Laser plasma thruster

The laser plasma thruster (LPT) is a new microthruster for small satellites. We report on development and testing of a prototype LPT. Some advantages of the LPT are: thruster voltage 4 V, mass less than 1 kg, power-to-thrust ratio 10 kW/N, and I_sp up to 1000 s. Typical thrust level is 250 μN with PVC fuel. Thrust of 1 mN is expected with energetic fuel. The pre-prototype continuous-thrust experiment includes the laser mount and heat sink, lens mounts, and focusing mechanism, which are coupled to the target-material transport mechanism. The target material is applied to a transparent plastic tape, and the laser is focused on a series of tracks on the tape. The tape-drive hardware and laser-drive electronics are described. Design, construction, and calibration of the thrust stand are described. During continuous operation, the exhaust plume is deflected in the direction of the moving tape. When the laser is operated in pulsed mode, the exhaust plume is perpendicular to the tape (parallel to the optical axis). This provides some thrust-vector control. Measured thrust is 70 μN with PVC fuel, and 550 μN with energetic fuel, with a specific impulse of 350 s. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-505-272-7203, E-mail: luke@iera.nmt.edu     

Application of lidar in ultraviolet,visible and infrared ranges for early forest fire detection

The efficiencies of direct lidars operating at 355, 532, 1064 and 1540-nm radiation wavelengths for early forest fire detection were compared. For each wavelength, the range for reliable smoke-plume detection was estimated on the basis of a computer simulation plume using a one-dimensional “top-hat” gas dynamic model for the calculation of the backscattering and extinction-coefficient profiles within the plume. The agreement between the predicted signal-to-noise ratio (SNR) and experimental results for 532 and 1064-nm wavelength radiation is good. The decrease of the signal-to-noise ratio with distance is maximum for 355 nm and minimum for 1064 nm. At 1540 nm, the decay of SNR with distance is slightly faster, but the SNR is higher than for other wavelengths, leading to the highest detection efficiency for the same energy of the probing laser pulse. For a burning rate of 2 kg/s and a laser beam divergence of 2.5 mr, the maximum distance for reliable detection varies between 6 and 12 km, depending on the wavelength. Received: 15 May 2002 / Revised version: 13 September 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +351-21/841-8120, E-mail: rui.vilar@ist.utl.pt     

Dynamics of the plume formation and parameters of the ejected clusters in short-pulse laser ablation

The dynamics of the early stages of the ablation plume formation and the mechanisms of cluster ejection are investigated in large-scale molecular dynamics simulations. The cluster composition of the ablation plume has a strong dependence on the irradiation conditions and is defined by the interplay of a number of processes during the ablation plume evolution. At sufficiently high laser fluences, the phase explosion of the overheated material leads to the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets, gas-phase molecules, and small clusters. The ejection of the largest droplets is attributed to the hydrodynamic motion in the vicinity of the melted surface, especially active in the regime of stress confinement. Spatially resolved analysis of the dynamics of the plume formation reveals the effect of segregation of the clusters of different sizes in the expanding plume. A relatively low density of small/medium clusters is observed in the region adjacent to the surface, where large clusters are being formed. Medium-size clusters dominate in the middle of the plume and only small clusters and monomers are observed near the front of the expanding plume. Despite being ejected from deeper under the surface, the larger clusters in the plume have substantially higher internal temperatures as compared to the smaller clusters. The cluster-size distributions can be relatively well described by a power law Y(N)∼N^-τ with exponents different for small, up to ∼15 molecules, and large clusters. The decay is much slower in the high-mass region of the distribution. Received: 13 October 2001 / Accepted: 18 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-434/982-5660, E-mail: lz2n@virginia.edu     

Femtosecond pulse shaping in the mid infrared by difference-frequency mixing

The generation of programmable complex femtosecond pulses in the mid infrared (3–10 μm) with high precision is reported. Designed pulse shapes in the near infrared (1–1.6 μm) are transferred to the mid infrared via difference-frequency mixing with a second infrared pulse spectrally narrowed in a zero-dispersion compressor. In particular, pulse sequences with variable relative phases have been obtained. The control of the pulse properties is achieved purely electronically, allowing for implementation into a feedback loop. Received: 12 December 2003 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-89/32905-200, E-mail: mcm@mpq.mpg.de     

