Incident Laser Fluence Research Articles

Surface patterning on periodicity of femtosecond laser-induced ripples

Properties of femtosecond laser-induced ripples on patterned metal surfaces are investigated through using a crossed two-step line-scribing method. It is found that the ripple periodicity tends to decrease with larger surface roughness but increase with higher laser fluence. For the increased roughness, the change in ripple periodicity becomes more sensitive to the incident laser fluence. A cut-off surface roughness that prevents from altering the ripple periodicity is also revealed to increase with the lower laser fluence. These phenomena are discussed in the view of surface plasmon polaritons in the laser-induced plasma and the modified dielectric constant of the roughened surfaces.

Supermolecular-Chromophore-Sensitized Near-Infrared-to-Visible Photon Upconversion

Selective near-IR (NIR) excitation (780 nm) of the conjugated supermolecule ruthenium(II) [15-(4'-ethynyl-(2,2';6',2''-terpyridinyl))-bis[(5,5',-10,20-di(2',6'-bis(3,3-dimethylbutoxy)phenyl)porphinato)zinc(II)]ethyne][4'-pyrrolidin-1-yl-2,2';6',2''-terpyridine] bis(hexafluorophosphate) (Pyr(1)RuPZn(2)) in solutions containing N,N-bis(ethylpropyl)perylene-3,4,9,10-tetracarboxylicdiimide (PDI) or tetracene gives rise to a substantial anti-Stokes energy gain (PDI, 0.70 eV; tetracene, 0.86 eV). Experimental data clearly demonstrate that this upconverted fluorescence signal is produced via Pyr(1)RuPZn(2)-sensitized triplet-triplet annihilation (TTA) photochemistry. The TTA process was confirmed by the quadratic dependence of the integrated (1)PDI* emission centered at 541 nm derived from 780 nm laser excitation. The T(1)→T(n) excited state absorption decay of Pyr(1)RuPZn(2), monitored at 900 nm as a function of PDI concentration, revealed Stern-Volmer and bimolecular quenching constants of 10 048 M(-1) and 5.9 × 10(8) M(-1) s(-1), respectively, for the PDI triplet sensitization process. The T(1)→T(n) PDI extinction coefficient at 560 nm (ε(T) = 6.6 × 10(4) M(-1) cm(-1)) was determined through the triplet energy transfer method utilizing anthracene as the donor chromophore. (3)PDI* transient triplet absorption dynamics observed as a function of 485 nm incident nanosecond pump laser fluence demonstrate a bimolecular (3)PDI*-(3)PDI* TTA rate constant (k(TT) = 1.0 ± 0.2 × 10(9) M(-1) s(-1)). The maximum quantum yield of the supermolecule-sensitized PDI upconverted emission (Φ(UC) = 0.0075 ± 0.0002) was determined relative to [Os(phen)(3)][PF(6)](2) at an incident laser power of 22 mW at 780 nm. This study successfully demonstrates NIR-to-visible photon upconversion and achieves a new record anti-Stokes shift of 0.86 eV for sensitized TTA, using the supermolecular Pyr(1)RuPZn(2)sensitizer. The stability of the Pyr(1)RuPZn(2)/PDI chromophore combination is readily apparent as continuous irradiation at 780 nm produces 541 nm centered fluorescence with no significant decrease in intensity measured over time domains exceeding several hours. The molecular components of these NIR-to-vis upconverting compositions illustrate that substantial anti-Stokes energy gains via a TTA process can be effortlessly realized.

Laser ablation of liquid surface in air induced by laser irradiation through liquid medium

The pulse laser ablation of a liquid surface in air when induced by laser irradiation through a liquid medium has been experimentally investigated. A supersonic liquid jet is observed at the liquid–air interface. The liquid surface layer is driven by a plasma plume that is produced by laser ablation at the layer, resulting in a liquid jet. This phenomenon occurs only when an Nd:YAG laser pulse (wavelength: 1064 nm) is focused from the liquid onto air at a low fluence of 20 J/cm2. In this case, as Fresnel’s law shows, the incident and reflected electric fields near the liquid surface layer are superposed constructively. In contrast, when the incident laser is focused from air onto the liquid, a liquid jet is produced only at an extremely high fluence, several times larger than that in the former case. The similarities and differences in the liquid jets and atomization processes are studied for several liquid samples, including water, ethanol, and vacuum oil. The laser ablation of the liquid surface is found to depend on the incident laser energy and laser fluence. A pulse laser light source and high-resolution film are required to observe the detailed structure of a liquid jet.

