Laser Energy Fluence Research Articles

Effects of ablation energy and post-irradiation on the structure and properties of titanium dioxide nanomaterials

Nanomaterials of titanium oxide were prepared by pulsed laser ablation of a titanium metal target in distilled water. The ablation was performed at different laser energy (fluence) using a nanosecond pulsed Nd:YAG laser output of 1064 and 532nm. A post-irradiation of titanium oxide nanocolloids obtained by ablation using 532nm was carried out to explore its effects on the structure and properties. Analysis of morphology, crystalline phase, elemental composition, chemical state, optical and luminescent properties were performed using Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), UV–-vis absorption spectroscopy and room temperature photoluminescence spectroscopy. It was found that titanium oxide nanomaterial morphologies and optical properties were determined by ablation wavelength and fluence. Further, nanocolloids prepared by 532nm ablation showed a crystalline phase change by laser post-irradiation. The results showed that pulsed laser ablation in liquid as well as post-irradiation were effective in modifying the final structure and properties of titanium oxide nanocolloids.

Patterned perforation of polyester films by excimer laser ablation

The effects of 248 nm KrF excimer laser irradiation on combinations of different thickness polyethylene terephtalate (PET) films and different stainless steel mask mesh opening sizes were considered as a possible method to achieve different size perforation patterns on different thickness PET films. The findings of this work will provide an effective and controllable perforation technology. Therefore, ablation behavior of PET films was investigated for different laser energy fluence and number of laser pulses. The morphology of irradiated samples was observed by optical microscopy, and atomic forced microscopy was employed to study the topography and microstructures after laser ablation. An attempt has been made to correlate these findings with the orientation, strength, and mechanical properties of the polymer. The experimental results reveal that the percentage of perforation and the average perforated area increase with increasing number of pulses for all film thicknesses. The affected area, defined as...

Tin sulfide nanoparticles by pulsed laser ablation in liquid

Tin sulfide nanoparticles were synthesized by pulsed laser ablation of a tin sulfide (SnS) target in different liquid media using the 532 and 1064 nm output wavelengths from a pulsed (10 ns, 10 Hz) Nd:YAG laser. Analysis of the morphology, crystalline structure, chemical state and elemental composition, optical absorption and luminescent properties were done using transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV–Vis absorption spectroscopy and room temperature photoluminescence spectroscopy. TEM analysis demonstrated that change in the liquid medium and laser energy fluence resulted in tin sulfide nanoparticles with different morphologies. XRD analysis confirmed the crystallinity of SnS nanoparticles obtained. XPS results confirmed the compositions and chemical states of these nanoparticles. The optical absorption spectra and band gap energies for SnS nanocolloids were evaluated. These SnS nanocolloids showed emissions in the blue–green region of the visible range.

Effects of Substrate Temperature, Oxygen Pressure and Laser Fluence on Structural and Magnetic Properties of Pulsed Laser-Deposited Cobalt Ferrite Thin Films

Cobalt ferrite (CFO) thin films were grown on fused quartz substrate using pulsed laser deposition technique. The effects of various process parameters viz. substrate temperature, oxygen pressure, and laser energy fluence on structural, morphological, and magnetic properties were studied using X-ray diffraction (XRD), atomic force microscope (AFM), and vibrating sample magnetometer (VSM), respectively. The film deposited at the lower substrate temperature of 400 °C showed growth of CFO along preferred [110] direction and becomes textured for the substrate temperature of 800 °C with increase in grain size from 21 to 46 nm measured using AFM. The value of saturation magnetization (4 πMs) increases from 1940 to 2460 G with increase in substrate temperature from 400 to 800 °C, respectively. The CFO thin film deposited at 400 °C showed a coercivity value of 1880 Oe, which decreased to 670 Oe for 800 °C. The 4 πMs values first decrease with increase in oxygen pressure up to 0.4 mbar then increased for higher oxygen pressures. The H c value increases from 670 to 1290 Oe when oxygen pressure is increased from 0.2 to 0.4 mbar, which finally decreased to 1100 Oe for an oxygen pressure of 0.8 mbar. The structural and magnetic properties of CFO thin films are also observed to be influenced greatly by laser energy fluence. The varied magnetic properties of CFO thin films at different deposition conditions in pulsed laser deposition are interesting and may be useful for possible applications.

