-mm-diameter self-threading screw implant was placed. Rats were killed humanely either 3 weeks or 3 months after surgery, and the skulls were processed in paraffin wax for histological analysis. Laser-prepared defects: At 3 weeks, the screw was surrounded by vital woven bone. The dura mater was perforated, and cystic change was present in the underlying brain tissue. There was active bone formation adjacent to the screw surface, deposited on a thin zone of necrotic bone. At 3 months, the screws were osseointegrated, and the brain tissue was healed by gliosis. Bur-prepared defects: At 3 weeks, there was extensive remodeling around the prepared defect. The dura mater was intact, and there was no damage to the underlying brain. At 3 months, the screws were successfully osseointegrated with bone adjacent to the screw. Osseointegration of titanium screws can be achieved using an erbium-YAG laser to prepare the implant bed.

Laser–tissue interaction with a high-power 2-μm fiber laser: Preliminary studies with soft tissue

Recent developments in fiber laser technology have introduced highly efficient, compact sources with high output beam quality. The first laser–tissue interaction studies with a high‐power 2‐μm fiber laser were conducted.

Particle sizing and flow measurement using self-mixing interferometry with a laser diode

New applications are presented for self-mixing interferometry, based on optical scattering, feedback and self-mixing in laser diodes. The self-mixing interferometry method has been developed for the determination of sub-micron particle sizes and for the measurement of flows in narrow diameter tubes. The rate equations for laser diodes subjected to frequency-shifted feedback are reviewed and extended to include a normalized frequency distribution to be characterized by the backscattered light spectrum. Experimental investigations are presented for (a) particle sizing using polystyrene particles in water from 0.02 to 0.20 mu m in diameter and (b) for transverse velocity profile determination using a 1.4 mm diameter tube with flow rates ranging between 10 and 100 ml h(-1). The observed frequency band increases for smaller particles when observing Brownian motion and for higher flow rates when observing flowing liquids. This experimental technique is inherently simple and low cost. Further potential applications of this technique include blood flow measurement in medicine, electrophoresis investigations in biology and particle characterization in process engineering and chemistry.

Effect of target biological tissue and choice of light source on penetration depth and resolution in optical coherence tomography

The effectiveness of an optical coherence tomography (OCT) system depends largely on the light source chosen. Published data on the optical properties of tissues are used to quantify the exponential attenuation of broadband light on transport through tissue. The effective attenuation coefficient is taken to be the sum of the absorption and scattering coefficients. This is used to demonstrate the effect on the spectra of a wide range of published OCT sources and the change in system resolution induced, and hence to comment on the suitability of different sources for OCT. The tissues studied include skin dermis, liver, and gallbladder. Sources at higher wavelengths are shown to be capable of high-resolution OCT imaging at greater depths. Titanium:sapphire lasers would be most suited for high-resolution OCT over comparatively shallow depths into tissue. For lower-resolution applications of OCT, a semiconductor optical amplifier and ytterbium fiber sources have better powers and bandwidths than superluminescent diodes. The resolution of OCT systems is not reduced significantly with imaging depth.