P.E. Dyer

Dynamics of excimer laser ablation of superconductors in an oxygen environment

The plume dynamics for excimer laser ablation of Y‐Ba‐Cu‐O in an O2 atmosphere have been studied using streak photography and spectroscopy. At pressures ≳1 mbar the expansion resembles a blast wave driven by the ablation products with mixing and reaction at the contact surface. A simple model for the plume range is developed which shows agreement with experiments.

Analysis of grating formation with excimer laser irradiated phase masks

Abstract Excimer laser irradiated phase masks provide a convenient and effective method for writing micron-scale gratings for optoelectronic device applications. Here we analyze the interference field produced by a periodic mask and assess the near-field energy density and fluence distribution for varying degrees of order content when exposed using an excimer laser with finite spatial and temporal coherence. Results are compared with experimental findings for gratings produced on ablated polymers and in optical fibres.

Periodic surface structures in the excimer laser ablative etching of polymers

Periodic surface structures have been observed on organic polymers when photoablated using low coherence excimer lasers. Both unpolarized and linearly polarized radiation at 248 and 308 nm produce well ordered, micron‐scale structures over dimensions greatly exceeding the coherence area of the laser at the surface. Surface scattered wave effects appear to explain these structures.

An investigation of laser ablation and deposition of Y-Ba-Cu-O in an oxygen environment

Abstract Streak photography, visible-UV emission spectroscopy, infrared emission and time-integrated photography have been used to study the laser ablation of Y-Ba-Cu-O in an oxygen environment. At O 2 pressures ≥ 1 mbar the dynamics of the plume become relatively complex, with the expansion resembling a blast-wave with strong mixing and reaction at the contact surface. The range of the ablated material has been measured and compared with an adiabatic expansion model which gives a reasonable description of the results. Preliminary experiments suggest that the quality of deposited films is critically dependent upon the relative position of substrates with respect to the plume range.

Excimer laser ablation of polymers and glasses for grating fabrication

In this paper we describe studies carried out related to grating formation for optoelectronics applications using 248 nm and 193 nm excimer lasers. These include basic studies of UV laser induced incubation and ablation of various glasses, and an evaluation of laser-induced-periodic-surface-structures (LIPSS) and UV holographic techniques based on a phase mask for forming gratings on polymers and in fibres. Modelling and experiments show that the use of excimer laser illuminated phase masks in a contact printing mode or Talbot reimaging arrangement provide a particularly effective approach to forming submicron period gratings.

Analysis and application of a 01 order Talbot interferometer for 193 nm laser grating formation

Abstract We report the analysis, design and application of a Talbot interferometer in which the zero and first order beam from a grating are recombined. This interferometer produces fringes of the same period, d, as the master grating or phase mask rather than d 2 when the ±1 orders are employed. Experiments using the 0 1 Talbot interferometer with 193 nm ArF laser illumination to write gratings on polymers by ablation and photosensitive Bragg gratings in fibres are reported.

IONIZATION MODELLING OF MATRIX MOLECULES IN ULTRAVIOLET MATRIX-ASSISTED LASER DESORPTION/IONIZATION

A scheme of matrix ionization in UV matrix-assisted laser desorption/ionization is presented. Excitation to an upper lying electronic singlet state by the absorption of two photons and subsequent thermionic emission by vibronic coupling is proposed as the primary route for the formation of unprotonated matrix ions within the desorbed gas-phase plume. Under conditions where matrix/matrix collisions are sufficiently frequent to establish a thermal distribution of energy amongst the matrix vibrational states, thermionic emission is assumed to proceed as a unimolecular reaction that draws upon the vibrational energy reservoir of individual molecules. The dominant free-electron loss mechanism is assumed to be electron/neutral attachment, thus providing an equal number of positive and negative matrix ions whilst maintaining the overall ionization level. Predictions based on excited-state thermionic emission and a photoionization model shows that the former provides a considerably better agreement with experimentally determined ion yield vs. fluence characteristics of matrices. For example, for a 3 ns (full width at half maximum) Gaussian N2 laser (337 nm) incident fluence (F) of 15 mJ cm−2, the predicted fractional ion yield is ∼9.8 × 10−4 scaling as F8.5.

Transient photoacoustic studies of laser tissue ablation

Measurements of thermoelastic and ablative stress transients generated by pulsed lasers in tissue samples have been made using fast time-response (nanosecond) piezoelectric film transducers. Studies of the ablation of cornea using excimer and CO2 lasers, and of vascular tissue using excimer, dye and solid-state lasers are described. It is shown that useful information on the pulsed laser interaction can be gained from these measurements.

Fast time-response photoacoustic studies and modelling of KrF laser ablated YBa2Cu3O7

Abstract The transient pressure produced at the surface of YBa2Cu3O7 samples ablated using a KrF laser has been measured with nanosecond time resolution. It is observed that ablation commences within the ∼ 10 ns laser pulse and leads to large amplitude stress waves propagating in the sample. The results are analysed using a vaporization model and it is tentatively concluded that relatively high average mass species are produced in the initial ablation step. A scheme for explosive removal to account for this behaviour is put forward.

Study and analysis of submicron‐period grating formation on polymers ablated using a KrF laser irradiated phase mask

Submicron‐period gratings have been formed in polyethylene terephthalate and polyimide films using a KrF laser irradiated, zero suppressed, phase mask with a period d=533 nm. The imprinted grating has a dominant period of d rather than d/2, a result which is shown to be due to recording the pattern by a threshold surface ablation process.