Kevin Lyon

Nanosecond-laser plasma-assisted ultradeep microdrilling of optically opaque and transparent solids

A mechanism of ultradeep (up to tens of microns per pulse, submillimeter total hole depths) plasma-assisted ablative drilling of optically opaque and transparent materials by high-power nanosecond lasers has been proposed and verified experimentally using optical transmission and contact photoacoustic techniques to measure average drilling rates per laser shot versus laser intensity at constant focusing conditions. The plots of average drilling rates versus laser intensity exhibit slopes which are in good agreement with those predicted by the proposed model and also with other experimental studies. The proposed ultradeep drilling mechanism consists of a number of stages, including ultradeep “nonthermal” energy delivery into bulk solids by the short-wavelength radiation of the hot ablative plasma, bulk heating and melting, accompanied by subsurface boiling in the melt pool, and resulting melt expulsion from the target.

Dynamics of laser-induced surface phase explosion in silicon

Time-resolved ultrasonic studies revealed a second, delayed ablative pressure pulse after the first primary plasma pressure pulse in a silicon wafer irradiated by a UV nanosecond laser. The intensity-dependent delay time for the second pulse indicates the existence of a corresponding intensity-dependent homogeneous vapor bubble nucleation time in the superheated molten silicon prior to its phase explosion and ablative removal, since the integral pressure correlates with the ablation rate. A transient hot ablative plasma with calculated peak temperature ∼30–90 eV and pressure ∼20–110 GPa is suggested to superheat the bulk silicon via short-wavelength recombination and Bremsstrahlung emission.

Laser ablation of optically thin absorbing liquid layer predeposited onto a transparent solid substrate

Ablation of optically thin liquid 2-propanol layers of variable thickness on IR-transparent solid Si substrate by a nanosecond CO2 laser has been experimentally studied using time-resolved optical interferometric and microscopy techniques. Basic ablation parameters—threshold fluences for surface vaporization and explosive homogeneous boiling of the superheated liquid, ablation depths, vaporization (ablation) rates, and characteristic ablation times versus laser fluence—were measured as a function of alcohol layer thickness. The underlying ablation mechanisms, their thermodynamics, and microscopic details are discussed.

Picosecond multiphoton STIRAP detection of gas phase species: a test with sodium

Laser detection technologies offer obvious benefits for the standoff detection of hazardous or energetic materials where safe detection at a distance is the goal. Of the many optical standoff detection methods available, multiphoton fluorescence techniques have been studied less extensively. Multiphoton fluorescence allows high selectivity relative to the background while preserving the larger signal of laser induced fluorescence (LIF). Using sodium vapor as a test system, we demonstrate that stimulated Raman adiabatic passage (STIRAP) is capable of providing more than a factor of ten improvement in population transfer efficiency to the final state when compared to stimulated emission pumping (SEP). The two sodium transitions used are the 3p (2P1/2) ← 3s (2S1/2) and 5s (2S1/2) ← 3p (2P1/2). The light used to couple the states was produced with two synchronously pumped OPG/OPAs pumped by the 355 nm light from a picosecond tripled Nd:YAG.

Parametric Generation of Multimegahertz Acoustic Oscillations in Laser-generated Multibubble System in Bulk Water

Using a nanosecond CO2 laser for explosive surface boiling of bulk water, oscillatory acoustic transients from steam bubbles were recorded using a contact photoacoustic technique. Multiple well-resolved, high-amplitude multimegahertz spectral features reflecting parametric interactions between oscillations of cavitating steam bubbles were revealed in the fast Fourier transformation spectra of these transients. A potential parametric generation mechanism for these oscillation modes of steam bubbles is discussed.

Limitations and guidelines for measuring the spectral width of a single pulse of light with a Fabry–Perot interferometer

We present a method of analyzing the output of a single pulse of light from a Fabry-Perot interferometer (FPI). Together with an independent measurement of the pulse width and shape, the analysis enables the determination of the linewidth, TBP, and, consequently, the degree of coherence of the individual light pulses. The analysis presented builds on the method presented by Marzenell et al. [Appl. Phys. B 71, 185-191 (2000)] by analyzing the ring pattern of the FPI.

Picosecond standoff multiphoton detection of gas phase species: initial results

In the implementation of laser-induced fluorescence (LIF) for the detection of vapor-phase organic compounds that accompany hazardous materials, multiphoton excitation offers a significant advantage over single photon methods. In particular, if the absorption spectra of unwanted background molecules overlap that of the target molecule, single photon LIF is plagued by false positives. Multiphoton methods alleviate this difficulty by requiring that the target molecule be in resonance with multiple molecular transitions. A promising multiphoton method is stimulated Raman adiabatic passage (STIRAP). This method involves a counterintuitive sequence of laser pulses which is capable of transferring 100% of the target molecules to the desired excited state from which fluorescence is to be observed. As a precursor to more complex molecules, we demonstrate the STIRAP technique on sodium vapor using the 3p (2P1/2) ← 3s (2S1/2) and 5s (2S1/2) ← 3p (2P1/2) transitions. This is the first time STIRAP has been achieved on a vapor using picosecond lasers. We produced light to couple the states using two synchronously pumped OPG/OPAs (pumped by the 355 nm light from a picosecond YAG). We measured the fluorescence from the 5s state to both 3p states (2P1/2, 2P3/2) and from both 3p states to the 3s state with monochromator using a gated CCD to eliminate Rayleigh scattered light. Our results indicate a four to five-fold increase in the transfer efficiency to the 5s state when the laser pulse that couples the 3p and 5s states precedes the laser pulse tuned to the 3p ← 3s transition.

Near-Field Thermal Radiative Transfer and Thermoacoustic Effects from Vapor Plumes Produced by Pulsed CO2 Laser Ablation of Bulk Water

Submillimeter deep heating of bulk water by thermal radiation from ablative water plumes produced by a 10.6μm transversely excited atmospheric CO2 laser and the related acoustic generation has been studied using a contact time-resolved photoacoustic technique. Effective penetration depths of thermal radiation in water were measured as a function of incident laser fluence and the corresponding plume temperatures were estimated. The near-field thermal and thermoacoustic effects of thermal radiation in laser-ablated bulk water and their potential near-field implications are discussed.

Pulsed laser cleaning of sub- and micron-size contaminant particles from optical surfaces: cleaning versus ablation and damage

Pulsed laser cleaning of sub- and micron-sized (0.3-10 μm) monodispersed model spherical polystyrene particles and fused silica particles from fused silica and glass optical surfaces was performed by means of ns TEA CO2 laser. Efficient removal of these absorbing particles has been demonstrated in certain laser fluence range, which is below the threshold for ablative damage of the fused silica and glass substrate. Removal mechanisms of dry and steam laser cleaning of various critical optical surfaces are discussed.

Pulsed selective laser removal of nano- and micro-particles

Selective laser removal of micro-particles of one chemical composition from their mixture with micro-particles of another chemical type pre-deposited on hydrophobic or hydrophilic surfaces have been demonstrated by means of steam laser cleaning method realized with nanosecond IR laser and various liquid energy transfer media (ETM). Microscopic imaging of particle mixture deposition, ETM dosing and final particle removal has been performed with the help of timeresolved optical microscopy. Optimal ETM/particle combinations for selective targeting and removal of specific particles from their mixture on the surfaces have been revealed.