Synthesis and Properties of Platinum Nanoparticles by Pulsed Laser Ablation in Liquid

Platinum (Pt) nanoparticles were synthesized by pulsed laser ablation in liquid (PLAL) technique in different liquids (acetone, ethanol, and methanol). Ablation was performed using a Q-switched Nd:YAG laser with output energy of 230 mJ/pulse for 532 nm wavelength. Ablation time and laser energy fluence were varied for all the liquids. Effects of laser energy fluence, ablation time, and nature of the liquid were reported. The mean size, size distributions, shape, elemental composition, and optical properties of Pt nanoparticles synthesized by PLAL were examined by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV-Visible absorption spectroscopy.

Investigation of laser-fired point contacts on KOH structured laser-crystallized silicon by conductive atomic force microscopy

A conductive atomic force microscope is used to study the local topography and conductivity of laser-fired aluminum contacts on KOH-structured multicrystalline silicon surfaces. A significant increase in conductivity is observed in the laser-affected area. The area size and spatial uniformity of this enhanced conductivity depends on the laser energy fluence. The laser-affected area shows three ring-shaped regimes of different conductance depending on the local aluminum and oxygen concentration. Finally, it was found that the topographic surface structure determined by the silicon grain orientation does not significantly affect the laser-firing process.

CuInGaSe2 nanoparticles by pulsed laser ablation in liquid medium

Pulsed laser ablation in liquid medium (PLALM) is a nanofabrication technique to produce complex nanostructures. CuInGaSe2 (CIGS) is an alloy with applications in photovoltaic industry. In this work, we studied the effects of laser ablation wavelength, energy fluence and liquid medium on the properties of the CIGS nanoparticles synthesized by PLALM. The nanoparticles obtained were analyzed by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS) and UV–vis absorption spectroscopy. XPS results confirmed the chemical states and composition of the ablated products. TEM analysis showed different morphologies for the nanomaterials obtained in different liquid media and ablation wavelengths. The optical properties for these CIGS nanocolloids were analyzed using UV–vis absorption spectroscopy. The results demonstrated the use of PLALM as a useful synthesis technique for nanoparticles of quaternary photovoltaic materials.

Cell perforation mediated by plasmonic bubbles generated by a single near infrared femtosecond laser pulse.

We report on transient membrane perforation of living cancer cells using plasmonic gold nanoparticles (AuNPs) enhanced single near infrared (NIR) femtosecond (fs) laser pulse. Under optimized laser energy fluence, single pulse treatment (τ = 45 fs, λ = 800 nm) resulted in 77% cell perforation efficiency and 90% cell viability. Using dark field and ultrafast imaging, we demonstrated that the generation of submicron bubbles around the AuNPs is the necessary condition for the cell membrane perforation. AuNP clustering increased drastically the bubble generation efficiency, thus enabling an effective laser treatment using low energy dose in the NIR optical therapeutical window.

IR laser fragmentation of homogeneous (CF3I)n clusters and (CF3I)n clusters located inside of (Xe)m large clusters or on their surface

We have studied the fragmentation of homogeneous clusters ( is the average number of molecules per cluster) and of clusters that are located either inside of or on the surface of large clusters ( is the average number of atoms per cluster) as a result of resonant excitation of clusterized molecules by IR laser radiation. We have detected that these three types of clusters are differently stable with respect to the fragmentation and exhibit quite different dependences of the fragmentation probability on the IR radiation energy fluence. Whereas homogeneous clusters efficiently fragment at comparatively low laser energy fluences ( mJ cm−2), to fragment clusters that are localized inside clusters, higher energy fluences ( mJ cm−2) are necessary, and even higher energy fluences are required ( mJ cm−2) to fragment clusters localized on the surface of clusters. We have shown that small clusters localized on the surface of clusters do not fragment up to energy fluences of mJ cm−2. Possible reasons why the considered clusters have this fragmentation character are discussed.

Slowing of femtosecond laser-generated nanoparticles in a background gas

The slowing of Pt nanoparticles in argon background gas was characterized by Rayleigh scattering imaging using a plume of nanoparticles generated by femtosecond laser through thin film ablation of 20 nm-thick Pt films. The ablation was performed at threshold laser energy fluences for complete film removal to provide a well-defined plume consisting almost entirely of nanoparticles traveling with a narrow velocity distribution, providing a unique system to unambiguously characterize the slowing of nanoparticles during interaction with background gases. Nanoparticles of ∼200 nm diameter were found to decelerate in background Ar gas with pressures less than 50 Torr in good agreement with a linear drag model in the Epstein regime. Based on this model, the stopping distance of small nanoparticles in the plume was predicted and tested by particle collection in an off-axis geometry, and size distribution analysis by transmission electron microscopy. These results permit a basis to interpret nanoparticle propagation through background gases in laser ablation plumes that contain mixed components.

