Nathan Madsen

Fabrication and characterization of low loss rib chalcogenide waveguides made by dry etching

We report the fabrication and characterization of rib chalcogenide waveguides produced by dry etching with CF4 and O2. The high index contrast waveguides (Deltan ~1) show a minimum propagation loss of 0.25 dB/cm. The high refractive nonlinearity of 100 times silica in As2S3 allowed observation of a pi phase shift due to self-phase modulation of an 8 ps duration 1573 nm pulse in a 5 cm long waveguide.

Picosecond high-repetition-rate pulsed laser ablation of dielectrics : the effect of energy accumulation between pulses

We report experiments on the ablation of arsenic trisulphide and silicon using high-repetition-rate (megahertz) trains of picosecond pulses. In the case of arsenic trisulphide, the average single pulse fluence at ablation threshold is found to be >100 times lower when pulses are delivered as a 76-MHz train compared with the case of a solitary pulse. For silicon, however, the threshold for a 4.1-MHz train equals the value for a solitary pulse. A model of irradiation by high-repetition-rate pulse trains demonstrates that for arsenic trisulphide energy accumulates in the target surface from several hundred successive pulses, lowering the ablation threshold and causing a change from the laser-solid to laser-plasma mode as the surface temperature increases.

Ablation of metals with picosecond laser pulses: Evidence of long-lived non-equilibrium surface states

Experiments on laser ablation of metals in air, in vacuum, and in similar irradiation conditions, revealed that the ablation thresholds in air are up to three times lower than those measured in vacuum. Our analysis shows that this difference is caused by the existence of a long-lived transient non-equilibrium surface state at the solid-vacuum interface. The energy distribution of atoms at the surface is Maxwellian-like but with its high-energy tail truncated at the binding energy. We find that in vacuum the rate of energy transfer from the bulk to the surface layer to build the high-energy tail, which determines the lifetime of this non-equilibrium state, exceeds other characteristic timescales such as the surface cooling time. This prohibits thermal evaporation in vacuum for which the high-energy tail is essential. In air, however, collisions between the gas atoms and the surface markedly reduce the lifetime of this non-equilibrium surface state allowing thermal evaporation to proceed before the surface cools. It was experimentally observed that the difference between the ablation depth in vacuum and that in air disappears at the laser fluencies 2–3 times in excess of the vacuum threshold value. The material removal at this level of the deposited energy density attains the features of the non-equilibrium ablation similar for both cases. We find, therefore, that the threshold in vacuum corresponds to non-equilibrium ablation during the pulse, while thermal evaporation after the pulse is responsible for the lower ablation threshold observed in air. This paper provides direct experimental evidence of how the transient surface effects may strongly affect the onset and rate of a solid-gas phase transition.

Cluster formation through the action of a single picosecond laser pulse

We demonstrate experimentally and describe theoretically the formation of carbon nanoclusters created by single picosecond laser pulses. We show that the average size of a nanocluster is determined exclusively by single laser pulse parameters and is independent of the gas fill (He, Ar, Kr, Xe) and pressure in a range from 20mTorr to 200 Torr. Simple kinetic theory allows estimates to be made of the cluster size, which are in qualitative agreement with the experimental data. We conclude that the role of the buffer gas is to induce a transition between thin solid film formation on the substrate and foam formation by diffusing the clusters through the gas, with no significant effect upon the average cluster size.

Subpicosecond and picosecond laser ablation of dental enamel: comparative analysis

We report the use of sub-picosecond near-IR and ps UV pulsed lasers for precision ablation of freshly extracted human teeth. The sub-picosecond laser wavelength was ~800nm, with pulsewidth 150 fs and pulse repetition rate of 1kHz; the UV laser produced 10 ps pulses at 266 nm with pulse rate of ~1.2x105 pulses/s; both lasers produced ~1 W of output energy, and the laser fluence was kept at the same level of 10-25 J/cm2. Laser radiation from both laser were effectively absorbed in the teeth enamel, but the mechanisms of absorption were radically different: the near-IR laser energy was absorbed in a plasma layer formed through the optical breakdown mechanism initiated by multiphoton absorption, while the UV-radiation was absorbed due to molecular photodissociation of the enamel and conventional thermal deposition. The rise in the intrapulpal temperature was monitored by embedded thermocouples, and was shown to remain low with subpicosecond laser pulses, but risen up to 30°C, well above the 5°C pain level with the UV-laser. This study demonstrates the potential for ultra-short-pulsed lasers to precision and painless ablation of dental enamel, and indicated the optimal combination of laser parameters in terms of pulse energy, duration, intensity, and repetition rate, required for the laser ablation rates comparable to that of mechanical drill.