Ablation with a single micropatterned KrF laser pulse: quantitative evidence of transient liquid microflow driven by the plume pressure gradient at the surface of polyesters

A microscopic flow of a transient liquid film produced by KrF laser ablation is evidenced on targets of PET and PEN. Experiments were done by using single pulses of the excimer laser beam micropatterned with the aid of submicron projection optics and grating masks. The samples of various crystalline states, ablated with a grating-forming beam (period Λ=3.7 μm), were precisely measured by atomic force microscopy, in order to evidence any deviation from the ablation behavior predicted by the current theory (combination of ablation curve and beam profile). This was confirmed by comparing various behaviors dependent on the polymer nature (PC, PET and PEN). PC is a normally ablating polymer in the sense that the ablated profile can be predicted with previous theory neglecting liquid-flow effects. This case is called ‘dry’ ablation and PC is used as a reference material. But, for some particular samples like crystalline PET, it is revealed that during ablation a film of transient liquid, composed of various components, which are discussed, can flow under the transient action of the gradient of the pressure of the ablation plume and resolidify at the border of the spot after the end of the pulse. This mechanism is further supported by a hydrodynamics theoretical model in which a laser-induced viscosity drop and the gradient of the plume pressure play an important role. The volume of displaced liquid increases with fluence (0.5 to 2 J/cm²) and satisfactory quantitative agreement is obtained with the present model. The same experiment done on the same PET polymer but prepared in the amorphous state does not show microflow, and such an amorphous sample behaves like the reference PC (‘dry’ ablation). The reasons for this surprising result are discussed. Received: 31 October 2002 / Accepted: 4 November 2002 / Published online: 22 January 2003 RID="*" ID="*"Present address: ST Microelectronics, Crolles, France RID="**" ID="**"Corresponding author. Fax: +33-556/84-6645, E-mail: s.lazare@lpcm.u-bordeaux1.fr     

Hot-electron-induced plasma formation on the rear surface of a foil

The plasma jet formed on the rear surface of a foil in laser–solid interaction is investigated by laser probing. The rear plasma jet, which is in line with the laser, formed a few picoseconds after the incidence of the focused laser, is due to a beam of fast electrons propagating through the target and is collimated by a strong magnetic field in the plasma. Received: 14 January 2003 / Revised version: 2 April 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +86-10/8264-9531 E-mail: jzhang@aphy.iphy.ac.cn     

Transient-diffusion effects

Transient effects on diffusion and activation during post-implantation anneals are a major obstacle for the further miniaturization of ultra-large-scale integrated semiconductor devices. The article reviews recent developments in the simulation of such phenomena with particular emphasis on models for the kinetics of self-interstitial agglomerates and boron–interstitial clusters. Received: 21 August 2002 / Accepted: 21 August 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-9131/761-212, E-mail: pichler@iis-bfhg.de     

Femtosecond pulsed-laser deposition of BaTiO<Subscript>3</Subscript>

The elemental composition and the surface morphology of thin films grown by laser ablation of barium titanate with femtosecond pulses at 620 nm laser wavelength have been systematically studied according to the experimental pulsed-laser deposition parameters : laser energy density, oxygen pressure, substrate temperature, target–substrate distance and substrate position (in- and off-axis geometry). Firstly, even at high temperature (700 °C), the deposits consist of coalesced particles up to 1-μm in size, mixed in a poorly crystallised tetragonal BaTiO₃ thin film. The particles formed in femtosecond pulsed-laser deposition induce a high surface roughness, which is observed whatever the experimental growth conditions and does not correspond to the droplets often observed during laser ablation in the nanosecond regime. As shown by plasma expansion dynamics, these particles propagate toward the substrate in the plasma plume with a low velocity, and are assumed to be produced by gas-phase reactions. Moreover, the cationic concentration evaluated through the Ba/Ti ratio strongly depends on the oxygen pressure in the ablation chamber and the angular position of the substrate along the normal to the target at laser impact. Indeed, the films appear to be enriched in the heavy element (Ba) when the substrate is located at high angular deviation. This fact is correlated to an increase in the lighter species (i.e. Ti) in the central part of the plasma plume. Received: 30 April 2002 / Accepted: 26 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +33-1/4354-2878, E-mail: millon@gps.jussieu.fr RID="**" ID="**"Also at: LSMCL, Université de Metz, 57078 Metz Cedex 3, France     

