Combined dielectrophoretic field cages and laser tweezers for electrorotation

Symmetric rotating octode field cages do not allow simultaneous particle trapping and electrorotation. Laser-trapped particles can stably be held in inhomogeneous rotating electric fields. Therefore, in a combination of laser tweezers and field-cage (electro-optical cage) dielectric single-particle spectroscopy (DSPS) of particles and cells can be carried out at low electric field strength. The electro-optical cage overcomes the limitation of low enough effective particle conductivity ensuring repelling dielectrophoretic forces in conventional field cages. At low power, forces in field cages and laser tweezers hardly interfere and can be used to calibrate each other. Received: 3 May 1999 / Revised version: 16 August 1999 / Published online: 27 October 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     

Absolute absorber quantification in turbid media at small source–detector separations

The aim of our studies was to develop a method to determine the absorption coefficient of a turbid medium in a reflection geometry with small source–detector separations. Therefore, the time-integrated microscopic Beer–Lambert law (MBL) was modified in order to obtain the absolute absorption coefficient from mean time of flight and dc-intensity measurements. The new technique was evaluated using turbid phantoms having varying scattering (μ_s ^′ between 0.2 and 2.2 mm^-1) and absorbing properties (μ_a between 0.04 and 0.14 mm^-1), comparable to many biological tissues at various source–detector separations between 3 and 11 mm. The measurements were performed at a wavelength of 1064 nm. We found that this new method was able to determine the absolute absorption coefficient of the selected phantoms with a standard error of less than 0.005 mm^-1 over the range of optical properties investigated. Received: 23 November 2001 / Revised version: 28 February 2002 / Published online: 14 May 2002     

Tailored ablation processing of advanced biomedical hydroxyapatite by femtosecond laser pulses

The micromachining of hydroxyapatite (HAp) is highly important for orthopedics and dentistry, since human bone and teeth consist mainly of HAp. We demonstrate ultrashort Ti:sapphire laser ablation of HAp, using pulse-widths of 50 fs, 500 fs, and 2 ps at a wavelength of 820 nm and at 1 kpps. The crucial medical issue is to preserve the chemical properties of the machined (ablated) surface. If the chemical properties of HAp change, the human bone or tooth cannot re-grow after laser processing. Using X-ray photoelectron spectroscopy, we observe chemical properties of HAp ablated in air. The HAp is ablated at laser fluences of 3.2 J/cm² (6.4×10¹³ W/cm² at 50 fs), 3.3 J/cm² (6.6×10¹² W/cm² at 500 fs), and 9.6 J/cm² (4.8×10¹² W/cm² at 2 ps), respectively. As a result it is found that the ablated surface is unchanged after laser ablation over the pulse-width range used in this experiment. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-45/566-1533, E-mail: obara@obara.elec.keio.ac.jp     

Nonlinear side effects of fs pulses inside corneal tissue during photodisruption

In order to evaluate the potential for refractive surgery, fs laser pulses of 150-fs pulse duration were used to process corneal tissue of dead and living animal eyes. By focusing the laser radiation down to spot sizes of several microns, very precise cuts could be achieved inside the treated cornea, accompanied with minimum collateral damage to the tissue by thermal or mechanical effects. During histo-pathological analysis by light and transmission electron microscopy considerable side effects of fs photodisruption were found. Due to the high intensities at the focal region several nonlinear effects occurred. Self-focusing, photodissociation, UV-light production were observed, leading to streak formation inside the cornea. Received: 12 October 2001 / Revised version: 13 February 2002 / Published online: 14 March 2002     

Quantitative oxygen imaging in an engine

Simultaneous imaging of laser-induced fluorescence of toluene and 3-pentanone was used to determine the local absolute oxygen and residual gas concentrations present within an engine. The technique utilizes the different sensitivities of the laser-excited molecules to quenching by molecular oxygen as a means to determine quantitative images of in-cylinder oxygen concentrations. The difference in the amount of oxygen available between two operating conditions was investigated. Results are in agreement with measurements in the exhaust gas. Received: 4 June 2002 / Published online: 8 August 2002     

Morphology of native and reconstituted biological membranes and their components analysed with atomic force microscopy

Received: 1 July 1998     

Measurement of gas jet flow velocities using laser-induced electrostrictive gratings

We used time-resolved light scattering of cw probe laser radiation from laser-induced electrostrictive gratings for the determination of flow velocities in air at room temperature. Some possibilities of the technique have been experimentally demonstrated with submerged planar air jets in atmosphere, both for accumulated and single-shot measurements. The range of investigated flow velocities was 5–200 m/s. The method of data treatment and of the estimate of the experimental parameters is described. Received: 8 Febuary 2000 / Revised version: 2 May 2000 / Published online: 2 August 2000     

