Non-thermal laser-induced desorption of metal atoms with bimodal kinetic energy distribution

Laser-induced desorption of metal atoms at low rate has been studied for pulsed excitation with wavelengths of = 266, 355, 532 and 1064 nm. For this purpose sodium adsorbed on quartz served as a model system. The detached Na atoms were photo-ionized with the light of a second laser operating at = 193 nm and their kinetic energy distribution was determined by time-of-flight measurements. For = 1064 nm a distribution typical of thermal bond breaking is observed. If desorption, however, is stimulated with light of = 266 or 532 nm, the kinetic energy distribution is non-thermal with a single maximum atE _kin = 0.16 ± 0.02 eV. For = 355 nm the non-thermal distribution is even bimodal with maxima appearing atE _kin = 0.16 ± 0.02 and 0.33 ± 0.02 eV. These values of the kinetic energies actually remain constant under variation of all experimental parameters. They appear to reflect the electronic and geometric properties of different binding sites from which the atoms are detached and thus constitute fingerprints of the metal surface. The non-thermal desorption mechanism is discussed in the framework of the Menzel-Gomer-Redhead scenario. The transition from non-thermal to thermal desorption at large fluentes of the laser light could also be identified.     

Femtosecond pulse crystallization of thin amorphous hydrogenated films on glass substrates using near ultraviolet laser radiation

Femtosecond laser treatments (second harmonic of Ti-sapphire laser, λ ≈ 400 nm wavelength, <30 fs pulse duration) were applied for crystallization of thin hydrogenated amorphous silicon films on glass substrates. The concentration of atomic hydrogen in the films was varied from 10 to ≈35%. The energy densities (laser fluences) for crystallization of the films with thicknesses from 20 to 130 nm were found. Assumedly, non-thermal processes (plasma annealing) take place in phase transition caused ultra-fast pulses. The developed approach can be used for creation of polycrystalline silicon films on non-refractory substrates.     

Variable UV laser exposure processing of photosensitive glass-ceramics: maskless micro- to meso-scale structure fabrication

A novel variable UV laser processing technique was developed that enables the concurrent fabrication of structures in photosensitive glass-ceramic (PSGC) materials that range from the micro-scale to the meso-scale domains. This technique combines the advantages of direct-write volumetric laser patterning and batch chemical processing. The merged non-thermal laser fabrication approach relies on the ability to precisely and selectively alter the chemical etch rate of the PSGC by varying the laser exposure during pattern formation. The present study determined that the chemical etch rate of a commercial photosensitive glass-ceramic (Foturan^TM, Schott Corp., Germany) in dilute hydrofluoric (HF) acid is strongly dependent on the incident laser irradiance during patterning at λ=266 nm and λ=355 nm. For low laser irradiances, the etch rate ratio (R_exposed/R_unexposed) increased nearly linearly with laser irradiance. The slopes of the linear ranges of the etch rate ratios were measured to be 435.9±46.7 μm²/mW and 46.2±2.3 μm²/mW for λ=266 nm and λ=355 nm, respectively. For high laser irradiances, the measured etch rate ratio saturated at ∼30:1 with a maximum absolute etch rate of 18.62±0.30 μm/min. The maximum absolute chemical etch rate was independent of the exposure wavelength. Consequently, variation of the laser exposure during direct-write patterning permits the formation of variegated and proximal high and low aspect ratio structures on a common substrate. The results show that adjacent microstructures with aspect ratios ranging from <1:1 to ∼30:1 can be fabricated in a single, simultaneous batch chemical etch step without the need for a complex masking sequence or post-process ablation step. This new technique facilitates rapid prototype processing with pattern and component uniformity, and achieves material processing over large areas without incurring high cost. PACS 42.62.-b; 42.79.-e; 81.05.Kf; 81.20.-n     

A three-wavelength Ti:sapphire femtosecond laser for use with the multi-excited photosystem II

We have constructed a three-wavelength Ti:sapphire femtosecond laser with an independent tunable wavelength (λ₁) and two variable central wavelengths (λ₂ and λ₃) for use with the multi-excited photosystem II. Stable sub-40-fs pulses are generated. The λ₁-wavelength pulses can be tuned independently from 750 nm to 850 nm. The center wavelengths λ₂ and λ₃ can be varied from 760 nm to 840 nm. Received: 14 April 2000 / Revised version: 5 September 2000 / Published online: 27 April 2001     