A simple model of laser-induced fluorescence under arbitrary optical thickness conditions at the excitation wavelength

A simple model is worked out to investigate laser-induced fluorescence in media of arbitrary optical thicknesses at the excitation wavelength. Close-form expressions are obtained for laser fluence, absorbed energy density and emitted fluorescence signal as a function of optical thickness and incident laser fluence. It is shown that in optically thick media saturation curves are not proportional to density of the species excited by the laser, except at saturation, and that the saturation fluence increases with optical thickness. Experimental saturation curves of lead in laser-produced plasmas of brass are discussed in the light of this model.

Detailed studies of aluminum thin films morphology deposited by pulsed laser ablation

In the present paper we report the investigations by scanning electron microscopy of Al thin films prepared by pulsed laser ablation deposition experiments in vacuum. Laser parametric studies show that the poor thickness uniformity of Al films arises from surface roughening and structure formation on the irradiated Al target surface. Moreover, it has been confirmed that the plume deflection effect, the morphological changes of the irradiated target surfaces and of the deposited films strongly depend on the laser fluence (4.6–14.7 J/cm 2). At low laser fluence, plume deflection angles up to 12° and columnar structures on the target surface have been observed. The variation in the target morphology and in the droplets density of the deposited films can be associated with competitive ablation processes related to different values of local incident laser fluence. Systematic profilometric investigations of the deposited Al films revealed a strong asymmetry during the deposition process. These results confirm once again our previous achievements obtained with Si laser ablation experiments.

Laser surface micro-engineering for industrial applications in Singapore

Some recent research in laser-induced surface ripple structures, surface ablation, surface colouration, and their potential industrial applications are discussed in this article. Both wavelength and sub-wavelength periodic surface structures were observed on semiconductors such as InP and GaN/sapphire surfaces after irradiation of femtosecond laser pulses. The orientation of the periodic structures was dependant on the laser beam polarization, and the period was dependent on the incident laser fluence. Such surface periodic structures may find applications in controlling surface hydrophobic and hydrophilic properties. Excimer laser ablation of epoxy compounds from wafer level chip size packages was found to be a feasible method to expose the micron-size Au bumps for solder reflow, which is a critical process in manufacturing portable electronic products. Studies on controlled surface oxidation by laser pulses showed that a range of colours can be achieved on a stainless steel surface. Selective Cr oxidation and iron oxides were detected. The appearance of colour is the result of the light constructive interference of the transparent oxide layer and is determined by the oxide thickness and the refractive index. The potential industrial applications of the techniques are discussed.

Hydroxyapatite thin films synthesized by pulsed laser deposition and magnetron sputtering on PMMA substrates for medical applications

Functionalized implants represent an advanced approaching in implantology, aiming to improve the biointegration and the long-term success of surgical procedures. We report on the synthesis of hydroxyapatite (HA) thin films on polymethylmetacrylate (PMMA) substrates – used as cranio-spinal implant-type structures – by two alternative methods: pulsed laser deposition (PLD) and radio-frequency magnetron sputtering (MS). The deposition parameters were optimized in order to avoid the substrate overheating. Stoichiometric HA structures were obtained by PLD with incident laser fluences of 1.4–2.75 J/cm 2 , pressures of 30–46.66 Pa and 10 Hz pulses repetition rate. The MS depositions were performed at constant pressure of 0.3 Pa in inert and reactive atmospheres. SEM-EDS, XRD, FTIR and pull-out measurements were performed assessing the apatitic-type structure of the prepared films along with their satisfactory mechanical adhesion. Cell viability, proliferation and adhesion tests in osteosarcoma SaOs2 cell cultures were performed to validate the bioactive behaviour of the structures and to select the most favourable deposition regimes. For PLD, this requires a low fluence of 1.4 J/cm 2 , reduced pressure of water vapours and a 100 °C/4 h thermal treatment. For MS, the best results were obtained for 80% Ar + 20% O 2 reactive atmosphere at low RF power (∼75 W). Cells grown on these coatings exhibit behaviour similar to those grown on the standard borosilicate glass control: increased viability, good proliferation, and optimal cell adhesion. In vitro tests proved that HA/PMMA neurosurgical structures prepared by PLD and MS are compatible for the interaction with human bone cells.