Analysis of hydroxyapatite laser ablation plumes in a water atmosphere

4 m/s, shows the development of a shock wave in its front, resulting from the interaction between the species released from the target and the background gas. The water vapor slows down this component up to values of about 3×10³ m/s following a behavior that can be well described by the dynamics of a spherical shock wave. The low intensity of emission of the second component has not allowed us to analyze its dynamics. The third and slowest component expands at a constant velocity of 5×10² m/s and is constituted by hot particulates leaving the target. Spectra recorded in the shock front have shown the presence of emission lines arising from Ca I, Ca II, P I and some impurities, and two strong emission bands that can be assigned to some sort of calcium oxide. Received: 7 November 1997/Accepted: 17 February 1998     

Detection of small traces of 15N2 and 14N2 by Faraday LMR spectroscopy of the corresponding isotopomers of nitric oxide

15 N₂ in a gas phase sample is described. The nitrogen is transformed by a microwave discharge into nitric oxide NO. The latter is analyzed by recording both a ¹⁵NO and a ¹⁴NO Faraday LMR signal. The determination of the transformation rate from N₂ into NO is described. The method of measurement and the achieved sensitivity (∼0.1 ppm ¹⁵N₂≈4 nanomoles ¹⁵N₂/litergas) of the spectrometer are discussed. An application in pharmacology, where ¹⁵N is used as a tracer for metabolism is indicated. First experiments with the exhalation of a rat show that the apparatus is useful to give a new quality of results. Received: 30 April 1996/Revised version: 29 July 1996     

The mechanism of nanostructuring upon nanosecond laser irradiation of a STM tip

Received: 20 April 1998/Accepted: 2 June 1998     

Pulsed holmium laser ablation of tissue phantoms: correlation between bubble formation and acoustic transients

5 Pa served as tissue phantoms to evaluate such effects. Holmium laser pulses (wavelength: 2.12 μm, duration: 180 μs FWHM), were delivered through 400 and 600 μm diameter optical fibers inserted into cubes of clear gel. Bubble effects were investigated using simultaneous flash micro-videography and pressure recording for radiant exposures of 20–382 J/cm². Bubble formation and bubble collapse induced pressure transients were observed regardless of phantom stiffness. Bubbles of up to 4.2 mm in length were observed in gels with a Young’s modulus of 2.9×10⁵ Pa at a pulse energy of 650 mJ. An increase of Young’s modulus (reduction in water content) led to a monotonic reduction of bubble size. In the softest gels, bubble dimensions exceeded those observed in water. Pressure amplitudes at 3 mm decreased from 100±14 bars to 17±6 bars with increasing Young’s modulus over the studied range. Theoretical analysis suggested a major influence on bubble dynamics of the mass and energy transfer through the bubble boundary. Received: 26 August 1996/Revised version: 10 February 1997     

Oxygen-distribution imaging with a novel two-tracer laser-induced fluorescence technique

We introduce a new technique for imaging oxygen concentrations in fuel/air mixtures that takes advantage of the different responses of toluene and 3-pentanone to collisional quenching by molecular oxygen. Since laser-induced fluorescence signals from both tracers upon excitation at 248 nm are spectrally well separated, simultaneous detection is possible. The technique is first applied to instantaneous imaging in turbulent mixing processes of interacting seeded air and nitrogen flows. Received: 1 August 2001 / Revised version: 29 October 2001 / Published online: 29 November 2001     

13CO2/12CO2 isotopic ratio measurements using a difference frequency-based sensor operating at 4.35 micrometers

A portable modular gas sensor for measuring the ¹³C/¹²C isotopic ratio in CO₂ with a precision of 0.8‰(±1σ) was developed for volcanic gas emission studies. This sensor employed a difference frequency generation (DFG)-based spectroscopic source operating at 4.35 μm (∼2300 cm^-1) in combination with a dual-chamber gas absorption cell. Direct absorption spectroscopy using this specially designed cell permitted rapid comparisons of isotopic ratios of a gas sample and a reference standard for appropriately selected CO₂ absorption lines. Special attention was given to minimizing undesirable precision degrading effects, in particular temperature and pressure fluctuations. Received: 16 April 2002 / Revised version: 28 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-713/5245237, E-mail: fkt@rice.edu     