Single-frequency Ti:Er:LiNbO3 distributed Bragg reflector waveguide laser with thermally fixed photorefractive cavity

The first single-frequency Ti:Er:LiNbO₃ distributed Bragg reflector waveguide laser with two thermally fixed photorefractive gratings as resonator mirrors is reported. The optically pumped (λ_p=1480 nm,120-mW incident power) laser emits up to 1.1 mW at λ_s=1561.1 nm. The threshold pump power is 70 mW. Received: 7 June 2001 / Published online: 30 October 2001     

Production of “dichroitic” diffraction gratings in glasses containing silver nanoparticles via particle deformation with ultrashort laser pulses

By repetitive irradiation of pico- or femtosecond laser pulses on glasses containing silver nanoparticles, dichroitic areas can be produced with different optical properties depending on the actual irradiation parameters. This effect, which is nanoscopically caused by permanent deformation of the initially spherical particles to non-spherical shapes and an additional formation of a halo of very small particles, is studied as a function of polarization and number of the applied pulses using two different laser systems (Ti:sapphire, λ=400 nm, t_p=150 fs; Nd:YLF, λ=523.5 nm, t_p=4 ps). A very special diffraction grating produced by this local deformation, which has strongly polarization- and wavelength-dependent features, is introduced and discussed. Received: 20 July 2001 / Published online: 10 October 2001     

Continuous frequency up-conversion in a double-Λ scheme of Na2

 In a double-Λ level configuration of Na₂ molecules, involving rotational–vibrational levels of the X, A and B bands, continuous resonant frequency mixing ω₄=ω₁−ω₂+ω₃ is demonstrated. A DCM dye laser at 661 nm (λ₁) pumps a molecular Raman laser at 746 nm (λ₂) in a sodium heatpipe, which is used to generate the molecular vapour. In the same heatpipe, both fields are mixed with the radiation of an argon-ion laser at 514 nm (λ₃) to generate up-converted laser radiation at 473 nm (λ₄). For laser powers of 200 mW (λ₁), 700 mW (λ₂, internal power) and 140 mW (λ₃), an output power of 120 μW (λ₄) has been achieved. Dependences of the generated radiation on the pump fields (powers and detunings) and polarization features are presented; influences of coherent coupling and population transfer mechanisms are discussed. Received: 7 October 1996     

Self-assembled organic monolayers as high-resolution resists in rapid nonlinear processing with single femtosecond laser pulses

Femtosecond laser patterning of alkanethiol monolayers on gold-coated silicon substrates at λ=800 nm, τ<30 fs and ambient conditions has been investigated. Single-pulse processing allows one to selectively remove the organic coating. Subsequently, pattern transfer into the gold film via wet etching in ferri-/ferrocyanide solution is achieved. As demonstrated, burr-free patterning can be carried out over an extremely wide range of laser pulse fluences from above 2 J/cm² down to 0.5 J/cm². Moreover, at low fluences, sub-wavelength processing down to λ/5 is feasible. In particular, at a 1/e laser spot diameter of about 1 μm, holes with diameters of 160 nm and step edges below 80 nm are fabricated. These results emphasize the prospects of organic monolayers as high-resolution resists in rapid nonlinear femtosecond laser processing.     

Pulsed laser-induced ablation of absorbing liquids and acoustic-transient generation

2 CrO₄ are irradiated by a KrF excimer laser (λ=248 nm, FWHM=24 ns) with moderate energy density (up to 100 MW/cm²) below the plasma-formation threshold. The ablation process, including the vapor-cavity formation and the acoustic-wave propagation is visualized by laser-flash photography. The ablation thresholds are determined by measuring the generated pressure transients and vapor-phase kinetics using a broadband piezoelectric pressure transducer and a simultaneous optical-transmission probe, respectively. The mechanisms of liquid ablation and acoustic-pulse generation are investigated based on the thermoelastic behavior of the liquid medium and the evaporation dynamics. A numerical model is proposed to describe the explosive-vaporization process at high laser fluences. The computation results are compared with the experiment. In short-pulse heating, ablation can be initiated at low laser fluences by the tensile component of the thermoelastic stress without a significant increase in the liquid temperature. On the other hand, if the heating rate is rapid enough to achieve a high degree of superheating of the liquid, the abrupt increase of the homogeneous-bubble-nucleation rate leads to explosive vaporization, which then plays the major role in the ablation dynamics. The pressure transient in the liquid is generated thermoelastically at low laser fluences, but the contribution of the vapor-phase expansion and/or the recoil momentum exerted by the ablation plume becomes significant at high laser fluences. Shock waves are formed in the ambient air in the case of explosive vaporization. The propagation of these wave fronts is in good agreement with the numerical-computation results. Received: 8 February 1998/Accepted: 10 February 1998     