Biomolecular papain thin films grown by matrix assisted and conventional pulsed laser deposition: A comparative study

Biomolecular papain thin films were grown both by matrix assisted pulsed laser evaporation (MAPLE) and conventional pulsed laser deposition (PLD) techniques with the aid of an UV KrF∗ (λ=248 nm, τFWHM≅20 ns) excimer laser source. For the MAPLE experiments the targets submitted to laser radiation consisted on frozen composites obtained by dissolving the biomaterial powder in distilled water at 10 wt % concentration. Conventional pressed biomaterial powder targets were used in the PLD experiments. The surface morphology of the obtained thin films was studied by atomic force microscopy and their structure and composition were investigated by Fourier transform infrared spectroscopy. The possible physical mechanisms implied in the ablation processes of the two techniques, under comparable experimental conditions were identified. The results showed that the growth mode, surface morphology as well as structure of the deposited biomaterial thin films are determined both by the incident laser fluence value as well as target preparation procedure.

Surface microstructuring of Ti plates by femtosecond lasers in liquid ambiences: a new approach to improving biocompatibility

Microstructuring of Ti plates with femtosecond laser pulses is investigated in three different liquids. In these ambiences, complex microstructures with voids and islands are produced on the sample surfaces, whose feature sizes are controlled by the laser parameters. Through adopting supersaturated Hydroxyapatite suspension with higher incident laser fluences, it is for the first time to observe the firm deposition of biocompatible elements Ca-P on the microstructures. At lower laser fluence, only porous structure is present but without additional elements deposition. Both plasma-related ablation under the confinement of liquids and micro-bubbles striking are employed to discuss such structures formation. Tight combining elements Ca-P onto the structured surfaces provide a new way to improve the biocompatibility of body-embedded devices.

The influence of wavelength in extinction measurements and beam steering in laser-induced incandescence measurements in sooting flames

The accuracy of laser-induced incandescence (LII) measurements is significantly influenced by the calibration process and the laser profile degradation due to beam steering. Additionally, the wavelength used for extinction measurements, needed for LII calibration, is critical and should be kept as high as possible in order to avoid light absorption by molecular species in the flame. The influence of beam steering on the LII measurement was studied in turbulent sooting C2H4/air flames at different pressures. While inhomogeneities in the laser profile become smoothed out in time-averaged measurements, especially at higher pressure, the corresponding single-shot beam profiles reveal an increasing effect of beam steering. In the current configuration it was observed that the resulting local laser fluence remains within certain limits (30% to 200%) of the original value. A sufficiently high incident laser fluence can thus prevent the local fluence from dropping below the LII threshold value of approximately 0.3 J/cm2 at the cost of increased soot surface vaporization. A spatial resolution in the dimension of the sheet thickness of below 1 mm cannot be guaranteed at increased pressure of 9 bars due to beam steering. A feasibility study in a combustor at technical conditions demonstrates the influence of both effects beam steering and choice of calibration wavelength and led to the conclusion that, however, a shot-to-shot calibration of LII with simultaneously measured extinction can be realized.

