Sergey I. Kudryashov

Optical transmission measurements of explosive boiling and liftoff of a layer of micron-scale water droplets from a KrF laser-heated Si substrate

Water plume velocities were measured in air by optical transmission as a function of laser fluence using a KrF laser for explosive boiling and liftoff of a layer of micron-scale water droplets from a laser-heated Si substrate of interest for laser particle removal. The thickness of the superheated water layer near the water/Si interface determines acceleration and removal of the water droplets from the Si substrate.

Photoacoustic study of explosive boiling of a 2-propanol layer of variable thickness on a KrF excimer laser-heated Si substrate

The dynamics of explosive boiling of a 2-propanol layer of variable thickness on a Si substrate heated by a nanosecond KrF excimer laser was studied using a contact photoacoustic technique. The transition from acoustic generation at a free Si boundary to that at a rigid alcohol/Si boundary accompanied by a sharp increase of acoustic generation efficiency was found above a laser fluence threshold of 0.17 J/cm2 and a liquid layer thickness greater than 0.25 μm due to subnanosecond near-critical explosive boiling of the superheated liquid layer near the hot absorbing Si substrate. The gradual increase of the photoacoustic response of the superheated alcohol with increasing thickness of the liquid film at fluences above the explosive boiling threshold was attributed to a diffraction effect due to the fluence- and time-dependent increase of the area undergoing explosive boiling. A model describing photoacoustic generation and subsequent lift-off of the entire liquid layer in this experimental “thin transparent...

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.

Photoacoustic study of KrF laser heating of Si: Implications for laser particle removal

A photoacoustic study of KrF laser heating of Si has revealed that the dominant mechanism of acoustic generation is thermoacoustic with a considerable contribution from the concentration–deformation mechanism at laser fluences below the Si melting threshold of 0.5 J/cm2. Upon Si melting the contraction of the molten material contributes significantly to acoustic generation. At fluences above 1.4 J/cm2 laser ablation of the molten layer enhances the amplitude of the compression pulse and diminishes that of the rarefaction pulse. The results of photoacoustic measurements allow optimization of experimental conditions for dry laser particle removal.

Removal Versus Ablation in KrF Dry Laser Cleaning of Polystyrene Particles from Silicon

Direct absorption and melting of 0.2, 0.5 and 1.1 μm polystyrene particles on a Si substrate irradiated by 248 nm excimer laser radiation was found to contribute to their dry laser removal via a “hopping” mechanism at cleaning thresholds of 0.05, 0.1, and 0.16 J/cm2, respectively. Ablation of these particles, which starts near the beginning of substrate deceleration at fluences above 0.4–0.5 J/cm2, suppresses particle removal due to ablative recoil momentum. At fluences above a second cleaning threshold of 0.7 J/cm2 particles are completely evaporated without any visible surface damage of the Si substrate.

Experimental and Theoretical Studies of Laser Cleaning Mechanisms for Submicrometer Particulates on Si Surfaces

Submicrometer (0.1–1 μm) polystyrene and alumina particles were removed by single-shot nanosecond 248 nm KrF laser radiation from Si wafer surfaces with or without pre-deposited thin liquid layers. Nearly complete (>90%) single-shot laser cleaning has been achieved for combinations of polystyrene and alumina particles, respectively, with 2-propanol and water used as liquid energy transfer media, while for other combinations cleaning was absent or incomplete. Time-resolved optical microscopy studies have for the first time revealed important transient microscopic interactions between these particles and Si substrates in the pre-deposited micron-thick liquid layers, resulting, in some of these cases, in significantly reduced particle-substrate coupling. The visualization results give insight into microscopic particle removal mechanisms relevant to our laser cleaning experimental conditions, which are discussed along with their theoretical analysis. Theoretical modeling has been performed to interpret these experimental results and to provide insight into microscopic particle removal mechanisms.

Ablative modification of graphite surfaces by single intense femtosecond pulses

Several types of center-symmetrical (elliptical) microstructures of multinanometer heights are fabricated on a surface of quasicrystalline graphite ablated by single femtosecond laser pulses with peak intensities in the range of 1–102TW∕cm2. Potential underlying physical mechanisms for these high-intensity ablative micromodification phenomena are discussed.

Photoacoustics of individual live cells and particles

The photoacoustic (PA) technique has been employed to a number of new biomedical applications based of highly sensitive detection of laser-induced acoustic waves from individual live cells and single absorbing micro-particles or clusters of nanoparticles. These applications involve both linear and non-linear thermoacoustic phenomena initiated by focused nanosecond single laser pulse and detected with a fast PZT-ceramic acoustic transducer. Particularly, we present the following experimental results: 1) monitoring of linear and non-linear PA responses from red blood cells in suspensions in vitro; 2) detection of PA responses from breast cancer cell targeted with gold nanoparticles; 3) PA study of linear and non-linear interaction of laser with colored polystyrene micro-particles as model single absorbers; 4) monitoring of PA responses from moving absorbers in flow in vitro (PA flow cytometry in vitro); 5) recording of PA responses from blood flow in vivo on rat mesentery as animal model (PA flow cytometery in vivo); and 6) monitoring of sedimentation kinetics of particles and cells. The obtained results demonstrate the high sensitivity, low background, simple detection principle, easy data acquisition, and straightforward interpretation of the PA data.

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.

New mechanism of ultra-deep drilling of solids by high-power lasers

A new mechanism of ultra-deep drilling and related molten material expulsion during high-power short-pulse laser ablation of metals, semiconductors and dielectrics is proposed. In this mechanism ultra-deep (multi-micron) heat penetration and melting depths in these materials are assumed to result from their bulk absorption of thermal short-wavelength con-tinuous and characteristic radiation emitted by hot near-surface ablative laser plasmas. Multi-microsecond delays for expulsion of subsonic jets of micron-size droplets and for re-radiation of UV bursts from the irradiated targets are ex-plained by subsurface explosive boiling in bulk of the resulting ultra-deep melt pool.