Flame growth and wrinkling in a turbulent flow

High-speed planar laser-induced fluorescence (PLIF) and 3-D large eddy simulations (LES) are used to study turbulent flame kernel growth, wrinkling and the formation of separated flame pockets in methane/air mixtures. Turbulence was effected by a set of rotary fans situated in a cylindrical enclosure. Flame wrinkling was followed on sequential 2-D OH images captured at kHz repetition rates. Under stoichiometric conditions and low turbulence levels the flame kernel remains singly connected and close to spherical in shape. By increasing turbulence or reducing the stoichiometry of the mixture the formation of separated pockets could be observed and studied. The mechanisms behind these phenomena are investigated qualitatively by LES of a level-set G-equation describing the flame surface propagation in turbulent flows. Received: 12 April 2000 / Revised version: 26 June 2000 / Published online: 5 October 2000     

Detection of biogenic CO production above vascular cell cultures using a near-room- temperature QC-DFB laser

We report the first application of pulsed, near-room-temperature quantum cascade laser technology to the continuous detection of biogenic CO production rates above viable cultures of vascular smooth muscle cells. A computer-controlled sequence of measurements over a 9-h period was obtained, resulting in a minimum detectable CO production of 20 ppb in a 1-m optical path above a standard cell-culture flask. Data-processing procedures for real-time monitoring of both biogenic and ambient atmospheric CO concentrations are described. Received: 24 October 2001 / Published online: 29 November 2001     

Optoacoustic sensor head for depth profiling

-1 (for 0.25 mJ/cm²). The response function of the total experimental setup is derived from the optoacoustic signal of a black absorber, which is necessary to correct theoretical calculations with regard to the finite time response of the detection system. In the case of homogeneous samples, the theory of optoacoustic signal generation is verified, qualitative results are achieved with layered samples. Received: 14 October 1998 / Published online: 24 February 1999     

Analysis of the local conformation of proteins with two-dimensional fluorescence techniques

Two 2D fluorescence techniques are described which allow the study of conformational changes in proteins in their native form in μM solutions using aromatic amino acids (tryptophan, tyrosine) as intrinsic fluorescence markers. Simultaneous time- and wavelength-resolved fluorescence spectra are measured using a 80 ps laser source in conjunction with streak detection in the exit plane of an astigmatism-corrected spectrometer. This approach allows identification of different photophysical processes by their associated lifetime and spectral intensity distribution; errors due to the more common integration over a wider spectral range are avoided. Time-resolved spectra are sensitive to changes in the collisional environment (dynamic quenching) and can thus be used to monitor local conformation changes close to the respective fluorophors. This is demonstrated for the Ras protein which undergoes a drastic conformation change while binding to different nucleotides. Excitation-emission spectra are two-dimensional fluorescence images with one axis corresponding to the excitation and the other to the emission wavelength. Thus, they contain all conventional excitation and fluorescence spectra of a given substance. The 2D structure facilitates the interpretation of these spectra and allows the direct identification of resonance effects, scattering and the isolation of the contribution of different fluorophors to the complete spectrum. This is demonstrated for mixtures of tyrosine and tryptophan. In this case, both wavelength-resolved spectra and temporal decays are affected by energy transfer processes between the two amino acids. In a last example, both static and time-resolved spectral methods are combined to determine the respective contribution of static and dynamic quenching in calsequestrin. Evaluation of the fluorescence data is in good agreement with a recent crystallographic analysis which shows that all tryptophans are located in a conserved domain of the protein. Addition of Ca²⁺ ions leads to a more compact form of calsequestrin and to polymers. This information would not be obtainable from either of the two techniques alone. Received: 10 February 2000 / Published online: 13 September 2000     

Kerr-lens, mode-locked lasers as transfer oscillators for optical frequency measurements

We introduce a novel concept for optical frequency measurement and division which employs a Kerr-lens, mode-locked laser as a transfer oscillator whose noise properties do not enter the measurement process. We experimentally demonstrate that this method opens up the route to phase-link signals with arbitrary frequencies in the optical or microwave range while their frequency stability is preserved. Received: 23 July 2001 / Published online: 29 November 2001     

A laser-pumped magnetometer for the mapping of human cardiomagnetic fields

Magnetic fields produced by biological organisms contain valuable information on the underlying physiological processes and their pathologies. Currently, superconducting detectors cooled far below room temperature are required to measure these generally weak biomagnetic signals. We have developed a sensitive laser magnetometer based on optical pumping of cesium atoms that makes it possible to map the magnetic field produced by the beating human heart. A gradiometer formed by two identical sensors greatly reduces the influence of external stray magnetic fields. The magnetometer operates at room temperature and therefore opens the way to affordable and convenient monitoring of biomagnetic fields in research and medical diagnostics. Received: 8 January 2003 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +41-26/300-9631, E-mail: robert.wynands@unifr.ch