Low Power Visible-to-UV Upconversion

Low power visible-to-UV photon upconversion is demonstrated for the first time, achieved from two simple organic chromophores dissolved in benzene. Selective 442 nm excitation of the triplet sensitizer 2,3-butanedione (biacetyl) in the presence of the laser dye 2,5-diphenyloxazole (PPO) results in the observation of singlet fluorescence from the latter in the UV centered at 360 nm, anti-Stokes shifted by a record 0.64 eV with respect to the excitation. All of the experimental data are consistent with the upconverted singlet PPO fluorescence being produced as a result of biacetyl-sensitized triplet-triplet annihilation (TTA) of triplet excited PPO chromophores. Nanosecond laser flash photolysis performed under pseudo-first-order conditions revealed the bimolecular rate constant of triplet-triplet energy transfer between the biacetyl sensitizer and PPO acceptor, k(q) = 9.0 x 10(8) M(-1)s(-1). The TTA process was confirmed by the quadratic dependence of the upconverted integrated PPO emission intensity measured with respect to incident 442 nm light power density. The maximum quantum yield of the upconverted emission (0.0058 +/- 0.0002) was determined relative to 1,8-diphenyl-1,3,5,7-octatetraene, both measured with 0.389 W/cm(2) incident power density. The PPO triplet-triplet annihilation rate constant (k(TT)) was determined from transient absorption decays monitored at the peak of its characteristic triplet-to-triplet excited-state absorption (500 nm) as a function of incident pulsed laser fluence; this process attains the diffusion limit in benzene at room temperature, k(TT) = 1.1 +/- 0.1 x 10(10) M(-1) s(-1).

Functional polyethylene glycol derivatives nanostructured thin films synthesized by matrix-assisted pulsed laser evaporation

We report the thin film deposition by matrix-assisted pulsed laser evaporation (MAPLE) of a polymer conjugate with an hydrophilic sequence between metronidazole molecules that was covalently attached to both oligomer ends of carboxylate poly(ethylene glycol) (PEG 1.5–metronidazole). A pulsed KrF* excimer laser was used to deposit the drug–polymer composite films. Fourier transform infrared spectroscopy was used to demonstrate that MAPLE-transferred materials exhibited chemical properties similar to the starting materials. The dependence of the surface morphology on incident laser fluence is given.

Internal state populations and velocity distributions of monatomic species ejected after the 1064 nm laser irradiation of barium

The plumes accompanying 1064 nm nanosecond pulsed laser ablation of barium in vacuum at three moderate incident laser fluences in the range of 5.3–10.8 J/cm2 have been studied using both wavelength and time resolved optical emission spectroscopy and time-of-flight laser-induced fluorescence. Neutral atoms and both singly and doubly charged monatomic cations in excited states up to near the corresponding ionization limits are identified in the optical emission spectra. The population distributions of low-lying (≤1.41 eV) “dark” states of Ba atoms measured by laser-induced fluorescence revel that the metastable D3J and D12 abundances in the plume are higher than predictions based on assuming a Boltzmann distribution. The D3J and D12 populations are seen, respectively, to decrease slightly and nearly no vary with raising fluence, which contrasts with the increasing trend that is observed in the ground-state Ba(S10) population. At all fluences, the time-of-flight distributions of the whole dark states and of various of the emitting levels are bimodal and well described by Maxwell–Boltzmann and shifted Maxwell–Boltzmann velocity functions, respectively, with different average translational temperatures ⟨T⟩ for each state. The ⟨T⟩ values for the dark states are insensitive to the fluence, while for all emitting species marked variations of ⟨T⟩ with fluence are found. These observations have been rationalized in terms of material ejection from the target being dominated by a phase explosion mechanism, which is the main contributor to the Ba(S10) population. Thermionic emission from the target surface can also release initial densities of free electrons and cations which, at the prevailing irradiances, will arguably interact with the incident laser radiation by inverse bremsstrahlung, leading to further excitation and ionization of the various plume species. Such a heating mechanism ensures that the energy injected to the plume will alter the propagation velocities of the primary inverse bremsstrahlung absorbers, i.e., cations, to a major extent than those of neutral atoms with increasing fluence. Electron-ion recombination occurring early in the plume expansion can lead to the generation of both neutral and ionic species in a manifold of long-lived Rydberg states, from which a radiative cascade will likely ensue. The distinct fluence dependences of the Ba(D3J) and Ba(D12) populations and velocity distributions show up the major complexity that distinguishes their populating mechanisms with respect to the remaining species.