Examination of the laser-induced variations in the chemical etch rate of a photosensitive glass ceramic

Previous studies in our laboratory have reported that the chemical etch rate of a commercial photosensitive glass ceramic (Foturan^TM, Schott Corp., Germany) in dilute hydrofluoric acid is strongly dependent on the incident laser irradiance during patterning at λ=266 nm and λ=355 nm. To help elucidate the underlying chemical and physical processes associated with the laser-induced variations in the chemical etch rate, several complimentary techniques were employed at various stages of the UV laser exposure and thermal treatment. X-ray diffraction (XRD) was used to identify the crystalline phases that are formed in Foturan following laser irradiation and annealing, and monitor the crystalline content as a function of laser irradiance at λ=266 nm and λ=355 nm. The XRD results indicate the nucleation of lithium metasilicate (Li₂SiO₃) crystals as the exclusive phase following laser irradiation and thermal treatment at temperatures not exceeding 605 °C. The XRD studies also show that the Li₂SiO₃ density increases with increasing laser irradiance and saturates at high laser irradiance. For our thermal treatment protocol, the average Li₂SiO₃ crystal diameters are 117.0±10.0 nm and 91.2±5.8 nm for λ=266 nm and λ=355 nm, respectively. Transmission electron microscopy (TEM) was utilized to examine the microscopic structural features of the lithium metasilicate crystals. The TEM results reveal that the growth of lithium metasilicate crystals proceeds dendritically, and produces Li₂SiO₃ crystals that are ∼700–1000 nm in length for saturation exposures. Optical transmission spectroscopy (OTS) was used to study the growth of metallic silver clusters that act as nucleation sites for the Li₂SiO₃ crystalline phase. The OTS results show that the (Ag⁰)_x cluster concentration has a dependence on incident laser irradiance that is similar to the etch rate ratios and Li₂SiO₃ concentration. A comparison between the XRD and optical transmission results and our prior etch rate results show that the etch rate contrast and absolute etch rates are dictated by the Li₂SiO₃ concentration, which is in turn governed by the (Ag⁰)_x cluster concentration. These results characterize the relationship between the laser exposure and chemical etch rate for Foturan, and permit a more detailed understanding of the photophysical processes that occur in the general class of photostructurable glass ceramic materials. Consequently, these results may also influence the laser processing of other photoactive materials. PACS  42.62.-b; 61.43.Fs; 81.05.Kf; 81.10.-h; 83.80.Ab     

Interaction of laser radiation with TNT with respect to the laser neutralisation of AP mines

According to UN estimations there are between 80 and 115 million activated landmines worldwide. These mines, or other unexploded ordnance (UXO), can be triggered accidentally and kill or injure more than 2000 civilians per month. The most common explosive in these mines is trinitrotoluene (TNT). In this paper, the potential of some of the most promising lasers for mine neutralisation is investigated, namely an ArF laser, a KrF excimer laser and a Nd:YAG solid-state laser. We have studied the interaction between laser beams emitting at λ=193 nm, 248 nm and 1060 nm and a bare solid sample of TNT of approximately 15 mg. Using pulsed excimer radiation at λ=193 nm, with an energy density up to 1 J/mm², ablation of the TNT without any deflagration has been achieved. At λ=248 nm, using the KrF excimer laser with a pulse duration of 30 ns and a repetition rate of 5 Hz, the TNT sample started melting and burning after an irradiation of 10 s. Preliminary results with the Nd:YAG solid-state laser operating in cw emission have shown that the irradiated sample exhibits the desired burning behaviour even after the exposure is stopped. Received: 14 December 2000 / Accepted: 18 December 2000 / Published online: 20 June 2001     

