Bragg spectroscopy and superradiant Rayleigh scattering in a Bose–Einstein condensate

Two sets of studies concerning the interaction of off-resonant light with a sodium Bose–Einstein condensate are described. In the first set, properties of a Bose–Einstein condensate were studied using Bragg spectroscopy. The high momentum and energy resolution of this method allowed a spectroscopic measurement of the mean-field energy and of the intrinsic momentum distribution of the condensate. Depending on the momentum transfer, both the phonon regime as well as the free-particle regime could be explored. In the second set of studies, the cigar-shaped condensate was exposed to a single off-resonant laser beam and highly directional scattering of light and atoms was observed. This collective light scattering was caused by the long coherence time of the quasi-particles in the condensate and resulted in a new form of matter wave amplification. Received: 26 June 1999 / Revised version: 21 September 1999 / Published online: 10 November 1999     

Hybrid laser processing for microfabrication of glass

Hybrid laser processing for the precision microfabrication of glass materials, in which the interaction of a conventional pulsed laser beam and a medium on the material surface leads to effective ablation and modification, is reviewed. A major role of the medium is to produce strong absorption of the conventional laser beam by the material. Simultaneous irradiation by a vacuum ultraviolet (VUV) laser beam that possesses an extremely small laser fluence and an ultraviolet (UV) laser greatly improves the ablation quality and modification efficiency for fused silica (VUV-UV multiwavelength excitation process). The metal plasma generated by the laser beam effectively assists high-quality ablation of transparent materials by the same laser beam, resulting in microstructuring, cutting, color marking, printing, and selective metallization of glass materials (laser-induced plasma-assisted ablation (LIPAA)). The detailed discussion presented here includes the ablation mechanism of hybrid laser processing. Received: 18 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-48/462-4682, E-mail: ksugioka@postman.riken.go.jp     

Laser polishing of diamond plates

Results are reported on laser polishing of 150–400-μm-thick free-standing diamond films with either a copper vapor laser (510 nm wavelength) or an ArF excimer laser (193 nm wavelength). Studies were focused on three particular goals. First, we aimed at a choice of optimum conditions for laser polishing of thick diamond films. It was shown that the laser polishing conditions and the resulting surface roughness were controlled by varying the angle of incidence of a scanning laser beam and by polishing time. Second, the laser ablation technique was applied to remove a defective layer from the “substrate” side of the diamond plates in order to reduce optical losses due to absorption in this layer. Third, the structure of the laser-graphitized diamond surface was studied using UV, visible, and IR optical spectroscopy techniques in the course of the “step-by-step” oxidative removal of the graphitic layer with increasing temperature of the oxidation in ambient air. Once the graphitic layer was removed, the optical transmission in the UV-visible-IR spectral range of the diamond films polished under optimum conditions was measured and compared with the optical transmission of the mechanically polished diamond films. It was shown that the optical quality (in the long-wave infrared region) of the laser-polished diamond plates was sufficient to reach the transmittance value very close to the theoretical limit. Received: 20 October 1998 / Accepted: 8 March 1999 / Published online: 5 May 1999     

Model for laser-induced thermal degradation and ablation of polymers

Ablation of organic polymers is described on the basis of photothermal bond breaking within the bulk material. Here, we assume a first-order chemical reaction, which can be described by an Arrhenius law. Ablation starts when the density of broken bonds at the surface reaches a certain critical value. In order to understand the ablation behavior near the threshold fluence, φ_th, non-stationary regimes must be considered. The present treatment reveals several qualitative differences with respect to models that treat ablation as a surface process: (i) Ablation starts sharply with a front velocity that has its maximum value just after the onset. (ii) The transition to the quasi-stationary ablation regime is faster. (iii) Near threshold, the ablated depth h has a square-root dependence on laser fluence, i.e., h∝(φ-φ_th)^1/2. The ablation velocity is very high even near φ_th. (iv) With φ≈φ_th ablation starts well after the laser pulse. (v) The depletion of species is responsible for the Arrhenius tail observed with fluences φ≤φ_th. (vi) Residual modification of material has maximum near the threshold. (vii) Stationary regimes of ablation demonstrate change of effective activation energy with laser intensity. The model calculations are applied to Polyimide (Kapton^TM H). Here, differences in single-pulse ablated depth determined from mass loss and profilometry should be about 10 nm. Received: 16 February 1999 / Accepted: 18 February 1999 / Published online: 28 April 1999     

Control of wettability of hydrogenated amorphous carbon thin films by laser-assisted micro- and nanostructuring

A flexible and rapid surface functionalization of amorphous carbon films shows a great potential for various application fields such as biological surfaces and tribological systems. For this purpose, the combination of thin film deposition and subsequent laser material processing was investigated. Amorphous carbon layers doped with hydrogen were deposited on silicon wafers by reactive direct-current magnetron sputtering. Films with three different hydrogen contents were synthesized. Subsequent to the thin film deposition process, UV laser material processing at wavelengths of 193 nm or 248 nm was performed with respect to chemical surface modification and surface structuring on micro- and nanometer scale. Depending on structure size and laser-induced chemical surface modification the adjustment of the surface energy and wetting behaviour in a broad range from hydrophobic to hydrophilic was possible. The chemical modification and the ablation mechanisms near the ablation threshold were strongly influenced by the hydrogen content in amorphous carbon thin films.Structural and chemical information of the as-deposited and modified films was obtained by Raman spectroscopy, X-ray photoelectron spectroscopy and contact angle measurements.     