Measurement of free SO2 in wine with 7.4-μm difference frequency spectrometer

A 7.4-μm difference frequency spectrometer is used for analysing the headspace above the liquid surface of a sample of red wine for SO₂. The analysis is based on previously determined line parameters for a number of SO₂ lines of the ν₃ band, and allows fast, interference-free determination of the concentration without the need for any calibration against a standard of known concentration. The potential for developing this technique into a method for determining free and total sulphur dioxide in beverages is discussed. Received: 16 January 2003 / Revised version: 19 February 2003 / Published online: 9 April 2003 RID="*" ID="*"Corresponding author. Fax: +45-4593/1137, E-mail: jh@dfm.dtu.dk     

Analytical functions for the optimization of second-harmonic generation and parametric generation by focused Gaussian beams

An analytical function involving four parameters is proposed to express the second-harmonic generation efficiency as well as the parametric generation gain coefficient in the Boyd–Kleinman theory. The analytical function clearly reveals the dependence of conversion efficiency on the focusing parameter and the walk-off parameter. Moreover, the optimum focusing parameter and its corresponding maximum efficiency are explicitly given in the analytical function, leading to a straightforward evaluation of a given crystal performance. Received: 20 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +886-35/7291-34, E-mail: yfchen@cc.nctu.edu.tw     

InAsSb/InAsSbP current-tunable laser with narrow spectral line width

The article describes the manufacture and testing of a new type of semiconductor laser working at low temperatures (12–100 K) in the wavelength range 3200–3300 cm^-1. This kind of laser can be tuned in the modal range up to 6 cm^-1 and is characterized by a narrow spectral line width (about 7 MHz). Received: 12 September 2002 / Final version: 29 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +420-286/591-766, E-mail: civis@jh-inst.cas.cz     

On the accurate determination of pressure- induced line shifts in the 2ν<Subscript>3</Subscript> band of H<Subscript>2</Subscript>O at 1320 nm

A simple technique is demonstrated for the accurate determination of pressure-induced line shifts of water in air. High- and low-pressure water samples are simultaneously probed on selected overtone transitions at 1.32 μm using a current-modulated distributed-feedback diode laser and harmonic detection. The resultant profiles yield an average line shift of -293±30 MHz/atm for the 3,3,0 (002)2,2,1 (000)transition at 227251 GHz and -134±7 MHz/atm for the 3,2,1 (002)2,1,2 (000) transition at 227027 GHz. Comparisons are made between first- and second-harmonic detection, and wavelength- and frequency-modulation regimes. The effect of modulation broadening on the returned line shifts is quantified. Received: 12 August 2002 / Revised version: 21 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +44-01865/275410, E-mail: gus.hancock@chemistry.ox.ac.uk     

Excimer laser polymer ablation: twenty years on

Research and development in excimer laser polymer ablation has been actively pursued for some twenty years, driven by interest in the basic science as well as by numerous applications that have emerged for this high-resolution technique for material removal. This paper reviews some of the basic mechanistic aspects of the UV laser–polymer interaction as a prelude to dealing with practical matters related to polymer processing by ablation. Applications in micro-machining and potential areas for future research are briefly covered. Received: 7 October 2002 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +44-1482/465-606, E-mail: p.e.dyer@hull.ac.uk     

High-resolution electron microscopy of individual metallofullerene molecules on the dipole orientations in peapods

Electron microscopy with atomic sensitivity enables us to obtain a direct image of the intra-molecular structure of metallofullerenes encapsulated inside single-walled carbon nanotubes. By a comparison of high-resolution images with a simulation to extract the relative atom positions for encaged metal atoms in each molecule, the distribution of the molecular orientations and interactions between adjacent molecules in metallofullerene peapods have been statistically analyzed. The results are suggestive of strong interactions between fullerene–fullerene and fullerene–tube in peapods at room temperature. Received: 10 October 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/61-6310, E-mail: suenaga-kazu@aist.go.jp     

Laser-driven vehicles – from inner-space to outer-space

Laser-supported propulsion of a micro-airplane with a water-covered ablator is demonstrated. The repetitive use of an overlay structure is experimentally demonstrated with a specially designed water supply. Various transparent overlays are investigated by the CIP-based hydrodynamic code and by experiments using a pendulum and using a semi-conductor load cell. A momentum-coupling efficiency of ∼10⁴ N s/MJ is achieved by water–exotic-target experiments, in agreement with the simulation code, which predicts a maximum efficiency of ∼10⁵ N s/MJ. The concept of laser-supported propulsion can also be used for driving a Mach 5 airplane in the stratosphere, a micro-ship inside the human body, and a robot in a nuclear power reactor accident, during which large numbers of neutrons make electronic devices useless. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-3/5734-2165, E-mail: yabe@mech.titech.ac.jp     