Some current research in femtosecond laser-induced surface ripple structures

Some current research in femtosecond (fs) laser induced surface nano/micro structures on compound semiconductors is discussed in this paper. Both wavelength and sub-wavelength periodic surface structures were observed on InP and GaN/sapphire surfaces irradiated with fs laser pulses. It was found that the morphology of the periodic structure was dependent on the polarisation of the laser beam and the number of laser pulses, and the period was dependent on the incident laser fluence. Through finely adjusting laser fluence and pulse number, uniform ripples can be achieved. The mechanisms involved in the formation laser induced surface structures are also discussed.

Fluorescence and Photodissociation of Rhodamine 575 Cations in a Quadrupole Ion Trap

The fluorescence and photodissociation of rhodamine 575 cations confined to a quadrupole ion trap are observed during laser irradiation at 488 nm. The kinetics of photodissociation is measured by time-dependent mass spectra and time-dependent fluorescence. The rhodamine ion signal and fluorescence decay are studied as functions of buffer gas pressure, laser fluence, and irradiation time. The decay rates of the ions in the mass spectra agree with decay rates of the fluorescence. Some of the fragment ions also fluoresce and further dissociate. The photodissociation rate is found to depend on the incident laser fluence and buffer gas pressure. The implications of rapid absorption/fluorescence cycling for photodissociation of dye-labeled biomolecular ions under continuous irradiation are discussed.

Dynamic force microscopy study of the microstructural evolution of pulsed laser deposited ultrathin Ni and Ag films

Ultrathin films (6–10 nm) of silver and nickel were deposited by pulsed laser deposition (PLD) in high vacuum (1 × 10−6 mbar). Microstructural evolution of these films as function of incident laser energy, substrate temperature, substrate material [borosilicate glass, fused silica, MgO(100) and Si (311)] and target–substrate distance was studied in detail using dynamic force microscopy. It is shown that with increase in laser energy incident on the target, there is a substantial increase in particle size in the film. The effect of increased laser energy on microstructure is much more drastic than that for the increase of substrate temperature. In general, denser packing of nanoparticles and increased clustering have been observed at elevated substrate temperature. Increase in laser energy gives rise to higher average grain size, packing density, and clustering in comparison to substrate temperature. It is observed that the aspect ratio of grains is dependent on incident laser fluence and substrate temperature, but more drastically on the substrate material. Substrate coverage and aspect ratio of the grains are particularly dependent on the nature of crystallinity of the substrates. It is demonstrated that PLD provides a greater degree of microstructural manipulation than other physical vapor deposition techniques.

Processable Hybrids of Ferrocene-Containing Poly(phenylacetylene)s and Carbon Nanotubes: Fabrication and Properties

A group of ferrocene-containing poly(phenylacetylene)s (PPAs) with different alkyl spacers were synthesized by using organorhodium complexes [Rh(diene)Cl](2) and Rh (+)(nbd)[C(6)H(5)B (-)(C(6)H(5))(3)] as catalysts. With the aid of pi-pi interactions between the walls of carbon nanotubes (CNTs) and the PPA skeleton together with the ferrocene pendants, the polymer (P 1, P2(5) and P2(10)) chains effectively wrapped round the shells of both single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). The "additive effect" of the PPA skeleton and the ferrocene pendants in dispersing the SWNTs and MWNTs resulted in the generation of highly soluble hybrids. The solubilities of P 1-functionalized SWNTs and MWNTs in tetrahydrofuran (THF) are up to 633 mg/L and 967 mg/L, respectively. They are much higher than the solubilities of M 1-modified SWNTs and MWNTs, which are only 167 mg/L and 133 mg/L in THF. The results indicate the existence of a powerful polymer effect on dispersing CNTs. The high solubilities of the hybrids in organic solvents allowed us to fabricate high-quality and large-area films. Meanwhile, the desirable loading of ferrocene-containing PPAs onto the CNTs offered polymer/CNTs hybrids with multiple redox centers and ferrocene-featured electrochemical properties. The P 1/MWNT hybrid exhibits evident optical-limiting properties. At high incident laser fluence, the optical-limiting power of P 1/MWNT is higher than that of C(60), a well-known optical limiter. Thermal analyses indicate that the decomposition temperatures ( T(d), the temperature at which a sample loses its 5% weight) for P1 and P1/MWNT are 342 and 346 degrees C, respectively, much higher than that for PPA (225 degrees C). Thus the attachment of a ferrocene pendant to a PPA backbone, followed by hybridization with CNTs, improved the thermal stability. Upon pyrolysis, both the polymer and the polymer/CNTs hybrid gave rise to superparamagnetic ceramics; the saturation magnetizations ( M(s)) of the ceramics derived from P1 and P1/MWNT are 29.9 and 26.9 emu/g, respectively. The latter datum is in the list of the best results reported for the magnetic nanocomposites obtained by the attachment of magnetic nanoparticles onto CNTs.