Optical properties of ion-modified metal surface in the infrared range

The results of study of ion-cleaned and -implanted surfaces of polycrystalline Be, Al, Ti, Fe, Cu, Mo, Zr, and W using optical and infrared (IR) spectroscopy are presented. It is shown that major changes in the optical reflectance spectra are observed in the range of 190–250 nm and are associated with both the formation of scattering centers and features of the chemical state of near-surface layers of materials. It is established using IR spectroscopy that treatment by Ar⁺ ion beam with a broad energy spectrum increases the reflection coefficient of the modified surface and additional resonance lines related to the formation of oxides themselves and hydroxyl groups are revealed in the reflectance spectrum in the case of formation of oxide film on the surface.     

Optical investigation of CdS quantum dots in Langmuir–Blodgett films

Structures with CdS quantum dots produced by the Langmuir–Blodgett (LB) technique were investigated by Raman, IR, and UV spectroscopies. The confinement effect of longitudinal optical (LO) phonons in CdS quantum dots was investigated by Raman spectroscopy. Surface vibrational modes of CdS quantum dots were observed in IR spectra. It was shown experimentally that the frequency of the surface vibrational modes depends on the properties of the surrounding media. An average size of CdS quantum dots of about 3–6.4 nm was obtained from the analysis of UV measurements. Received: 1 February 1999 / Accepted: 1 April 1999 / Published online: 19 May 1999     

Ultraviolet and visible Raman spectroscopy characterization of diamond–like carbon film growth by pulsed laser deposition

Laser-induced removal of flash from heat sinks in integrated circuit (IC) packages has been studied. It is found that flash can be effectively removed from heat sinks in plastic IC packages by laser deflashing. An optical microscope, an α-step surface profiler and X-ray photoelectron spectroscopy are used to analyze the deflashing efficiency. Laser deflashing of IC packages is based on laser ablation of flash materials. With an increase of laser fluence, the ablation rate increases. The laser fluence is selected between the ablation threshold of flash materials and that of heat-sink materials. An acoustic wave is generated by laser ablation of flash materials. Acoustic wave detection is used to monitor the surface cleanness during laser deflashing and to determine the ablation threshold of flash materials. Received: 18 April 2001 / Accepted: 14 September 2001 / Published online: 17 October 2001     

Femtosecond ablation of ultrahard materials

Several ultrahard materials and coatings of definite interest for tribological applications were tested with respect to their response when irradiated with fs laser pulses. Results on cemented tungsten carbide and on titanium carbonitride are reported for the first time and compared with outcomes of investigations on diamond and titanium nitride. The experiments were carried out in air, in a regime of 5–8 J/cm² fluences, using the beam of a commercial Ti:sapphire laser. The changes induced in the surface morphology were analysed with a Nomarski optical microscope, and with SEM and AFM techniques. From the experimental data and from the calculated incident energy density distributions, the damage and ablation threshold values were determined. As expected, the diamond showed the highest threshold, while the cemented tungsten carbide exhibited typical values for metallic surfaces. The ablation rates determined (under the above-mentioned experimental conditions) were in the range 0.1–0.2 μm per pulse for all the materials investigated. Received: 31 August 2001 / Accepted: 3 December 2001 / Published online: 20 March 2002     

Biomaterial immobilization on polyurethane films by XeCl excimer laser processing

The surface chemical modification of polyurethane (PU) films was performed by an UV laser-induced chemical reaction in a polysaccharide solution. This process may be applicable as hydrophilic packaging of implantable medical devices and in vivo sensors. When a PU film in contact with an aqueous alginic acid (AAC) solution was irradiated with a XeCl laser, the PU film turned hydrophilic. Contact angles of water on the film were reduced from 110° to 60°. Since light absorption of the AAC solution at 308 nm was negligibly small, reactive sites were generated solely on the PU surface. There, AAC could be immobilized by chemical bonds thus allowing for a nanometer-scaled grafting of this biomolecule. The mechanism was investigated by surface analyses with Fourier-transform infrared spectroscopy (FT-IR), dye staining, ultraviolet-visible (UV-VIS) spectroscopy, and scanning electron microscopy (SEM) techniques. A one-photon photochemical process could beidentified. Received: 30 June 2000 / Accepted: 4 July 2000 / Published online: 13 September 2000     