Compact diode-side-pumped Nd:YVO<Subscript>4</Subscript> laser in grazing-incidence configuration

The analysis of compact CW diode-side-pumped grazing-incidence-geometry Nd:YVO₄ laser designs is presented. An output power of 5 W (λ=1064 nm) was produced at 17 W of diode pump (conversion efficiency of 30%) in single transverse TEM₀₀ mode operation at high laser beam quality (M_x ²≈1.05 and M_y ²≈1.01). The resonator geometry was analyzed by applying generalized 4×4 matrix modeling of the spatial mode size, including the impact on the laser operation of cavity astigmatism and a thermal lens in the laser slab. The simplicity and compactness of the laser cavities allow their use for technological applications. Received: 31 July 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +44-20/7594-7744, E-mail: m.damzen@ic.ac.uk     

Effect of the ceramic grain size and concentration on the dynamical mechanical and dielectric behavior of poly(vinilidene fluoride)/Pb(Zr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript> composites

Laser-induced backside wet and dry etching (LIBWE and LIBDE) methods were developed for micromachining of transparent materials. Comparison of these techniques is helpful in understanding the etching mechanism but was not realized due to complications in setting up comparable experimental conditions. In our comparative investigations we used a solid tin film for dry and molten tin droplets for wet etching of fused-silica plates. A tin–fused-silica interface was irradiated through the sample by a KrF excimer laser beam (λ=248 nm, FWHM=25 ns); the fluence was varied between 400 and 2100 mJ/cm². A significant difference between the etch depths of the two investigated methods was not found. The slopes of the lines fitted to the measured data (sl_LIBDE=0.111 nm/mJ cm⁻², sl_LIBDE=0.127 nm/mJ cm⁻²) were almost similar. Etching thresholds for LIBDE and LIBWE were approximately 650 and 520 mJ/cm², respectively. To compare the dependence of etch rates on the pulse number, target areas were irradiated at different laser fluences and pulse numbers. With increasing pulse number a linear rise of depth was found for wet etching while for dry etching the etch depth increase was nonlinear. Secondary ion mass spectroscopic investigations proved that this can be due to the reconstruction of a new thinner tin-containing surface layer after the first pulse.     

Pulse-width influence on the laser-induced structuring of CaF2 (111)

We have investigated the morphology of CaF₂ (111) irradiated by 780 nm laser pulses of varying pulse width (200 fs-8 ns) with fluences above the damage threshold. Large differences can be observed which we relate to the mechanisms and dynamics of defect production in this wide band gap material. The best defined and most controllable ablation is obtained for laser pulse widths of a few picoseconds. For nanosecond and femtosecond pulses strong fracturing of the crystal is observed with damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps.     

Biostimulation of HeLa cells by low-intensity visible light

Summary The action of low-intensity red light with λ=633 nm (a He−Ne laser, a filament lamp with light filters a dye-laser pumped by a Cu laser) on the intensity of nucleic-acid synthesis in HeLa cells 1.5 hours after irradiation has been studied. It has been shown that the DNA synthesis is stimulated similarly after irradiation both by the He−Ne laser and by an ordinary source. The RNA synthesis intensity does not alter in both cases. A high-repetition-rate radiation at λ=633 nm acts in the opposite manner: it stimulates the RNA synthesis and the DNA synthesis remains constant. The action spectra of the DNA and RNA synthesis stimulation by continuous light in the range (570÷693) nm are presented.

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Riassunto è stata studiata l'azione della luce rossa a bassa intensità con λ=633nm (un laser a He−Ne, una lampadina a filamento con filtri per la luce, un laser a colorante pompato da un laser a Cu) sull'intensità della sintesi degli acidi nucleici in cellule HeLa un'ora e mezzo dopo irradiazione. Si è mostrato che la sintesi del DNA è stimolata in modo simile dopo irradiazione sia con laser a He−Ne che da una sorgente ordinaria. La sintesi del RNA non è alterata in entrambi i casi. Una radiazione ad alta frequenza di ripetizione a λ=633 nm agisce in maniera opposta: stimola la sintesi del RNA, mentre la sintesi del DNA resta costante. Si forniscono gli spettri d'azione della sintesi di DNA ed RNA mediante luce continua nell'intervallo da 570 a 693 nm.