Frequency domain photothermoacoustic signal amplitude dependence on the optical properties of water: turbid polyvinyl chloride-plastisol system

Photoacoustic (more precisely, photothermoacoustic) signals generated by the absorption of photons can be related to the incident laser fluence rate. The dependence of frequency domain photoacoustic (FD-PA) signals on the optical absorption coefficient (micro(a)) and the effective attenuation coefficient (micro(eff)) of a turbid medium [polyvinyl chloride-plastisol (PVCP)] with tissuelike optical properties was measured, and empirical relationships between these optical properties and the photoacoustic (PA) signal amplitude and the laser fluence rate were derived for the water (PVCP system with and without optical scatterers). The measured relationships between these sample optical properties and the PA signal amplitude were found to be linear, consistent with FD-PA theory: micro(a)=a(A/Phi)-b and micro(eff)=c(A/Phi)+d, where Phi is the laser fluence, A is the FD-PA amplitude, and a, ...,d are empirical coefficients determined from the experiment using linear frequency-swept modulation and a lock-in heterodyne detection technique. This quantitative technique can easily be used to measure the optical properties of general turbid media using FD-PAs.

Femtosecond pulsed laser ablation dynamics and ablation morphology of nickel based superalloy CMSX-4

Pump-probe shadowgraphic imaging of single pulse femtosecond laser ablation was performed to investigate the dynamics of material removal during femtosecond laser machining of the intermetallic superalloy CMSX-4. Time-resolved shadowgraphic images were collected, which showed the presence of an expanding shock wave in the air in front of the target, following the onset of laser ablation. The dimensions of the shock wave were measured as a function of time (0–10.3ns), following the onset of ablation. The energy release associated with the observed shock wave and the pressure at the shock wave front versus time as a function of incident laser fluence (1.27–62.8J∕cm2) were inferred from the shock dynamics. The measured shock wave dynamics and inferred shock energy release are discussed in light of the evolving ablation morphology and ablated crater depth as a function of incident laser fluence.

Immobilization of urease by laser techniques: Synthesis and application to urea biosensors

Urease thin films have been immobilized using matrix-assisted pulsed laser evaporation for biosensor applications in clinical diagnostics. The targets exposed to laser radiation were made of frozen composites that had been manufactured by dissolving urease in distilled water. An UV KrF* (lambda = 248 nm, tauFWHM congruent with 30 ns, nu = 10 Hz) excimer source was used for the multipulse laser irradiation of the targets that were cooled down to solidification using Peltier elements. The incident laser fluence was set at 0.4 J/cm2. The surface morphology and chemical bonding states of the laser immobilized urease thin films were investigated by atomic force microscopy and Fourier transform infrared spectroscopy. The enzymatic activity and kinetics of the immobilized urease were assayed by the Worthington method, which monitors urea hydrolysis by coupling ammonia production to a glutamate dehydrogenase reaction. Decreased absorbance was found at 340 nm and correlated with the enzymatic activity of urease.