Antireflection structures written by excimer laser on CVD diamond

Two-dimensional antireflective periodical microstructures for the IR range are fabricated on the surface of CVD diamond films. These structures are created using an ArF excimer laser (λ=193 nm) and a direct writing scheme consisting of a beam collimator and a microscope objective to focus the beam onto the sample. Two different arrays are investigated. One has a spacing of 3 μm and is produced with single shots and the other one has a spacing of 4 μm and is produced with three shots per spot. The hole depth and shape are measured with an atomic force microscope (AFM). The optical transmittance and the scattering properties of the structure at 10.6 μm are reported for a CO₂ laser beam. With a spectrometer further transmission measurements in the range of 5 to 20 μm are performed. Received: 16 September 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

XPS studies of short pulse laser interaction with copper

The effect of laser ablation on copper foil irradiated by a short 30 ns laser pulse was investigated by X-ray photoelectron spectroscopy. The laser fluence was varied from 8 to 16.5 J/cm² and the velocity of the laser beam from 10 to 100 mm/s. This range of laser fluence is characterized by a different intensity of laser ablation. The experiments were done in two kinds of ambient atmosphere: air and argon jet gas.The chemical state and composition of the irradiated copper surface were determined using the modified Auger parameter (α′) and O/Cu intensity ratio. The ablation atmosphere was found to influence the size and chemical state of the copper particles deposited from the vapor plume. During irradiation in air atmosphere the copper nanoparticles react with oxygen and water vapor from the air and are deposited in the form of a CuO and Cu(OH)₂ thin film. In argon atmosphere the processed copper surface is oxidized after exposure to air.     

Analysis of semiconductor surface phonons by Raman spectroscopy

Only recently Raman spectroscopy (RS) has advanced into the study of surface phonons from clean and adsorbate-covered semiconductor surfaces. RS allows the determination of eigenfrequencies as well as symmetry selection rules of surface phonons, by k-conservation limited to the Brillouin zone-center, and offers a significantly higher spectral resolution than standard surface science techniques such as high-resolution electron energy loss spectroscopy. Moreover, surface electronic states become accessible via electron–phonon coupling. In this article the fundamentals of Raman scattering from surface phonons are discussed and its potential illustrated by considering two examples, namely Sb-monolayer-terminated and clean InP(110) surfaces. Both are very well understood with respect to their atomic and electronic structure and thus may be regarded as model systems for heteroterminated and clean semiconductor surfaces. In both cases, localized surface phonons as well as surface resonances are detected by Raman spectroscopy. The experimental results are compared with surface modes predicted by theoretical calculations. On InP(110), due to the high spectral resolution of Raman spectroscopy, several surface modes predicted by theory can be experimentally verified. Surface electronic transitions are detected by changing the energy of the exciting laser light indicating resonances in the RS cross section. Received: 7 April 1999 / Accepted: 25 June 1999 / Published online: 16 September 1999     

Processing of micro-particles by UV laser irradiation in a field cage

An electric cage-laser micro-turning lathe was realised and applied to contact-free handling and mechanical processing of micro particles. Since particles with diameters of several micrometers cannot be fixed in mechanical chucks, an octode field cage was used to trap and rotate a single particle in a fluid without any mechanical surface contact. A pulsed nitrogen laser of high beam quality focused to about 1 μm in diameter could be adjusted independently of the cage position. The trapping forces (negative dielectrophoresis) acting on a bead of 5 to 15 μm are up to several hundred pN. This and the surrounding fluid damp down the effect of the laser pulses during bead processing. Examples demonstrating the possibilities of this technique are shown. Microsystems with high optical quality were fabricated photolithographically or by laser direct-write chemical vapor deposition (LCVD). Technical and biotechnological applications are discussed. Received: 20 October 1999 / Accepted: 27 October 1999 / Published online: 10 November 1999     

Laser ablation of polysulfone films: a laser ionization TOF mass spectrometric study

2 . Major peaks were assigned to direct fragments and recombination products ejected from the PS surface. The arrival profiles of these ablation products varied from product to product and were fitted by using a shifted Maxwell–Boltzmann distribution with a center-of-mass flight velocity [(1.4-5.6) ×10⁵ cm/s] and a Knudsen layer temperature (350–3810 K). Two types of ablation products, whose velocities and temperatures showed different dependences on laser fluence, were found to exist. Dynamical aspects in the decomposition of the polymer chain, the ejection of various fragments, and their expansion are discussed on the basis of a photothermal ablation model, where a heated surface layer with a temperature gradient along its depth plays an important role. Received: 11 August 1998/Accepted: 12 August 1998     