Effect of laser fluence on electrical properties of (Sr0.75,La0.25)TiO3 thin films grown by pulsed-laser-deposition.

We have grown Sr0.75La0.25TiO3 (SLTO) thin films using pulsed laser deposition (PLD) with various laser energy fluences. We investigated the effect of energy fluence on the compositions of SLTO films. The stoichiometry of SLTO films was controlled by adjusting the laser energy density. At low energy densities below 1.0 J/cm2, SLTO films become non-stoichiometric with Ti deficiency. The Ti deficiency increases with decreasing the laser energy fluence. We have also investigated the effect of laser energy fluence on the electrical properties of the thin films. The electrical resistivity and carrier density intimately depend on the laser energy fluence as a result of the non-stoichiometry. After eliminating the effect of oxygen vacancies by post-annealing, the electrical properties are dependent on the cation stoichiometry in the oxide films.

Tailoring out-of-plane magnetic properties of pulsed laser deposited FePt thin films by changing laser energy fluence

Abstract Magnetic properties of pulsed laser deposited (PLD) FePt thin films are investigated at three different laser energy fluences of 51, 136 and 182 J/cm 2 . Deposition at lower laser energy fluence (51 J/cm 2 ) yields softer out-of-plane coercivity (≤0.4 kG), whereas deposition at higher laser energy fluence (136 and 182 J/cm 2 ) results in harder out-of-plane coercivity (≥5.0 kG). The improved coercivity is found to be attributed to the formation of vacancy defects in thin films, which is indicated by stress change from tensile to compressive form with increasing laser energy fluence. Maximum out-of-plane saturated magnetization of 615 emu/cm 3 and remanent squareness ratio of 0.88 are achieved for 16 nm thick FePt thin films deposited at moderate laser energy fluence of 136 J/cm 2 , making them suitable for high density perpendicular data storage applications.

Photoacoustic detection and optical spectroscopy of high-intensity focused ultrasound-induced thermal lesions in biologic tissue.

The aims of this study are: (a) to investigate the capability of photoacoustic (PA) method in detecting high-intensity focused ultrasound (HIFU) treatments in muscle tissues in vitro; and (b) to determine the optical properties of HIFU-treated and native tissues in order to assist in the interpretation of the observed contrast in PA detection of HIFU treatments. A single-element, spherically concaved HIFU transducer with a centre frequency of 1 MHz was utilized to create thermal lesions in chicken breast tissues in vitro. To investigate the detectability of HIFU treatments photoacoustically, PA detection was performed at 720 and 845 nm on seven HIFU-treated tissue samples. Within each tissue sample, PA signals were acquired from 22 locations equally divided between two regions of interest within two volumes in tissue - a HIFU-treated volume and an untreated volume. Optical spectroscopy was then carried out on 10 HIFU-treated chicken breast specimens in the wavelength range of 500-900 nm, in 1-nm increments, using a spectrophotometer with an integrating sphere attachment. The authors' optical spectroscopy raw data (total transmittance and diffuse reflectance) were used to obtain the optical absorption and reduced scattering coefficients of HIFU-induced thermal lesions and native tissues by employing the inverse adding-doubling method. The aforementioned interaction coefficients were subsequently used to calculate the effective attenuation coefficient and light penetration depth of HIFU-treated and native tissues in the wavelength range of 500-900 nm. HIFU-treated tissues produced greater PA signals than native tissues at 720 and 845 nm. At 720 nm, the averaged ratio of the peak-to-peak PA signal amplitude of HIFU-treated tissue to that of native tissue was 3.68 ± 0.25 (mean ± standard error of the mean). At 845 nm, the averaged ratio of the peak-to-peak PA signal amplitude of HIFU-treated tissue to that of native tissue was 3.75 ± 0.26 (mean ± standard error of the mean). The authors' spectroscopic investigation has shown that HIFU-treated tissues have a greater optical absorption and reduced scattering coefficients than native tissues in the wavelength range of 500-900 nm. In fact, at 720 and 845 nm, the ratio of the optical absorption coefficient of HIFU-treated tissues to that of native tissues was 1.13 and 1.17, respectively; on the other hand, the ratio of the reduced scattering coefficient of HIFU-treated tissues to that of native tissues was 13.22 and 14.67 at 720 and 845 nm, respectively. Consequently, HIFU-treated tissues have a higher effective attenuation coefficient and a lower light penetration depth than native tissues in the wavelength range 500-900 nm. Using a PA approach, HIFU-treated tissues interrogated at 720 and 845 nm optical wavelengths can be differentiated from untreated tissues. Based on the authors' spectroscopic investigation, the authors conclude that the observed PA contrast between HIFU-induced thermal lesions and untreated tissue is due, in part, to the increase in the optical absorption coefficient, the reduced scattering coefficient and, therefore, the deposited laser energy fluence in HIFU-treated tissues.

