Brendan Doggett

Langmuir probe characterization of laser ablation plasmas

For laser ablation plumes that are significantly ionized, Langmuir probes have proved to be a useful tool for measuring the plume shape, ion energy distribution, and electron temperature. Typically in laser ablation plasmas the flow velocity is supersonic, which complicates the interpretation of the current-voltage probe characteristic. In this paper we describe some recent developments on the application of Langmuir probes for laser ablation plume diagnosis. We have investigated the behavior of the probe when it is orientated perpendicular, and parallel, to the plasma flow, and show how an analytical model developed for plasma immersion ion implantation, can quantitatively describe the variation of the ion current with probe bias for the case when the plasma flow is along the probe surface. The ion signal recorded by a probe in the parallel position is proportional to the ion density and the square root of the bias voltage. It is shown that the current varies as mi−1/2 so that by comparing the ion signal...

Expansion dynamics of laser produced plasma

We consider the applicability of the isentropic, adiabatic gas dynamical model of plume expansion for laser ablation in vacuum. We show that the model can be applied to ionized plumes and estimate the upper electron temperature limit on the applicability of the isentropic approximation. The model predictions are compared with Langmuir ion probe measurements and deposition profiles obtained for excimer laser ablation of silver.

Femtosecond ultraviolet laser ablation of silver and comparison with nanosecond ablation

The ablation plume dynamics arising from ablation of silver with a 500 fs, 248 nm laser at ∼2 J cm−2 has been studied using angle-resolved Langmuir ion probe and thin film deposition techniques. For the same laser fluence, the time-of-flight ion signals from femtosecond and nanosecond laser ablation are similar; both show a singly peaked time-of-flight distribution. The angular distribution of ion emission and the deposition are well described by the adiabatic and isentropic model of plume expansion, though distributions for femtosecond ablation are significantly narrower. In this laser fluence regime, the energy efficiency of mass ablation is higher for femtosecond pulses than for nanosecond pulses, but the ion production efficiency is lower.

Langmuir probe diagnosis of laser ablation plasmas

For laser ablation plumes which are significantly ionised, Langmuir probes have proved to be a relatively simple and inexpensive tool for measuring the plume shape, ion energy distribution and electron temperature. In this paper we describe some recent work on the development of Langmuir probes for laser ablation plume diagnosis. Typically in laser ablation plasma the flow velocity is supersonic, which complicates the interpretation of the I-V probe characteristic. We describe some new work on the behaviour of a flat probe lying parallel to the plasma flow. We also compare our measurements with theoretical models of laser ablation plume expansion and draw some conclusions as to which model is more appropriate for the low temperature plasmas which arise in pulsed laser deposition.

Langmuir probe investigation of plasma expansion in femto-and picosecond laser ablation of selected metals

A time resolving Langmuir probe was used to study the plasma plumes produced by the ablation of Ag, Ni and Al targets with laser pulses of different pulse durations (0.2 − 10 ps). These metals were chosen because their electron-phonon relaxation times, τe-ph, are of the order of the pulse durations used. The time of flight (TOF) signals have been used to establish the threshold fluences and plume expansion dynamics of the laser produced plasmas for the different pulse durations. The angular dependence of the magnitude of the ion flux was analysed on the basis of Anisimovs self-similar model of the plasma expansion. The amount of charge in the ablation plume is compared for the different pulse durations.

Langmuir probe study of plasma expansion in femtosecond pulsed laser ablation of silver

A time-resolving Langmuir probe has been used to study the plasma plumes produced by ablation of silver with 200 femtosecond laser pulses at fluences of 1-12 J cm-2 at a central wavelength of 775 nm. Initial results have shown that surface contamination, and subsequent recontamination, can significantly influence the time of flight (TOF) signals obtained using the Langmuir probes. Surface conditioning techniques have been developed to overcome these influences. The TOF signals have been used to establish that the threshold fluence for the laser produced plasma in silver, under the present operating conditions, occurs at 1.04 J cm-2. The angular dependence of the magnitude of the ion yields and energies, at the time when the ion flux is maximized, agree with the predictions of Anisimov’s self-similar isentropic model of the plasma expansion.

Model for laser ablation plume expansion in gas

A simple model for laser ablation plasma plume expansion in a background gas is presented. The expanding plume is approximated by a semi-ellipsoidal shell of infinitesimal thickness and with a mass corresponding to the amount of background gas that has been snowploughed by the shell as it expands. Furthermore, the internal pressure of the plume and external pressure of the background gas are taken into account. Electrical probe measurements of ns laser ablation of Ag were used to map out the angular dependence of plume expansion in argon at various pressures and the results compared with model predictions.

Langmuir probe study of laser ablation plume dynamics

For many applications of pulsed laser ablation it is necessary to have an understanding of the expansion dynamics of the ablation plume both in vacuum and in low pressure gases. Knowledge of the ablation plume hydrodynamics can also contribute to the understanding of the laser ablation process. In this paper we will consider some of the existing theoretical models of laser ablation plume expansion and draw some conclusions as to which model is most appropriate for the low temperature plasmas which arise in pulsed laser deposition. For ablation plumes which are significantly ionised, Langmuir probes have proved to be a relatively simple and inexpensive tool for measuring the plume shape, ion energy distribution and electron temperature. We describe some recent work on the development of Langmuir probes for laser ablation plume diagnosis. Typically in laser ablation plasma the flow velocity is supersonic, which complicates the interpretation of the I-V probe characteristic. We describe some new work on the behaviour of a flat probe lying parallel to the plasma flow. For nanosecond ablation of silver, we also show how a planar Langmuir probe can be used to obtain a fairly comprehensive description of the expansion dynamics of the ionised part of the ablation plume, including plume shape, ion energy distribution and electron temperature.

Interaction of surface plasmons with CdTe quantum dot excitons

A 5-fold enhancement in the luminescence of CdTe nanocrystal quantum dots (QDs) is observed when they are placed in proximity to a nanostructured Au film deposited by pulsed laser deposition technique. No enhancement is observed with a nanostructured Ag film. The enhancement is due to the interaction of the QDs excitons with the localized surface plasmons (LSP). The Au surface plasmon (SP) frequency is closer to the QDs emission frequency than Ag LSP frequency and this accounts for the differences in observed behavior. As the SP-QD interaction strongly depends on the geometric structure and shape of the metal nanoparticles, a comparison with QDs deposited on a film of Au colloidal nanoparticles is presented. In the case of QDs placed directly on the Au colloids the luminescence quenching is much stronger and with a spacer layer a 3.5-fold enhancement over the bare QDs luminescence is observed.