Molecular beam epitaxy of GaN on a substrate of MoS2 layered compound

The feasibility of MoS₂ layered compound as a substrate for GaN growth was investigated. GaN films were successfully grown on MoS₂ by plasma-enhanced molecular beam epitaxy and the crystal quality of GaN on MoS₂ was compared with that on Al₂O₃. For GaN grown on MoS₂ substrate, it was found that the surface flatness observed by atomic force microscopy, stress in the film measured by Raman spectroscopy, optical properties measured by photoluminescence spectroscopy, and threading dislocation density observed by transmission electron microscopy show superior properties compared with that grown on Al₂O₃. These results suggest the layered compound such as MoS₂, which has no dangling bonds on the surface and has lattice mismatching of 0.9% to GaN, has high potential for a substrate of GaN growth. Received: 1 March 1999 / Accepted: 8 March 1999 / Published online: 26 May 1999     

Manifestation of glass-crack fractal geometry in Raman spectra

A Raman spectroscopic investigation of the fractal structure on the surface of cracks of different size scales in glassy SiO₂ is reported. A study was made of the crack surface and of the distorted layer formed after grinding and polishing and comminution. The parameters of the fractal structure thus found reflect the properties of the material and the crack nature; namely, fractal dimension is of greater importance for more compact materials, and the geometric size of fractal units (i.e., the size in real space) correlates with crack dimensions. Fiz. Tverd. Tela (St. Petersburg) 41, 1030–1034 (June 1999)     

Micro-lens arrays generated by UV laser irradiation of doped PMMA

Micro-lenses with well-defined optical parameters are generated on polymethylmethacrylate (PMMA) substrates doped with diphenyltriazene (DPT) by controlled use of a swelling effect generated under conditions of subablative excimer laser illumination. The surface profiles depend on the laser spot size and energy density. A sensitively balanced combination of matrix softening, substrate volume expansion due to photochemical nitrogen release, and surface tension is responsible for the final shape of the lenses. Complete arrays of identical lenses with 15 μm diameters and a focal length of 30 μm are produced by irradiation of (0.25 wt. %) DPT-PMMA with a single laser pulse at a wavelength of 308 nm and a fluence of 3 J/cm². It is shown experimentally and theoretically that appropriate volume expansion is possible without introducing internal light scattering due to the formation of small bubbles. Received: 7 April 1999 / Accepted: 8 April 1999 / Published online: 5 May 1999     

Critical role of laser wavelength on carbon films grown by PLD of graphite

The structure of thin films deposited by pulsed laser ablation (PLD) is strongly dependent on experimental conditions, like laser wavelength and fluence, substrate temperature and pressure. Depending on these parameters we obtained various kinds of carbon materials varying from dense, mainly tetrahedral amorphous carbon (ta-C), to less compact vertically oriented graphene nano-particles. Thin carbon films were grown by PLD on n-Si 〈100〉 substrates, at temperatures ranging from RT to 800°C, from a rotating graphite target operating in vacuum. The laser ablation of the graphite target was performed by a UV pulsed ArF excimer laser (λ=193 nm) and a pulsed Nd:YAG laser, operating in the near IR (λ=1064 nm). The film structure and texturing, characterised by X-ray diffraction analysis, performed at grazing incidence (GI-XRD), and the film density, evaluated by X-ray reflectivity measurements, are strongly affected both by laser wavelength and fluence and by substrate temperature. Micro-Raman and GI-XRD analysis established the progressive formation of aromatic clusters and cluster condensation into vertically oriented nano-sized graphene structures as a direct function of increasing laser wavelength and deposition temperature. The film density, negatively affected by substrate temperature and laser wavelength and fluence, in turn, results in a porous bulk configuration and a high macroscopic surface roughness as shown by SEM characterisation. These structural property modifications induce a relevant variation also on the emission properties of carbon nano-structures, as evidenced by field emission measurements. This work is dedicated to our friend Giorgio who passed away 20th August.     

聚四氟乙烯材料表面激光改性与刻蚀

利用波长为248 nm的准分子激光束在不同激光能量密度下照射聚四氟乙烯(PTFE)材料的表面,并用扫描电镜(SEM)、X射线光电子能谱(XPS)、拉曼(Raman)光谱等手段对激光处理前后样品的表面形貌、化学成分和结构进行测量和分析,进而对激光与聚四氟乙烯相互作用的机理进行了研究。实验结果表明,激光辐照使聚四氟乙烯表面产生去氟效应,导致表面碳化、分子链的交联以及含氧基团的产生,随着激光能量密度的增加,C=C双键逐渐形成。这些结构的变化可以导致表面硬度和粘结性增强。激光能量密度的大小对照射后样品表面的物理性质和化学结构有着重要的影响,它是聚合物表面激光改性和烧蚀的关键因素。