Enhanced light scattering in Si nanostructures produced by pulsed laser irradiation

An innovative method for Si nanostructures (NS) fabrication is proposed, through nanosecond laser irradiation (λ = 532 nm) of thin Si film (120 nm) on quartz. Varying the laser energy fluences (425–1130 mJ/cm2) distinct morphologies of Si NS appear, going from interconnected structures to isolated clusters. Film breaking occurs through a laser-induced dewetting process. Raman scattering is enhanced in all the obtained Si NS, with the largest enhancement in interconnected Si structures, pointing out an increased trapping of light due to multiple scattering. The reported method is fast, scalable and cheap, and can be applied for light management in photovoltaics.

Elimination of impurity phase formation in FePt magnetic thin films prepared by pulsed laser deposition

The formation of impurity phases in FePt thin films severely degrades its magnetic properties. The X-ray diffraction patterns of FePt thin films, synthesized using pulsed laser deposition (PLD), showed peaks corresponding to impurity phases, resulting in softer magnetic properties. A systematic investigation was carried to determine the factors that might have led to impurity phase formation. The factors include (i) PLD target composition, (ii) substrate material, (iii) annealing parameters such as temperature, duration and ambience and (iv) PLD deposition parameters such as chamber ambience, laser energy fluence and target–substrate distance. Depositions on the different substrates revealed impurity phase formation only on Si substrates. It was found that the target composition, PLD chamber ambience, and annealing ambience were not the factors that caused the impurity phase formation. The annealing temperature and duration influenced the impurity phases, but are not the cause of their formation. A decrease in the laser energy fluence and increase of the target–substrate distance resulted in elimination of the impurity phases and enhancement in the magnetic and structural properties of FePt thin films. The energy of the ablated plasma species, controlled by the laser energy fluence and the target–substrate distance, is found to be the main factor responsible for the formation of the impurity phases.

Mold-free fabrication of 3D microfeatures using laser-induced shock pressure

This paper reports on the fabrication of microfeatures on metallic foils using laser-induced shock forming without the assistance of micromold patterns. A mold-free laser shock forming technique, Flexible Pad Laser Shock Forming (FPLSF) has been developed and demonstrated to fabricate near-spherical microcraters on thin copper foils through the laser-generated plasma shock inducing plastic deformation on the copper foil. It is found that the crater formation strongly depends on the laser energy fluence applied to ablate an ablative overlay which is on top of the copper foil for plasma shock generation. Microfeatures with deformation depth of 80μm to130μm and radius of 485μm to 616μm were formed on 25μm thick copper foils for the laser fluence of 7.3J/cm2 to 20J/cm2 while using aluminum foil as the ablative overlay and silicone rubber as a flexible support instead of a micromold. Fabrication of crater arrays on copper foils was also demonstrated successfully.

A new model of selective photothermolysis to aid laser treatment of port wine stains

Pulsed dye lasers equipped with cryogen spray cooling (CSC) are now widely used to treat vascular malformation such as port wine stains (PWSs). This paper presents a new integrated model that can quantitatively simulate the cooling of the skin and the heating of the targeted blood vessels in PWSs during laser treatment. The new model is based on the classical homogeneous multi-layer skin model that treats PWS-containing dermis as a mixture of dermal tissue and homogeneously distributed blood. Light propagation in skin and PWSs is simulated by a Monte-Carlo method, which provides accurate description of the light scattering and absorption in the skin. Thermal response of a targeted vessel in the new model is then obtained from the thermal analysis of a Krogh-unit that consists of the vessel and the surrounding dermal tissues and is buried in PWSs. The results from the multi-layer skin model provide appropriate laser influence input as well as the initial thermal condition for the micro-model of the Krogh-unit. A general dynamic relation is also introduced on the surface of skin to quantify the convective cooling of CSC. The model is then applied to dye-laser treatment (wavelength of 585 nm) of PWSs with CSC. Numerical results demonstrate that the present model is able to quantify thermal response of a deeply buried blood vessel in PWSs as a discrete blood vessel does, with a more realistically estimate of the sheltering effect of the dermal tissue (scattering) and blood vessels (absorption) in front of the targeted vessel. To understand the poor response of PWSs in clinic, the thermal characteristics of a targeted vessel was simulated under various conditions. The effects of two morphological parameters, the vessel diameter and the burying depth of the vessel, are then systematically investigated under various pulse durations and fluences of laser. A threshold fluence for given vessel diameter and depth is then estimated quantitatively under the condition of the optimal laser pulse duration. The results indicate that significant high threshold fluence is needed for large vessels buried deeply in the dermis, explaining the physics of the difficulty in clinic of complete clearance of PWSs. The results provide guidance to the clinic selection of the laser pulse duration and laser energy fluence for given PWSs.