     

Simultaneous detection of CO and CO2 using a semiconductor DFB diode laser at 1.578 μm

2 and CO at a wavelength of λ=1578 nm. Sensitivity measurements under different conditions have been performed and the detection limit of the apparatus was measured to be less than 10 mTorr over a 1-m path length. In addition, we measured for the first time environmentally and spectroscopically relevant self-broadening and nitrogen-broadening coefficients for CO₂ and CO in this spectral region and we discuss different possibilities for increasing the sensitivity of the apparatus. Received: 8 November 1996/Revised version: 11 December 1996     

Laser-assisted mechanical texturing of magnetic media

Mechanical texturing is the dominant technique for texturing magnetic media and is widely used in current hard disk drive manufacturing processes. This technique possesses the advantages of easy operation and low cost. However, it has an inherent disadvantage since mechanical texturing leads to jagged profiles randomly generated on hard disk surfaces, which precludes its further use in low-fly-height cases. A laser-assisted process working at near-threshold fluences for mechanical texturing was proposed to buff the irregular profiles of mechanical textures formed on Ni-P disk surfaces using a KrF excimer laser (λ=248 nm,τ=23 ns). This process, based on selective melting of the Ni-P surfaces due to inhomogeneous deposition of laser energy, was found to be capable of improving the surface characteristics of mechanically textured Ni-P disks. VSM and XRD analyses demonstrated that magnetic and structural properties of the Ni-P surfaces would not be affected in a detrimental way so as to influence the recording features of the magnetic media after laser buffing. Received: 28 November 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

Spin-dependent internal photo-electron emission over metal–semiconductor junctions – a study with circularly polarised infrared radiation

The spin asymmetry of internal photo-electron emission over Schottky barriers has been investigated for Fe/GaAs (001) having close-to-ideal current–voltage characteristics. Using a low-power circularly polarised YAG laser operated at its fundamental frequency (λ=1064 nm) as well as a visible diode laser (λ=670 nm) we demonstrate that, by eliminating the photo-current due to inter-band transitions in GaAs with the infrared source, a significant enhancement to the magnetic asymmetry could be achieved. The bias dependence of the asymmetry was also measured. It was found that the values were considerably different for the photo-electrons traversing the barriers in opposite directions. Received: 15 November 2001 / Revised version: 23 March 2002 / Published online: 2 May 2002     

A laser system providing tunable, narrow-band radiation from 35 nm to 2 μm

We describe a laser system that readily provides radiation tunable from 2 μm in the infra-red to 35 nm in the extreme ultraviolet spectral range. The broad spectral range is covered through a range of non-linear processes such as Raman shifting and high-order harmonic generation. Pulses with duration of tens of picoseconds are obtained. The relative bandwidth of the radiation is δλ/λ=10^-4, comparable with what can be achieved by using high-resolution monochromators at state-of-the-art synchrotron beamlines. We discuss different methods for characterising the radiation in this wide wavelength regime. We also discuss the capabilities of the system from the measured parameters. Received: 12 December 2000 / Revised version: 8 March 2001 / Published online: 27 April 2001     

Shape tailoring of hexagonally ordered triangular gold nanoparticles with nanosecond-pulsed laser light

In this contribution recent results on selective and precise tailoring of triangular gold nanoparticles (NPs) using ns-pulsed laser light are presented. The NPs were prepared by nanosphere lithography and subsequently tailored with ns-pulsed laser light using different fluences and wavelengths. The method is based on the size and shape dependent localized surface plasmon polariton resonance (SPR) of the NPs. We will demonstrate that the gap size between triangular NPs can be tuned from approximately 102±14 nm to 122±11 nm, due to a shape change of the NP from triangular to oblate. These morphological changes are accompanied by a significant shift of the surface plasmon resonance from λ_SPR=730 nm to λ_SPR=680 nm. Most importantly if the laser wavelength is chosen such that the dipolar SPR is excited, the hexagonal order of the NPs remains intact after irradiation, in contrast to excitation via the quadrupole SPR or within the interband transition. A tuneable gap size and the conservation of the hexagonal order of the NP array is the precondition for applications, where the NPs should serve as anchor points, e.g. for functional molecular nanowires, which can be used to utilize molecular devices.