Small signal gain measurements in a small scale HF overtone laser

The overtone gain medium of a small-scale HF overtone laser was probed using a sub-Doppler tunable diode laser. Two-dimensional spatially resolved small signal gain and temperature maps were generated for several ro-vibrational transitions in the HF (v=2→v=0) overtone band. Our results compare well with previous measurements of the overtone gain in a similar HF laser device. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-505/846-4807, E-mail: kevin.hewett@kirtland.af.mil     

Wetting and scanning force microscopy on rough polymer surfaces: Wenzel’s roughness factor and the thermodynamic contact angle

The influence of the surface roughness of polypropylene on the contact angle hysteresis is investigated by means of ethylene glycol drops in order to estimate the true Young’s equilibrium contact angle. A new relationship between the contact angle hysteresis and Wenzel’s contact angle is derived. In addition, the determination of Wenzel’s roughness factor by means of scanning force microscopy opens an alternative way to obtaining Young’s equilibrium contact angle without any surface manipulation. The experimental results presented verify this new approach. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-3328/352-452, E-mail: helmut.kamusewitz@gkss.de     

Surface nanostructuring of silicon

Irradiation with polarized laser light of 248-nm wavelength induces the formation of periodic undulations ∼10-nm-highon flat silicon substrates. The wavelength of these periodic structures is a function of the light wavelength and the angle of incidence of the laser beam. Linear arrays of silicon nanoparticles with fairly uniform size that extended up to a millimeter were formed if the irradiation was performed using polarized light. When non-polarized laser light with the same fluence was used to illuminate an initially flat surface, non-aligned nanoparticle strings were obtained. However, if part of the irradiated area was microstructured, nanoparticle linear arrays resulted in the vicinity of the microstructured region. An analysis on the evolution of these nanostructures is presented. Nanocolumns could be grown on top of every cone of a microstructured surface upon cumulative laser irradiation with non-polarized light, reaching a height of ∼3 μm and a diameter of 100–200 nm. The mechanisms of nanocolumn origin and growth are analyzed. Received: 16 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-865/974-4115, E-mail: apedraza@utk.edu     

Quantitative analysis of dilute mixtures of SO<Subscript>2</Subscript> in N<Subscript>2</Subscript> at 7.4 μm by difference frequency spectroscopy

We describe a 7.4-μm source based on difference frequency generation with 6.5 mW of 1278-nm radiation from an extended cavity laser and 66 mW of 1544-nm radiation from another extended cavity laser, amplified in an erbium-doped fibre amplifier. Optimum focusing of the input beams in the 5×5×10-mm³ AgGaSe₂ crystal, and the spatial and temporal characteristics of the output beam, are determined. The source is used for accurate determination of line parameters for selected lines of the ν₃ band of SO₂, centred at 1361 cm^-1. Subsequently, these lines are used for performing quantitative analysis of gas mixtures containing SO₂ at concentration levels down to 4 ppm without relying on any calibration with certified gas mixtures. This demonstrates the potential of infrared spectroscopy as a primary method for low-concentration gas analysis. Received: 16 January 2003 / Revised version: 19 February 2003 / Published online: 9 April 2003 RID="*" ID="*"Corresponding author. Fax: +45-4593/1137, E-mail: jh@dfm.dtu.dk     

Micromirror SLM for femtosecond pulse shaping in the ultraviolet

We present the application of a novel micro mirror array, which is based on a micro electro mechanical system (MEMS), as one- and two-dimensional phase-modulating spatial light modulator (SLM) for femtosecond pulse shaping in the spectral region from the deep-ultraviolet (DUV) to the near-infrared (NIR) (200–900 nm). Using such a high-resolution MEMS-SLM, we demonstrate one-dimensional pulse shaping at 400 nm, including THz-pulse train generation, chirp compensation, and phase wraps. Received: 7 April 2003 / Published online: 2 June 2003 RID="*" ID="*"Corresponding author. Fax: +49-3461/947-202, E-mail: hacker@ioq.uni-jena.de