The influence of the energy density and other clinical parameters on bond strength of Er:YAG-conditioned dentin compared to conventional dentin adhesion

The aim of this in vitro study was to optimise clinical parameters and the energy density of Er:YAG laser-conditioned dentin for class V fillings. Shear tests in three test series were conducted with 24 freshly extracted human third molars as samples for each series. For every sample, two orofacial and two approximal dentin surfaces were prepared. The study design included different laser energies, a thin vs a thick bond layer, the influence of adhesives as well as one-time- vs two-time treatment. The best results with Er:YAG-conditioned dentin were obtained with fluences just above the ablation threshold (5.3 J/cm(2)) in combination with a self-etch adhesive, a thin bond layer and when bond and composite were two-time cured. Dentin conditioned this way reached an averaged bond strength of 23.32 MPa (SD 5.3) and 24.37 MPa (SD 6.06) for two independent test surfaces while showing no statistical significance to conventional dentin adhesion and two-time treatment with averaged bond strength of 24.93 MPa (SD 11.51). Significant reduction of bond strength with Er:YAG-conditioned dentin was obtained when using either a thick bond layer, twice the laser energy (fluence 10.6 J/cm(2)) or with no dentin adhesive. The discussion showed clearly that in altered (sclerotic) dentin, e.g. for class V fillings of elderly patients, bond strengths in conventional dentin adhesion are constantly reduced due to the change of the responsibles, bond giving dentin structures, whereas for Er:YAG-conditioned dentin, the only way to get an optimal microretentive bond pattern is a laser fluence just above the ablation threshold of sclerotic dentin.

Mechanism of femtosecond laser nano-ablation for metals

Metals have three ablation threshold fluences (high,middle and low-threshold fluence, here called) forfemtosecond laser pulses. In order to investigatethe physics of metal ablation under an intenseoptical field, the ions emitted from a laserirradiatedcopper surface were studied by time-offlightenergy spectroscopy. The low laser fluenceat which ions are emitted, Fth,L is 0.028 J/cm2, andtwo higher emission thresholds were identified atfluences of Fth,M =0.195 J/cm2 and Fth,H =0.470J/cm2. The relation between the number of emittedions per pulse Ni and laser fluence F was in goodagreement with Ni ∝F4 for Fth,L - Fth,M, Ni ∝F3 forFth,M - Fth,H, and Ni ∝F2 for ≥ Fth,H. Thedependence of ion production on laser energyfluence is explained well by multiphotonabsorption and optical field ionization.For fluence levels near the middle to high ablationthreshold, the formation of grating structures onmetal surfaces has been observed. The interspacesof grating structures were shorter than the laserwavelength, and the interspaces depend on fluencefor Mo and W with a 160 fs laser pulse. Thisphenomenon is well explained by the parametricdecay model proposed by Sakabe et al.

Effects of laser energy fluence on the onset and growth of the Rayleigh–Taylor instabilities and its influence on the topography of the Fe thin film grown in pulsed laser deposition facility

The effect of laser energy fluence on the onset and growth of Rayleigh–Taylor (RT) instabilities in laser induced Fe plasma is investigated using time-resolved fast gated imaging. The snow plow and shock wave models are fitted to the experimental results and used to estimate the ablation parameters and the density of gas atoms that interact with the ablated species. It is observed that RT instability develops during the interface deceleration stage and grows for a considerable time for higher laser energy fluence. The effects of RT instabilities formation on the surface topography of the Fe thin films grown in pulsed laser deposition system are investigated (i) using different laser energy fluences for the same wavelength of laser radiation and (ii) using different laser wavelengths keeping the energy fluence fixed. It is concluded that the deposition achieved under turbulent condition leads to less smooth deposition surfaces with bigger sized particle agglomerates or network.