Effects of non-equilibrium energy distribution of surface atoms on the onset and rate of laser ablation: experiments and theory

We report here experimental results on laser ablation of metals in air and in vacuum in similar irradiation conditions. The experiments revealed that the ablation thresholds in air are less than half those measured in vacuum. Our analysis shows that this difference is caused by the existence of a long-lived transient non-equilibrium surface state at the solid-vacuum interface. The energy distribution of atoms at the surface is Maxwellian-like but with its high-energy tail truncated at the binding energy. We find that in vacuum the time needed for energy transfer from the bulk to the surface layer to build the high-energy tail, exceeds other characteristic timescales such as the electron-ion temperature equilibration time and surface cooling time. This prohibits thermal evaporation in vacuum for which the high-energy tail is essential. In air, however, collisions between the gas atoms and the surface markedly reduce the lifetime of this non-equilibrium surface state allowing thermal evaporation to proceed before the surface cools. We found that ablation threshold in vacuum corresponds to non-equilibrium ablation during the pulse, while thermal evaporation after the pulse is responsible for the lower ablation threshold observed in air. This paper provides direct experimental evidence of how the transient surface effects may strongly affect the onset and rate of a solid-gas phase transition.

FORMATION OF NANOCLUSTERS IN EXPANDING LASER PLUME

Formation of carbon nanoclusters in a laser-pulse created plume expanding in vacuum and in a noble gas environment at various pressures was studied. Experiments were performed with carbon nanoclusters formed by laser ablation of graphite targets with 12-picosecond 532 nm laser pulses at MHz-range repetition rate in a broad range of ambient He, Ar, Kr, and Xe gas pressures from 2 × 10-2Torr to 1500 Torr. The experimental results confirmed our theoretical prediction that the average size of the nanoparticles depends weakly on the type of the ambient gas, and is determined exclusively by the single-laser pulse parameters. The most important finding relates to the fact that in vacuum the cluster size is mainly determined by hydrodynamic expansion of the plume, while in the ambient gas it is controlled by atomic diffusion in the gas.

Positive magnetism in Carbon Nanoclusters produced by high-repetition Rate-Laser Ablation

Carbon nanoclusters produced by high-repetition-rate laser ablation of graphite and glassy carbon in Ar exhibits para- and ferromagnetic behaviour at low temperature. The results show that the degree of remanent order is strongly dependent on the magnetic history, i.e. whether the samples were cooled under zero-field or field conditions. Such behaviour is typical for a spin glass structure where the system can exist in many different roughly equivalent spin configurations. The spin-freezing temperature is unusually high (50–300 K) compared with ≤ 15 K for typical spin glasses. The maximum in the zero-field magnetic susceptibility experiments and their field dependence indicate that there is competition between ferromagnetic and antiferromagnetic exchange pathways, accounting for the spin glass behavior and/or a low-dimensionality of the system.

Expansion-Limited Nanocluster Growth in a Plume Formed by MHz-Pulse-Rate Laser Ablation

We study the formation of carbon nanoclusters created by MHz repetition rate - picosecond laser pulses. We show that the average size of a nanocluster is determined exclusively by single laser pulse parameters and is largely independent of the gas fill (He, Ar, Kr, Xe) and pressure, in a range from 20mTorr up to 200 Torr. We provide evidence of the formation of large clusters at higher pressures in excess of 400 Torr, where the gas fill density is comparable or higher to the density of carbons in the ablated plume, and use simple kinetic theory to estimate cluster sizes, which are in qualitative agreement with the experimental data. We conclude that at pressures well below 400 Torr, the role of the buffer gas is to induce a transition between thin solid film formation on the substrate and nanofoam formation by diffusing the clusters through the gas, with no significant effect upon the average cluster size. At the higher pressure the buffer gas serves as a confiner for the carbon plume, increasing the collision frequency between the carbon atoms and resulting in cluster size growth.

Magnetic ordering and spin-glass behaviour of carbon nanoclusters

Carbon nanoclusters produced by high-repetition-rate laser ablation of graphite and glassy carbon in Ar exhibits para- and ferromagnetic behaviour at low temperature. The results show that the degree of remanent order is strongly dependent on the magnetic history, i.e. whether the samples were cooled under zero-field or field conditions. Such behaviour is typical for a spin glass structure where the system can exist in many different roughly equivalent spin configurations. The spin-freezing temperature is unusually high (50-300 K) compared with < 15 K for typical spin glasses. The maximum in the zero-field magnetic susceptibility experiments and their field dependence indicate that there is competition between ferromagnetic and antiferromagnetic exchange pathways, accounting for the spin glass behavior and/or a low-dimensionality of the system.